ireland epa

7/28/2019 Ireland EPA

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WASTEWATER TREATMENT MANUALS

TREATMENT SYSTEMS

forSINGLE HOUSES

ENVIRONMENTAL PROTECTION AGENCYAn Ghnomhaireacht um Chaomhn Comhshaoil

P.O. Box 3000, Johnstown Castle Estate, Co. Wexford, Ireland.

Telephone : +353-53-60600 Fax : +353-53-60699Email: [email protected] Website: http://www.epa.ie/


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Environmental Protection Agency 2000

Although every effort has been made to ensure the accuracy of the material contained in thispublication, complete accuracy cannot be guaranteed. Neither the Environmental Protection Agencynor the author(s) accept any responsibility whatsoever for loss or damage occasioned or claimed tohave been occasioned, in part or in full, as a consequence of any person acting, or refraining fromacting, as a result of a matter contained in this publication. All or part of this publication may be

reproduced without further permission, provided the source is acknowledged.

WASTEWATER TREATMENT MANUALS

TREATMENT SYSTEMS FOR SINGLE HOUSES

Published by the Environmental Protection Agency, Ireland.

Mr. John Mulqueen, Teagasc and Dr. Michael Rodgers, NUI, Galway,are the external contributors to this manual.

Mr. Gerard OLeary and Mr. Gerry Carty, EPA are the internal contributors.

06/00/1,000ISBN 1 84095 022 6Price IR15.0019.05


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TABLE OF CONTENTS

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

1.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91.2 CHARACTERISTICS OF WASTEWATER FROM A SINGLE HOUSE SYSTEM . . . . . . . . . . . . . . .101.3 CRITERIA FOR SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101.4 SEPTIC TANK SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101.5 MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.6 POLISHING FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.7 SITE DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .141.8 SITE CHARACTERISATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

2. SITE CHARACTERISATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

2.1 DESK STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .172.2 ON-SITE ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .182.3 INTEGRATION OF THE DESK STUDY AND ON-SITE ASSESSMENT INFORMATION . . . . . . .25

3. TREATMENT OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

3.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .273.2 CHOOSING AN ON-SITE SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .273.3 CHOOSING THE OPTIMUM DISCHARGE ROUTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303.4 LICENCE REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

4. SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 31

4.1 SEPTIC TANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314.2 PERCOLATION AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .354.3 CONSTRUCTION REQUIREMENTS FOR PERCOLATION PIPES . . . . . . . . . . . . . . . . . . . . . . . . .384.4 MAINTENANCE OF SEPTIC TANKS AND PERCOLATION AREAS . . . . . . . . . . . . . . . . . . . . . . .384.5 FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .394.6 SOIL FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .404.7 SAND FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .404.8 PEAT FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .434.9 OTHER INTERMITTENT MEDIA FILTER SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .444.10 CONSTRUCTED WETLANDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .444.11 POLISHING FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

CONTENTS i


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5. MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

5.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

5.2 BAF SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .515.3 RBC SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .525.4 SEQUENCING BATCH REACTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .525.5 OTHER TREATMENT SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .535.6 LOCATION OF MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .535.7 POLISHING FILTERS FOR MECHANICAL AERATION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . .54

REFERENCES AND FURTHER READING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

APPENDIX A: SITE CHARACTERISATION FORM . . . . . . . . . . . . . . . . . . . . . . . .59

APPENDIX B: PERCOLATION TESTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

APPENDIX C: EVALUATION OF TREATMENT SYSTEMS . . . . . . . . . . . . . . . . .67

APPENDIX D: SOIL/SUBSOIL CLASSIFICATION CHART . . . . . . . . . . . . . . . . .68

APPENDIX E: INDICATOR PLANTS OF DRAINAGE . . . . . . . . . . . . . . . . . . . . . .69

USER COMMENT FORM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70

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PREFACE

The Environmental Protection Agency was established in 1993 to license, regulate and control activities for

the purposes of environmental protection. In Section 60 of the Environmental Protection AgencyAct, 1992, itis stated that "the Agency may, and shall if so directed by the Minister, specify and publish criteria andprocedures, which in th e op in io n of the Ag ency are reasonable and desi rable fo r th e pu rpos es of

environmental protection, in relation to the management, maintenance, supervision, operation or use of all or

specified classes of plant, sewers or drainage pipes vested in or controlled or used by a sanitary authority for

the.....treatment or disposal of any sewage or other effluent to any waters". The following is a list of themanuals published to-date:

Wastewater Treatment Manuals - Preliminary Treatment; Wastewater Treatment Manuals - Primary, Secondary & Tertiary Treatment; Wastewater Treatment Manuals - Characterisation of Industrial Wastewaters; and Wastewater Treatment Manuals - Treatment Systems for Small Communities, Business, Leisure

Centres and Hotels.

This manual has been prepared to provide guidance on the design, operation and maintenance of on-sitewastewater treatment systems for a single house. The National Standards Authority of Ireland publishedstandard recommendations in 1975 (revised in 1991) with the aim of achieving satisfactory practice in thedesign, construction and maintenance of septic tank drainage systems. This manual has been prepared havingregard to the above and will inter alia assist planning authorities, developers, system manufacturers, systemdesigners, system installers, system operators to deal with the complexities of on-site systems. Wherereference in the document is made to proprietary equipment, this is intended as indicating equipment type andis not to be interpreted as endorsing or excluding any particular manufacturer or system.

Chapter 1 of this manual contains an introduction to wastewater treatment and the types of on-site treatmentsystems available for a single house.

Chapter 2 outlines the steps which should be taken to characterise a site. Characterisation of a site is dividedinto a desk study followed by an on-site assessment. The on-site assessment is subdivided into a visualassessment, a trial hole and a percolation test. The significance of the information collected during the deskstudy and the on-site assessment is summarised at the end of this chapter.

Chapter 3 outlines a methodology for choosing the on-site treatment system and the optimum dischargeroute.

Chapter 4 includes information on the design, construction and maintenance of a septic tank,soil percolationarea, intermittent filters, constructed wetlands and polishing filters.

Chapter 5 includes information on mechanical aeration systems and polishing filters.

A site characterisation form for use with this guidance manual is included in Appendix A.

This manual was prepared following completion of a research study carried out under the direction of the EPAin the period 1995 to 1997. A seminar on the conclusions of the study was held on the 12th February, 1998.The Geological Survey of Ireland (GSI) in conjunction with the Department of Environment and LocalGovernment (DELG) and the EPA have developed a methodology for the preparation of groundwaterprotection schemes to assist the statutory authorities and others to meet their responsibility to protectgroundwater. Groundwater protection responses have been developed for on-site systems for single houses(DELG/EPA/GSI, 2000). These responses should be consulted when reading this document.

The Agency welcomes any suggestions which users of the manual wish to make. These should be returned to

the Environmental Management and Planning Division at the Agency headquarters on the enclosed UserComment Form.

PREFACE iii


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ACKNOWLEDGEMENTS

In order to examine the current position in relation to on-site systems (in Ireland and internationally) and to

produce draft guidelines for their future use, a research project apropos on-site systems was part-financed bythe European Union through the European Regional Development Fund as part of the EnvironmentalMonitoring, R&D sub-programme of the Operational Programme for Environmental Services, 1994 -1999.The sub-programme is administered on behalf of the Department of the Environment and Local Governmentby the Environmental Protection Agency, which has the statutory function of co-ordinating and promotingenvironmental research.

The consortium awarded the project was led by the Civil Engineering Department, National University ofIreland, Galway. The project leader was Dr. Michael Rodgers, assisted by Mr. John Mulqueen and Mr. BrianGallagher. Other members of the project team were: Ms. Angela Casey, Mr. John Kenny, Mr. PadraicBallantyne, Mr. Eamonn Waldron (P.J. Tobin & Co.), Mr. Brendan Fehily (Fehily Timoney & Co.), Ms. MaryHensey (Hensey Glan Uisce Teo.), Ms. Sheila Davey (Neptune Labs) and Ms. Patricia Brannick (CentralMarine Services Labs, NUI Galway).

The project was monitored by a Technical Steering Group established by the EPA and included representativesof the EPA, the Department of the Environment and Local Government, the County and City EngineersAssociation and the project consortium.

Members of the Technical Steering Group were (in alphabetical order):

Mr. Gerry Carty, EPA Mr. Tony Cawley, Department of Environment and Local Government Ms. Lorraine Fegan, EPA Mr. Frank Gleeson, Sligo Co.Co., representing the City and Co. EngineersAssociation Mr. John Mulqueen, Teagasc

Mr. John OFlynn, Waterford Co.Co., representing the City and Co. EngineersAssociation Mr. Gerard OLeary, EPA Dr. Michael Rodgers, NUI, Galway, Project leader

As part of this research study a detailed questionnaire was issued to local authority and health board personnel.The co-operation of those who returned a completed questionnaire is gratefully acknowledged. The outputfrom the research study formed the bas is for the development of this manual. The Agency wishes toacknowledge the assistance of Mr. Donal Daly, Geological Survey of Ireland and Mr. Billy Moore, MonaghanCounty Council in reviewing early drafts of the manual.

The Agency would like to acknowledge the National Standards Authority of Ireland for the use of materialand diagrams from SR6.

The Agency wishes to acknowledge the contribution of those persons listed below, who took the time to offervaluable information, advice, comments and constructive criticism on the draft manual.

Mr. Martin Beirne, Environmental Officers Association.

Mr. Dan ORegan, National Standards Authority of Ireland.

Ms. Louise Mulcair, National Standards Authority of Ireland.

Ms. Yvonne Wylde, National Standards Authority of Ireland.

Mr. Bruce Misstear, Trinity College Dublin.

Mr. Paul OConnor, Environmental Assessments.

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Mr. Garvan Ward, Biocycle.

Dr. Eugene Bolton, Bord na Mona.

Dr. Hubert Henry, Bord na Mona.

Mr. Jer Keohane, Geotechnical and Environmental Services.

Mr. John Molloy, John Molloy Engineering.

Mr. Seamus Butler, Butler Manufacturing Services.

Mr. Albert Sneider, Aswatec.

Mr. Terry OFlynn, Banks Douglas Environmental Science.

The Agency also wishes to acknowledge the contribution of the Engineering Inspectors of the Department ofthe Environment and Local Government, and the Sanitary Services sub-committee of the Regional Laboratory,Kilkenny, who commented on the draft manual. The authors would also like to acknowledge the assistanceof Ms. Margaret Keegan, Mr. Donal Howley and Ms. Jane Brogan.

ACKNOWLEDGMENTS v


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LIST OF FIGURES

Figure 1: A typical septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Figure 2: Illustration of biomat formation on the base of a percolation trench . . . . . . . . . . . . . . . . . . . . . .12Figure 3: Schematic diagram of a soil covered mound sand filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Figure 4: Types of constructed wetlands (Section) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Figure 5: Selecting an on-site treatment system for a single house . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Figure 6: Soil classification chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Figure 7: Types of soil structure illustrated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Figure 8: Relationship between structure type and water movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Figure 9: Flow diagram for choosing an on-site system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Figure 10: Longitudinal section of a typical septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

Figure 11: Plan and section of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

Figure 12: Section of a percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Figure 13: Plan and section of a conventional septic tank system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Figure 14: Plan and section of a typical distribution box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

Figure 15: Illustration of an intermittent filter or constructed wetland system . . . . . . . . . . . . . . . . . . . . . . .39

Figure 16: Schematic diagram of a soil covered intermittent sand filter for an impervious soil . . . . . . . . . .42

Figure 17: Sub-Surface (SFS) horizontal flow wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

Figure 18: Vertical flow wetland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

Figure 19: Intermittent filter overlying and loading a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . .47

Figure 20: Secondary treatment unit followed by a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . .47

Figure 21: Secondary treatment unit followed by a percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Figure 22: Secondary treatment unit followed by a sand polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . .49

Figure 23: Schematic cross section of a sand polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49Figure 24: Mechanical aeration and polishing filter system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

Figure 25: Percolation test hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

Figure 26: Percolation test hole for shallow soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66

LIST OF TABLES

Table 1: Characteristics of domestic wastewater from a single house . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Table 2: Attributes of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Table 3: Factors to be considered during a visual assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Table 4: Minimum separation distances in metres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19Table 5: Soil/Subsoil textures and typical percolation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Table 6: Factors to be considered during a trial hole examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

Table 7: Trial hole - site requirements which indicate adequate percolation characteristics . . . . . . . . . . .24

Table 8: Information obtained from the desk study and on-site assessment . . . . . . . . . . . . . . . . . . . . . . .26

Table 9: Typical capacities of septic tanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

Table 10: Typical design features of a septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Table 11: Minimum gradients for drain to septic tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Table 12: Minimum percolation trench length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Table 13: Details of a typical percolation trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Table 14: Design criteria for intermittent sand filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43Table 15: Minimum trench lengths in a soil polishing filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Table 16: Design criteria for the SBR process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

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List of Abbreviations

C Capacity

C Degrees Celsius

Agency Environmental Protection Agency

BAF Biofilm aerated filters

BOD Biochemical oxygen demand

BOD5 Five-day biochemical oxygen demand

COD Chemical oxygen demand

DELG Department of the Environment and Local Government

d Day

DO Dissolved oxygen

DWF Dry weather flowEPA Environmental Protection Agency

FOG Fats, oils and g rease

FWS Free-water surface

g Gram

GSI Geological Survey Of Ireland

h Hour

kg Kilogram

ISO International Organisation for Standardisation

l Litrem Metre

m3 Cubic metres

m/s Metres per second

mg Milligram

mm Millimetre

NHAs National Heritage Areas

NUI National University of Ireland

p.e. Population equivalent

PFP Preferential flow paths

RBC Rotating biological contactors

s Second

SACs Special Areas of Conservation

S.I. Statutory instrument

SBR Sequencing batch reactor

SFS Sub-surface flow system

SS Suspended solids

TSS Total suspended solids

TWL Top water level

LIST OF ABBREVIATIONS vii


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1. INTRODUCTION

1.1 GENERAL

In Ireland, the wastewater from over one third of thepopulation - principally those living in dwellings notconnected to municipal sewers - rely on systemsdesigned to treat the wastewater at or near thelocation where it is produced. These wastewatertreatment systems are called on-site systems.

Many on-site systems are available for the treatmentof wastewater from single houses and are designedto:

treat the wastewater to minimisecontamination of soils and water bodies;

protect humans from contact withwastewater;

keep animals, insects, and vermin fromcontact with wastewater;

prevent direct discharge of untreatedwastewater to the groundwater;

minimise the generation of foul odours; and

prevent direct discharge of untreatedwastewater to surface water.

The biological treatment of the wastewater in on-sitetreatment systems occurs, in the main, under aerobicconditions. For example, in a soil percolation area,aerobic conditions are present due to the unsaturatednature of the soil.

Public health is threatened when on-site systems fail

to operate satisfactorily. System failures can resultin wastewater ponding or forming stagnant pools onthe ground surface when the wastewater is notabsorbed by the soil. In such circumstances ofsystem failure, humans can come in contact with theponded wastewater and be exposed to pathogens andfoul odours can be generated.

The three documents commonly used in relation tothe design of on-site systems in Ireland are:

SR6: 1991, Septic tank systems:Recommendations for domestic effluent

treatment and disposal from a single dwellinghouse (National Standards Authority ofIreland);

BS 6297: 1983, Design and installation ofsmall sewage treatment works and cesspools(British Standards Institution) deals mainlywith the design of small sewage treatmentworks serving small communities, notprimarily concerned with septic tank systems;and

US EPA/625/R-92/005 Manual:WastewaterTreatment/Disposal for Small Communities.

In order to examine the current position relating toon-site systems (in Ireland and internationally) andto establish guidelines for their future use, so as toensure sustainable development, a research s tudywas carried out between 1995 and 1997 (as part ofthe Department of the Environment OperationalP rogramme for Env i ronmental Serv i c e s , 1 9 9 4 -

1 9 9 9 ) . This study was co-ord i n ated by theD ep a rtment of Civil Engi n e e ri n g, The Nat i o n a lUniversity of Ireland, Galway under the direction ofthe Environmental Protection Agency (EPA) and wasfunded through the E nv i ronmental Monitori n g,

Research and DevelopmentSub-programme of theOperational Programme.

Some of findings of the research regarding singlehouse treatment systems were:

conventional septic tank systems (septic tankand percolation area), properly installed andmaintained, are satisfactory where suitablesubsoil conditions exist;

where suitable subsoil conditions do not

initially exist for treatment by means of aconventional septic tank system, sitedevelopment works may improve the subsoilconditions and make the subsoil suitable incertain circumstances;

in certain situations such as when unsuitablesubsoil conditions exist, other systems, whichinclude mechanical aeration or intermittentfilters for secondary treatment and followedby a polishing filter can be used;

all treatment systems including wastewater

collection systems must be designed,constructed, commissioned and maintained inaccordance with recognised standards; and

1 INTRODUCTION 9


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Parameter Typical concentration(mg/l unless otherwise stated)

Chemical Oxygen Demand COD (as O2) 400

Biochemical Oxygen Demand BOD5 (as O2) 300

Total solids 200

Total Nitrogen (as N) 50

Total Phosphorus (as P) 10Total coliforms (MPN/ 100 ml)* 107 - 108

all surface water and groundwater should beexcluded from entering any treatment system.

1.2 CHARACTERISTICS OF WASTEWATER

FROM A SINGLE HOUSE SYSTEM

For the purposes of this manual, a single housesystem refers to a dwelling house of up to ten peoplewith toilet, living, sleeping, bathing, cooking andeating facilities. Under no circumstances shouldrainwater, surface water or run-off from paved areasbe discharged to on-site single house systems. Toprevent the quantity of wastewater generated in ah o u s e h o l d, water reducing measures should beadopted. Such measures include: minimising the useof high water using equipment such as automatic

washing machines and dishwa s h e rs , the use ofshowers instead of baths, the use of dual flushcisterns in WCs, and the prompt fixing of leaks inhousehold plumbing system.

The s t re n g t h of the infl ow in terms of BOD( B i o chemical Oxygen Demand) into an on-sitesystem will largely depend on the water usage in thehouse; for example, houses with dishwashers mayhave a wastewater strength reduced by up to 35%due to dilution even though the total organic load tothe treatment system (kg/day) remains the same.

Household garbage grinders can increase the BODloading rate by up to 30% and because theseappliances are becoming more popular their use is animportant consideration.

Other important constituents in domestic wastewaterinclude nitrogen, phosphorus and microorganismss u ch as colifo rms. Table 1 gives typicalconcentration values for a number of parameters indomestic wastewater.

Typical daily hydraulic loading to an on-site systemfor single houses is 180 litres per person.

1.3 CRITERIA FOR SELECTION

When selecting a tre atment system to tre atwastewater from single houses, the system chosen:

should protect public health;

should protect the environment;

should be economical;

should operate with minimal maintenancefrom the owner; and

should have a long (> 20 years) lifespan.

On-site systems for single houses can be divided intotwo main categories:

septic tank systems; and

mechanical aeration systems.

1.4 SEPTIC TANK SYSTEMS

1.4.1 CONVENTIONAL SEPTIC TA N KSYSTEM

A conventional septic tank system comprises a septictank followed by a soil percolation area. The septictank functions as a primary sedimentation tank,removing most of the suspended solids from thewa s t ewater; this re m oval is accompanied by alimited amount of anaerobic digestion. It is in thep e rc o l ation area that the wa s t ewater undergo e ssecondary treatment and is purified. The wastewaterfrom the septic tank is distributed to a suitable soilpercolation area, which acts as a bio-filter. As thewastewater flows into and through the subsoil, itu n d e rgoes surface fi l t rat i o n , s t ra i n i n g, p hy s i c o -chemical interactions and microbial bre a k d ow n .

TABLE 1: CHARACTERISTICS OF DOMESTIC WASTEWATER FROM A SINGLE HOUSE

* MPN Most Probable Number

10 WASTEWATER TREATMENT MANUALS TREATMENT SYSTEMS FOR SINGLE HOUSES


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After flowing through a suitable percolation area thewastewater is suitable for discharge.

A typical septic tank is illustrated in Figure 1 and theattributes of a septic tank are given in Table 2. Thetank, which should be two-chambered, allows thewastewater from the dwelling house time to settleout into three layers viz. the sludge layer, the liquidlayer and the scum layer (Figure 1). The sludge layeris a blanket of heavy solids and some coagulatedmaterials, which settle out on the tank floor. Theliquid laye r, while re l at ive ly free of coars esuspended solids, is high in decomposable dissolvedand colloidal organic matter and contains bacteria,viruses, worm eggs, larvae etc.; it is allowed to flowto the perc o l ation area through a tee-pipe fo r

distribution and secondary treatment. The scumlayer consists of greases, oils and gas-buoyed solidswh i ch accumu l ate as a layer on the surfa c e.Detention times should be in excess of 24 hours.

The subsoil through which the wastewater percolatesacts as an at t a ched growth medium fo rmicroorganisms. As the percolation trenches are

loaded with wastewater from a septic tank, a biomatlayer quickly develops along the base and wettedsides of these trenches (Figure 2). The biomat layerconsists of a deposit of microorganisms, slimes andsludge which coats the floor and walls of the trenchand enters the subsoil for a short distance inside theinfiltrative surface. The biomat drastically lowers thei n fi l t ration through the base and sides, c a u s i n gponding in the tre n ches. The ponding causeswastewater to flow over the entire trench base and ina short time leads to a uniform distribution of thewastewater over the total length of the trenches.Ponded wastewater gradually rises in the trenchesaccompanied by the development of a biomat alongthe wetted walls of the trench until an equilibrium isreached, causing flow through the sides and base. An

adequate depth of gravel aggregate in the trench isimportant for hydraulic function. The biomat layerthen determines the hydraulic loading. Therefore forl o n g - t e rm successful operation of a perc o l at i o nsystem, the system should be designed to cope withthe impedance caused by the development of thebiomat layer along the base and wetted walls of thepercolation trench.

OutletTWL

Sludge layer

Inlet

Liquid layer

CHAMBER NO. 1 CHAMBERNO. 2

Scum Layer

Manhole cover with ventilation Manhole cover with ventilation

SECTION A - A

FIGURE 1: A TYPICAL SEPTIC TANK

1 INTRODUCTION 11


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initial operation - no biomat

Biomat formation

Biomat formation

Biomat formation with

extension of clogging of

the base

and adjoining walls

of trench

Distribution box

(Order of weeks)

A properly constructed septic tank will:

Retain and remove 50% or more solids; outflow from tank contains about 80 mg/l solids

Allow some microbial decomposition

Accept sullage (i.e. water from baths, wash hand basins etc.)

Accept water containing detergents

Reduce clogging in the percolation area

Not fully treat domestic wastewater

Not work properly if inadequately maintained

Not significantly remove microorganisms

Not remove more than 15 - 30 % of the BOD

Not operate properly if pesticides, paints, thinners, solvents, disinfectants or household hazardoussubstances are discharged to it

Not accommodate sludge indefinitely

Not operate properly if surface waters (i.e. roofs etc.) are discharged to it

TABLE 2: ATTRIBUTES OF A SEPTIC TANK

FIGURE 2: ILLUSTRATION OF BIOMAT FORMATION ON THE BASE OF A PERCOLATION TRENCH



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A percolation area is considered "failing" when (i) itcauses a backing up of wastewater in the distributionbox or (ii) it does not keep untreated wastewaterbelow the surface of the land or (iii) it does not treatthe wastewater be fore it reaches groundwater orsurface water.

In Ire l a n d, a significant number of septic tanksystems do not function properly, mainly because

t h ey have been poorly constru c t e d, i n s t a l l e d,operated, maintained or, are located in areas withunsuitable subsoils, or percolation of the septic tankeffluent is through a soakaway. It is important tonote, however, that in the absence of a connection toa sewer system, one of the most appropriate and costeffective means of treating wastewater in a suitablesite is a pro p e rly constructed and maintainedconventional septic tank system.

1.4.2 FILTER SYSTEMS

Where the subsoil is unsuitable for tr eating thewastewater from a septic tank, filter systems may beused. These include intermittent soil filters, sandfilters, peat filters and other filters using materialssuch as plastic foam filters and geosynthetic strips.Intermittent soil filters comprise suitable soils placedoften in the form of a mound, through which septictank effluent is filtered and purified. Intermittentsand filters consist of one or more beds of gradedsand underlain at the base by a filter gravel orpermeable soil layer to prevent outwash or piping ofthe sand. Soil covered intermittent sand filters maybe undergro u n d, p a rt underground and part

ove rgro u n d, or ove rgro u n d. The latter twoconstructions are commonly referred to as moundsystems (Figure 3). Fibrous peat and plastic media

for the other fi l t e rs are usually installed inprefabricated containers (prefabricated intermittentfilters).

1.4.3 CONSTRUCTED WETLANDS

C o n s t ructed wetlands can also be used for thet re atment of wa s t ewater from single houses.Wetlands are areas with high water t ables which

promote aquatic vegetation or water tolerant plantssuch as reeds.

Primary treatment by a septic tank is used prior todischarge to a constructed wetland. In the wetland,the wastewater from a septic tank is treated by acombination of physical, chemical and biologicalprocesses that develop through the interaction of theplants (re e d s ) , the growing media (gravel) andmicroorganisms. These processes include settlementand filtering of suspended solids, biodegradation,plant uptake and chemical interactions.

There are two different types of constructed wetlandsand they are characterised by the flow path of thewater through the system (Figure 4). In horizontalflow constructed wetlands, wastewater is introducedat one end of a flat to gently sloping bed of reeds andflows horizontally across the bed to the outfall end.In the second type, called the vertical-flow wetland,the wa s t ewater is dosed unifo rm ly ove r, a n dintermittently onto the media, and gradually drainsvertically to a drainage network at the base of themedia. Constructed wetlands should be securelyfenced off to prevent access by unauthorised persons,

especially children.

Distribution gravel

GeotextileDistribution laterals

Filter sand

Gravel, fractured bedrock, high water table or impervious soil

filter gravel or permeable soil

Soil

cap

Top soil

Washed gravel

FIGURE 3: SCHEMATIC DIAGRAM OF A SOIL COVERED MOUND SAND FILTER

1 INTRODUCTION 13


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1.5 MECHANICAL AERATION SYSTEMS

In recent years, many mechanical aeration systemshave come on the market; these offer a solution insome cases where a site may be unsuitable fortreating the septic tank wastewater, or an alternativeto the conventional septic tank system. Th e s esystems include the following:

biofilm aerated (BAF) systems;

rotating biological contactor (RBC) systems;and

sequencing batch reactors (SBR) systems.

BAF systems may consist of a primary settlementtank, aerated filter media and a secondary settlementtank. RBC systems consist of a primary settlementtank, a biological treatment compartment and as e c o n d a ry settlement tank. These systems are

similar to conventional trickling filter systems in thatthe micro o rganisms carrying out the secondarytreatment are attached to an inert media surface.Sequencing bat ch re a c t o rs (SBR) consist of aprimary settlement tank and a reactor in whichbiological treatment and clarification occur.

1.6 POLISHING FILTERS

Polishing filters should be used to treat wastewaterfrom intermittent filters, constructed wetlands andmechanical aeration systems. These filters consist of

either soil or sand and are employed to reducemicroorganisms, phosphorus, and nitrate nitrogen.Soil polishing filters may comprise in-situ,improved

soil or imported soil, whereas sand polishing filterscomprise stratified layers of sand.

1.7 SITE DEVELOPMENT

Where a site is initially unsuitable for a septic tanksystem, site development works may improve thesite and make it suitable for the development of anon-site system.

Site development works could include lowering thewater table, raising the ground surface by filling withsuitable soil, part replacement of the subsoil bysuitable soil or subsoil loosening. After carrying outthe necessary improve m e n t s , the site should bereassessed to establish whether the improved soil issatisfactory.

1.8 SITE CHARACTERISATION

The objective of a site characterisation is to obtain

sufficient information to determine if the site can bedeveloped for an on-site system. Characterising thesite involves a number of s tage s . These shouldinclude:

a desk study, which collects any informationthat may be available on maps etc. about thesite;

a visual assessment of the site, which definesthe site in relation to surface features;

a trial hole to evaluate the soil structure, depth

to rock and water table; and

percolation tests.

Septic tank

Horizontal flow wetland

Septic tank

Vertical flow wetland

FIGURE 4: TYPES OF CONSTRUCTED WETLANDS (SECTION)



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DESK

STUDY

SITE

IMPROVEMENT

UNSUITABLE*

ON-SITE

INVESTIGATION

VISUAL ASSESSMENT

TRIAL HOLE

PERCOLATION

TESTS

UNSUITABLE**

NO SITERESTRICTIONS

SITE

CHARACTERISATION

SUITABLE

* This option may not always be available

NOT SUITABLE

Or

Or

SITE

RESTRICTIONS

** Site may not always be suitable for an

on-site system

Fi g u re 5 below summarises the protocol to befollowed to select and design an on-site system.

The concepts of risk, risk assessment and risk

management have recently become important toolsin environmental protection. Risk can be defined asthe likelihood or expected frequency of a specifieda dve rse consequence. Applied for example togro u n dwat e r, a risk ex p resses the likelihood ofc o n t a m i n ation arising from a proposed on-sitetreatment system (called the hazard). A hazardpresents a risk when it is likely to affect somethingof value (the target, e.g. surface water). It is thec o m b i n ation of the pro b ability of the hazard

occurring and its consequences that is the basis ofrisk assessment. Risk management involves siteassessment, selection of options and implementationof measures to prevent or minimise the consequences

and probability of a contamination event (e.g. odournuisance or water pollution). The methodology forselection and design of an on-site system in thismanual embraces the concepts of risk assessmentand risk management.

The remainder of this manual sets out how a sitecharacterisation should be completed and a choice ofon-site system made.

FIGURE 5: SELECTING AN ON-SITE TREATMENT SYSTEM FOR A SINGLE HOUSE

1 INTRODUCTION 15

FILTER SYSTEM

AND

POLISHING FILTER

MECHANICAL AERATION SYSTEM

AND

POLISHING FILTER

FILTER SYSTEM /MECHANICAL AERATION SYSTEMAND

POLISHING FILTER

CONVENTIONAL SEPTIC TANK SYSTEM


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The purpose of a site assessment is to determine thesuitability of the site for an on-site treatment system.The assessment will also help to predict thewastewater flow through the subsoil and into thesubsurface materials.

The key to installing a reliable on-site system thatminimises the potential for pollution is to select anddesign a suitable tre atment system fo l l owing athorough site assessment. For a subsoil to beeffective as a med ium for treating wastewater, it

must retain the wastewater for a sufficient length oftime, and it must be well aerated.

Only after a site evaluation has been completed canan on-site system be chosen. The info rm at i o ncollected in the evaluation will be used to select theon-site system.

In designing a soil perc o l ation area to tre atwastewater, three factors must be considered:

the suitability of the site;

the suitability of subsoil and groundwaterconditions, and

the permissible hydraulic load on the subsoil.

To determine these considerations a sitecharacterisation is undertaken. This includes:

1) a desk study; and

2) an on-site evaluation, consisting of :

a visual assessment;

a trial hole; and

percolation tests.

2.1 DESK STUDY

The purposes of a desk study are to:

obtain information relevant to the site, whichwill assist in assessing its suitability;

identify targets at risk; and

establish if there are site restrictions.

A desk study involves the assessment of availabledata pertaining to the site and adjoining area thatmay determine whether the site has any restrictions.Information collected from the desk study shouldi n clude mat e rial re l ated to the hy d ro l ogi c a l ,hydrogeological and planning aspects of the site,wh i ch may be ava i l able in maps or rep o rt s .Hydrological aspects include locating the presence(if any) of streams, rivers, lakes, beaches, shellfishareas and/or wetlands while hydrogeological aspectsinclude:

soil type;

subsoil type;

bedrock type;

aquifer type;

vulnerability class; and

groundwater protection response (refer to theDELG/EPA/GSI groundwater protection

scheme and groundwater protectionresponses for on-site systems for singlehouses).

The Gro u n dwater Protection Schemes prov i d eguidelines for developers in assessing groundwatervulnerability and for the planning authorities incarrying out their functions, and a fr amework toassist in decision-making on the location, nature andcontrol of developments and activities (includingsingle house treatment systems) in order to protectgroundwater. The density of on-site systems should

be considered also at this stage. The protectionresponses required to protect groundwater from on-site systems should be satisfied. Where no schemeex i s t s , i n t e rim measures as set out in theGroundwater Protection Schemes should be adopted.Each site is specific and local factors should be takeninto account in using this guideline information.

Planning aspects include:

zoning in the development plan;

presence of significant sites (archaeological,natural heritage, historical etc.); and

past experience of the area.

2 SITE CHARACTERISATION 17

2. SITE CHARACTERISATION


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2.1.1 INTERPRETING THE RESULTS OF THEDESK STUDY

The information collected from the desk study

should be examined and the following should beconsidered for all treatment options:

zoning (including groundwater protectionschemes): Zoning for groundwater protectionschemes outlines the aquifer classification inthe general area and the vulnerability of thegroundwater. The groundwater protectionresponses will provide an early indication ofthe probable suitability of a site for an on-sitesystem. The on-site assessment will laterconfirm or modify such responses;

presence of significant sites: Determinewhether there are significant archaeological,natural heritage and/or historical featureswithin the proposed site. To avoid anyaccidental damage, a trial hole assessment orpercolation tests should not be undertaken inareas, which are at or adjacent to significantsites (e.g. SACs, NHAs etc.), without prioradvice from Duchas, the Heritage Service;

nature of drainage: A high frequency ofwatercourses on maps indicates high orperched watertables; and

past experience: Is there evidence ofsatisfactory or unsatisfactory local experiencewith on-site treatment systems?

2.2 ON-SITE ASSESSMENT

2.2.1 VISUAL ASSESSMENT

The purposes of the visual assessment are to:

assess the potential suitability of the site;

assess potential targets at risk (adjacentwells); and

provide sufficient information to enable adecision to be made on the suitability of thesite for the wastewater to be treated and thelocation of the proposed system within thesite. The principal factors which should beconsidered are listed below.

Topography and landscape: Topography reflectsthe relief of the site. Landscape position reflects thelocation of the site in the landscape e.g. crest of hill,valley, slope of hill. Sites which are on level, well

d rained are a s , or on convex slopes are mostdesirable. Sites which are in depressions, or on thebottom of slopes or on concave slopes are lessdesirable.

The principal factors which should be consideredare, relief, shape and form, rock outcrops, wells andwat e rc o u rs e s , land use, vege t at i o n , t ra m p l i n gdamage to the soil by livestock, seepage, boundaryof property, and old building foundations.

Slope: It is more difficult to install pipework andensure that the wastewater will stay in the soil if theland has an extreme slope. Where there is surfacewater run-off and interflow, low-lying areas and flatareas generally receive more water. This accounts to

some extent for the occurrence of poorly drainedsoils in low-lying areas. Soils with poor drainage,however, may also be found on good slopes wherethe parent mat e rial or the subsoil is of lowp e rm e ab i l i t y. Provision must be made for theinterception of all surface run-off and seepage, andits diversion away from the proposed percolationarea.

P roximity to surface fe at u re s : M i n i mu mseparation distances as set out in th e followingch ap t e rs should be maintained from specifi e dfeatures. The presence/location of surface featuressuch as wells/springs, watercourses, dwelling houseson adjacent sites, site boundari e s , ro a d s , s t e epslopes, etc. should be noted.

Wells: Wells should be considered as targets at risk.The groundwater flow direction, where it can beinferred; the number of wells; the presence of anywetlands, and presence of any karst features shouldbe noted.

Drainage: A high density of streams or ditchestends to indicate a high water table and potential risk

to surface water. Low density of streams indicates afree draining subsoil and or/bedrock.

Type of vegetation: Rushes, yellow flags (irises)and alders indicate poor percolation characteristicsor high water table levels. Grasses, trees and fernsm ay indicate suitable perc o l ation ch a ra c t e ri s t i c s .Plants and trees indicating good drainage and poordrainage are illustrated in Appendix E.

Ground condition: The ground conditions duringthe on-site investigation should be noted. Trampling

damage by livestock can indicate impeded drainageor intermittent high water tables, especially whereaccompanied by widespread ponding in hoof prints.The factors examined during a visual assessment and



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their significance are summarised in Table 3 above.

2.2.2 INTERPRETING THE RESULTS OF THEVISUAL ASSESSMENT

The minimum separation distances that should beused in the visual assessment are set out in Table 4.

These apply to any on-site system. If any of these

requirements cannot be met, on-site systems cannotbe developed on the si te. The re c o m m e n d e dminimum distances from wells should satisfy there q u i rements of the gro u n dwater pro t e c t i o nresponse, which should have been reviewed duringthe desk study. In some cases, the requirements ofthe groundwater protection scheme and responses

may be greater than the distances set out in Table 4.

Factor Significance

Water level in ditches and wells Indicates depth of unsaturated subsoil

Shape, slope and form of site May indicate whether water will collect at a siteor flow away from the site

Presence of watercourses May indicate low permeability or a high watertable

Presence and types of rock outcrops Insufficient depth of subsoil to treat wastewaterallowing it to enter the groundwater too fast

Proximity to adjacent percolation areas and/or houses May indicate too high a loading rate for thelocality and/or potential nuisance problems

Land use and type of grassland surface (if applicable) Indicator of rate of percolation or groundwaterlevels

Vegetation type Indicator of the rate of percolation orgroundwater levels

Proximity to wells on-site and off-site, water supply Indicates targets at risksources, groundwater, streams, ditches, lakes,surface water ponding, beaches, shellfish areas,and wetlands

TABLE 3: FACTORS TO BE CONSIDERED DURING A VISUAL ASSESSMENT

Type of system Watercourse/ Wells/ Lake Any Site Road Slope

stream springs* Dwelling boundary breaks/

cuts

Septic tank;prefabricatedintermittent filters; 10 10 50 7 3 4 4mechanical aerationsystems

In situ intermittentfilters; percolation 10 30 50 10 3 4 4area; polishing filters

TABLE 4: MINIMUM SEPARATION DISTANCES IN METRES

* This applies to wells down-gradient or where flow direction is unknown. For more information on wells alongside orup-gradient consult DELG/EPA/GSI ground water protection scheme1.

1 Department of Environment and Local Government, Environmental Protection Agency, Geological Survey of Ireland2000. Groundwater Protection Responses for On-site Systems for Single Houses.



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2.2.3 TRIAL HOLE

The purposes of the trial hole are to determine:

the depth of the water table;

the depth to bedrock; and

the soil and subsoil characteristics.

The trial hole will also help to predict the wastewaterflow through the subsoil.

The trial hole should be as small as practicable, e.g.1.0 metre x 0.75 metre in plan, and should beexcavated to a depth of at least 1.2 m below the invert

of the lowest percolation trench. In the case of a levelsite the depth of the trial hole should be a minimumof 2.1 m below ground surface. In the case of asloping site it is essential that an estimate of thedepth of the invert of the percolation trench be madebeforehand. The hole should remain open for 48

hours to establish the depth to the water table (ifpresent) and should be securely fenced. The soilch a ra c t e ristics assessed are : t ex t u re, s t ru c t u re,p resence of pre fe rential fl ow pat h s , d e n s i t y,

compactness, colour, layering, depth to bedrock anddep th to the watertable. If items of suspec teda rch a e o l ogical interest are discove re d, c o n t a c tshould be made with the relevant authorities.

Depth to bedrock and depth to water table: Adepth of 1.2m of suitable free draining unsaturatedsubsoil, to the bedrock and to the water table belowthe base of the percolation trenches, must exist at alltimes to ensure sat i s fa c t o ry tre atment of thewastewater. Sites assessed in summer when thewater table is low, should be examined below the

proposed invert of the perco lation p ipe for soilmottling - an indicator of seasonally high watertables.

Soil texture: Texture is the relative proportions ofsand, silt and clay particles in a soil after screeningthrough a 2 mm size sieve. The rate and extent ofmany important physical processes and chemicalreactions in soils are governed by texture. Physicalprocesses influenced by texture include drainage andmoisture retention, diffusion of gases and the rate oftransport of contaminants. Texture influences theb i o film surface area in wh i ch biochemical andchemical reactions occur. The soil texture may becharacterised using the chart in Figure 6 .

To classify a soil/subsoil, it should be wetted andsqueezed between the fingers. Soils/subsoils high insand feel sandy, soils/subsoils high in silt are silky tofeel and soils/subsoils high in clay are sticky andh ave tensile strength. A guide to assist thecl a s s i fi c ation of soil/subsoils is included inAppendix D. Va rious soil/subsoil tex t u reclassifications schemes exist; Table 5 compares threesuch classifications and indicates typical percolation

rates.

FIGURE 6: SOIL CLASSIFICATION CHART

Soil Class Subsoil Unified Class Typical Percolation

Classification Classification Classification Rate *

(min/25mm)

sand medium fine SAND sand; silty sand; clayey sand 1 - 5

loamy sand silty, clayey SAND sand; silty sand; clayey sand 6 - 10

sandy loam silty SAND silty sand; clayey sand 6 - 30

loam / silt loam sandy SILT silty fine sands - low plasticity 31 - 50**

TABLE 5: SOIL/SUBSOIL TEXTURES AND TYPICAL PERCOLATION RATES

* typical for soil in an uncompacted state and not indurated or hard.** upper limit of 50 may need to be reviewed in the light of on-going research findings.


Example x: 50% Clay30% Sand20% Silt

x


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Structure: Soil structure refers to the arrangementof the soil particles into larger units or compoundparticles in the soil. The soil particles,sand, silt, clayand organic matter, are generally clumped together

to form larger units called peds. The shape and sizeof the peds have a large effect on the behaviour ofsoils. A ped is a unit of soil structure such as anaggregate, a crumb, a prism, a block or granulesformed by natural processes. Soil texture plays amajor part in determining soil stru c t u re. Th es t ru c t u re of the soil influences the pore space,a e ration and dra i n age conditions. Types of soilstructure (shape of the ped) are illustrated in Figure7 and are:

Crumb - peds have curved surfaces. The

faces of peds do not fit into the faces ofneighbouring peds. Commonly found in topsoils.

Granular- peds composed of single grainswith curved surface e.g. sands.

Blocky - the faces of each ped are nearlyequal and fit into the faces of neighbouringpeds. They are common in loamy soils.

Prismatic - the soil particles are arrangedabout a vertical axis and are bounded byrelatively smooth vertical faces; the verticalfaces are longer than the horizontal faces andfit into neighbouring peds; commonly foundin clayey soils.

Platy - peds consist of thin flat plates and areformed where soils dry out rapidly (rare inIreland).

Structureless - massive - soil is not separatedinto structural units but occurs as one large(often plastic) mass; typical of clays and silts.

Structureless - single grain - soil has novisible aggregation; on immersion in water,soil readily disintegrates into its single grainsof gravel, sand, silt and clay.

The rate of flow of water through soils of the variousstructures is in the following order:

crumb faster than blocky; blocky faster thanstructureless-single grain.

Structureless massive structure have very low flowrates and can often be regarded as impervious ( e.g.with a permeability < 10 mm/day). The relationshipb e t ween stru c t u re type and water movement is

illustrated in Figure 8. Where water is supplied to asoil at a rate less than its permeability, as in the caseof septic tank effluent, the r ate of flow through thesoil equals the rate of supply in soils of adequate

permeability.

The preferred structures from a wastewater treatmentperspective are granular(as fine sand), blocky andstructureless-single grain sandy loams, loams andsilt loams. Unstru c t u red massive plastic soilsindicate seasonal or continuous saturation and areunsuitable. Likewise soils with extensive, large andcontinuous fissures and thick lenses of gravel andcoarse sand may be unsuitable; this suitability willbe assessed in the percolation test.

Preferential flow paths: Preferential flow paths(PFPs) are formed in soils by biological, chemicaland physical processes and their interactions. Theymay be randomly distributed or their formation maybe systematic, reflecting the influence of agriculturalpractices. Research in recent years indicates thatPFPs can have a significant influence on them ovement of ponded or perched water insoil/subsoils where free (non capillary) water is indirect contact with PFPs. The presence of PFPsshould be noted during the trial hole assessmentbecause their presence may influence the percolationrate of the subsoil.

Soil density: this refers to how tightly the soil grainsare packed together. Dry bulk density is commonlyclassified as low, medium or high.

Low - loose and easily disintegrated intostructural peds when dry to moist; typical ofmany topsoils;

Medium - dry bulk density of intermediatemagnitude (e.g. 1.3 tonne/m3); typical ofmany permeable soils; and

High - compact and strong and resistant topenetration; typical of some deep permeablesoils.

Soils of low and medium dry bulk density are best aspercolation soils.

Colour: This is a good indicator of the state ofa e ration of the soil/subsoil. Free dra i n i n gunsaturated soils/subsoils are in the oxidised state atall times and exhibit brown, reddish brown and

yellowish brown colours. Many free draining soils oflimestone origin with deep water tables are grey atdepth. Saturated soils/subsoils are in a reduced stateand exhibit dull grey or mottled colours. Mottling



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FIGURE 7: TYPES OF SOIL STRUCTURE ILLUSTRATED

FIGURE 8: RELATIONSHIP BETWEEN STRUCTURE TYPE AND WATER MOVEMENT



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(comprising a reddish brown or rusty staining) of thesoil layers can indicate the height to which the watertable rises in periods of high rainfall; mottling in agrey mat rix (grey with re ddish brown mottles)

indicates aeration along old root channels and crackswhile the matrix remains reduced; this soil layer issaturated during part of the year.

Layering: This is common in soils, arising duringdeposition and/or subsequent weathering. In soils,that are free draining in the virgin state, weatheringcan result in downward movement of some of theclay fraction leading to enrichment of a sub-layer

with clay. In some areas a thin, hard, rust colouredimpervious layer can develop (iron pans) as a resultof the downward leaching of iron and manganesecompounds and deposition at shallow depth (less

th an 1m ). The underlying subsoil often has asatisfactory percolation rate. Enrichment with clayparticles and precipitation of iron and calcium andm agnesium compounds can lead to ve ry lowpercolation rates. Such soils can often be improvedby loosening or by breaking the impervious layer.

The factors that are evaluated from the trial hole andtheir significance are summarised in Table 6 below.

Factors Significance

Soil structure and texture Both influence the capacity of soil to treat and dispose of the wastewater; silts and clays are generally unsuitable

Mottling Indicates seasonal high water tables

Depth to bedrock Subsoil must have sufficient depth to treat wastewater

Depth to water table Wet subsoils do not allow adequate treatment of wastewater

Water ingress along walls Indicates high water table

Season Water table varies between seasons

TABLE 6: FACTORS TO BE CONSIDERED DURING A TRIAL HOLE EXAMINATION



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2.2.4 INTERPRETING THE RESULTS OF THETRIAL HOLE TEST

Table 7 sets out the subsoil characteristics which

i n d i c ate sat i s fa c t o ry perc o l ation and other

ch a ra c t e ris tics necessary for the tre atment ofwastewater. The percolation characteristics will beconfirmed later by examining the percolation testresults.

Subsoil Characteristics Requirements

Minimum depth of unsaturated permeable subsoil 1.2 mbelow base of all percolation trenches Percolation trench cross section

for a level site

Minimum depth of unsaturated subsoil to bedrock 1.2 mbelow invert level of all percolation trenches

Minimum depth to water table below invert of all 1.2 mpercolation trenches*

Texture of unsaturated soil/subsoil Sand (medium fine SAND),Loamy sand (silty, clayey SAND),Sandy loam (silty SAND),Loam and silt loam (sandy SILT);

Structure of unsaturated soil/subsoil Granular, blocky; and structurelesssingle grain

Colour of unsaturated soil/subsoil Greyish brown, reddish brown, andyellowish brown; grey in the case of manyfree draining limestone soils

Layering in the walls of a percolation trench or No gravel or clay layer should be presentbelow its invert

Bulk density of unsaturated soil/subsoil Low to medium

TABLE 7: TRIAL HOLE - SITE REQUIREMENTS WHICH INDICATE ADEQUATE PERCOLATION CHARACTERISTICS

Ground level

Topsoil

GravelGravelDistribution pipe &

Gravel

UnsaturatedSubsoil

300 mm

150 mm

100 mm

250 mm

1200 mm

450 mm2000 mm

*where the dimensions of the percolation trench and unsaturated soil are as shown, the minimum depth to the watertable is 2 m below ground surface.



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2.2.5 PERCOLATION TESTS

A perc o l ation (perm e ability) test assesses thehydraulic assimilation capacity of the subsoil i.e. the

length of time for the water level in the percolationhole to fall from a height of 300 mm to 200 mmabove the base of the test hole in a percolation area.The permeability of each soil class may vary withinan order of 10 - 100 fold depending primarily on thep a rt i cle size grading wh i ch re flects the va ry i n gamounts of fine particles, the structure and void ratioin each soil class. The procedure for carrying out apercolation test is set out in Appendix B.

The results of percolation tests are expressed as a "T"value. This is the average time in minutes for the

water level to fall 25 mm in each of two percolationtest holes over the water depth range of 300 mm to200 mm in the proposed percolation area.

To carry out a percolation tes t (which should bewithin the proposed percolation area), a 30 0 mmsquare percolation test hole is excavated to a depth of400 mm below the invert of the proposed distributionpipe.

To establish the percolation value for shallow soilsthat may be used for polishing filters (discussedlater) a modification of the T test is required. Forthis, the test hole is 400 mm below the groundsurface as opposed to 400 mm below the invert of thedistribution pipe for the T test. To avoid confusionwith the previous test, this test is called a P test, andthe values are referred to as P values.

2.2.6 INTERPRETING THE RESULTS OF THEPERCOLATION TEST

A "T" value greater than 50 suggests that wastewaterentering such subsoils would cause ponding on-site.A "T" value less than 1 suggests that the retention

time for the wastewater would not be long enough toprovide satisfactory treatment. If the percolation Tvalue is within the range 1-50* then the site shouldbe suitable for the development of a conventionalseptic tank system.

Where the "T" value is less than 1 or greater than 50

the site is not suitable for the treatment of septic tankwastewater by soil percolation. Other options shouldbe considered such as a constructed percolation area,mechanical aeration systems, intermittent filters or

constructed wetlands. Where mechanical aerationsystems, intermittent filters or constructed wetlandsare used, the treated wastewater from such systemsshould discharge to rece iving waters (surface orgroundwater) through a polishing filter.

Where shallow or impervious soils exist, a soilpercolation area may still be possible by importingsuitable soil and placing it in lift s in the proposedp e rc o l ation area such that there is a minimu mthickness of 2.0 m of unsaturated soil with drainageover the bedrock or impervious soil. A trial hole and

percolation tests (T - tests) should then be carried out(se e Appendix B - Percolation Tests for furtherdetails) in the same way as for in situ soils. Wheresuch soil filling is not feasible, alternative systemsfollowed by a polishing filter may be suitable.

Where an alternative system and a polishing filter aree m p l oye d, the nat u re of the soil or bedro cku n d e rlying the polishing filter determines thedisposal route of the treated wastewater. For apolishing filter overlying impervious soils or rocks,the treated wastewater is collected in a suitabledrainage system and discharged to surface waters.Polishing filters overlying permeable soils, gravelsor bedrock with a T/ P value less than 50 mayd i s ch a rge the tre ated wa s t ewat e rs to thegroundwater. A flow diagram to assist in the choiceof an on-site system is shown in Figure 9.

2.3 INTEGRATION OF THE DESK STUDY

AND ON-SITE ASSESSMENT INFORMATION

Table 8 summarises the information that can beobtained from the data collected from the desk study

and the on-site assessment. This information is usedto characterise the site and used later to choose anddesign an on-site system. An integrated approachwill ensure inter alia that the targets at risk areidentified and protected.

* upper limit of 50 may be reviewed depending on experience.



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To assist in the selection of the on-site system and tos t a n d a rdise the assessment pro c e s s , a sitecharacterisation form has been prepared (AppendixA ) . The completed form should accompany all

planning applications for on-site systems for a singlehouse. A verification section is included at the endof the form and this should be completed by theplanning authority.

Information Collected Relevance Factor Determined

Zoning (County development plan,groundwater protection scheme,groundwater protection response etc.);

Hydrological features;

Density of houses;

Proximity to significant sites;

Experience of the area;

Proximity to surface features;

Depth to bedrock

Texture;

Structure;

Bulk density;

Layering;

Colour;

Mottling;

Depth to water table;

Drainage (permeability);

Percolation test;

TABLE 8: INFORMATION OBTAINED FROM THE DESK STUDY AND ON-SITE ASSESSMENT

Identifies planning controlsand targets at risk

Indicators of the suitability ofthe subsoil for percolationand of its percolation rate

A minimum thickness of 1.2 mof unsaturated soil is required

to successfully treat septictank effluent

Identifies suitable soils that haveadequate but not excessive

percolation rates(T or P value)

Depth of the water table

Unsuitability if prismatic,structureless-massive

silt or clay.

Sufficient subsoil to allowtreatment of wastewater

Depth to bedrock

Site restrictions



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3.1 INTRODUCTION

The information collected from the desk study andon-site assessment should be used in an integratedway to determine whether an on-site system can bedeveloped, and if so, the type of system that isappropriate and the optimum final disposal route ofthe tre ated wa s t ewat e r. Depending on thecharacteristics of the site, more than one option maybe available.

3.2 CHOOSING AN ON-SITE SYSTEM

Figure 9 sets out how the information from the deskstudy and on-site assessment is used to choose an on-site system. The procedure for deciding how todispose of the treated wastewater, i.e., whether it canbe disposed of by soil percolation to groundwater orbe discharged directly to surface water, is set out inthe lower half of the diagram. The wastewater froman on-site system cannot be disposed of by soilp e rc o l ation to gro u n dwater unless the subsoilcharacteristics are suitable for this purpose.

The desk study information is first examined. Areaswith significant sites e.g. archaeological, naturalheritage or historical features should be ruled out offurther consideration. If past experience indicatesthat there have been problems with the proposedsystem in the locality, further investigation may bewarranted before proceeding to the next step. If theDesk Study conclusion is that the site is potentiallysuitable for an on-site system, an on-site assessmentshould be carried out.

As previously mentioned, the on-site assessment

consists of a visual assessment, a trial hole andpercolation tests. The minimum separation distancesto pass the visual assessment are set out in Table 4 .These apply to any treatment system. If any of theserequirements cannot be met, on-site systems cannotbe developed on the site. Where a site is marginal,further investigation may be warranted to identifywhether or not site improvements will suffice. If thevisual assessment indicates that the site is potentiallysuitable, proceed to the trial hole stage of the on-siteassessment.

Table 7 sets out the subsoil characteristics whichindicate satisfactory percolation and other subsoil

ch a ra c t e ris tics suitable for the tre atment ofwastewater. The percolation characteristics will beconfirmed later by examining the percolation testresults. It is important to remember that the subsoilcharacteristics (i.e. depth and type of subsoil) as setout in Table 7, and the control measures outlined inthe groundwater protection responses should bothbe satisfied. In some cases the requirements of thegroundwater protection responses will be greaterthan 1.2 m of subsoil below the inve rt of thepercolation trench.

3.2.1 SYSTEMS USED WHERE ON-SITEASSESSMENT IS SUCCESSFUL

If the site satisfies all the specified requirements andhas a T value between 1 and 50, the site is suitablefor the development of a septic tank with a soilpercolation area (conventional septic tank system).

A septic tank followed by either an intermittent filteror a constructed wetland, or a mechanical aerationunit can also be developed on sites that have suitablepercolation characteristics.

3.2.2 SYSTEMS USED IN THE EVENT OF ON-SITE ASSESSMENT FAILURE

For sites which fail the on-site assessment (refer toFigure 9), site improvement works may allow thed evelopment of an on-site system. Th e s eimprovement works may include lowering the watert able by dra i n age, i n c reasing the soil depth byi m p o rting suitable soil or replacing ex i s t i n gunsuitable soil . The conditions that give rise to ahigh water table are site specific; these include

topography, nature of soils, bedrocks and outfalls.Some of the pro blems resulting from theseconditions are re a d i ly solve d. Detailed designprocedures are available in drainage manuals2 .

I m p o rted soil may be placed in mounds - asillustrated in Figu re 3 - or level wi th the groundsurface. The mounds may be constructed partially ortotally overground. Free draining unsaturated soilsas detailed in Table 5 should be used. The fill shouldbe placed in layers not exceeding 300 mm thick andlightly compacted. Great care should be taken not toovercompact the soil as this will lead to ponding.After each lift is placed, percolation tests should be

3. TREATMENT OPTIONS

2 Mulqueen, Rodgers, Hendrick, Keane, McCarthy (1999). Forest Drainage Engineering. COFORD Dublin.

3 TREATMENT OPTIONS 27


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carried out. A 150 mm square hole is excavated to adepth of 150 mm in the placed soil. After presoakingto completely wet up the soil, 0.5 litres of water ispoured into the hole and the time in minutes for the

water to soak away is recorded. This time should bebetween 10 minutes and 2 hours. After these workshave been completed, a second test hole must beexcavated in an appropriate location in the improvedsoil and a percolation test carried out.

Where a minimum of 0.6 m of permeable soil ispresent or can be placed over the rock and/or thewater table (i.e. shallow soils) and all otherrequirements of the on-site assessment are satisfied,an on-site system can be developed using this soil asa polishing filter in anyone of the following:

a septic tank and an intermittent filterfollowed by a polishing filter;

a mechanical aeration system followed by apolishing filter; or

a septic tank and constructed wetland system

followed by a polishing filter may be installedon the site.

The treated wastewater from systems other than aconventional septic tank system should be percolatedthrough a polishing filter to reduce microorganisms.

The polishing filter may comprise the in situ soil,wh e re there is adequate depth of suitable soil,imported suitable soil or a combination of the in situand imported soil. Polishing filters may also beconstructed from medium fine sands placed in layers

alternating with layers of 10-20 mm clean washedgravels. Typical designs for polishing filters aregiven in Chapter 4.



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Pass

PercolationTest

(T- Test)

VisualAssessment

Pass

Trial Hole

Pass

On-site Assessment

Desk studyFail

Desk study

Conventional septictank system

Mechanicalaeration system

Intermittentfilter system

Wetlandsystem

Pass Fail

Unsuitable

Siteimprovement

Archaeologicalsite, NHA

Fail

Discharge togroundwater

Discharge**

soilpolishing

filter

sandpolishing

filter

PercolationTest

(P- Test)*

Fail

This option may not

always be available

Site may not be

suitable for any on-

site system

FIGURE 9: FLOW DIAGRAM FOR CHOOSING AN ON-SITE SYSTEM

3 TREATMENT OPTIONS 29

* A P (or T) test is required to design a soil polishing filter. The hydraulic loading r ate depends on the soil or bedrockand recommended loading rates are as follows: up to 20 l/m2.d for P/T values of 20 or less; up to 10 l/m2.d for P/Tvalues from 21 to 40 and up to 5 l/m2.d for P/T values 41 -50.

** The treated wastewaters from the polishing filter may discharge to groundwater or surface water depending on thenature of the strata underlying the filter. The discharge of treated wastewater from the polishing filters overlyingpermeable soils or bedrocks may go to groundwater. A soil or bedrock with a P/T value of 50 or less is suitable topercolate effluent from a polishing filter to groundwater. The treated wastewater from the polishing filters overlyingsoils or bedrocks with P/T values greater than 50 is collected in a suitable drainage system and discharged to surfacewaters.

Desk Study

On-site Assessment

Archaeologicalsite, NHA


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3.3 CHOOSING THE OPTIMUM DISCHARGE

ROUTE

Once the on-site treatment system has been decided

upon, the disposal of the treated wastewater needs tobe considered. For septic tank systems with a soilpercolation area the treated wastewater will normallybe discharged to the groundwater. In the case offilters, mechanical aeration systems and wetlandsystems, treated wastewater from the polishing filtermay be discharged to the ground or to surface water(Figure 9).

In the case of a discharge to surface water a licenceto discharge is required from the local authorityunder the Water Pollution Acts 1977-1990. Where

such a licence is required, the final wastewaterquality from the on-site system should comply withthe requirements set out in the licence.

3.4 LICENCE REQUIREMENTS

The discharge of any sewage effluent to "waters3"requires a licence under the Water Pollution Acts1977-1990. Licence applications are processed bythe local authorities. Domestic sewage, however, notexceeding 5 m3/day, which is discharge d to anaquifer from a septic tank or other disposal unit, bymeans of a percolation area, soakage pit or othermethod is not subject to the licensing provisions ofthe 1977-1990 Acts. If an on-site system does notcomply with all the conditions above, a dischargelicence is required for the on-site system. However,it should be noted that a "soakage pit" or similarmethod is not an acceptable means for treating septictank effluent and does not comply with therequirements set out in this document.

Where it is proposed to discharge was tewater to"waters", local authorities should assess the impactof the discharge from the on-site system on thereceiving water. The parameters to be examined

should include:

Flow;

BOD;

Nitrates;

Ammonia;

Phosphates; and

Microorganisms.

When assessing the impact of an on-site system,local authorities should consider the beneficial usesof the receiving water. The principal beneficial usesof surface water are, water intended for humanconsumption after treatment, agriculture, bathing,b o at i n g, c o a rse fi s h e ry, c o o l i n g, game fi s h e ry,general amenity, or industry. Principal beneficialuses of gro u n dwater are agri c u l t u re, d ri n k i n g,i n d u s t ry and raw water intended for humanconsumption after treatment.

Once the benefi cial use of the water has beenestablished, local authorities should consult relevantRegulations, water quality management plans anda ny published standards to obtain the re l eva n tdischarge standard. The treated wastewater from theon-site system should comply with the water qualitystandard set for the receiving waters.

3 includes any (or any part of any) river, stream, lake, canal, reservoir, aquifer, pond, watercourse or other inland waters,whether natural or artifical.



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4.1 SEPTIC TANKS

Septic tanks are primary settlement tanks providinga limited amount of anaerobic digestion. Septic tanksshould comprise two ch a m b e rs. For bestperformance, septic tanks should have the followingcharacteristics:

septic tanks should be much longer than theyare wide to promote settlement of suspendedsolids;

larger septic tanks are better than smallertanks because of greater settlement of solidsand larger storage volume for liquid andsolids;

properly designed baffles provide quiescentconditions and minimise the discharge ofsolids to the percolation area; and

the inlet and outlet of the septic tank shouldbe separated by a long flow path for thewastewater; if the outlet is too close to the

inlet, solids settlement and grease separationmay be inadequate.

Septic tanks must be able to (i) withstand corrosion(ii) carry safely all lateral and vertical soil pressuresand (iii) accommodate water pressure from insideand outside the tank without leakage occurring.S eptic tanks must be wat e rtight to prevent (i)wastewater escaping to the s oil outside, and (ii)surface water and groundwater entering the tank. Aleaking tank can cause pollution problems.

4.1.1 SEPTIC TANK CAPACITY

The septic tank should be of sufficient volume top rovide a retention time for settlement of thesuspended solids while re s e rving an adequat evolume for sludge storage. The volume required forsludge storage is the determining factor in sizing theseptic tank and this sizing depends on the potentialoccupancy of the dwelling which can be estimatedfrom the maximum number of people that the housecan accommodate taking into account the number

and types of bedrooms. The tank capacity may becalculated from the following formula:

where

C = the capacity of the tank (litres)

P = the design population with a minimum of 4persons

A minimum capacity of 2720 litres (2.72 m 3) shouldbe provided. This assumes that desludging of theseptic tank is carried out at least once in every 12-month period. When kitchen grinders are installed,additional sludge solids are discharged with thewastewater and the capacity of the septic tank shouldbe increased by 70 litres for each additional person.Typical septic tank capacities and dimensions forrectangular tanks are shown in Table 9, Table 10,Figure 10 and Figure 11.

4. SEPTIC TANKS, PERCOLATION AREAS AND OTHER

FILTER SYSTEMS

C = 180 . P + 2000

4 SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 31


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Outlet T-Piece

OUTLETINLE

T

Cover

Scum layer

2200 1000

liquid layer

200

Sedimentation

Gas Buoyed

Flotation

Inlet T-piece

300 Freeboard

Sludge Layer

TWL75

350C

Ventilation cowl

350

550

SECTION A-A

No of Required Dimensions (m)

Persons Storage Capacity(litres)

length width depth

C = 180 . P + 2000 a* d* b* c*

3 2720 2.2 1.0 1.0 1.2

4 2720 2.2 1.0 1.0 1.2

5 2900 2.4 1.0 1.0 1.2

6 3080 2.5 1.0 1.0 1.2

TABLE 9: TYPICAL CAPACITIES OF SEPTIC TANKS

* refer to Figure 11

FIGURE 10: LONGITUDINAL SECTION OF A TYPICAL SEPTIC TANK (ALL DIMENSIONS IN MM)

INTLET



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TWL

450

200

Section B-B

B

B

AA

10001000

2200

Floor Plan

d

c

a

b

FIGURE 11: PLAN AND SECTION OF A SEPTIC TANK (ALL DIMENSIONS IN MM)

4 SEPTIC TANKS, PERCOLATION AREAS AND OTHER FILTER SYSTEMS 33


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4.1.2 CONSTRUCTION OF A SEPTIC TANK

The design features of a septic tank sys tem areoutlined in Table 10.

S eptic tanks may be cast in situ or may beprefabricated from steel, reinforced concrete, glassfibre reinforced concrete or plastic. Two-chamberedtanks may be accommodated by having two separate

tanks connected together using a tee-piece baffle ineach tank. The principles given for rectangular tanksshould be fo l l owed for cy l i n d rical tanks wh e rereasonably practical. Some upward adjustment tovolumes may be necessary. The roof, outer wallsand floors of the tank and all joints should bewatertight.

4.1.3 WATERTIGHTNESS OF SEPTIC TANKS

A septic tank should be watertight up to the top ofthe tank. Methods employed to test such tanksshould be in accordance with CEN E u ro p e a n

Standard EN 12566 watertightness test method4.

For concrete septic tanks the loss of water measuredafter 30 min. should be 0.1 litre per m2 of theinternal wet surface area of external walls. Forp o lye t hylene and glass re i n fo rced plastic (GRP)septic tanks, no leakage is permitted.

4.1.4 IN SITUTANKS

The fo l l owing construction standards arerecommended for in situ tanks:

the floors should be of concrete with aminimum thickness of 225 mm;

the walls should be a minimum of 100 mmthick reinforced concrete or equivalent suchas 225 mm solid block rendered wall;

the roof should be either cast in situ or

precast reinforced concrete; and

Tank Characteristics Recommended Requirements

Tank capacity 2720 litres for 4 persons

Tank length to width ratio 2 to 3 : 1

Number of compartments 2

Volume of inlet compartment 2/3 to 3/4 of the total tank capacity

Concrete compressive strength 35 N/mm2 at 28 days, minimum

Wall thickness 100 mm minimum reinforced concrete orequivalent

Roof thickness 125 mm minimum

Interior height 1.2 m minimum

Liquid depth 0.9 m minimum

Freeboard (roof height above liquid) 300 mm

Baffle wall liquid opening 450 mm to centre of opening from floor of tank(Figure 10 and Figure 11)

Inlet and outlet pipes Minimum internal diameter of 100 mm

Bottom end of T-piece 550 mm above floor of tank

Difference in elevation of inlet and outlet 75 mm

Joints Watertight joints required

Ventilation 100 mm diameter pipe in roof with a cowl ineach chamber

Access covers 600 mm x 600 mm (2 no.)

TABLE 10: TYPICAL DESIGN FEATURES OF A SEPTIC TANK

4 CEN/TC 165 "Wastewater Engineering" is preparing a series of European standards on small wastewater treatmentsystems.



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for safety, the roof should be strong and stableenough to prevent interference from childrenand others.

4.1.5 PREFRABICATED TANKS

Prefabricated tanks should be manufactured froms u i t able mat e rials (e. g. pre-cast concre t e, g l a s sreinforced plastic, glass reinforced concrete) and therequirements stated above for capacity, h

ireland epa

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