design and operation of uasb

1

WAGENINGEN UNIVERSITY Sub-department of Environmental Technology

Lettinga Associates Foundation

Design and operation of UASB for treatment of domestic wastewater

Jules B. van LierWageningen University /Lettinga Associates Foundation (LeAF)



influent

screening grit chamber

sludge drying bedreuse

effluentU A S B

watersludgebiogas

biogas use

- polishing pond- Trickling filter- RBC- etc.

General lay-out of an anaerobic WWT plant

Post treatment

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sludgewithdrawal

biogas

treatedwater

wastewater

feedinlet

deflectorbeam

Distribution box

effluentgutter

gascollector

sludge bed

sludge blanket

biogas

settling zone aperture

baffle

The UASB Reactor



Internal view 1200 m3 UASB, Cali, Colombia

3



sludgewithdrawal

biogas

Treated water

wastewater

feedinlet

deflectorbeam

Distribution box

effluentgutter

gascollector

sludge bed

sludge blanket

biogas

settling zone aperture

baffle

density

minimum SRT

concentration

concentration

detention time

angle

loading rate

upflow velocity

number

angle

velocity

Design Criteria for the UASB Reactor



Critical Parameters for Design of a UASB Reactor

Vieira et al, 1986

4



SRT as prime design criterion:SRT is directly linked to the amount of viable, active biomass in the system:

SRT = X / (dX/dt),with X = concentration of viable biomass (e.g.methanogens ).

SRT is determined by:

- incoming suspended solids- solids digestion in the reactor - filtering capacity sludge bed (upflow velocities + sludge characteristics)- growth of new sludge- sludge retention in the settler (upflow velocities)- withdrawal of excess sludge

SRTmin. ≥ 3 * Td (doubling time) of critical biomass (e.g. methanogens)



0

20

40

60

80

100

120

140

160

180

10 15 20 25 30 35 40 45

Temperature [°C]

SR

T fo

r st

abili

zed

slu

dg

e [d

ays]

Required SRT for Hydrolysis in Reactor

5



Impact of Reduced Hydrolysis

At low temperatures (< 15ºC) and/or strong temperature fluctuations between summer (25ºC) and winter (15ºC), the singlestep UASB design needs reconsideration:

Limited Hydrolysis

Accumulation of particulate organic matter

Deterioration of the reactor performance

Low removal efficiency



Conventional Design of Single Stage UASB reactors

Temperature.: > 20 °CCOD infl.: < 1000 mg/lSS-influent: < 500 mg/l

6



Relationship between pollution strength and reactor volume

Assumptions:Θmin = 4 h Q = 250 m3 · h-1

rv= 15 kg COD · m-3 · d-1

hydraulic load = 6 m3 · m- 3 · d-1

2000

1500

1000

500

01 2 3 4 50

Vr = Θ · Q Vr = (c · Q) · rv-1

c (kg COD · m -3)

Vr(m

3 )

Assessment of the size of a UASB Reactor

Normal COD-range for domestic sewage



The feed inlet distribution system is a crucial part of the reactor

It is important to accomplish optimal contact between sludge and

waste water, i.e.

The danger of channelling will be bigger at low gas production rates

(less than 1 m3 · m3 · day1)

Feed Inlet System

to prevent channelling of the waste water through the sludge bed

to avoid the formation of dead corners in the reactor

7



Rough guidelines for the number of inlet points required in UASB reactors

treating mainly soluble waste waters

Type of sludge Area (m2)

per feed inlet point

Granular sludge

Loading rate

(kg COD . m-3 . day-1)

< 2

2 - 4

> 4

Dense flocculant sludge (>40 kg DS/m3)

< 1

1 - 2

> 2

Medium thick flocculant sludge

(20 - 40 kg DS/m3)

<1 - 2

> 3

0.5

1 - 2

>2

0.5 - 1

1 - 2

2 - 3

1 - 2

2 - 5

Number of Inlet Points



Drawing of UASB for Sewage Treatment

Feed inlet pipes

Influent

Influent

Effluent

Gas

8



Feed Distribution Systems



Design of a Rectangular Inlet Box

9



Full-scale anaerobic sewage treatment plant, Bucaramanga, Colombia

Gas outlet

effluent

2nd influent distribution box

3rd influent distribution boxes



Concrete plates for the GSS-device

Problem: leakage!

Full-scale anaerobic sewage treatment plant, Bucaramanga, Colombia

10



Polyester Circular Distribution Box

Maintenance (declogging)

Clogged inlet pipe



Feed Inlet Tubes

Sludge discharge pipes

Warning: poor design!

Long inlet tubes on bottom may cause clogging

11



The use of specific nozzles or perforated inlet pipes placed

at the bottom of the reactor

Intermittent supply of the feed

Every feed-inlet system should be easy to clean

A conically shaped reactor bottom, or a multi-cone reactor

bottom is attractive

Other Aspects of the Feed Distribution System



separation of the biogas and discharging this from the reactor

to prevent as efficiently as possible the wash out of viable bacterial

matter

to enable the sludge to slide back into the digester compartment

to serve as a kind of barrier (stopper) for rapid excessive

expansions of a sludge blanket (which is mainly composed of

flocculant sludge) into the settler

to provide a polishing effect

to prevent the wash out of floating granular sludge

Main Objectives the Gas Liquid Solids Separator (GLSS) Device

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foaming (when proteins and/or fats are present)

Remedy:

install anti-foam sprayers

scum layer formation (particularly in presence of fats) in the settler or

gas bowl

Remedies:

do not place a baffle in front of the effluent launder (will lead to high effluent SS)

install a skimmer in the settler compartment

remove the scum layer from the gas bowl

install agitators for settling down and/or mixing-up floating matter with active sludge

Problems with the GLSS Device



The slope of the settler bottom (i.e. the inclined wall of the gas collector) should be between 45-60o

The surface area of the apertures between the gas collectorsshould be 15-20% of the reactor surface area.

The height of the gas collector should be between 1.5-2 m at reactor heights of 5-7 m.

To facilitate the release and collection of gas bubbles and to combat scum layer formation, a liquid-gas interface should be maintained in the gas collector.

Tentative Guidelines for the Design of the GLSS Device (1)

13



To avoid up-flowing gas bubbles to enter the settler compartment, the overlap of the baffles installed beneath the apertures should be 10-20 cm.

Generally, scum layer baffles should be installed in front of the effluent weirs.

The diameter of the gas exhaust pipes should be sufficient to guarantee the easy removal of the biogas from the gas collectioncap, particularly in case of foaming.

In the upper part of the gas cap, anti-foam spray nozzles should be installed in the case the treatment of the waste water is accompanied with heavy foaming.

Tentative Guidelines for the Design of the GLSS Device (2)



UASB design for industrial WW (Biopaq process)

Increase of the (critical) aparture width by placement of 3 small settlers above each other

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UASB design for industrial WW (Biothane process)

gas

sludge

water

Effect of “mammoth flow”: indirect increase of aperture



Settler Equipped With Tiltable Plate Settler (TPS)

15



Tiltable plate settlers (TPS’s) in Biotim UASB-reactors



Cross-flow TPS of Biotim

gas

sludge

water

16



Discharge excess sludge: equip the reactor with sludge

discharge pipes 1) nearby the bottom, 2) halfway and 3)

approximately half a meter beneath the GSS device.

Assessment of the total quantity of sludge in the reactor: install a

number (5 or 6) of valves over the height of the reactor to be able to

make a sludge profile.

Sludge Discharge



measurement and registration of the influent flow

measurement/control of temperature and pH

measurement and registration of the reactor temperature and pH

(particularly in the lower part of the reactor)

measurement and registration of the gas production rate and the

gas composition with respect to the CO2 content and the H2S

content

Auxiliary Equipment Requirements (1)

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installations for the addition of essential nutrients (N, P and S),

alkalinity, and trace elements

heat exchangers (also desirable for occasionally heating the

reactor content in the case accumulated biodegradable solids

have to be removed from the sludge)

Auxiliary Equipment Requirements (2)

Tentative requirements:



1. The biggest problem takes place above the water level:

Air oxygen oxidizes H2S to sulphate. This can lead to very

low local pH conditions. This will affect both concrete and

steel.

2. Corrosion also occurs under the water level:

Corrosion by dissolved CO2: CaO from concrete will

dissolve as H2CO3 is present.

Corrosion of Construction Materials (1)

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stainless steel

plastics

proper coatings

coated concrete rather than coated steel

plastic covered with impregnated hardwood for the settler

plastic fortified plywood

asbestos

Corrosion of Construction Materials (2)

Prevention of corrosion by using proper construction materials:



Collection and treatment of gases

Reduction of corrosion

Prevention of the Release of Malodorous Compounds

Covering the anaerobic reactor allows:

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Virtual Wastewater Data & Assumptions for UASB Design (1)

Wastewater dataAverage flowMaximum flowPeak flowCODBCOD (degradable COD)BODTSSAsh contentSO4

PO4

TKN

4009002400225250

m3/h

m3/htimes max. flowmg/lmg/lmg/l

4025325

%mg/lmg P/lmg N/l

200 mg/l

Calculation example using an automated approach



Sludge production assumptionsY totalY methaneDegradation non-sludge VSSSludge ash content

0.080.034055

Kg VSS.kg CODinKg VSS.kg CODin%%

UASB operational aspectsTemperaturepH

227.5

°C-

Biogas aspectsCOD conversion effieciencyCH4 percentage in biogasDissolved in the waterDissolved in the water

60652263

% of BCODin into CH4

% biogasNml CH4

Mg CH4-COD

Virtual Wastewater Data & Assumptions for UASB Design (2)

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Case: Design of UASB Reactors for 50,000 PE (1)

Reactor dimensionsNumber of reactorsWidthLengthHeightDefinitive number of feed inlets

224.0018.004.50108

-mmm-

Gas collectorTotal widthAperture percentageWidth gas collectorPlate projectionPlate heightg

3.00232.300.901.07

m%mmm

Model output:



Feed inlet boxesMaximum servicing areaSuitable number of feed inlets per boxNumber of feed inlet boxesMinimum feed pipe diameter

50.0012947

m2

--mm

Effluent guttersLength of guttersV-notches per meterV-notch depthGutter widthGutter depthWater depth

28848.020.020.010.2

mPer mcmcmcmcm


Model output:

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Reactor loadingVolumetric loadingVolumetric loading rateOrganic loading rateBiogas loading rate

0.990.560.030.40

Kg COD.m-3.d-1

Kg BCOD.m-3.d-1

Kg BCOD.kg VSS-1.d-1

m3 .m-2.d-1

Biological parametersSolids retebtion timeMaximum methanogenic activityActual methanogenic activityBiogas productionH2S content

67.50.090.023500.04

DaysKg CH4-COD/kg VSS.dayKg CH4-COD/kg VSS.daym3/day% in biogas

Sludge productionEffluent TSS =>to be setSludge growthSludge to be removedSludge volume to be removed

302304201629

mg TSS/lKg/dayKg/daym3/day




Sludge drying bedsSludge bed loading => to be setSludge dried to => to be setSludge productionDrying bed surface area, totalDrying bed sideDrying bed surface areaNumber of drying bedsDried sludge to be removed

0.8612.02016235818.00324916.8

Kg.m -2.day-1

%kg/daym2

mm2

-m3/day


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upflow velocity

Economic Evolution of Design Criteria

feed inlet distance

solids retention time



0

20

40

60

80

100

120

140

0.00 0.50 1.00 1.50

UPFLOW VELOCITY (m/h)

RE

LA

TIV

E C

OS

TS

(%

)

Influence of Upflow Velocity on UASB Reactor Costs

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30

40

50

60

70

80

0.0 0.5 1.0 1.5 2.0

Vup (m/h)

EC

OD

(%)

Treatment Efficiency As a Function of Upflow Velocity

COD-total

COD-soluble

BOD-total



80

90

100

110

120

1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40

FEED INLET DISTANCE (m)

RE

LATI

VE

CO

STS

(%

)

Influence of Feed Inlet Distance on UASB Reactor Costs

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80

100

120

140

160

80 100 120 140 160

RELATIVE SURFACE/HEIGHT (%)

RE

LATI

VE

CO

STS

(%

)

Influence of Reactor Size on UASB Reactor Costs

surfaceheight



Difficulties in Introducing AWWT for Sewage

Ignorance with the system in practiceAbsence of required infra structure, regarding:

Experienced contractors / consultants

non or poorly informed authoritieslittle if any full scale experiencelittle experience in research institutes and universities

Commercial disinterest at established consultants specialised on conventional systemsUniversity research is considered academic (not for practice)

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Enhancing Implementation of AWWT

Establish contacts with universities, experienced contractors and consultants Start co-operation with polluting industries: either for environmental protection or energy recoveryAcquire subsidiary projects for pilot or demosAwareness amongst industries, authorities, and politiciansIncrease (scientific) research activities at universitiesEducate engineers and operatorsRaise specialised consultants / contractorsHave well informed engineers in agencies and water control boards

design and operation of uasb

Documents

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new sludge sludge retention

uasb reactorvieira et

prime design criterion