sewage discharges

8/19/2019 Sewage Discharges

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Sewage Discharges and

Oxygen Depletion in Natural Waters


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Outline of Topics

• Introduction

– overview of probles due to huan and anial waste pollution – cobined sewer overflows

– pathogens

• Oxygen Depletion – saturated DO

– oxygen deand !"OD# $OD%

– sources of "OD waste

– oxygen sag curves

• Sewage Treatent

– priary and secondary treatent – tertiary treatent

– disinfection

– sewage sludge


3/26

&ollution due to 'uan ( )nial Waste

• Sources* – +unicipal wastewater !ie# sewage%

• ,specially fro cobined sewer overflows !$SOs%• Treated sewage discharge fro &OTW !publicly owned treatent wor-s%

– Septic tan-s• Subsurface disposal.treatent syste for each hoe

– When sewer lines are not available !eg# sparsely populated areas%

• $onsists of a buried septic tan- and either a leaching field or seepage pit

– /ro ,"0 1Solids are decoposed by the anaerobic bacterial action of thesludge2 )fter several years the accuulation of sludge interferes with the action#and a scavenger unit ust pup the sludge out of the tan- for disposalelsewhere23

– 4ivestoc- waste• )ny agricultural operation that includes livestoc-• )nial feeding operation !)/Os%

– $onfined livestoc-

– No crops

• 4arge )/Os are a particular concern – Concentrated animal feedlot operations !$)/Os% are defined by the nuber of

anials in the )/O – )s of 5667# regulated as point sources under the N&D,S portion of the $lean

Water )ct


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The Nature of Sewage Discharges

• Lecture Question – What pollutants are present in raw sewage discharges* What are the resulting

effects on ecosyste and huan health* – &athogens

• $olifor count is usually 869 – 86: .4 in raw sewage• $ause various waster;borne diseases !see later slide%2 The ost serious water

pollution proble in developing countries2

– Degradable Organic &ollution• 1'igh "OD waste23 "OD of raw sewage0 866;


5/26

$obined Sewer Overflows

• $ase Study0 Seattle

– 8585 overflow events in 5668

– Over 966 illion gallons stor water = sewage flowed into &uget Sound


6/26

Typical &athogens in'uan Waste

• W'O0 pathogens in water fro

huan waste -ill 72< illion

people every year

+any of these are children under 9

years


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Saturated Oxygen 4evels in /resh Water

• 4i-e all gases# O5 is less soluble as

teperature increases

• Saturated level of water exposed to the

atosphere is typically >;8< g.4


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,ffects of Oxygen Depletion

• ,ffects of low DO on ecosystecounities and populations – 'ow uch DO is enough*

– ?apid decrease in DO cancause assive fish kills

– So;called dead zones for if

DO level falls enough

DO level (mg/L) Qualitative effect

: – 89 O@

< – : Stressed

5 – < $ho-ing

8 –5 Dying

6 – 8 Dead

• ,ffects of low DO on cheical coposition

– $onverts cheicals to their to reduced state

• +ethane !$'


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"OD Waste Water

• Question –

What is 1high;"OD3 waste water* – What are the aBor sources of high;"OD waste water*

– "OD C biocheical !or biological% oxygen deand• When organic aterial is decoposed !ostly icrobial aerobic respiration%

it 1deands3 oxygen

– Oxygen demand represents a potential loss of DO in a water body• Iportant factor0 relative rates of oxygen loss and replenishent• If the rate of oxygen loss due to decoposition exceeds the rate of oxygen

replenishent !eg due to dissolution of gaseous O5%# then the DO level falls

– Oxygen deand can be Auantified by easureent of "OD# $OD !orTO$%

• BOD measurement 0 !i% collect a saple of -nown volue !ii% easure the

DO level !iii% seal and incubate at constant teperature for 9 or E days !iv%easure the DO level again2• The "OD is the difference between the two DO easureents2• "OD is deterined by the aount of degradable aterial present in the

water2 Fsually# it is ostly due to organic aterial !1$"OD3% but can also bedue to other cheicals in their reduced state !eg# aoniu%2


10/26

$oon Sources of 'igh;"OD Wastewater


11/26

Oxygen;Sag $urves

• Question

– What is an oxygen sag curve*

– )n oxygen sag curve is the dip in dissolved oxygen observed

when "OD waste water is discharged continuously into a river2

• The extent of the sag is deterined by "OD level in the wastewater

strea# by the rate of discharge# and by other factors such asteperature and river characteristics !flow rate# turbulence# etc%2

– )n oxygen sag curve is also observed due to a one;tie

discharge of "OD waste into a la-e

• In that case# the DO drop is with time instead of distance downriver2

• $ontinuous discharge of "OD waste into a la-e results in adecrease in steady;state DO level !not a 1sag3 followed by a

recovery%2


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Oxygen;Sag $urves


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+ore Oxygen Sag $urves

,ffect of teperature0

• sag deepens and shortens• ay cause a portion of river to

have unhealthy DO levels

,ffect of "OD 4evel0• sag becoes ore severe• longer distance !or tie% at

unhealthy DO levels

D i f O


14/26

Dynaics of Oxygen

Depletion ( Dissolution

/igure on left shows a odel fraewor- to

calculate0• DO as a function of distance downstrea froa point source discharge or

• DO as a function of tie after a single 1spi-e3

discharge of "OD wastewater

• DO falls when decoposition rate G dissolution rate

and DO rises when decoposition rate H

dissolution rate

• ?ate of decoposition !deoxygenation%

• 4inearly proportional to "OD level

• "OD falls exponentially with tie

• ?ate of oxygen dissolution !reaeration%

• 4inearly proportional to the oxygen deficit 0 DOsat ; DOactual


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Sewage Treatent

• Lecture Question – 'ow does the $lean Water )ct regulate sewage discharges*

– $W) passed in 8E5• )ctually# they were coprehensive aendents to an existing statute

– regulates sewage discharges !aong other things% as point sources ofpollution

• ) aBor proble at the tie• ?eAuires all &ublicly;Owned Treatent Wor-s !&OTWs# ie 1sewage

treatent plants3% to obtain discharge perits

• Fnder the National &ollutant Discharge ,liination Syste !N&D,S%

– ?eAuires that all &OTWs eet a iniu of secondary treatment level of sewage

• ) big help in reducing "OD# but• &robles reain

– "OD still a little high

– Nutrient levels not reduced very uch with only secondary treatent levels

– )nd neither are toxics !etals# organics%


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Sewage Treatent &lants !&OTWs%

• Lecture Question

– Describe in soe detail the processes in the primary and secondary

treatent of sewage2

– Primary treatment 0 physical separation for reoval of bul-y solids and

oil.grease

– Secondary treatment 0 bioreactor priarily intended to reduce "OD


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Secondary &OTW0 Tric-ling /ilter +ethod


18/26

Secondary &OTW0 )ctivated Sludge +ethod


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,ffects of &riary and Secondary Treatent 4evels


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)dvanced !Tertiary% Treatent


– What are the ethods used in advanced !tertiary% sewagetreatent* 'ow do they help safeguard water Auality*

– )dvanced treatent consists largely of chemical treatent

ethods designed to do a nuber of things0

• ?eove nutrient pollution – &hosphate reoved by treatent with lie# $a!O'%5

– )onia reoved by basification followed by sparging !accelerated

outgassing%

• /urther reduce "OD

– $oprecipitation# activated charcoal# further decoposition

• ?eove toxic organics

– )ctivated charcoal filter

• ?eove toxic etals

– Ion exchange resin filter


21/26

,ffect of Tertiary Treatent 4evel


22/26

Disinfection


– What are the ain ethods of disinfection used in sewage treatent*

– $hlorination

• )pplied either as chlorine gas or as a hypochlorite !O$l;% salt

– p' control is iportant

• )dvantages

– $heap

– ?esidual disinfection

• Disadvantages

– +any disinfection byproducts !D"&s%0 T'+s# '))s# chloraines

– )lternatives

•OJonation

• uv light

• With alternative treatents# a lesser aounts of chlorine often also used for

residual disinfection !or as a bac-up if colifor counts get too high%2


23/26

D"&s &roduced by $hlorination


24/26

$hlorinated D"&s in Teated Water


25/26

Sewage Sludge !"iosolids%


– What is sewage sludge !KbiosolidsL% and what is done with it*

– Wastewater treatent generates large Auantities of solid waste• $ollectively this is called 1sludge3 or# ore eupheistically#

1biosolids3

• $ontains all solid aterial reoved fro the waste strea# including – 'uan waste# icroorganiss# and toxic cheicals

• Molue dwarfs that of unicipal solid waste !ie# 1trash3%

– Sludge is very watery• 4oo-s essentially li-e uddy water in original for

• Only 8;86 solid

– Fsually KdewateredL at the treatent plant• Texture of a wet sponge

• 88;


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Disposal of Sewage Sludge

• ,ventual /ate*

– 4and application.recycling !

sewage discharges

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