Stabilization Ponds

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APresentation

on

Stabilization Ponds


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Definition It is a shallow basin used to treat raw

water by natural processes

Where rate of waste oxidation is slower


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Three categoriesAnaerobic ponds removes BOD by

sedimentation, sludge is then digested

in bottom layer

Facultative ponds bacteria degradewaste ,uses O2 and generates CO2,

algae use CO2 and generate O2 Maturation ponds Disinfection is used


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Arrangement


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Anaerobic pond Sedimentation pond

High waste water loading depletes allO2

Solids settles to pond

Anaerobic digestion of sludge occurs inpond bottom


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Anaerobic digestion process Hydrolysis - Complex organics (proteins

and fats broken down to simpler

compounds by various bacteria

Acidogenesis (Fermentation) Fattyacids and alcohols oxidized ,amino acids

and carbohydrates fermented ,formsvolatile fatty acids and hydrogen


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Anaerobic digestion processActeogenesis conversion of complex

fatty acids to acetic acid

Methnogenesis - conversion of aceticacid to methane and CO2 and CO2 ,H2to methane


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Process When temperature rises above 15

degree centigrade digestion generates

enough biomass which causes pondsurface to bubble biogas (70% CH4and 30% CO2)

Digested solids accumulate cleanoutin 1 to 3 years


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Pond Characteristics Hydraulic detention time is short 1 day Depth 2 to 5 m (usually 3m)

Design is highly empirical based onvolumetric load (gm BOD/m3/day)

for e.g. 100gm BOD/m3.day to 3 m deep pond=3000kg/ha/day

For air temp.(Degree centigrade)

10 Loading(gm BOD/m3/day)

100

20 300

25 350


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Role of pH pH= 7.5 (S HS-)

Below pH 7- H2S forms and causesodor

Below ph 6.2conditions are toxic


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Facultative ponds Much lower aerial loading rates- 100 to 400

Kg/ha/day (vs. 3000+ for anaerobic ponds of

BOD) Pond water is aerobic and supports and

supports very high density algal population

Beneath water is an anaerobic bottom layer(sludge) in which sludge is digested


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.Algae generate O2 by photosynthesis

during the day ,which is useful for

oxidation of waste by bacteria


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. It may operate as a primary pond (with

no anaerobic pond for pretreatment) or

secondary ponds (after anaerobicponds)

Performance depends upon

1)time(diurnal )2)vertical space


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Vertical variations Algae requires light but depth of photic zone

(light penetration) may be limited in pondwith heavy algae growth and turbid water

Light penetrates about 30 cm(1 foot) Pond algae tend to be motile species which

swim to optimum level Non-motile species requires wind mixing to

circulate through photic zone Wind mixes O2 from photic zone to pond

depth


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In tropics , ponds go through diurnalstratification pattern

Morning wind mixing ,uniform temperaturethrough depth

Mid-day- warming of surface water, onset ofstratification, thermocline forms (hot water

above ,cold water below and no mixing)Late afternoon or evening-top layer cools,

mixing occurs during wind mixing


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..

Motile algae optimum depth-not very upperwater (too hot) ,not too deep (very dark)-30

to 50cm below surface Ponds have oxypause (depth at which DO

goes to zero), moves up and own during day

Besides generating O2,bacteria consume CO2to the point that depleted CO2 alterscarbonate equilibrium. This raises pH andkills fecal bacteria


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Diurnal pattern

Leads to diurnal treatment performance

Rise in DO ,pH during day

Decrease in coliform

Rise in BOD,TSS and chlorophyll-a

BOD is high but it is algae and notorganic waste-chlorophyll-a=500 to1000 micro gm /l


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Effect on H (Depth)

H 1.8 m- oxypause is too shallow relativeto overall depth (pond is more anaerobicthan aerobic)

Optimal H=1.5 m (1m


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Maturation ponds

Used to remove pathogenic bacteriaand viruses(99.9997%)- e-Coli removal

of 6 log units using three maturationponds in series in brazil


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Mechanism

Solar disinfection using UV light

Raising pH


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Design

1 m deep

Total detention time of the order of 10

days


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Stabilization Ponds

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