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FACULTATIVEFACULTATIVEPONDSPONDS

 

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 acu a ve pon sacu a ve pon s

Terminology: Primary fac. ponds:Primary fac. ponds: receive raw

removal)

Secondary fac. ponds:Secondary fac. ponds: receive effluent

from anaerobic onds or se tic tanksUASBs; or from settled sewerage

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Profuse presence of various speciesProfuse presence of various species

of mainlyof mainly motilemotile microalgaemicroalgae~~   −−  

So facultative ponds are (or shouldSo facultative ponds are (or shouldbe) dark greenbe) dark green

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--O2, used by heterotrophic bacteria which

2   .

thin sludge layer in facultative ponds that

digestion & CH4 production).

 – – 90%). This “algal” BOD is very different90%). This “algal” BOD is very different

“ ”“ ” .  .

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 Algae produce O2, but only during daylight hours

 

diurnaldiurnal variation in DO levels in pond:variation in DO levels in pond:

○ ~10 cm● ~ cm

below

surface

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Diurnal variation of pH as well:Diurnal variation of pH as well:

• Algal photosynthesis consumes CO2

• 2   2  

bacterial metabolism or from atmosphere, thenbicarbonate and carbonate ions dissociate:

2HCO2HCO33−− → →  COCO33

22−− + H+ H22O +O + CO2

COCO3322−− + H+ H22OO → →  CO2 ++ 2OH2OH

−−

CO used b al ae; OHOH−− accumulates, soaccumulates, so HH

risesrises

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  , .

Im ortant for dieIm ortant for die--off of faecal bacteria:off of faecal bacteria: 

pH >9.4pH >9.4 is rapidly lethal to faecal bacteria,

including E. coli−

 main exception is

r o c o erae w c s e o y

sul hides in anaerobic onds[see later in ‘Maturation Ponds’]

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ep o acu a ve pon sep o acu a ve pon s• Usuall 11– –2 m2 m commonl 1.5 m

• If

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LengthLength--toto--breadth ratio of fac. pondsbreadth ratio of fac. ponds

 – – o pon rece ves raw was ewa er o pon rece ves raw was ewa er 

if more than this slud e accumulates near

inlet (rather than being reasonably well

distributed across pond bottom) and can

block the inlet] can be >3 to 1 if pond receives anaerobiccan be >3 to 1 if pond receives anaerobic

pond effluentpond effluent

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Design of Facultative PondsDesign of Facultative Ponds

Man methods − mostl bad!

The best is based on:The best is based on:

the surface BOD loading ratethe surface BOD loading rate

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λλ , kg BOD/ha daykg BOD/ha day L = influent BOD m /l * 

Q = flow, m3/day

 A = pond area, m2ss ii

NOTE UNITS !!NOTE UNITS !!mpor anequation!

 

Design loading is a function of temperature:Design loading is a function of temperature:

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McGarry & Pescod’s

envelope of failure

O = failed pond

λλss ==60(1.099)60(1.099)TT

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McGarry & Pescod’s

envelope of failure

O = failed pond

Shape of McG&PShape of McG&P

λλss ==60(1.099)60(1.099)TT

 

influenced by:influenced by:

points at ~0points at ~0 °°C, andC, and

“ ”“ ” 

points at atypicallypoints at atypically

(>>500 kg/ha d)(>>500 kg/ha d)

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• But ponds should not be designed to

opera e a e r po n o a ure

• Therefore various desi n e uationsdesi n e uations have

been introduced

• es s ara s g o a es gn equa onara s g o a es gn equa on:

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MARA’ L BAL DE I N E ATI NMARA’ L BAL DE I N E ATI N 

λλss = 350(1.107 − 0.002T)= 350(1.107 − 0.002T)T−25T−25

Note:Note:

2525°°CC Another important equation Another important equation

 

(a) T = 8 °C, λs = 80 kg/ha day [experience in Europe & NZ]

(b) T = 25 °C, λs = 350 kg/ha day [experience in NE Brazil]

(c) T = 35 °C, λ = 500 kg/ha day [arbitrary upper limit]

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The McGarry & Pescod (A) and Mara (B) equations

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•• MinMinθθ

f f = 4 days= 4 days

•   ,then take θf = 4 days, and

AA = Q= Qθθ /D /D

•   .

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B D removal in facultative ondsB D removal in facultative onds 

λλr r = a= aλλss + b+ b

=r    ,

= 10(Li – Le)Q/A

•

λλr r = 0.725= 0.725

λλss + 10.75+ 10.75

close to zeroUsuall 0.7Usuall 0.7−−0.8 in0.8 in

warm climateswarm climates

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McGarry & Pescod’s data:McGarry & Pescod’s data:

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r    swhere α = 0.7−0.8 usuall

ie, BOD removal is some 70ie, BOD removal is some 70––80%80%********************************************************

McGarry & Pescod’s data were for

 – e, eye, ey nnc u e e o e a gae nc u e e o e a gae n

the effluentthe effluent

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E r n ni n:E r n ni n:

Urban Waste WaterUrban Waste Water

Treatment DirectiveTreatment Directive

For WSP effluentsFor WSP effluents:: 

andand ≤150 mg SS/l≤150 mg SS/l

If OK in the EU, why not inIf OK in the EU, why not in

developing countries ???developing countries ???

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PURPLE PONDSPURPLE PONDS

 

(or red or reddish brown) colour. This is due

to the growth of photosynthetic bacteria……photosynthetic bacteria…… 

ponds:1. Sulphate reduction to hydrogen sulphide

y anaero c su p a e-re uc ng ac er a

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. x a on o y rogen su p e osulphur and sulphate by anaerobic

photosynthetic purple and green bacteria

 –   22 u yu y

bacteria oxidize Hbacteria oxidize H22S, not HS, not H22OO

 – Equations:

2121HH22SS + 10CO+ 10CO22 + 2NH+ 2NH33

lightlight  -

2(C2(C55HH88OO22N) +N) + 2121SS +16H+16H22OO

and, when no H2S........

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2121SS ++ 30CO30CO22 + 6NH+ 6NH33 + 36H+ 36H22OO++lightlight  

two important groups of photosynthetic

bacteria in WSP:

 – purple sulphur bacteriasu p ur granu essu p ur granu es

depositeddeposited withinwithin

 – green sulphur bacteriasulphursulphur

ChlorobiaceaeChlorobiaceae 

outsideoutside cellscells

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Thios irillum Thios irillum s .s .

CC

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Chromatium Chromatium sp.sp.

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Thi hrixThi hrix ..

[Oscillatoria ]

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 wavelength than algal chlorophyll – ,

So photosynthetic bacteria occur inSo photosynthetic bacteria occur in nonnon--overloaded facultative ondsoverloaded facultative onds belowbelow thethe 

algae andalgae and aboveabove the anaerobic zonethe anaerobic zone

They thus act as a sulphide filter sulphide filter –pro ec ng e a gae rompro ec ng e a gae rom su p e ox c ysu p e ox c y(see later), andand also helping to controlalso helping to controlo our re easeo our re ease..

f

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Anoxic facultative pond treating sugar mill

,