Natural WastewaterTreatment & Reuse

ANAEROBICPONDS

ANAEROBIC PONDS

high organic (BOD) loading devoid of dissolved oxygen

depth range 15 m; depends on groundconditions usually 24 m

no (or very few) algae surface area notimportant

OPERATION OF ANAEROBIC PONDS sedimentation of settleable solids and

intense anaerobic digestion (>15oC) andcopious biogas (CH4 & CO2) production

high BOD & SS removal

functions much like an open septic tank

ALGAE:occasional surface

film of sulphide-tolerant

Chlamydomonas

same groups of anaerobic bacteriainvolved as in septic tanks and anaerobicdigesters require same environmentalconditions (eg, pH >6.5)

Biogas: CH4, CO2

sludge layer

scum layer

About 30% of influent BOD leaves as biogas

Anaerobic BiodegradationPolymers

(proteins, polysaccharides)

Monomers(sugars, amino acids, peptides)

butyratepropionate

H2 + CO2 acetate

CH4 + CO2

h

h

111

1

2 2

2

3

3

44

4Hydrolytic enzymesFermentative bacteriaSyntrophic acetogenic bacteria

Homoacetogenic bacteriaMethanogens

Methanogenic Reactions

CH3COOH

CO2 + H2

CH4 + CO2

CH4

acetate carbondioxide

methane

methanecarbondioxide

hydrogen

acetoclasticmethanogenesis

autotrophicmethanogenesis

Methanogenesis is the rate-limitingstep in anaerobic treatment

Methano-gens areArchaea

DESIGN OF ANAEROBIC PONDSbased on volumetric BOD5 loading, v

V = Li Q/V g/m3d= Li/a

VLiQ

Li = influent BOD5mg/l (= g/m3)

Q = flow, m3/d

V = volume, m3

V/Q = (retention time, days) a 1 d

Design range for normal domesticwastewater:

100 V 400

if 400, then risk of odour release

ODOUR

caused mainly by hydrogen sulphide gas(H2S)

sulphates (SO42) reduced in anaerobicponds by obligately anaerobic sulphate-reducing bacteria (eg, Desulfovibrio spp)to sulphides (H2S, HS, S2)

proportion of sulphide existing as H2Sdepends on pH:

Effect of pH on hydrogen sulphide bisulphidesulphide equilibrium

H2S

S=HS-

Odour, continued odour not usually a problem if sulphate

concentration 7 (or byrecirculating maturation pond effluent, butexpensive and pump needs maintenance)

Recommended Design Procedure forAnaerobic Ponds

Performance should increase withtemperature, but insufficient reliable fielddata available to develop good designequation

Design temperature: mean air temp. incoldest month (slightly conservative aspond temp. some 23oC warmer)

Minimum retention time of 1 day

DESIGN VALUESBOD5 Percentage

Temp (oC) loading BOD(g/m3 day) removal

10 100 40

1020 20T 100* 2T + 20*2025 10T + 100* 2T + 20*>25 350 70

* Linear interpolation (T = temperature, oC)

PERFORMANCE OF ANAEROBIC PONDS

Northeast Brazil, ~25 C

Raw wastewater

A2

306 g/m3d

A3

A4129 g/m3d

A2

Retention BOD5 SS FCtime (mg/l) (mg/l) (per

(days) 100 ml)

Raw ww 245 310 4.7 107

0.8 59* 82** 8.1 106

A3 0.4 45 64 5.0 106

A4 1.9 49 57 4.7 106

* 76 percent removal ** 74 percent removal Not worth having (at least for domestic ww)

A2

Performance of Anaerobic PondsNortheast Brazil 25oC

Pond Retention Volumetric PercentageCode time BOD load BOD

(days) (g/m3 day) removalA2/1 0.8 306 76A6 1.0 215 76A4/1 1.9 129 80A1/3 2.0 116 75A1/2 4.0 72 68A1/1 6.8 35 74

Anaerobic ponds

very good removal of BOD & SS

this has effect of reducing land arearequirement for overall pond system, soanaerobic ponds should always be used(except at small works serving only a fewthousand people)

Operation and maintenance1. fly breeding in scum layer may be a

problem: use a suitable larvicide2. desludge when full of sludge this

occurs every n years:

Sludge accumulationrate, m3/caput year (Population)

(Pond volume, m3)n =

~0.04 m3/cap.yr in tropics

~0.08 m3/cap.yr in Europe

For small communities, use septic tanksrather than anaerobic ponds

Largest mega septic tank is 72,000 litres.One of these followed by one of 36,000 litres

can serve a population P of:P = [(72000 + 36000) 2000]/200 = 530 (say, 500)

(Equation from British Standard on Septic Tanks)

Centralslaughterhouse

CYPRUS

Average BOD = 15002000 mg/l

Anaerobic pond, Cyprus

Anaerobic ponds,Murcia, Spain

Strong odourof H2S !

.only became a problem whenmotorway built next to ponds.

Edge of an. pond

Badsolution !

O2

.totally ineffective !

O2 into pond 30 cm below surface:

Anaerobic ponds, Murcia Strong hydrogen sulphide odour Source of sulphates?

The local drinking water had a sulphateconcentration of 6001200 mg/l

(far in excess of EU Drinking WaterQuality Directive max: 250 mg SO4/l)So TREAT THE DRINKING WATER !! and the odour problem will vanish !! successfully done in the next province!