Effective Decentralized Sewage Sanitation with Low CO2 Footprint

Aaron A. Forbis-Stokes, Joan Colón, Lilya S. Ouksel, Marc A. DeshussesDepartment of Civil and Environmental Engineering

Duke University, Durham, North Carolina, USA

Anaerobic Digestion Pasteurization Latrine

Pit latrine/Human wastes

DigesterBiogas

Additional organic wastes(optional)

Heat exchanger

Treated and sanitized effluent

30°C 45°C

75°C60°C

(Extra biogas/ cooking/lighting)

Anaerobic digestion of concentrated waste

Biogas poweredheater

Self-sustaining pasteurization system with sufficient pathogen inactivation

Efficient and simple construction and operation

Field efficacy

PROOF OF CONCEPT

Anaerobic Digester

• 10 person system– Lab scaled 1:33

• Feed stock: – 120 g Feces/d & 300 mL

Urine/d

• 17 L volume, 40 day HRT• OLR = 1.8 g COD/(Lreactd)

0.13 g N/(Lreact d)• T = 30 °C• No Mixing

Feed

Biogas

Overflow

Anaerobic Digester

Non-acclimated inoculum

Effect of Acclimation on Biogas ProductionBiogas potential test with different N concentrations

8 g TAN/L

5 g TAN/L

5 g TAN/L

8 g TAN/L

Acclimated inoculum

0 50 100 150 200 250 3000.00

2.00

4.00

6.00

8.00

10.00

12.00

0

1

2

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8

Biogas Methane OLR

Time (d)

Gas

pro

dcuti

on (N

L ga

s/g

COD

)

Load

ing

(g C

OD

/L re

acto

r/d)

and

(g

N/L

)

Start-up and Biogas Production

• Average biogas yield at steady state conditions of 0.37 NLbiogas/gCOD • Methane content 62 ± 3 %

S1 S2 S3 S41:1 u:wStart-up No dilution High N

10 person household

=400 L biogas/day

COD Removal Efficiency

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

70

80

0

10

20

30

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Inlet Total DissolvedSuspended Removal efficiency

Time (d)

COD

(g/L

)

Rem

oval

effi

cien

cy (%

)

VFA Accumulation

Total Ammonia Nitrogen and Total Free Ammonia

S1 S2 S3 S4

Effluent coming from the digester30°C

Heat Sterilization System

-Designed to treat intermittent loads (1 load of 0.6 L each hour, 14 L/d)-Optimized for heat efficiency-No moving parts, simple materials

Thermal Efficiency of the Heater

Methane calorific value: 33.9 KJ/LTemperature range: 55-75 °CThermal efficiency: 55-70 %

Heat Sterilization TestsApproach: -Test with E. coli (intermittent flow, 14 L/d in 0.6 L loads once per hour)-Calibrate and validate flow, heat and heat deactivation model-Simulate performance for helminth and virus*

*kill rate constants from Popat et al. Wat. Res. 2010, 44, 5965-5972.

Heat Sterilization Tests

Before heater

EffluentAfter heater

CumulativeInlet230-280 Lbiogas/d for

65≤T≤75 °C

PILOT STUDY

Implementation• Sogomo Estate, Eldoret, Kenya– 1/8 acre, ~20 residents, municipal water tap, borehole

well, shared pit latrines– Near University of Eldoret

Community Entry

• Questionnaires– Shallow wells often used for

drinking water (boil)– Squat-style toilet, wipers– Pits used for solid waste

disposal– System resources initially

unappealing• Owner interest– Protective of current

systems

Toilet & Digester• Prefabricated plastic latrine labs

• 2 sites with standard slabs• 1 site with urine diversion

• SimGas Gesi2000 floating dome digester

• <1L pour flush

Heating system

• Fabricated in local labor market– 16 gage galvanized

steel and welded seams

• Heating tank – Left– 7.7 L

• Heat exchanger – Right– 9.4 L

Complete System• Acclimated seed sludge and gradual loading • In use – plots with 17, 24, and 35 residents• Fully enclosed treatment system, 3x3 meter footprint• User advantages – Less odor & flies, well-lit, resources• Disadvantages – Height inconvenience, solid waste, pour flush

System Cost

Description Quantity Total Cost Cost/SystemGesi2000 Biodigester 3 $2421* $807Digester connections 3 $283 $94Heating system 3 $289 $97Latrine 3 $1474 $491Total $4632 $1544†

*Gesi2000 Biodigester was donated by SimGas.

†With a conservative estimate of 5 year lifespan and average of 25 users per system, system cost = $0.03/p/d

Conclusions

• Anaerobic digestion of undiluted human waste self-sustains effective pasteurization system– A yield ≈ 0.3-0.4 NLbiogas/g COD can be expected– 100% of E.coli inactivation was achieved at a working T ≥ 65 °C– 10 p system - produce 400 Lbiogas/d and require 230-280 Lbiogas/d to operate

65 ≤ T ≤ 75 °C

• Systems were built in Eldoret, Kenya, using all local materials and are currently in operation– 3 systems, serving 15-35 people each– System outputs (nutrient-rich treated effluent & excess biogas)

desirable to community members• System is simple with no moving parts, operates by gravity

flow, and requires little maintenance & operation

Future Work

• Monitoring and evaluation of system in Eldoret, Kenya– Assess needs for design improvements

• Cost assessment and scale-up– Resource evaluation of effluent for fertilization and excess biogas

for multiple purposes (cooking, lighting, biogas generator)– Mass reproducible components and installation

• Tertiary effluent filter for reuse purposes– Turbidity and odor removal

• Different contexts and measures– Rural vs peri-urban vs urban; single family vs shared– Use, diarrheal disease, other?

Thank you!

Special thanks to:• Bill & Melinda Gates Foundation• SimGas• University of Eldoret• Wataalamu Repair & MechanicsContact:aaron.forbis-stokes@duke.edu or marc.deshusses@duke.eduhttp://deshusses.pratt.duke.edu/

Extra Slides

Anaerobic Digestion 101

Source: Syed Hashsham, PhD, lecture notes, Michigan State University

Faecal Sludge Simulant PropertiesProperties

Simulant feces

Real feces Properties

Simulant urine Real urine

Moisture (%) 80 65-85(1) Moisture (%) 97.6 95-98(4)

TS (%) 20 15-35(1) TS (%) 2.4 2.5-3.7(4)

VS (%) 80 - VS (%) 60 -COD (g COD/g TS) 1.23 1.24 (2) COD (g COD/l) 4.8 3.8-8.2(2)

CODs (g COD/g TS) 0.85 - CODs (g COD/l) 0 -CODdis (g COD/g TS) 0.38 - CODdis (g COD/l) 4.8 -Ntot (% dry matter) 2.55 2-3(3) N-tot (mg/l) 5200 5000-8000(4)

N-NH3 (% Ntot) 3.02 <7(2) N-NH3 (mg/l) 197 <100(2)

P-total (mg/l) 400 400-1000(4)

pH (1:5 w:v) 5.3 4.6-8.4 pH 6.05 6-8.2(4)

Conduct. (1:5 w:v, mS/cm) 5.7 -

Conduct. (mS/cm) 23 16-22(4)

(1) Wignarajah et al. (2006)        (2) Jönsson et al. (2005)(3) Ganong (1983)(4) Putnam (1974) Example:

Simulant excreta: 7.2 g N p-1 d-1 Real excreta: 5.2 -8.2 g N p-1 d-1 (Uganda, Haiti, India, South Africa)

Metals?

Energy in Faecal Sludge… a Few Relevant Numbers

• Heat 1 kg = 1 L water by 1 °C 4180 J• Vaporize 1 kg water 2,260,000 J• Burn 10 L = 6.4 g methane 358,000 J• Burn 10 g wood ~200,000 J• Burn 80 g dry feces (~400 g wet) ~1,600,00 J• Dry 400 g wet feces, burn solid ~880,000 J• Dry 400 g wet feces + 1 L urine, burn solid requires 1,400,000 J

• Waste of 1 person digested anaerobically 860,000 J or about 10-15 Watts continuous (or 240-360 Wh per day)

1 person: 400 gwet feces and 1 L urine per day