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Effective Decentralized Sewage Sanitation with Low CO 2 Footprint. Aaron A. Forbis-Stokes , Joan Colón, Lilya S. Ouksel, Marc A. Deshusses Department of Civil and Environmental Engineering Duke University, Durham, North Carolina, USA. Anaerobic Digestion Pasteurization Latrine.

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Effective decentralized sewage sanitation with low co 2 footprint

Effective Decentralized Sewage Sanitation with Low CO2 Footprint

Aaron A. Forbis-Stokes, Joan Colón, Lilya S. Ouksel, Marc A. Deshusses

Department of Civil and Environmental Engineering

Duke University, Durham, North Carolina, USA

Anaerobic digestion pasteurization latrine

Anaerobic Digestion Pasteurization Latrine

Anaerobic digestion of concentrated waste

Field efficacy

Pit latrine/



(Extra biogas/ cooking/lighting)



Heat exchanger







Biogas powered




Efficient and simple construction and operation

Self-sustaining pasteurization system with sufficient pathogen inactivation

Treated and sanitized effluent

Proof of concept

Proof of Concept

Anaerobic digester

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



Anaerobic digester1

Anaerobic Digester

Effect of acclimation on biogas production biogas potential test with different n concentrations

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

Non-acclimated inoculum

Acclimated inoculum

5 g TAN/L

5 g TAN/L

8 g TAN/L

8 g TAN/L

Start up and biogas production

Start-up and Biogas Production

No dilution


High N

1:1 u:w





10 person household =400 L biogas/day

Average biogas yield at steady state conditions of 0.37 NLbiogas/gCOD

Methane content 62 ± 3 %

Cod removal efficiency

COD Removal Efficiency

Vfa a ccumulation

VFA Accumulation

Total ammonia nitrogen and total free ammonia

Total Ammonia Nitrogen and Total Free Ammonia





Heat sterilization system

Heat Sterilization System

Effluent coming from the digester


-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

Thermal Efficiency of the Heater

Methane calorific value: 33.9 KJ/L

Temperature range: 55-75 °C

Thermal efficiency: 55-70 %

Heat sterilization tests

Heat Sterilization Tests


-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 tests1

Heat Sterilization Tests

230-280 Lbiogas/d for

65≤T≤75 °C


Before heater

After heater



Pilot study

Pilot Study



  • Sogomo Estate, Eldoret, Kenya

    • 1/8 acre, ~20 residents, municipal water tap, borehole well, shared pit latrines

    • Near University of Eldoret

Community entry

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

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

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

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

System Cost

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

*Gesi2000 Biodigester was donated by SimGas.



  • 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

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 vsperi-urban vs urban; single family vs shared

    • Use, diarrheal disease, other?

Thank you

Thank you!


aaron.forbis-stokes@duke.edu or marc.deshusses@duke.edu


Special thanks to:

Bill & Melinda Gates Foundation


University of Eldoret

WataalamuRepair & Mechanics

Extra slides

Extra Slides

Anaerobic digestion 101

Anaerobic Digestion 101

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

Faecal sludge simulant properties

Faecal Sludge Simulant Properties



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)

Energy in faecal sludge a few relevant numbers

Energy in Faecal Sludge… a Few Relevant Numbers

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

  • Heat 1 kg = 1 L water by 1 °C 4180 J

  • Vaporize 1 kg water2,260,000 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)

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