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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/

Human

wastes

(Extra biogas/ cooking/lighting)

Biogas

Digester

Heat exchanger

30°C

45°C

Additional

organic

wastes

(optional)

Biogas powered

heater

75°C

60°C

Efficient and simple construction and operation

Self-sustaining pasteurization system with sufficient pathogen inactivation

Treated and sanitized effluent



Anaerobic digester
Anaerobic Digester

Biogas

  • 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

Overflow

Feed



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

Start-up

High N

1:1 u:w

S4

S2

S1

S3

10 person household =400 L biogas/day

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

Methane content 62 ± 3 %



Vfa a ccumulation
VFA Accumulation



Heat sterilization system
Heat Sterilization System

Effluent coming from the digester

30°C

-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

Approach:

-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

Inlet

Before heater

After heater

Effluent

Cumulative



Implementation
Implementation

  • 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.


Conclusions
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
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!

Contact:

[email protected] or [email protected]

http://deshusses.pratt.duke.edu/

Special thanks to:

Bill & Melinda Gates Foundation

SimGas

University of Eldoret

WataalamuRepair & Mechanics



Anaerobic digestion 101
Anaerobic Digestion 101

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


Faecal sludge simulant properties
Faecal Sludge Simulant Properties

Metals?

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)


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 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)


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