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
3
4
5
6
7
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
40
50
60
70
80
90
100
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:[email protected] or [email protected]://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