Human beings and animals produce waste products. Yet this waste is a source of both energy and nutrients. Our repugnance to waste has blindsided us to its value. Adopting the Cradle-to-Cradle philosophy of Waste = Food, a new generation of human waste bioreactors is solving multiple problems at once:
- eliminate human waste as a source of disease and pollution
- generate energy
- create fertlizer
- create potable water
By the end of 2011, in Africa and Asia 430,000 households were using biogas digesters, benefiting over 2.5 million individuals by providing a safe, inexpensive and hygienic source of power.
Figure 1: EPA Agstar Biogas Flow Diagram (Source: EPA AgStar)
Biogas recovery system components:
- Manure collection systems.
A system is needed to collect manure and transport it to the digester. Existing liquid/slurry manure management systems can readily be adapted to deliver manure to the anaerobic digester.
- Anaerobic digesters.
Anaerobic digesters, commonly in the form of covered lagoons or tanks, are designed to stabilize manure and optimize the production of methane. A facility for digester effluent storage is also required.
- Biogas handling systems.
Biogas (a product of the decomposition of the manure, typically comprising about 60 percent methane and 40 percent carbon dioxide) is collected, treated, and piped to a gas use device.
- Gas use devices.
Biogas can be used to generate electricity, as a boiler fuel for space or water heating, upgraded to natural gas pipeline quality, or for a variety of other uses. Flares are also installed to destroy extra gas and as a back-up mechanism for the primary gas use device.
- Digester byproducts.
In addition to the biogas recovery system, the effluent of the anaerobic digestion can be used to create a number of digester byproducts.
SNV is a Netherland-based NPO that has been working on innovate Base of the Pyramid solutions in agriculture, Renewable Energy, and Water, Sanitation & Hygiene. In particuliar, SNV is at the forefront of low cost, clean and efficient animal and human waste biogas systems.
One of the big success stories is the Biogas Program for the Animal Husbandry Sector in Vietnam is implemented by the Livestock Production Department (under MARD) in cooperation with Netherlands Development Organization – SNV. Overall objectives of the project are to effectively exploit biogas technology and to develop a commercially viable biogas sector in Vietnam; and to contribute to rural development and environmental protection via provision of clean and affordable energy to rural households, improvement of community sanitation and rural people’s health, creation of rural jobs and reduction of greenhouse gas emissions. It has just won the World Energy Award 2012 because since 2003 it has:
- supported the construction of over 140,000 biogas digesters
- created employment for more than 5,700 masons
- improved the lives of more than 700,000 people through enabling access to clean, safe and sustainable energy
Now present in more than 52 of Vietnam’s 63 provinces and municipalities, the programme targets low-income, rural livestock farmers.
Figure 2: Map showing success of the Vietnamese program
Through promoting the development of a commercially viable biogas sector, the program contributes to both rural development and environmental protection:
- providing a source of clean energy
- improving community health and sanitation
- creating jobs
- preventing deforestration
- reducing greenhouse gas emissions
SNV works in partnership with the African Biogas Partnership Program to promote biogas in six African countries:
- Bukina Faso
As of August 2012, the ABPP programme has reached 18,000 plants (16,015 precisely). All countries have been able to consolidate their operations and are scaling up the operations. The ultimate target for the first phase which ends in 2013 is 70,500 plants.
Engineering Calculations for Construction of a Human Waste Bioreactor
The following calculation is reprinted in full without modifications with permission from Stanford student Paul Andrew Cook from the Stanford website here
Design of a Household Human Waste Bioreactor
Paul Andrew Cook
September 24, 2010
Submitted as coursework for Physics 240, Report II, Stanford University, Fall 2010
|Fig. 1: Simplified Bioreactor Process Diagram A household with a HWB would deposit organic waste into the reactor and then use the byproducts for heating, cooking, and fertilization.|
This report examines using human waste as feedstock in a small-scale bioreactor to produce methane gas for cooking and heating. While the use of biogas produced from livestock manure is commonplace, I am interested in the feasibility of building a household reactor that instead utilizes human waste as its primary input.
When organic material (including human feces, animal waste, and plants) is digested by microorganisms in the absence of oxygen (anaerobic digestion) a gas is released consisting of 60% methane and 40% carbon dioxide. This gas is typically called biogas and because it can be ignited, it may be used as a cooking and heating agent.
A human waste bioreactor (HWB) can be used in developing nations by families who do not have access to electricity nor livestock manure. In addition, the household-sized HWB can be built without industrial construction equipment and access to industrial materials. It could be employed in war torn areas where the electrical grid has been destroyed. The HWB would be especially useful in refugee camps where there is no sanitary sewer, people are already exposed to waste-borne illnesses, and there is a high-density living environment without access to electricity.
Human feces can carry parasites and diseases such as cholera and giardia. In this report, I do not consider the pathology of creating a HWB. Anyone building a household-sized HWB should analyze its potential to spread disease.
|Fig. 2: Diagram of a Potential Biogas PlantDirect contact with human waste could be avoided by having the latrine directly input into the digester. The inlet and mixer are for supplementing the human waste with livestock manure or food scraps. Source: Wikimedia Commons.|
Construction and Operation
A household HWB could be a simple vessel dug into the ground with latrine and manure inputs and a solid-waste outlet. (A secondary output of the HBW is a nutrient-rich fertilizer called the digestate. (See Figs. 1 and 2.) The walls could be concrete or clay brick. The HWB needs to be sealed to create anaerobic conditions, but must have a cover with a release valve to trap the biogas and allow for its release. Human waste would enter the bioreactor near the bottom and continually feed the anaerobic digestion process. The methane containing biogas would rise to the top of the container; the user would extract the gas as needed for cooking and heating. The digested sludge or digestate would be removed and employed as fertilizer.
How many people would have to contribute to a bioreactor to meet the energy demands of a modern household? In 2007, 111,609,629 US households consumed 101,527,000 Bbtu of electricity. Converting to joules, the per household consumption for one day was
|= 2.63 × 106 kJ/day|
But, only 45.2% of this energy went towards cooking and heating. Therefore, the total energy the bioreactor would have to produce would be
|2.6 × 1006 kJ/day × 0.452 = 1.19 × 106 kJ/day|
One human produces 0.25 lbs of volatile waste per day that can be fully utilized in the reactor. [6-8] The mass branching ratio between methane and carbon dioxide is 0.35. With a conservative estimate, 50% of the waste will burn as methane, which has a specific heat (amount of energy released when the material is ignited) of 5.55 × 104 kJ/kg.  The usable energy one human produces in one day is
|× 0.50 × 0.35 × 5.55 × 104 kJ/kg = 1102.2 kJ/day|
Therefore, the number of people that should contribute to the bioreactor is simply,
|1.19 × 106 kJ/day
If we were just putting human waste into the digester, how large would it need to be to accommodate 1080 people? On average, one human produces 2.2 lbs (0.998kg) of urine and 0.5 lbs (0.227 kg) of fecal matter or a total of 2.7lbs (1.224 kg) waste in one day. [6-8] Assuming that the average density of human waste slurry is 1.0 g/cm3, the volume of the primary basin would need to be at least
|1080 people × 1.224 kg/person ×||1000 g/kg
1 g/cm3 × 106 cm3/m3
|= 1.32 m3|
This is the minimum volume necessary, as an anaerobic reaction proceeds optimally when the carbon to nitrogen ratio (C/N) is near 30. [6,7,8] The C/N for human feces is between 6 – 10 and urine has 18% nitrogen, making the C/N ratio decrease as more urine is added.  By combining the human feces with sawdust, C/N of 200 – 500, we can increase the overall ratio inside the digester.
With the above C/N ratios, human feces have 6% nitrogen (sawdust contains only .1%). The total nitrogen content of human feces is
|1080 people × 0.227 kg/person × 0.06 = 14.6 kg|
for the number of people contributing to the reactor. The total carbon content is
or eight (the median between the 6-10 C/N ratio) times the amount of nitrogen. For human urine the total nitrogen content is
|1080 people × 0.998 kg/person × 0.18 = 193.9 kg|
with a carbon content of
because of its low C/N ratio. Considering both the feces and urine, the addition of 2000 kg of Sawdust (nitrogen content of 1kg, carbon content of 350 kg) would raise the C/N ratio to
|113.6 + 15 + 700
14.2 + 187.4 + 2
inside the digester. This ratio is well below the needed ratio, but removing the urine the C/N ratio is
|113.6 + 350
14.2 + 1
with only 1000 kg of sawdust. This brings the C/N ratio near optimum for an anaerobic reaction.
The energy usage of the household would have to decrease in order to limit the input needed to meet the daily demand. The high nitrogen content of human waste necessitates the addition of other materials to the bioreactor to enable anaerobic digestion. Because of its high nitrogen content, urine should not be added to the digester. Considering only the addition of human feces, 1000 kg/day of sawdust must be added to the reactor for the creation of biogas. Therefore, human feces is not an ideal primary material for a bioreactor, but one is capable of using it if there is access to a large amount of a secondary component with a high C/N ratio.
© Paul A. Cook. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 C. Sawyer, P. McCarty and G. Parkin, Chemistry for Environmental Engineering and Science, 5th Ed. (McGraw-Hill, 2002).
 C. Sawyer, P. McCarty and G. Parkin, “Chemistry for Environmental Engineering and Science,”Fifth Edition, McGraw Hill, p. 573-574 (2002).
 U.S. Census Bureau, “Statistical Abstract of the United States: 2010 (129th Edition),” (U.S. Government Printing Office, 2009), p. 593.
 “Energy Statistics of OECD countries, 2010 Edition,” (OECD Press, 2010).
 “Energy Efficiency Trends in Residential and Commercial Buildings,” U.S. Department of Energy, October 2008.
 J. Fry, “ Methane Digesters For Fuel Gas and Fertilizer,” New Alchemy Institute, 1973.
 C. Polprasert, Organic Waste Recycling (IWA Publishing, 2007).
 C. R. Prasad et al, “Bio-Gas Plants: Prospects, Problems and Tasks,” Economic and Political Weekly 9, 1347 (1974).
 C. S. Ferreira, “Refractive Index Matching Applied to Fecal Smear Clearing,” Rev, Inst. Med. Trop. S. Paulo 47, 347 (2005).
 L. Hopwood, “Anaerobic Digestion,” UK National Non-Food Crops Centre, November, 2009.