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.[10]

Introduction

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.

Background

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.

Usage

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 Plant Direct 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.

Design

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

101,527,000 Bbtu/year
111,609,629 households
× 1.055 kJ/Btu
356 days/y
= 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. [1] The usable energy one human produces in one day is

.25 lbs/day
2.205 lbs/kg
× 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
1102.2 kJ/day
= 1080

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
1m3
106cm3
= 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. [7] 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

14.6 kg × 8 = 116.6 kg

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

193.9 kg × 0.08 = 15.5 kg

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
= 4.0

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
= 29.9

with only 1000 kg of sawdust. This brings the C/N ratio near optimum for an anaerobic reaction.

Summary

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.

References

[1] C. Sawyer, P. McCarty and G. Parkin, Chemistry for Environmental Engineering and Science, 5th Ed. (McGraw-Hill, 2002).

[2] C. Sawyer, P. McCarty and G. Parkin, "Chemistry for Environmental Engineering and Science,"Fifth Edition, McGraw Hill, p. 573-574 (2002).

[3] U.S. Census Bureau, "Statistical Abstract of the United States: 2010 (129th Edition)," (U.S. Government Printing Office, 2009), p. 593.

[4] "Energy Statistics of OECD countries, 2010 Edition," (OECD Press, 2010).

[5] "Energy Efficiency Trends in Residential and Commercial Buildings," U.S. Department of Energy, October 2008.

[6] J. Fry, " Methane Digesters For Fuel Gas and Fertilizer," New Alchemy Institute, 1973.

[7] C. Polprasert, Organic Waste Recycling (IWA Publishing, 2007).

[8] C. R. Prasad et al, "Bio-Gas Plants: Prospects, Problems and Tasks," Economic and Political Weekly 9, 1347 (1974).

[9] C. S. Ferreira, "Refractive Index Matching Applied to Fecal Smear Clearing," Rev, Inst. Med. Trop. S. Paulo 47, 347 (2005).

[10] L. Hopwood, "Anaerobic Digestion," UK National Non-Food Crops Centre, November, 2009.