Biogas from human waste
Treatment of human waste in a biodigester
 Human waste as a resource
Treating human waste through Anaerobic Digestion is an incredibly ethical sanitation technology. Anaerobic Digestion occurs in biodigesters and produces a fuel (biogas), removes Biochemical Oxygen Demand (BOD) from sewage, conserves nutrients (especially nitrogen compounds) and most importantly reduces pathogens. Excessive concentrations of human waste damages the environment, because it is loaded with BOD, nutrients, and anthropozoonotic diseases. This can cause a host of environmental problems that can lead to ecosystem collapse such as rendering a water body uninhabitable for many organisms. Untreated sewage causes algal blooms, red tide, and so called dead zones. Humans also suffer from untreated sewage (also called black water). Waterborne disease transmitted through human excrement is a leading cause of death worldwide, especially in the so-called developing world. Some diseases caused by untreated human sewage are Cholera, Typhoid fever, Paratyphoid fever, Salmonella, Dysentery, Gastroenteritis, Leptospirosis, Meningitis, Hepatitis, and various parasitic diseases.
The amount of biogas that can be yielded from human waste is limited in comparison with livestock manure and other feedstocks. Our stomachs are just too efficient! David House states in his excellent book that 1000 lbs of human waste produces about 0.6 cubic meters of biogas (enough cooking fuel for about 1 to 2 persons). But that amount quickly adds up, please reference the internet for example projects especially in Rwanda, India and Thailand.
Untreated sewage, along with causing a prevalence of disease, developing countries are also disposing of valuable nutrients in places where fertilizers aren’t available. Biodigesters turn waste into a biofertilizer. There is also a major flaw in the sewage treatment systems of developed countries where enormous amounts of energy are used to aerate and treat sewage; Anaerobic Digestion treats sewage and also produces energy rather than consumes it. This article discusses considerations for human waste treatment and various options are outlined.
 Important considerations
A handful of considerations need to be made for treating human waste. There are IMPORTANT disease related issues and some common physical considerations. The number 1 issue is handling human waste. Operators that handle human waste without any precautions will inevitably get sick. The waste handling process must consider the handlers. Ideally a waste treatment system will eliminate any direct handling by humans.
Typical biodigester effluent is NOT sterile. Anaerobic digestion creates a competitive environment where pathogens are out competed by non-infectious microorganisms and therefore are edged out in terms of populations. This means that pathogens are REDUCED, but not entirely eliminated. However, studies in thermophilic biodigesters (45-55 degrees C) have shown a much greater reduction of pathogens than in ambient temperature and lower temperature biodigesters (see biodigesters capable of controlling pathogens section). A waste treatment system needs to address the issue of disease during the process via pre or post treatment or the effluent needs to be disposed of accordingly.
One common consideration in designing biodigesters to fit into an already existing system is that usually human excrement is heavily diluted to facilitate movement. Toilet flushes consume large volumes of water (range from 1.3 to 2.5 gallons but about 2 gallons in the US) and designing a biodigester with for example a 30-day hydraulic retention time (HRT) for treating flushed waste requires a very large volume biodigester at a 2 gallon per flush dilution. There are biodigester designs, however, that can handle an HRT, or the amount of time a biodigester retains a waste, of only a few hours. These designs are sludge retaining reactors such as an Upflow Anaerobic Sludge Blanket (UASB) and even better performing Fixed Film Reactors. One last important factor to consider is ammonia toxicity as human waste has been reported to have a low C: N ratio. This problem can be solved via dilution and co-digestion of a carbon rich feedstock such as molasses. Animal waste is inherently safer to treat then human waste because they tend to carry less human pathogens, though consideration for some manure born pathogens ought to be made as well.
 Treatment methods
 Heat pre-treatment
During this process human excrement would be pasteurized to 70 degrees C before entering the biodigester. This would be done best before dilution to reduce energy costs and can be done using waste steam, passive solar heating, or direct combustion of biogas or any other fuel source. The process would make more of the human excrement available for Anaerobic Digestion and would in fact likely increase the amount of biogas produced. Heat pre-treatment can also lower the HRT. Sterilization upfront will deal with any pathogen related effluent issues down the line and produce a biofertilizer for comestible (fit for human consumption) crops.
 Treatment through retention
Very long retention times for sewage have the ability to virtually destroy pathogens. The amount of time human excrement should be retained varies. In a very warm climate you may want to retain the waste for 60-90 days, however in cold climates (20 degrees C and below) 150 or more days of retention are recommended. Retention time can be controlled via the biodigester HRT or by holding the effluent for an additional period of time. The option that is the most economic should be considered as well as safety factors such as the access to holding tank and any other issue that involves potential exposure to humans and animals. Safety Warning: Retention methods to destroy pathogens should be confirmed by lab results before adoption.
 Post treatment and sterilization
Biodigester effluent may also be treated in a secondary treatment phase such as Ultrafiltration, Ultraviolet Light (UV), a Treatment Wetland, Composting, or Aerobic Treatment. Ultra filtration consists of running the effluent through a membrane that only allows solubles to pass through. At the moment this technology is more likely to be used in the developed world but appropriate solutions using materials such as mangroves and other plants might be used. Ultrafiltration is practical for concentrated wastewaters that have had most solids settled out. UV treatment is a common water treatment technology however may only be practical for dilute effluents where turbidity is not an issue. A treatment wetland provides additional treatment as well as habitat for wildlife. Essentially a movement gradient is created and planted with wetland plants that facilitate nutrient and pathogen removal. This is the way wastewaters, such as storm runoff, are naturally treated in the environment. A composting process maybe allowed used to treat the effluent however it must first be dried to facilitate aeration, which is land and energy intensive. Care must be made to ensure that no one breathes in the dust from the fresh effluent during this process. The effluent may also go through an aerobic treatment process to polish the effluent however this is expensive, intensive, and removes nutrients from a productive system. Other waste treatment options may include sand filters and clarifiers.
 Biodigesters capable of controlling pathogens
As previously alluded to, some biodigester processes are able to control virtually all the pathogens found in sewage. These are thermophilic biodigesters, phase biodigesters, and staged biodigesters. In a thermophilic biodigester the environment within the biodigester is so hot that many pathogens are unable to survive. The environment is also far more competitive than in a regular biodigester. Pathogens are usually acclimated and most happy around body temperature. Fortunately many of the organisms capable of carrying out Anaerobic Digestion are thermophiles, or heat loving organisms. However caution must be made with the previously mentioned ammonia toxicity, as thermophilic biodigesters are far more sensitive to this issue than ambient and lower temperature biodigesters. A phase biodigester separates the respective phases that material must undergo during the anaerobic digestion process. Organic material undergoes hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Essentially a container can facilitate the conversion of organics to solubles (hydrolysis), the production of acids (acidogenesis and acetogenesis) or methane production (methanogenesis). In phase Anaerobic Digestion two or more containers are used to separate the phases. This can be done physically (removing organics as they are hydrolysed), chemically (inhibiting methane production or buffering acids to a pH where methanogenesis can occur) or biologically (acidifying the first reactor(s)). If a reactor is allowed to acidify to inhibit methane production the low pH will also create an extreme environment where some pathogens are unable to live. After an acidic environment they will be introduced to a methane-producing environment that additionally removes pathogens through microbial competition. A two-phase biodigester capable of eliminating pathogens might have an acidifying first tank, which is then fed into a thermophilic, methane producing second tank. Staged biodigesters can work in the same way by changing the competition mechanisms in various stages (reactors) though still not quite separating the phases.
 Applying effluent
Completely eliminating pathogens is not necessary when adequate care is given to applying the effluent. Biodigester effluent that still contains pathogens can be applied into subterranean leachfields (with a clarifier), used for non-edible crops and in some cases forage crops, and applied directly to land. However all these things require safety considerations. The amount of human exposure needs to be taken into consideration. Groundwater and water body contamination are all potential threats to releasing effluent not completely void of pathogens into the environment. Direct land application needs to take direct exposure into account such as use of land by children and adults. Non-edible crops are another option and also allow for nutrient capture. Crops could include energy crops, biomass production, and many others. Exposure to humans however is again a risk that must be accounted for. The simplest and safest way to dispose of effluent is to simply inject it in an already existing sewer system.
Biodigesters offer a variety of benefits to the person interested in ethical treatment of human waste. The most important consideration, which has not necessarily always been effectively managed, is the danger pathogens in human waste pose to health. These systems are scalable from the household, community level to the larger industrial scale applications. Successful applications can be found worldwide and as well as in history. Best of all, Anaerobic Digestion offers to turn waste into a resource.
 Further Reading
- Bitton G. Wastewater Microbiology. 3rd Ed.Wiley-Liss 2005
- van Haandel, A.C., Lettinga, G. Anaerobic Sewage Treatment: A Practical Guide for Regions with a Hot Climate J Whiley 1994
- House, D. The Complete Biogas Handbook 3rd Ed 2007 www.completebiogas.com
- Speece, R. E. Anaerobic Biotechnology for Industrial Wastewaters Archae Press 1996
- Production of acetate from soluble organic materials (i.e. products of hydrolysis)
- Production of organic acids from soluble and insoluble organic material
- Aerobic Treatment
- The treatment of organic waste via cellular biomass production and respiration. This process requires constant input of relatively insoluble oxygen into aqueous solutions for wastewaters.
- Ammonia Toxicity
- Inhibition of anaerobic activity due to high concentrations of ammonia. Ammonia interferes with transport of soluble organics in cell walls.
- Anaerobic Digestion
- A microbial process carried out by a naturally occurring community of microorganisms that process organic matter into biogas, a liquid biofertilizer, and recalcitrant fiber.
- Diseases that originate from humans to animals. Unfortunately vice versa is almost always the case and can lead to difficult to control disease outbreaks.
- Biochemical Oxygen Demand (BOD)
- The biodegradability of organic matter in a substance. Determining the amount of oxygen needed to degrade a substance over time usually tests BOD.
- Biodigester Effluent
- The liquid by-product from an anaerobic digester.
- Biodigester Process
- A process that specially facilitates anaerobic digestion. The process may have various advantages such as sludge retention or polishing of effluent.
- A biodigester is a sealed container that facilitates anaerobic digestion.
- A biologically derived nutrient source.
- A biofuel derived from organic matter mostly containing methane and carbon dioxide but also containing trace amounts of hydrogen sulfide, water vapour, and ammonia.
- Black Water
- Wastewater containing human excrement. Usually refers to the portion of wastewater coming from a toilet.
- C to N Ratio
- Carbon to nitrogen ratio. The ration of carbon to nitrogen can determine toxicity factors in an anaerobic system and facilitation of degradation in an aerobic system. C:N is also a measure of odor potential.
- Settles or floats the insoluble fraction of the waste stream.
- Anaerobic digestion of a source of organic material along with an additional organic material. Co-digestion can be used to solve toxicity factors, nutrient deficiencies, and increase the robustness of the fermentation.
- An aerobic treatment process of organic waste that uses cellular biomass production to convert organics and nutrients into a stabilized product (compost). Most commonly used in homes or centers to treat food waste and turn into an organic fertilizer.
- Dead Zone
- An area in a water body where nutrients have triggered an algal bloom that causes the complete consumption of Oxygen on a nightly basis rendering the area uninhabitable for higher organisms.
- Fixed Film Reactor
- An anaerobic digestion process where organic material is passed over a fixed community of organism on inert media.
- Forage Crop
- A crop raised and harvested to feed animals.
- Hydraulic Retention Time (HRT)
- The amount of time that a substance is retained in a biodigester for treatment. HRT prevents microbial washout and helps mediate the amount of biogas produced in a system.
- The microbial process of turning insoluble organic material such as fats, starches and proteins into soluble by-products.
- An area where treated water is slowly dispersed.
- Process of producing methane from a variety of sources but mainly acetate and carbon dioxide+ hydrogen.
- Passive Solar Heating
- Using the instant solar radiation rather than other forms of energy to heat.
- Sterilization of a product by bringing the temperature to 71.7 degrees C for 15 to 30 seconds.
- A disease causing vector such as a bacteria or virus.
- Phase Biodigester
- A biodigester that separates one or more of the microbial processes into a separate phase.
- Red Tide
- A bloom of organisms that can sometimes produce toxins that leads to death of marine life and minor to severe skin irritation in humans.
- Sand Filter
- Uses biological and physical removal processes to treat wastewaters.
- Staged Biodigester
- a series of biodigesters that while not separating out phases is capable of achieving more control of the process and a more polished effluent.
- A heat loving organism that exist at temperatures mostly ranging from 45 to 55 or even 60 degrees Celsius. These are found in many extreme environments such as hot springs and geothermal vents.
- Thermophilic Biodigester
- A biodigester operating at temperatures of around 45 to 55 degrees Celsius.
- Treatment Wetland
- A waste treatment technology that uses natural functions in a wetland to remove BOD, nutrients, heavy metals, and pathogens
- Cloudiness caused by suspended particles.
- Two-Phase Biodigester
- A biodigester that processes wastes in two phases, usually an acid producing phase (controlled by pH) and a methane producing phase.
- Filtration that only allows solubles to pass through the membrane (i.e. about 106 daltons).
- Ultraviolet Light (UV)
- UV light can be used to destroy pathogens although there is sensitivity to turbidity and danger involving larger organisms.
- Upflow Anaerobic Sludge Blanket (UASB)
- Wastewaters are passed up through a blanket of granulated microorganisms. A tortuous path leaving the biodigester causes granules to fall out of suspension.