与牲畜粪便和其他原料相比，从人类粪便中产生的沼气量是有限的。我们的胃太高效了！David House 在他的优秀著作中指出，1000 磅人类排泄物可产生约 0.6 立方米沼气（足够 1 至 2 人做饭的燃料）。但这个数字很快就会增加，请参考互联网上的项目示例，尤其是在卢旺达、印度和泰国的项目。
在设计生物消化器以适应现有系统时，一个常见的考虑因素是，通常人类排泄物被严重稀释以促进移动。厕所冲水消耗大量的水（范围为 1.3 至 2.5 加仑，但在美国约为 2 加仑），并且设计具有例如 30 天水力停留时间 ( HRT) 的生物消化器来处理冲洗废物需要一个非常大容量的生物消化器每次冲洗稀释 2 加仑。然而，有些生物消化器设计只能处理几个小时的 HRT（即生物消化器保留废物的时间）。这些设计是污泥截留反应器，例如上流式厌氧污泥床 (UASB)以及性能更好的固定膜反应器。最后一个需要考虑的重要因素是氨毒性，因为据报道人类排泄物的碳氮比较低。这个问题可以通过稀释和共消化富含碳的原料（例如糖蜜）来解决。动物粪便本质上比人类粪便处理起来更安全，因为它往往携带较少的人类病原体，尽管也应该考虑一些粪便传播的病原体。
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.
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.
- 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