通过厌氧消化处理人类废物是一种令人难以置信的道德卫生技术。厌氧消化发生在生物消化器中，产生燃料（沼气），去除污水中的生化需氧量（BOD），保存营养物质（尤其是氮化合物），最重要的是减少病原体。人类排泄物浓度过高会破坏环境，因为其中含有大量的生化需氧量、营养物质和人畜共患疾病。这可能会导致一系列环境问题，从而导致生态系统崩溃，例如使水体无法适合许多生物居住。未经处理的污水会导致藻华、赤潮和所谓的死区。人类还遭受未经处理的污水（也称为黑水）的困扰。通过人类粪便传播的水传播疾病是全世界死亡的主要原因，特别是在所谓的发展中国家。未经处理的人类污水引起的一些疾病有霍乱、伤寒、副伤寒、沙门氏菌、痢疾、胃肠炎、钩端螺旋体病、脑膜炎、肝炎和各种寄生虫病。
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。
未经处理的污水除了导致疾病流行之外，发展中国家还在没有肥料的地方丢弃了宝贵的营养物质。生物消化器将废物转化为生物肥料。发达国家的污水处理系统还存在一个重大缺陷，需要消耗大量的能源来曝气和处理污水；厌氧消化处理污水并产生能量而不是消耗能量。本文讨论了人类废物处理的注意事项，并概述了各种选择。
重要考虑因素
处理人类排泄物需要考虑一些因素。有重要的疾病相关问题和一些常见的身体注意事项。第一个问题是处理人类排泄物。不采取任何预防措施处理人类排泄物的操作人员将不可避免地生病。废物处理过程必须考虑处理者。理想情况下，废物处理系统将消除人类的任何直接处理。
典型的生物消化器流出物不是无菌的。厌氧消化创造了一个竞争环境，病原体在竞争中被非感染性微生物击败，因此在种群数量上被淘汰。这意味着病原体减少了，但并未完全消除。然而，高温生物消化器（45-55 摄氏度）的研究表明，与环境温度和较低温度生物消化器相比，病原体的减少量要大得多（参见能够控制病原体的生物消化器部分）。废物处理系统需要通过预处理或后处理来解决处理过程中的疾病问题，或者需要对废水进行相应的处理。
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。
治疗方法
热处理
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.
Conclusion
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