On biodigestors and human waste

Making Biogas from Human Waste
From Appropedia

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. 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. Are stomachs are just too efficient! David House states in his excellent book that 1000 lbs of humans 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.


[edit] 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.