Methane Digester better than composting


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  • F H Mughal
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The pros and cons of anaerobic processes (biogas) compared to other systems

(note by moderator: I have moved Mughal's posting about the factsheet of WG 3 here, as it makes important points about anaerobic systems in general)

WG3 - Links between sanitation, climate change and renewable energies

The factsheet should have, towards its end, a section on “constraints and limitations in biogas production in developing countries,” containing the write-up reflecting the following views:

In some developing countries, low-level of technological advances and lax attitude of the operation staff, demand careful use of the anaerobic systems for the generation of methane gas. There have been incidents of anaerobic digesters’ explosion causing damage, as the methane is an explosive gas.

Anaerobic digestion is a three-stage process. In the first stage, the complex insoluble organics are hydrolyzed to simple soluble organics by the extra cellular enzymes. During this stage, the cellulose and starch are hydrolyzed to simple sugars, while proteins bifurcate into amino acids. In the second stage, called the acid phase, the acid formers (various species of Pseudomonos, Alcaligens, Flavobacterium, Escherichia and Aerobacter) convert simple organics to organic fatty acids (acetic acid). This results in higher acid concentration and low pH. In the third stage, called methane phase, methane formers utilize the organic acids and metabolized them to methane and carbon dioxide. Amino acids give rise to ammonia, which in turn, neutralize the remaining acids.

Methane formers (species of Methanobacterium, Methanococcus and Methanosarcina) are highly frail in nature. They are strict anaerobes, grow over a wide range of temperature, difficult to cultivate and, they remain inalienable. They are highly sensitive to low pH conditions.

These concomitant reactions in actual digestion occur simultaneously. Flawless performance of the digester will take place only when there is a balanced bacterial population of acid formers and methane formers, or, in other words, the volatile acids production equals volatile acids breakdown. If the volatile acids formation is greater than its breakdown (which usually is the case, in actual digesters), the pH lowers, inhibition or wash-out of methane formers occurs and, the process fails, biologically.

As an example, in Karachi, Pakistan, trickling filters wastewater treatment plants were constructed in 1966. Because of lack of expertise at operators’ level, the anaerobic digesters went out of order, during its first year of operation, due to acidic conditions.

As a second example, in early 1990s, large anaerobic ponds were constructed in Karachi for wastewater treatment. As was expected, within a short span of time, the plant started producing poor quality effluent. In addition to the industrial constituents of the wastewater (toxic metals inhibit methane formers), the volatile acids formation was more than its breakdown, due to organic overloading and shock-loading.

The lesson learnt is that in those developing countries, where the level of technological knowledge relatively is low, it is advisable to go for aerobic systems. If at all anaerobic systems are preferred, great care and caution needs to be adopted, to prevent their failure.

F H Mughal (Mr.)
F H Mughal (Mr.)
Karachi, Pakistan
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  • Richard
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Re: Methane Digester better than composting

I had emailed this organization and I was asked to open a discussion on this topic. A methane digester has several advantages to composting of poop (fecal matter) and pee. 1.) Nitrogen is spared and not released as ammonia as can happen in composting. 2.) There are no nutrients lost through leaching. 3.) The digester solves the problem of energy as it is a natural by-product of waste treatment in a closed anaerobic digester. 4.) A digester can also accept kitchen waste that has been slurried and left to ferment for a couple of days. 5.) Where there are farm animals their waste can also be added to produce an additional amount of Methane and high quality fertilizer.

A methane digester solves the problem of dealing with human or animal waste and food waste all-in-one. In return it produces a fuel that can be used to cook with and even light a home for a few hours at night while producing a superior fertilizer over composting. As a digester is continuously fed the liquid can be put directly onto the garden, or land, to grow food. Once the slurry is added the soil community begins to grow in numbers further enhancing the available of nutrients for the plants to take up.

India (and other countries) has been tackling the problem of waste management for years by adding a methane digester of a particular design to solve a particular problem: at a single home, within the city or a small town. A digester can either be small which serves a single home or larger where it serves a small community of homes. A methane digester requires little maintenance beyond the daily addition of new material and the collection of finished material. Occasionally it would have to be emptied for inspection, possible repair in larger units that are built underground or cleaned out - this should never be more than once a year, or even every few years - it is rarely required.

I for one am in favor of creating a valuable fuel source and a high quality fertilizer in a methane digester over composting. I would like to hear your comments.
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  • JKMakowka
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Re: Methane Digester better than composting

Hello Richard

I guess it is general consensus here that anaerobic digesters are a very good potential solution to deal with household waste-water etc.

However relatively speaking, the costs to build these are quite high, maintenance and operation is compared to simple composting pit solutions still a bit of a problem, and last but not least the slurry from anaerobic digesters is not quite as pathogen free as sometimes suggested.

But especially for rural HH with some amounts of cow manure it is definitely a proven and well working solution.
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  • christian.rieck
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Re: Methane Digester better than composting

Hello Richard,
I would like to emphasize on the point made by Julius that the hygienic quality of digester effluent that is fed by human waste bears healt risks. I do not have studies at hand to prove but it is natural that with the given low retention times of digesters of a few days to up to 1 month a lot of pathogens will remain in the effluent and slurry for example Ascari eggs. Hence the handling of slurry or waste water can become a complicated and costly business that might outbalance the benefits of engery production.

In this sense and with regard to construction costs composting and digester options should be well evaluated prior to implementation.

Best regards,
GIZ Uganda
Enhanced Water Security and Sanitation (ENWASS)
Sanitation for Millions
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  • Richard
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Re: Faecal sludge management - the role of private and public sector?

(note by moderator: I have moved Richard's topic to here as it fits better into this existing discussion; it was in the thread on role of private sector in FSM)

I agree, fecal and waste management in general is very important. We all know what fecal matter is, but waste consists of many different types. Waste in this context is food waste from the kitchen and any vegetable matter, non-woody, that can be incorporated into producing methane. I have always been a huge advocate of methane using a digester for a few big reasons: 1) it deals with the waste (poop & vegetable matter), 2) it turns waste into an energy source - methane, 3) what comes out is a high quality fertilizer that can be diluted and put right into the garden or field to within two weeks of harvest. Pee should be diluted 1 to 3 water every day and used right on the garden or field. It is an excellent fertilizer high in nitrogen.
A methane digester is one of the simplest types of technologies that can remove the need to find wood for cooking and lead to rebuilding an ecosystem, trees and all. A single home or village can become a of hub of life (no pun intended).
So, please consider very strongly installing a methane digester for each home or at the village level, it would be to the benefit of everyone, all life.
For people who think it would cost too much, think again. A simple home sized two drum type would cost between $60 and $180 US all depending on its size. Size depends on the size of the home and amount of waste. A digester for a village is dependent on the number of homes, animals and vegetable matter. A typical one that they have installed in India costs around $1,000 US for around 10 - 12 homes, cows and vegetable waste all slurried together to produce enough gas for each family to cook three meals and still have enough gas at night to run a generator to turn several lights on in each home and street for about 3 to 4 hours.
This type of investment can save each family from having members go blind from the smoke or have breathing trouble. They also will not have to spend hours each day collecting wood. The time not spent collecting wood can be better used growing more food for the family and even have extra to sell. The gas is a much cleaner burning so people don't have health problems. All very good reasons to have a methane digester.
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