Very interesting discussion. I have few questions.

1. When adding shredded kitchen waste is it necessary to add water?

2. If partially digested organic solid waste (after keeping few days in some waste bin), could it help to increase c/n ratio or does it increase biogas production without adding extra water?

3. I am in favour of adding kitchen waste, because it will ease the disposal of the kitchen waste in a dumping site. Without secondary treatment of slurry, does it suitable for use as agricultural manure in case of without or with adding kitchen waste?

4. Again whatever the biogas production, it will treat fecal sludge that is the main goal. In addition we are getting gas which is a by-product adding extra benefit.

Thanks,
Sadiq

Dear Patrick,
Factors that can increase biogas production are:

1. Effect of Temperature: Warmer temperatures of digesters favours gas production (see attached paper. Sectt: please run a check). It is known that the biological reactions doubles for every rise of 10 degrees C, so long as the components themselves remains unaltered due to heat energy (McKinney, 1962; Dadue et al., 1966). According to Speece and Kem (1970), the gas production shows a linear response to the increase in temperature. They found 50 per cent increase in gas production, when the digester temperature was raised from 30 to 37 degrees C.

2. Effect of Nutrients: Nitrogen and phosphorus are the essential requirements. Other colleagues have discussed the nutrients, in a way, above.

3. Effect of Loading: Increased organic loading, beyond a stage, will decrease the efficiency of the digester, lowering the gas production, as methane formers cannot utilize the volatile fatty acids, produced by the acid-forming bacteria. At high loadings, the sludge tends to become filamentous, hindering in the gas production.

4. pH: pH should be maintained between 6 to 8. Low pH will hinder gas production


Regards,

F H Mughal

FYI: energypedia.info/wiki/Parameters_and_Pro...imisation_for_Biogas
energypedia.info/wiki/Biogas_Manuals

Except for a limited number of cases (for example government buildings) where some advanced treatment can be made mandatory and funds are generally available, bio-gas systems are often a mean to encourage the construction of some sort of treatment where there would be otherwise none.

In the first case a regular DEWATS system with as little maintenance requirements as possible is probably the better option, while with the latter it should be primarily a bio-gas system with *some* faecal sludge treatment as a positive side effect in my opinion.

Current systems are often rather the opposite, e.g. waste water treatment with bio-gas as a small side effect (as others have explained above), so their maintenance often gets neglected as the perceived benefit is to low too be worth the trouble. And if you are using an alternative fuel for 70% of your needs anyways, the step back to 100% if the system breaks down is small, thus repairs are rarely done.

Maybe it would make sense to look into dry batch reactors (maybe with solar passive heating?) as a supplement to dry toilets. Especially in areas where there is a lot of post harvest organic waste available, a series of such systems which are also fed with the dried feces and some of the urine for optimal C:N ratio could be a very simple option which fulfills the "perceived benefit" requirement.

Dear Christoph,

I think you are right, especially if you look at the system as a waste water treatment unit. I guess we come to a crucial point here in the whole concept of black water treatment with a biogas digester. The gas will always just be a positive side effect, but it seems unlikely that the gas produced can really offset the additional cost of including a biogas digester. I think in many cases people and organizations do not realize this ahead of time and are therefore disappointed with biogas technology.

Thinking out loud a bit here, there is a difference between a system with an ABR/septic tank and one with a biogas digester. (It seems DEWATS systems now look at modified ABR with biogas capture concepts to circumvent this.) Being that in the first system most of the solids stay in the system and the reactors needs to be emptied. In the case of a biogas digster most of the solids come out in the slurry. As I understand it, no biogas digester will produce an effluent that has a carbon loading low enough to release it into surface waters, and thus a second treatment for the effluent is needed. The question is if the secondary treatment system can cope with the extra carbon load resulting from co-feeding organic waste.
It is possible to design systems from the start to work like this, but whether co-feeding is an option for existing plants should be evaluated on a case by case basis. Whether it is feasible or not will, in my view, depend on how the slurry from the biogas plant is treated and on the impact on the retention time of the digester.

Thoughts anyone?

Regards

Marijn

From 1000 daily users of toilets 30 cum of biogas can be produced per day. It can't fulfill the requirement of cooking purpose. In case of centralised cooking like prison- it can serve the purpose by maximum 30-40% of the total requirement. Animal dung or kitchen wastes (shredded with pretreatment) can be added to enhance biogas production. Human waste biogas plant linked with toilet can't fullfil the requirement of biogas for cooking purpose. It is suitable when feeding is mixed- human wastes, animal wastes and kitchen wastes.
PK JHA

Yes, that is a very valid point.

But at least adding shredded kitchen-waste that is also produced on site I would consider ok, as it would otherwise rot somewhere in a pile and leach out organic load to nearby water-bodies also.

The question is basically what happens to the organic material otherwise...

Especially growing maize etc. to add to the bio-gas digester is obviously total nonsense and only done in developed countries due to the high subsidies or because one does not have to pay for the externalities this causes.

Dear Patrick and Krishan,
for me it is a critical point to put MORE organic load to a Wastewater treatment. DEWATS should be in first a Wastewater treatment, not a biogas production. So everything which gives higher organic carbon to the wastewater stream seems contra productive to me. A co-fermentation does only work (from a sanitation stand point) when you have a side stream - digester for sludge - but not in the main stream (biogas dome for wastewater, ABR, UASB).
One might argue that cutting wood is worse than some organics in the effluent of a wastewater treatment, but than... why invest in a treatment.
Just some numbers to clarify my point: If you have in mind a 1000 pe plant, this would be an organic load of about 60 kg/d, if you have an efficiency of 60% in the dome you have an ef-fluent of 24 kg/d. If you tripple the organic carbon load (which you could do from a standpoint of biogas production) in order to get the sufficient gas, than the effluent load would be 60 kg/d * 3 (3 times) * 40 % (rest load) = 72 kg/d….therefore higher as your original influent load. I consider this critical, therefore I would not recommend to put high organic load to a DEWATS.

What do you think?

Yours
Christoph

Dear Patrick,

I think the low gas production for the digester volume is always a problem when using black water for biogas production. Even with low flush toilets the dilution factor is very high. Therefore Julius' suggestion of adding other feedstock is a very sensible one, and probably the only one that is really going to boost gas production in a significant way.

Some things to keep in mind:
1.) There needs to be a way to feed the extra feed stock into the digester, obviously
2.) If it is food waste or other organic waste, it should be shredded first.
3.) Some pre-fermentation of the waste may be needed to keep the process stable (depends much on the inlet design and the types of waste).
4.) You may need to add a significant amount of organic material, which could influence the retention time and thus the slurry coming out of the digester may be less sanitized.

Regards

Marijn

Thanks for the hints. It will give me a point to start.

Patrick Onyango

Some ideas were posted here:
forum.susana.org/forum/categories/35-bio...lic-biogas-digesters

But in general you will have to add additional energy rich substrate to it, optimizing the process only will not give you an additional 50%.

Adding finely shredded organic waste, or waste-water from food-processing industries (for example sugar-cane) will probably do it though.

Dear All,

Most institutions in Kenya like boarding school and prisons use a lot of firewood for cooking purposes which is causing environmental degradation.

We have introduced DEWATS (fixed dome biodigesters and ABRs covering populations between 600-1000 people) on a pilot scale in a few schools and prisons here in Kenya however our major problem is that the biogas produced can only replace about 20-30% of the woodfuel energy. In order to conserve the environment, we need to enhance the gas production to cover up to 70% of the needs of these institutions. How can this be done using DEWATS and wastewater (blackwate and greywater)?

Thanks and best regards,

Patrick.