CHP septic tank

[Deadlywonky]

Hi all,

I've been wondering about the possibility of using bio-waste (human, food and garden waste, i.e. lawn clippings) in an aerobic digester to generate methane to run a CHP unit. Does anyone have any experience of such a system, is it even feasible?

I should stress that this is planning for the future, something i want to implement on my next house if possible.

All advice gratefully received.

Cheers

Tom

 

[dcawkwell]

I saw a video years ago in the late 70s
about an old guy who used pig manure and straw put it into a digester
and generated methane. The compressed this up. He then coverted his morris minor to run on it.
The guy was decades ahead.

Ok if you have the time for it. I believe this sort of thing is more common
out in the far east and such places.

 

[Deadlywonky]

I was hoping to have a semi-automated system, i.e it only needed attention to empty the fertiliser from the tank.

looking on the web suggests that a relevant size CHP gas boiler is available (it states for LPG etc) i was just hoping to have:

A large bio-reactor fed directly from the waste system with the ability to flush waste food and chipped wood/leaves/grass etc in manually,

A pump to draw off the CH4 and storage tank for the gas with pumping to the boiler (obviously it will be professionally installed).

My ideal is to live off grid, if i can

 

[Boilerman2]

I used to look after some commercial gas boilers for Anglian Water Plc that used CH4 sludge gas, the biggest problem was the variance in Calorific value (the amount of heat released by a given volume of gas)
this used to cause some operating problems the blown gas burners would become unstable, the flue gas emissions would be variable and the products of combustion contained a large amount of white scale that was very difficult to clean off the boiler flue ways ! the boilers were Tasso with a rating of 500Kw each and were fired by Dunphy blown gas burners model TG4 - the atmoshere in the digester building also had to be constantly monitored for methane build-up which could cause an explosion

all in all quite an undertaking for a small scale domestic project!
but lots of luck !

 

[Hugh Jaleak]

There are a number of commercial AD plants on stream around the country, Anglian Water have several, producing gas from sludge, Biffa and Biogen are 2 of a number that use foodwaste. AD is a complicated process, and without a great deal of research I would be very cautious about attempting a small scale project.

Your first stumbling block may be that AD produces 'Biogas', a mixture of MH4, CO2, and traces of other gases. CO2 wont burn, so the gas has to be 'cleaned' in order to provide a gas supply the CHP can burn!

I think it is also a process that requires a lot of monitoring. The digester(s) need to be 'fed' little and often, get it wrong and it simply wont work. Then you're faced with a vessel full of stinking waste, no gas, and all you can do is start again. Problem is, what to do with the contents of the vessel that are now of absolutely no use.....

 

[Onetap]

Not enough bio-mass to run an engine with a worthwhile output, I'd suspect. The only working schemes that I know of are sewage works and landfill. The control system to get a tiny engine running reliably would be much the same as that required for a battery of huge engines.

http://scrapbook.channel4.com/bookmarkBar/506ea40ce4b09b0852863736

Anaerobic digestion doesn't seem to be a process that lends itself to a small scale applications. ISTR it was featured in Kevin McCloud's Man Made Home programme but only for gas production(episode 3, still available on 4OD).

 

[75pete]

The digester will in it's self need a heat in put. A temp. of about 34c for best gas production. The gas will change as and when, not only with the digester temp. but the type of waste added. With each change in gas, the chp engine will need to be able to operate across a range of gas quality. 64% methane is a good starting point. Feeding as already said, small feeds over each 24 hr. the digested waste will also need to be remove as per. Over feeding and under feeding will kill the process. This is not an easy process and very dangerous.
Pete

 

[Onetap]

ISTR it was featured in Kevin McCloud's Man Made Home programme but only for gas production(episode 3, still available on 4OD).

I watched that; I thought it was quite suspect as a practical process, the digester tank wasn't insulated and there was no means of warming the slurry. The large tank (several cubic metres, I'd think) produced a small volume of gas, adequate for camping but not enough to keep an engine running.

 

[Hugh Jaleak]

Biogen have a plant in this area running purely on foodwaste, another nearby also accepts pig slurry from the company's farm nearby, there is also Biffa and Agrivert who run large AD plants processing foodwaste at Cannock and Oxford respectively, and a number of other operators nationwide. Water companies are using sludge to run similar setups.

One point of interest I have noted, is all the large scale plants have a large spherical gas holder, this (I assume) is to not only only the gas, but to ensure any peaks and troughs in the gas production are ironed out and a smooth supply is available for the engines. (A flare is also provided to burn off any excess gas.)

Not only does the slurry need to be heated for optimum digestion, but the residue has to be pasteurised to remove any remaining pathogens prior to it being stored for spreading on the land. Also, I am pretty sure, as food is involved, DEFRA have very strict criteria that need to be met when running this type of operation.

It does seem an awful lot of effort, and may take many years to repay the initial outlay for a small home based AD setup.