Converting an old septic tank into a biogas plant, some questions

Greetings fellow biogas enthusiasts! I just purchased some land for my future homestead that has an older septic tank in place, but no permit to be found. Since it is in a rural area, this is a fairly common situation. It was originally installed for a small mobile home, so I am guessing the size is 750-1,000gal. I am assuming it is concrete, but I suppose it could also be plastic or fiberglass. But supposing I am right, how viable would it be to convert a small, concrete septic tank into a biogas plant? I am thinking I could have it pumped out, then excavate around it, bore a hole on one side to install an inlet tube, construct a ground level mixing pit, attach an outlet tank on the other side, reseal it to be as gas-tight as possible and then bolt on a rubber top, ala Warren Weisman's design ( http://www.hestiahomebiogas.com/workshops/ ). Is this feasible? Better than building a digester from scratch? I also want to utilize a pedal-powered biomass shredder to prepare materials for input. The main inputs would be: human excreta (directly added from one flush toilet, using original septic pipe), old cow and sheep bedding (daily on pasture, so only dung left overnight), pig manure, kitchen scraps, garden weeds, fall leaves, winter hay.

A few other random questions for any biogas experts-- Does it have to be fed every day? Would there be significant performance losses if instead of adding 10lbs per day we add 30lbs every three days? 70lbs once a week? Also, if we did not use the gas quickly enough, what is the max pressure threshold before something bad would happen? That's all for now..

I am in a similar situation to you, and I also have propane tank. I am currently heating my house with propane, but I would like to utilize the methane which I am already generating. Here is a comparison of a septic tank and a digestor.

I believe that with a few modifications a septic tank could be turned into a continuous loading bio-digestor. I am a believer in grey water, so I will route that out of the tank, but the black water could be routed through some kind of a one way valve and into the tank. Then a piece on the top could easily be added. Even through the access hole. another one way valve could be used to allow methane to exit and be guided into a bag for storage. The bag could be used as is (with a weight on top) or in my case, I would like to run the gas through a compressor and into my propane tank, which is already fully hooked up to the house. I have been considering the possibility of using an old mechanical compressor from a car AC system and run with an electric motor. This compressor could be turned on when the bag is full, and the methane would be injected into the metal tank.

I do not think that the waste must be mixed before it enters the digestor, but this process does aid in breakdown by the microbes... but just about every biogas system uses the mixer, so it would be easy to route the black water into a mixer which would then flow into the digestor. I would prefer that the mixer is automatic, but it could also be manual. An automatic system could be as simple as a 50 gallon tank which has a float switch on top. When activated a motor moves turbines and blends the waste. then it would be flused into the system. By the way if this is what you are trying to achieve, then obviously antimicrobial soap, and many other toxic chemicals cannot be allowed to enter the digestor, as this will kill the dragon.

As a septic tank is so similar to a continuous anaerobic digestor then there are many different variations on continuous digestors as from wikipedia "In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds and internal circulation reactors."