Digesters by definition are either in-ground, or above ground lagoons that are primary process vessels for waste generated by humans, animals, or fowl. The general expectation by owners, is that biogas will be the constant effluent from these digesters. The layman may assume that magically, bacteria converts the organic material to biogas, without much sophisticated technology required. Nothing could be further from the truth.
Most animal waste digesters need to operate at the mesophilic bacteria ideal range, which is above 100 F to 110F. This not only requires commitment to OPEX in the form of energy addition, but also implies that lower temperatures will yield lower biogas outputs as well. This is true, asymptotically in reverse.
In addition to primary limitations to biogas production, there are the other secondary players, such as ammonia that is a death warrant to the biogas producing bacteria, thus chicken waste is very difficult to convert with ordinary digester technology. Ammonia needs to be removed prior to introduction to the digester.
Produced biogas almost always contains levels of hydrogen sulfide, and without preliminary treatment in the digester, will have to be removed post production, because pipeline specifications limit the levels of hydrogen sulfide. These maximum levels are typically under .25 grains/100 SCFM. If the gas is not going to a pipeline, then other limits on the gas quality will be defined by the end user.
One method of control of hydrogen sulfide is to inject air into the digester. Also pure oxygen is a viable solution. If we are going to RNG with the biogas, the air injection is very problematic, due to the nitrogen introduced to the biogas. Only the oxygen in air is used for the reduction of the hydrogen sulfide. The nitrogen must be removed prior to the injection site. Next, digesters are prone to upset. This condition requires re-inoculation, or even more dramatic remedies.
So, now that we have entertained all the negatives, where do we go from here? It is not an exaggeration that RNG from any organic waste requires careful design, and real time controls. Without these, the question is when the failure occurs, not if.
Expert chemistry and process consultants are the necessary partners required for the most successful outcome of an installation that converts organic waste to RNG.