Methane gas generated from anaerobic digester that is designed to treat the wastewater sludge coming from primary and secondary treatment plant can be used a fuel source and the volume can be estimated based on a simple calculation that takes into account the percentage of volatile solids reduction. Basically the gas production can vary considerably depending on the sludge loading rate and how well the biological system is controlled to optimize breakdown of organic matter.
The collection of gas involves a risky process as there is a safety concern that must be taken into account when coming up with the design of the digester. Basically, methane is high combustible and air must be prevented from entering the digester at all cost caused by reduction in liquid volume. Similarly as the volume in the anaerobic digester increases and causes displacement, there must be a safety valves to avoid overpressure and loss of methane out from the system. Currently there are two different types of design for the cover of the digester, one is the floating type and another is a fixed cover. Floating covers will keep the reactor air tight but at the same time allows expansion and contraction to take place which caters to the change in volume of the reactor. For fixed cover system, usually it needs another external pressure vessel so that gas can be stored and when there is a volume change taking place; gas will be drawn in instead of air. These are something that needs to be put into consideration during the designing part before the anaerobic digester comes into operation.
Digester mixing is another important area to look into as proper sludge distribution is needed to achieve maximum process capability if let’s says the reactor is a high-rate process type or a two-stage combined system utilizing both conventional standard rate and high-rate together into one. There are different mixing technologies using several types of configuration which are currently in use such as the mechanical stirring system, confined and unconfined gas injection systems and the gas circulation pumping systems. In a mechanical system, a simple top mixer with a rotating impeller is used to create the mixing effect in the digester. Normally a low speed turbine system is put in place and it can be used for both fixed and floating cover anaerobic digester.
For confined gas injection system used for mixing process, methane collected at the roof of the reactor is first compressed and then discharged through an internal submerged gas pipes, thus creating a mixing effect that carries the sludge together with it. In an unconfined gas injection, similarly gas is compressed and instead of using gas pipes, the methane is distributed using bottom diffusers or top mounted lances. Compared between both systems, generally based on experience, the bottom diffuser unconfined method allows better mixing and thus leads to higher gas output due to the faster reaction rate. Finally for the circulation pump system, it will prevent bottom stagnant sludge buildup, thus creates better mixing efficiency. Weighing back each option in terms of ease of operation and maintenance requirements, the proposed configurations have their own advantages and disadvantages.
Related: Using anaerobic digester for biogas production