**5.2 Anaerobic plug-flow reactors (APFRs)**

Anaerobic plug-flow reactors (APFRs) are generally long rectangular channels, with the flow entering one end and leaving at the distant end. There is roughly seldom mixing in the flow direction. The channels, or tanks, are mostly placed above ground. Both thermophilic and mesophilic operations are utilized [50, 127]. APFRs are considered one of high rate digester and commercially used for treating different types of organic wastes involving slurries of animal manure, distillery wastewater, and the organic fraction of municipal solid waste [128, 129]. Compared to a single-stage CSTR, plug-flow reactors are mostly more efficient in converting the substrate to biogas and are more stable to operate [50, 130].

The two generally utilized reactor kinds are: continuous stirred-tank reactors (CSTR, using biogas recirculation for mixing or mechanical agitation or effluent), and plug-flow reactors (PFR, where the reactor content is shoved along a horizontal reactor). In dry digestion processes, PFRs are usually utilized to treat substrates with high solid content [131], whereas CSTRs are applied in wet digestion systems. The choice of wet or dry digestion technology relies on the total solid content (TS) of the material treated [50]. Recovery of biogas from manure is widely applied with CSTR and PFR systems in developed countries, likewise covered lagoons, and other kinds of anaerobic reactors are also used [74].

## **5.3 Anaerobic contact reactor (ACR)**

Anaerobic contact reactor (ACR) is consistently a fully mixed mechanically stirred tank with recycle of sludge. The effluent from the tank flows into some kind of a solid-liquid separator (e.g., gravity sedimentation tank, sludge flotation device, lamella clarifier) and the recovered solids are returned to the anaerobic digester. ACRs are efficient of treating high-strength waste with a high concentration of digestible solids due to high concentration of active microbial biomass [132–134]. Hydraulic retention times are short and fluctuations in organic loading are well tolerated. The ACRs are relatively less affected to souring and other inhibitors [128, 135, 136]. Stirred digesters coupled to some type of membrane-based cell retention have proved highly effective in biogas production [50, 137, 138].

## **5.4 Biofilms**

Biofilms are microbial consortia attached to a support material. The support surface is usually inert and may be fixed or suspended. Anaerobic microbial biofilms can effectively digest organic material to produce biogas [139]. A huge mass of immobilized biofilm and mass-transfer upgrading motion of liquid around the film let biofilm reactors to hold high organics loading and bear well any fluctuations in hydraulic or organics loads. Once the biofilm has produced, start-up periods are short compared to the other traditional anaerobic treatment systems [139, 140]. The support material nature affects the improvement of the biofilm and its intensity of attachment, or mechanical steadiness [50, 141].
