High concentrations of active micro-organisms can be used to treat industry waste. There are two ways to achieve high concentrations of micro-organisms in the process. Using a stirred reactor assists micro-organisms to maintain close contact with the waste to improve efficiency of hydrolytic activity. A settler is used in the reactor to recycle the micro-organisms into the reactor. At the methanogenic reactor phase, immobilization is used to achieve a high density of organisms [98]. During this process, temperature plays an important role [99]. Bacteria (Bacillus subtilis) can effectively remove soluble organic matter and total ammonia nitrogen from wastewater [100]. Methanosarcina spp. is the main aceticlastic methanogen in unstable co-digesters with high levels of acetate, while Methanosaeta concilii is the main organism in stable digestion systems. Syntrophobacter wolinii growth can be improved during stabilization of a co-digester with a well-developed population of Methanobacteriaceae, as sufficient levels of these methanogens increases the syntrophic oxidation of propionate [101]