**4.2 Enhancement: Hydrogenotrophic methanogens pathway**

The other bioaugmentation pathway is to enhance hydrogenotrophic methanogens which are affected by ammonia inhibition, where the syntrophic acetate oxidation coupled with hydrogenotrophic methanogens are influenced by ammonia inhibition [71]. The bioaugmentation via syntrophic oxidation bacteria that operate in mesophilic and thermophilic conditions such as *Clostridium ultunense* [72]*, Syntrophaceticus* [73]*, Tepidanaerobacter acetatoxydans* [74]; and *Thermacetogenium phaeum* and *Thermotoga lettingae* [75–77], respectively. The syntrophic oxidation bacteria were syntrophic cooperation with hydrogenotrophic methanogens, which could use its ability of acetate digestion into H2 and CO2 and surmount the energy barriers [78]. Those systems are based on interspecies hydrogen transfer by reducing the hydrogen partial pressure which purposed H2 and formate transfer [79]. In this case, formate plays an important role as an electron carrier at the time when the hydrogenotrophic methanogen and oxidizing bacteria have a distance gap through the Wood-Ljungdahl pathway [78, 80]. The bioaugmentation strategy was to present the syntrophic oxidation bacteria and hydrogenotrophic methanogens that could resist high ammonia levels. Tian et al. [72] was used a syntrophic acetate oxidizing bacteria i.e., *C. ultunense* that resists high ammonia levels with about 7 g NH4 + -NL − 1 and significantly increased with high activity in line with *Methanoculleus sp* as a hydrogenotrophic methanogen. Fotidis et al. [81] were used a combination to hire a bioaugmentation agent for syntrophic acetate oxidation association with hydrogenotrophic methanogen i.e., *C. ultunense* and *Methanoculleus bourgensis* respectively. That study was operated in mesophilic condition resulted in the increase of growth rate and incubation period of syntrophic acetate oxidation agent with 42 and 33%, respectively. Another bioaugmentation strategy to alleviate the ammonia inhibition under thermophilic conditions using *Methanoculleus thermophilu* resulted in 45 to 52% VFAs decreasing and 11 to 13% methane production improvement. This condition was described as the condition that the addition *M. thermophilu* could handle the ammonia inhibition which was proven by high activity and positive growth *of T. phaeum* is a syntrophic acetate oxidizing bacterium that stimulated by those additions [82]. The other report that examined the *M. bourgensis* as hydrogenotrophic methanogen bioaugmentation alone, added in CSTR with ammonia concentration at 5 g NH4 + -NL − 1 which enhanced 31.3% methane production [83].
