Archaea obtain a limited amount of energy from methanogenesis and possess the slowest growth rate among the anaerobic digester microorganisms. Maintaining valid environmental and operational parameters for archaea is one of the key factors for effective methane production. The main environmental factors are temperature, pH, alkalinity, and redox potential. Operational parameters, such as C:N:P ratio, the presence of essential micronutrients, organic loading rate (OLR), hydraulic (HRT) and solids retention time (SRT), and incoming salts and toxicants concentration are subject to tight control and regulation. The accumulation of certain intermediates or byproducts, such as VFAs, ammonia, and hydrogen sulfide, can lead to inhibition of methane production [62].

1.1.1 Temperature

Temperature is one of the most important environmental factors for methanogene — sis. The methanogenic archaea can be classified according to the temperature ranges for maximum growth and substrate utilization rates. The optimal temperature for growth of psychrophilic organisms is between 10 and 15°C, mesophilic between 35 and 40°C, and thermophilic between 58 and 68°C [63, 64]. The rate of methane generation by psychrophilic microorganisms is significantly slower compared to mesophiles and thermophiles, and therefore the psychrophilic regime is rarely used for large-scale methane production. The methanogens are the most sensitive organ­isms to temperature variation. A sudden temperature change as small as 2-3°C causes an accumulation of VFAs and a decreasing methane generation rate espe­cially at thermophilic conditions [65]. A significant temperature drop affects the activity of all anaerobic microorganisms and ceases methane production, but the microorganisms are able to recover after temperature stabilization [65-67].