Anaerobic digestion from the viewpoint of microbiological, chemical, and operational aspects — a review

Anaerobic digestion (AD) is intrinsically a sequential complex chemical and biochemical process, and many factors (microbiological, operational, and chemical) can affect its performance. The great complexity of AD may lead to many serious problems (such as instability, long retention times, low efficiency, and high polluted supernatant) that prevent this technique from being widely used and commercialized. The aim of this paper is to review the present knowledge of the AD process in view of its microbiological, operational, and chemical aspects. Different groups of anaerobic microorganisms with specific growth conditions, physiological properties, and metabolic activities are involved in the AD process. Interactions of anaerobic microorganisms are incredibly complicated, and the effective performance of AD strongly depends on the balance of these relationships. The syntrophic interaction of acetogens and methanogens is the most important relationship in AD because acetogenesis and methanogenesis reactions under thermodynamic standard conditions are endergonic and naturally do not occur. Moreover, operational and chemical factors affect the anaerobes in certain ways. It is believed that there are many ambiguous points in AD that are not yet known. Therefore, by controlling and monitoring each of the microbiological, operational, and chemical parameters, AD performance may be enhanced.