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.

ABSTRACT:Volatile fatty acids are the most important intermediates in anaerobic digestion, and their degradations are extremely complicated thermodynamically. In this research, syntrophic anaerobic digestion of volatile fatty acids using enriched acetogenic and methanogenic cultures in a batch reactor at mesophilic conditions was investigated. Interactive effects of key microbiological and operating variables (propionic, butyric and acetic acids, retention time and methanogen to acetogen populations ratio) on the anaerobic degradation of volatile fatty acids were analyzed. Acetogenic and methanogenic anaerobes in the granular sludge from an up-flow anaerobic sludge blanket reactor were enriched at mesophilic conditions within a period of four weeks, separately. Enriched cultures were mixed with known proportions and then used in the bioreactor. Experiments were carried out based on central composite design and analyzed using response surface methodology. Four parameters (final concentrations of propionic, butyric and acetic acids and biogas production) were directly measured as response. Also, the optimum conditions for volatile fatty acid degradation were found to be 937.5 mg/L, 3275.5 mg/L, 2319.5 mg/L, 45 h and 2.2 proportions for propionic acid, butyric acid, acetic acid, retention time and methanogen to acetogen populations ratio, respectively (corresponding to maximum volatile fatty acid removal efficiencies and biogas production). The results of the verification experiment and the predicted values from the fitted correlations at the optimum conditions were in close agreement at a 95% confidence interval. The present study provides valuable information about the interrelations of quality and process parameters at different values of microbiological and operating variables.

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