Anaerobic treatment process stability

Controlled anaerobic treatment processes are being considered today as one possible means of recovering energy in the form of methane gas while at the same time reducing the pollu tional load of organic-waste slurries. McC? rty x estimated that the resulting methane produc tion from the anaerobic digestion of municipal sludges and refuse, ariimal wastes, and crop residues in the U. S. would yield an equivalent of about 20% of the current natural gas con sumption. Anaerobic treatment of human wastes and utilization of the resultant methane gas have been practiced since the latter part of the nine teenth century. Various anaerobic process configurations have found widespread usage in the treatment of municipal sludges and more limited application in the treatment of organic industrial wastes including fruitand vegetable processing wastes, packinghouse wastes, and animal manure slurries. Despite its usage since the beginning of the present century, anaerobic treatment is still one of the least understood processes in waste treat ment. Widespread application has been ham pered by a lack of understanding of factors associated with stability of the biological pro cesses involved. The problem has been ac centuated by a perceived lack of reliability in the process. Although recently escalating energy costs have given increased emphasis to anaerobic treatment processes, further develop ment of anaerobic-process technology is de pendent upon a better understanding of process biochemistry and its relationship to process stability than currently exists. The anaerobic fermentation process involves two major groups of bacteria that decompose organic matter in two major phases. In the first stage, numerous species of saprophytic bacteria collectively called "acid formers" hy drolyze and degrade the complex organic mat ter to fatty or volatile acids. The second stage of the process is accomplished by methanogenic bacteria that utilize the volatile acids and pro duce methane gas as one of the byproducts of their metabolism. The two processes occur simultaneously and process stability is depen dent upon maintenance of a delicate biochem ical balance between the fast-growing "acid formers" and the more fastidious "methane formers. " Process instability is usually indi cated by a rapid increase in the concentration of volatile acids with a concurrent decrease in methane gas production. Operational factors that have usually been associated with process failure include insuf ficient acclimation of the methane formers to new substrates, overloading, and rapid temper ature fluctuations. Many of the laboratory and pilot-scale studies reporting failure of the an aerobic process have reported very short ac climation periods of one month or less when switching to new substrates. Kotze et al.2 monitored the enzymatic activity of anaerobic digesters receiving different substrates and con cluded that adaptation of a substrate takes more than five weeks. Rapid increases in the rate of organic loading and rapid process temperature changes have also been implicated as contributing to process imbalance. A number of organic and inorganic materials that may be present in the waste play a sig nificant role in process inhibition and toxicity. These include excessive concentrations of vola tile acids, ammonia, alkaline earth-metal salts, heavy metals, and Sulfides. Inhibition by these materials is usually indicated by a decrease in the steady-state rate of methane gas production and volatile-acids concentration, while toxicity is indicated by a total cessation of methan ogenic activity. Pilot-plant studies at the University of Mani toba on anaerobic digestion of swine manure have indicated that extreme process stability could be achieved despite a digester chemical