Anaerobic digestion of organic matter to methane is a widespread process in natural environments. Methane production is a syntrophic process depending upon the action of several types of microorganisms, for example anaerobic bacteria.
It is currently accepted that there are four steps in the process, as shown in FIG. 1.
The initial stage of hydrolysis is performed by a variety of organisms, chiefly Clostridia. The majority of the intermediary products of the second stage, acidogenesis, are short chain fatty acids, hydrogen and carbon dioxide. The third stage (acetification) is the result of metabolism of fatty acids by H.sub.2 -producing acetogenic bacteria. These organisms are unable to grow at partial pressures of hydrogen &gt;10.sup.-3 atm. Thus, their maintenance within the methanogenic consortium depends on the continual removal of hydrogen by methanogens. This is known as interspecies hydrogen transfer.
About 70% of the methane generated by the anaerobic digestion of organic matter is produced from acetate by the acetoclastic methanogens. The remainder is derived from H.sub.2 and CO.sub.2 by the action of hydrogenotrophic methanogens. The acetoclastic methanogens are very slow growing and, thus, a high retention time is necessary for maximum methane production.
The pH for optimum growth of each member of the anaerobic consortia differs widely. In the acidogenic phase, the optimal pH is 5, and the optimal pH is 7 for the methanogenic phase.
The digestion of organic matter is now described.
Early digesters, see FIG. 2, resemble septic tanks in design. Most traditional digesters, FIG. 2a, were constructed in the form of up-right cylinder tanks into which the feedstock was pumped and from which the effluent was removed. Gas was vented from the top of the cylinder. In this type of digester very little mixing occurs, and a distinct sludge layer settles at the bottom of the digester. A thick layer of scum forms on top of the liquid fraction, which restricts the working volume of the digester.
The introduction of stirred digesters, FIG. 2b, resolved the problem of sludge settling and scum formation and, in addition, produced a relative increase in methanogenic activity. The increased metabolic turnover is due to better mixing of the methanogenic flocs with the substrate. Most digesters used for treatment of human or animal wastes are now of this basic design.
The use of anaerobic digestion as a treatment for low-strength, high volume wastes such as those from food processing plants causes the problem of washout of the methanogenic consortia from the digester, because of the high hydraulic loading rates necessary when the substrate is very dilute. One remedy for this problem is the immobilisation of the methanogenic flora on solid particles in the fixed film digester, shown in FIG. 2d. In this type of digester, the influent is pumped through a column of granules on which the methanogenic consortia develop. It was soon found that the settling properties of the methanogenic flocs were such that the solid particles in fixed film digesters were in many cases superfluous. A recent development is the up flow anaerobic sludge blank (UASB) digester, shown in FIG. 2c. In this design, the sludge layer is allowed to settle and the influent is pumped up through the sludge from the bottom of the digester. Mixing occurs as the gas is produced and rises through the sludge.
In all the above digesters, the entire process takes place within one chamber or stage, as a batch process. There are certain disadvantages associated with conventional single stage digester designs. One-stage digesters are susceptible to the effects of substrate overloading. When this occurs the digester pH falls as the volatile fatty acids accumulate in excess, and both acetogenesis and methanogenesis are inhibited. This imbalance can be corrected by stopping the flow of substrate in the digester until the flora has equilibrated and methanogenesis recommences. Inhibition due to overloading is costly in terms of both labour and lost methane production.
One approach which has been adopted to overcome this and other problems is the construction of two-stage digesters. In digesters of this design, hydrolysis and acidification are separated from acetification and methanogenesis. The two stage digester has the following potential advantages:
(i) it allows better operation of both phases of digestion; PA0 (ii) it reduces inhibition of methanogenesis due to substrate overloading; and PA0 (iii) it is less susceptible to changes in feedstock composition.
However, a two-stage batch process remains large in volume and is more complex (and therefore more costly) both to establish and control.
The above digesters are capable of digesting small suspended particles of organic matter. However, the treatment of a high solids contents material is inefficient due to the inadequate solids mixing mechanisms. Mass transfer at high solids loadings is inefficient, requiring a substantial energy inventory. For example, various trials have been conducted to assess the feasibility of digesting sorted municipal waste. Efficient mixing and mass transfer of the high solids has not been achieved in conventional single and two-stage digesters, see FIG. 3, and consequently methanogenesis is inhibited.