160 Organic waste recycling: technology and management
accumulation of volatile fatty acids. Such an accumulation may occur when the
organic loading rates are excessively high and/or when toxic materials are
present in the digester, all producing inhibitory effects to the methanogenic
bacteria.
Appropriate measures should be taken promptly when there is a lowering of
pH in an anaerobic digester due to accumulation of volatile fatty acids or
increase in H 2 partial pressure, and the rate of CH 4 production decreases. In
general, the feeding of the digester should be stopped to allow the methanogens
to utilize the accumulated volatile fatty acids and H 2 at their pace. When the
optimal gas production rates are re-established, the normal loading of the
digester can be resumed. In addition, the pH of the digester needs to be adjusted
to neutrality by the addition of lime or other basic materials. If the alkalinity of
the digester slurry is maintained within the range of 2,500 to 5,000 mg/L, a good
buffering capacity is normally obtained in the digester.
4.3.3 Nutrient concentration
Most of the information available in this area has been obtained from studies of
rumen bacteria. Energy for growth of most rumen bacteria is obtained through
anaerobic fermentation of carbohydrates. Nitrogen is utilized for cell structure.
To guarantee the normal biogas production, it is important to mix the raw
materials in accordance with a proper C/N ratio. Bacteria use up carbon 25 - 30
times faster than they use nitrogen. Therefore, at this ratio of C/N (25-30/l) the
digester is expected to operate at the optimal level of gas production, similar to
that required for composting (Chapter 3). Importance of other elements such as
P, Na, K and Ca in the gas production is also indicated. However, C/N ratio is
considered the essential factor.
The C/N ratios of various organic wastes are given in Table 3.1. Since human
nightsoil, animal manures and sewage sludge have C/N ratios lower than the
optimum values; they may be mixed with other agricultural residues that have
high C/N ratios. Examples of these residues are wheat straw, rice straw, water
hyacinth and duckweeds, all of which are usually biodegradable, and can be
made more so by physical size reduction (e.g. shredding) or by precomposting.
However, problems can arise with these agricultural residues as they float to the
top and forming a hard layer of scum on the slurry surface inside the digester
(Polprasert et al. 1982)
When combination of organic wastes is considered for anaerobic digestion,
Table 3.1 should be referred to select the proper combination and appropriate
C/N ratio according to the method shown in Example 3.1.