The secret is out. There is a recipe for successfully treating waste anaerobically. Anaerobic digestion is one of the free gifts of nature. Micro-organisms break down waste products into innocuous sludge while liberating various gases.

Anaerobic digestion takes place in tanks in the absence of air (oxygen). Particular species of micro-organisms that can live in such an environment are needed to accomplish this process. Anaerobic decomposition of waste regardless of the source is a fermentative process rather than an oxidative process. Fermentation takes place in the absence of free oxygen molecules while oxidation occurs in the presence of oxygen-using micro-organisms.

An anaerobic digester may be a tank with or without a roof or a lagoon. The choice is usually determined by available land and cost of construction and operation. As usual, a lagoon is cheaper to construct and to operate than a large concrete or steel tank. However, a lagoon may be more difficult to trouble shoot because there is not a single discharge point for gases that can be collected and analyzed. Do not be dismayed, however, because an anaerobic lagoon can also be analyzed to determine if it is operating properly.

Anaerobic digesters are ideal for treating high strength waste because digestion can occur in a much smaller area and volume than treating the same waste aerobically. It is particularly suited for high strength waste such as that generated in food processing plants. It can also be used to further process sludge generated by a domestic treatment plant. Obviously, there is a huge savings in power cost as well since aeration is not required. Furthermore, some return on investment can be realized if methane gas is captured and used for heating the digester tank or for other purposes. Finally, even though the process is much slower than aerobically treated waste, the conversion of organic material is more complete and results in less sludge.

Waste products either untreated high-strength or sludge resulting from aerobic treatment of waste is fed into a tank or lagoon where the fermentation process begins. There are four distinct steps in the fermentation process as described below:

• STEP 1 - The conversion of complex organic matter being converted into lower molecular
  weight soluble compounds is called HYDROLYSIS. Proteins are converted into amino acids, carbohydrates are transformed into soluble sugars, and lipids are converted into long-chain fatty acids and glycerin. Under low temperature conditions, this step alone can be the limiting factor for the entire anaerobic process.
• STEP 2 -- A diverse group of fermentative bacteria further break down the products of hydrolysis through ACIDOGENESIS into simple organic compounds. These compounds include volatile fatty acids, alcohols, and lactic acid (this is the acid that makes our muscles hurt when we over exercise), carbon dioxide, hydrogen, ammonia, and hydrogen sulphide gas.
• STEP 3 - At the completion of Step 3, ACETOGENESIS, approximately 70% of the influent COD has been converted to organic acetate, propionate, carbon dioxide, and hydrogen. The formation of acetate, carbon dioxide, and hydrogen is very important because they form the basis for the final step.
• STEP 4 - The final step, METHANOGENESIS, is the conversion of organic acids to methane gas. There are three pathways in which this may occur. One of them is the use of the carbon dioxide and hydrogen that was formed in Step 3. A second pathway occurs in the conversion of acetate that was also formed in Step 3. Both these pathways are used by micro-organisms in the conversion of substrates to methane gas in anaerobic digestion of sludge or high strength waste. The actual conversion process is carried out by a group of micro-organisms that sometime display the characteristics of bacteria and eukaryotes, but are neither. These "bugs" are strict anaerobes and function best under harsh conditions. They, too, have their limitations, however.

Finally, there are some additional comments that should be made. One of the most important considerations is buffering of the system to maintain alkalinity. If the formation of organic acids formed in Step 3 gets ahead of the formation of methane gas in Step 4, Step 4 will never occur. pH of the anaerobic digester will fall and the methane-formers will no longer function and a higher than normal organic loading will be dumped on the next unit process. Therefore, the system must be buffered to maintain a pH of 6.8 or better. Soda bicarbonate is recommended because it replaces the natural carbonate alkalinity, is safe to handle, and is completely soluble. The popular use of lime has its limitations in that it is only slightly soluble in water, will not raise the pH above 6.8, and creates a sludge disposal issue.

Monitoring a lagoon system daily or at least twice weekly will provide the information needed to watch for trends and to determine when an upset may occur or has occurred. Parameters to check include daily sludge feed rate, gas production and carbon dioxide content. It should also include influent and effluent total solids, volatile solids, temperature, pH, volatile acids, and alkalinity. These parameters can then be compared to normal operating ranges to determine what adjustments must be made to correct the digester problems. Like all such operating systems, the only good data is historical data, so now is the time to start developing a data bank regarding the performance of all unit processes. So when an expert has to be called in, data will be available for him to analyze and the curing process will be shortened.

Operating an anaerobic digester should not be a mystery. Understanding the information contained in this Technical Note will get an operator on the right track for successful treatment. If you have any further questions about this article or would like help in improving the operation of your anaerobic system please contact the author Dr. Donald G. Adams at (318)243-1022.