Stuck digesters

Upset An upset digester does not decompose organic matter properly. The digester is characterized by low gas production, high volatile acid/alkalinity relationship, and poor liquid-solids separation. A digester in an upset condition is sometimes called a "sour" or "stuck" digester. 

In sludge digester startup, this condition is known as a stuck digester and can be cured by the slow addition of alkaline buffers to the mix. Strong alkalis can take the mix well out of the sludge range, where all activity stops. 

Figure 5 (17) illustrates that acetate is the least toxic anion of any of those commonly found in wastes. However, at the levels used (2000-4000 mg/liter) which are levels reported in faihng or stuck digesters. 

To prevent shock and permit development of new species at each level, if such were involved, pH levels were automatically assumed at 0.2 pH unit, and after a drop the controlled units were equilibrated for one to two weeks. If the units responded successfully, the next incremental drop was programmed. Results for one year are compressed into Figures 13 and 14. The digestion units were finally operated successfully at pH 4.6 without making any change in the environment, except as noted above. There was no failure, and no stuck digester developed. In the same way these units were brought back to full gas productivity at pH 7.0. 

Frank decided that he would try taking this medicine. Although plagued with multiple side effects that affected mainly his bowels, he lost weight. He knew that if he ate fewer fatty foods, the digestive side effects would lessen. He did this and stuck with the medication. He has lost 15 pounds and would like to lose more. Frank has already seen a change in his blood pressure, and his diabetes medicine dosage is lower. 

The studies of McCarty and co-workers have shown clearly that volatile acids are not toxic to methane bacteria at concentrations that would occur in stuck or sour digesters. On the contrary, evidence has been elucidated which indicates propionate retards the acid formers. Thus, the use of alkaline substances to maintain an adequate buffer capacity in an anaerobic waste treatment unit is a valid procedure. A word of caution is necessary pH control is not a universal palliative. Its only advantage is to prevent a bad situation from getting out of hand. The basic cause of the digester biochemical imbalance must be discovered and rectified. Unless this is done, pH control is worthless in the long run. In addition, care must be exercised in selecting an alkaline material that will not produce a toxic reaction. 

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