Digestion of disintegrated sludges

In order to accelerate the digestion of sewage sludge, to raise the degree of degradation and thus to decrease the amount of sludge to be disposed of mechanical disintegration was investigated (Kunz, 1992; Müller and Schwedes, 1992). By applying mechanical disruption the break-up of cells occurs in minutes instead of days. The intracellular components are set free and are immediately available to biological degradation which leads to an acceleration of the process. Facultative anaerobic micro-organisms are disrupted as well and become degradable, thus resulting in a higher degree of degradation. Similar investigations are carried out by several researchers (Tiehm et al., 1997; Dohanyos et al., 1997; Baier et al., 1997; Choi et al., 1997).

Reliable results based on experiments in a half-technical scale only exist for the disintegration methods SBM, HPH and UH. They show that excess sludge containing a great portion of bacteria is the most suitable for disintegration. Using primary sludge only a small increase in the degradation process can be observed because it consists of easily degradable components. Disintegration of anaerobically degraded sludge leads to a significant increase in gas production. The results obtained in large scale investigations using the HPP- and LC-Techniques have shown that significant improvement of the degradation process can only be obtained at higher energy input.

Fig. 1.  Degree of degradation of organic matter as a function of digestion time

 

Kopp et al. (1997) investigated the pollution of sludge water after an anaerobic digestion of the sludge. While the concentrations of carbon and phosphorous are not influenced by the disintegration, the content of Kjeldahl-nitrogen is increased. The reason for this is the extended degree of degradation leading to a solution of nitrogen from the degraded biomass (Müller et al., 1998).