Mechanical disintegration to reduce final sludge production

J. A. Müller

Institute of Sanitary and Environmental Engineering and Institute of Mechanical Process Engineering, Post Box 3329, Technical University of Braunschweig, D-38023 Braunschweig, Germany, Tel. +49/531/391-7943, Fax +49/531/391-7947, E-mail: jo.mueller@tu-bs.de

 

Sewage sludge disintegration can be defined as the destruction of sludge by external forces. These forces can be of physical, chemical or biological nature. For mechanical disintegration various methods are used like stirred ball mills, high-pressure homogenizers, ultrasonic disintegrators, lysate-centrifugal technique, high pulse power technique and others.

A result of the disintegration process are numerous changes of sludge properties, which can be grouped in three main categories:

- Destruction of floc structures and disruption of cells

- Release of soluble substances and fine particles

- Biochemical processes

Because of these changes in sludge properties the sludge production can be reduced according to the following effects:

- Improvement of aerobic or anaerobic stabilisation processes

- Reuse of disintegrated sludge as external carbon-sources for the denitrification process

- Higher solids content of the dewatered sludge

The most important application today is the improvement of digestion. Full scale investigations have been finished recently comparing a stirred ball mill, a lysate centrifuge, an ultrasonic disintegrator and a chemical disintegration by partial oxidation with ozone. The thickened surplus sludge was disintegrated and in combination with primary sludge digested. In a second, parallel operating digester, the untreated raw sludge was stabilized. The disintegration process carried out with ozone showed the highest specific energy demand Fifty percent of the energy demand is used up for the provision of pure oxygen as an input element. The achieved degree of disintegration is the highest in this process as well and an increase of 20% in the degree of degradation was observed. The lysate centrifuge only showed little increase of the degree of degradation (8%), but therefore has a lower energy demand. The ultrasonic disintegrator shows similar results to the lysate centrifuge concerning the degradation (10%) and energy demand. The pollution of process water after dewatering of the digested sludge is increased due to the disintegration. Especially the ozone treatment leads to a remarkable increase of more than 50% for COD, while with the other methods an increase between 3 and 18% was measured. The increase in ammonia varied between 5 and 17% and the enhancement of the phosphorus content was lower then five percent in just about all cases. The increase in the demand of flocculant is less than 10% except for the ozone treatment. The dry substance content after dewatering in a pilot-scale chamber filter press showed little influence due to the disintegration.

Based on these results the cost effectiveness has been assessed taking into account different conditions (size of WWTP, cost for disposal etc.). The calculations have shown that disintegration technology, as an instrument of digestion improvement is only efficient, if the total sludge disposal costs are high. Because the investment and energy costs are the largest sums within the cost calculation, it seems to be beneficial to increase the efficiency of disintegration units. This would not only reduce energy costs, but lower investment costs as well, because more compact disintegration units would be used. The specific conditions of each treatment plant may provide further arguments for or against an application of disintegration. Especially operational problems are important factors, e.g. difficult digestion properties of the sludge, poor dewatering characteristics, overload of digesters, foaming problems due to filamentous micro-organisms in excess sludge and other more.

Source: J. A. Mueller (2003). Mechanical desintegration to reduce final sludge production, IWA Leading Edge Conference - Drinking Water & Wastewater - Treatment Technologies, Nordwijk/ Amsterdam, 26-28 May 2003, 100