New processes for the improvement of sludge digestion and sludge dewatering

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Courtesy of Cambi AS

INTRODUCTION
The examination of numerous sewage sludges from municipal wastewater treatment plants leads to the conclusion that on the one hand dewaterability of these sludges worsens and that on the other hand dewaterability of sludges becomes more and more important due to increasing disposal costs. Especially wastewater treatment plants with inflows from the food processing industry often face sludges with problematic dewatering characteristics.

A new treatment procedure, the CAMBI Thermal Hydrolysis Process (THP), will be discussed which can significantly improve the dewatering behaviour of sewage sludges and the gas-production in anaerobic digestion. In the CAMBI Process, sludges are treated thermally, which includes the proteins contained in the sludge. Exemplary results of full-scale tests will be presented in this paper.

PROTEINS IN SEWAGE SLUDGE
As a consequence of the above mentioned sludge problems, the possible influence of proteins on the dewaterability of sludges was investigated. Proteins and polysaccharides belong to the exopolymeric substances (EPS) which can bind extremely large water quantities. Most publications do not differentiate exactly between proteins, polysaccharides and EPS. It can be deducted from various investigations that proteins can bind up to 4-5 grams of water for each gram of protein. EPS serve as a biofilm for the bacteria and strongly influence dewatering results and biodegradability of the sludges. Previous investigations already proved a direct correlation between the anionic surface charge of the particles and the amount of EPS found in sewage sludge. The protein content in waste activated sludges depends on sludge loading and comes to 35-40% of DS (at 0.2 kg COD/kg DS day). It must be taken into consideration that the group of proteins or EPS, is very large and that the proteins differ widely in structure and solubility.

Proteins can be contained in the wastewater inflow or can be produced by bacteria contained in the wastewater. Since proteins and EPS as biofilm are supposed to function as acid protection, they are hardly degradable in the biological process steps. Polysaccharides and proteins have a gel-like structure in their water-soluble state and are therefore a relevant factor in regard to the processes of thickening and dewatering.

During the research work, various proteins were specifically added to digested sludges. A significant decrease in dewaterability could be observed with increasing protein content. Therefore, it can be deduced that it is important for treatment procedures, which aim at improving dewatering characteristics, to alter the physical characteristics (structure and water binding capacity) of the proteins contained in the sludge.

If fats are added as a co-substrate to the digestion process, degradation of fats is preferred over degradation of proteins. While co-digestion leads to a high biogas yield, dewaterability of the digested sludges decreases by 2-3% and polymer demand for dewatering of these sudges increases.

During a joint research project with the Technical University Braunschweig1, various proteins were added specifically to the digested sludges. Figure 1 and Figure 2 show the coorelationcorrelation between protein content and specific resistance to filtration and polymer demand. A distinct decrease in dewaterability can be observed with increasing protein content.

It was found to be of great importance for processes, which aim at an improvement of dewaterability, to alter the physical characteristics (structure and water binding capacity) of the proteins contained in the sludge.

THE CAMBI-PROCESS
Contrary to thermal conditioning, the CAMBI process precedes sludge digestion. It is a thermal disintegration
procedure which substantially hydrolyses the sludge. The CAMBI process has been applied on a large scale for several years in Scandinavia, England, Ireland, Poland and Japan2. It provides for thermal hydrolysis with saturated steam at a temperature of 165 degrees centigrade for 30 minutes without the addition of chemicals.

Upon practical application of the process at German sewage treatment plant3, a variant is offered which does not treat the complete sludge volume, but only the waste activated sludge (WAS).

After disintegration the hydrolysed sludge is cooled to the required digestion temperature. The heat released during the cooling process may be used again for preheating the sludge to be hydrolysed in systems equipped with two or more reactors. The hydrolysed sludge is fed into the digester as usual and exposed to the anaerobic process. Provided the complete sludge is subjected to thermal hydrolysis, the digester can be fed with sludge of up to 12% DS because the sludge viscosity dramatically declines as a result of hydrolysis, thus maintaining its ability to be pumped and stirred/mixed.

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