Why dewaterabilty of sewage sludge occurs upstream – a model to quantify the effects
Kopp, J., Kläranlagen Beratung Kopp, Germany
(free)
Mechanical dewatering and its optimization of sludge is a key technology to reduce disposal costs. It can be observed that ignition loss of sewage sludge in the last ten years increase continuously and dewatering results decrease even if good degrees of degradation are achieved in anaerobic digesters. This can be explained by change in organic compound in primary sludge (PS) itself and increasing amount of waste activated sludge (WAS). (Fig 1).
To optimize sludge dewatering processes, the dewaterability of sewage sludge needs to be understood. There are two categories of water in a sewage sludge suspension: the free water content with no physical binding to sludge particles (Fig. 2). By mechanical dewatering of sewage sludge only this free water content can be separated. The second category is the bound water, which is the sum of internal water, surface water and chemical bound water. The internal water is bound in the interstices inside flocs and particles physically by capillary forces in pores up to approx. 10 µm. Surface water is bound on the surfaces of particles by physical and chemical adhesive forces. At least chemical bound water is fixed in cells, gels, EPS and by hydration. The structure of flocs mainly determines the amount of bound water and dewatering results. Therefore dewaterability occurs upstream. A model was generated to explain the main influences on dewaterabilty in wastewater treatment plant. Further the effect of new technologies like vacuum degassing, thermal hydrolysis and phosphorus precipitation as struvite on the water binding will be quantified.
To predict dewaterability parameter DS(A)KBKopp is used. Parameter DS(A)KBKopp predicts the DS in cake after separation of the free water fraction and marks the limit of mechanical dewatering. The parameter is determined by thermogravimetric measurements and the accuracy is about ±0,5%DS (Fig.3). DS(A)KBKopp is used to control guaranty values, accepted by court and described in German technical guideline DWA M-383.
Due from database from 20 years of sludge analysis it was possible to create a model to explain dewaterability and treatment plants especially the changes during the summer and winter seasons. The main influence on dewaterability is the freight fraction of WAS to PS before digestion. The WAS bind approx. three time more water than PS. The water in WAS is fixed in the particles in proteins and extra-cellular polysaccharides. The second main influence is the influence of phosphate on to water-binding of the WAS itself. Fig. 4 and Fig. 5 show these effects and the correlation between the DS(Mod-A) and the measuring results DS(A)KBKopp. Fig 6 shows the dewaterability and the full scale results of 15 year old centrifuges. Of course the old centrifuges do not achieve the optimum dewatering result, but the model and full scale results fits together and changes can be explained with upstream circumstances like dosage of PAC to reduce filamentous bacteria. Further examples with full scale results will be shown in presentation.
The paper proposes to illustrate these upstream effects on dewaterability and the model is a helpful tool to optimize dewatering process in full scale, by comparing dewatering result and dewaterability with the aim to reduce disposal costs. Further the change of new technologies on sludge treatment and the effect onto dewaterability can be by quantified. Comparison of dewaterability and full scale dewatering results can be realized.
