Prediction of full-scale dewatering results of sewage sludges by the physical water distribution.

The dewaterability of sewage sludge can be described by the total solids concentration of the sludge cake and the polymer-demand for conditioning. The total solids concentration of the sludge cake depends on the physical water distribution. The various types of water in sewage sludge are mainly distinguished by the type and the intensity of their physical bonding to the solids. In a sewage sludge suspension four different types of water can be distinguished. These are the free water, which is not bound to the particles, the interstitial water, which is bound by capillary forces between the sludge flocs, the surface water, which is bound by adhesive forces and intracellular water. Only the share of free water can be separated during mechanical dewatering. It can be shown, that by thermo-gravimeteric measurement of the free water content, an exact prediction of full-scale dewatering results is possible. By separation of all free water during centrifugation the maximum dewatering result is reached. Polymer conditioning increases the velocity of the sludge water release, but the free water content is not influenced by this process. Furthermore it is not possible, to replace the measuring of the water distribution by other individual parameters such as ignition loss.

Comparative investigations on COD-removal in sequencing batch reactors and continuous flow plants.

Investigations on enhanced COD removal from municipal wastewater were performed over a period of 2.5 years, comparing three different types of reactor. The main idea was to determine the influence of the mixing characteristics of the reactor on the treatment processes and the effluent quality. Therefore three pilot plants (a completely mixed reactor, a cascade of three reactors and a SBR) were operated under equal conditions (wastewater, hydraulic load, temperature, sludge age) in parallel to each other. Investigations were carried out at different sludge ages. It could be shown that within one sludge age the CODf removal efficiency increased, when mixing characteristics came closer to the plug flow and it also increased with higher temperatures. A significant correlation was observed between the COD removal efficiency and the sludge load. The higher the sludge load was the greater the CODf concentration in the effluent. Especially the SBR reactor showed an excellent performance under the given operating conditions. Dynamic simulation calculations were carried out, to investigate whether the influence of the type of reactor on the COD-elimination could be described theoretically by combining growth kinetics and the mixing characteristics of the individual reactors. The results showed that performance was better when mixing characteristics came closer to plug flow.

Influence of the free water content on the dewaterability of sewage sludges.

Dewaterabilty of sewage depends on the physical water distribution. The various types of water in sewage sludge are mainly distinguished by type and intensity of their physical bonding to the solids. In a sewage sludge suspension different types of water can be distinguished. These are free water, which is not bound to the particles, interstitial water, which is bound by capillary forces between the sludge flocs, surface water, which is bound by adhesive forces and intracellular water. Only free water can be separated during mechanical dewatering. It can be shown, that thermo-gravimteric measurement of the free water content leads to an exact prediction of full-scale dewatering results. Maximum dewatering results are reached by separating all free water during centrifugation. Furthermore on the basis of the drying curve an estimation of water binding energies can be achieved. The binding energy for free water is less than 0,28 kJ/kg water. The binding energy for bound water (sum of surface and intracellular water) is higher than 5 kJ/kg water.

Effects of disintegration on anaerobic degradation of sewage excess sludge in downflow stationary fixed film digesters.

The effects of mechanical disintegration on anaerobic digestibility of sewage excess sludge in downflow stationary fixed film (DSFF) digesters were investigated on laboratory scale. Mechanical pretreatment using a high pressure homogenizer led to significantly enhanced concentrations of soluble proteins and carbohydrates in the feed sludge. Using DSFF digesters with two different tubular plastic media as support material it was shown that a stable digestion process could be achieved at hydraulic retention times (HRT) down to 5 days. Compared to conventional digesters at 10 d and 15 d HRT respectively, the degradation of volatile solids was enhanced up to 25%, also resulting in a higher specific biogas production. Further investigations on degradation of soluble proteins and carbohydrates showed that a slowly degradable fraction of carbohydrates was released via disintegration. Using the distribution of chain length and the concentrations of volatile fatty acids as process parameters, the dependability on the HRT and the degree of disintegration (the release of soluble COD) predominated the effects of specific surface area of the support media.