![]() Primary sludge from municipal wastewater treatment plants with substantial industrial wastewater contributions from: Machine and metallurgical plants Synthetic rubber plants Textile plants Various industrial plantsĥ0-300 200-400 300-700 300-1,000 400-8,000 400-2,000 Specific resistance varies depending on the type of sludge and its characteristics the values are shown in Table 2.6 for various types of sludge. The dewaterability of sludge can be evaluated by measuring its specific resistance, which is determined in a Buchner funnel test by measuring the volume of filtrate collected from sludge and the time it takes to filter. Oils, fats, and nitrogen compounds intensify the anaerobic sludge digestion but interfere with the conditioning and dewatering processes. Compounds of iron, aluminum, chromium, and copper, as well as acids and alkalis, improve the process of precipitation and dewatering and reduce the consumption of chemical reagents for conditioning of sludge before dewatering. The chemical composition of sludge also exerts a significance influence on its treatment and dewaterability. Therefore, any sludge treatment process that reduces the size of suspended solids particles has a negative effect on the conditioning and dewa-tering of sludge. ![]() The separation of water from sludge depends on the size of solid particles the smaller the particles, the poorer the water separation from sludge. The greater the bound water present in sludge, the more the energy or reagents it takes to condition the sludge for removal of the bound water. Water may be present in sludge as free water or bound to the particles physically or chemically. In the process of thickened activated sludge digestion, 15 MJ of heat is produced per kilogram of volatile suspended solids.ĭewatering is the process of natural or mechanical removal of water from sludge. Sludge burns at a temperature of 430 to 500☌ (800 to 930☏) however, to eliminate odors, the temperature needs to be raised to 800 to 850☌ (1470 to 1560☏). The heat value of combustion of sludge dry solids equals 16.7 TABLE 2.5 Thermophysical Characteristics of Sludge The specific heat of a mixture of primary and thickened activated sludge is 3.5 to 4.7 x 103/kg-K. ![]() Thermophysical characteristics of sludge are shown in Table 2.5. They are also thixotropic, which means that they become less viscous when mixed. Primary sludge at solids concentrations above 5% and activated sludge at solids concentrations above 3% are non-Newtonian, which means that head losses in piping are not proportional to the velocity and viscosity. The density of dry sludge solids is 1.2 to 1.4 g/ cm3. The density of primary sludge is 1.0 to 1.03 g/cm3, and the density of activated sludge is about 1.0 g/cm3. The organic part of the sludge decays more rapidly, with an increase in the quantity of finely dispersed and colloidal particles and bound water resulting in a decrease in the separation of water from the sludge and poor dewaterability. In activated sludge, the approximate distribution is: 90% below 0.2 mm, 8% between 0.2 and 1 mm, 1.6% between 1 and 3 mm, and 0.4% over 3 mm. Raw primary sludge particle size distribution is: greater than 7 mm (5 to 20%), 1 to 7 mm (9 to 33%), and smaller than 1 mm (50 to 88%), of which about 45% is less than 0.2 mm.
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