Tables 2.2 through 2.4 also list the characteristics of raw activated sludge. Activated sludge contains mostly bacterial cells that are viscous and difficult to dewater. The sludge is light gray or dark brown in color. Suspended solids concentration is 0.4 to 1.5%. Factors affecting the concentration of solids have been discussed in Section 2.2.2. Thickening of activated sludge is a very important process because of its low solids concentration and high volume. Mechanical thickening such as gravity belt thickening can increase solids concentration to about 5% and reduce the volume to about one-fifth. For approximate computations, the quantity of the mixture of primary sludge and thickened activated sludge at the average solids concentration of about 4% can be assumed to be 0.6 to 1.0% of the volume of wastewater treated.
As can be seen from the tables, compared to primary sludge, activated sludge contains lower amounts of grease and fats and cellulose, but higher amounts of nitrogen, phosphorus, and protein. The phosphorus content will be at the higher end of the range in plants using a biological process for phosphorus removal. The alkalinity of activated sludge is 580 to 1100 mg/L as CaCO3 and the pH is 6.5 to 8.0. Organic acids range from 1100 to 1700 mg/L as HAc.
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