The major operations, equipment, and chemicals used for pretreatment and primary treatment are listed in Table 6. From Figure 2 and Table 6, it can be seen that a sludge stream from primary settler enters a series of sludge processes elaborated in later sections. In general, settled grits discharged from the grit chamber are not defined as sludge in the literature. They are usually transported directly to final landfill sites.
In a gravity sedimentation process, organic solids generally will not compact to more than 5-10%, while inorganic solids compact to 20% or 30%. Approximately 50-60% of the raw sewage suspended solids, and as much as 30-35% of the raw sewage BOD may be removed in the primary tank.
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