The goal of primary treatment is the removal of settleable solids through opportunely designed sedimentation tanks. Primary sedimentation tanks or clarifiers may be circular or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 1 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks, by sludge rakes that scrape the sludge to a central well, from which it is pumped to sludge processing units. Scum is swept across the tanks surface by water jets or mechanical means from which it is also pumped to sludge processing units. The particle material removed during this treatment has normally dimensions superior to 50 mm. This method is also effective to get a modest reduction of the organic nitrogen, the organic phosphorus and the heavy metals. On the contrary, colloidal and dissolved constituents cannot be removed with this treatment. A great reduction of phosphorus and heavy metals can be obtained using chemical coagulants and particular polymers. In the following Table 1 the typical removal percentages of the principal constituents, after a primary treatment, are presented. As regards to the removal of the microorganisms (such as protozoa, parasites eggs, cysts) and virus this method of treatment cannot obtain acceptable removal percentages (see Table 2 ). For this reason, limiting the treatment to the only primary phase does not represent an appropriate solution to satisfy the quality requisites for most of the reuse applications.

Table 1.  Removal effectiveness (%) of principal wastewater contaminants after primary treatments (source: adapted from Water Pollution Control Federation, WPCF, 1989).

Table 2. Removal effectiveness (%) of some microorganisms after primary treatment (Source: adapted by Crook, 1990).

TREATMENT COSTS

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