Improving STP Performance by Lime Addition

• COD in Roma-Nord sewage is predominantly associated with settleable and supracolloidal particles, each size class containing about 40% of total COD. • A large fraction of COD associated with supracolloidal particles is characterised by slow degradability, therefore suggesting that removal of these particles prior to biological treatment may greatly improve the overall treatment scheme. • Pilot plant coagulation tests with lime at pH 9 showed that the lime-enhanced primary treatment may increase COD removal efficiencies from typical 30-35% up to 65–70%, by inducing an almost complete removal of the supracolloidal COD fraction. • An additional benefit of lime-enhanced primary treatment may be a certain degree of disinfection (1–2 log reduction in E. coli), which is not typically obtained in conventional primary treatment. • The main drawback of sewage coagulation with lime is the consumption of a large amount of lime and the consequent production of excess sludge, most likely due to the incomplete dissolution of lime. This drawback may be overcome by using a semi-batch coagulation process in place of the continuous flow mode.