Dosing upstream from primary settlement has the advantage that coagulation removes a considerable proportion of solids and colloids together with some soluble material, resulting in a substantially reduced organic load on the secondary treatment plant. This results in reduced aeration costs and secondary sludge production, but at the expense of greater chemical usage and a significantly increased primary sludge production (typically 50-100%) which could overwhelm an existing facility. The relative benefit of these effects must be determined on a site-specific basis. Another disadvantage of dosing upstream from primary settlement is that organically-bound P is not easily precipitated and therefore full P removal may not be achieved. Conversely, excessive P removal could impair secondary treatment performance. The use of iron salts and alum reduce the pH, and optimum phosphorus removal is achieved in slightly acid conditions. This would have an adverse impact on a downstream nitrification stage and could require pH and alkalinity restoration at some sites.
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