The first step in the conditioning of surface waters is the removal of suspended solids and colloids (particles <1 μm in diameter). For this purpose the surface water is subjected to coagulation and/or flocculation steps with the aid of proper chemical agents. The optimum pH for this treatment is usually below pH 7.4. Raw waters of medium to high alkalinity therefore benefit from a pH adjustment with acids in order to reduce the consumption of expensive chemicals and the associated production of waste sludge. In addition, when coagulating with aluminum salts, a well-adjusted pH value helps to keep the residual aluminum content low. This also reduces the aluminum load of any downstream activated carbon filter.