One of the major problems connected with the biological phosphorus removal process is that the phosphorus absorbed by the activated sludge may be released if the sludge is handled incorrectly, and this makes it difficult to treated sewage efficiently. Therefore, phosphorus in the return water from the sludge treatment process must be carefully controlled and reduced. Lime and ferric chloride were added to fix phosphorus in the digestion tank. Phosphorus fixation improved with the addition of lime. The digestion ratio reached a maximum at about 0.8 mol/mol to total phosphorus (T-P) with lime addition and decreased thereafter. Lime or ferric chloride was added to the supernatant to fix phosphorus. Ferric chloride removed suspended solids (SS) and phosphorus more efficiently than lime. A combination of these chemicals gave the most favourable results: a phosphorus fixation ratio of 90% or more was attained at a dosage of about 1 mol/mol. Magnesium ammonium phosphate (MAP) was efficiently formed by adding magnesium to the supernatant, and by increasing the system pH to at least 8 by the addition of sodium hydroxide or by aeration. The phosphorus removal ratio in pilot plant experiments was 70%, and granulated MAP was produced.

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