Figure 11.8.The product of phosphate and ammonium concentration, taken from Fig. 11.7, is plotted versus pH in a double logarithmic diagram.

The effluent quality at optimum dosage is plotted versus pH for each of the selected précipitants to find the optimum pH; see Fig. 11.10. Here the nitrogen concentration of the effluent is used as quality parameter.

Another question to raise is of course: what is the economic optimum? An answer to this question requires that the relationship between the cost of waste water discharge and the effluent quality is known and can be compared with the cost of the added chemicals (precipitant, polyflocculant) and acid (to obtain the right pH). Recovery of proteins might, on the other hand give an income, which should be included in the financial calculations on the various alternatives. The experience gained from such calculations shows that the economic optimum very often is close to the technical optimum, i.e. it pays to utilize the technical possibilities to obtain the best effluent quality with the lowest nitrogen concentration.

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