Now we search for the treatment line with the lowest flow rate. To achieve this, we have to know the desired degree of removal:
The highest removal a follows from curve 3:
cin cout a
cin a + b it characterises the three triangle in Fig. 13.13d. The lowest treatment line 1 and the middle line 2 are possible solutions of the wastewater treatment, but the flow rates Q1 and Q2, the reciprocal inclinations, are relatively high. Only treatment line 3, which touches the wastewater stream line, represents a solution of the problem using the lowest possible flow rate Q3 (Fig. 13.13d). A straight line with a larger slope crosses the wastewater stream line at two points (not plotted here). It represents an impossible solution because mass balances are not fulfilled (Wang and Smith 1994).
After obtaining the fundamental solution for Qmin, cin and cout, a study of the design of the treatment system must follow. We will not go further into details here and will take up this question in Problem 13.2. We are convinced that there is a need to use this type of method to save water in industry.
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