We assume that a production unit of a factory generates four wastewater streams which are polluted by nearly the same components, giving a COD which can be removed by the same method or type of treatment. The treatment must take place before discharging the treated water into an end-of-pipe treatment plant (WWTP). The COD must be removed down to ce. An example of such a situation is given in Fig. 13.11, showing WWTP 1, 2, 3 and 4 as a situation of a development for a decentralized treatment system. It is assumed that the costs of the pretreatment are proportional to the flow rate. Therefore, the aim of the optimization is to minimize the treated flow rate. This assumption is only one of several possible objectives. However, it makes it easier to explain the procedure. In Fig. 13.11, four different ways to reach the objective are discussed.
In Fig. 13.11a, the streams flow into one treatment plant, where they are mixed and treated together. This method is problematic because a stream with a high flow rate and a low concentration may be mixed with a stream with a low flow rate and a very high concentration.
A better way may be to treat the four streams separately (Fig. 13.11b). The retention time in treatment plant 1 (TP 1) can now be lower because of the low COD and the high flow rate. In contrast, it may be possible to treat another stream in TP 2 or TP 3 using a higher retention time. But there are several reasons to be dissatisfied with this treatment concept and we have not decreased the reactor volume.
This is possible if we reduce the COD in each TP below ce, pumping only part of the four streams each in parallel to the TPs and mixing them all together to obtain the required ce (Fig. 11.13c).
A further way to reduce the reactor volume is presented by Fig. 13.11d. The TPs are connected in parallel and in series. The part of the wastewater which is treated in parallel can be decreased by increasing the part being treated in series. Several possibilities are given to optimize this treatment process. The part of the recycled wastewater in WWT 1 can influence that of the following WWTs. A strategy is necessary to find an optimized solution to a given problem. The method described in the next section was published by Wang and Smith (1994).
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