Page 238 - Membranes for Industrial Wastewater Recovery and Re-Use

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overall cost. All purification techniques previously outlined are permitted and
the pinch methodology is used to target the optimal solution.
As is already obvious from Section 4.2.4, recovery of the sizing agent and
recovery of alkali with simultaneous recovery of a purified water is economically
very beneficial, with water available at good quality at the cost price of
groundwater. Looking at the proposed target (1016 m3 dayp1 offreshwater), the
water network suggested is operated at a cost price of €48 19, lower than the base
case. The target is achieved by implementing the membrane technique for
treatment of the printing paste. The final effluent is discharged to the sewer
system without implementing other purification processes to achieve a
wastewater that can be discharged at a lower cost to the surface water. One of
the flow rates suggested is very small and transpires to be an internal recycle for
one process. The question of whether or not the overall water flow through the
process should be lowered can then be addressed.
As can be seen in Table 4.11 for several processes multiple inlet streams
should be connected. This often implies additional cost, not yet accounted for by
the analysis, for pipework, storage tanks and control. It is advisable to ignore
these costs when embarking on the analysis, since they make the problem too
complex. When the optimum network is selected these additional costs can be
added manually or can be taken into account in the program.

Scenario 3: targeted minimum flow rate when a// treatments are allowed but discharge on
surface water is imposed. The limits set for the discharge to surface waters are
much stricter than those for discharge to sewer. When the company is faced with
these stricter values, proper solutions have to be accounted for. Using the
software for the case study and restricting discharges to surface water yields a
daily operational cost of €6551. The solution indicates that the constraints can
only be met when allowing good quality water, i.e. the outlet of the RO
installation, to be discharged to the surface water, Indeed, the RO is essential for
the strict regulations set to be met, It is observed that this good quality water is
not being reused in the process. Careful examination of the problem reveals this
is due to the low cost of the fresh water! Indeed, should the company be forced to
cut down on the groundwater use, tap water with additional treatment to reduce
the hardness, has to be used. A higher price of this water source would make the
RO installation competitive.

Remarks
When critically evaluating the case study it is apparent that not all of the
contaminants behave in a way that is consistent with necessary assumptions
made in the pinch analyses. Indeed, it is assumed by the program that, for each
contaminant, there is a linear relationship between the measurable
concentration and the actual mass per unit volume when different streams are
combined. For colour, this is not always valid. When running the program it is
observed that, depending on the initial conditions, the software sometimes
provides a solution that is indicated not to be the optimal one. Running the
software again with different initial values can result in the optimal solution.