that 131 m3 d1 of fresh water are needed (the 26% of the current
water consumption), which represents a specific consumption of
0.46 m3 tMSW
1 . This implies a water saving of 377 m3 d1
.
Nevertheless, this solution is complex and expensive because it
requires the installation of a heat exchanger, a reverse osmosis section,
and the additional pre-treatments (e.g. filtration, pH adjustment,
biocide and antiscalant dosing, ...) and post-treatments
(e.g. pH adjustment). In particular, to mix the organic effluent (sanitary
facilities) with non-organic effluents could create problems
on the operation of reverse osmosis, because a higher cost of maintenance
will be needed. Therefore, the reuse of this particular flow
should be carefully considered.
The heat exchanger is required because, in a closed loop, heat
accumulates and temperature increases. Assuming data in
Table 2 and flow rates in Fig. 3, the energy discharged with the
WWTP effluent results in an interval between 5 107 and
6 107 kJ d1
. Moreover, reverse osmosis needs to operate in a
well-defined temperature range, which depends on manufacturer
specifications.
It should be mentioned that incineration is a suitable way to
dispose the concentrate flow of a reverse osmosis. However, the
incineration of the concentrate in the same WtE plant where it is
produced determines a mass loop (mainly for chloride and sulfate),
because the pollutants removed by the flue gas cleaning system are
brought into the system again. Incineration of the concentrate flow
also implies an increase of the pollutant concentration in the various
sections of the WtE plant (process water and flue gas), that
should be planned carefully in order to anticipate possible malfunctioning.
Therefore, the designer or the plant managers should
consider this option only after an analysis of the whole WtE plant
layout.
As for the reverse osmosis, it can be assumed that a
well-designed process produces an amount of concentrate equal
to 25% of the influent flow rate. The RO management cost per cubic
meter of treated water ranges from 0.5 to 1 € m3
, and the disposal
of the concentrate approximately from 2 to 5 € m3 in an
45 m3