Page 219 - Membranes for Industrial Wastewater Recovery and Re-Use

188 Membranes for Industrial Wastewater Recovery and Re-use

interest. A contaminant, in this context, is defined as any property of the water
that prevents its direct reuse, and can thus include heat content (i.e.
temperature) as well as the usual physicochemical attributes such as suspended
solids, acidity and hardness. Increasing the allowable influent concentration
results, in general, in an increased effluent concentration. In most cases basic
rules can be applied to determine the maximum allowable effluent concentration
and, as such, the maximum allowable influent concentrations, based around
such fundamental properties as mass transfer, solubility of scalants, corrosivity
and (organic) fouling. This relaxing of concentrations allows flow rates to be
determined that are most appropriate for efficient water use.

The limiting water flow rate concept (limiting water line)
An important difference can be observed between two basic precepts concerning
flow rate. The flow rate may either be fixed at some value or can be assumed to
take on some limiting value whereby the maximum allowable inlet and outlet
concentrations are obtained according assumptions or measurements made
based on deterioration of water quality through a unit. In Fig. 4.9 these
approaches are presented for a simple single-contaminant case. The
physicochemical properties of the process and of the equipment allow a
maximum inlet and effluent concentration of 50 ppm and 150 ppm respectively.
However, when increasing the inlet concentration up to 50 ppm, and keeping to
the existing flow rate of 2 t/h, the maximum effluent concentration is not
reached for this fixed flow rate. Permitting a variable flow rate for the process
considered allows the maximum effluent concentration to be reached. The water
pinch methodology was initially presented as a problem without flow rate

200 -

Non-fixed flowrate approach: both inlet and outlet
Concentration are allowed to attain their physico-chemical
/ -
/ -0
Cout, max
of 1 tnl. 75-
h
E
4 ./ /-
C
.- /-
/