Page 193 - Membranes for Industrial Wastewater Recovery and Re-Use

162 Membranes for lndustrial Wastewater Recoverg and Re-use

possibly preceded by coagulation. In many cases such an option is only
marginally economically viable given the current level of effluent charges, cost of
plant maintenance and the incentives provided by water companies to keep
discharging to drain. However, having treated the effluent to a reasonable level,
the costs of treating to a standard suitable for recycling and thereby avoiding
water costs often shifts the economics in favour of the plant recycling option.
The economic case is further enhanced if (as in the case of the food and drinks
industry) the water is reused in high-quality applications such as boiler feed.
This is because the quality of water produced by an RO plant is generally of a
higher quality, with reference to key parameters such as hardness, silica and
total dissolved solids, than mains water. It is almost certain that the company
will already have to treat the mains water separately for boiler use. Often the
quality of the RO recycled water will be better than the current feedwater and so
that savings in boiler chemicals and heat can be made. When all of these factors
are evaluated, the economic case for effluent treatment and recycling may be
justified. Several effluent recovery and reuse plant have already been installed in
the UK, and it is likely that once the treatment and pretreatment regimes have
been established and proved many more will follow.

General aspects of plant design and operation
Many plants have used biological treatment plants with filtration and
chlorination prior to cellulose acetate-based reverse osmosis. This can work
successfully providing the level of filtration (sometimes dual media filtration) is
sufficient. The cellulose acetate (CA) membranes are less prone to fouling and
can tolerate a chlorine residual (Table 2.3), so biological fouling is reduced.
Unfortunately CA membranes are not as widely used within the industry
because of higher power costs, and lower rejections and pH tolerance. Composite
polyamide membranes are more commonly used and can be used successfully,
but pretreatment becomes more critical since they are more prone to fouling
than CA membranes. In a few plants employing tertiary media filtration severe
problems have been encountered in maintaining the flux within the RO plant. In
most cases either a membrane biological treatment process (i.e. a membrane
bioreactor, MBR) is required or alternatively ultrafiltration must be used as the
basic pretreatment step. The UF configuration and cleaning regime will depend
on the upstream process, as determined by pilot trials.
With an effective UF plant or an MBR as pretreatment, the use of polyamide RO
membranes should not cause a problem. It is advisable however to use much
higher fouling allowances (such as 30-SO%), such that the pump pressures are
significantly higher than design projections based on osmotic pressure alone. It
has been found in some plants that following initial organic fouling the
membrane flux drops to a sustainable level. This must be allowed for in the plant
design, and can normally be determined by pilot trials.
Biological fouling can normally be controlled by biocide addition. This can
be carried out periodically on line using a non-oxidising biocide, but can only be
employed if, as in most cases, the water is not being used for potable applications.
In some cases chloramine formation has been used successfully to protect the