Page 192 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 161

Malting steep water
A significant amount of water is used as steep water in maltings, where the water
is used to soak the barley. The effluent contains a high concentration of organics,
making it expensive to dispose of. Trials at several maltings have shown that
nanofiltration membranes or some types of reverse osmosis membranes can
produce water suitable for reuse (PCI Memtech). The cost of the effluent disposal
is not reduced, however, as the organic loading rate from the retentate stream to
drain remains the same.

Milk processinglcondensate
Membranes have been used for many years in the dairy industry for process
separation applications (Cheryan, 19 9 8). Across the whole food industrial
sector dairy applications probably account for the largest proportion of installed
membrane capacity. Indeed, it is the selectivity of the membrane filtration
processes, in terms of retentate molecular size, which allows fractionation of
milk to produce cream and skimmed milk by microfiltration and protein from
lactose by ultrafiltration. Since a key membrane property is its thermal
stability, generally to around 50-55°C to permit operation at lower fluid
viscosities, and chemical stability, to permit more aggressive chemical cleaning
and sanitation, polysulphone, polyethersulphone or PVDF membranes (Table
2.3) are generally used.
The use of membranes for other applications such as condensate recovery is
now an established technology. In powdered milk production facilities a large
amount of steam is used for evaporation purposes, and the condensate recovered
from the evaporators is both hot and relatively pure. This makes it ideal for make-
up water for the boilers. To remove the organic contaminants special high-
temperature reverse osmosis membranes (Duratherm Excel by Desal) are used
and the reverse osmosis plants are equipped with sophisticated CIP systems to
clean the units on a daily basis.

Other applications
Other membrane applications have been used in various industries for effluent
reduction and product recovery. These include cross-flow microfitration for beer
recovery from tank bottoms (Vivendi Memcor) and vibrating membranes for
beer recovery from spent yeast processing (Pall VMFm, Section 2.1.4). These
applications can normally be economically justified from savings based not on
water but on product recovery and reuse.

3.4.2 End of pipe recovery opportunities
As mentioned in previous sections, once the effluent has been mixed and the risk
of contamination has increased, the potential uses of the water are reduced.
Effluent from most UK food and beverage industries is discharged direct to sewer
and the associated costs paid. To recycle effluent would first involve an effluent
treatment plant, generally employing primary biotreatment for high organic
loadings with downstream polishing using depth and/or membrane filters,