Page 262 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studies 231

(a) (b)
Figure 5.2 (a) Microfiltrationmd (b) reverseosmosispilotplnntsat Flag Fcn

3.7 for suspended solids and 3.3 for BODj. Smaller variations are observed for
COD (2.2) and total phosphorus (1.5). Seasonal variations occur due to a
vegetable processing factory that discharges effluent of very different quality
depending on the food being processed.
The permeate quality from both membrane plants was both reliable and in line
with previous values found from pilot plant work. Feed quality to the RO plant
can show large variation with rainfall: for instance, a conductivity of between
800 and 1200 pS cm-'. Permeate quality from the RO plant is 38.5 pS cm-'
representing a rejection of 96.7% on average. Overall rejection of TDS is
93% with specific rejections of sodium and calcium of 95 and 99% respectively
(Table 5.20).
The main issue of concern was membrane fouling reducing output from the
MF plant and increasing pressure on the feed stream of the RO plant. Initial
operation of the plant showed excellent water quality but high RO fouling rates.
The RO feed pressure increased at a rate of 1 bar per hour at worst due to rapid
build up of calcium phosphate. Changes to upstream operation has effectively
controlled the problem and membrane cleaning is now required every 6 months
on average.
The Flag Fen plant is currently operating well and exceeding the target water
qualities set down in the original negotiations. The quality of the delivered
water has enabled ultrapure water production at the power station to increase
by 20%. Coupled to this is a reduction of over 90% in the costs of ion exchange
regeneration due to an increase in the operating cycle of the twin bed
demineralisation plant from 8 to 60 hours. Overall, 12 50 m3 of tap water per day
have been saved which has reduced the station's total water use by 11%. Client
receptivity to the scheme is very high as supply is guaranteed and operating costs
decreased. The high-purity water plant cost around €1 000 000 ($1 524 000)
and each membrane stage has an operating cost of 7.5~ m-3 ($0.11 m-3). The
success of the scheme has attracted media attention and the project has won a
number of prestigious awards including the 2 000 Water UK/Environment
Agency water, the 2001 IChemE innovation efficiency award and the 2001
Green apple award.