Page 260 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studies 2 2 9

Table 5.1 Contractual water quality
~
Parameter Units Contractual Expected
Conductivity ps cm-' 70 50
Silica dissolved mg I-' Si02 1 0.5
Sodium mg I-' Na 20 15
Calcium mg I-' CaCO3 5 2
TOC mg I-' C 2.5 <2
Polysaccharides Yg I-' 100 50

sewage eMuent which has a naturally variable effluent quality. A similar
situation exists in the USA at Water Factory 2 1 where a multiple treatment train
is used. This comprises of pH adjustment to 10, coagulation, super chlorination,
clarification, depth filtration, pH adjustment to 6 and finally dechlorination. This
approach suffers from a number of drawbacks including a large chemical use,
large footprint and high operating costs. The original process train also included
activated carbon but was discontinued due to fines coming out of the filter beds
and blocking the RO tubes. As an alternative to the above, AWG investigated the
use of microfiltration membranes. Previous research had shown membrane
processes to be a suitable unit operation for the removal of solids, bacteria and
microorganisms capable of robustly producing a silt density index below 2. Four
commercial systems were tested including the specific pre screening
requirements which ranged from 1 mm screens to sand filters (-50 pm
equivalent screen).
The final process train selected consisted of a pre-screen, microfiltration plant
and a reverse osmosis plant (Fig. 5.1). Wastewater from the screens and the
microfiltration plant is returned to the head of the sewage works. Concentrate
disposal is achieved by dilution with final emuent from the sewage works in the
final effluent chamber and discharged into the tidal section of the river Nene.

Pre-screens
Appropriate pre-screening was found to be important to ensure suitable
operability of the plant. Assessment of the different options resulted in a
Hydrotech HDF 830 drum screen being installed. The screen removes solids
larger than 150 pin and has a self-cleaning facility to remove the retained
solids by back flushing with screen effluent. The screen is manufactured from
stainless steel with a polyester cloth insert material. To ensure constant flow to
the membrane plant the screens were fitted with variable speed pumps which
controlled the level in the screened water tank.

Microfiltration
The screened effluent then passes to the microfiltration plant. The plant
comprises two parallel streams, each containing 17 modules. Each module
contains 50 m2 of PVDF hollow fibre membrane operating in a semi dead-end
mode with the flow from outside to in. The membrane material is rated with a
nominal pore size of 0.1 pm and fabricated into individual membrane units with