Page 264 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studies 2 3
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Table 5.2 Specifications for water quality

Parameter Maximum
value
Suspended solids (mg IF1) <1
Silt density index <3
Reovirus (100 ml-') Nil
Enterovirus (100 m1-l) Nil
Total coliforms (100 I&') < 10
Faecal coliforms (100 ml-I) <1
Faecal streptococci (100 ml-l) <1

Demineralising feed water has two requirements. The first is that the water
must not contain any organic matter that will foul the ion exchange resin,
particularly anion resin. The second is that the TDS should not impose any
additional load on the plant over that of domestic feedwater. Benefits can be
easily obtained if the TDS is in fact lower than the original supply as it can
significantly reduce costs through fewer regenerations of the demineraliser
plant. A prime requirement for all end uses, and one required by the New South
Wales Environmental Protection Agency, is that the water be disinfected (Table
5.2). A number of technologies were considered including ponding, wetlands
and UV disinfection but all required additional treatment before the water was
suitable for reuse at the plant and so membranes were ultimately used.

5.2.2 Description of plant
Secondary effluent is supplied from the Dora Creek sewage treatment plant and is
initially stored in a 8 M1 holding tank before being transferred to the treatment
plant. The flow is supplied at a rate of 3.5 M1 d-' and blended with tertiary
effluent from the station's sewage works and contaminated plant water which
has had the oil and grit removed. The flow then passes through a motorised
screen before being pumped to the microfiltration plant (Fig. 5.3).
Filtrate from the MF plant is dosed with sodium hypochlorite en route to a
storage tank to control biological growth. Sulphuric acid (4%) is also added to
reduce pH and minimise hydrolysis of the RO membrane. Water is then pumped
from the storage tank, dosed with anti-scalant, and screened through a 5 pm
disposable cartridge filter before passing to the RO plant. Permeate is pumped to a
degasser unit before being fed preferentially to the demineraliser plant because of
its low TDS. The RO reject stream is dosed with ferrous chloride before being
passed to the station's ash dam together with waste from the demineraliser plant
and the wastewater sump.

Microfiltration
The microfiltration plant incorporates two parallel streams each containing 90
modules. Each module houses 15 m2 of polypropylene hollow fibre membrane
supplied by Memcor and rated at a nominal pore size of 0.2 pm. The plant