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Temperature

30 to 35 °C

Figure 2 ZeeWeed™ Microfiltration Unit 2.2 RO Two Stage Concentration

Filtered water from the MBR enters the first stage of the RO plant. The feed is pressurised to 40 bar g by an inverter controlled multi-stage centrifugal pump to maintain a fixed feed flowrate before entering a 2:1 array of pressure tubes each containing six high rejection thin film composite membranes supplied by Koch - Fluid Systems. The pressure is boosted between the two arrays by a fixed speed centrifugal pump.

The RO process works by pressurising the feed side of the membrane with a pressure greater than the natural osmotic pressure of the solution. Where the natural process of osmosis results in the movement of water from a high osmotic pressure to a lower osmotic pressure through the semi-permeable membrane, the application of a net driving pressure in excess of the natural osmotic pressure reverses the process to produce a small volume of concentrated solution. The amount of permeate produced divided by the volume treated is termed the recovery of the RO system. A two stream RO plant provided for treatment of a borehole supply is depicted in Figure 3 on page 225.

The first stage recovers 70% of the MBR filtrate as permeate for re-use and utilisation in the wool scouring process. The typical permeate quality required is shown in Table 2 below. An Allen-Bradley programmable logic controller automatically controls the recovery rate of the RO plant along with all other control interlocks for safe and efficient operation of the system. Operator monitoring and control initiation is provided via a user interface screen.

The concentrate stream from the first stage RO is further processed by the second stage RO plant. A fixed feed flowrate is maintained using a positive displacement plunger type pump capable of generating pressures up to 80 bar g. The feed is thus pressurised onto a single pressure tube comprising five membranes suitable for the saline concentrate from the first stage of the RO system. 50% of this concentrate is recovered from the second stage to add to the first stage permeate. The overall recovery of both RO systems in this two stage concentrator is thus 85%.

RO permeate recycled to the wool scouring process is recovered at a continuous rate of 11.05 m3h"' with a TDS of less than 300 mg/1 and a COD of less than 10 mgl"1. The level of dissolved solids is less in the recycled water than in the mains supply.

Table 2 RO Permeate Typical Specification

Parameter

Value/Unit

Feedwater flowrate

13m3h-'

Permeate flowrate

11.05m3h"'

Total hardness

<25 mgl"1

Silica

<0.5-3.0 mgl

Iron

<0.1 mgl"1

Manganese

<0.02 mgl1

Alkalinity

<50 mgl"1

Total Dissolved Solids

<300 mgl"1

Colour

<20 Hazen

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