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4 ERARING POWER STATION, AUSTRALIA

This is a municipal/industrial re-use project in which secondary treated effluent from a local Sewage Treatment Plant undergoes tertiary treatment at Eraring Power Station in New South Wales. The plant design combines Memcor continuous microfiltration (CMF) and reverse osmosis (cellulose acetate membranes) to produce high purity water for all power station uses except for drinking and showering.

The power station is located on the shore of Lake Macquarie, a tidal saline lake near the coast and approximately one hour from Sydney. The project consisted of connecting the outlet from the Dora Creek sewage treatment works to a new membrane treatment system incorporating microfiltration and reverse osmosis. The treated sewage is then used instead of potable water for boiler feed, cooling water, dust suppression and fly ash handling.

This 2-stage membrane system has been in operation since March 1995 producing 63m3/h with a planned expansion to 168m3/hr. Effluent from Dora Creek Sewage Treatment plant flows under gravity to centrifugal pumps, which deliver the feedwater via a single in-line motorised 500 micron self cleaning strainer to two Memcor CMF units.

Virtually all suspended solids, faecal coliforms and giardia cysts are removed. The SDI of the CMF filtrate water is consistently reduced to 1.5 allowing RO membranes to be operated at approximately 40% higher flux than is possible with a pretreatment based on the conventional lime coagulation/sedimentation/filtration process.

Filtrate from the CMF units is dosed with sodium hypochlorite, for downstream control of biological growth. Sulphuric acid is also dosed to reduce pH and minimise hydrolysis of the cellulose acetate RO membranes. Microfiltered water is drawn from the storage tank, dosed with antiscalant and passed through a 5 micron disposable cartridge guard filter for feed to the RO plant.

The RO system comprises two trains (2 x 50%), each with two stages - the first stage comprising 6 RO pressure vessels and the second stage comprising 3 (6:3 array). Each pressure vessel houses seven RO membrane elements. The membrane elements themselves are 8.5-in. diameter x 40 inches long cellulose acetate membranes rated at 98% salt rejection. The RO membrane, allowing only water to pass through, rejects salts and organics. Permeate (treated water) which is virtually free of all salts and micro-organisms, is piped to a degasser tower to increase pH by flashing off CO2 and is then stored in a 60m3 treated water tank.

4.1 Process Design

4.1 Process Design

Figure 4 CMF System
Figures RO System

The system provides the following benefits:

1) The treated water (permeate) from the RO plant has a TDS of < 40 mg/1 when treating secondary sewage of 500-1500 mg/1 TDS . The existing deioniser capacity has been increased by more than 3 fold as a result of the membrane pretreatment resulting in over A$150,000 savings per annum in chemical costs.

2) The membrane system removes bacteria and viruses for health risk minimisation and to protect the downstream equipment from biofouling.

3) RO permeate can be used directly for washdown, dust suppression and gland seal, with no health risks.

4) The compact size of the installation enabled an existing building to be used.

5) The total system is fully automatic, operated through the SCADA system and has self diagnostic data logging.

6) Membrane integrity tests automatically take place daily. Clean in Place (CIP) frequency is determined automatically by the control system. All cleaning chemicals are recovered and reused automatically on the microfiltration system.

7) All effluents from the system are recycled or used for dust suppression.

5 ZERO DISCHARGE PAPER MILL, NEW MEXICO, USA

One of the best known "Zero Effluent" Paper mills is McKinley Paper Company's mill in New Mexico, USA, producing test liner from OCC (old corrugated container). U.S.F supplied the water reclamation plant project with Continuous Microfiltration (CMF), RO and with Crystallization (USF HPD S.A.).

The mill has operated since 1994 and it was originally designed for production of 135 000 ton/year of 100 % recycled fiber based board.Today the production is approximately 165 000 ton/year.The raw water consumption is approximately 1.5 m3/ton of product. This raw water is reverse osmosis treated deep well water from the Plains Electric Generation and Transmission company next door. There is no foul sewer near the mill in order to discharge the effluent and the raw water availability is restricted. These matters were the original driving forces for the "Zero Effluent" concept.

The effluent from board production and OCC handling is treated biologically in an SBR (Sequenced Batch Reactor) type of activated sludge treatment plant. The biologically treated effluent is aerated and clarified in the same basin, in sequences. During clarification the aeration is simply turned off and the sludge is settled to the bottom of the basin. Excess sludge is removed from the bottom for dewatering and the clear effluent is removed from the surface of the basin. The water flow to the biological treatment is approximately 1500 m3/d. Typical COD content of the water is 3000-5500 mg/1 in the inlet, TSS approx. 250 mg/1. COD in the outlet is 450-500 mg/1 and TSS 40-50 mg/1. The effluent is cooled to suitable level (<37 °C) before the SBR treatment in order to optimize the biological activity.

5.1 Membranes After Biological Treatment

The biologically treated effluent contains approximately 40-50 mg/1 suspended solids of which is removed by the CMF microfiltration system.

The CMF treated water has TSS < 1 mg/1 and SDI < 3, which is very suitable for reverse osmosis treatment, in order to remove salts. Reverse osmosis treatment in McKinley is for approx. 660 m3/d, with 75% recovery, resulting in approx. 1000 m3/d water recycled after microfiltration. The CMF treated water is low in BOD and COD, low in TSS (< lmg/1), low in bacterial (103 CFU/ml).

An HPD Crystallizer evaporates the concentrate from the RO plant in order to turn the "last liquid form of effluent" to solid waste. All TDS left over from biological, CMF and RO treatment is concentrated to RO brine. The amount is approx. 168 m3/d with TDS concentration as high as 8000 mg/l.The Crystallizer is a tube type of falling film evaporator where the driving force for evaporation is generated by compressing the vapor pressure higher and simultaneously heightening the vapor temperature with mechanical single stage centrifugal vapor compressor. The condensate from the crystalliser is the cleanest treated water fraction from the whole water reclamation plant and it is recirculated back to the process.

The final solid waste, practically 80 % Na2S04 salt crystals, is disposed with the rest of the solid wastes by incineration in the multi fuel boiler in the coal mine next door.

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