M. Thompson^a, D. Powell^b,

(a Senior Process Engineer, Memcor Australia, Windsor, NSW, Australia, b Project Manager, Veolia Water Systems, Singapore)

Abstract: Increased water demand from population and economic growth, environmental needs, change in rainfall, flood contamination of good quality water and over abstraction of groundwater are all factors that continue to create water shortage problems. These changes combined with new legislation are encouraging the development of sustainable water resource strategies. Many national policies now include the promotion of wastewater reuse.

Singapore, like many other major cities throughout Asia, is running short of a key resource – Potable Water. Consequently, the Singapore Ministry of the Environment and the Public Utilities Board (PUB) are planning to reuse up to 200 ML/d of treated effluent, both for industry and for indirect potable reuse.

This paper discusses the dual membrane treatment system installed at Kranji in the north of the Singapore Island, and in particular the microfiltration pretreatment as a practical and economical choice for reuse. Initially, the Kranji High Grade Water Reclamation Plant (HGWRP), or NEWater plant, will supply industry including the microelectronics industry, with a small amount supplied for indirect potable reuse. The Singapore Government intend to eventually supplement up to 2.5% of it’s drinking water with NEWater from reuse plants like Kranji.

1.0 Project Background

A joint venture between the Public Utilities Board (PUB) and the Ministry of the Environment (ENV) of Singapore initiated a Water Reclamation Study (NEWater Study) in 1998. The primary objective of the joint initiative was to determine the suitability of using NEWater as a source of raw water to supplement Singapore's water supply. NEWater is treated used water that has undergone stringent purification and treatment process using advanced dual-membrane (microfiltration and reverse osmosis) and ultraviolet technologies. NEWater could be mixed and blended with reservoir water and then undergo conventional water treatment to produce drinking water (a procedure known as Planned Indirect Potable Use or Planned IPU).

1.1 The Bedok Demonstration Plant

Previous studies have identified Microfiltration and Reverse Osmosis as the preferred process to meet the stringent treated water standards required when treating secondary effluent for potable reuse. To both ensure the effectiveness of this chosen technology, and determine design parameters for the future full scale plants, the Ministry and PUB, commissioned their consultant, CH2M-Hill, to carry out a design and health effects study on a full scale plant. Veolia Water's Asian and Australian operations combined to design and construct the 10,000m3/day demonstration plant, using Memcor's CMF technology, and USF Reverse Osmosis systems.

The combined treatment systems of CMF, RO and UV units provide a multiple barrier to pathogens in wastewater. A key benefit of this approach is the integrity testing ability of CMF, which has been shown in many applications to achieve greater than log 4 removal of particles greater than 0.2 micron. The Memcor Pressure Decay Test is used to measure the integrity of the CMF system. To further validate the process, the Ministry and PUB carried out a challenge test with MS2 bacteriophage across the Reverse Osmosis and UV disinfection part of the plant.

1.2 Panel Review

An expert panel that consisted of both local and foreign experts in: engineering; biomedical science; chemistry and water technology was formed in January 1999, and oversaw the NEWater study. After evaluating plant performance and extensive testing of final water quality (including physical/chemical analyses; pesticide/herbicide analyses; radionuclides; synthetic and natural hormones and microbiological testing) for a period of 2 years, the expert panel concluded that:

· NEWater is considered safe for potable use (and meets the latest requirements of the US EPA’s National Primary and Secondary Drinking Water Standards and the WHO’s Drinking Water guidelines; and

· Singapore should adopt the approach of indirect potable reuse.

1.3 Study Outcome

The outcome of the NEWater study led PUB to embark on new initiatives to supply NEWater to wafer fabrication plants and other industries for non-potable use. In January 2002, PUB awarded Vivendi Water Systems Asia the contract to supply a 40,000 m3/day dual-membrane high grade water reclamation plant (HGWRP) at Kranji. The plant combines Memcor’s CMF-S (Microfiltration) with Reverse Osmosis (RO) and UV to produce high purity water from secondary effluent. The CMF-S Submerged Continuous Microfiltration process combines Memcor’s proven pressurised CMF product know-how with a submerged configuration to achieve increased product scale and improved operating economies. This plant entered service at the end of December 2002, and is now operated by the PUB. The pant is designed to allow future expansion of capacity up to 72,000 m³/day.

2.0 The Kranji NEWater Plant & Process Description

The Kranji HGWRP is a 40,000 m3/day dual-membrane wastewater reclaim plant. The plant combines Memcor’s CMF-S (Microfiltration) with Reverse Osmosis (RO) and UV to produce high quality water from secondary effluent. A multiple barrier process to ensure pathogen removal in wastewater. The main unit processes in the plant include:

· Secondary effluent pumping c/w chlorine dosing and Equalisation tank;

· Microfiltration – 6 x 480S10T CMF-S cells;

· Filtered water storage c/w chlorine dosing;

· 5 x 2 stage (49 vessels 1^st stage, 24 vessels 2^nd stage, 7 elements/vessel) RO trains;

· 3 x UV units for disinfection; and

· Product water storage and pumping c/w pH and chlorine control

2.1 Microfiltration

Chloraminated secondary effluent (MF Feedwater) is drawn from the flow equalisation tank. It is pumped through a feed strainer to the CMF-S feed channel. Chloramine residual is controlled via dosing at the secondary effluent pump discharge. The CMF-S feed channel distributes the MF feedwater to the CMF-S cells at the bottom of each cell and it is then drawn through the 0.2 micron porous membranes by suction from the filtrate pump. The speed of the pump is controlled to maintain a specific filtrate flow rate for that cell. The membrane removes all the suspended solids, bacteria and pathogens from the feed water.

The CMF-S system incorporates one train of six 480S10T CMF-S cells and the ancillary process and control systems required for operation. Each cell has 448 modules installed (14 racks each containing 32 membrane sub modules) and one “Filler Rack” (a rack that contains no membrane sub modules allowing for future expansion). A cell incorporates a filtrate pump and the local valving to effect individual flow control, backwash and CIP. The cells are positioned adjacent to the inlet feed channel. A backwash pipe is located underneath the cells. The cell is drained by gravity to a backwash waste sump during the backwash process. The CMF-S system includes the CMF-S cell and ancillary equipment required for operation of the cells. The CMF-S system design is:

· Modular (6 independent cells, including one redundant cell);

· Compact footprint;

· Easily accessible; and

· Has a purpose built Service Access Platform (MEMSAP) to minimise membrane installation and service times.

2.1.1  CMF-S Backwash

The CMF-S cell is backwashed after 20 – 45 minutes of filtration. The backwash uses low-pressure air, to scour the fibre surface, while filtrate is back flushed through the membrane. Once the surface of the membrane has been scrubbed and backwashed, the dirty water is drained completely and quickly from the cell, the cell is refilled with feed water before returning to filtration. The backwash water is returned to the head of the Kranji Water Reclamation Plant.

2.1.2  CMF-S CIP

After 3-6 weeks of filtration a Clean-In-Place (CIP) can be carried out on a cell to remove any residual fouling on the membrane that is retained after backwash. The CIP consists of a soak/recirculation in a caustic and Memclean CÔ solution before rinsing and then returning to filtration. The caustic/Memclean CÔ solution is highly effective at removing organic foulants from the membrane. About once a quarter, the CIP will also include an acid CIP to remove any inorganic foulants on the membrane.

The CIP is conducted by pumping heated RO permeate from a hot water tank to the CMF-S Cell. The water is then recirculated through the CIP manifold where it is dosed with chemicals to the desired concentration. The CIP manifold has an inline heater to maintain the temperature of the solution, as well as instruments to monitor the pH and conductivity of the solution while the solution is recirculated back to the Cell.

2.1.3  Memcor^Ò Service Access Platform (MEMSAP)

To minimise the installation, service and maintenance times for the CMF-S membranes and equipment, the row of cells has a Service Access Platform (MEMSAP). Each rack of membranes is suspended in the cell and can be detached and lifted from the cell using the MEMSAP. The MEMSAP has the following features:

· Mounted directly on the filter cell structure;

· VSD motor-driven travel across the top of the CMF-S cells;

· Motor-driven raising/lowering of CMF-S racks;

· Tooling for unlocking rack from the cell and for isolating membranes;

· Pneumatic tool on-board for removal of membrane clovers and;

· Pin repair vessel for on-board service of membrane sub-module.

The MEMSAP allows individual sub-modules within a cell to be directly accessed and tested in a matter of minutes. The repaired sub-module can then be returned to service.

2.1.4  CMF-S Rack Inserts

A feature of the CMF-S system at Kranji is the use of Rack inserts. The CMF-S sub-modules are manifolded together and submerged in an open tank (the CMF-S cell). Since the sub-modules are cylindrical, there is a void space around each module when submerged in a rectangular tank. The CMF-S rack inserts are designed to fill this space, reducing the operating volume of the CMF-S cell with several positive impacts on CMF-S performance, including:

· Increasing recovery by reducing backwash waste volume;

· Increasing production for a given instantaneous flux by shortening the backwash duration by reducing cell fill and drain times;

· Reducing CIP chemical use by reducing the Cell volume;

· The rack inserts also channel the air scour within each membrane bundle during backwash providing more efficient use of the air.

2.2 Final stages: Reverse Osmosis and UV

The filtrate produced from the CMF-S system flows to the downstream filtrate storage tank and on to the RO plant. The RO permeate is then treated with UV for disinfection, and pH adjusted before end use.

The Reverse Osmosis system consists of 5 by 2 stage reverse osmosis trains. Each train consists of 49 vessels in the first stage and 24 vessels in the second stage. Each vessel contains 7 eight-inch Hydranautics LFC-1 elements. Both stages operate at a 50% recovery, thus each train has an overall recovery of 75%. The RO units have a normal operating pressure of approximately 10 bar. The RO system has the added feature of being able to separate the first and second stage, depending upon raw water conductivity. If the RO feed conductivity goes high for any reason, permeate from the second stage can be directed back into the RO feed tank. This allows the RO to lower the conductivity of the feed to the first stage, and hence produce/maintain higher quality (lower conductivity) waters. The RO system also has its own Clean in Place system, for recovering permeability of the system. Anti-Scalant is used to prevent scaling on the surface of the RO membranes.

Following the RO, there are 3 UV chambers for disinfection. Two of the UV units are duty and the other is for standby. The UV doses at approximately 90 mJ/cm².

2.3 Chlorine Dosing and Chloramine Residual Control

Typically, there is 1 to 3 mg/L of ammonia in the secondary effluent feeding the NEWater plant.  5 mg/L of chlorine is dosed into the secondary effluent prior to the equalisation tank. The equalisation tank not only buffers flow from the wastewater treatment plant to equalise flow to the NEWater plant it also provides reaction time for the chlorine. 2-3 mg/L of the chlorine is consumed by the organics in the secondary effluent, the remaining 2-3 mg/L combines with the ammonia to form chloramines, which is then fed to the CMF-S system.

After the CMF-S, the chloramine levels are slightly lower, 1-2 mg/L, as more is consumed in the CMF-S system. After CMF-S more chlorine is dosed to achieve 2 mg/L of chloramines in the feed to the RO. The RO permeate contain residual amounts of chloramine and more chlorine is added to reach the required residual for the distribution system.

The chloramine residual greatly controls the biological fouling in both the CMF-S and RO systems, increasing run times between CIP's. In this application the chloramines do not oxidise the polypropylene membranes due to the high levels of organics in the water, however, free chlorine will and control of the chlorine dosing is critical. Chloramine levels are measured using Total Chlorine analysers together with ORP meters to measure the oxidation potential of the effluent.

3.0 Water Quality Analyses

3.1 Raw Water

The table below contains the raw quality data for the month performance trial, 25^th Nov to 25^th Dec 2002. The plant had some difficulties maintaining a stable chloramine level in the MF Feed due to large fluctuations in the raw water ammonia levels.

3.2 CMF-S Filtrate

The table below contains the microfiltered water quality data for the month performance trial, 25^th Nov to 25^th Dec 2002.

3.3 RO Permeate/Product Water Quality

The table below contains the average product water quality as measured during the performance trial.

4.0 CMF-S Performance

The CMF-S system has performed well since commissioning. All cells operate for more than 25 days without requiring a CIP, when the required chloramine levels are maintained. The chemical cleaning has been successful at recovering the permeability of the membrane. The hydraulic performance is generally quite stable, however, due to periods of low ammonia levels in the secondary effluent the chloramine residual feeding the CMF-S has been low resulting in higher biological fouling rates.

4.1 CMF-S Design Validation

During the 30 day performance trial in December 2002, the CMF-S either met or exceeded all of the design criteria that were required as part of the contract.

4.2 Membrane Integrity

The combined treatment systems of CMF, RO and UV units provide a multiple barrier to pathogens in wastewater.  A key benefit of this approach is the integrity testing ability of CMF-S, which has been shown in many applications to achieve greater than log 4 removal of particles greater than 0.2 micron.  The Memcor Pressure Decay Test is used to measure the integrity of the CMF-S system and all cells have had stable integrity greater than 4.4 log.

5.0 RO Performance

RO plants are very sensitive to the quality of their feedwater. Microfiltration is a reliable source of high quality treated water.  The water it produces is substantially better than that of traditional pretreatment systems.  The proof is that it improves the performance of RO systems. The RO plant performance has been stable with no appreciable change in flow or salt passage.  The performance has been so stable that no chemical cleans have been required in the first 6 months of operation.

6.0 Conclusion

As treatment of traditional drinking water supplies become more expensive and waters sources become scarcer, water recycling increases in importance. The Singapore Government is actively developing, testing and installing new approaches for drinking water treatment and supply. In particular, wastewater reuse for indirect potable reuse with the installation of the 40,000 m3/day Kranji NEWater plant.

The Kranji NEWater plant is a prime example of how wastewater reuse can be employed to produce high grade water for industrial use. This has the benefit of both providing industry with higher quality water and reducing potable water usage in industry by replacing it with NEWater.  Key design advances in microfiltration technologies continue to provide reliable, stable and high quality feed to RO whilst further reducing operating costs.

Reference:

1. Singapore Public Utilities Board (PUB), “Singapore Water Reclamation Study, Expert Panel Review and Findings Report”, June 2002.

2. Bruce Durham, Stephanie Rinck-Pfeiffer (PhD) Dawn Guendert, ”Integrated Water Resource Management – through reuse and aquifer recharge”, IDA Water Reuse and Desalination, February 2003 Singapore.

3. Koh W.K., Thompson, G., Biltoft B., Durham, B., “Water Reuse & Zero Liquid Discharge – A Sustainable Water Resource Solution”, IDA Water Reuse and Desalination, February 2003 Singapore.

4. Durham, B., Koh W.K., Thompson, G., Biltoft B., “Membrane Filtration - An Effective Pretreatment to RO in Water Reclamation Experience in the Municipal & Industrial Sectors”, Water / Wastewater Management Conference, 20-21 Nov 2002, Shangri-la Hotel, Singapore.