Innovative membrane technology for potable water production in the Channel Islands, by Jack Noble, Managing Director at Zenon Environmental (UK) Ltd.
In this article we describe two recent potable water projects in the Channel Islands where Zenon ZeeWeedTM ultrafiltration (UF) membranes have been retrofitted to meet the challenges of variable feed water quality and removal of suspended solids and pathogens such as Cryptosporidium. We describe a 13 MLD system for The States of Guernsey Water Board (SGWB), installed at St. Saviours, that was retrofitted into existing clarifier basins, and a UF system followed by reverse osmosis (RO) that produces 4 MLD of an RO permeate / raw water blend at Le Mourier on Jersey. The blended water is added to Jersey's resources to meet the peak requirement of 30 MLD. This system was retrofitted into an existing storage tank. The UF / RO system reduces the nitrate level in the blended water to below the European Commission guideline level of 50 mg/l. We also describe the configuration of the ZeeWeedTM membrane and show how on-line measurements are used to help maintain system integrity.
Membrane Configuration and Operation
ZeeWeedTM UF membranes installed on both Channel Island projects are supplied assembled into ready-to-use standard cassettes (Figure 1). Thousands of membrane hollow fibres, manufactured from PVDF, hang loosely in the cassettes which are immersed directly into the raw water for ease of installation and subsequent access. ZeeWeedTM systems operate under a gentle negative pressure (by applying a slight vacuum to the inside of the hollow fibres) and offer many economic advantages over positive-pressure systems.
The cassettes are connected by permeate and aeration couplings and the ZeeWeedTM fibres, with a nominal pore size of 0.04 µm, filter raw water from outside to inside. The main advantage of this “outside-in” flow is that suspended particles remain on the outside of the membrane fibre preventing fibre blockage. Figure 2 shows a schematic of the ZeeWeedTM membrane.
Filtration efficiency is maintained by periodically reversing the flow of permeate (backwashing) combined with intermittent coarse bubble aeration which causes a slight motion of the membrane fibres to assist in removing particles. Periodic maintenance cleaning is used to ensure membranes function at optimal performance.
Because the ZeeWeedTM membranes are immersed in the raw water, system footprints are often much smaller than in conventional systems. Membranes are designed with the unique ability to retrofit existing infrastructure (as in the case of both examples below) such as clarifiers, raw water tanks, and activated sludge wastewater treatment processes.
ZeeWeedTM 500 and 1000 series membranes are approved by the UK Drinking Water Inspectorate for use in potable water treatment systems. Zenon UK can supply the membrane cassettes and ancillary equipment, and manage the overall project. Alternatively, cassettes can be supplied for installation by others, as is the case in the two Channel Islands projects described below where Purac Limited were the principal contractor. Commissioning support and ongoing system monitoring using ZenonTrackTM software for remote optimisation is provided by Zenon’s engineers.
The widespread threat of pathogens, such as Cryptosporidium and Giardia, to public health through contamination of potable water supplies is a problem that was addressed in the Channel Islands by installing Zenon membrane technology. The ZeeWeedTM systems include an inbuilt membrane integrity testing system that enables water utilities to maintain confidence in treated water quality.
The efficiency of pathogen removal is generally measured in terms of the log removal value (LRV), which is defined as:
|LRV = Log10
Where CFeed = concentration of pathogen in the feed to the process.
CFiltrate = concentration of pathogen in the filtrate from the
The ZeeWeedTM system typically achieves a LRV of at least 4, corresponding to 99.99 % removal of Giardia and Cryptosporidium. This compares to a maximum LRV of 2 (99% removal) achieved by conventional processes such as coagulation/sedimentation or rapid gravity filtration.
The Zenon integrity testing system utilises a pressure decay test (PDT) consisting of initially pressurising the inside of the hollow fibres with air and isolating the permeate. The rate of pressure decay is then measured and the result is used in the system control software to calculate the LRV.
This integrity test complies with the US Environment Protection Agency’s Surface Water treatment Rule which requires the PDT be conducted at a test pressure sufficiently high to have 3 µm sized leaks contribute to the pressure decay. This ensures that the PDT can verify the rejection of Cryptosporidium and Giardia since Cryptosporidium oocysts have a minimum diameter of 4 µm, with Giardia cysts at 6 µm.
St. Saviours Project, Guernsey
The SGWB specified the ZeeWeedTM membrane after extensive pilot trials. These were conducted to fully characterise the nature and variability of the feed water. The technology was required to ensure absence of the pathogens described above, and to eliminate the effects of algal blooms and turbidity spikes which caused repeated problems for the pre-existing clarification system.
The feed water originates from St. Saviours reservoir (Figure 3) which has been the major water storage site on Guernsey since 1938. The reservoir provides one third of the Island’s storage with the remainder comprising old quarry sites and small underground storage reservoirs with pumped connections to the treatment plant which purifies half the potable water requirement for Guernsey’s population of 60,000.
Compact ZeeWeedTM 500 membrane cassettes were retrofitted by Purac into existing clarifier basins at St. Saviours water treatment works (Figure 4) in early 2004. In addition to compactness, membranes were selected for their robustness, ease of operation, and straightforward maintenance procedures.
Raw water is screened to 40 µm and dosed with coagulant and flocculant before being drawn through the membranes by self-priming extraction pumps.
Particulate contamination is retained in the existing basins using a combination of backwashing and coarse bubble aeration before being transferred to an existing wash water recovery system. Filtrate is discharged to an existing filtrate collection channel.
At the end of the six-month optimisation phase, Mr Andrew Redhead, Director of Water Services, SGWB, commented regarding the installation that “¬hing would cause us to doubt going down the same route again.” Mr Redhead advised that the overall recovery of the system at the time was in excess of 95% and no chemical cleaning had been required during system optimisation, the anticipated frequency being 4 times per year.
Le Mourier Project, Jersey
The Le Mourier project consists of a ZeeWeedTM system followed by reverse osmosis (RO) demineralisation which produces around 2 MLD of permeate with a nitrate level of less than 10 mg/l. This is blended with raw water at a typical ratio of 50:50 to meet a target nitrate requirement of less than 50 mg/l and produce 4 MLD of potable water to supplement Jersey’s resources. Raw and treated water details are shown in Table 1.
Table 1 – Water Parameters at Le Mourier
Raw Water Treated Water Colour (Hazen) 12 Not Detectable Turbidity (NTU) 5 Not Detectable Nitrate (mg/L) 60-100 < 50 The plant was designed by Purac to reuse as much existing infrastructure as possible to both reduce cost and environmental impact by maintaining the appearance of the site, which is located near a popular coastal walking route and area of natural beauty. Following pre-screening at 50 µm, water is filtered by four modules of ZeeWeedTM 500 membranes immersed in an existing storage tank that used to supply cooling water to now abandoned diesel pumps.
UF permeate transfer is again achieved by creating a vacuum on the membranes to transfer the product to the permeate tank. Permeate pumps are periodically reversed to enable backwashing. Chlorine can be added to backwashes and coarse bubble aeration serves to enhance the cleaning efficiency.
The RO plant contains Koch thin film composite ultra-low pressure spirally wound membranes, arranged in two streams – duty and assist. The use of ZeeWeedTM UF membranes as pre-treatment to RO membranes ensures that they can operate under difficult raw water conditions (e.g. during turbidity spikes). RO plants can operate at higher fluxes (permeate production rates per unit area) leading to less RO membrane surface required and reduced fouling potential.
Other advantages of UF pre-treatment include lower RO pressure drop and cleaning frequency, decreased membrane warranty exposure and extended lifetime of RO membranes, increased system reliability and up-time, simplified operation, and reduced footprint. The UF and RO systems were supplied as compact skid mounted systems for ease of transportation around Jersey’s narrow lanes.
Both the St Saviours and Le Mourier installations demonstrate the versatility of the ZeeWeedTM membrane for effective use in municipal potable water applications. Both examples have shown that these membranes can be easily retrofitted into existing infrastructure to reduce costs and minimise the impact of the installation on the surroundings. In addition to potable water supply, Zenon systems are also effectively employed in the UK and around the world in wastewater treatment, and in various industrial and water re-use applications.