Innovative membrane technology solutions for municipal water and wastewater treatment
In this article we review two recent municipal projects where ultrafiltration (UF) membranes have been used to enable the challenges of variable feed water quality and stringent treated water requirements to be met. We describe a 13 MLD potable water system for The States of Guernsey Water Board (SGWB)
Figure 1 (Top) – St. Saviours Water Treatment Works
Figure 2 – Membrane System Retrofitted into Clarifier Basins
Figure 3 – ZeeWeedTM Membrane Cassette
Figure 4 (Bottom) – Immersed MBR at Nordkanal, Germany
Potable Supply Upgrade
The SGWB specified the DWI-approved Zenon ZeeWeedTM membrane following extensive on-site pilot trials to fully characterise the nature and variability of the feed water supply. The technology was required to ensure the absence of Cryptosporidium and Giardia in the potable water supply, and to mitigate the effects of algal blooms and turbidity spikes.
The feed water originates from St. Saviours reservoir which has been the major water storage site on the Island since 1938. The treatment plant purifies half of the potable water required by Guernsey’s population of 60,000.
The project was undertaken by Purac Limited who retrofitted the ZeeWeedTM system into existing clarifier basins at St. Saviours water treatment works (see figures 1 and 2) in the Spring of 2004. Membranes were selected for their compactness, robustness, ease of operation, and straightforward maintenance procedures.
The membranes were supplied configured into standard cassettes (Figure 3). Thousands of ZeeWeedTM membrane hollow fibres, manufactured from PVDF, are suspended in the cassettes which are immersed directly into the existing clarifier basins. The fibres, with a nominal pore size of 0.04 µm, filter the water from outside to inside by applying a slight vacuum to the membrane. Intermittent coarse bubble aeration, periodic backwashing and maintenance cleaning are used to ensure membranes function at optimal performance.
The system is supplied with an automatic membrane integrity check that utilises a pressure decay test (PDT) consisting of pressurising the inside of the hollow fibres with air, isolating the permeate and then measuring the rate of pressure decay over time.
This integrity test complies with the US Environment Protection Agency’s Surface Water Treatment Rule which requires that the PDT be conducted at a test pressure sufficiently high to have 3 µm sized leaks or larger contribute to the pressure decay. This ensures that the PDT can verify the rejection of Cryptosporidium and Giardia.
Reservoir water is screened to 40 microns and dosed with a combination of coagulant and flocculant before being drawn through the membranes by means of dedicated self-priming extraction pumps.
Filtrate is discharged to an existing filtrate collection channel and particulate contamination is retained by the membrane fibres and subsequently removed from the fibre surfaces by an automatic back-pulse system using low pressure air.
The system has now been operating effectively for six months. At the end of this period, Andrew Redhead, Director of Water Services, SGWB, commented that “…nothing would cause us to doubt going down the same route again.”
The current overall recovery of the system is in excess of 95% and no chemical cleaning has been required for six months, the anticipated frequency being 4 times per year.
Municipal Wastewater Treatment Upgrade
Zenon’s 48 MLD wastewater treatment membrane bio-reactor (MBR) system at Nordkanal, Kaarst, Germany, was commissioned in December 2003. The German water management company Erftverband was the first in Germany to turn to hollow fibre membrane technology and, at the time of commissioning, this project was Europe’s largest immersed MBR facility.
The Nordkanal is a small artificial canal used to receive effluent from the Kaarst Sewage Treatment Plant. Outdated and running at full capacity, the existing works utilised conventional activated sludge technology. As a requirement to improve the overall quality of the canal, the new plant had to produce bathing water quality with total coliforms less than 500 per 100ml, faecal coliforms 100 per 100ml, and salmonella zero per 1000ml. This was a major driving force for the installation of a UF system.
The aeration tank containing the immersed ZeeWeedTM membranes, again supplied in standard cassettes, is shown in Figure 4. System footprint is minimised by placing the membrane fibres directly into the aeration tank.
Raw sewage is collected and passed through a 6 mm step screen before being processed through aerated sand and grit removal, followed by a 0.5 mm fine wedge-wire screen. Primary sedimentation is not required, which drastically decreases the overall footprint. After ferric chloride dosing the wastewater flows under gravity to an anoxic zone where most of the denitrification occurs. The mixed liquor then flows to an aerobic/anoxic zone, and then to a final aerobic zone for nitrification. This final aerobic zone contains the immersed membranes.
Filtration is achieved as at St. Saviours by drawing effluent through the surface of the membrane under a low-pressure vacuum. Once drawn through the membrane fibre, treated water is transferred to the main effluent discharge pipes. The UF membrane acts as a physical barrier, preventing suspended solids and colloidal material from being released in the final effluent. Typical raw and treated water quality is:
Biochemical Oxygen Demand (mg/l) 300 (influent) < 3 (effluent)
Chemical Oxygen Demand (mg/l) 600 (influent) < 20 (effluent)
Total Suspended Solids (mg/L) 350 (influent) Not Detectable in effluent
Turbidity (NTU) Variable in influent < 0.05 (effluent)
Both the St Saviours and Nordkanal ZeeWeedTM installations demonstrate the versatility of the membrane for effective use in municipal applications. However, in addition to potable water supply and wastewater treatment, Zenon systems are also effectively employed world-wide in various industrial and water re-use applications.
Jack Noble is Managing Director at Zenon Environmental (UK) Limited. Contact Jennie Peace at Zenon’s offices in Sheffield (where all UK projects are managed) for further details on 01226 760600 or email firstname.lastname@example.org.
This article was produced by Clarity: www.clarityauthoring.co.uk
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