Neat solution gives high quality effluent

Hollow-Fibre Membrane Bioreactors are helping to raise the standard of wastewater treatment in UK. Jack Noble, Managing Director, ZENON Environmental (UK) looks at two rural, and highly sensitive case studies.

Membrane bioreactor (MBR) technology is bringing fundamental changes to wastewater treatment in the UK, enabling communities and industries to
produce high-quality effluent from treatment works that are significantly smaller and more efficient than comparable conventional systems.

MBR systems can ensure that wastewater treatment works (WwTW) comply with the Urban Wastewater Treatment Directive with effluent that will raise the overall standard of water quality in the UK as required by new EU Drinking Water Directive. Whether retrofitting existing facilities or constructing new wastewater treatment works, MBRs are critical to upgrading the UK's wastewater treatment capabilities and improving the environment.

MBRs are one of the most efficient and cost-effective methods to achieve high quality water that can be safely discharged to the environment or reused for irrigation, industrial process water, and groundwater recharge.

Urban or rural applications
Available for about the same cost as a conventional wastewater treatment technology, MBR applications can range from small, modular systems that treat less than 100 metres3/day to very large systems that can process well over 50,000 metres3/day. Both municipalities and industries can benefit from MBR technology to cope with stringent discharge standards, space constraints for wastewater treatment plants, and residential, commercial or recreational development in remote locations.

Reinforced, hollow-fibre ultrafiltration membranes are at the heart of today's most advanced and cost-effective MBR systems, providing many benefits to municipalities and industry. Systems such as the ZENON ZeeWeed® MBR combine the UF membranes with biological wastewater treatment processes and eliminate the need for primary and secondary clarifiers-instead providing solids separation by filtration rather than settling.

This means that MBRs can produce high quality effluent at all times, regardless of the settling characteristics of the sludge. Since sludge settling is not required MBRs can operate at much higher mixed liquor suspended solids (MLSS) concentrations, typically in the range of 8,000 to 15,000 mg/L enabling the system to function with a relatively small bioreactor volume.

Hollow-fibre membranes are immersed directly into the mixed liquor and draw treated effluent into the fibre using a gentle suction. The membrane acts as a physical barrier, preventing suspended solids and pathogens from entering into the final effluent. MBR treatment works produce tertiary quality effluent that has very low levels of BOD, nitrogen, phosphorus and suspended solids.

The simple design of an MBR WwTW means that complete wastewater treatment can be carried out in one or two steps which significantly reduces the footprint of the plant and the operational equipment. Main equipment for a typical MBR wastewater treatment works will include a fine screen, membrane cassettes, a bioreactor, permeate pumps, blowers for process and membrane scouring, and clean-in place equipment to handle membrane backpulsing and washing.

The operation of the system is highly automated and many smaller facilities do not require operators on site at all times. Fibres can be easily cleaned with a clean-in-place method or using an optional backpulsing process that delivers non-chlorinated permeate water back through the membranes.

This dislodges any particles that may adhere to the membranes. Intermittent aeration of the membranes is also used to scour debris from the fibres and provides mixing within the process tank to maintain solids in suspension. In-situ membrane cleaning can also be automatically performed if membrane permeability falls below a specified level.

As many conventional WwTWs in the UK consider upgrades to comply with new water quality objectives, MBR systems offer these facilities the means cost-effectively meet the requirements. Modular ZeeWeed membrane cassettes are designed to fit into existing process tanks, enabling existing plants to increase capacity by up to four times within the current footprint while reusing some conventional treatment
equipment.

Design considerations
New MBR facilities can be constructed in virtually any location and can occupy as little as 1/4 of the space that a conventional WwTW requires, offering significant savings in land acquisition costs. Moreover, the small space requirements and modular construction provides a great deal of flexibility in design. Often the entire WwTW can be enclosed within a single building that can be architecturally crafted to blend in with the surrounding area.

When completed, the discreet design, low odour, and quiet operation of MBR plants makes them barely noticeable to passersby.

Several UK communities have already selected MBR technology for new WwTWs or for upgrading existing facilities to conform to new water quality objectives. The following case studies offer two examples of successful applications of MBR systems in UK and the benefits to the communities.

Campbeltown, Scotland
One of the first immersed membrane bioreactor plants to be constructed in the UK, the Campbeltown WwTW was required by the growing community to meet the Urban Wastewater Treatment Directive. Located at the head of a deep loch in western Scotland, Campbeltown had surpassed the 15,000 P.E. limit and, according to the Directive, needed to implement a minimum standard of secondary level wastewater
treatment.

Avoiding long sea outfall
Campbeltown considered several options for a wastewater treatment plant including a conventional activated sludge system, but this system was costly because it required a long sea outfall to discharge effluent to the open sea rather than into Campbeltown Loch.

Zenon ZeeWeed® MBR technology was selected as the best available solution since it provides high quality effluent that can be discharged directly to the Loch, thereby eliminating the need for the long sea outfall.

Campbeltown WwTW was constructed in only 13 months under The Coastal Towns Partnership schemes between the West of Scotland Water Authority and Biwater Treatment Ltd. The 23,000 P.E. plant was commissioned in November, 2001 and can treat up to 6,480metres3/day of wastewater.

Influent is pretreated with grit removal followed by 6 mm and 3 mm screens. The plant is equipped with three MBR process trains to provide biological treatment and filtration. ZeeWeed UF membranes are immersed directly into the mixed liquor of the final aerobic zone. Effluent is drawn through the surface of the hollow-fibre membranes using a low-pressure vacuum and flows to the main effluent discharge pipes.

The membranes prevent bacteria, viruses, and other pathogens from passing through the microscopic porous layer, so a high quality effluent can be released into the Campbeltown Loch without the addition of any chemical disinfectants. Effluent released into Loch consistently meets European Bathing Water Quality Standards. Average effluent quality produced by the plant is BOD of 2 mg/L, COD of 20 mg/L and TSS of 2 mg/L

Buxton, England
Located on the limestone cliffs that rise above Buxton's Wye River, the community's aging wastewater treatment plant was considered unable to comply with a new water quality regulation that combined Fishery and River Quality Objective consents. Several obstacles posed problems for an expansion, including the adjacent A6 trunk road and the unusual topography of the site.

MBR technology and biologically active filtration (BAF) were considered for the expansion since each offers compact plant footprints; however, concerns were raised about the ability of BAF to comply with the ammonia consent. ZENON ZeeWeed MBR was selected as the best available solution for Buxton because of its compact size and the high quality effluent that would meet all of the town's water quality objectives.

The new plant, recently commissioned in August, 2004, is designed for approximately 30,000 P.E., and can treat up to 16,700 metres3/day of wastewater. Because of the limited space at the existing site, Biwater Treatment Ltd. designed an innovative solution for Buxton, placing the storm tanks, headworks, and sludge handling processes on the previous site, and constructing the aeration plant and MBR in a nearby redundant quarry that is 12 metres above the existing site.

Influent is pretreated with grit removal and 6 mm screens. Excess storm flows that exceed three times dry weather flow (DWF) are held in storage tanks or discharged if the storage tanks are full. Below 3 DWF, influent continues to a 2 mm screen and is pumped under the A6 trunk road to the aeration plant and MBR in the quarry.

Compliant river discharge
Wastewater enters the denitrification process and flows into three aeration trains. Following denitrification, the mixed liquor from the three trains flows into six filtration tans. Each tank holds 11 ZeeWeed cassettes that produce permeate for discharge into the environmentally sensitive Wye River.

For most low-pressure ultrafiltration systems, permeate pumps are used to create the vacuum required to produce permeate, but at Buxton a natural siphon effect is used to produce permeate. The difference in elevation between the membrane tanks and the discharge pipe creates a hydraulic gradient that is sufficient to consistently draw water through the membranes and achieve the desired operational flux. This configuration dramatically simplifies the design, reduces the number of controls required, minimizes floor space and reduces costs.

Additional space in the membrane tanks has been provided to accommodate for the growth of the community. Up to 12 additional ZeeWeed membrane cassettes can be added in a just-in-time fashion as flows to the WwTW increase


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