Filtration+Separation June 2007 Feature 18 Membranes and wastewater: Modular treatment systems offer solutions hris King, GE Water & Process Technologies, outlines why upgrades may be required at wastewater treatment works (WWTW) to improve effluent quality, increase the potential to re-use water or expand treatment capacity. Membrane ultrafiltration (UF) technology, in combination with appropriate bioreactor systems seem to offer a viable solution. In this article, we look at the main features of UF membrane technology and the advantages it offers in terms of performance, costs and space requirements over conventional activated sludge-clarifier treatment systems are discussed. Highly automated modular membrane treatment systems are available that can be rapidly incorporated to upgrade an existing plant, or alternatively comprise part of a new-build treatment works. These systems are robust, and consistently produce a high quality effluent, even with variable influent loads. We then look at two case studies in Wales, UK, and see how these systems worked to their advantage. In both cases, a factory- tested Z-MOD modular membrane UF plant from GE Water & Process Technologies was adopted as the solution to distinctive problems. Strict planning constraints severely limited the space available for WWTW upgrading at Dale (Pembrokeshire), whilst at Mathry (South West Wales) a small municipal WWTW with a highly variable load was upgraded. The need for improvements Wastewater treatment requirements change with time, driven by changing regulatory regimes aiming to effect environmental improvements or sustainable water use. In some circumstances treatment plant expansion is required to increase treatment capacity where local development dictates or where the composition of the wastewater has changed. Improvements to the quality of treated effluent have recently been required as discharge consents are reviewed under Integrated Pollution Prevention and Control (IPPC) and improvement conditions are implemented. IPPC regulations also require that controlled installations (covering a wide range of industries) adopt best available technology (BAT) in order to achieve their discharge limits. Furthermore, IPPC permitting also requires routine water audits and the implementation of any necessary water recovery and reuse conditions. Further regulation of discharges to the environment are likely as the Water C Figure 1 – Modular pre-packaged membrane UF systems (Z-MOD Type S)
Transcript
Feature18Filtration+Separation June 2007
Feature1818 Feature18
Membranes and wastewater:
Modular treatment systems offer solutions
hris King, GE Water & Process Technologies, outlines why upgrades may be required at wastewater treatment works (WWTW) to improve effluent quality, increase the potential to re-use water or expand treatment capacity. Membrane ultrafiltration (UF) technology, in combination with appropriate bioreactor systems seem to offer a viable solution.
In this article, we look at the main features of UF membrane technology and the advantages it offers in terms of performance, costs and space requirements over conventional activated sludge-clarifier treatment systems are discussed. Highly automated modular membrane treatment systems are available that can be rapidly incorporated to upgrade an existing plant, or alternatively comprise
part of a new-build treatment works. These systems are robust, and consistently produce a high quality effluent, even with variable influent loads.
We then look at two case studies in Wales, UK, and see how these systems worked to their advantage. In both cases, a factory-tested Z-MOD modular membrane UF plant from GE Water & Process Technologies was
adopted as the solution to distinctive problems.
Strict planning constraints severely limited the space available for WWTW upgrading at Dale (Pembrokeshire), whilst at Mathry (South West Wales) a small municipal WWTW with a highly variable load was upgraded.
The need for improvements
Wastewater treatment requirements change with time, driven by changing regulatory regimes aiming to effect environmental improvements or sustainable water use. In some circumstances treatment plant expansion is required to increase treatment capacity where local development dictates or where the composition of the wastewater has changed.
Improvements to the quality of treated effluent have recently been required as discharge consents are reviewed under Integrated Pollution Prevention and Control (IPPC) and improvement conditions are implemented. IPPC regulations also require that controlled installations (covering a wide range of industries) adopt best available technology (BAT) in order to achieve their discharge limits. Furthermore, IPPC permitting also requires routine water audits and the implementation of any necessary water recovery and reuse conditions. Further regulation of discharges to the environment are likely as the Water
C
Figure 1 – Modular pre-packaged membrane UF systems (Z-MOD Type S)
Feature 19Feature 19Feature 19Feature 19Filtration+Separation June 2007
Framework Directive is implemented, where surface water quality is assessed on a catchment-wide basis and total emission inventories are collated and reviewed. This will particularly focus on total COD, solids and nutrient (nitrogen and phosphorus) fluxes to the receiving surface waters and habitats.
Whilst conventional physical and biological treatment systems (such as conventional activated sludge, CAS) with clarification of the final effluent can often meet current requirements, under stricter discharge limits, alternative technologies need to be considered to meet the increased challenge.
Further regulation of discharges to the environment are likely as the Water Framework Directive is implemented, where surface water quality is assessed on a catchment-wide basis and total emission inventories are collated and reviewed.In recent years membrane bioreactors (MBR) have proved one such viable alternative. MBR offers particular advantages in the flexibility of deployment, either as complete new-build treatment systems or integrated into existing WWTW. Capital and operational costs are competitive, and there are significant space savings relative to CAS. MBRs consistently produce a high quality effluent. The improved performance over CAS means that MBR technology not only meets today’s requirements, but will also ensure compliance with future, tighter, regulatory controls. The exceptionally high quality of treatment possible using MBR broadens the potential to reuse of water within industrial processes. The technology can also be used for irrigation (such as in landscapes or on golf courses) or for aquifer recharge, and can readily generate an effluent suited to reintroduction into the general water supply.
Membrane technology: reliable high performance
Membrane technologies provide high quality water because they act as a physical barrier to suspended particles in the mixed liquor. Membranes are thus inherently resistant to upset or variable wastewater conditions. A small pore size prevents the passage of colloids, resulting in a very effective removal of solids and a concomitant reduction in final effluent COD.
MBR systems are now available that are well suited for use at unmanned or remote WWTW. Self-diagnostic systems and automated self-cleaning capabilities, combined with low maintenance requirements, reliably and consistently
produce a high quality final effluent. Membrane UF systems are easy to install because of flexible configuration, and can be incorporated even where space is at a premium. Pre-engineered, factory-tested modular membrane filtration systems are available to facilitate rapid deployment, with treatment capacities tailored to suit most scenarios.
The exceptionally high quality of treatment possible using MBR broadens the potential to reuse of water within industrial processes.One such system is the Z-MOD packaged plant from GE Water & Process Technologies (Figure 1), incorporating ZeeWeed hollow fibre membrane technology. The ZeeWeed fibres are manufactured from reinforced resilient PVDF to enhance mechanical robustness, and are gathered together as membrane cassettes (see Figure 2), which are directly immersed into the process tank. A mild
suction applied from a low-energy permeate pump at the head of the cassettes filters the water from outside inwards through billions of pores (nominally 0.035 µm). Impurities rejected at the membrane surface remain in the process tank.
To maintain optimal performance, the membranes undergo intermittent automated coarse bubble aeration and back-flushing to eliminate impurity accretion. Only occasional manual maintenance is required using acid-based or caustic-based solutions. Performance and membrane integrity is automatically monitored by means of using turbidimeters. This affords a high degree of confidence in ensuring treatment consistency.
The permeate is of very high quality. Treated water turbidity of <0.2 NTU, transmissivity of >75% and TSS <2 mg/l are readily achievable for typical MBR effluent. BOD (<2 mg/l) and faecal coliform (<10 CFU/100 ml) concentrations are low, and, using appropriate biological design and chemical dosing,
Figure 2– GE ZeeWeed Membrane Cassette. ZW500 hollow fibre membranes collected as modules; modules are assembled into cassettes
effective treatment of total nitrogen (<3 mg/l) and total phosphorus (<0.05 mg/l) is readily achieved.
ZeeWeed hollow fibre technology is also used in GE Z-BOX packaged plant, for potable water applications. A higher density of fibres in the Z-BOX yields further improvements in treatment performance, effectively excluding pathogens and, with the adoption of appropriate pre-treatment equipment, can effectively treat metals, TOC (50-70% removal) and improve colour (to typically <5 PCU), to satisfy stringent public health concerns.
Incorporating membrane technology to WWTW
Some of the distinctive advantages of MBR technology are highlighted by its utilisation in order to upgrade WWTW at two locations Dale and Mathry, both in Wales, U.K. In both cases Meica Process Limited (under contract from Welsh Water) elected to use GE Z-MODTM UF packaged plants to achieve their WWTW upgrade solutions.
Dale WWTW, situated in the Pembrokeshire National Park, West Wales, serves a village (resident PE 250) that attracts significant numbers of tourists (increasing PE by 750). The WWTW is contained within a purpose built building with landscaping to reduce the visual impact in its centralised location. The original WWTW consisted of a small CAS system, with a 6mm inlet screen, aeration tank, final settlement tank and UV disinfection system. This suffered from settlement problems adversely affecting UV disinfection. Upgrade was required to
provide a consistent supply of high quality, disinfected effluent, within the strict planning constraints.
Commissioned in early 2006, Z-MOD MWW8 UF packaged plants effect physical, rather than gravitational separation of the biomass from the aeration tank. Two UF systems operate in a duty-assist configuration with automated changeover to ensure even use of the membranes, treating approximately 390 m3/d.No longer constrained by settling limits, higher biomass concentrations are used in the aeration tank during the peak tourist season to maintain high treatment standards at all times.
The UF systems replaced the settlement tank, easily fitting within the building structure, and screening was upgraded to 3mm to protect the membranes. A tidal diversion system was installed to prevent shock-loading by a sudden inflow of saline water to the bioreactor. Diverted water is normally slowly bled back into the system; the WWTW performance was not adversely affected by a 20-year high tide that occurred soon after commissioning. Planning constraints on access hatches to the WWTW roof were alleviated by mounting the Z-MOD on wheeled skids to permit vertical access for maintenance.
The upgraded WWTW achieves a typical effluent quality of <5 mg/l BOD (consent 50 mg/l), <3 mg/l TSS (consent 60 mg/l), whilst eliminating prior problems with the UV disinfection stage.
A small municipal WWTW at Mathry, South West Wales, in receipt of domestic and industrial effluent was designed and built around the ZeeWeed MBR process and
control requirements in order to upgrade an existing CAS system (with BOD 85 mg/l and TSS 150 mg/l). Two immersed Z-MOD systems were compactly installed (see Figure 3). The WWTW is subject to highly variable influent loading, and has been designed to operate with a flow ranging from 2.9 to 22.3 m3/h. Installation of the MBR resulted in a vastly improved effluent quality, easily meeting discharge consents of <32 mg/l BOD, TSS <57 mg/l and ammonia <7 mg/l.
Conclusions
Membrane technology offers several advantages over conventional CAS treatment systems. MBR has a superior performance, consistently generating high quality effluent of a standard easily complying with a tightening regulatory regime. Good quality final effluent has a higher potential for reuse and recycling.
MBR can readily be incorporated or retrofitted into existing systems for WWTW upgrade, and offers a competitively priced alternative as part of new-build. Pre-tested modular packaged plant, such as the GE Z-MOD can be rapidly deployed in most situations. The highly automated, self-cleaning systems are well suited for remote or unmanned locations, or those with limited space.
The advantages of MBR technology are illustrated by Meica Process Limited adoption of the Z-MOD at two locations in Wales to solve specific WWTW upgrade problems. At Dale, variable effluent loads from transient tourists were tackled using physical (membrane) rather than gravitational biomass separation. The MBR fitted within the tight space limitations imposed by planning constraints. At Mathry, marked improvements to effluent quality were achieved by replacing the CAS with MBR in a WWTW receiving highly variable domestic and industrial influent loads. •Contact: GE Water & Process Technologies, a unit of General Electric Company, is solving some of the world’s most pressing water challenges by providing industrial, agricultural and potable water, while lessening our dependence on fresh water sources. Technologies to accomplish this include desalination, advanced membrane, separation solutions, and water reuse and wastewater management and process technologies. GE delivers value to customers by improving performance and product quality, reducing operating costs and extending equipment life.
For further information on GE Water & Process Technologies (ZENON Membrane Solutions), contact Jennie Peace in the UK office on +44 (0)1226 760600, email [email protected], or visit www.zenon.com.
Figure 3 – Immersed membrane cassettes as part of the compactly installed Z-MOD system at Mathry (image courtesy of Meica Process Limited)
