Improving Effluent Qualities using Immersed UF Membrane Technology
In this article
Figure 1 (Top) – Zeeweed® Membrane Cassette.
Figure 2 – Membrane cassettes are immersed directly into the mixed liquor at the Varsseveld site.
Figure 3 (Bottom) – Aerial view of the recently constructed Varsseveld wastewater treatment works.
Membranes for Wastewater Treatment
Existing wastewater treatment processes are facing enhanced quality requirements such as lower biological oxygen demand (BOD), suspended solids (SS) and ammonia concentrations in the final effluent, and improved phosphorus removal efficiency.
To meet these challenges, three process options utilising UF membranes are available:-
— upgrade conventional activated sludge systems to enhance treatment performance and hydraulic throughput of existing assets;
— provide an effective tertiary process for treating a variety of final effluents;
— install MBR applications in new wastewater treatment projects.
The ZENON ZeeWeed® UF membrane is a patented immersed membrane technology using hollow-fibre membranes operating in an outside-in flow direction. Typically, a vacuum of 0.2-0.3 bar is generated by the permeate withdrawal pumps to pull the permeate through the membrane. In some applications, gravity withdrawal will be used providing site conditions allow sufficient differential head across the membrane.
The membrane is manufactured using polyvinylidene fluoride (PVDF), so is highly chemical and oxidant resistant and tolerant of variations in pH (can tolerate a pH range between 2 to 10.5 during cleaning operations). The membrane is compatible with a variety of coagulants and powdered activated carbon.
The hollow fibre membranes have a nominal pore size of 0.04 ìm and are contained in bundles called modules, which are then assembled into cassettes. The membrane cassettes are directly immersed within the medium to be separated. As permeate is withdrawn through the membranes, solids are rejected at the surface of the membrane.
The membranes can be automatically back-pulsed regularly using collected permeate. Coarse bubble air diffusers are located at the base of each membrane cassette; the airflow generated scours the external surface of the membrane and transfers the rejected solids away from the membrane surface. The airflow also supplements biological oxygen requirements when used as part of a biological system. The membrane scour air typically provides 30-50% of the total oxygen requirements in municipal MBR applications.
Two cassette designs, ZeeWeed® 500 and ZeeWeed® 1000, have been developed for high and low solids filtration applications. The ZW500 membrane cassette operates with cyclic aeration which lowers energy consumption and keeps air diffusers clear of sludge fouling. The ZW1000 cassette is only air scoured during an intermittent back pulse and cleaning cycle.
ZeeWeed® 500 membranes use reinforced fibres to provide greater mechanical strength when used to separate permeate from mixed liquor suspended solids (MLSS) in MBR or high solids tertiary applications. ZeeWeed® 1000 membranes are non-reinforced and are utilised in low solids separation processes such as in tertiary treatment. The packing density of the membranes reflects the operational application – ZeeWeed® 500 cassettes have much greater spacing between the modules to allow solids to be more easily removed from the fibre bundle.
Depending upon the nature of medium the rejected solid material is either returned back to the process (as mixed liquor in MBR applications) or displaced through the use of a backpulsing cycle (as in some tertiary applications). Typically ZENON membrane cassettes are immersed in a separate tank from the bioreactor or process, resulting in a number of advantages:
— simplified construction in retrofitting applications with self-contained membrane systems;
— easier isolation of membranes for cleaning;
— reduction of energy consumption at smaller sites where periods of no flow may occur – the membrane system may automatically reduce or stop throughput;
— optimum conditions are achieved within separate biological treatment and solids separation stages;
— reduction of civil construction costs with existing structures retained or adapted.
Tolerance to solids loadings is conditional on filterability of the solids material. In low solids filtration, coagulation or flocculation may be required depending on the nature of the process fluid.
Asset Enhancement by MBR Upgrade
New regulations are requiring wastewater treatment facilities to achieve lower concentrations of effluent nutrients; nitrogen and phosphorus. Also, where the numbers of intermittent discharges are being rationalised within a water catchment, the hydraulic load on existing processes may also be expected to increase.
An existing activated sludge process may be retrofitted with a membrane separation stage as a replacement of the existing clarification process. The membranes offer a physical barrier to suspended solids, which permits the process to be operated for maximum biological and hydraulic performance rather than the settling conditions dictated by conventional clarification.
As volumetric loading rates are high, systems are compact, which gives membrane technology a significant advantage in retrofitting new locations where space is at a premium or increasing the capacity of existing tankage.
For example, where conventional activated sludge requires upgrading to produce a nitrified effluent, the membrane separation process permits a higher operational MLSS and sludge age within the volumetric confines of the existing aeration system and, therefore, creates conditions where nitrification can occur.
The effective sludge age is generally maintained at a minimum of 15 days in colder climates to ensure that filterability is not impaired. At high MLSS concentrations, the sludge age is high, and thus activated sludge generation is lower in comparison to processes with lower sludge ages. The waste sludge is also generated at a high concentration, meaning there is a lower volumetric demand on downstream processing equipment. The additional oxygen demand which results from increasing the MLSS is generally offset by the oxygen supplied as a result of membrane air scouring.
If phosphorus levels need to be reduced, iron or aluminium salts may be added directly to the aeration process. The majority of phosphorus discharged from a conventional activated sludge process is particulate. Effluent soluble phosphorus concentrations can be reduced to less than 0.05 mg/l through the aeration process, rather than prior to the primary settlement tanks, therefore reducing the quantity of primary sludge generated. Due to the small membrane pore size, the discharge of particulate material is virtually eliminated allowing for consistent achievement of effluent total phosphorus concentrations < 0.1 mg/l. For larger systems, it may be economical to include biological phosphorus removal in the design.
The ZeeWeed UF membrane can remove over 4 log bacteria as well as most viruses. This has particular benefits for discharges at coastal locations where bathing water or shellfish environments are protected.
Tertiary Treatment of Final Effluent
The use of membrane systems for tertiary filtration of final effluent will ensure very low solids, and the subsequent reduction of any particulate BOD, in the final effluent. Membrane filtration systems can be used for effluent polishing of most biological processes such as trickling filters, rotating biological contactors or other advanced forms of biological treatment.
Whilst the ZeeWeed® 500 would be used exclusively for high solids wastewater applications, the ZeeWeed® 1000 is suitable for tertiary applications when the solids loading is consistently less than 30 mg/l. Two basic alternatives for tertiary treatment in municipal applications are:
1. SOLIDS REMOVAL: Removal of suspended solids has a significant effect on the concentration of the associated BOD. Typically, tertiary membrane systems operate with recoveries between 90-95% depending on solids load and characteristics.
2. PHOSPHOROUS REMOVAL: When utilised with coagulation, the risk of coagulant floc carryover is minimised due to the physical barrier provided by the membranes prior to discharge. This opens possibilities for coagulant dosing after the biological process or reducing the initial coagulant dose prior to primary settlement. This helps with management of the coagulant dose and reduces the quantity of primary sludge or concentrate generated, limiting the expansion in sludge handling and treatment facilities.
In 2002, at IWVA in Belgium, ZENON commissioned a membrane system that provides UF pre-treatment of municipal secondary effluent prior to reverse osmosis (RO). Treated water from the RO is injected into the ground to act as a barrier to sea water contamination of the aquifer, and this provides a raw water source for potable use following a below-ground retention period of 40 days. The facility indirectly meets around 40 % of the region’s drinking water needs, producing 2,500,000 m³/year.
Effluent is treated by a 1 mm pre-screen before chlorine dosing prior to storage in a feed supply reservoir. Chlorinated water from the reservoir then flows into the UF process tanks from where permeate is extracted from the membranes. The UF system is designed for a maximum flow of 450 m³/h, which equates to a flux rate of 36.7 l/m2h. The facility consists of five UF trains, each with five ZW500 membrane cassettes.
Post-chlorination and ammonia dosing are used to form monochloramines prior to storage in the UF reservoir. Prior to RO treatment, dosing of anti-scalant, sulphuric acid (for pH adjustment) and sodium bisulphite (for dechlorination) is undertaken and the UF permeate passes through 15 µm-rated cartridge filters before being pumped through the RO.
RO permeate flows into the treated water reservoir where infiltration water is produced from a blend of 90 % RO permeate and 10 % UF filtrate. This is treated by UV prior to pumping into the ground. Following the 40 day retention time, groundwater is extracted and treated by aeration and sand filtration before being utilised for drinking water.
New MBR Systems
Where the choice exists to install MBR systems in new wastewater treatment works, membrane technology is becoming the preferred solution.
In May 2005, the first full-scale MBR system in the Netherlands was opened. The Varsseveld MBR project included ZENON membranes and was completed in close collaboration between the Rhine and Ijssel Water Board, the Dutch Foundation for Applied Water Research and DHV Consultancy and Engineering. The project involved extensive on-site pilot studies over a five-year period and was made possible through a LIFE subsidy from the European Union. The selection of ZENON technology was based on results from the demonstration programme and positive experiences with hollow fibre membranes in a number of Dutch treatment plants, including one in Beverwijk.
The Varsseveld plant was designed for 755 m3/h of wastewater projected in 2015, relating to 23,150 population equivalents and a flux rate of 37.5 l/m2h. The design was based on stricter effluent requirements than stipulated by Dutch law: < 5 mg total N/l and < 0.15 mg total P/l. The design features needed to accommodate the presence of a nearby cheese factory and a maximum hydraulic load of three times the average supply due to high rain drainage in the sewer system.
Significant attention was paid in the design to pre-treatment, with leaves and other components removed through a screen with a rod distance of 6 mm, an aerated sand and grease trap and finally through 0.8 mm perforated microsieves.
Membranes were installed in four separate compartments, with specially designed inflow and outflow facilities to ensure uniform distribution of the wastewater loading. Also, the division into compartments made it possible to take membranes out of service in case of low supply levels, when full capacity was not needed.
Coarse bubble aeration was included to prevent attachment of MLSS to the membrane surface, and the inclusion of a back-flush step in the process cycle prevented membrane pore blockage. This option saved energy because the membranes did not have to be continually aerated and were given sufficient time for relaxation. Periodic chemical cleaning is also required, in addition to the continuous cleaning of the membrane surface.
UF membrane bioreactor systems have been successfully employed in the enhancement of conventional biological processes and in making best use of existing plant assets, as well as in new-build situations. A number of different scenarios are possible for upgrading conventional activated sludge processes to nitrifying, de-nitrifying or phosphorous removal processes.
The use of UF membranes in tertiary applications can reliably produce effluents with minimal SS and particulate BOD. Coagulation may also be employed to reduce phosphorus concentrations in the final effluent. The membrane acts as an effective barrier to particulates, ensuring that metal flocs are retained within the system.
Due to the compact nature of ZENON membrane systems, footprint and land availability issues are minimised.
Jack Noble is Managing Director at ZENON Environmental (UK) Limited. Contact Jennie Peace for further information at ZENON’s offices in Sheffield (where all UK projects are managed) on +44 1226 760600 or email email@example.com.
This article was produced by Clarity: www.clarityauthoring.co.uk
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