Understanding ultrafiltration

Ultrafiltration membrane processes can improve wastewater treatment effectiveness and capacity. This can be either through upgrade of existing plants or as a complete new-build membrane bioreactor treatment system. Jack Noble of Zenon Membrane Solutions reports.

The review of discharge consents by environmental regulators, notably with the implementation of integrated pollution prevention and control regulations, may require the improvement of effluent quality with respect to COD, suspended solids and ammonia.

Similarly, as the Water Framework Directive is addressed by regulators, attention to nitrogen and phosphorus in aqueous effluent will be a particular focus on a river-catchment-wide basis. Discharges to sewer (and thus to the environment via municipal WwTWs) and directly to the environment are subject to review and tighter control.

To meet these challenges, ultrafiltration (UF) membranes can be used to upgrade conventional activated sludge (CAS) treatment systems to enhance their performance and improve effluent quality. The treatment capacity of existing plants can be increased by adopting membrane technology to cope with increased production demand. Alternatively, membrane bioreactors (MBR) can be used in new-build applications as complete treatment systems. Existing plants can also be retrofitted with MBR technology.

Zenon Membrane Solutions, a part of GE Water & Process Technologies, has developed UF MBR systems, incorporating GE ZeeWeed membranes. The company has more than a decade of experience with these systems. ZeeWeed fibres are manufactured from resilient PVDF, each fibre having billions of pores of nominal 0.04µm. Groups of fibres are gathered together as modules (the fibre density is varied to suit the intended application), and modules are combined into membrane cassettes (see Figure 1). The cassettes are directly immersed into the wastewater process tank, which is normally installed separately from any existing plant. A slight suction is applied to the head of the membrane cassettes, and the wastewater is filtered through the physical membrane barrier from the outside of the fibre to the inside to achieve UF.

Fibre geometry provides a high UF surface area, and permeate is drawn from both ends of the cassette which improves membrane permeability. The inside of the membrane comes into contact only with high-quality permeate, and is not subject to fouling. Impurities and removed solids remain outside the hollow fibres. Air diffusers located at the base of each membrane cassette generate an airflow that scours the external membrane surface with coarse bubbles, removing rejected solids. This airflow supplements oxygen requirements in a biological system, typically providing 30-50% of the total oxygen requirements in municipal MBR applications. The membrane cleaning process is completed by periodic automated back-pulsing of permeate through the membrane to maintain a clean exterior surface.

ZeeWeed hollow fibre membranes are manufactured in two ways, either as standard or as reinforced. Reinforced fibres are suited to high suspended solids environments such as wastewater treatment; reinforced (or supported) fibres are strengthened by the infusion of braid into the hollow fibres. This improves the robustness and mechanical strength of the fibres, and improves the life span of the membranes.

ZeeWeed membranes are highly chemically resistant and are tolerant to variable pH. And they can be safely used in combination with those chemical and coagulating agents typically used at WwTWs. Cassette designs include the ZeeWeed 500, available with the supported membranes and suited to wastewater treatment applications, and ZeeWeed 1000 with a higher density of non-reinforced fibres suitable for low-solids applications such as tertiary treatment, and drinking water applications, which can include direct filtration, enhanced coagulation and reverse osmosis feed-water supply. ZeeWeed 500 cassettes have a greater spacing between the modules allowing solids to be more easily removed from the fibre bundle.

Because ZeeWeed 500 cassettes are normally immersed in a separate tank from the bioreactor, this simplifies construction and allows for the separate optimisation of hydraulic and biological treatment processes. This is especially advantageous for applications where variable effluent strength and quantity is anticipated. In addition, retro-fitting these modules to existing plant is facilitated.

ZeeWeed technology is available in ready-to-install pre-packaged modules. The Z-MODTM range utilises ZeeWeed 500 supported cassettes, and is available in three sizes, which can be flexibly linked together to provide a tailored wastewater treatment capacity. Modules are factory tested and each comes with automated membrane self-cleaning initiation technology, suitable for reliable unmanned operation.

Improvement of existing plant
Changes to discharge consents, perhaps including nutrients (nitrogen and phosphorus) for the first time, or consolidation of intermittent discharge locations resulting in increased capacity at fewer treatment sites may require the expansion of existing CAS processes at a plant, or enhanced treatment requirements.

An existing CAS plant may be retrofitted with a membrane separation stage as a replacement of the existing clarification process. ZeeWeed UF membranes provide a physical barrier to suspended solids. This allows the treatment process to be operated for maximum biological and hydraulic performance rather than being limited by the settling conditions of a clarifier; high volumetric loading rates can thus be used.

ZeeWeed MBR systems are compact (a cassette has dimensions only 2.1m wide by 1.75m deep by 2.5m tall). The need for separate clarifiers is eliminated, so membrane technology has a significant advantage at locations where space is limited. Where effluent phosphorus concentration needs to be reduced in small and medium treatment plants, coagulants may be added directly to the aeration process. The majority of phosphorus discharged from a CAS process is particulate. Due to the small membrane pore size, the discharge of particulate material is virtually eliminated guaranteeing less than 0.1mg/l phosphorus, with less than 0.05mg/l readily achieved. GE ZeeWeed MBR also removes bacteria as well as most viruses at more than 4 log removal by physical exclusion in the UF process. This has particular benefits for wastewater discharges at coastal locations subject to Bathing Water and Shellfish Water Directives with statutory microbiological consent limits. Membrane technology can be used in-line with existing plant to effect tertiary treatment, primarily by polishing to ensure low solids with associated removal of fine particulate COD. While ZeeWeed 500 supported membranes would be used for high solids wastewater applications, ZeeWeed 1000 membranes are suitable for such tertiary treatment applications when the feed-water is already treated for solids, but final effluent loadings of less than 30mg/l are required.

Complete ZeeWeed MBR systems
Installation of MBR systems at new-build WwTWs is becoming a preferred solution. GE ZeeWeed MBR provides for complete UF systems, to be used in conjunction with appropriate aeration. This combines the aeration, secondary clarification and filtration stages of conventional CAS-clarifier technology into one process. The advantages of membrane technology over CAS include space savings, flexible, fast and easy installation, enhanced treatment performance even with variable feed-water, and lower capital and operational costs.

The first full-scale MBR system in the Netherlands was opened at Varsseveld in May 2005, and included ZeeWeed membrane technology. Successful extensive pilot studies at this site over a five-year period confirmed its excellent treatment performance and robustness. In combination with positive experiences with hollow fibre membranes across a number of Dutch treatment plants, the pilot study resulted in the selection of GE ZeeWeed technology at Varsseveld. The Varsseveld plant was designed for anticipated treatment requirements in 2015 of 755m3/h wastewater serving approximately 23,000 population equivalents and accommodating a maximum hydraulic load of three times the average supply due to high rainwater drainage.

A treated effluent quality less than 5mg/l total nitrogen and less than 0.15mg/l total phosphorous is achieved. Pre-treatment, removing leaves and other components by screening, also includes an aerated sand and grease trap, and perforated microsieves, to protect the MBR system.

MBR systems have been successfully employed both in improving the performance of existing WwTWs, as well as in complete new-build situations. A number of different scenarios are possible for upgrading CAS processes to further reduce COD and solids in final effluent (polishing to tertiary treatment standards) as well as effecting phosphorous removal by adopting a coagulation stage at the same time as aeration.

The membrane acts as an effective barrier to particulates, ensuring that coagulant flocs as well as fine organic solids and bacteria are retained within the treatment system.

GE ZeeWeed MBR technology offers several advantages over conventional treatment approaches, including improved treatment performance, a small plant footprint, flexible and quick deployment options and low capital and operational costs.

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