New immersed membranes for upgrading
conventional filter beds

By Rafael Simon, and Michael S. Stadnyckyj, Zenon Environmental

Municipalities often have substantial investments in drinking water treatment infrastructure that may not be able to achieve required water quality standards. The new ZeeWeed^® 1000 membrane system allows existing conventional filtration plants to be upgraded to ultrafiltration facilities, by replacing the existing media with membranes. In addition to saving capital and/or operational costs over other upgrade options, the system can simultaneously achieve a high quality effluent and increase plant capacity in the existing plant footprint.

The building blocks of this filtration system are parallel piped membrane elements. Elements are assembled into cassettes by stacking them in vertical and horizontal dimensions. In the vertical dimension, a standard stack has three elements, but stacks of one and two elements can be assembled to fit in shallow tanks. In the horizontal dimension, cassettes of one, sixteen and thirty-two elements, allow the flexibility to fit in virtually any size tank.

An element can be inserted into, or removed from the cassette by sliding it like a book into a bookcase. Each stack of three elements is connected to a permeate manifold that runs horizontally above the cassette. Cassettes are suspended in the process tank and do not require any bottom connections or support. The system incorporates ultrafiltration membranes that remove suspended solids, parasites, bacteria and viruses.

Filtration is achieved by immersing cassettes into the existing media filter basin. Raw water is fed from the bottom of the basin and flows vertically through the individual cassette elements. Impurities are rejected and remain in the process tank, while pure water is drawn through the surface of the membrane fibre under an ultra-low pressure suction and conveyed to the main permeate collection system. At specific time intervals, the membrane fibres are back pulsed and the reject water and impurities are removed through a backwash channel.

Given the similarity of operation between ZeeWeed^® 1000 and conventional sand filters, existing troughs, backwash equipment, pipes, blowers, and other infrastructure can often be re-used in the upgrade, extending the life of capital structures and equipment that have often already been fully depreciated. In fact, operators may find that the system's operation is more similar to a sand filter than to other membrane systems.

Figure 2a. Existing rapid sand filter (5 MGD)

As an example of upgrading existing media filter beds, Figure 2a displays a conventional rapid sand filter that can no longer meet demands for effluent flow and quality. The 110 m² (1,200 ft.²) of filter beds have a rated capacity of 19,000 m³/d (5 MGD) with a filtration rate of 7.3 m³/m²-hr (3 gpm/ft.²). Figure 2b shows a ZeeWeed 1000 upgrade where membrane cassettes have been inserted directly into the filter beds.

Figure 2b. ZeeWeed®1000 retrofit in same footprint (18 MGD)

Much of the existing equipment is reused, resulting in a substantial cost savings. The overall plant footprint is essentially unchanged as any additional equipment needed can be fitted in existing spare space. Membrane quality water is achieved while simultaneously increasing the plant capacity by a factor of 3.5 to 70,000 m³/d (18 MGD). Other than pre-screening, no pre-treatment for the membrane system is required. Rather than adding a new unit process (membrane filtration) after the sand filters, the granular media has simply been replaced by membranes, resulting in a much higher throughput and enhanced treated water quality.

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