Immersed Membranes Offer Simplicity and Versatility

Zenon Environmental offers a new immersed membrane, shell-less filter module system where the driving pressure is supplied by suction. Low trans-membrane pressures and moderate fluxes result in a system that has reduced energy costs and a longer life when compared to traditional membrane systems.

The companys ZeeWeed® modules consist of hollow fibers mounted on a vertical frame with permeate extraction from headers at the top and bottom. The modules are assembled into cassettes that can be lowered into new or existing tanks to form a treatment unit. As an example, several cassettes can be installed in an existing sedimentation basin. This approach allows the construction of very large trains of up to 5 mgd. The design is flexible and can be upgraded for increased flow by simply adding cassettes to the tank.

A typical immersed membrane filtration plant has parallel process tanks into which the cassettes are placed. Each process tank normally corresponds to one production unit and has a dedicated permeate pump. A complete system also includes a number of shared ancillary equipment, such as blowers, back-pulse pumping unit, permeate storage tank, clean-in-place unit and membrane integrity testing unit.

Immersed membrane systems are designed to be as easy to operate as sand filters. This is possible because the membrane is operated in quasi-direct filtration (dead-end), at low trans-membrane pressure and moderate fluxes. Gentle aeration agitates the hollow fibers and induces an airlift circulation pattern in the tank. Periodic back-pulses are used to deconcentrate the surface of the membrane.

Operation under suction imposes a practical limit to the trans-membrane pressure and operating flux, which remain in the stable pressure-controlled region of the filtration curve. These operating conditions result in less membrane fouling, reduced energy consumption and a long membrane life.

Direct membrane filtration will remove greater than 99.9 percent of particulate and colloidal material, but will not remove dissolved species. When required, the removal of dissolved species can be achieved through coagulation, oxidation, adsorption or biodegradation. Thus, immersed membranes can be applied to a variety of water treatment applications, including:

• Disinfection of surface water where chlorination has limited efficiency for parasites such as Cryptosporidium and Giardia.
• Filtration of water sources that show sudden variations of turbidity.
• Filtration of cold and low alkalinity water where coagulation has limited efficiency and sedimentation is very slow.
• Iron and manganese removal from ground water by coupling membranes with oxidation.
• Color removal by combining membranes with coagulation.
• Concentration of conventional water treatment plant residuals.
• The polishing of drinking water in existing large plants to control microbiological risks.
• Wastewater treatment (both industrial and municipal) by coupling membranes with biological treatment.