Fibra filters have been installed at several precast concrete plants.
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    Credit: Fibra Solutions

    Plot of Turbidity

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Fibra Solutions, the British division of Tarmac, has been developing ways to reduce wastewater discharge volumes while meeting stringent UK Environment Agency (EA) requirements.

In the last few months, it has installed Fibra filters at several precast operations to remove wastewater contaminants. According to David Griffiths, the system's co-developer, the solution is particularly effective at sites when the goal is discharging difficult-to-treat wastewater to the municipal sewer system.

The Fibra filter is a low-energy, chemical-free progressive depth filter. Widely applied for sub-micron particle removal, it removes particles and can incorporate efficient gas transfer through micro-bubble production for uses such as pH correction using CO2 dosing.

In contrast to existing technology, the Fibra filter is parallel to the water flow. The filter comprises an in-line fiber bundle; in filtration mode, the flow moves to a point where a flexible compression bladder acts to constrict flow.

During filtration, CO2 can be dosed into the feed liquid to allow accurate and simultaneous pH reduction, a feature useful in the precast industry.

Particles are retained in gaps between compressed fibers; the degree of compression is varied by altering bladder pressure. The progressive narrowing of spaces between fibers results in low pressure loss and natural grading through size exclusion. Optimization of bladder pressure and fiber size allows precise particle removal control.

The process has an efficient, automated flushing mode during which bladder pressure is removed to open up fibers. The sequence takes five to 10 seconds and uses a small quantity of feed liquid.

The filter is supplied complete with instrumentation and control equipment in a frame, weatherproof enclosure for custom installation in the field. The filtration technology can be easily scaled to meet specified field requirements.

Engineers performed extensive pilot testing before Tarmac purchased a full-scale system. The feed turbidity was around 1000 Nephelometric Turbidity Units (NTU) during the trial, and maximum suspended solids were 150 mg/l. The treated water had to meet an EA discharge standard of 20 mg/l.