A flexible approach to liquid filtration

Fibra technology combines three processes in one fibrous bed device: filtration, coalescence and gas contacting. Dr Christopher Flinn describes some of the benefits of the product, and explains its multiple flexibility.

Fibra says its in-line filtration offers distinct advantages over alternative technologies. According to the company, operating in line with the flow (as opposed to cross flow), with the potential to maintain flow pressure through the graded compression zone, high-performance filtration can be achieved with low energy consumption.

Filtration can be optimised for each application by controlling the degree of compression and fibre characteristics. With Fibra technology, no expensive consumables are necessary to effect treatment. The filtration units undergo automated flush-cleaning procedures for kittle down time before treatment resumes.

The fibres and housings are robust, the company says, and can be safely used for a range of effluents. And they are suitable for use at either low or high pH (between 2 and 13). The fibres require only occasional maintenance, typically monthly, using environmentally friendly Fibraclean and Fibracare cleaners.

Liquid filtration using Fibra technology is achieved by means of compressing in-line fibre bundles, aligned parallel to the effluent flow, contained within a cylinder. A flexible compression bladder surrounds the fibre bundle; by application of controlled pressure to the bladder, the flow path is constricted causing particles to be retained in the gaps between the compressed fibres.

Through progressive increase in pressure along the length of the fibre bundle, particles can be retained in a graded way, resulting in limited overall flow-rate loss (see Fig 1).

The Fibra unit is cleaned periodically by releasing the compression bladder pressure and flushing the solids to waste using the same feed liquid. Only low volumes of feed are used for flushing while re-circulating the flow to the feed holding/settlement tank.

Flushing typically takes less than 10s. Pressure is then reapplied to the fibres, flow diversion stops and filtration resumes. This mode of operation means that there is no loss of treated liquid.

The Fibra device is available in different housing diameters and shapes to suit various treatment capacities. The filtration technology is readily scaled up by means of filter unit manifolds to meet higher throughput requirements.

Filter units are suitable for installation in retrofit applications, or can be supplied mounted on a robust frame together with controls and instrumentation.

According to the company, micro-bubble generation using a Fibra system offers further advantages over alternative processes. For example, CO2 gas contacting can neutralise alkaline waste, almost instantaneously, in-line.

Gas mixing
Using only compressed gas cylinders, this approach removes the necessity for expensive specialised acid storage, such as a secure tank, bunding, fume scrubber, dosing equipment and the associated operator safety, handling and personal protective equipment. Gas mixing is essentially instantaneous, consuming as little as 10% of the gas required by less-efficient CO2-based pH reduction systems such as pipe flocculators.

Gas contacting using other gases is possible; air or oxygen injection can reduce wastewater COD/BOD and reduce humic discoloration.

Although the Fibra unit acts as a stand-alone high-performance filtration technology with additional gas contacting treatment capability, in certain retrofit applications, the efficient mixing characteristics are advantageous for chemical dosing. For example, to neutralise acidity, sodium hydroxide can be effectively mixed into the effluent liquor via the filter inlet.

Other applications, including the case study described below, use the Fibra system as a final effluent quality polisher in combination with gravity settlement.

Applications for this technology include industrial process water and wastewater treatment - including metals processing, food and drink, laundry, construction - as well as groundwater clarification, landfill leachate and mine wastewater treatment.

Fibra treatment systems are also suited for membrane pre-filtration.

The technology has met stringent effluent treatment requirements for an Integrated Pollution Prevention and Control (IPPC) Improvement Programme.

By controlling the bladder compression pressure, varying degrees of particle retention can be achieved, allowing filtration efficiency to be optimised to suit each individual application. Overall filtration efficiency is a function of fibre material, fibre diameter and bladder pressure, so a preliminary evaluation is made to assess the requirements. Although details vary for each application, it is possible to remove solids to sub-micron levels.

A development from Fibra is the combination of precise filtration together with gas contacting to achieve further treatment benefits. Compressed gas - CO2 or oxygen for example - injected into the Fibra unit forms micro-bubbles as it passes through the compressed fibre bundle.

Gas micro-bubbles can be used to effect in-line effluent pH reduction (using CO2) or COD/BOD reduction (using air/oxygen), among other potential uses.

Tarmac Pre-cast Concrete, of Kirkby-in Ashfield, Nottinghamshire, manufactures pre-cast tunnel sections that require the neutralisation of alkaline wastewater derived both from process water and from site drainage of a considerable hard-standing area.

Reviews of discharge consents for IPPC licensing required a maximum of 50mg/l total suspended solids (TSS). Typical effluent flows were 0.65m3/h, although this could increase significantly at times of high rainfall.

Treated effluent
Surface and subsurface drainage together with process water are collected at the site's effluent treatment plant. This consists of primary and secondary settlement tanks (part of the original treatment plant design). These include absorbent oil interceptor baffles and flocculant dosing and a compartmentalised clarifier tank. And these, together with Fibra filtration units, form part of the improved plant design (see picture above).

Dual Fibra filter units, containing bundles of 0.1mm diameter fibres, treat effluent pumped from the penultimate clarifier compartment, discharging treated effluent to the final clarifier tank for subsequent release direct to the environment.

Filter bundles are compressed at 7bar, undergoing 5min filtration stages followed by 10s fibre flushing; flushings are diverted back to the primary settlement tank.

In this way, raw influent solids concentrations of up to 1,000mg/l are reduced to 15-35mg/l (reflecting removal rates of up to 93% TSS). Neutralisation of the effluent from an initial pH 11-12 down to final effluent 7-8 is also achieved.

At Tarmac, the Fibra filter units operate as part of a retrofit treatment system, acting as a kidney to clean-up non-settled particles. Final effluent quality is continuously monitored.

Should influent variations cause a deterioration in effluent quality, automated valve switching prevents discharge in breach of the consent, instead recirculating effluent through the treatment plant, allowing the Fibra system to reduce TSS (see Fig 2).

The filtration technology developed by Fibra achieves particle retention by compressing fibre bundles, which are aligned parallel to effluent flow.
Through controlled compression pressure, the degree of particle retention can be precisely controlled and optimised for each application.

Automated compression pressure relief allows periodic flushing of retained material from the Fibra units to waste/recirculation using raw feed liquid so no loss of filtrate.
Additional treatment can be derived using gas injection; CO2 reduces pH more cost-effectively and more efficiently than alternatives, while air injection aerates the effluent for oxidation to take place.

Dr Christopher Flinn is technical sales manager at Fibra.
T: 0118 984 4077

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