David Griffiths describes an innovative in-line process from Fibra Limited. A high degree of particle size exclusion control allows users to optimise their system to minimise power consumption, maximise treatment flow rates, whilst, uniquely, additional gas contacting can be simultaneously incorporated to adjust pH or achieve additional effluent treatment.
Liquid filtration using Fibra technology is achieved by the controlled compression of fibres which form the filter bed. Particles are retained in the gradually decreasing interstitial gaps before the compression point (Figure 1). Through progressive increases in compression pressure , particle retention can be closely controlled ensuring maximised flow rates for in-line filtration. In contrast to many microfiltration systems this process occurs parallel to the liquid flow, not cross-flow. This means the filtration unit, acting either stand-alone or incorporated as part of existing treatment systems, has low energy requirements and can be scaled to suit individual treatment capacity needs without requiring the expensive consumables incurred using membrane filtration.
Figure 1
Filtration (0.5-20 µm) is achieved through controlled compression of the fibre bundle, with particles retained in the interstitial gaps between the fibres. The process is carried out parallel to (in line with) the influent flow.
Overall filtration efficiency is a function of fibre material, fibre size and compression pressure. A preliminary assessment of each specific application is recommended to optimise the system. Typical performance results in final effluent suspended solids in the low-mg/l range, excluding 0.5-20 µm particles, and achieving >90% removal of 1µm or larger particles. This is an order of magnitude better than granular media filters; Fibra technology conveniently covers a range between conventional filtration and membrane filtration.
Filter unit manifolds are manufactured for a fully scalable system. For larger units with a rectangular section filter bed, compression is normally achieved by mechanical means rather than by an inflatable bladder. Filter units can be supplied mounted on a robust frame together with controls and instrumentation, or are alternatively readily fitted within existing treatment plant housing. The fibres and housing are robust and safe for a wide range of effluents, being chemically inert and suitable for use at either low or high acidities (pH 2 to 13).
Figure 2
The Fibra automated unit, here a frame-mounted single 100mm filter unit together with instrumentation and controls
The Fibra unit is cleaned of retained particles by periodically relaxing the fibre bundle compression pressure and flushing retained particles to waste or for recirculation. Typically the forward flow of feed effluent is increased and flushes the unit for 5 to 10 seconds. Fibre compression is then reapplied, flow is re-diverted to treated effluent and the treatment process is resumed. There is no need to use treated effluent for flushing, and the flushing process can be fully automated, initiated at timed intervals or by pressure differential measurements. The interval between flushing events depends upon influent characteristics, but can be up to several hours since fibre bundles have a high load capacity. Occasional manual cleaning of fibre bundles will be required, taking about 30 minutes every 1-2 months (depending upon influent loads); special fibre cleaning and maintenance kits are available for this purpose.
The Fibra unit is more than a filtration device;, it is also a coalescer. Droplets are captured on the fibre bundle, coalesce, and form a separate phase which is drawn off periodically. Removal efficiencies of up to 90% are achieved, meaning that many treatment requirements are achieved in a single pass.
A third process, gas contacting, can be incorporated into the treatment process within the single Fibra unit with the fibre bundle acting as a contactor bed. Compressed gas, e.g. carbon dioxide or oxygen, is injected into the unit along with the influent, forming micro-bubbles on the fibre contactor bed (Air at atmospheric pressure may also be used). Under compression increased partial pressures in the gas improve molecular exchange efficiencies, and keep micro-bubble dissipation rates low. Gas contacting can effect effluent pH adjustment (CO2), COD reduction (O2) and humic discoloration, amongst other uses. Utilising only compressed gas cylinders, this approach removes the need for expensive specialised chemical storage, bunding, dosing equipment and associated operator PPE. Compressed gas (including cylinder rental) costs as little as 25% of alternative dosing chemicals. Gas mixing, absorption and reaction is highly efficient, for example consuming 10% of the carbon dioxide required for less efficient pH reduction systems. The efficient mixing characteristics of the Fibra process are advantageous for chemical dosing should this be important in specific applications. Also, gas contacting can be used for bubble size control to improve efficiency in physical processes such as flotation. In recognition of the success and potential of this revolutionary technology Fibra has attracted substantial investment in the company from a Shell Technology Ventures fund
Useful Links
Kenda Capital website: www.kendacapital.com
Fibra website: www.fibrasolutions.com
About the Author
David Griffiths is Managing Director at Fibra Limited. Contact via their website.