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Screen the stink - properly

Water Technology

A house next to a sewage treatment works no longer need come with a complimentary clothes peg. Alan Sparks took in a lung full of a new filtering system that could consign the stench to history.

Large, obtrusive and notoriously smelly is how most people justifiably think of sewage treatment works. In sensitive areas, where high quality outflow is required, building such a plant can be counterproductive. Today though, a spanking new tungsten driver has landed in the golf bag of solutions available to developers.

Conventional civils schemes next to housing are no longer acceptable in modern society. But for smaller schemes, where exceptionally high quality is needed and the site footprint might be restricted, membrane bio-reactors (MBR) offer a practical solution.

MBR systems use plastic panels each holding a filter screen, arranged like toast in a rack. This block of panels is submerged in a tank and, when aerated sewage is pushed through them, clean water is filtered out.

High technology obviously carries an extra cost, so MBR systems are unable to compete with conventional civils systems in their own backyard - ie large plants producing outflow of good water quality. But UK specialist Aquator sees its market in smaller scale developments where high quality water is vital.

A recently commissioned plant at Picnic Bay on Magnetic Island in Australia opted for the MBR system. 'As it was flowing out into the Great Barrier Reef - a world heritage site - guaranteed quality was imperative, ' says Aquator director, Ross Severn.

Another big market for Aquator is Ireland, where protecting the salmon stocks and maintaining a green image is key to attracting the tourist Euro. 'What we are able to achieve, in contrast to conventional plants, is the ability to release water which is purer than the watercourses it is flowing into, ' says severn The efficiency of the MBR system means that more sewage, five times more concentrated than normal, can be treated - basically cutting out two stages from a conventional process. This is where the land savings can be made, and as there is no need for an open air tank there are no nasty whiffs to offend the neighbours.

'Waste produced is typically 30% less than alternative schemes.

which is one of the reasons why the Royal Navy uses MBRs on its type 23 frigates, ' says Severn. 'The potential of then using the treated water for non-potable uses within the ship rather than dumping it in the sea is another attractive feature.'

Aquator proposals manager Alan Ford explains how these land savings can turn potential schemes into reality. 'At a 28,000 population scheme at Swanage in Dorset, a conventional plant would need a 4.5ha area, while the MBR system needed just 0.7ha.'

Another notable scheme at Porlock, Somerset, flows directly into Exmoor National Park, so again conventional plants were unacceptable. Installed five years ago, only seven membranes out of 3,600 have had to be replaced.

'Our experience at Porlock has led us to today offer a ten year guarantee, ' says Severn proudly.

The MBR is ideally suited to the removal of organic contaminants, with applications fitted to food, dairy, tannery and chemical production facilities. So far there are just 23 operational plants in the UK, yet over 800 exist worldwide - with arid regions particularly taken with the MBR's ability to re-use the outflow. 'This is because the system was not marketed here, but that is all about to change, ' says Severn.

Aquator was born out of a management buyout of Wessex Water's MBR technical division last year and with this new found freedom is actively spreading the gospel about the option today open to developers.

INFOPLUS

On the rack Membrane bio-reactors use sewage tanks similar to those already used in treatment works. Each panel is 1m high, 400mm wide and 7mm thick.

These comprise an impermeable plastic sheet with a polymer membrane material ultrasonically welded to it. A field of 150 panels with a top box has dimensions of 2.4m long, 2m tall and 600mm wide.

'This 'rack of toast' is then submerged in a tank of mixed liquor suspended solids with 1.5m head of fluid, ' explains Aquator proposals manager, Alan Ford. 'This pressure drives the mix through the wicking layer - the gap between the plastic sheet and the membrane.'

The membrane has a pore size of 0.4 microns, which when you allow for the dynamic cellular protein layer that forms on the surface leaves a pore of 0.1 microns able to cut out viruses.

To feed the bacteria is an aeration system that will drive the air/sludge mix at 0.5m 3/sacross and upward through the panels. This also keeps the solids in suspension, preventing the sludge from caking the panels. A discharge pipe above each panel then removes the clean water that has been pushed through the membrane.

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