Spheres for clears

The UK water industry is backing research into a potential new low-energy water purification technique that uses floating glass beads.

Research starting next month at Cranfield University will investigate a concept patented by its inventor Barrie Woodbridge, in which the beads are incorporated into a floc — a clump of solids formed during the purification process.

The floating spheres themselves are of a type currently used in a number of industrial and engineering applications, such as forming lightweight solid structures when mixed with a resin.

During conventional water treatment processes, raw water enters the front end of the treatment plant and a coagulant chemical, such as ferric sulphate or aluminium salt, is added to destabilise the mixture.

This causes the formation of a solid particle that aggregates and enlarges, which is then removed using either a sedimentation or a flotation system.

Certain types of low-density floc aggregate lend themselves more readily to flotation. One of the common ways of removing these particles is by introducing micro-bubbles using a compressor/ saturator system to dissolve air into water at high pressure.

This is then released into the tank at normal pressure, forming micro-bubbles. These attach to the floc aggregate, floating it to the top of the tank and allowing the solids to be removed.

Dr Peter Jarvis, academic fellow in sustainable systems at Cranfield, explained: ‘Our concept is to get rid of the bubble generation system and incorporate these floating spheres into the floc instead.

‘The proposed method will replace the need for any saturator system in a dissolved air flotation type of solid/liquid separation process by replacing the need for the micro-bubbles.’

The beads would be added at the coagulation stage where a floc aggregate is produced after being destabilised by the metal salts. These types of floc are usually very low-density, so lend themselves to floating very well to incorporate the beads. This makes their average density lower than that of water — so they float to the top of the tank.

‘We estimate that you could potentially get around 80-90 per cent energy saving overall, and we would recycle the beads that are incorporated in the floc back to the front end of the process,’ said Jarvis.

‘To do that you need some sort of solid separation system which would require a certain amount of energy, but much less than currently used in a saturator system.’

The researchers have so far carried out bench scale testing using the beads. By the end of the project, they aim to have proven the concept and created a full-scale demonstration continuous treatment system using the glass beads, which would be the first of its kind.

The three-year research project is sponsored by UK Water Utilities, the organisation representing the country’s water suppliers. ‘What’s attractive to the water industry is having a process that can be incorporated into existing treatment schemes without having to change them,’ said Jarvis.

Although the main applications for the system would be treating ground and surface water for drinking, Jarvis said it could eventually be used in a sewage system as well. ‘It gets a bit complicated as the solids concentrations are higher in sewage, so in the first instance we’re just looking at potable water production,’ he added.

Berenice Baker