Process & Production: Breaking the ice for cleaner pipes

A method that utilises a mass of crushed ice particles to displace debris could result in crystal-clear drinking water

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A technique developed to help clean food processing equipment effectively could lead to a major change in the way that drinking-water pipes are maintained. It uses a material that sweeps away debris and sediment without the need for harsh chemicals that require further processing.

In addition, it is simple to dispose of and is completely hazard free. What is this wonder material? Ice.

Cleaning out pipework is known in the industry as pigging and the ‘ice pig’ came about after Joe Quarini, a professor of process engineering at Bristol University, decided to turn his attention to the problem of cleaning in the food industry. ‘I went around a lot of food factories and, absolutely everywhere, cleaning was the problem,’ he said. ‘Let’s say you have a factory producing some expensive creamy substance. Every time you clean it, first of all you lose any of the product that’s in the equipment – and that’s bad news. Then you have the problem of the biological oxygen demand of the residue and you have to use chemicals such as hypochlorite or paracetic acid to clean it out, or the shelf life of the next batch is reduced.’

Quarini thought, rather than flushing all the equipment through with water, the best method would be to send something through that could displace the material inside the plant. ‘The more I thought about it, the more I realised that they were actually doing the right thing in using water, but it just flowed through and didn’t clean efficiently,’ he said. ‘So what if I stiffened up the water? That’s how the ice pig was born.’

The ice pig is actually a mass of crushed ice that is pumped into the piping. Ice particles are abrasive and so they clean the inside of the pipe as they go. Crushed ice is easy to pump and is self-lubricating; it behaves like a semi-solid, penetrating and filling the most complex of topologies. It cannot get stuck and when it has done its work it melts into water.

‘It’s not quite as simple as just crushing ice and pumping it in,’ said Quarini. His team used commercial ice makers that freeze water into a slurry and stir it as it forms, keeping the ice crystals small. They also use a freezing-point depressant, which is necessary to stop the crystals freezing together and forming into a solid plug. ‘We started off using sugars, because we were looking at the food industry, but we’ve tended to come down on the option of common salt,’ said Quarini. ‘But you tailor it to the application – if they don’t want salt in the system, we can use sorbitol, which we developed for cleaning a toothpaste plant. You could also use alcohol; we’re looking at that to develop a beer pig.’

He also looked into the delivery system. ‘It’s easy to push the ice slurry once it’s in the pipe, but getting it there can be a problem,’ added Quarini. ‘You can’t suck it through, because you tend to pull the lubricating liquid from the ice slurry.’ There is also a problem with a phenomenon known as Ostwald ripening, when small ice crystals weld themselves together to form bigger ones. ‘The material ages and you want it to be young and fresh, or you have to sort out how to rejuvenate it; we have to resort to mechanical crushing, which is quite difficult to do with 10 tonnes of ice,’ he said.

While developing the technique, Quarini made contact with the research and development department of Bristol Water, which applied for funding from OFWAT to develop the technique for the water industry. ‘We thought it could clean up the problem of the discolouration of drinking water by getting rid of the sediment in the pipes,’ he said. ‘You can never mess up because the pig just melts; it’s as environmentally friendly as you can imagine.’

Initial tests used small ice makers and glass piping. ‘We’d put sand in the pipe, see if we could get it to move using water at different velocities, then we’d put an ice pig in and get rid of it,’ added Quarini. ‘Then we moved on to using about half a tonne of ice down real 3in [7.6cm] water pipes.’

“You can never mess up, because the pig just melts. It is very eco-friendly”

Bristol Water’s engineers then designed a full-scale delivery system based on a scaled-up version of the university’s apparatus, consisting of a large stirred ice maker and pump on the back of an articulated lorry, and set about testing the technique on real water pipes. ‘Bristol Water had a team of six people, working at night,’ said Quarini. ‘We did the work around 02:00, as there’s a much lower demand for water. The team valved off the pipe sections so it could clean them and we would be taking samples to see how much sediment it had got out.’

The project wouldn’t have been possible without Bristol Water’s involvement, he added. ‘The collaboration has been extremely productive and successful and certainly ice pigging wouldn’t be where it is in the water sector without Bristol Water’s involvement.’

The results of the tests, which are still ongoing, are encouraging. Committed through the grant from OFWAT to cleaning 30km of live trunk mains between 2005 and 2010, the university/utility team has found that ice pigging is more efficient than water flushing – the conventional technique for reducing water discolouration. ‘We get cleaning capacities typically 100 to 1,000 times better then flushing with water,’ said Quarini. ‘That is, we use 100 to 1,000 times less water to get the same degree of cleanliness.’

The technique uses hydrants or other inlets to get the ice into the pipes, which is convenient in towns and cities; out in the countryside, however, extra hydrants would need to be installed. ‘But that’s a very simple operation; it takes about half a morning and only costs about £300 for each hydrant,’ Quarini added.

Nick Haskins, strategic operations manager of Bristol Water, is an ice-pigging enthusiast. ‘It fills a niche that hasn’t been addressed before,’ he said. ‘We’ve recently been out to Barcelona to demonstrate it to AgBar – the Barcelona water utility that owns Bristol Water – and it is keen on rolling it out in Spain and its other subsidiaries.’

While Bristol Water holds the licensing rights for ice pigging in the water sector, Bristol University can exploit it elsewhere. Quarini is in talks with the nuclear industry, where he started his career, to develop the technique for use in decommissioning, where the ice would sweep away grouting and metal components in piping. The careful manipulation of particle size and the freshness of the ice allow the technique to be used on tubes down to 0.7mm in diameter, which could open it up to the medical market.

However, Quarini believes that the biggest market is still likely to be the food industry. ‘The stuff it displaces from the pipes is good, wholesome product that hasn’t been diluted and there’s less effluent,’ he said.