Ohmic heater treats waste

A UK company has developed a small-scale heating system that can treat certain categories of animal by-products, prior to them being put through an anaerobic digester, to enable safe disposal.

C-Tech Innovation, based near Chester, has developed ohmic heating, a method of rapidly and uniformly heating electrically-conducting or moist materials. The company has so far used the technology in the food industry and in sewage treatment applications.

'It's a very simple electrical technology.' said C-Tech's project manager Dr Rachel James. 'Basically put any pumpable fluid between electrodes and pass a current between them — the material needs to be conductive so that a current can be passed through it. The resistance of the material gives you the heating mechanism,' added James.

The company identified the new application of the technology after consulting farmers.

Animal by-products are classified in three categories. Category one is high-risk material (for example, containing diseases such as BSE). Other diseased animals or those that have died of unidentified causes fall under the second category, and category three — considered as low risk — includes material such as food waste. There are stringent regulations regarding the treatment of each type.

'There is not really a way of getting category two animal by-products into anaerobic digester processors at the moment — it is something that is allowed under the animal by-product regulations but no one really does it.

'If we can treat it through an ohmic heater then we can put it through an anaerobic digestion facility,' said James.

Unlike category three by-products, which only have to be heated up to 70º C for an hour, stricter rules govern the treatment of category two materials.

'These have to be heated to a minimum of 133º C, which means it has got to be under pressure — so you have got three bar of pressure and high temperature,' said James.

Regulations state that the material has to be held at the high temperatures for 20 minutes, so C-Tech plans to make its system work as a batch operation, where a cylindrical vessel containing electrodes will automatically fill and empty.

'We would have a macerating pump at the front end which would pull the material through to the heater and then we can have a feed system that takes it from the heater through to the anaerobic digester,' said James.

According to James, one of the advantages of the heating technology is its efficiency.

'Ohmic heating generally has an advantage in that it is very efficient. Basically, the power you put in is basically the power that goes into heating it — it is 95 per cent efficient,' she claimed.

'It is also a very robust technology — because you are heating things up with electrodes rather than hot surfaces you can't get material burning on to surfaces as much. We've had these systems operating on site — although not for this particular application — for a couple of years with no maintenance needs,' said James.

Anh Nguyen