Articles

An excess of sludge in a sewage treatment plant is not a new problem. For a long time, "end of pipe" techniques were the only way of restricting the production of sludge. Recently new techniques have been developed which reduce sludge at source, in the sewage plant itself. One of these techniques – reducing biomass using ozone – has gradually emerged from its pilot phase to become a practical solution. But it does require close attention in order to preserve the fragile biological processes.

Over the last few years, several applications of ozone have become common practice. Examples are the treatment of drinking water and water in cooling systems, the bleaching of cellulose, and decolorizing. Using ozone to reduce high levels of chemical oxygen demand (COD) is only economic in certain very specific circumstances. Using ozone to reduce biomass on sewage stations is a relatively new technique. Traditionally, wet oxidation and more efficient dehydration methods have been used to reduce sludge. All these solutions treat the excess sludge at the end of the treatment cycle. Ozone technology aims to act at source: the formation of excess sludge is simply prevented.

The technique relies on the highly oxidising properties of ozone. The principle is simple. Part of the return flow of the thickened sludge is extracted in the sewage station and treated with ozone in a reactor tank according to specific conditions. The treatment is designed to reduce the vitality of bacteria in the flow. The weakened micro-organisms are then pumped back into the anaerobic basins, where they can be metabolised by the more powerful bacteria. The secret of success with this technique lies in ensuring the correct doses of ozone are applied. Jean Parmentier, of Air Liquide Europe, says that his company calls them "homeopathic doses of ozone. The action of the ozone on the biomass is only a secondary process. The real work is done by the active sludge in the reactor which takes care of the decomposition of the freed cellular matter."

Source: European Environmental Press (EEP)