In the past few years, legislation, increased awareness of the cost factor by the public and private sector and technological progress in compact water treatment plants have enabled the development and decentralisation of waste water treatment.

Decentralised plants are however subject to stricter conditions than large water treatment plants since they are more affected by the following factors:

• Hydraulic load surges
  Large swings in the amount of waste water with daily peaks and the low buffering volume of the plant lead to surges in the hydraulic loads;
  
• Pollutant loads
  As far as the load of pollutants is concerned, compact water treatment plants must be able to cope with non-heterogeneous maximum loads and wide variations in the loading rate;
  
• Under-loading operation
  As well as maximum loads, decentralised plants must be able to cope with periods where there is little or no load (school holidays, seasonal variations etc).

Henceforward, the proper operation of a large number of compact and decentralised water treatment plants is proof that reliable and simple treatment of domestic or municipal waste water is possible using modern equipment thanks to advances in technology, equipment and processes.

It is precisely in the especially difficult conditions created by decentralised waste water treatment that efficient use can be made of the high-performing properties of submerged aerobic fixed culture.

25 years' experience with over 250,000 plants in operation in Europe of which 40,000 are in Belgium.