It is well understood that natural wetland systems have a dramatic effect upon the quality of water as it passes through them. Particulate matter that is suspended in the water column, dissolved organic nutrients, coliform bacteria and even industrial and agricultural chemical pollutants are all significantly reduced and their subsequent impact in the 'down-stream' receiving waters is ameliorated by passage through a wetland system. This cleaning process continues to work throughout the year in both tropical and temperate regions.

Over the last twenty years, this natural water treatment process that is exhibited in wetlands has prompted considerable interest and research. Initially, research centred upon the combined physical, chemical and bio-chemical processes operating within the wetland system. Eventually applied researchers began to examine the practical application of constructing wetlands as wastewater treatment systems.

Over the last ten years the concept of integrating constructed utility wetlands into the water treatment provision of a county or region has become widespread - with examples existing across Europe, the United States, Canada and Australia.

There are a number of advantages in using wetlands in this way, most notably:

1. no chemical or additives are used in the treatment process the construction, maintenance and running costs are much reduced over conventional water treatment plants.
2. even the smallest constructed wetland, will have a positive impact upon wildlife and the conservation goals of an area.
3. the aesthetic value is considerably greater than conventional wastewater treatment systems.
4. any constructed wetland will have a positive impact upon the hydrological aspects of waste processed in its locality. Water retention, absorption and ground water recharge are all improved by the construction and maintenance of wetland systems.

Aquaculture waste from land based farms lends itself well to treatment in constructed wetlands, and fresh water systems have been used successfully in the US and Canada. However, their use is not widespread and the relatively large demands upon space that this technology requires is probably the reason why their use is, as yet, restricted.

Design is, of course, of critical importance, particularly when space is at a premium. Constructed wetlands have to be developed to maximise the water retention time , oxygen transmission through the substrate to produce both aerobic and anaerobic conditions, and suitable growing conditions for the chosen combination of aquatic plants and algae.

Surface flow, vertical flow, reciprocating and most recently recirculating systems have all been developed to reduce the amount of space required and to increase the effectiveness of waste treatment. No single planting plan or hydrological system has been found to treat all of the various pollutants equally well; there is no single 'one stop design', which will suit all situations.

During the AquaEtreat programme, we will be examining and developing a range of different wetland and algal ponds to maximise the treatment processes, by adapting the known design criteria which work successfully in standard waste treatment systems to the range of aquaculture operations represented in the programme.