The development of a series of three purification tanks of standard design and their use with artificial seawater

In England and Wales purification tanks are built to comply with broad operating criteria specified by the Ministry of Agriculture Fisheries and Food. These have been developed over many years with limited availability of data and has resulted in design restrictions which have limited the economic viability and effectiveness of the tanks. The approved method for the large scale purification of mussels has been to spread them in a single shallow layer in large outdoor tanks with access to a supply of clean sea water. This results in a purification plant that requires a lot of land, is labour intensive and generally remote from the harvesting area. In addition such tanks are exposed to ambient conditions and can result in filtration activity, upon which purification depends, being reduced or even prevented by wide fluctuations in water temperatures. To overcome these problems Seafish have developed a modular tank design of 1500 kg capacity incorporating multi-layer stacking of mussels and the re-use of artificial sea water. This tank has a reduced land and labour requirement and enables a purification plant to be housed in a building, offering much improved control of environmental and hygiene conditions. The use of artificial sea water is not normally considered for large scale mussel purification, but with this development it is possible to extend its re-use over a period of one month. The purification plant can, therefore, be sited near to the mussel supply, reducing transport and handling costs and undesirable delay. Following this successful development Seafish went on to develop two further modular designs for the small processor who may also wish to purify other bivalve molluscs such as oysters and clams. One incorporates the same multi-layer stacking design in a much smaller tank with a capacity of 750 kg of mussels. The other uses the concept of mounting boxes one above the other in a frame and cascading water down through them. In this system the boxes have been carefully designed to maintain uniform water flow and has a capacity of 2000 oysters. Both systems use artificial sea water. The development of these modular tanks has enable Seafish to investigate and re-appraise the design and operating criteria for the purification of both mussels and oysters, but the work has not been limited to tank design alone. If molluscs are damaged or in physical stock then the effectiveness of purification and subsequent storage life may be significantly reduced. Trials with mussels have been carried out to develop new equipment to separate and sort at sea and also to make use of boxes, compatible with the purification tanks.