Recirculating dosed system

This system is based upon continuous water circulation through mechanical and biological filters (biofilter), thus providing continuous removal of solid and nitrogenous wastes. This assures low levels of ammonia and nitrite and avoids the daily peaks that occur in open and static closed systems.Furthermore, this system does not involve major water replacement, and water quality conditions are stable enough to ensure good conditions for the larvae.

The dynamic closed system used in M. rosenbergii hatcheries includes the following components: larval rearing tanks, mechanical and biofilters and, sometimes, disinfection through ultraviolet (UV) lights or ozone (Fig. 5.1). System efficiency is dependent upon filter and tank shape and size, type of biofilter media, and rate and pattern of water circulation, among others. Culture systems can have individual biofilters or one common biofilter for all larval rearing tanks. The latter is more vulnerable to contamination of the whole system in case of infection in one tank.

The rate of water recirculation is fundamental for system efficiency. The biomass of nitrifying bacteria present in the biofilter depends on the total amount of ammonia entering the biofilter, instead of its concentration in the water (Brune & Gunther 1981). Therefore, higher flow rate through the filter introduces more ammonia per unit of time and increases the efficiency of the nitrification pro cess (Kaiser &Wheaton 1983). Valenti et al. (1998) recommended a high rate of recirculation to reduce the biofilter size. According to these workers, the total water volume in the larval rearing tanks should circulate through the filter 10 to 24 times daily (recirculation rate of 1000 to 2400% daily). Aquacop (1983) and Griessinger et al. (1989) developed systems with recirculation rates of approximately 2400 and 1200% daily, respectively. However, many systems often cited in the literature have lower rates, which vary from 140 to 500% daily (Sandifer & Smith 1975; Singholka & Sukapunt 1982; Ong 1983; Castro et al. 1988; Chowdhury etal. 1993).

A variation of the traditional recirculating system, called the static closed system, was developed in Brazil in 1984 and has been used by inland hatcheries (Valenti 1992, 1993, 1996, 2007). Tanks and general management protocols are similar to those employed in open systems. However, daily removed water from larval rearing tanks is transferred to other tanks for processing and then to a biological filter instead of being discarded. After the nitrification process is completed, the water is returned to the rearing tanks. Carvalho & Mathias (1998) described the static closed system in detail. Half of the rearing tank water is exchanged daily. Water removed from the larval tank is clarified by filtering it through a mesh screen (5 |im). It is then chlorinated (2.5 mg/L active chlorine) for 40 minutes in the treatment tank. The residual chlorine is removed with sodium thio-sulphate (Na2S2O3). After filtering the water again through a mesh screen (1 |im), the water is transferred to a large biological filter where it is circulated through the substrate for 2 to 3 days to complete the nitrifying process. The water is then returned to the larval rearing tanks. The system is composed of three biological filters, which are used on consecutive days and always in the same order. Each filter has a volume capacity equivalent to the total volume of the larval rearing tanks of the hatchery.

The same problems described by Valenti (1996) for flow-through systems occur in the static closed system. In addition, continuous chlorine application can cause excessive chloramine in the system, which is highly toxic to M. rosen-bergii larvae. Chloramine is a stable compound formed through the reaction of chlorine and ammonia, and is not removed by aeration (Noga 1995). Continuous addition of thiosulphate to de-chlorinate may also lead to stress and mortality since the 'thio'-groups in sodium thiosulphate are used as insecticides to kill other arthropods.

In general, recirculating systems are believed to be the best systems for M. rosenbergii hatcheries. They operate at low cost, save water, have less environmental impact and provide higher productivity. A comprehensive and useful text covering all aspects of recirculating systems in aquaculture is provided in Timmons et al. (2002).

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