Partial Reuse and Recirculating Aquaculture Systems incorporate many of the same system components. Recirculation systems however typically include additional treatment processes to maintain the same high quality of water at higher rates of water reuse.
Although raceway culture vessels may be used in flow-through aquaculture systems, circular culture tanks are more desirable for use in Partial Reuse and Recirculating Aquaculture Systems. The main disadvantages of raceways include the longitudinal degradation in water quality and poor mixing of water due to plug flow operation, and the poor solids removal due to slow flow velocities. Circular culture tanks with rotational flows and dual drain systems provide superior mixing for optimum use of the culture volume, and rapid concentration and removal of solid wastes which is necessary for good water quality when water is reused.
In all aquaculture systems, the rapid and continuous removal of solid wastes is necessary to prevent the accumulation and degradation of these solids within the system which will increase concentrations of carbon dioxide, ammonia, and fine particulate while consuming valuable oxygen. As water reuse rates increase, solids removal becomes increasingly important because there is more opportunity for the solids to degrade. Gravity separation technologies such as swirl separators or radial flow separators, and filtration technologies such as drum filters are integral to maintain water quality in recirculating systems. Many partial reuse systems rely on flushing for solids removal instead and do not require treatment. In this case, reuse rates may need to be limited to maintain the concentrations of particulate below the acceptable limit.
In recirculating systems, additional treatments may be required to prevent the accumulation of fine suspended and dissolved solids that can not be removed through gravity separation or filtration processes. Oxidation with ozone can be an effective method of controlling of dissolved solids, color and odour. Foam fractionators or protein skimmers are especially effective in salt water systems but may also be effective in freshwater when coupled with ozone injection.
Ammonia (NH3) is a toxic end product of both fish metabolism and the bacterial consumption of waste organic solids. In Flow-through or Partial Reuse Aquaculture Systems, ammonia is flushed from the system before concentrations build to toxic levels. However, in Recirculating Aquaculture Systems, ammonia must be removed through the biofiltration treatment process. Biofiltration uses the metabolic activity of bacteria, cultured on a media substrate, to convert toxic ammonia-nitrogen to less toxic forms using a process called nitrification. Many biofiltration technologies, using different media types, exist. Fluidized sand bed biofilters use inexpensive sand as the media and provides much more bacteria growing surface per unit volume than other media types.
In fish culture systems, both fish respiration and bacterial consumption of wastes will deplete dissolved oxygen and produces carbon dioxide. Carbon dioxide removal and oxygen addition are integral treatment components of both Partial Reuse and Recirculating Aquaculture Systems. The PR Aqua forced ventilated CO2 Stripper combines both aeration with enhanced CO2 removal performance. Post aeration, the Low Head Oxygenator (LHO™) can effectively and efficiently boost dissolved oxygen concentrations to above saturation levels without raising total gas pressure. PR Aqua has combined these two unit processes in one small footprint in the OxyTower™. This unit is ideal as part of a recirculation treatment process, or as a stand-alone treatment solution for either centralized or tank-by-tank partial reuse systems. Supplementation of dissolved oxygen concentrations may also be achieved using oxygen cones, oxygen saturators, or diffusers.
Economies of scale can push water treatment system design from multiple isolated tank systems into a single centralized water treatment package. When blended water from multiple tanks is to be redistributed back to multiple tanks, there is a significant increase in the risk of pathogen transfer between tanks. To mitigate this risk, most multi-tank systems incorporate a disinfection system, typically ozone addition or UV disinfection or both.
Monitoring systems are important components of a modern Partial Reuse and Recirculating Aquaculture System. Multifunction monitoring systems can provide both instantaneous measurements and trending of key water quality parameters, providing operators with the information they need to maintain optimum conditions for fish health and growth. Integrated alarms can warn operators before failures occur and can automatically implement backup systems such as oxygenation diffuser systems to mitigate impacts of equipment failure.