Degassing, aeration and stripping
Before the water runs back to the fish tanks the accumulated gases must be removed. This degassing process is carried out by aeration of the water, and the method is often referred to as stripping. The water contains carbon dioxide from the fish respiration and from the bacteria in the biofilter in the highest concentrations, but free nitrogen (N2) is also present. Accumulation of carbon dioxide and nitrogen gas levels will have detrimental effects on fish welfare and growth. Under anaerobic conditions hydrogen sulphide can be produced, especially in saltwater systems. This gas is extremely toxic to fish, even in low concentrations, and fish will be killed if the hydrogen sulphide is generated in the system.
Aeration can be accomplished by pumping air into the water whereby the turbulent contact between the air bubbles and the water drives out the gases. This underwater aeration makes it possible to move the water at the same time, for example if an aeration well system is used.
The aeration well system is however not as efficient for removing gases as the trickling filter system. In the trickling system gases are stripped off by physical contact between the water and plastic media stacked in a column. Water is led to the top of the filter over a distribution plate with holes, and flushed down through the plastic media to maximise turbulence and contact, the so called stripping process. The trickling filter is often referred to as a CO2 –stripper.
The aeration process of the water will add some oxygen to the water through simple exchange between the gases in the water and the gases in the air depending on the saturation of the oxygen in the water. The equilibrium of oxygen in water is 100% saturation. When the water has been through the fish tanks, the oxygen content has been lowered, typically down to 70%, and the content is reduced further in the biofilter. Aeration of this water will typically bring the saturation up to around 90%, in some systems 100% can be reached. Oxygen saturation higher than 100% in the inlet water is however often preferred in order to have sufficient oxygen available for a high and stable fish growth. Higher saturation levels call for an oxygenation system using pure oxygen. Pure oxygen is often delivered in tanks in the form of liquid oxygen, but can also be produced on the farm in an oxygen generator. There are several ways of making super-saturated water with oxygen contents reaching 200-300 %. Typically oxygen cones or deep shafts are used. The principle is the same. Water and pure oxygen are mixed under pressure whereby the oxygen is forced into the water. In the oxygen cone the pressure is accomplished with a pump creating a pressure of typically around 1.4 bar in the cone. Pumping water under pressure into the oxygen cone consumes a lot of electricity. In the deep shaft the pressure is reached by digging a pipe loop down to for example 6 metres depth, and injecting the oxygen at the bottom of the loop. The pressure from the water column above, in this case 0.6 bar, will force the oxygen into the water. The advantage of the deep shaft is that pumping costs are low, but the installation is troublesome and more expensive.
UV disinfection works by applying light in wavelengths that destroy DNA in biological organisms. In aquaculture pathogenic bacteria and one-celled organisms are targeted. The treatment has been used for medical purposes for decades and does not impact the fish as UV treatment of the water is applied out of the fish production area. It is important to understand that bacteria grow so rapidly in organic matter that controlling bacterial numbers in traditional fish farms has limited effect. The best control is achieved when effective mechanical filtration is combined with a thorough biofiltration to effectively remove organic matter from the process water, thus making the UV radiation work efficiently.
“The UV dose can be expressed in several different units. One of the most widely used is micro Wattseconds per cm2 (μWs/cm2). The efficiency depends on the size and species of the target organisms and the turbidity of the water. In order to control bacteria and viruses the water needs to be treated with roughly 2,000 to 10,000 μWs/cm2 to kill 90% of the organisms, fungi will need 10,000 to 100,000 and small parasites 50,000 to 200,000 μWs/cm2.” UV lighting used in aquaculture must work under water to give maximum efficiency, lamps fitted outside the water will have little or no effect because of water surface reflection.
Today, ozone (O3) is seldom used in fish production itself as the effect of over-dosing can cause severe injury to the fish. In fish farms placed inside buildings ozone can also be harmful to the people working in the area as they may inhale too much ozone.
However ozone treatment is an efficient way of destroying unwanted organisms by the heavy oxidation of organic matter and biological organisms. Ozone treatment can be preferred when the intake water to a recirculation system needs to be disinfected. In many cases, however, UV treatment is a good and safe alternative.
The nitrifying process in the biofilter produces acid and the pH level will fall. In order to keep a stable pH a base must be added to the water. In some systems a lime mixing station is installed dripping limewater into the system and thereby stabilizing pH. An automatic dosage system regulated by a pH-meter with a feedback impulse to a dosage pump is another option. With this system it is preferable to use sodium hydroxide (NaOH) as it is easy to handle making the system easier to maintain. Anyone handling acids or bases must be careful as it can severely burn eyes and skin. Safety precautions must be taken, and glasses and gloves must be worn while handling the chemicals.
Maintaining an optimal water temperature in the culture system is most important as the growth rate of the fish is directly related to the water temperature. Using the intake water is a fairly simple way of regulating the temperature from day to day. In a closed recirculation system inside an insulated building the heat will slowly build up in the water, because energy in the form of heat is released from the fish metabolism and the bacterial activity in the biofilter. Heat from friction in the pumps and the use of other installations will also accumulate. High temperatures in the system are therefore often a problem in an intensive recirculation system. By adjusting the amount of cool fresh intake water into the system, the temperature can be regulated in a simple way.
In the wintertime in cold climates simple heating using an oil boiler connected to a heat exchanger to heat up the recirculated water is most often sufficient. The use of energy for this kind of heating depends mostly on the amount of cool intake water used and its temperature, although some heat also escapes from the building. In some cases, a heat recovery system, consisting of a titanium plate exchanger, can also be installed. The process water in the recirculation system is used to heat up (or cool down) the intake water by passing the water through the plate exchanger. The system is regulated by the use of a water temperature sensor connected to a temperature control unit that regulates the function of the titanium plate exchanger.
Different types of pumps are used for circulating the process water in the system. Pumping requires electricity, and low lifting heights and efficient and correctly installed pumps are important to keep running costs at a minimum.
The lifting of water should preferably occur only once for every recirculation cycle, whereby the water runs by gravity all the way through the system back to the pump sump. Pumps are most often positioned in front of the biofilter system and the degasser as the water preparation process starts here. In any case, pumps should be placed after the mechanical filtration to avoid breaking the solids coming from the fish tanks.
Calculation of the total lifting height for pumping is the sum of the actual lifting height and the pressure losses in pipe runs, pipe bends and other fittings. This is also called the dynamic head. If water is pumped through a submerged biofilter before falling down through the degasser, a counter pressure from the biofilter will also have to be accounted for. Details on fluid mechanics and pumps are beyond the scope of this guide.
The total lifting height in most systems today is less than 2 metres, which makes the use of low pressure pumps most efficient. However, the process of dissolving pure oxygen into the process water requires centrifugal pumps as these pumps are able to create the required high pressure in the cone.
In some systems, the water is driven by blowing air into aeration wells. In these systems the degassing and the movement of water are accomplished in one process, which makes low lifting heights possible. The efficiency of degassing and moving of water is however not necessarily better than that of pumping water up over the degasser, because the efficiency of aeration wells in terms of using energy and the degassing efficiency is lower than using lifting pumps and stripping or trickling the water.
Figure 2.18 An oxygen probe (Oxyguard) is calibrated in the air before being lowered into the water for on-line measurement of the oxygen content of the water. Surveillance can be computerized with a large number of measuring points and alarm control.
Monitoring, control and alarms
Intensive fish farming requires close monitoring and control of the production in order to maintain optimal conditions for the fish at all times. Technical failures can easily result in substantial losses, and alarms are vital installations for securing the operation.
In many modern farms, a central control system can monitor and control oxygen levels, temperature, pH, water levels and motor functions. If any of the parameters moves out of the preset hysteresis values, a start/stop process will try to solve the problem. If the problem is not solved automatically, an alarm will start. Automatic feeding can also be an integrated part of the central control system. This allows the timing of the feeding to be coordinated precisely with a higher dosage of oxygen as the oxygen consumption rises during feeding. In less sophisticated systems, the monitoring and control is not fully automatic, and personnel will have to make several manual adjustments.
Whatever the case, no system will work without the surveillance of the personnel working on the farm. The control system must therefore be fitted with an alarm system, which will call the personnel if any major failures are about to occur. A reaction time of less than 20 minutes is recommended, even in situations where automatic back-up systems are installed.
The use of pure oxygen as a back-up is the number one safety precaution. The installation is simple, and consists of a holding tank for pure oxygen and a distribution system with diffusers fitted in all tanks. If the electricity supply fails a magnetic valve pulls back and pressurized oxygen flows to each tank keeping the fish alive. To back up the electrical supply, a generator is necessary. In many cases the toxic ammonia will build up in the system when the water is not circulating. This problem will be the next to overcome after the oxygen availability has been solved by the oxygen back-up system. It is therefore important to get the water flow up and running within an hour or so.
Water used for recirculation should preferably be from a disease-free source or sterilised before going into the system. In most cases it is better to use water from a borehole, a well, or something similar than to use water coming directly from a river, lake or the sea. If a treatment system for intake water needs to be installed, it will typically consist of a sandfilter for microfiltration and a UV or ozone system for disinfection.