Incubation systems have traditionally been designed with single-pass, flow-through water use strategy however, recirculation systems for incubation are fundamentally simple. Water conservation and the economical control of water temperature are key motivations for considering the use of recirculation technology in incubation systems.
In the first part of this article, we identified some of the water quality criteria that should be considered in the design of any incubation system. This part reviews the available technologies and operational parameters considered necessary to successfully implement a water reuse strategy. Typically, incubation rooms should be quiet and, for biosecurity reasons, should see as little traffic as possible. Treatment equipment and pumps should be located in an isolated but adjacent mechanical room. The key processes to be considered in the design of a recirculation system for incubation are solids filtration, ammonia removal, disinfection, temperature control, and aeration.
Although very little particulate matter is generated within the system prior to hatching, the addition of solids filtration will protect against fine silt that may enter with the influent water, remove egg shells during and after the hatch, and provide prefiltration upstream of disinfection processes to optimize pathogen control. Typical filtration options include microscreen filters, media filters, or cartridge filters. Although they will all provide suitable treatment, these technologies differ in the operating pressure (and cost), the amount of backwash water consumed, and in maintenance requirements. Microscreen drum filters have a low operating cost, consume little backwash water, and backwash automatically without interrupting the process flow. Drum filters may be better than other filters because they can be used to treat the gravity-fed incubator discharge water prior to pumping, preventing the accumulation of egg shells in the pump reservoir. If shells do accumulate, it is important that there be a means of purging the reservoir to prevent the growth of fungus. Pressurized media filters have a higher operating cost and consume greater quantities of backwash water, but when the media is sized appropriately, they may provide better protection from fine silt. Pressurized cartridge filters are simple, effective barriers but require regular, manual cleaning to prevent clogging and reduced flows. If either media- or cartridge filters are used, it is important to have a second filter in parallel, plumbed so that water flow may be continued while one filter is being backwashed.
Ammonia Control In incubation systems, ammonia production prior to hatch will be minimal. However, immediately after hatch, ammonia excretion increases dramatically. Biofilters, which are commonly used in recirculating aquaculture systems, use cultured bacterial populations grown on a media substrate to remove ammonia and other waste products from the water. The effectiveness of a biofilter is dependant on having a suitable bacterial culture that takes time to develop under a consistent ammonia load. The low production of ammonia prior to hatch will likely result in there being insufficient bacteria within the biofilter to combat the rapid build-up of ammonia that occurs once eggs have hatched. Also, chemical treatments that may be used periodically to combat fungus or other pathogens within an incubation system may have a negative impact on the biofilter’s bacterial culture. Biofilters may not therefore be a suitable for controlling ammonia levels in a recirculating incubation system. In the absence of biofiltration, ammonia is best controlled through the exchange of system water, or in other words, by varying the recirculation rate. Prior to hatch, minimal water exchange is required to maintain water quality. After hatch, when solids and ammonia production are high, water exchange may be increased to flush the waste products from the system. Regular
testing of the water quality will help determine the maximum recirculation rate that may be maintained without compromising water quality. Typically, the amount of additional water required to account for the flushing is not large and the duration of use is relatively short.
In systems where water temperature is adjusted to delay or advance hatching, increased water exchange will make it more difficult to maintain the culture temperature. However, a more median temperature is often desirable during the hatching period so increased water use is often not an issue. If water consumption and/or temperature control are issues during the hatching period, adsorptive ammonia removal methods may be used. Zeolite is a microporous alumino-silicate mineral compound that has been
used with success in recirculating aquaculture systems to remove ammonia through ion exchange.
Biosecurity and pathogen control are critical to the successful management of an incubation system. Effective disinfection of the water supply is the first and most important step in disease prevention. However, disinfection technologies are also commonly incorporated into the recirculation system to reduce the risk of disease transmission between incubators as the water is recycled. Disinfection treatments commonly used include ultraviolet (UV) irradiation or the use of chemical oxidants such as ozone (O3) or hydrogen peroxide (H2O2). Regardless of the technology selected, identification of the target organisms is important to ensure that sufficient disinfection dosage is provided. Fungal infection on fish eggs is one of the more common and problematic issues encountered in incubation systems. The problem is exacerbated in recirculation systems where fungal outbreaks may proliferate and spread between incubators due to the shared water supply. UV treatment at relatively low dosages is effective at reducing, if not eliminating, Saprolegnia infestations on eggs within recirculating systems. It is safe, effective, and easily integrated, and is often the preferred method disinfecting incubation water. Ozone and hydrogen peroxide are also effective but, unlike UV, these products are dangerous to eggs and alevins at low residual concentrations, and also have inherent health and safety issues for operators.
Beyond the water treatment processes used, chemical treatments are also periodically used to combat Saprolegnia and other pathogens in incubation systems. However, these chemical therapeutants may circulate throughout the system for a long time. Care should be taken to avoid overdosing and to ensure that therapeutants are sufficiently diluted and flushed away at the end of the treatment cycle.
Control of water temperature at the incubation stage is a powerful management tool. By chilling or heating the water, egg development and hatching may be delayed or accelerated to meet specific production date requirements, or to develop multiple cohorts. Through recirculation of the water, temperature control is more stable, and achieved more economically.
Commercial water chillers are available as either air- or water-cooled units. Air-cooled units have a larger condenser which must be placed on an external wall or even outside to allow for sufficient ventilation. Water-cooled units are more compact and quieter, but will require a source of clean, cold water. Heating small recirculation systems may be achieved with electric immersion heaters, while larger systems will likely require a boiler and heat exchangers. Because the effluent water from incubation systems is generally very clean, it may often be fed to a heat exchanger to pre-chill or heat the influent water.
Regardless of the equipment used, temperature controllers should be specified that provide the highest accuracy possible, particularly in systems where thermal otolith marking is performed. Air temperature and humidity within the building may also have a significant effect on the heat loss or gain of the process water, particularly in stacked-tray incubators which expose large water surface areas to the building air. Special attention should be given to insulating incubation rooms, and to moving only enough air to keep the room dry. Mechanical components, which generate heat and are more frequently accessed, should be isolated in a separate room.
The oxygen consumption rates of eggs prior to hatching are relatively low. However, as with ammonia production, oxygen consumption increases dramatically in the period between hatching and the end of yolk sack absorption. Generally, increasing flow rates and providing simple aeration are all that are used to provide sufficient oxygen to the fish. As alevins have little tolerance to oxygen supersaturation, supplementation with pure oxygen is not recommended. In heated systems, aeration of the incubator supply water, after heating, is a critical requirement to prevent supersaturation. If stacked tray incubators are used, and typical recommended flow rates are maintained, the aeration that occurs as water falls between trays is often enough to provide sufficient oxygen to the eggs. However, other types of incubators may require a separate aeration process for the supply water. During this period of high oxygen consumption, dissolved oxygen should be monitored regularly, and the water flow rate adjusted accordingly to ensure
an adequate supply of oxygen to the fish. The design of a water reuse system for an incubation system dependent on influent water availability and quality, the species involved, and the water quality required for the different life-stages that will be raised. The equipment used to support the eggs or contain the alevins may affect accessibility or water consumption, but has a very limited impact on water quality. Facility operators who are prepared to manage water reuse systems by flushing at critical times, can achieve significant improvements in water quality control and reductions in water consumption with very simple designs. For those interested in minimal water consumption, very precise control of water quality, or an automated system that relies on technology to maintain water quality, more complex designs built out of an assembly of treatment equipment options can be customized to a specific site or requirement.
By KC Hosler, P.Eng and Stephen Piggott M.Eng, P.Eng