Find out the categories of grow-out systems that help employ deriving from the resources available to the grower and the regulatory environment of the location.
There are numerous options for grow-out system design, with decisions on which to employ deriving from the resources available to the grower and the regulatory environment of the location. In general, grow-out systems can be grouped into the following categories:
Flow-through systems are characterized by the capture of water either through pumping from an aquifer or collection of naturally flowing surface waters and the direction of these waters through rearing units in a single pass. As water quality naturally diminishes during the passage of water through a production system, methods such as falls, settling basins and in some cases mechanical filtration are used to improve the critical parameters of dissolved gas and fish metabolite concentrations. Flow-through systems require less intensive water quality improvement measures by relying on an abundance of fresh water constantly flowing into the system, leaving water re-purification unnecessary. Flow-through systems are often the easiest to manage due to their relative simplicity, requiring only that the infrastructure be maintained, and any screens or inlets be kept free of debris that would hinder flow.
Figure 1. Water source supplying the farm using gravity vs pumping. (Lekang, 2007, pg296)
Historically flow-through systems began as earthen structures, gradually moving toward rearing units of more regular shape (raceways, circular tanks, etc) to allow for easier management and harvest. The style of rearing unit may depend on factors such as local topography, water resources available, and capital on hand to use in new construction.
Rectangular concrete raceways have been a popular design choice in the US and elsewhere due to their relative ease of management. In this design water from a head flume is distributed equally among a series of first-use raceways designated for the stock of smallest average size. As the water moves through the raceway dissolved oxygen levels decrease and metabolite/manure concentrations increase, decreasing water quality and causing most of the stock to occupy the upper two-thirds of the raceway. In many cases the tail screen is set well ahead of the actual end of the raceway to create a quiescent zone for the settling and ultimate removal of any solids generated in the raceway. As water moves from one series to another, some measure of water quality improvement is implemented to regain productive capacity for the subsequent series. A common and simple water quality improvement method is to allow the water to plunge or cascade into the next series of raceways. A direct plunge can entrain air bubbles deeper in the receiving raceway, allowing for oxygen infusion at greater pressure, while installation of a splash plate or other device to diffuse the water over a larger area before entering the receiving raceway can increase the surface area of the water droplets with respect to their volume – allowing for increased diffusion of detrimental gases to the atmosphere and infusion of oxygen into the receiving water. Waters can be used and recharged in this manner through subsequent series of raceways until their productive capacity can no longer be adequately increased, at which point they undergo treatment (settling of biosolids, re-oxygenation, etc.) before being discharged to a receiving water.
The primary advantages of raceways are the ability to view, manage and harvest stock with ease. Unlike in sea cages or earthen impoundments fish can be surveyed for signs of disease, behavior abnormalities, and level of appetite. Should clinical signs of disease be present, an attentive member of the production team can quickly detect them and begin the process of recuperating the stock.
Although they are more popular in the Recirculating Aquaculture Systems of today, circular tanks can also be incorporated into flow-through aquaculture if local circumstances permit or require. Circular tanks have many of the same management advantages as raceways (ease of observing stock, applying therapeutics and harvest) while bringing the benefits of self-cleaning characteristics and increased uniformity of water quality through the unit. Influent water is typically introduced parallel to the tank wall, helping to create a spinning or swirling effect as water moves through the tank and concentrating solids further toward the center drain over time. As water swirls in the tank it will mix with influent water, preventing the development of a water quality gradient and letting the stock spread more evenly throughout the unit.
Recirculating aquaculture systems are managed with the intention of re-using as much water as possible in the production process to avoid discharging any effluent to the environment and to reduce the cost of pumping water continuously. To accomplish this the water used in the facility must be filtered and treated in between uses in rearing units to remove biosolids, destroy pathogens, strip carbon dioxide, add dissolved oxygen and purify any toxic dissolved metabolites. The next few sections will discuss what technologies are applied toward water treatment in RAS.
Figure 2. A centralized re-use system serving several fish tanks
Particulates generated in aquaculture production systems range in size, thus different methods are used for separating out these particulates. Coarse, dense particulates can be effectively removed from a system through physical manipulation of the water (radial flow separation, swirl separation, settling basins) or mechanical filtration (drum filter, parabolic screens, sand filtration). Selection of which system is most appropriate should be undertaken with site-specific considerations in mind.
When coarse waste removal is accomplished, ultraviolet (UV) sterilization can occur. UV sterilizers use intense, constant supply of ultraviolet light directed into water as it passes through a plumbing system to kill any pathogens present in the water. It is important this technology is used only after coarse solids have been removed from the water as any turbidity in the water will limit the penetration of the UV waves through the entire volume of water, wasting the effort of UV sterilization.
Following UV sterilization, dissolved metabolites such as ammonia (NH3/NH4) and nitrite (NO2) need to be converted via nitrification to the far less toxic form of nitrogen in water – nitrate (NO3). Biofiltration is a common technique used to perform this transition, wherein nitrifying bacteria are cultivated in an aerobic environment through which water is constantly passed. The amount of water requiring treatment as well as its nitrogen load will determine the appropriate size of the biofilter and its required footprint in the facility.
Once toxic metabolites have been processed from the effluent water, any remaining harmful dissolved gases such as carbon dioxide and dissolved nitrogen that may remain in the system must be removed, while oxygen must be added. Techniques for stripping water of dissolved gases typically involve pumping to the top of a column packed with media through which the water falls to allow diffusion. The same diffusion effect can be generated through any number of means that cause agitation of the water and thus an increase in the area of the water-air interface, but the packed column remains a simple, passive method that requires a small footprint in the RAS facility.
Closed-pond systems are those that have no constant influent and effluent, and no water treatment systems in place. Closed-pond systems are almost always earthen in nature and typically possess a larger water surface area than other production systems. These systems rely on natural productivity of the water body in many ways, from providing early feeding opportunities to fry/juveniles to allowing phytoplankton to oxygenate the water during the day. Closed-pond systems need less intensive management than recirculating systems, but still require diligence from the grower to keep conditions in balance (most of all during night-time hours).
A fundamental necessity in establishing a productive closed-pond system is the cultivation of a steady, managed plankton population (zooplankton during early rearing, and phytoplankton during all stages). By establishing a well-managed phytoplankton population, the water will maintain strong dissolved oxygen levels during sunshine hours while not overly consuming this oxygen during the night. Closed-pond system growers rely on various aeration technologies during the low-oxygen night, most commonly one of the various paddlewheel designs and others such as compressed air diffusion.
Due to the typically high algal populations in these systems, observation of stock is extremely limited. Disease events are typically only recognized when feeding ceases, mortalities appear on the surface, or fish behavior changes dramatically and drives them to the surface. This inability to observe stock effectively poses a major challenge to fish health management, and due to the typically large surface area of these systems applying therapeutants is in many cases an uneconomical recourse even if a disease is identified early.
Despite these challenges, closed-pond systems offer several advantages to the producer. In well-balanced ponds, much of the water quality management and zooplankton forage production is taken care of by natural processes, meaning a grower will only need to monitor environmental conditions and allow nature to do the work. The generally larger size of these units offers growers the ability to stock them with a number of fingerlings equivalent to the number they plan to harvest, considering mortality losses in the interim. Whereas growers in raceways, tanks, and other systems are often forced to split and transfer their stock to different rearing units as they grow to economize space, closed-pond growers avoid this effort by stocking for harvest, lowering their manpower effort per kilogram harvested.
Cage system aquaculture, popular for many of the largest and most advanced growers of finfish in the world, is characterized by the raising of stock in net pen cages located in open or near shore waters favorable to the cultural requirements of the stock. Cages offer the opportunity to use the natural flow of an open water body to replace water in the net pen, and as long as site selection is done well growers may benefit from consistently good water quality without the effort of treating or constantly monitoring water quality that RAS and closed-pond system growers face. Provided sites are properly selected, cage system producers can benefit from an ability to use very large cages for production – increasing the production capacity of a farm without having to perform expensive earthwork or other infrastructure installation.
Proper site selection is paramount in the productivity and success of a cage production operation. Growers often look for sites that are deep, maintain predictable and gentle water current patterns, and offer easy access from shore to allow for transport of feed and personnel to the site as well as harvested biomass from the site. Depth of the water body is important to cages in that feces will be deposited far below the stock being grown, and if appreciable current occurs below the cages feces will be deposited over a wide area of the bottom – preventing the accumulation of feces in a concentrated area, which can lead to the development of an anoxic mound of organic material beneath the cages. An appreciable level of predictable current through the site, often as a result of tidal movement in marine environments and wind patterns in freshwater bodies, allows for a steady replenishment of water within the cage.