Aquaculture Wastewater Treatment

Proper treatment of aquaculture wastewater prior discharge to natural water bodies is important to minimize the environmental impacts of aquaculture. Additionally, in RAS where culture water quality depends on the treatment of wastewater, the main reason to treat wastewater is to maintain sufficient water quality in the system. (Cripps and Bergheim 2000). Traditionally the control of pollutant levels in aquaculture systems and the collection of the wastewater has been

carried out by changing large volumes of water in and out of the system. This method is, however, unsustainable due to resulting large amount of dilute wastewater and the large amount of energy that is required to transfer the waters. (Castine et al. 2013). Currently aquaculture wastewater treatment is diverse and various options are available to suit the needs of different aquaculture systems. Physical, chemical, and biological treatment techniques from the field of conventional wastewater treatment have been adapted to treat aquaculture wastewater. These treatment techniques include sedimentation, mechanical and biological filtration, flotation, and the use of constructed wetlands. (Turcios and Aydin Temel 2018).

The treatment of aquaculture wastewater is challenging due to the high volume of wastewater and the low concentration of pollutants (Cripps and Bergheim 2000, Castine et al. 2013, Turcios and Papenbrock 2014, Mongirdas et al. 2017). Especially the removal of SS from dilute wastewater is difficult as the interaction between the particles and the formation of larger aggregates is minimal in low SS concentration. It is therefore beneficial to use pre-treatment to concentrate the solids before actual treatment. Pre-treatment increases the efficiency of the subsequent SS removal units and reduces required treatment capacity. If performed within the culture system, pre-treatment also improves the quality of the culture water. Hydrodynamics of the culture system impact the movement of SS, which can be utilized to create quiet collection points for SS to gather in. In addition, particle concentrators can be used at the culture system outlet to aid the settlement of particles. The formed sludge is directed from the concentrator to the treatment through a separate outlet from the primary flow. (Cripps and Bergheim 2000).

Sedimentation is a SS removal method used both within the aquaculture systems as a pre-treatment and in separate settling basins or ponds as the first stage of SS removal. Sedimentation is based on the gravitational settling of SS, and it requires low flow rate and sufficient residence time to remove SS successfully. Large particles are removed with sedimentation most effectively. Settling ponds can remove 60% of total suspended solids (TSS), 20% of TN, and 30% of TP when they first are constructed, but over time the removal efficiencies decrease, if the removal of sludge from the bottom of the pond is not carried out properly. (Castine et al.

2013). Settling basin designs range from simple ponds to more complex systems utilizing baffles to slow and direct water flow. Even though sedimentation is used for the first stage SS removal from untreated wastewater, it is not well suited for this purpose, and should only be used within the aquaculture system for pre-treatment. This is because of the high flow rates from aquaculture systems often result in settling problems like insufficient residence times and

short circuiting of the water from the inlet of the sedimentation basin to the outlet. (Cripps and Bergheim 2000). The nutrient-rich sludge formed in sedimentation and other aquaculture wastewater treatment processes can be used as a slow-release fertilizer in agriculture (Turcios and Papenbrock 2014).

SS removal can also be achieved by using microscreens, tube settlers, or flotation. Microscreens are the most commonly used method among mechanical particle separation techniques. Several microscreen designs are available, but the SS removal mechanism is same in all of them: to capture the particles on a fine mesh screen with pore size of 60-200 μm. In a drum microscreen the mesh is cylinder-shaped and allows water to flow in from one end of the drum and out through the walls of the drum. Rotating disc screens on the other hand are flat circular screens placed perpendicular to wastewater flow. The particles trapped on the microscreens are backwashed to a collection trough by spraying water on the screens from the downstream side of the screen. (Cripps and Bergheim 2000). The removal efficiencies of microscreens with different designs range between 50-74%, 10-42%, and 49-63% for solids, TN, and TP, respectively (Tepe and Aydin Temel 2018). Tube settlers differ from traditional settling-based treatment techniques by utilizing settling plates. The plates that are angled at 45-60° force water to flow upwards and collect SS on the undersides of the plates. (Castine et al 2013). While other SS removal techniques are effective in removing large particles, flotation is the most functional method for the removal of smaller particles under 50 μm in size. In flotation small bubbles produced at the bottom of the contact chamber flow upwards, and capture surface active particles from the downwards flowing wastewater on their way to the water surface. When the bubbles reach water surface, they form foam which can be removed from the system over a weir. To achieve optimal SS removal different SS removal unit processes should be combined with proper management of the feeding. (Cripps and Bergheim 2000).

Dissolved nutrients can be removed from the wastewater by using biological or chemical methods. Biological methods utilize microorganisms in the removal of nutrients. When the conditions are optimal for microbial processes, microorganisms remove nitrogen from the wastewater through nitrification and denitrification, and also assimilate nutrients for their growth. (Castine et al. 2013). Anaerobic fixed and fluidized bed reactors, activated sludge treatment, rotating biological contractors, and trickling filters are some of the possible biological treatment methods (Tepe and Aydin Temel 2018). Certain filters including granular, porous media, and bead filters can function both as biological and mechanical filters by

providing growing surface for microorganisms and capturing particles at the same time (Castine et al. 2013). The designs of the filters vary. As an example, bead filter consists of a bead bed formed by small polyethylene beads. The beads are fluidized by the wastewater flowing upwards through the bed. (Cripps and Bergheim 2000). Physico-chemical methods used in nutrient removal from aquaculture wastewater include sorption, ion exchange, reverse osmosis, and electrodialysis (Tepe and Aydin Temel 2018). In addition, some physico-chemical methods such as neutralization, coagulation, sterilization, and oxidation can be used to dispose pathogens (Cao et al. 2007).

Lastly, algae, plants, or filter-feeding organisms (e.g. mussels) can be used to assimilate nutrients from aquaculture wastewater. Growing these organisms alongside the culture animals in IMTA system also creates profitable biomass that can be harvested. (Cao et al. 2007).

Another option for the utilization of plants is the use of constructed wetlands. Constructed wetland consists of filter material, which supports the growth of wetland vegetation and hosts microbial communities. While wastewater flows through the wetland, the removal of organic matter, solid particles, dissolved nutrients, pathogens, and pharmaceutical compounds occurs through physical, chemical, and biological mechanisms. Constructed wetlands are divided to two main types: free water surface constructed wetlands and subsurface flow constructed wetlands. Free water surface constructed wetlands are similar to the natural wetlands and have the wastewater surface in contact with the atmosphere. In subsurface flow constructed wetlands the wastewater surface remains below the surface of the filter material. The treatment efficiency of the wetland varies depending on the type of the wetland and the characteristics of the treated wastewater. Using different wetland designs the removal efficiencies of 34-99%, 10-69%, and 16-91% have been achieved for TSS, TN, and TP, respectively. (Tepe and Aydin Temel 2018).

Aquaculture systems can be divided to three types regarding the ability to control the wastewater and its environmental impacts. Aquaculture systems located in natural water bodies are classified uncontrollable, flow-through aquaculture systems hold a limited ability of control and RAS can be fully controlled. (Mongirdas et al. 2017). As the wastes from aquaculture practiced in water bodies are released directly to the environment, the only feasible way to diminish their impacts is to culture algae or filter-feeding organisms in the vicinity of the aquaculture system (Cao et al. 2007). The controlling of wastewater from flow-through aquaculture systems is complicated due to the large volume of the wastewater. As flow-through aquaculture system designs are usually simple, also the wastewater treatment is conducted by

simple methods including sedimentation within the aquaculture system or in separate sedimentation basin and constructed wetlands. (Castine et al. 2013). RAS are closed systems, which makes them easily controllable. Due to the need to maintain sufficient culture water quality in the system, the wastewater treatment in RAS is more advanced compared to flow-through systems. In RAS, SS are removed using sedimentation basins, microscreen filters (Mongirdas et al. 2017), tube settlers (Castine et al. 2013), flotation, and bead filters, or a combination of some of them (Cripps and Bergheim 2000). Biological reactors or filters including granular and porous media filters are utilized for the removal of dissolved nutrients (Castine et al. 2013). Also constructed wetlands can be used, and in some cases it may even be possible to discharge the wastewater to centralized wastewater treatment facilities (Mongirdas et al. 2017).

Various advantages and disadvantages are associated with different aquaculture wastewater treatment techniques. In general, physical, chemical, and biological methods have high energy consumption and maintenance requirements, and the treatment results in the generation of sludge. Also, the initial capital investment for physical and chemical methods is high, and the performance of physical and biological methods is unstable since the efficiency of the treatment ranges widely. Among physical methods especially sedimentation requires large area for the treatment, but on the other hand its design principles and operation are simple. The major advantage of both biological and chemical methods is the high treatment efficiency that they can offer. (Tepe and Aydin Temel 2018). Another treatment technique that has large area requirement is constructed wetlands. In addition, constructed wetlands require low hydraulic loading rate and long retention time to function efficiently. (Turcios and Papenbrock 2014).

Low energy consumption and maintenance requirement, simple operation, cost-effectiveness, landscape esthetics, and increase in habitats for wildlife are considered the advantages of constructed wetlands (Tepe and Aydin Temel 2018, Turcios and Papenbrock 2014).