1. Preface
With the improvement of seawater aquaculture technology and the expansion of market demand, China's seawater factory aquaculture has developed rapidly in the past 10 years. The residual feed, chemical residues, and biological discharges rich in nitrogen, phosphorus, organic matter, and toxic substances contained in aquaculture wastewater will exacerbate the eutrophication and water pollution of adjacent seawater, causing harmful red tide and other marine ecological environment problems. At the same time, water pollution in turn restricts the development of aquaculture. Therefore, the treatment and recycling of aquaculture wastewater are gradually receiving attention. In recent years, scholars at home and abroad have conducted applied research on conventional physical, chemical, and biological treatment technologies for the characteristics of seawater factory aquaculture wastewater, and have achieved many practical results. After physical, chemical, and biological treatment, the concentrations of chemical oxygen demand (COD), suspended solids (SS), and ammonia nitrogen (NH3-N) in aquaculture wastewater are reduced and then recycled.
2. Physical treatment technology for aquaculture wastewater
Conventional physical treatment techniques mainly include filtration, neutralization, adsorption, precipitation, aeration and other treatment methods, which are important components of wastewater treatment processes. Mechanical filtration, foam separation technology and ozone purification are effective for the discharge and recycling of industrial aquaculture wastewater.
2.1 Mechanical filtration
Due to the fact that most of the remaining feed and excrement from aquaculture wastewater exist in the form of suspended large particles, physical filtration technology is the fastest and most economical method for removal. Common filtering equipment includes mechanical filters, pressure filters, sand filters, etc. In practical engineering, mechanical filters (microfiltration machines) are widely used and have good filtration effects. There is a type of filter machine in Japan, which works by sucking up pool water with a water pump and spraying it into a filter tank through a spray pipe. The filter tank contains a layer of small particle zeolite and a specially designed filter, and the filtered water flows back to the fish pond.
2.2 foam separation technology
Foam separation technology has been widely used in industrial wastewater treatment. It can not only remove organic substances such as proteins before they are mineralized into ammoniates and other toxic substances, avoid the accumulation of toxic substances in water bodies, but also provide necessary dissolved oxygen to aquaculture water bodies, which has a good role in maintaining the ecological environment of aquaculture water bodies.
2.3 Ozone Purification
The intermediate substance hydroxyl radicals (· OH) decomposed by ozone in water have strong oxidizing properties and can decompose organic compounds that are difficult to decompose with general oxidants. Therefore, using ozone to treat wastewater can not only quickly eliminate harmful substances such as bacteria, viruses, and ammonia, but also increase dissolved oxygen in water, thereby achieving the goal of purifying aquaculture wastewater. There are reports that ozone has a significant effect on fish and shrimp farming. Japan's Ito Shingo's research on using ozone to treat seawater shows that 99.9% of various bacteria in seawater can be eliminated by ozone. The combination of ozone and biofilters results in high dissolved oxygen content in the effluent, which can be reused to increase aquaculture density.
3. Electrochemical treatment
The research results on the removal of dissolved nitrite and ammonia nitrogen in water by electrochemical method show that the time and energy consumption for complete removal of nitrite decrease with the increase of conductivity. When the maximum input current is 2A, the energy consumption is the lowest, and pH has almost no effect on the input current and conductivity; Under acidic conditions, it is beneficial for the removal of nitrite, while under alkaline conditions, it is beneficial for the removal of ammonia. The removal rate of ammonia is lower than that of nitrite.
4. Biological processing technology
Biological treatment of aquaculture wastewater is a typical way to stabilize organic pollutants, including activated sludge process and biofilm process.
Mainly utilizing the absorption and metabolism of microorganisms to degrade organic matter and nutrients in water bodies, it is currently the most economical and effective way to treat dissolved pollutants. The feed and excrement released during the breeding process are mainly composed of carbohydrates, proteins, fats, and other elements such as carbon, nitrogen, and phosphorus, which have good biodegradability. Therefore, biological treatment technology can be effectively used to treat industrial aquaculture wastewater, among which the efficiency of biological strains and their fixed growth mode are two important aspects that determine the treatment effect.
4.1 Activated Sludge Process
The activated sludge treatment system is one of the main technologies in sewage biological treatment technology. It is composed of good microorganisms and their adsorbed and adhered organic and inorganic substances, and has the ability to adsorb and decompose organic pollutants in water, demonstrating its biochemical oxidation activity. The traditional activated sludge process has been developed into oxidation ditch intermittent activated sludge process (SBR) and AB method treatment processes. Meske et al. studied the treatment of aquaculture recycled water using activated sludge process, and found that the NH4+- N content could not meet the requirements for reuse. Umbl et al. used an operating method close to SBR for aerobic anaerobic treatment in aquaculture drainage channels, and the effect was good. Nugul et al. used SBR method to treat seawater aquaculture wastewater and explored the effect of salinity. The results showed that under low salinity conditions, the denitrification effect was good.
4.2 Biofilm method
The biofilm method mainly includes biofilters, biological turntables, biological contact oxidation equipment, and biological fluidized beds. Due to the diversity of microorganisms, these technologies have been applied in the closed cycle use of aquaculture wastewater. The key to effectively treating industrial aquaculture wastewater is to select efficient and rapidly proliferating microbial communities that can thrive and grow in seawater environments. At present, the application of photosynthetic bacteria, Yulei bacteria, and nitrifying bacteria in the treatment of aquaculture wastewater has been mainly studied both domestically and internationally [9]. Due to its high density, strong activity, and fast reaction rate, immobilized microorganisms have significant removal effects on ammonia nitrogen and certain difficult to biodegrade organic compounds compared to conventional microbial biofilm biological treatment technologies [10]. Therefore, this technology is expected to become an important biochemical treatment technology for seawater industrial aquaculture wastewater treatment.
4.2.1. Biological filter
The biological filters used in intensive fish farming facilities include horizontal flow, upward flow, and downward flow. The most critical part of the operation of a biofilter is membrane formation. If a biofilm cannot form on the surface of the filter material, it is impossible to discuss the treatment of wastewater by the filter. From a microbiological perspective, biofilm formation refers to the inoculation of bacterial cells, which allows microorganisms to adsorb onto the surface of the filter material. The packing material in the biological filter is the carrier of organisms, mainly including crushed stones, pebbles, coke, coal slag, plastic honeycomb, and various artificially synthesized products; The biological filter can be used continuously without the need to replace the filter material. The selection of packing materials is also important in the design of biofilters, and the structure and surface area of the packing materials should be conducive to the growth of biofilms and the capture of organic suspended particles. China and other countries use the sedimentation tank → biological filter → secondary sedimentation tank → biological filter process, in which the filler is mixed fiber, which can be reused after treating large-scale intensive aquaculture water in estuaries. Sauthier et al. used pond (aeration) → mechanical filter → ultraviolet disinfection → submerged biological filter (denitrification tank) → fish pond reuse, and the treatment effect was very good. Tian Wenhua and others have found that the use of zeolite as a filter material in aerated biofilters for wastewater treatment is effective.
4.2.2. Biological Rotating Disc
The biological turntable consists of a series of disks fixed on the shaft, with a gap between the disks. One half of the disks is placed in water, and the other half is exposed above the water surface. Microorganisms in water and air attach to the surface of the disc, forming a biofilm. When rotating, the disc immersed in water is exposed to the water surface, and the water on the disc flows down along the surface of the biofilm due to its own weight. The oxygen in the air is absorbed, mixed, diffused, and infiltrated into the water through the rotation of the turntable, increasing the dissolved oxygen in the water and purifying the water quality.
4.2.3. Biological drum
The bio rotating drum is a variant of the bio rotating disc, which developed in the mid-1970s and rapidly expanded in Denmark and Germany. Denmark has developed a single drum type, while Germany has developed a multi drum type. The packing inside the drum includes plastic balls, plastic rings, and corrugated discs. Some biological rotors are also equipped with gas collection devices outside to increase the dissolved oxygen in water. The typical three forms of biological rotors are: (1) the outer shell structure is made of hard polyethylene plastic, with polyvinyl chloride corrugated discs inside, and the rotor consists of 16 small rotors; (2) The outer shell of the cylinder is made of steel, and the surface of the hard polyethylene ripple fixed on the shaft inside the cylinder is polygonal; (3) There are small containers around the body of the rotary drum. When the rotary drum rotates upwards, the small containers are filled with water. When it rotates downwards, the water is sprinkled on the plastic ball, and the empty container is filled with air entering the water. The volume of purified water is 15-25 times that of the biological rotary drum.
4.2.4. Biological fluidized bed
Biological fluidized beds (BFBS) are a high load biofilm method applied in the secondary treatment of wastewater (organic matter oxidation, partial nitrification) for the treatment of organic wastewater and denitrification. Michael et al. used a reactor that combines aerobic nitrification drip filtration with anaerobic denitrification fluidized bed. The suspended nitrate rich and dissolved organic matter on the surface was sent to the sulfide bed, and the treatment effect was good. Jewell et al. utilized the nitrification and denitrification effects of an expanded bed in aquaculture water circulation, while treating BOD5, SS, and nitrogen, resulting in effluent ammonia nitrogen levels below 0.5 mg/L. The technology is widely used in the oxidation, nitrification, and denitrification treatment of organic matter in water and wastewater. As an innovative technology in water treatment methods, the biological fluidized bed process will play a greater role in water treatment engineering.
4.3 Natural biological treatment of aquaculture technology
The use of natural organisms to treat aquaculture water bodies mainly includes wetlands, stabilization ponds, and land treatment systems. Its advantage is that it can achieve a relatively thorough treatment effect for water bodies containing nitrogen and phosphorus. The natural water area of non intensive aquaculture is a typical wetland system with good self purification ability. As long as its self purification ability is utilized and strengthened reasonably, it will have good environmental and economic effects. The aquatic ecosystem of fish ponds itself has a strong ability to purify pollutants, and in the treatment of aquaculture water bodies, the purification ability of fish ponds for pollutants can be fully utilized to purify sewage.
5. The process flow of recycling aquaculture wastewater
There are various types of water treatment devices with different structures and process flows. Below are several typical processes. Fish pond drainage → collection pond → oxidation pond → sedimentation pond → warming and oxygenation pond → fish pond reuse, in this process, the oxidation pond is a biological rotary drum; Fish pond drainage → sedimentation tank → upflow biological filter → water spray tower oxygenation → heating and disinfection → fish pond reuse, can remove 99% ammonia nitrogen, fresh water/reuse water is 1/9; Fish pond drainage → oxygenation → upflow limestone filter → sedimentation tank → oxygenation → reuse, with fresh water/circulating water accounting for 1/5; Fish pond drainage → Upflow gravel filter → Downflow gravel filter → Warming tank → Reuse; Fish pond drainage → catch basin → upflow zeolite filter → downflow zeolite filter → replenishment of fresh water, temperature regulation → fish pond reuse. Based on the basic principles of ecological design and aquaculture environmental engineering technology, Liu Changfa et al. [17] found that with the goal of zero wastewater discharge in aquaculture systems, ecological engineering and process design can be carried out for aquaculture systems, and a typical zero wastewater discharge industrialized composite aquaculture system can be developed.
6. Summary
With the increasing severity of global water scarcity and environmental pollution, countries will adopt closed loop aquaculture methods in the future. Among them, the comprehensive utilization and harmless discharge technology of aquaculture wastewater has great research and development value and broad application prospects. The diversity of pollutants in seawater factory aquaculture wastewater determines the complexity of its treatment process. Therefore, when designing the treatment process for seawater factory aquaculture wastewater, the principles of efficiency and economy should be followed. Organic combination of physical, chemical, and biological treatment technologies should be used to meet the water quality requirements after treatment, which can achieve good treatment effects and achieve the goal of circular aquaculture.
