Advances in theory and practice of aquaculture wastewater treatment
Written By: Jasmine
Contact email: Kate@aquasust.complastic.com
Aquaculture wastewater is mainly composed of animal urine, manure and management water for aquaculture, and contains high concentrations of organic matter, nitrogen, phosphorus and suspended solids, as well as some elements that constitute salt. In order to have a clearer understanding of the key breakthroughs in my country's aquaculture wastewater technology and the problems encountered in practical applications so far, this paper additionally summarizes the entanglement between resource utilization and advanced treatment of aquaculture waste in addition to the theme of this field. The content of pollutants that have attracted much attention, as well as the progress in some technical fields. Finally, some suggestions are put forward for the development and application of aquaculture wastewater treatment technology.
Animal husbandry is an important part of my country's agricultural economy. However, with the rapid development of mechanization and scale of animal husbandry, serious environmental problems have arisen, among which breeding wastewater is one of the main pollution sources. Aquaculture wastewater is high-concentration organic wastewater containing organic matter, nitrogen, phosphorus and suspended solids, as well as heavy metals, antibiotics, antibiotic resistance genes and pathogenic microorganisms. If not properly treated, it will lead to changes in the surrounding environment and ecology, threatening animals. and human health. At present, there are two main treatment modes for aquaculture wastewater: one is the advanced wastewater treatment (standard discharge) mode, which is mainly used in southern farms with less land facilities. The aquaculture wastewater undergoes solid-liquid separation, anaerobic/aerobic treatment and After advanced treatment, it is discharged up to the standard or recycled; the other is the resource utilization (fertilizer, energy) treatment mode, which is mainly used in northern farms with more land facilities, and the wastewater is harmless through sedimentation, anaerobic fermentation, etc. After treatment, biogas is used for energy utilization, and biogas slurry is used for farmland resource utilization. This paper briefly summarizes the current situation and technical problems to be overcome in the implementation of wastewater treatment in large-scale aquaculture enterprises in my country, for the reference of personnel engaged in production, scientific research and management.
1 The hesitation between resource utilization and advanced treatment of aquaculture waste
Aquaculture wastewater treatment is still the field that has received the most attention and invested the most in environmental protection in the aquaculture industry in the past decade. Large-scale aquaculture enterprises must choose between resource utilization and advanced treatment when disposing of aquaculture waste. Although the combination of planting and breeding and the utilization of waste resources have been advocated and encouraged in recent years, due to various reasons, advanced treatment of aquaculture wastewater, standard discharge or zero discharge is still necessary for many aquaculture enterprises to survive.
The solution of environmental protection problems and the utilization of resources are not completely equivalent concepts. For enterprises, to solve environmental protection problems, they must first obtain the environmental impact assessment permit, and then take measures to dispose of waste according to the requirements of the environmental impact assessment and meet the requirements; legal compliance, economical and effective It is not as simple as "turning waste into treasure" verbally. First of all, it is necessary to have sufficient land resources within an economical and effective radius (in line with the principle of local and nearby utilization), and more importantly, to "turn treasures" , that is, the value increase at the back end of the industrial chain is realized through the harvested products. If the harvested products are only theoretical yields without realizing their own utilization or converting them into market value, the feasibility study report of resource utilization will be distorted; From the perspective of environmental protection, prevent secondary pollution (including water, soil, and air). At present, it is difficult to promote the resource utilization of aquaculture waste in my country, which is also related to the following factors: First, there is a lack of environmental assessment guidelines for the aquaculture industry, and there are many relevant standards. For example, most places require that aquaculture wastewater must meet the "Water Quality Standards for Farmland Irrigation" (GB 5084-2005) before the resource utilization. Second, due to historical reasons, many large-scale farms no longer have enough supporting land resources around them.
2 Research on hot spot pollutants
In the treatment of aquaculture wastewater, in addition to the indicators for the current environmental protection requirements [such as chemical oxygen demand (COD), ammonia nitrogen, total phosphorus (TP), etc.], research and practice in recent years have shown that it is necessary to pay more attention to the following pollutants: Drug bacteria and resistance genes (ARGs), salinity (salinity), total nitrogen (TN), and sludge generated during wastewater treatment. Sludge is a normal product in the water treatment process. Due to the change of the manure removal mode and the improvement of the back-end effluent standard requirements, the sludge production generally increases. The difficulty of sludge treatment lies in its high water content. Many studies have shown that although the chemical indicators of the effluent at the end of the current water treatment process meet the standards, there are still environmental risks of drug-resistant bacteria and drug-resistant genes. The accumulation of salt will cause harm to soil and crops, so it is necessary to guard against it in the process of resource utilization. Some places restrict the discharge of total nitrogen from aquaculture wastewater, which will greatly increase the cost of water treatment under the current technical level and significantly increase the burden on enterprises.
3 Developments and breakthroughs in important technical fields
At present, the commonly used aquaculture wastewater treatment processes include anaerobic biological treatment, aerobic biological treatment, natural treatment and advanced treatment technologies, microalgae, membrane separation and other treatment technologies under research and development, as well as farm cleaning related to back-end water treatment. The dung process, etc., has been featured in other articles in this special issue. This article only briefly describes anammox, simultaneous nitrification and denitrification, and short-range nitrification and denitrification.
3.1 Anammox technology
Anammox technology is a new type of anaerobic biological treatment technology, which is a process in which anammox bacteria directly convert ammonia nitrogen and nitrite into nitrogen gas in an anaerobic environment. The key bacteria of anammox technology are anammox bacteria, which can convert ammonia nitrogen in aquaculture wastewater into nitrogen gas through biochemical reaction under anaerobic conditions to realize the removal of ammonia nitrogen. Therefore, anammox technology is an anaerobic biological treatment technology, and also belongs to the type of simultaneous nitrification and denitrification technology. Due to the slow growth of anammox bacteria and many influencing factors, fixed beds, activated sludge beds and membrane bioreactors are often used in production to increase the retention of anammox bacteria and combine with other treatment technologies , improve the efficiency and stability of wastewater treatment. Anammox technology has the advantages of high efficiency and economy, and has great application prospects in the direction of denitrification of aquaculture wastewater, but there are problems such as long start-up time and many interference factors, which need to be further solved. Under the conditions of field work, further breakthroughs are needed in the exploration and regulation of anammox technical conditions.
3.2 Short-path nitrification and denitrification technology
Anoxic/oxic process (Anoxi/oxic, A/O) mainly realizes denitrification (NH+4→NO2→NO3) and nitrification (NO3→NO2→N2) by setting anoxic pool and aerobic pool, respectively. Ammonia nitrogen removal from wastewater. However, studies have shown that the accumulation of nitrite nitrogen will occur in the traditional nitrification and denitrification process [3]. To this end, the theory of short-range nitrification and denitrification is proposed. By promoting the growth of ammonia oxidizing bacteria (nitrite bacteria) and inhibiting the growth of nitrite oxidizing bacteria (nitrifying bacteria), the process of short-range nitrification and denitrification (NH+4→NO2) is realized. →N2). The growth cycle of ammonia oxidizing bacteria is shorter than that of nitrite oxidizing bacteria, among which mud age, temperature, pH and dissolved oxygen are the main factors affecting ammonia oxidizing bacteria and nitrite oxidizing bacteria. When the temperature is higher than 28 °C, it is conducive to the growth of ammonia oxidizing bacteria and inhibits the growth of nitrite oxidizing bacteria; pH around 8.0 is also conducive to the accumulation of ammonia oxidizing bacteria; the affinity of ammonia oxidizing bacteria to low concentrations of dissolved oxygen is greater than that of nitrite oxidizing bacteria[4-6] . Theoretically, short-range nitrification and denitrification shorten the reaction time, save the supply of oxygen and carbon sources, and reduce the sludge production [7]. However, during the operation of the water treatment facility, a large amount of sludge is produced every day due to the need to increase the sludge discharge to reduce the sludge age. In addition, due to many influencing factors, its stability also needs further improvement.
3.3 Simultaneous nitrification and denitrification technology
Simultaneous nitrification and denitrification technology realizes simultaneous nitrification and denitrification by controlling parameters such as dissolved oxygen, pH and temperature in the biological pool, and improves the efficiency of wastewater treatment [8]. The mechanism of simultaneous nitrification and denitrification includes macro environment theory, micro environment theory and microbiology theory [9]. The macro-environmental theory refers to controlling the concentration and uniformity of dissolved oxygen in the reactor, creating an environment suitable for both nitrifying bacteria and denitrifying bacteria to grow, and synchronizing the nitrification and denitrification processes [10]. The microenvironment theory refers to controlling parameters such as dissolved oxygen concentration, activated sludge particle size and biofilm thickness, forming a dissolved oxygen gradient on the surface and inner layer of activated sludge particles and biofilms, the surface aerobic nitrification reaction, and the inner layer hypoxia. denitrification reaction. Microbiology theory refers to the utilization of microorganisms that can perform nitrification and denitrification simultaneously. Studies have shown that there are aerobic denitrifying bacteria and anaerobic nitrifying bacteria in the environment, such as anammox bacteria, which can directly convert ammonia nitrogen into nitrogen.
In addition to the above technologies, research and application of high-efficiency microorganisms in wastewater treatment process, control of product inhibition in anaerobic process, optimization and automatic control of fermentation process conditions, cracking of phosphorus crystallization causing pipeline blockage in wastewater treatment system, prevention and control of odor in wastewater treatment process Breakthroughs in technologies such as breeding, diffusion and anti-seepage will help to control risks and reduce costs and increase efficiency.
4 Summary and Outlook
Farm wastewater treatment technologies include aerobic biological treatment, anaerobic biological treatment, advanced treatment and natural treatment. Among them, A/O, Upflow Anaerobic Sludge Bed (UASB), Upflow Solid Anaerobic Reactor (USR) ), biogas digesters, oxidation ponds, chemical oxidation and coagulation and other process technologies are relatively mature and widely used. Each treatment method has its own advantages and limitations. Different technical combinations can be selected according to the characteristics of farm wastewater and local policies. For example, farms with higher wastewater discharge standards can choose anaerobic + aerobic + advanced treatment. The combination of technologies, farms with enough land can give priority to anaerobic treatment technology for harmless treatment of wastewater. In addition, some new treatment technologies such as short-path nitrification and denitrification, simultaneous nitrification and denitrification, anammox, microalgae treatment and membrane separation have high application prospects, but their treatment parameters and stability parameters need further research and optimization or outdoor Engineering Applications.
With the increase of environmental protection, people put forward higher requirements for the research and application of aquaculture wastewater treatment technology. The research and development of new wastewater treatment technology is still the focus of future research, especially the strong market demand for efficient, stable and low-cost wastewater treatment technology; the improvement of existing wastewater treatment technology is also the research focus in the future, such as aerobic Or the development of functional microorganisms in anaerobic biological treatment technology, and the research and development of high-efficiency and durable membranes in membrane separation technology; at the same time, the recycling and energy utilization of aquaculture wastewater is an important research direction, such as the safety assessment in the process of recycling wastewater The research and development of energy utilization technologies such as biogas bioenergy and biodiesel has important reference significance for the safe treatment and utilization of aquaculture wastewater.