AbstractLargescale pigraising can discharge a great deal of wastewater, which contains high content of organic matter, ammonia nitrogen and suspended solids. The improper treatment of the piggery wastewater can lead to serious environmental problems. As a liquid fertilizer, piggery wastewater is relatively low in fertilizer efficiency and high in transportation cost, so it is very necessary to treat it in situ. Energy plants have the advantages of rapid growth, large biomass, strong tillering ability and developed root system. Therefore, energy plants can be used to absorb and transform the pollutants (like nitrogen and phosphorus) in piggery wastewater into the components of plants, as well as form the rhizosphere environment which is conducive to microbial growth, so as to enhance the effects of nitrogen and phosphorus removal. The obtained energy plants can be recycled as the raw materials for biogas to increase the production of biogas, which brings economic benefits while solving the environmental problems caused by piggery wastewater.
　　Key wordsEnergy plants; Largescale pigraising; Recycling; Wastewater treatment; Nitrogen and phosphorus removal
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　　Received: November 23, 2018Accepted: December 28, 2018
　　Supported by the National Natural Science Foundation of China (41263006, 2014BAC04B02); the Fund Project of Science and Technology Department of Jiangxi Province (20124ACB01200, 20122BBG70086, 20133ACF60005); the Fund Project of Jiangxi Academy of Sciences (gankeyuan(2013)No.1906, 2012YYB 01, 2013XTPH114, 2013H003).
　　Ziqiang AO (1975-), male, P. R. China, associate researcher, Ph.D., devoted to research about environmental science and engineering, Email: aoziqiang628@163.com.
　　* Corresponding author.
　　The largescale development of pigs has brought about significant agglomeration benefits. Great improvements have been made to the breeding technology, disease prevention and management levels of pigs, whose competitiveness has increased significantly. The traditional scattered breeding has been gradually eliminated, and largescale pig raising farms (of more than 10 000 pigs) are constantly appearing. At the same time, a series of problems have arisen, especially in the treatment of piggery wastewater. Since pig raising is a lowprofit industry, it is obvious that the natural wastewater treatment experience in traditional scattered breeding is not applicable for the wastewater treatment of pig farms with such large scales. Currently, the industrialized sewage treatment mode popular in large cities has high construction costs and high operating costs, which account for a great deal in pigraising costs. Once the market price of pig price fluctuates, the builtup treatment facility becomes useless. Therefore, the economical and effective treatment of piggery wastewater has become the current direction of pigraising entrepreneurs and researchers. Piggery wastewater contains a large amount of organic matter and nutrient elements such as nitrogen and phosphorus. These elements are not only easy to result in the eutrophication of water bodies, but also essential elements for plant growth. It needs to undergo a series of biological and ecological processes to transform the pollutants in the piggery wastewater into the matters needed for the production and living of human beings. The common economical and effective treatment processes include anaerobic treatment, constructed wetlands and other technologies or technical combinations. Realizing the harmless, recycling and reducing treatment of piggery wastewater is the basis for ensuring the sustainable development of the pig industry. It is to carry out the harmless treatment of the piggery wastewater and to maximize the utilization of resources, which can solve the wastes of N, P resources and bring in great economic benefits. Nowadays, energy plants have been used for the construction of constructed wetlands, in which the piggery wastewater is treated. In this way, it not simultaneously achieves the harmless wastewater treatment, maximal resource utilization and reducing the cultivation costs for energy plants. Moreover, the obtained energy plant biomass can be used as raw material for biogas or green feed, which creates great economic benefits for the pig farms and guarantees the sustainable development of pig farms［1］. 　　Characteristics of Piggery wastewater
　　Large discharge amount of water, containing high organic matter content, ammonia nitrogen content and suspended matter content
　　With the rapid development of animal husbandry in China, the scale of the pig industry is expanding and the pollutants generated are increasing, masking the environmental problems become more and more serious. According to the statistics in 2012, there were 37 million pigs, which discharged 13.5 million t of solid manure, and 81 million m3 of waste water. The annual discharge of CODcr, NH3N and TP to the environment was 1.6 million t, 40 000 t and 2 500 t, respectively. The direct discharge of piggery wastewater into the water without effective treatment could cause serious pollution to the local environment. The wastewater from largescale pig raising is mainly the pig manure and urine and from the piggery cleaning, and the water quality varies depending on the cleaning process. At present, the cleaning of excrement of largescale pig farms in China mainly adopts the following 3 kinds of washing processes: water flushing, water soaking and dry collection. The water flushing process has the advantages of high labor efficiency, low labor intensity and low labor input cost, but the flushing water consumption is large, the concentration of organic matter in the discharged piggery wastewater is high, and the sewage treatment is difficult. Currently, most areas in China adopt such processing method. The water soaking process is to store a part of the water in the septic ditch in the piggery, which can store the pig manure and urine after fall into the ditch, and then the excrement and urine waste is discharge when the septic ditch is full. Compared with the water flushing process, water soaking processes saves water and labor, but the organic matter content is high, making it difficult to treat. The dry collection process is to collect the waste mechanically or manually once generated, and then the excrement is cleared out through the clearing channel, while the urine and wastewater flow out through the sewer. In this way, the discharged piggery wastewater is a mixture of pig urine, some pig excrement and piggery cleaning wastewater. The organic matter content in the wastewater is low, but it requires more mechanical and labor input. The piggery wastewater is usually composed of pig feces and piggery cleaning water, which has the following characteristics: ① large discharge amount: a large scale pigraising farm with 10 000 pigs can produce 150-250 m3 of wastewater using the waster flushing process every day, which can generate great shock load. ② The wastewater has high organic matter content, many suspended solids, deep color, and contains a large number of microorganisms and high concentration of NH+4N. ③ The wastewater contains the pollutants which have high chemical oxygen demand (COD), biochemical oxygen demand (BOD), solid suspended solids (SS) and so on. ④ The piggery wastewater from largescale pig raising contains a lot of organic matter and nutrients such as nitrogen and phosphorus, which is biodegradable. However, after anaerobic treatment, ammonia nitrogen increases, and the further reduction of carbon and nitrogen is not conducive to the growth of microorganisms. The reduction in the denitrification effect needs to supplement suitable carbon sources［2-4］. 　　High cost for fertilizer transportation, but suitable for onsite disposal according to local conditions
　　Piggery wastewater contains a large amount of organic matter, and high nitrogen and phosphorus content, making it an excellent natural fertilizer. However, the fertility of piggery wastewater is not as concentrated as that of chemical fertilizer, and it takes time to show effects. In the meantime, it is impossible for the farms around the pig farms to digest such a great deal of piggery wastewater. Largescale planting enterprises are far away from most pig farms, so it costs a lot to transport the piggery wastewater. On the other hand, as fertilizers, piggery wastewater is not as convenient as the chemical fertilizers, since it needs to input a lot of labor force, which results in the increase of labor input. Thus, the local farmers are not quite willing to purchase and use the piggery wastewater. Moreover, the pigraising enterprises rarely consider converting piggery wastewater into liquid fertilizer for sale. At present, most of the piggery wastewater is treated on the spot according to the local conditions of the pig farm.
　　Great difficulty in treatment, promising great potential harm to environment
　　At present, the treatment process of pig wastewater usually includes 3 working procedures of solidliquid separation, anaerobic digestion and aerobic degradation. Some anaerobic treatments use underground biogas digesters, some use aboveground anaerobic reactors, and most of aerobic treatments are done using Sequencing Batch Reactor (SBR), Membrane Bioreactor (MBR), Contact Oxidation Process, Activated Sludge Process, Integrated Oxidation Ditch and so on［5］. The biggest problem with this kind of treatment method is that the aerobic process has high energy consumption that it consumes 2-4 kWh to treat 1 m3 of wastewater. Moreover, there are many mechanical equipments, large quantities of maintenance and management work, resulting in high operating costs, and the running cost for 1 m3 of wastewater is about 2.0 Yuan［5］. According to our calculations, using such mode to treat the piggery wastewater, a pig farm that produces 10 000 pigs a year needs to pay about 100 000 Yuan for the running costs. Piggery wastewater is a kind of highconcentration organic wastewater rich in N and P nutrients, which poses potential pollution harms to the environment［6］. The direct discharge into the water body or flushing into the water body can easily lead to the mass growth of algae and the phenomenon of eutrophication. In the end, a large number of aquatic organisms die, seriously destroying the aquatic ecosystem. In addition, the nitrogenous compounds in the piggery wastewater are infiltrated into the ground, and under the action of soil microorganisms, the compounds can become NO3N after a series of chemical reactions such as ammoniation and nitrification, resulting in groundwater pollution［3-4］. 　　Advantages of Energy Plants
　　The nutrient elements such as nitrogen and phosphorus after the insitu treatment of piggery wastewater can be further transformed by the constructed wetland which is constructed with the plants which can grow fast and have great biomass. Energy plants have such characteristics. Energy plants are plants that can produce biomass energy, also known as biofuel plants or petroleum plants, including woody plants and herbaceous plants that live on land and various aquatic plants. The use of energy plants to construct wetlands to treat piggery wastewater can produce biomass energy and solve the environmental problems caused by piggery wastewater and reduce greenhouse gas emissions［7］. According to the characteristics of components, energy plants can be divided into 3 categories: first, carbohydraterich energy plants, mainly plants rich in starch and cellulose, including Miscanthus sinensis, Panicum virgatum, Phalaris arundinacea, Sorghum dochna, Manihot esculenta crantz, which are mainly used in the production of fuel ethanol, biodiesel and gas; second, oilrich energy plants, including Cyperus esculentus, mainly used to produce biodiesel; third, energy plants rich in petroleumlike components (mainly terpenes, olefins), including Euphorbia lathyris, which are mainly used to produce biodiesel and gas［7］. Energy plants can be used to treat piggery wastewater, mainly because energy plants have the following distinct advantages:
　　Fast growth and large biomass
　　Biomass is the most important indicator to measure the growth status of plants, and the growth of plants can be directly expressed by changes in biomass［8］. The plant that grows fast has high demand for nutrients such as nitrogen and phosphorus, so that the removal rate of piggery wastewater is high. Energy plants grow rapidly and have large biomass. For example, the hay yield of P. virgatum is 18 t/hm2, and the annual yield of Pennisetum purpureum grassy weeds can reach 54 t/hm2. The yield of planting energy plants is much higher than that of food crops. At the same time, the energy plant has simple breeding method and high survival rate. It can store light energy in plants through photosynthesis, and the growth cycle is short. Generally, it can harvest 2-4 times a year. The energy plant has a large biomass and can obtain higher organism quality［7］.
　　Strong tolerance to piggery wastewater stress
　　Piggery wastewater is a kind of adversity for plant growth. Therefore, the selected energy plants must have strong vitality, pollutionresistance, resistance to aquatic and wet environment, resistance to pests and diseases. Moreover, perennial plants are preferred. The roots of wetland plants are submerged in water and have longterm contact with pollutants. Thus, plants should have certain adaptability and resistance to pollutants, but different plants have different resistance to pollution and stress. The pollution resistibility has become one of the important indicators for the selection of constructed wetland plants. The selected energy plants for constructed wetlands should maintain the ability of normal growth and development, so as to remove the pollutants［9］. Therefore, it should select proper energy plants according to the ammonia nitrogen resistances of the plants in the treatment of piggery wastewater with high ammonia nitrogen concentration. 　　Short growth cycle
　　The selected energy plants are generally overyear living or perennial plants, and can be planted at one time but harvested multiple times. The plants should have short growth period and few cultivation times, which can not only save human and financial resources, but also contribute to the increase of soil nutrients, thereby improving soil texture and reducing soil erosion caused by surface exposure.
　　Highly developed root system
　　According to the removal principle and actual situation of piggery wastewater pollutants, the selected energy plants should have developed root systems. Because the developed root system can make the energy plants secrete more root exudates, and therefore provide sufficient microbial growth conditions like carbon sources for the rootzone microbes, which can promote the degradation of rhizosphere pollutants of energy plants. The secreted enzymes can participate in a variety of biochemical processes to accelerate the degradation of some pollutants, thereby improving the purification effect of energy plants. The developed root systems of plants also help to fix the soil surface and gather soil, which is of great significance for maintaining the stability of the wetland system.
　　Easy to store and transport
　　Most energy plants have low water content, generally 30%-40%, so they can be harvested in sunny days and stored after some brief dry in the sun. The energy plants are also easy to transport and the transportation cost is low, which are conducive to the concentration and mass production of biomass. In a word, it can reduce the input in planting, water and fertilizer as well as the processing costs to plant energy plants［10］.
　　Economic and effective in solving the environmental problems caused by piggery wastewater
　　The energy plants can reduce the nitrogen and phosphorus in the piggery wastewater by absorbing and transforming the nitrogen and phosphorus, which are essential part composed the plants. The absorption by the root systems can also reduce the migration diffusion of nitrogen and phosphorus in wastewater, and the root exudates from plant roots can provide the carbon sources for the microbial growth as the biomass, enhancing the effect of removing nitrogen and phosphorus from the wastewater. The anaerobic fermentation of the obtained energy plants with the piggery wastewater can produce more biogas, thereby obtaining more biomass energy. The produced biogas can be used as cooking water for civilian use, and it can also be used to provide the energy for winter warming equipment for pigs as well as to produce more electricity through biogas power generation. 　　Ziqiang AO et al. Research Progress on Energy Plants in Piggery Wastewater Treatment
　　Screening and Applications of Energy Plants
　　As mentioned above, piggery wastewater has high organic matter and high ammonia nitrogen, and it is not proper for longdistance transportation but suitable for onsite treatment. In view of these characteristics, the use of energy plants to treat piggery wastewater can solve the environmental problems caused by piggery wastewater, and can also recycle the biomass energy, thus producing significant economic benefits. At present, the following energy plants have been widely used in constructed wetlands to treat piggery wastewater.
　　Miscanthus floridulu (Labnll.) Warb.
　　There are 14 species in Miscanthus, including M. sinensis, M. floridulu (Labnll.) Warb, M. transmorrisonensis, which are mainly distributed in East Asia, Southeast Asia, Pacific Islands and Africa. M. floridulu (Labnll.) Warb is the most common wild grasses in daily life. It is a rare perennial evergreen grass plant that grows all year round, avoiding the winter plant wilting which can reduce the removal capability of pollutants from piggery wastewater. M. floridulu (Labnll.) Warb has a biomass production of 6.3-31.2 t/hm2 (dry weight), cellulose content of 25.69%-43.59%, hemicellulose content of 27.45%-42.80%, lignin content of 4.61%-15.35%, and ash content of 1.72%-8.17%. Developed as an energy crop, M. floridulu (Labnll.) Warb has the advantages of high biomass yield, excellent quality, low planting cost, strong adaptability, long growing period and good ecological effect. The ecological width and strong adaptability makes it able to survive in all types of soil. Moreover, the biomass of M. floridulu (Labnll.) Warb can be decomposed by microbial flora after mixing with piggery wastewater under anaerobic conditions, producing more biogas［11-12］.
　　Pennisseium sp.
　　Pennisseium sp. is a plant species introduced from abroad. It is a typical fourcarbon plant. A perennial bunch type grass, it has strong tillering ability. It is a variety of high quality Juncao suitable for the growth and cultivation in tropical, subtropical and temperate zones. Pennisseium sp has high plant height, generally 4-5 m, and the highest can reach up to 7.08 m. With strong resistance to stress, it can adapt to the cultivation in the soil of various types. It also has high yield, and the fresh grass yield can reach 300-500 t/hm2 in southern China. Moreover, it can be harvested for years after planting. The high crude protein and sugar content makes it have wide applications. For example, it can be used as the feed for pigs, cows, sheep and fish, or the raw material for biogas production. With developed root systems, it requires huge amounts of fertilizers. In addition, it is easy to breed Pennisseium sp, which can be bred through the vegetative propagation of auxiliary buds or sexual propagation of seeds［13-17］. It is believed that the mixed fermentation of energy plants with livestock manure can ensure the stability of the system［18-19］. The use of Pennisseium sp as energy plants for anaerobic fermentation to produce biogas has great development potential. At the same time, Pennisseium sp has strong tolerance to anaerobic piggery wastewater (COD of 1 075-1 800 mg/L, NH+4N of 643-745 mg/L, TP of 63-70 mg/L). The growth becomes more and more vigorous with the increase of soil fertility. 　　Myriophyllum spicatum.
　　M. spicatum is a member of plant of Myriophyta in the genus of Dioscorea. It has become one of the preferential pioneer species for water treatment and aquatic vegetation restoration project because of its strong adaptability, rapid biomass accumulation and strong pollution resistance. M. spicatum is distributed in both the north and south of China, mainly born in water bodies such as ponds or lakes. It has high ornamental value, fast growth rate, good effect of purifying water bodies, and can effectively remove organic matter as well as nitrogen, phosphorus and other nutrients from the wastewater. M. spicatum can grow in all parts of southern China all year round, but the growth in winter is slightly poorer［20］. The Institute of Subtropical Agriculture of the Chinese Academy of Sciences has built a M. spicatum wetland with an area of 100 000 m2 in a pig farm in Shangyu District, Shaoxing, Zhejiang, which has a stock amount of 50 000 pigs. The wetland system treats 500 t of piggery wastewater daily, with the COD of 70 mg/L and ammonia nitrogen of 1.4 mg/L in the discharged water, which reaches the national standard of emissions for breeding industry in China. M. spicatum can have radial oxygen secretion and generate root exudates, which can provide carbon sources and favorable external environment for wetland microorganisms. M. spicatum also has strong decontamination ability and has strong absorption capacity for nitrogen, phosphorus and other substances in wastewater. It is estimated that M. spicatum wetland can absorb 1-2 t of nitrogen and 100-300 kg of phosphorus per hectare each year. In addition, M. spicatum has high nutritional value and can be used as a highquality feed, which can realize the recycling of nitrogen and phosphorus in the process of absorbing and transforming the piggery wastewater［20］.
　　Conclusion
　　Piggery wastewater contains a large amount of nitrogen and phosphorus, which are essential elements for the growth of plants, and these substances are also the main factors that result in environmental problems such as eutrophication of water bodies. Energy plants with fast growth and large biomass can be used to construct wetlands, which can be used to transform the pollutants in piggery wastewater. In this way, it achieves further recycling, which not only solves the water environmental problems caused by piggery wastewater but also brings great economic benefits, promoting the sustainable development of largescale pigraising industry. Among the energy plants, M. floridulu (Labnll.) Warb, Pennisseium sp. and M. spicatum have been gradually applied to the treatment of piggery wastewater, because they have strong resistances to piggery wastewater, quick absorption and transformation of the nutrient elements like nitrogen and phosphorus, fast growth and large biomass. Moreover, these plants can also be used as the greenfeed or biogas raw materials for recycling. 　　References
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