Discharge Distribution and Quality of Tail Water for Mariculture in Tianjin

Daiyan WEI Jiahong LIU Xiaoming CHEN Meng SUN

Abstract [Objectives]This study was conducted to understand the pollution load of mariculture in Tianjin. [Methods]The tail water discharged by the main mariculture methods in Tianjin was monitored， and the tail water quality was evaluated. According to the aquaculture area and distribution and the discharge destination of the aquaculture tail water， a preliminary evaluation of the pollutant discharge situation was carried out， and different aquaculture methods were compared and analyzed for the differences in discharged pollutants. [Results]The main pollution factors in pond mariculture were chemical oxygen demand （CODMn）， active phosphate and total phosphorus. The main pollution factors in factory mariculture were inorganic nitrogen， active phosphate and total nitrogen. Pond mariculture showed higher up-to-standard rates of inorganic nitrogen and active phosphate contents and lower up-to-standard rates of suspended solids， CODMn， and total phosphorus content than factory mariculture， and the up-to-standard rates of total nitrogen and total phosphorus contents in the flowing water type of factory mariculture were higher than those in the circulating water type. The discharge of pond mariculture was less than factory mariculture， and mariculture had the greatest impact on Fuzhuang Drainage Canal in that 92.3% of the total pollutant discharge monitored went to Fuzhuang Drainage Canal. The circulating water type could reduce pollutant discharge compared with the flowing water type of factory mariculture. [Conclusions]This study proposed countermeasures and suggestions for reducing pollution from marineculture.

Key words Pond; Factory; Mariculture; Pollutants; Evaluation

Tianjin is close to the Bohai Sea， from Qikou in Hebei Province in the south to Jianhekou in Hebei Province in the north， with a coastline of 153 km and a sea area of 3 000 km[1]. It is rich in geothermal resources， and the sea water contains high organic matter and nutrient content. It has extensive salt fields and abundant brine resources， and has good natural conditions for the development of mariculture. In recent years， with the high-intensity development of the mariculture industry and the transformation of aquaculture methods to intensive[2]， marine aquaculture pollution has become one of the main marine environmental problems currently concerned by people[3-6]. Most aquaculture species are heterotrophic species， and require input of baits during the breeding process. Residual baits and metabolites of aquatic products during the aquaculture process are one of the main sources of nitrogen and phosphorus pollution in the breeding system[7]. In recent years， the Bohai Bay has suffered from serious eutrophication[8]， and most of the tail water produced by aquaculture is directly discharged without purification， which further aggravates the occurrence of red tides[9-10]. At present， there are also reports on the pollution load of mariculture in Tianjin， but there are few studies on the pollution of different aquaculture methods and the impact on rivers entering the sea. Based on this， in this study， we conducted monitoring and analysis of the water quality of mariculture tail water in different aquaculture methods， and proposed corresponding measures to reduce mariculture pollution.

Materials and Methods

Monitoring object

According to the current status of mariculture in Tianjin， mariculture methods mainly include pond and factory farming. The factory farming is divided into circulating water aquaculture and flowing water aquaculture. Seawater ponds in Mapengkou of Dagang District， Yangjiabo Town of Hangu District， and Zhaishang Street in Tianjin were selected as the monitoring objects. A total of 37 drainage stations were set up for monitoring during the pond drainage period from September to October 2019. A total of 17 drainage stations were set up for seawater factory farming， including 9 circulating water ones and 8 flowing water ones， which were monitored from September to October 2019.

Monitoring indicators and analysis methods

In view of the characteristics of mariculture tail water， suspended solids， chemical oxygen demand （CODMn）， inorganic nitrogen， active phosphate， total nitrogen and total phosphorus were selected as monitoring indicators. Among them， the analysis of suspended solids， CODMn， inorganic nitrogen， and active phosphate referred to GB 17378.4-2007 The specification for marine monitoring—Part 4： Seawater analysis[11]; the total nitrogen analysis referred to HY/T 147.1-2013 Code for practice for marine monitoring—Part 1： Seawater[12]; the total phosphorus analysis referred to GB/T 12763.4-2007 Specifications for oceanographic survey―Part 4： Survey of chemical parameters in sea water[13].

Evaluation method

The chemical analysis method is a simplified method of on-site measurement， which mainly depends on the difference in the concentration of pollutants entering and exiting the aquaculture system， combined with the drainage volume to estimate the discharge of pollutants in aquaculture itself[14]. This assessment only considered the quality of mariculture tail water discharge， and the calculation formula was as follows：

P=Q×Cout×10-6.

In the formula， P is the pollutant discharge load; Q is the discharged water; and Cout is the pollutant concentration in the discharged water.

Evaluation standards

The evaluation of the pollution factors of suspended solids， CODMn， inorganic nitrogen and active phosphate referred to the first level standard in SC/T 9103-2007 Water drainage for sea water mariculture[15]. The evaluation of total nitrogen pollution factors followed the V-class water environmental quality standard in GB 3838-2002 Environmental quality standards for surface water[16]. The evaluation of total phosphorus pollution factor was performed according to the second level standard in DB12/356-2018 Integrated wastewater discharge standard[17].

Results and Analysis

Regional distribution of mariculture

Maritime aquaculture in Tianjin is distributed in Binhai New Area. According to the Planning for Aquaculture Waters and Tidal Flats in Binhai New Area and relevant administrative department statistics， the area of mariculture ponds was 3 028 hm2， mainly distributed in Yangjiabo Town and Zhaishang Street， Hangu， Tianjin， followed by Tanggu District and Dagang District. The aquaculture tail water was mainly drained into the sea through Fuzhuang Drainage Canal， Qingjinghuang Drainage Canal and North Drainage River in Tianjin City. Among them， the aquaculture area drained through Fuzhuang Drainage Canal was 2 430 hm2; the aquaculture area drained through Qingjinghuang Drainage Canal was 442 hm2; and the aquatic breeding area drained through the North Drainage River was 132 hm2. The species of pond mariculture was relatively single， which were Chinese prawn and Litopenaeus vannamei.

In recent years， due to the limited extraction of groundwater， the number of factory-based marine aquaculture and seedling production enterprises had shrunk from 95 to 36， with a total aquatic breeding area of 78.8 hm2， of which 6 had circulating water facilities， with an aquatic breeding area of 11.7 hm2， and 3 had zero tail water discharge， with an aquatic breeding area of 11.3 hm2. Existing industrial seawater aquaculture tail water was all drained into the sea through Fuzhuang Drainage Canal， and the main breeding species were Cynoglossus semilaevis and Scophthalmus maximus.

Whereabouts and discharge of mariculture tail water

According to surveys， the average water depth of the pond mariculture areas was about 1.0 m， and the water was drained once a year， and all drained. The average water depth of the factory mariculture ponds was about 0.5 m. Specifically， the daily water exchange volume of the circulating water type accounted for approximately 20% of the total water volume， and the daily water exchange volume of the flowing water type accounted for approximately 100% of the total water volume. According to calculations， the annual drainage of marine aquaculture in Tianjin was about 136.434 million m3， of which the annual drainage of pond marine aquaculture was about 30.28 million m3 （Table 1）， and the annual drainage of factory mariculture was about 106.154 million m3 （Table 2）. Marine aquaculture tail water was mainly discharged into the sea through surface rivers such as Fuzhuang Drainage Canal ， Qingjinghuang Drainage Canal and North Drainage River， and 80.3% was drained into Fuzhuang Drainage Canal.

Water quality of mariculture tail water

It can be seen from Table 3 and Fig. 1 that the up-to-standard rates of suspended solids， CODMn， inorganic nitrogen， active phosphate， total nitrogen， and total phosphorus in pond mariculture were 94.6%， 18.5%， 48.4%， 27.0%， 46.2%， and 38.7%， respectively. The pollution factors that exceeded the standard were mainly CODMn， active phosphate， and total phosphorus， which exceeded the standard by 0.04-1.30， 0.06-15.94， and 0.08-2.80 times， respectively. For the flowing water factory-based marine aquaculture， the up-to-standard rates of suspended solids， CODMn， inorganic nitrogen， active phosphate， total nitrogen and total phosphorus were 100%， 85.7%， 12.5%， 0%， 57.1% and 100%， respectively; and as to the circulating water factory mariculture， the up-to-standard rates of suspended solids， CODMn， inorganic nitrogen， active phosphate， total nitrogen and total phosphorus in mariculture were 100%， 100%， 0%， 0%， 12.5%， and 50.0%， respectively. The pollution factors exceeding the standard were mainly inorganic nitrogen， active phosphate and total nitrogen. The contents of inorganic nitrogen， active phosphate and total nitrogen in flowing water factory mariculture exceeded the standard by 0.73-9.32， 1.06-4.22， and 0.07-0.61 times， respectively， and the circulating water factory mariculture exceeded the standard by 1.03-9.34， 4.10-7.82 and 0.09-3.23 times， respectively. Compared with circulating water factory farming， the average concentrations of suspended solids and CODMn in flowing water factory farming were higher， and the average concentrations of inorganic nitrogen， active phosphate， total nitrogen and total phosphorus were lower.

It can be seen from Fig. 2 that compared with factory mariculture， the average concentrations of suspended solids， CODMn， and total phosphorus in the tail water of pond mariculture were higher， the average concentration of inorganic nitrogen was lower， and the average concentrations of active phosphate and total nitrogen were not much different. As shown in Fig. 1， the up-to-standard rates of inorganic nitrogen， active phosphate and total nitrogen in pond mariculture were relatively high， but not more than 50%， and the water quality was poor.

Discussion

Effects of mariculture on surface rivers

The marine aquaculture tail water in Tianjin is mainly discharged into the sea through surface rivers such as Fuzhuang Drainage Canal， Qingjinghuang Drainage Canal， North Drainage River， etc. According to the discharge amount of aquaculture tail water and the average concentration of the tail water， the pollutant discharge conditions of marine aquaculture were calculated （Table 4）. The annual discharge amounts of suspended solids， CODMn， inorganic nitrogen， active phosphate， total nitrogen and total phosphorus were 1 701.1， 1 155.4， 185.0， 28.2， 301.5 and 46.8 t. Based on the total amount of pollutants， the amounts of pollutants discharged into Fuzhuang Drainage Canal， Qingjinghuang Drainage Canal， and North Drainage River respectively accounted for 92.3%， 5.7%， and 1.7% of the total amounts of pollutants， respectively. The discharge amounts of various pollutants from factory-based marine aquaculture were higher than those from pond aquaculture， and the discharge amounts from flowing water factory aquaculture were higher than those from circulating water factory aquaculture （Fig. 3）.

Comparison of different mariculture methods in emission reduction

It can be seen from Table 5 that if marine aquaculture must meet the emission standards， pollutants from pond and factory marineculture must be reduced annually by 14.5 t for suspended solids， 228.9 t for CODMn， 126.6 t for inorganic nitrogen， 21.6 t for active phosphate， 90.9 t for total nitrogen， and 9.8 t for total phosphorus， and the reduction ratios were 0.9%， 19.8%， 68.4%， 76.6%， 30.1%， and 20.9%， respectively.

All pollutants in pond aquaculture need to be greatly reduced. The circulating water-type factory farming needs to significantly reduce inorganic nitrogen， active phosphate， total nitrogen and total phosphorus， and the flowing water-type factory farming needs to significantly reduce CODMn， inorganic nitrogen， active phosphate and total nitrogen （Fig. 4）.

Effects of different factory mariculture modes on pollutant discharge

The flowing-water factory farming has a large water exchange proportion and high water exchange frequency， so although the concentrations of nitrogen and phosphorus are low， the tail water discharge is large， and the pollutant discharge is relatively high compared with the circulating-water factory farming. If all the flowing-water factory farming is transformed into circulating water factory farming， it will greatly reduce the discharge of various pollutants according to the current activity levels of the two methods， and the annual discharge reductions will be： 862.0 t of suspended solids， 577.5 t of CODMn， 91.1 t of inorganic nitrogen， 11.2 t of active phosphate， 102.9 t of total nitrogen， and 16.3 t of total phosphorus， which led to the discharge reduction ratios of 92.0%， 83.8%， 59.8%， 61.5%， 55.2% and 63.9%， respectively （Table 6）.

Countermeasures for pollution prevention and control in mariculture

We should reasonably plan the aquiculture waters. Determining the appropriate production scale of waters by environmental capacity is an effective way to solve the contradiction between aquiculture and environment. Large-scale intensive farming has led to the input of a large amount of exogenous nutrients， and the residual bait and manure produces water pollutants， which seriously affects the water environment[18]. It is necessary to scientifically plan the aquaculture water surface to determine the loading capacity of an aquaculture water body for nutrient elements， thereby determining the aquaculture capacity of the water body， and effectively reduce the impact on the water environment[19].

High-quality feed should be chosen. Baits contain a lot of nutrient elements， and contain nitrogen as high as 6.56% and phosphorus as high as 1.0%[20]. Residual baits will not only settle to the bottom of ponds， resulting in nutrient enrichment of the sediment， and the accumulation of residual baits promote the strengthening of microbial activities， which accelerates the regeneration of nutrients. In addition， some nutrients will dissolve in the water， seriously polluting the water environment. Reducing the bait coefficient and reducing the residual baits is an important measure to effectively control water pollution from the front stage of the aquaculture process[21].

We should strengthen mariculture tail water treatment and aquaculture water recycling technology[22]. On the one hand， more than 90% of mariculture tail water in Tianjin is directly discharged without treatment. Factory farming is mostly continuous drainage， but the proportion of tail water treatment is low， only 6.3% according to the survey. Due to seasonal influences， pond farming is cleaned and drained in September and October every year， and the tail water is directly discharged into the sea and rivers without purification treatment. Because a large amount of tail water is discharged within a period of time， which has an important impact on some rivers that enter the sea， and then affects the water quality of the seawater around the estuary. It is necessary to adjust measures according to local conditions and choose appropriate aquaculture tail water treatment technology to strengthen the treatment of aquaculture tail water. On the other hand， the popularizing rate of circulating water is low and the tail water discharge is large. None of the currently adopted circulating water treatment technologies consider the removal of nitrogen and phosphorus nutrients， and a longer circulation time leads to a higher concentration， a higher discharge level， and more serious impact on the water environment[23]. Therefore， the protection of the water environment should be considered in the design of circulating water facilities.

We should strengthen supervision and management. At present， Tianjin has not issued standards related to marine aquaculture， and the standards issued by the Ministry of Agriculture and Rural Affairs are recommended standards， which are not applicable to a certain extent in terms of control items and limits. Relevant management departments should strengthen the supervision and management of mariculture tail water discharge， and implement effective prevention and control of water pollution in the form of systems[24].

Conclusions

This study showed that the main pollution factors exceeding the standard of pollutants in mariculture ponds in Tianjin were CODMn， active phosphate and total phosphorus， and the main pollution factors exceeding the standard of pollutants in factory mariculture were inorganic nitrogen， active phosphate and total nitrogen. The pollutant discharge of factory-based mariculture was greater than that of pond mariculture， and the circulating water type could greatly reduce pollutant discharge compared with the flowing water factory-based aquaculture. Mariculture had the greatest impact on Fuzhuang Drainage Canal. To achieve discharge satisfying the standards， mariculture needs to greatly reduce pollutant discharge， and we should vigorously develop the circulating water culture model， while scientifically feeding baits and setting a reasonable aquaculture density.

References

[1]JIA WP， FU ZR， ZHANG W. The status quo and suggestion of sea water factory farming in Tianjin[J]. Hebei Fisheries， 2018（5）： 27-29. （in Chinese）

[2]XIN NH， ZHANG SW， YANG YH， et al. Analysis on development process and current situation of Tianjin industrial marine aquaculture[J]. Fishery Modernization， 2019， 46（2）： 1-6. （in Chinese）

[3]ZONG HM， YUAN XT， WANG LJ， et al. Preliminary evaluation on the nitrogen and phosphorus loads by mariculture in China[J]. Marine Environmental Science， 2017， 36（3）： 336-342. （in Chinese）

[4]CUI Y， CHEN BJ， CHEN JF. Evaluation on self-pollution of marine culture in the Yellow Sea and Bohai Sea[J]. Chinese Journal of Applied Ecology， 2005， 16（1）： 180-186. （in Chinese）

[5]CAI HW， ZHUO LF， WU CW. Review of waste loadings generation from marine aquaculture[J]. Journal of Zhejiang Ocean University： Natural Science， 2014， 33 （6）： 558-567. （in Chinese）

[6]CAO L， WANG WM， YANG Y， et al. Environmental impact of aquaculture and countermeasures to aquaculture pollution in China[J]. Environmental Science and Pollution Research-International， 2007， 14（7）： 452-462.

[7]YUAN XT， ZHANG SL， LIU SX， et al. Self-pollution in Ruditapes philippinarum bottom-cultured area of Zhuanghe coast[J]. Chinese Journal of Applied Ecology， 2011， 22（3）： 785-792. （in Chinese）

[8]KAN WJ， FAN DJ， ZHANG QF， et al. Seasonal variation and assessment of water quality in Bohai Bay in Tianjin Shore Sea Areas[J]. Transactions of Oceanology and Limnology， 2016（1）： 25-29. （in Chinese）

[9]XIA LH， XU S， CHEN ZB， et al. Mariculture pollution contribution rate of coastal water in Guangdong Province[J]. Journal of Guangzhou University： Natural Science Edition， 2013， 12（5）： 80-86. （in Chinese）

[10]CAO JH. Current situation of aquaculture industry's own pollution and its control countermeasures[J]. Social Scientist， 2018（2）： 46-50. （in Chinese）

[11]Standardization Administration， General Administration of Quality Supervision， Inspection and Quarantine of the People's Republic of China. GB 17378.4—2007 The specification for marine monitoring—Part 4： Seawater analysis[S]. Beijing： China Standards Press， 2007： 88-91， 101-103， 109-120. （in Chinese）

[12]State Oceanic Administration. HY/T 147.1—2013 Code for practice for marine monitoring—Part 1： Seawater[S]. Beijing： State Oceanic Administration， 2013： 23-25. （in Chinese）

[13]Standardization Administration， General Administration of Quality Supervision， Inspection and Quarantine of the People's Republic of China. GB 12763.4—2007 Specifications for oceanographic survey—Part 4： Survey of chemical parameters in sea water[S]. Beijing： China Standards Press， 2007： 25-26. （in Chinese）

[14]ZHANG YZ， HONG HS， CHEN NW， et al. Discussion on estimating nitrogen and phosphorus pollution loads in aquaculture[J]. Journal of Xiamen University： Natural Science， 2003， 42（2）： 223-227. （in Chinese）

[15]Ministry of Agriculture. SC/T 9103—2007 Water drainage for sea water mariculture[S]. Beijing： The Ministry of Agriculture of the People's Republic of China， 2007： 1-2. （in Chinese）

[16]State Environmental Protection Administration， General Administration of Quality Supervision， Inspection and Quarantine of the People's Republic of China. GB 3838—2002 Environmental quality standards for surface water[S]. Beijing： State Environmental Protection Administration， 2002： 2. （in Chinese）

[17]Tianjin Administration for Market Regulation， Tianjin Ecology and Environment Bureau. DB12/ 356—2018 Integrated wastewater discharge standard[S]. Tianjin： Tianjin Ecology and Environment Bureau， 2018： 5. （in Chinese）

[18]DNES G， FERENC P， VA K. A survey on the environmental impact of pond aquaculture in Hungary[J]. Aquaculture International， 2016， 24（6）： 1543-1554.

[19]LIU MQ， XI YG， CHEN QH， et al. Policy recommendations for aquaculture environmental management and pollution reduction[J]. Chinese Journal of Environmental Management， 2019（1）： 90-94. （in Chinese）

[20]ZHANG W， ZHUO JH. Preliminary study on estimation of nitrogen and phosphorus pollution load of aquaculture in Tianjin[J]. Scientific Fish Farming， 2018（6）： 54-56. （in Chinese）

[21]WANG J， JIANG B， HAN JB， et al. Distribution and impact of effluents in aquaculture ponds on aquatic environments in Liaoning Province[J]. Fisheries Science， 2013， 32（3）： 165-170. （in Chinese）

[22]WU Y， ZHAO HT， SUN GQ， et al. Research and evaluation of pollutant emission reduction effect in seawater factory aquaculture[J]. Hebei Fisheries， 2018（12）： 1-4. （in Chinese）

[23]WANG XH， GENG XY， LIU Y， et al. The status quo and development countermeasures of industrial aquaculture in Hangu， Tianjin[J]. Tianjin Fisheries， 2013（1）： 13-18. （in Chinese）

[24]YU RG. Discussion on the pollution of aquaculture environment and its control countermeasures[J]. Jiangxi Agriculture， 2016（7S）： 28. （in Chinese）