Suitable substrates to improve efficiency of biological aerated filter for greenhouse turtle breeding wastewater treatment

Abstract: The greenhouse turtle breeding wastewater contains high concentration of nitrogen and phosphorous. Its unordered discharge has seriously affected the quality and safety of daily and production water supply of the surrounding residents. The greenhouse turtle breeding wastewater has become one of the main pollution sources in the rural area of the Yangtze River Delta, especially in the compact district of turtle aquaculture. In order to provide some experiences for the practical application in a turtle aquaculture farm, the experiment would explore the feasibility of a biological aerated filter (BAF) for greenhouse turtle breeding wastewater treatment, and select a suitable substrate for the BAF. In this paper, we have investigated three types of substrates filling in the BAF, i.e. clinoptilolite, bio-ceramic and gravel, in terms of their capacity of adsorbing the nitrogen and phosphorous in the greenhouse turtle breeding wastewater, and the performance of purifying wastewater. The results have showed that the nitrogen adsorption capacities in the three substrates are similar in the range of 0.469-0.563 mg/g, but that the phosphorous adsorption capacity greatly varies from 0.003 to 0.114 mg/g with the best one from the clinoptilolite. A pilot scale of BAF for greenhouse turtle breeding wastewater treatment was built in one of the typical greenhouse turtle aquaculture farms in the Yangtze River Delta. After 122 days running in the BAF with different influent concentration, the results showed that the BAF with clinoptilolite, bio-ceramic and gravel substrate have good removal efficiency for COD, NH4+-N, TN, TP and so on. The result of pilot engineering has showed that average COD, NH4+-N, TN and TP removal efficiencies of a biological aerated filter are 59.4%-61.1%，93.2%-97.2%，54.4%-71.1% and 62.7%-84.3%, respectively. The clinoptilolite filter excels in the removal of total nitrogen (TN) compared to the others, whereas the bio-ceramic and the gravel filters have the better removal capacity of total phosphorous (TP) than the clinoptilolite filter. In addition, the nitrogen and phosphorus removed from greenhouse turtle breeding wastewater by BAF was the combined result of substrate adsorption and microbial functions, which extended the useful life of substrate in BAF. In addition, the types of substrate have little influence on the pollutant removal efficiency of the BAF. Simultaneously, considering the cost and resource of substrate, the gravel substrate would be an ideal substrate of BAF for greenhouse turtle breeding wastewater treatment. However, the clinoptilolite substrate that has a higher nitrogen removal capability would be a suitable substrate for a greenhouse turtle aquaculture farm located in an environmental sensitive area. Therefore, our results suggest the high value of application and dissemination of biological aerated filter treatment in the greenhouse turtle breeding wastewater in China.