An Investigation On The Water Pollution From Aquaculture And Wastewater Treatment Mechanism Of Multi-soil-layer (MSL) System

With an increase demand of aquatic product, the high density farming manner with intensive culture and feeding replaced traditional aquaculture these days, which caused depletion of dissolved oxygen and consequently eutrophication and water deteriorating, further negatively affected the aquaculture industry and the ecosystem. Understanding the pollution situation of aquaculture and controlling the aquaculture wastewater discharge is of great importance to the environment protection. In this study, we focused on the typical aquaculture plant from Zhejiang and Jiangsu as the representatives, tracking for the main pollution resources and their transport pattern, evaluating the feasibility of using multi-soil-layer (MSL) system and subsurface constructed wetland to treat the wastewater of marine cultured Penaeus Vannamei and studied the turtle aquaculture effluent treatment efficiency by the Multi-soil-Layer system (MSL). We then constructed a pilot aquaculture wastewater treatment plant based on the MSL system. The experiment results are listed below: Our main research results are listed below:(1) The water monitoring sites were: Penaeus Vannamei(pond), Palaemon (Exopalaemon) and carinicauda Holthuis(pond) for marine culture; Carassius auratus gibelio(pond), Micropterus salmonides (pond), Channa argus (pond), turtle (industrial aquaculture), breeding larva of Macrobrachium rosenbargii (industrial aquaculture), Bluntnose black bream(enclosure aquaculture, feed), Hypophthalmichthys molitrix(enclosure aquaculture, No feed), and Bluntnose black bream(pond) for fresh-water culture. In general, the pollution in fresh-water culture was more serious than marine culture. Results showed that the compute pollution fluxes of COD, TP, TN and NH4+-N were the highest in turtle industrial aquaculture pond. In Marine Culture, the CPF of COD, TP, TN and NH4+-N were higher in Penaeus Vannamei pond. Every tonne of aquatic products, the discharged quantity of COD, TP, TN and NH4+-N were the highest in Penaeus Vannamei pond. Under the same enconomic income, the discharged quantity of COD, TP, TN and NH4+-N were the highest in Palaemon (Exopalaemon) carinicauda Holthuis (pond). It was suggested that the adjustment of aquaculture industrial structure and industrial distribution should take pollution discharged per unit area, pollution discharged per unit of output and pollution discharged per unit of profit into consideration, to choose aquaculture which is the least negative impact on environment and most beneficial, which make the coordination between aquaculture and environment development.(2) Both the multi-soil-layer (MSL) system and subsurface constructed wetland were able to remediate the wastewater of marine culture, with the MSL system had higher efficiency. The MSL system had average removal rates of chemical oxygen demand(COD), total phosphorus(TP), total nitrogen(TN), ammonia nitrogen (NH4+-N) and nitrate (NO3--N) at 80.38±2.14%,68.14±3.51%,40.79±3.10%,42.68±2.90% and 54.19±5.15%, respectively, all higher than the constructed wetland systems. But we suggested to choose the wetland in which was land rich in natural resourse, and the MSL system is better for the aquaculture factory which was the land resource scarcity.(3) We studied the sludge addition's effect on the wastewater treatment efficiency by MSL system by adding sludge at dry weight ratios of 0%,5%,10% and 20%(MSL1, MSL2, MSL3 and MSL4, respectively). The results showed that the four MSL systems could effectively remove the COD, NH4+-N and TN, among which, MSL4 showed significantly improved NH4+-N removal efficiency, suggesting the potential of sludge addition to improve turtle aquaculture effluent treatment. The average COD, TN and NH4+-N removal efficiencies of MSL4 were 70.3%,66.5% and 72.7%, respectively. To further interpret the contribution of microorganisms to the removal, the microbial community composition and diversity of the four MSL systems were analyzed. Comparison of the denaturing gradient gel electrophoresis (DGGE) profiles revealed that the amount of nitrifying bacteria and diversity in MSL4 was higher than in the other three systems. We conclude that adding 20% of sludge improved Nt4+-N removal and stability of the system for nitrification are due to enrichment of nitrifying bacteria in MSL4.(4) The pilot MSL plant had an average effluent concentration of COD at 65.8 mg/L, NH4+-N at 4.46mg/L, TP at 7.30mg/L during the running period, with removal rates of COD at 97.4%, NH4+-N at 96.6%, and TP at 93.7%. The pilot experiment showed that MSL performed a good result in the treatment of aquaculture wastewater. Due to the concentration of TP exceeded the designed parameter in influent, TP in effluent could not meet the sstandard.(5) We studied the microbial community composition in pilot MSL which has been operated stably for 2 years. The results of 454 high throughput sequencing analyses showed that the predominant phylum of bacteria in pilot MSL were similar with which in other wastewater treatment plant, while the predominant genus of bacteria were significant different between which in the pilot MSL and in other wastewater treatment plant. The results validated that the Bacteroidetes bacteria in PI layer participated in COD removal process. It also found that the Planctomycete, Proteobacteria, and Chloroflexi bacteria were coexisting in the pilot MSL, suggesting the anaerobic ammonium oxidation and denitrification were coexisting in the pilot MSL. The results of Quantitative Real-time PCR analyses showed that:the nirK and nirS genes were detected in the system, indicating there were various kinds of denitrification colonies and nitrogen removal pathways. Most of the denitrification functional genes in MSL were detected in SMB, and amoA gene was only detected in SMB, while nxrB was only detected in P2. It indicates that the ammonia oxidation and denitrification reaction in the system may mainly occur in SMB, while nitrite oxidation may occur in P2 layer.