Studies On Construction And Operation Performance Of An Ecological Indoor Recirculating Aquaculture System

In this study, an integrated indoor recirculating aquaculture system (RAS) was established based on the theory of ecological engineering, which combined the technology of constructed wetland (CW) wastewater treatment and the art of intensive indoor aquaculture. The system consisted of primary sedimentation tanks, combined-type constructed wetlands, aeration tanks and culture tanks. The core compartments of the wetland system were three-stage, i.e. down-, up-and horizontal in turn, subsurface flow CWs. The whole experiment covered two stages:during the first one, studies focused on purification efficiency of the CWs without aeration, variation of water quality in the culture tanks, and culture efficacy; during the second one, the research concentrated on transport and purification mechanism of wastewater pollutants within the CWs and the impact of different aeration manners (continuously and intermittently) on purifying efficiency. The main results were listed as follows.1. Under a high hydraulic loading rate (HLR) of 6.6 m/d for the CWs, no obvious variation in water temperature or pH was observed after the wetland treatment, but the level of dissolved oxygen (DO) declined dramatically. Removal of nitrite mainly occurred in the first wetland (down flow), nitrate and total nitrogen (TN) in the second wetland (up flow), and ammonium and total phosphorus (TP) in the third wetland (horizontal flow). The overall percentage removal rates of NH4+-N, NO3--N, NO2--N, TN, TP and CODMn in the wastewater by the wetland system were 46.12%,38.03%,71.84%,21.28%,52.60% and 33.65%, respectively. The average temperature, DO and pH of the effluent from the CWs were 16.5℃,1.18 mg/L and 7.88, respectively. The mean concentrations of NH+-N, NO3--N, NO2--N, TN, TP, and CODMn of the effluent were 1.30 mg/L,0.68 mg/L,0.03 mg/L,5.30 mg/L,0.59 mg/L and 7.06 mg/L, respectively. The wetland effluent (except DO) could generally meet the requirements of water quality for sturgeon culture. The survival rate of reared sturgeon reached up to 93.3%, with a specific growth rate of 0.74 and feed conversion ratio of 1.30. Production of per kilogram fish consumed electric energy of 6.84 kWh and water of 3.77m3.Under the condition of uniform water exchange rate, fish reared under low density grew faster compared to high density, while the shape of culture tanks displayed no significant impacts.2. Under the HLR of 6.6 m/d for the CWs, and the ratio of gas versus water flow being 0.7. the distribution condition of DO was significantly improved after artificial aeration, which accordingly resulted in obvious aerobic\anoxic zones inside the CWs. Accordingly, the purification efficiency was improved to different degrees with the removal rates of NH4+-N, NO3---N, NO2--N. TN, TP and CODMn increased by more than 9.02%,24.19%,4.33%,7.47%,7.49% and 15.24%, respectively. There were significant differences in measured water quality parameters between continuous, intermittent and non-aeration, except nitrite and TSS. Continuous aeration had a better removal effect for NH4+-N, TN, TP, intermittent aeration had a better removal effect for NO3--N and NO2--N, while there was only significant difference in areal removal rate of TP between continuous and intermittent aeration, but no significant difference for other parameters. Considering saving energy and purifying effect, intermittent manner was more suitable for aeration in this system.3. Considering the above experimental results wholly, the art of the constructed wastewater treatment facilities was simple, but with high treatment efficiency. The ratio of water reuse in the RAS amounts to more than 94% and extremely improved the reuse of water resources; during the whole rearing process, there was no incidence of fish disease, and the survival rate for sturgeon amounts to more than 95%, and the total yield reached 3133 kg corresponding to an areal production of 20 kg/m-. During the whole rearing period, the RAS consumed electric energy of 21523 KWh corresponding to a consumption of 6.87KWh/kg (fish). Hence, the RAS possesses the characteristics of low investment and energy consumption, safety and easy management. The study project carried out the aims of healthy culture, resource conservation, and environmental friendly, which fit the current national conditions well and therefore, has a prospect for expansion and application to some extent across the country.