Purification Efficiency and Influencing Factors of Combined Bio-filters for Aquaculture Wastewater

Abstract

As an effective technology for wastewater treatment, bio-filter has been widely used. Nevertheless, there is still a lack of systematic report on purification efficiency and influencing factors of combined bio-filters. To this end, a novel combined system that consisted of aerated vertical-flow filter (AVF) followed by baffled horizontal-flow filter (BHF) was designed. After setting a series of hydraulic loading rates (131, 94 and 60 mm·d-1) and diversion ratios (8:2、6:4、4:6), we comprehensively assessed the impact of running condition adjustment on treatment performance by multiple statistical analyses. The results showed that, the average removal rates of organic matter, ammonia nitrogen and dissolved nitrogen in AVF were all above 80%, while the average removal rates of ammonia nitrogen, total nitrogen and dissolved nitrogen in BHF were all below 40%. Different running conditions had a significant (P<0.05) impact on treatment performance. Meanwhile, there were significant differences in purification efficiency between the two different kinds of filters. Oxidative degradation was one of the main ways to remove organic matter in the two kinds of filters. There were obvious nitrification and denitrification processes within the two kinds of filters. Nitrification followed by denitrification was the main way to remove total nitrogen since ammonium occupied the most portion of total nitrogen in the synthetic wastewater. Meanwhile, the intensity of nitrification and denitrification in AVF was obviously higher than that in BHF. Phosphorus removal was mainly controlled by hydraulic loading rate, temperature, dissolved oxygen, organic matter, etc. This might indicate that microbial absorption was one of the main ways to remove phosphorus for the two filters. Compared to the sole AVF, the removal of total organic matter and total phosphorus in the combined system was increased by 4.4% and 23.2%, respectively, but the removal of total nitrogen was reduced by 12.1%. Reducing the diversion ratio was helpful to improve the denitrification intensity in BHF. However, due to the introduction of excessive ammonia from the raw wastewater, as well as the limited nitrification capacity in BHF, the removal rate of total nitrogen for the combined system was decreased. Therefore, according to the composition of treated raw wastewater, the control of appropriate diversion ratio, residence time and redox conditions inside the filter bed was the key to enhance the overall performance of the combined system.