Design and performance of Multiway Gravity Device on removing suspended solids in aquaculture water

Abstract: Suspended solid (mostly comes from feed residue, fish feces and biological floc) is the most primary pollutant of aquaculture water, especially in the recirculating aquaculture system (RAS), which will indirectly increase the BOD and oxygen cost and also influence the fish health. Therefore, rapid removal of solid wastes is the most basic unit process in the RAS. Many different kinds of solids removal devices have been developed in these years, such as swirl separator, settling basin, micro-screen drum filter, foam separator. In all these filtrate methods, sedimentation method is one of the most effective and the simplest technology. However, most of the settling devices have the disadvantages of low water treatment capacity and efficiency. For these reasons, a new kind of solids removal device, based on gravity settling, was developed, named as multi-way gravity settler (MGS) which comprises of settling basin, sloping tube material, spray cone and etc. Influent water will firstly flow through a vertical pipe to the bottom of the settling basin. Reflected by the spray cone, the water will uniformly flow through the sloping tube material which can effectively increase the hydraulic retention time of the water and make the suspended solids settling on the surface of the material. In this paper, several tests have been made to evaluate the performance of MGS in different conditions, which use TSS removal efficiency as the test index. Five kinds of hydraulic retention time were designed which were 5, 10, 20, 30 and 40 minutes respectively. The result showed that by the increasing of hydraulic retention time, the TSS removal efficiency between the inlet and outlet of the MGS was significantly increased (P＜0.05). The best removal ratio was 58.57%±10.12%, which appeared when the hydraulic retention time reached 20 min. The difference is not significant although the removal efficiency increased after 20 min. Additionally, four specifications of sloping tube material were designed, which respectively were 35 mm×500 mm, 35 mm×600 mm, 50 mm×500 mm and 50 mm×600 mm (diameter×height). The result showed that the smaller tube diameter 35 mm could significantly increase the TSS removal efficiency to 49.12% (height=500 mm) and 50.90% (height=600 mm). And there is no significant difference for the removal efficiency between the material height sizes of 500 mm and 600 mm. Combined with the earlier results, the TSS removal ratio for different particle diameters was researched. It showed that for the diameter smaller than 20 μm, the TSS removal ratio was 19.5 %, while for the diameter bigger than 60 μm, it could reach more than 90%.