Minimizing phosphorus discharge from aquaculture earth ponds by a novel sediment retention system

Abstract

Freshwater fish farms in Quebec are facing stringent phosphorus discharge limits of 4.2 kg P per tonne of fish produced. Most phosphorus in fish farm effluents is found in particulate form (uneaten food, feces, etc.). Physical separation systems such as microscreens, filter beds, Cornell-type circular tanks and settling tanks have been proposed to remove solids from raceway and recirculation fish farm effluents but these technologies are relatively expensive and labour intensive for small pond based production facilities, as mostly found in Quebec. A novel sediment retention system (SRS), consisting of a 1 m 3 truncated pyramid, was installed at the bottom of an earth pond, below a surface aerator. The objective of this study was to determine the feasibility of collecting and removing sediments by using the SRS and to determine its particulate phosphorus removal efficiency. Solids accumulated in the SRS were quantified and characterised weekly, for 10 weeks. Fish production, food supply, rain events and fish harvesting were also monitored over the course of the study period. The total solids (TS) accumulation rate in the SRS was, on average, 4.0 kg/d with a volatile solids fraction of 7.8% and the P accumulation rate was, on average, 12.4 g P/d. The P removal efficiency obtained with the SRS was 24% of the total P not taken up by fish (effluent P, P EFF). Assuming that 50% of the P EFF was in the particulate form, the removal efficiency of the SRS was 47%. Lab-scale results correlated with the P mass balance calculations to show that sediments from earth ponds can play an active role in the sorption of soluble phosphorus. Approximately 30% of the P removed from the SRS was attributed to previous sorption/precipitation of soluble P into inorganic forms. The Langmuir model fitted the sorption isotherm of phosphorus onto earth pond soil and the maximum sorption constant obtained was 1.3 mg P/g soil. Fish harvesting was identified as the main external factor affecting sediments and phosphorus accumulation in the SRS. It was concluded that an SRS located under the aerator surface is a suitable and efficient strategy to collect and remove particulate phosphorus generated from fish production in earth ponds.