Treatment of rainbow trout (Oncorhynchus mykiss) raceway effluent using baffled sedimentation and artificial substrates

The treatment performance of a 6 m wide by 67 m long by 0.8 m deep, baffled sedimentation basin receiving rainbow trout (Oncorhynchus mykiss) raceway effluent was evaluated with and without the installation of artificial substrates (Aquamats®). Treatment efficiency was also determined using normal rearing condition effluent loading versus cleaning and harvesting events. Total suspended solids (TSS) removal for the total basin averaged 79% and 71% during normal rearing conditions, as compared to 92% and 79% during cleaning and harvesting operations, when the Aquamats® were installed versus removed, respectively. Total phosphorus (TP) removal by the total basin, with and without Aquamats®, was 20% and 23% during normal rearing conditions as compared to 55% and 65% under cleaning and harvesting conditions, respectively. Higher TP removal during cleaning operations was attributed to sedimentation of particulate fractions. Dissolved nutrient removal (ortho-phosphate (OP), total ammonia nitrogen (TAN), nitrate, nitrite, and total organic carbon (TOC)) was not consistent throughout the basin and did not improve when the Aquamats® were installed. A short contact time and periphyton grazing by isopods may have limited the capacity of the Aquamats®.Calculated retention times with and without Aquamats® for the first half and total basin were 37% and 32% and 27% and 17% less than theoretical values, respectively based on a rhodamine WT dye study. Average surface overflow rates were adjusted accordingly and measured 19.1 m3/m2 day when the Aquamats® were installed, versus 14.8 m3/m2 day when the Aquamats® were removed for the overall basin. These rates are lower than previous recommendations for treating aquaculture effluents, but resulted in high solids removal and consistently low TSS effluent (average <2 mg/L), which may be necessary for strict discharge permits. Use of the overall basin minimized the occurrence of TSS measurements >2 mg/L by 50%. For the first half of the sedimentation basin, the overflow rate averaged 44.1 m3/m2 day with Aquamats® versus 35.8 m3/m2 day without Aquamats®. The majority of effluent treatment occurred within the first half of the basin, which was responsible for 84% and 94% of overall TSS removal, 42% and 100% of overall NH3-N removal and 61% and 80% of overall TP removal during normal and cleaning/harvesting conditions, respectively.