Factors influencing the efficiency of constructed wetlands used for the treatment of intensive trout farm effluent

Abstract

In this paper the factors influencing treatment performance of subsurface flow constructed wetlands (SSF wetlands) treating aquaculture effluents were identified and quantified. The financial impact of advanced aquaculture effluent treatment with SSF wetlands was calculated. It is the first long-term, commercial-scale trial of SSF wetland treatment for effluents from intensive trout farming, a highly diluted effluent at very high flow rates (mean total phosphorous concentration 0.34 mg L −1 at 14.3 L s −1). The 12-month survey provided the opportunity to generate calculation fundamentals for the commercial application of SSF wetlands for aquaculture. Treatment efficiencies of up to 75–86% for total ammonia nitrogen (TAN), biological oxygen demand (BOD 5) and total suspended solids (TSS) were achieved. The daily area retention rate per square meter wetland area was between 2.1 and 4.5 g for TAN and between 30 and 98 g for TSS. The performance of the six wetland cells comprising three replicated hydraulic loading groups (14.5, 6.9, 3.3 m 3 m −2 day −1) was monitored, offering the possibility to identify factors influencing treatment efficiency through multifactor analysis. These factors turned out to be nutrient inflow concentration, hydraulic loading rate and accumulation of TSS within the wetland bed, the only time-dependent factor. Factors such as vegetation period and fish harvesting were shown to be of significant but negligible importance. Inflow nutrient concentration is determined by production intensity, husbandry conditions, feed quality and any pre-treatment of effluent. Hydraulic load is determined by the space and budget available for SSF construction. TSS accumulation in the wetland is influenced by pre-treatment of the solid fraction prior to the wetland and determines the wetland service lifetime. From these factors the expenses of commercial wetland application can be estimated, leading to a cost increase around €0.20 kg −1 fish produced (less than 10% of production costs) and therefore confirm the commercial feasibility of SSF wetland treatment.