A constructed wetland for treatment of marine aquaculture wastewater

• Main Author: Webb, Julie Marie
• Published: Bangor University 2012
• Subjects: 628.43
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.574541

This study sought to assess the potential of Salicornia europaea (L) agg. to act as biofilter for nitrogen and phosphorus and to determine its suitability as a halophytic replacement for emergent macrophyte species ego Typhus, Juncus in a constructed wetland (CW) for treatment of wastewater produced by a commercially-operating marine fish and shrimp recirculating aquaculture system (RAS). Assessment included, defining an ideal nitrogen concentration for optimum plant growth and nutrient removal; designing and running a pilot scale CW and evaluating its efficiency through quantification of N and P in the influent and effluent and measurement of plant biomass and tissue nitrogen and phosphorus content, with particular attention given to the impact of regular harvests on CW efficiency. Further investigations focussed on the effect of planting on nutrient removal rates, comparing the performance of planted beds to unplanted controls and looking into the effect of planting density on removal rates and yield. Under experimental conditions, at 2 mmol N r', or less, plants had inhibited growth and showed signs of nitrogen deficiency. In contrast, plants grown at 4 and 6 mmol N r' showed signs of an excess of available nitrogen. Treatment nitrogen levels> 2 mmol N r' had little effect on above-ground tissue N content of S. europaea which would indicate that a plants capacity to sequester nitrogen is finite and has to be assessed in relation to its biomass. Application of nitrogen concentrations up to 4 mmol r' would promote production of the most nitrogen rich part of a Salicornia plant, thereby increasing N removal rates and yield of the most commercially valuable part of the plant. When processing fish farm wastewater at ambient concentrations (93 to 439 umol r' TDIN and 34 to 90 umol r' DIP) the pilot-scale constructed wetland planted with S. europaea was highly effective, removing 97 to 100 % TDIN and 41 to 88 % DIP. The addition of ammonium nitrate fertiliser to the fish farm effluent (increasing TDIN loading from 16.9 ± 4.6 mmol m·2 d-I to 139.0 ± 61.2 mmol N m2 d-I) saw CW performance drop to 30 to 58 % TDIN and 19 to 40 % DIP. However, overall assimilation (max. 263 mmol m-2 d-I) of nitrogen by the CW was significantly higher than that reported under ambient N loading. Over the entire 88 day experiment, the CW removed in total 1.28 ± 0.05 mol N m-2 and 0.11 ± 0.01 mol P m-2. Plant uptake accounted for 85% of total N and 73 % of total P removed. The mean nitrogen content in the tissues of the root, shoot and re-growth of S. europaea were all significantly different from each other, 0.61 ± 0.14, 0.80 ± 0.23 and 1.27 ± 0.36 mmol g-I (root, shoot and re-growth respectively), with highest levels measured in the re- growth. A significant 297 DW g m-2 biomass response to addition of inorganic N could be an indication that plant growth was limited by available nitrogen under ambient loading. Croppping was found to reduce CW performance but encouraged production of the succulent tips preferred by the food market whilst removing nitrogen and phosphorus bound in plant tissues from the CW in exchange for an economic return. By comparison, un-cropped had a greater seed yield and uptake potential. When processing fish farm wastewater at ambient TDIN concentrations between 745 to 4087 umol r' the presence of plants within the beds did significantly improve TDIN uptake over the control beds, the removal rates reflected this and the control beds managed 19.9 ± 18.7 mmol m-2 d-I whilst planted beds removed 62.0 ± 25.5 mmol m-2 d-I TDIN over the experimental period. TDIN removal efficiencies ranged between 34 and 73 % for the planted beds and between 0 and 41 % for the unplanted control beds. Planting density had no apparent effect on the rate of TDIN removal. Planted beds produced a yield of 2.6 ±1.1 and 2.2 ± 0.7 FW kg m-2 (high and mid respectively) VI