What They Did Back Then
* Master of Science in Aquaculture completed 2000, University of Guelph
Advisor: Professor Richard D. Moccia
Section 1: Literature Review of Phosphorus removal technologies, and their feasibility at Recirculation Fish Farms in Ontario.
Section 2: Bench Scale Experiment on Phosphorus Removal Using Calcium Hydroxide as a Precipitation Agent.
Statement of Problem:
The refusal of the provincial government to adopt an alternative method of phosphorus monitoring has created problems for potential recirculation facilities in Ontario. If the phosphorus testing were based on cumulative discharges instead of absolute values (0.05 mg/l), farms would be able to meet the compliance limits. This is because over time their phosphorus outputs are not more than those of farms that utilize a flow through water system. As it stands recirculation facilities cannot meet the current phosphorus effluent guidelines because a cost effective reliable method to remove the phosphorus from the system before it is discharged has not been put into use.
There are methods currently used in waste treatment plants or in a developmental stage that remove phosphorus at the secondary and tertiary waste treatment levels. These include the use of chemical coagulants such as alum, ferric and calcium, the use of wetlands, biological methods, membrane technologies and the use of phosphate binding hydrogels. These technologies have not yet been utilized on a large scale at Ontario recirculation facilities for a variety of reasons including cost, ineffectiveness, and lack of development. To solve the phosphorus problem in aquaculture waste either and effective method of phosphorus removal must be adopted or the provincial government must change the set phosphorus compliance limits for recirculation farms.
There is no sign that the government is going to change the phosphorus guidelines for recirculation farms. Consequently, the goal of this project is to identify and develop a technology suitable for removing phosphorus from aquaculture waste. To accomplish this goal the following objectives must be met:
- Complete literature review of main strategies for phosphorus removal and their feasibility at Ontario recirculation farms.
- Perform laboratory testing on most practical phosphorus removal technique.
- Measure ambient total phosphorus levels of actual recirculation facility.
- Speculate on suitability of chosen phosphorus removal technique at an Ontario recirculation farm.
The effectiveness of calcium hydroxide as a precipitation agent for phosphorus was assessed in lab scale experiments. Calcium hydroxide was added in increasing amounts to fourteen synthetic wastewater samples that were spiked with phosphorus levels of 6.43 mg/l. The samples were stirred and allowed to settle for 20 min. A maximum phosphorus removal efficiency of 85.54% was achieved through the addition of 309.9 mg/l of Ca (OH)2, which corresponds to a pH of 11.57. The phosphorus removal efficiencies increased from 38.10% at a Ca(OH)2 concentration of 73.15 mg/l and a pH of 9.39 to 66.25% at a Ca(OH)2 concentration of 83.20 mg/l and a pH of 9.61. These results confirm that the reaction is pH dependant and a minimum pH of 9.5 is required to achieve significant phosphorus precipitation. The removal efficiencies from the lab experiments were used on actual phosphorus effluent levels from the Northern Tilapia recirculation farm. Using these efficiencies the use of calcium hydroxide to precipitate phosphorus out of water resulted in a decrease in phosphorus levels from 4.8 mg/l to 0.69 mg /l.