Field and Lab Scale Performance of Phosphorus Removal in Post-Wetland Slag Filters Treating Aquaculture Wastewater in Cold Climates
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Eutrophication, caused by phosphorus and nitrogen overloading, is a global pollution problem for our fresh water lakes and streams. Regulatory bodies have developed treatment guidelines for point-source pollution to address this concern, creating a need for small and remote waste producers to develop low cost solutions for nutrient removal – in particular phosphorus. Constructed wetlands have often been implemented as a low cost alternative to treat various sources of wastewater, however, their efficiency in removing phosphorus has been questioned. Post-treatment blast furnace slag filters (known to have a high phosphorus sorption capacity) have been in operation since 2008 in Haliburton, Ontario, Canada, treating aquaculture wastewater following treatment by a constructed wetland. Phosphorus removal performance of three filters of varying configuration have been assessed: • a random packed filter, packed randomly with unsieved blast furnace slag removed only 26% of phosphorus overall and became severely clogged likely due to the presence of fines, poor flow distribution and formation of calcium carbonate precipitate; • a screen filter, designed as twenty individual filters separated by 3 inches of space to provide more uniform flow throughout removed 55% of the total phosphorus added and maintained a uniform flow throughout the study; and • an anaerobic filter, designed to be sealed from the atmosphere in an effort to prevent the formation of calcium carbonate and subsequent filter clogging, removed 19% of the total phosphorus added and revealed signs of poor flow distribution. Lab-scale flow through cells were designed to parallel the field scale studies and were operated at varying configuration, temperature and residence times to assess these factor effects on phosphorus removal. The results of this research provide a valuable contribution to research involving blast furnace slag filters for phosphorus removal. It has been demonstrated that cold temperatures reduce the slag filter’s efficiency to remove phosphorus (but cold-climate filters can still be effective in removing phosphorus), the removal of slag fines reduces the potential for slag clogging, a screen design is effective in promoting uniform flow and offsetting the effects of clogging and an optimum residence time may exist for phosphorus removal beyond which phosphorus removal is minimal.