Struvite Control: An Alternative Approach

A major issue encountered in dewatering facilities of anaerobic digested sludge that also impacts downstream facilities is the precipitation of magnesium ammonium phosphate (MAP) commonly known as struvite, a problem that plagues many wastewater treatment plants. The most common method to reduce struvite formation is addition of ferric chloride (FeCl3) to the digested sludge prior to dewatering or directly to centrate. An alternate control method that may be considered is to induce preferential struvite precipitation prior to dewatering by air stripping the carbon dioxide from the digested sludge which will raise the pH causing preferential formation of struvite and subsequent capture in the dewatered cake.The City College of New York (CCNY) in conjunction with New York City Environmental Protection, (NYCEP), conducted a bench-scale study to assess the two struvite control methods. The digested sludge used in all tests was collected from the digesters at the Wards Island wastewater treatment plant, (WWTP). The ferric chloride dosing test procedure duplicated the practice underway at the Wards Island WWTP using the traditional Jar test apparatus. The air stripping of carbon dioxide method used was similar to the “AirPrex” process developed by Berliner Wasserbetribe (BWB) and utilized at the Waßmannsdorf WWTP in Germany. Several air flow rates were tested with and without supplementing the magnesium available in the sludge by adding magnesium chloride. Based on the experimental results, air stripping appeared to be equally effective in controlling subsequent struvite formation compared to the commonly used method of dosing with ferric chloride. This method has additional potential advantages that include removal of both ammonia and phosphate, incorporation of struvite within the cake produced, less sludge and cake produced, and elimination of corrosive conditions caused by the low pH values experienced with ferric chloride dosing. In addition the method is primarily a physical process where the design of air stripping processes is well understood, easier to control by adjusting air flow rates and/or aeration time and when necessary struvite formation can be further enhanced by supplementing the availability of magnesium in the sludge.