A Theoretical and Practical Evaluation of Struvite Control and Recovery: Two Case Studies

The Miami-Dade Water and Sewer Department's (MDWASD) operating staff has identified struvite deposits in the solids handling process equipment at the Central District Wastewater Treatment Plant (CDWWTP) and the South District Wastewater Treatment Plant (SDWWTP).Struvite, also known as magnesium ammonia phosphate (Mg NH4-PO4-6H2O) is a hard compound similar in appearance to fiberglass which precipitates out of solution, forms crystals and adheres to the piping system components. The anaerobic digestion of WAS releases into solution ammonia, phosphate and sometimes magnesium (otherwise present in the background) ions, which precipitate as struvite.Struvite formation is a common problem in wastewater treatment plants and can cause significant damage to equipment, pumps, and piping. It is important to address any struvite problems as a significant struvite accumulation can lead to mechanical and process failures. Struvite deposits have been identified in the digester units, the digester piping system including valves and heat exchangers and the centrifuge dewatering units downstream of the digester system at both plants.The CDWWTP has had more severe struvite problems than the SDWWTP. During the period from December 2009 through March 2010, the CDWWTP had at times relatively high concentrations of effluent total suspended solids (TSS) which were linked to struvite precipitation in the plant.Currently, MDWASD is feeding Ferric Sulfate (Fe2(SO4)3 (they have also tried Ferric Chloride (FeCl3) to the influent of the centrifuges at the CDWWTP as a short term fix for resolving struvite precipitation issues as well as improving the solids capture rate. Ferric is used to bind up the soluble orthophosphate preventing it from forming struvite. This solution, however, has been costly with an estimated ferric sulfate cost of $47,000 per month. The CDWWTP is also feeding gas scrubber water and chlorinated flushing water to the centrate in order to reduce the pH and reduce struvite precipitation.The SDWWTP has not added Ferric to the plant to reduce struvite precipitation, but has had to do extensive preventative maintenance (jetting out pipe lines) on a weekly basis to control struvite accumulation.As a result, several short and long term solutions to struvite precipitation were evaluated including the OSTARA nutrient recovery process (as OSTARA Pearl® fertilizer), through pilot testing. In addition, bench scale tests were conducted to determine the most cost effective Ferric Sulfate dose to prevent struvite precipitation. In conjunction, struvite scaling coupons were placed (inside pipes) at both plants to determine the rate of struvite precipitation and to confirm the effectiveness of the Ferric Sulfate dose. A BioWin process simulation model was also used to evaluate site specific struvite precipitation potential for the different long-term struvite reduction alternatives for the SDWWTP and CDWWTP, including Fe addition and sidestream treatment (Pearl®).The purpose of this paper is to review the results of this study with utility community and discuss the most cost effective and sustainable solution to struvite precipitation.