The Miami-Dade Water and Sewer Department’s (MDWASD) operating staff has identifiedstruvite deposits in the solids handling process equipment at the Central District WastewaterTreatment Plant (CDWWTP) and the South District Wastewater Treatment Plant (SDWWTP).Struvite, also known as magnesium ammonia phosphate (Mg NH_4-PO_4·6H_2O) is a hard compoundsimilar in appearance to fiberglass which precipitates out of solution, forms crystals andadheres to the piping system components. The anaerobic digestion of WAS releases into solutionammonia, 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 significantdamage to equipment, pumps, and piping. It is important to address any struvite problems asa significant struvite accumulation can lead to mechanical and process failures. Struvite depositshave been identified in the digester units, the digester piping system including valves and heatexchangers 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 periodfrom December 2009 through March 2010, the CDWWTP had at times relatively high concentrationsof effluent total suspended solids (TSS) which were linked to struvite precipitation in theplant.Currently, MDWASD is feeding Ferric Sulfate (Fe_2(SO_4)_3 (they have also tried Ferric Chloride(FeCl_3) to the influent of the centrifuges at the CDWWTP as a short term fix for resolving struviteprecipitation issues as well as improving the solids capture rate. Ferric is used to bind up thesoluble orthophosphate preventing it from forming struvite. This solution, however, has beencostly with an estimated ferric sulfate cost of $47,000 per month. The CDWWTP is also feedinggas scrubber water and chlorinated flushing water to the centrate in order to reduce the pH andreduce struvite precipitation.The SDWWTP has not added Ferric to the plant to reduce struvite precipitation, but has had todo extensive preventative maintenance (jetting out pipe lines) on a weekly basis to control struviteaccumulation.As a result, several short and long term solutions to struvite precipitation were evaluated includingthe 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
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