Struvite Reduction via PRI-TECH®

Scope

In May 2020, San Antonio Water Systems (SAWS) initiated testing to quantify the impacts and economics of Peroxide Regenerated Iron-Technology (PRI-TECH™) for reducing orthophosphate in the solids dewatering facility at the Steven M Clause Wastewater Treatment Plant (WWTP) for the purpose of struvite reduction.

Background

For nearly 20 years, SAWS has managed H2S and reduced sulfur compound-based odors in the sludge thickening phase (prior to digestion) at the Steven M Cause WWTP using hydrogen peroxide (H2O2) PRI-TECH®. In addition to odor control, treatment with peroxide regenerated iron upstream of the digesters also resulted in a recovery in digester gas production and volatile solids reduction performance, and a reduction in polymer demand. Positive results from the 2020 jar testing indicated that an additional PRI-TECH™ application at the dewatering step could potentially achieve an 80% reduction in filtrate orthophosphate, and other case studies indicated a further potential for reductions in polymer usage. The dewatering facility handles 0.72 MGD of solids containing an average of 700 lbs/day of ortho-P and experiences significant struvite issues in the belt filter press filtrate lines and equipment. The 80% reduction observed in the jar testing can potentially reduce the ortho-P to 135lbs/day, greatly reducing the struvite generation potential. Ultimately, SAWS implemented a PRI-TECH™ demonstration program with goals of struvite reduction, improved dewatering performance, and polymer reduction.

Technology

PRI-TECH™ is a proprietary odor control technology that utilizes iron salts and oxidants in a fashion that reduces sulfides to elementary sulfur and reduces sulfur compounds to non-odorous compounds. This program was implemented by adding ferrous chloride (FeCl2) as the primary sulfide control agent into the primary sludge line upstream of the sludge blend tank. Hydrogen peroxide (H2O2) was added downstream at the sludge blend tank recirculation pumps to regenerate iron from ferrous sulfide (FeS) to either free ferrous (Fe+2) and/or ferric (Fe+3) iron. The H2O2 was also added to the online belt press feed pumps discharge piping to provide additional iron regeneration and oxidation to ferric (Fe+3) in order to sequester orthophosphate into the solids cake.

Solution

USP installed a hydrogen peroxide storage and feed system at the outlet of the digested sludge holding tank prior to the day tanks and polymer. The ortho-P reduction was analyzed by using the HACH™ TNT method 846 analyzing the filtrate before chemical addition (avg. 130mg/L) and the filtrate from the belt filter presses (61mg/L), a 53% reduction on average at start-up with a dosage of 200-250mg/L of H2O2.

To optimize performance, target peroxide dose rate was increased to 400mg/and resulted in 80% removal of orthophosphate (173mg/L) observed in digestate and 29 mg/L observed in filtrate. As a result of operational changes in the day tank levels, it remains difficult to maintain a consistent peroxide dose rate of 400 mg/L.

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Download the Full Struvite Case Study here.