Chemical Control of Struvite Precipitation

Abstract

The control of struvite precipitation was investigated using a range of chemical inhibitors and chelating agents. Initial experiments investigated the mass of struvite that precipitated on jar tester impellers used to mix anaerobically digested sludge liquors. Mixing the liquors resulted in degassing of CO2 increasing the pH from below 7 to above 8. It was found that the greater the liquors’ supersaturation ratio (Ω) the greater the mass of struvite formed on the impellers. Liquors with a Ω=1.7 and 5.3 produced mean final masses of 73.8 and 136g/m2, respectively, after 40 h of mixing. Experiments using the same impellers were performed with a range of scale inhibitors and chelating agents. The scale inhibitors did not prevent struvite from forming, with only one product, Polystabilis, showing a reduction in the mass of scale formed on the impellers. Of the chelating agents tested, ethylenediaminetetraacetic acid (EDTA) was found to be the most effective. Further experiments showed that an EDTA: struvite ratio of 2:1 dissolved >90% of struvite back into solution even at pH values greater than 7.5.