Solutions to a combined problem of excessive hydrogen sulfide in biogas and struvite scaling

Abstract

The Woodman Point Wastewater Treatment Plant (WWTP) in Western Australia has experienced two separate problems causing avoidable maintenance costs: the build-up of massive struvite (MgNH₄PO₄· 6H₂O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H₂S) levels in the digester gas to levels that compromised gas engine operation and caused high operating costs on the gas scrubber. As both problems hang together with a chemical imbalance in the anaerobic digester, we decided to investigate whether both problems could be (feasibly and economically) addressed by a common solution (such as dosing of iron solutions to precipitate both sulfide and phosphate), or by using separate approaches. Laboratory results showed that, the hydrogen sulfide emission in digesters could be effectively and economically controlled by the addition of iron dosing. Slightly higher than the theoretical value of 1.5 mol of FeCl₃ was required to precipitate 1 mol of dissolved sulfide inside the digester. Due to the high concentration of PO₄³⁻ in the digested sludge liquor, significantly higher iron is required for struvite precipitation. Iron dosing did not appear an economic solution for struvite control via iron phosphate formation. By taking advantage of the natural tendency of struvite formation in the digester liquid, it is possible to reduce the risk of struvite precipitation in and around the sludge-dewatering centrifuge by increasing the pH to precipitate struvite out before passing through the centrifuge. However, as the Mg²⁺/PO₄³⁻ molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO₄³⁻ was limited by the amount of cations (Ca²⁺ and Mg²⁺) available in the sludge. Although this would reduce struvite precipitation in the centrifuge, it could not significantly reduce PO₄³⁻ recycling back to the plant. For long-term operation, maximum PO₄³⁻ reduction should be the ultimate aim to minimise PO₄³⁻ accumulation in the plant. Magnesium hydroxide liquid (MHL) was found to be the most cost-effective chemical to achieve this goal. It enhanced struvite precipitation from both, digested sludge and centrate to the point where more than 95% PO₄³⁻ reduction in the digested sludge was achieved.