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

Abstract

The Woodman Point Wastewater Treatment Plant (WWTP) at Western Australia has experienced two separate problems causing avoidable maintenance costs, the build-up of massive struvite (MgNH 4 PO 4 . 6H 2 O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H 2 S) 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 FeCl 3 was required to precipitate 1 mol of dissolved sulfide inside the digester.Due to the high concentration of PO 4 3-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 Mg 2+ /PO 4 3-molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO 4 3-was limited by the amount of cations (Ca 2+ and Mg 2+ ) available in the sludge. Although this would reduce struvite precipitation in the centrifuge, it could not significantly reduce PO 4 3-recycling back to the plant. For long-term operation, maximum PO 4 3-reduction should be the ultimate aim to minimise PO 4 3-accumulation in the plant. Magnesium hydroxide liquid (MHL) was found to be the most costeffective chemical to achieve this goal. It enhanced struvite precipitation from both, digested sludge and centrate to the point where more than 95% PO 4 3-reduction in the digested sludge was achieved.

Post-depositional formation of vivianite-type minerals alters sediment phosphorus records

“…9-10). In contrast to vivianite, struvite is observed in sulfide-rich waste water (e.g., Charles et al, 2006) and may thus be stable in the presence of sulfide. This is confirmed by our sulfidization experiment, in which, for example, only 25 % of precipitate 4 (estimated struvite fraction: 20 mol% P) dissolved in sulfide-rich seawater (Fig.…”

Application of waterworks sludge in wastewater treatment plants

“…Increased struvite formation in the digester is advantageous since its formation decreases the recycling of ammonia and phosphorous back to the wastewater treatment process, however, in practice, many WWTPs experience problems with struvite precipitation in the centrifuges and pipe network resulting in significant maintenance costs (Charles et al 2006).…”

Gaseous Emissions from Wastewater Facilities

“…Sattler and Rosenberk (2006) initial concentration greatly influence the efficiency of the process. Charles et al (2006) present results that 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.…”