Charles W, Cord-Ruwisch R, Ho G, Costa M, Spencer P
Centre for Organic Waste Management, Murdoch University, South Street, Murdoch, WA. 6150, Australia.
Water Sci Technol. 2006;53(6):203-10. doi: 10.2166/wst.2006.198.
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 PO4(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 Mg2+/PO4(3-) molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO4(3-) 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 PO4(3-) recycling back to the plant. For long-term operation, maximum PO4(3-) reduction should be the ultimate aim to minimise PO4(3-) 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% PO4(3-) reduction in the digested sludge was achieved.
西澳大利亚州的伍德曼角污水处理厂（WWTP）遭遇了两个导致可避免维护成本的独立问题：厌氧消化器下游大量鸟粪石（MgNH4PO4·6H2O）结垢，以及消化器气体中硫化氢（H2S）含量升高，影响了燃气发动机运行，并导致气体洗涤器的运营成本高昂。由于这两个问题都与厌氧消化器中的化学失衡相关，我们决定研究这两个问题是否可以通过一个通用解决方案（例如投加铁溶液以沉淀硫化物和磷酸盐），或者采用单独的方法（可行且经济地）解决。实验室结果表明，通过投加铁可以有效且经济地控制消化器中的硫化氢排放。在消化器中沉淀1摩尔溶解的硫化物，所需的FeCl3略高于理论值1.5摩尔。由于消化污泥液中PO4(3-)浓度较高，鸟粪石沉淀需要显著更多的铁。通过形成磷酸铁来控制鸟粪石，投加铁似乎不是一个经济的解决方案。利用消化器液体中鸟粪石形成的自然趋势，通过提高pH值，使鸟粪石在进入污泥脱水离心机之前沉淀出来，可以降低污泥脱水离心机内部及周围鸟粪石沉淀的风险。然而，由于消化污泥中Mg2+/PO4(3-)摩尔比很低，仅通过提高pH值（使用NaOH），PO4(3-)的沉淀受到污泥中可用阳离子（Ca2+和Mg2+）数量的限制。虽然这会减少离心机中的鸟粪石沉淀，但无法显著减少PO4(3-)回流到工厂。对于长期运行而言，最大程度减少PO4(3-)应是最终目标，以尽量减少PO4(3-)在工厂中的积累。发现氢氧化镁液体（MHL）是实现这一目标最具成本效益的化学品。它增强了消化污泥和离心液中鸟粪石的沉淀，使消化污泥中PO4(3-)减少超过95%。
