* Metal in pickling waste (mg/L) Pickling waste dosing to control excessive hydrogen sulfide in anaerobic digester and its impact on biosolids reuse Wipa Charles, Goen Ho and Ahmet Kayaalp Murdoch University, Faculty of Sustainability, Environmental and Life Sciences, School of Environmental Science, Murdoch University, Murdoch, WA 6150, Australia. BACKGROUND The Woodman Point wastewater treatment plant (WWTP) in Western Australia experiences excessive hydrogen sulfide (H2S) levels in biogas from anaerobic digesters. Current iron-based chemical oxidant gas scrubber is expensive and labour intensive. OBJECTIVE To investigate the feasibility of using pickling waste as an economic form of iron salt to control H2S emissions. To determine the impact on biosolids reuse The effect of Fe3+ in FeCl3 and pickling waste on dissolved sulfide precipitation and pH in anaerobic sludge Estimated heavy metal concentration (mg/kg) in biosolids before and after pickling waste dosing * Metal in pickling waste (mg/L) Average metal already in biosolids (mg/kg) **WA Guidelines before pickling waste dosing Additional metal from pickling waste dosing (mg/kg) Metal in biosolids after pickling waste dosing (mg/kg) after pickling waste dosing % metal addition from pickling waste dosing Maximum Minimum Average Aluminium 140 61 105 4527 C3 0.5544 4527.56 0.012 Arsenic <1 3.82 C1 <0.00396 0.104 Cadmium 0.91 1.98 0.200 Chromium 95 52 67.5 72.24 0.3762 72.61 0.351 Copper 3.4 1 2.2 1298.9 C2 0.01346 1298.98 0.001 Iron 190000 97000 136750 4133.3 752.4 4885.73 12.455 Lead 300 180 235 55.1 1.188 56.29 2.156 Mercury <0.01 2.0 2.00 0.198 Molybdenum 5.5 4.1 4.75 21.67 0.02178 21.69 0.090 Nickel 48 92 43.15 43.70 1.285 Selenium <2 4.40 Zinc 130000 55000 878.77 514.8 1393.57 58.582 * Results from a survey of ferric and other metals in picking waste from 4 local galvanizers **Based on biosolids acceptance concentration thresholds (Guideline for Direct Land Application of Biosolids and Biosolids Product, Department of Environmental Protection, 2002). Grade C1 is the highest quality (suitable for unrestricted use) while C3 is the low quality biosolids CONCLUSIONS Direct dosing of FeCl3 (1.5 mol of FeCl3 per mol of H2S) can control H2S emissions. The estimated cost of FeCl3 is $44,000 per year. Pickling waste is as effective as FeCl3 in controlling H2S emissions and can be used as an alternative Fe3+ (approximately 0.5 l/m3 anaerobic sludge). Due to low quantity of pickling waste required for H2S control, heavy metal addition through pickling waste dosing is minute compared to existing heavy metals already in biosolids. No change of the contaminant grade in biosolids from pickling waste dosing, therefore no impact on biosolids reuse. The quality of pickling waste may fluctuate from batch to batch; it is advisable to have the batch analyzed for appropriate metals before its use. ACKNOWLEDGEMAENTS The authors would like to thank Water Corporation of Western Australia for their financial and technical support of this research. IWA World Water Congress & Exhibition Vienna, 7 - 12 September 2008