1. * 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
0.012
Arsenic
<1
3.82 C1
<
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
0.001
Iron
190000
97000
136750
4133.3
752.4
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
21.69
0.090
Nickel 48 92
43.15
43.70
1.285
Selenium
<2
4.40
Zinc
130000
55000
878.77
514.8
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