Water Journal : Water Journal November 2012-1
refereed paper odour management water NOVEMBER 2012 63 in the resulting cake, while a 4% dose gave a 40% reduction in the peak TVOSC concentration of the biosolids cake, relative to the control (Figure 7). Addition of iron at the 2% dose resulted in only a slight decrease (23%) in peak TVOSC concentration, while addition of iron at the 4% dose resulted in a 50% increase in peak TVOSC concentration, relative to control (Figure 8). The observed increase in TVOSC concentration obtained with the 4% iron dose in our trials is somewhat consistent with earlier findings of the WERF study. Results from the Phase III WERF study showed that, in general, an increase in iron concentration in the sludge or biosolids resulted in higher TVOSC concentrations in the dewatered biosolids headspace, especially if iron was added prior to or during digestion (Adams, et al., 2008). It was also found that addition of ferric chloride to anaerobically digested sludge before dewatering did not reduce TVOSC emissions from cake until the iron dose was at least 8% on a dry mass- mass basis (Adams, et al., 2008). Results from recent laboratory studies, using batch anaerobic digestion, have shown that iron addition to the digester feed reduced TVOSC concentrations in the resulting biosolids cake by 50 to over 95% for most of the sludges (Novak et al., 2010). Direct addition of iron (4% dose) to biosolids cake also significantly reduced the TVOSC concentrations (Higgins, 2010). The contradictory results obtained from various studies using iron are most likely due to the sludge properties, location of iron addition and polymer-iron interactions (Higgins, 2010). Comparison of TVOSC profiles in Figures 6 to 8 showed that in all three cases the TVOSC concentrations peaked within the first week of incubation and then decreased, which was consistent with previously reported research (e.g . Higgins et al., 2003; Adams et al., 2008). Chemical addition to plant-dewatered cake Higgins (2010) reported that adding metal salts directly to the cake gave a better TVOSC reduction compared to adding the salts during the conditioning and dewatering step. However, addition to the cake also resulted in a greater reduction in the pH of the cake to levels probably below those desirable for land application. In our laboratory trials, a 2% dose of aluminium sulphate (based on aluminium) resulted in a 24% increase in peak TVOSC concentration, while addition of 4% aluminium sulphate resulted in approximately 70% decrease in peak TVOSC concentration, relative to the control sample. However, the pH of the cake treated with 4% aluminium sulphate was also significantly reduced (pH 4.2) to levels that may not be suitable for land application. These results are consistent with the results reported by Higgins (2010). Centrifuge speed trials Reducing the centrifuge bowl speed and/or torque can reduce the amount of shear imparted on biosolids, thereby reducing the odour of the dewatered cake (Adams et al., 2008). In a full-scale test, a 10% reduction in centrifuge bowl speed on one high-solids centrifuge resulted in 20% reduction of TVOSC emissions from dewatered cake with no observed reduction in cake solids concentration (Adams et al., 2008). In our laboratory trials, a 20% reduction in centrifuge speed (3080rpm) resulted in an approximate 30% decrease in peak TVOSC concentration, relative to the control. However, the solids content of the resulting cake was also significantly reduced which would not be desirable from the point of view of WWTP operations. Analysis of OVACs in biosolids samples from the odour reduction trials No significant concentrations of OVACs were detected in biosolids samples derived from chemically treated digested sludge. In most cases compounds were either at or below limits of quantification for the method. However, traces of geosmin were detected in all biosolids samples. Conclusions and Future Work This study identified some of the major odorous compounds in biosolids samples obtained from a Western Australian WWTP and investigated chemical addition and reduction of centrifuge speed as potential odour reduction strategies. In this study all experimentation was limited to laboratory scale work. Aluminium sulphate addition (4% based on aluminium) to digested sludge prior to dewatering offered the best odour reduction strategy among the options that were investigated, resulting in approximately 40% reduction in peak TVOSC concentration, relative to a control sample. Reduction of centrifuge speed would not be a viable option for our test WWTP as it resulted in a reduction in the solids content of the resulting biosolids cake. In most cases, results obtained from the HS SPME-GC-MS analyses were in general agreement with qualitative observations by a single trained odour assessor. In future studies, it would be beneficial to include dilution olfactometry measurements to obtain a more rigorous assessment of the overall odour generated from biosolids cake and to correlate/compare the results with measurements obtained using HS SPME- GC-MS. In addition, it would be useful to determine the nature of odour compounds in aged biosolids in which the very 0 50 100 150 200 250 300 350 400 0125678121315161920 TVOCSconcentration(ng/g) Day of analysis Control 2% PAC 4% PAC Figure 7. Effect of polyaluminium chloride addition to digested sludge on TVOSC production. 0 100 200 300 400 500 600 700 800 0125678121315161920 TVOCSconcentration(ng/g) Day of analysis Control 2% Fe 4% Fe Figure 8. Effect of ferric chloride addition to digested sludge on TVOSC production.
Water Journal December 2012
Water Journal September 2012-1