Water Journal : Water Journal November 2012-1
odour management refereed paper technical features 62 NOVEMBER 2012 water For the majority of the sulphur compounds there was no evidence of degradation products or “scrambled” compounds. However, methanethiol was oxidised to DMDS (major peak) and DMTS (minor peak). Ethanethiol (ET) was oxidised to DEDS (major) with only a very minor peak visible for ET. A mixture of MT and ET showed major peaks for DEDS and the “scrambled” compound methyl ethyl disulphide (MEDS) as well as smaller peaks for DMDS and DMTS. Since MT and ET were oxidised to DMDS and DMTS, and DEDS, respectively, and also reacted with each other, they were excluded from the mixed standard solution. Based on these results, it was assumed that any MT present in the biosolids would be transformed to DMDS and DMTS. Similarly, any ET present in the biosolids would be converted to DEDS. Odorous compounds identified in biosolids samples from Woodman Point WWTP Analysis of a relatively fresh biosolids sample (less than a week old) using the HS SPME-GC-MS method for the analysis of sulphur compounds showed the presence of DMS, DMDS and DMTS. No EMS or DEDS were observed in biosolids samples. A typical chromatogram of compounds detected in a biosolids sample is shown in Figure 4. A biosolids sample, which had been stored at room temperature for a few months, exhibited a very strong faecal/ nauseating odour, probably caused by indole and skatole, which showed strong peaks in chromatograms obtained using HS SPME-GC-MS (Figure 5). These compounds were not detected in the fresh biosolids samples. This finding is consistent with WERF reports that one of the major sources of odours during the first 1–2 weeks of biosolids storage is due to the production of VOSCs by microbial degradation of sulphur-containing amino acids (Higgins, et al., 2003, 2006; Chen et al., 2006), while the OVACs start to accumulate only after VOSCs have been depleted (Chen et al., 2004; 2006). Using our method for the analysis of OVACs, the presence of geosmin was also detected in fresher biosolids samples, which still contained some sulphur compounds but exhibited a more earthy/musty odour. Other types of compounds which were tentatively identified based on their mass spectra and/or library matches, but not confirmed with authentic analytical standards, included various long chain aliphatic hydrocarbons, terpenes, alkyl benzenes and other aromatic compounds, and some of these may well have contributed to the earthy musty odour. Analysis of biosolids samples from the odour reduction trials In this preliminary study, we have focussed only on analysing odorous compounds in the headspace of wet biosolids. Thus, the biosolids samples were analysed as “aqueous” samples as described in the Methods Section. Although the method is not fully quantitative at this point, it is reproducible, simple, relatively quick and fully automated. Odours of the biosolids samples from the subsequent odour reduction trials were assessed in terms of the concentration of total volatile organic sulphur compounds (TVOSC), measured as the sum of the DMS, DMDS and DMTS concentrations present in the biosolids sample, and expressed as nanogram per gram of moist biosolids sample used (ng/g). Thus, the odour reduction (or increase) was considered to be the reduction (or increase) in the TVOSC concentration relative to a control sample. Chemical addition to digested sludge prior to dewatering A 37% reduction of peak TVOSC concentration was observed for an alum dose of 2% (based on aluminium), while a 4% alum dose resulted in a 40% reduction of peak TVOSC concentration, relative to the control sample (Figure 6). The odour reductions observed in our laboratory trials were lower than the odour reductions observed by the WERF research team. In their laboratory trials, Adams et al. (2008) reported that a dose of 0.5% alum (based on aluminium) added to digested sludge prior to dewatering resulted in approximately 80% reduction of peak TVOSC concentration, while a 2% alum dose gave approximately 90% reduction in peak TVOSC concentration. The reasons for the observed differences in the odour reductions obtained in our laboratory trials and those reported by Adams et al. could be due to a number of different factors, namely the sludge properties, type of polymer used, chemical contact time, mixing, shear and interactions between the metal and polymer. A dose of 2% polyaluminium chloride (based on aluminium) resulted in an 11% increase in the peak TVOSC concentration 0 20000 40000 60000 80000 100000 120000 140000 4.0 5 .1 6.2 7 .3 8 .4 10.3 10.8 11.3 12.1 13.2 14 .3 15.4 16.5 17.6 18.7 Abundance Retention time (min) DMS DMDS DMDS-d6 DMTS Figure 4. Typical chromatogram of a biosolids sample, showing peaks for DMS, DMDS and DMTS. Sample analysed using the HS SPME-GC-MS method for analysis of sulphur compounds in selected-ion monitoring mode using a ZB-5MS capillary column. 0 1000000 2000000 3000000 4000000 5000000 6000000 7000000 8000000 9000000 188.8.131.52.78.08.48.79.19.49.810.110.510.911.211.611.912.312.613.013.413.714.114.414.815.115.515.816.216.616.917.317.618.018.318.7Abundance Retention time (minutes) skatole indole Figure 5. Typical chromatogram of a stored biosolids sample showing the presence of indole and skatole. Sample analysed using the HS SPME-GC-MS method for analysis of OVACs in selected-ion monitoring mode using a ZB-5MS capillary column. 0 50 100 150 200 250 300 350 0125678121315161920 TVOSCconcentration(ng/g) Day of analysis Control 2% Al 4% Al Figure 6. Effect of aluminium sulphate addition to digested sludge on TVOSC production.
Water Journal December 2012
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