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Bacterial pathogen indicators regrowth and reduced sulphur compounds’ emissions during storage of electro-dewatered biosolids
Chemosphere 113 (2014) 109–115
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Bacterial pathogen indicators regrowth and reduced sulphur compounds’ emissions during storage of electro-dewatered biosolids Tala Navab-Daneshmand, Samia Enayet, Ronald Gehr, Dominic Frigon ⇑ Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, Quebec H3A 0C3, Canada
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Regrowth of coliforms not observed
in all electro-dewatered biosolids after 7 d. Regrowth of total coliforms observed in inoculated heat-treated biosolids. Lower odour detection and recognition thresholds for electrodewatered biosolids. Little volatile organic sulphur compounds above electro-dewatered biosolids. pH decrease, nutrient removal and inhibitor formation may explain observed effects.
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 3 April 2014 Accepted 4 April 2014
Handling Editor: Hyunook Kim Keywords: Bacterial pathogen indicators Biosolids Electro-dewatering Odour production Reduced sulphur compounds Regrowth
a b s t r a c t Electro-dewatering (ED) increases biosolids dryness from 10–15 to 30–50%, which helps wastewater treatment facilities control disposal costs. Previous work showed that high temperatures due to Joule heating during ED inactivate total coliforms to meet USEPA Class A biosolids requirements. This allows biosolids land application if the requirements are still met after the storage period between production and application. In this study, we examined bacterial regrowth and odour emissions during the storage of ED biosolids. No regrowth of total coliforms was observed in ED biosolids over 7 d under aerobic or anaerobic incubations. To mimic on-site contamination during storage or transport, ED samples were seeded with untreated sludge. Total coliform counts decreased to detection limits after 4 d in inoculated samples. Olfactometric analysis of ED biosolids odours showed that odour concentrations were lower compared to the untreated and heat-treated control biosolids. Furthermore, under anaerobic conditions, odorous reduced sulphur compounds (methanethiol, dimethyl sulphide and dimethyl disulphide) were produced by untreated and heat-treated biosolids, but were not detected in the headspaces above ED samples. The data demonstrate that ED provides advantages not only as a dewatering technique, but also for producing biosolids with lower microbial counts and odour levels. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Production of biosolids and their handling and disposal costs have increased substantially over the past few decades (Higgins ⇑ Corresponding author. Tel.: +1 (514) 398 2476; fax: +1 (514) 398 7361. E-mail address: [email protected] (D. Frigon). http://dx.doi.org/10.1016/j.chemosphere.2014.04.012 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
et al., 2007). One of the most common methods for biosolids disposal is land application. According to the United States Environmental Protection Agency (USEPA) regulations, biosolids can be land applied if they fall under Class A or Class B, which are defined based on the levels of specific bacterial, viral and eukaryotic indicators (USEPA, 2003). In Canada, regulations follow similar principles and approaches (CCME, 2010). Another obstacle for biosolids land
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application is nuisance odours. While the USEPA does not regulate biosolids odour production for land application, certain Canadian jurisdictions categorize fertilizing biomaterials into four groups from less odorous than cow manure to more odorous than pig manure. Biosolids in the latter category cannot be land applied. Municipal biosolids are typically classified between cow and pig manures (Hébert, 2008). In addition to specific regulatory restrictions, emissions of odours during biosolids storage may irritate populations living close to treatment plants or land application sites. This could reduce public support for land application and limit the flexibility which operators have for biosolids management. Electro-dewatering (ED) is a relatively new technology that can increase the solids content of biosolids up to 70 wt%, while using less energy than heat drying. It has been shown to reduce total coliforms and Escherichia coli to below detection limits, producing Class A biosolids (Esmaeily et al., 2006; Saveyn et al., 2006; Navab-Daneshmand et al., 2012), however, the USEPA regulations on the microbiological requirements for biosolids, mentioned above, are to be met at the time of application, and not immediately after the process. Low bacterial counts after treatment processes do not guarantee that inactivation is irreversible, and regrowth can occur during storage. For example, centrifuged anaerobically digested biosolids have shown increases of 2–4 logs of faecal coliforms and E. coli when incubated at 25–37 °C for 24 h (Higgins et al., 2007; Qi et al., 2008). Regrowth of bacterial pathogen indicators by several orders of magnitude during storage can be affected and controlled by several environmental parameters. One such factor is biosolids moisture content, as lower bacterial pathogen growth rates are expected in drier environments. A second factor that could impact bacterial regrowth is the availability of oxygen as an electron acceptor. Typically, biosolids are stored in deep containers or in large piles. The surface layer of the pile is aerobic due to atmospheric exposure. Oxygen diffusion, however, is limited downwards and it penetrates only a few centimeters from the surface, resulting in anoxic or anaerobic conditions lower down (Yamada and Kawase, 2006). A third factor affecting bacterial regrowth is the pH of the sludge matrix. The ambient pH affects nutrient availability and changes the solubility of substances consumed by or inhibitory to bacteria; it directly impacts microbial metabolism and enzyme activities (Sidhu et al., 2001). There is an optimal pH for microbial growth that is specific for each species; for example, E. coli growth rate increases by 4–5 times from pH 4 to pH above 6 (Presser et al., 1997). During ED, a pH gradient is developed in the sludge cake from low pH near the anode (as low as 2.2; Huang et al., 2008), to high pH near the cathode (as high as 7.7; Navab-Daneshmand et al., 2012). This pH gradient is generated by electrolysis on the electrode surfaces that produces hydrogen ions at the anode and hydroxide ions at the cathode. We are not aware of regrowth studies for ED biosolids, but regrowth has been shown to be substantial for biosolids from other sources such as anaerobic digestion. Thus, the first objective of this work is to determine the effect of ED treatment on the regrowth of bacterial pathogen indicators during storage. It has been noted that ED produces biosolids with less objectionable odours (Eschborn et al., 2011; Bureau et al., 2012), but detailed information was not reported. Several classes of compounds have been identified as the causes of odours from biosolids; the main class is volatile organic sulphur compounds (VOSCs) including methanethiol (MT), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) (Murthy et al., 2003; Forbes et al., 2004; Krach et al., 2008). These compounds are commonly reminiscent of sewer odours, and they have a very low odour threshold. The VOSCs are formed from precursors released during the breakdown of readily extractable proteins in biosolids, including cysteine and methionine, that are subsequently degraded to MT and hydrogen sulphide (Higgins et al., 2006). While some authors have argued that DMDS formation was the result of abiotic oxidation of MT in the presence
of molecular oxygen (Higgins et al., 2006), the presence of DMDS under strict anaerobic conditions has also been reported (Turkmen et al., 2004). Another important class of odorants is amines and ammonia that cause fishy odours. They were reported to be the main contributors to the odour profile of lime-stabilized biosolids (Kim et al., 2003). The high acid dissociation constant (typically pKa > 9) of protonated amine groups means that a high pH is required for the efficient volatilization of these compounds, as it is the non-ionized form that is volatile (Chang et al., 2005). Finally, a third group of compounds responsible for biosolids odours is volatile fatty acids, which contribute to rancid or vinegary odours (Rosenfeld et al., 2001; Murthy et al., 2003). They are, however, typically considered to be minor contributors. In addition, the source, dewatering process and conditioning of biosolids impact the availability of precursors to the formation of odorants and the levels of oxygen that modulate biotic and abiotic pathways of odorant formation (Forbes et al., 2004; Murthy et al., 2006). There have been few – if any – comprehensive descriptive or quantitative data on odours or odour classes from ED biosolids in the published literature. Thus, the second objective of the current study is to establish the principal differences in odour generation during storage between untreated and ED biosolids. In this study, ED biosolids were compared to untreated biosolids, and to heat-treated biosolids because it had been shown that heat was the main inactivation mechanism during ED (NavabDaneshmand et al., 2012). Regrowth was studied under both aerobic and anaerobic storage conditions. Additionally, to mimic possible contamination with bacterial pathogen indicators from raw biosolids at treatment plants or during transport, some ED and heat-treated samples were inoculated with untreated biosolids before incubation. Odour production was investigated only under anaerobic conditions because they typically favor odour production. Odour profiles after 7 d of incubation were assessed by olfactometric tests, and the dynamics of selected VOSCs concentrations were determined by a GC–MS assay. 2. Materials and methods 2.1. Biosolids Biosolids were sampled from an activated sludge wastewater treatment plant without primary clarification near Montréal (Québec, Canada). The plant treats a flow of 60 000 m3 d 1 on average, with a hydraulic retention time (HRT) of 12 h, and a solids retention time of 6 d. At the time of the study, a cationic polymer (PAM C-65 L; Jes-Chem, Guelph, Ontario, Canada) was added at a concentration of 10–15 kg t 1 of solids before the centrifuge dewatering units. Biosolids samples were taken immediately after centrifugation, brought to the laboratory on ice and stored at 4 °C for up to 4 d. 2.2. Electro-dewatering and heat treatment of biosolids The laboratory ED unit was a CINETIK CK-lab model (Ovivo, Boucherville, Québec, Canada), which used a direct current electrical field. Similarly to our previous study (Navab-Daneshmand et al., 2012), the maximum voltage and current were set at 60 V and 5.5 A, respectively. For each ED experiment, 165 wet g biosolids were placed on a filter medium (100% PPS Ryton, woven) over a stainless steel perforated cathode. The ceramic-coated titanium anode applied 140 kPa constant pressure. The 10 min ED cycle reduced total coliforms to below detection limits. Heat-treatment was the control process as it had been shown that the inactivation of bacterial pathogen indicators during biosolids ED is due to high temperatures (>75 °C; Navab-Daneshmand
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et al., 2012). For heat-treatment, approximately 40 wet g of biosolids were placed in a 50 mL glass tube, and incubated in a water bath at 80 °C. It took 6 min for the biosolids samples to reach 80 °C; hence to have 10 min heat-treatment, the tubes were incubated in the water bath for a total of 16 min. 2.3. Seeding To mimic possible field contamination of treated biosolids, some ED and heat-treated samples were inoculated with untreated biosolids. For this purpose, 2 wet g of untreated biosolids were added to 40 mL distilled water and homogenized with a disperser (ULTRA-TURAX S10N-10G, IKA Works Inc., Wilmington, NC). 2 mL of the slurry were added to 100 wet g of ED or heat-treated biosolids in 50 mL sterilized glass tubes and mixed thoroughly by hand. 2.4. Biosolids incubation About 50 wet g of sample was placed in a 1 L media bottle that was closed by a silicon septum. The bottles were placed on a roller apparatus (Wheaton Industries Inc., Millville, NJ, USA) to maintain sample uniformity and covered to avoid light exposure. Incubation proceeded at room temperature (22 ± 0.5 °C). For regrowth experiments, aerobically incubated bottles were left open vertically on the bench for 10 min daily, while the anaerobically incubated ones were flushed daily for 10 min with pure nitrogen. Four sets of experiments lasting 7 d each were performed 3–8 weeks apart. For the odour production experiments, anaerobically incubated bottles were flushed with nitrogen at the beginning of the experiments (Day 0) and were kept closed for their entire duration (14 d). Anaerobic indicator strips were placed in each bottle (BD GasPak, Franklin Lakes, NJ); the blue color of the humid strips turned white to confirm the absence of oxygen. Headspace samples were taken through the silicon septa for odour analysis. 2.5. Physical–chemical parameters Standard Method No 2540-B (APHA, 2012) was used for total solids measurement. Following Navab-Daneshmand et al. (2012), pH of biosolids was measured by creating a slurry (0.8 wet g in 9.2 mL of distilled water). Water activity was measured using a manometric method (HygroLab Set 2 equipped with an AW-DIO probe, ROTRONIC, Switzerland). Water activity of samples was measured in sealed containers every 15 s and the value was recorded when the vapour pressure was stable for at least 10 min. If condensation was present on the container walls before the test, water activity was recorded as 1.0 without taking measurements. 2.6. Total coliforms enumeration A modified microplate MPN method (Navab-Daneshmand et al., 2012) was used to enumerate total coliforms using Standard Method No. 9223 (APHA, 2012; Colilert reagent, IDEXX Laboratories, Westbrooke, ME) with a microplate reader (SpectraMax M5, Molecular Devices, Sunnyvale, CA). An online MPN calculator was used to calculate MPN g 1 total solids (Curiale, 2004). The detection limit was determined to be 3 positive wells in the first row corresponding to 75–750 MPN g 1 TS, depending on the dryness. 2.7. Odour analyses The impact of ED on odour production was analysed in several ways. In one series of tests, olfactometric analyses of the headspaces above samples were done following ASTM E679-04 standard
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protocol and British Standard 13725, using 5 panellists (BSI, 2003; ASTM, 2011). Samples were sent overnight to Pinchin Environmental Ltd. (Mississauga, Ontario, Canada) for analysis of the following parameters: (a) Detection threshold: dilution ratio at which the sample was detected by at least 50% of the panellists. (b) Recognition threshold: the dilution ratio at which the sample was correctly recognized by at least 50% of the panellists. (c) Hedonic tone: a reference scale ( 10 to + 10) ranking odours from most unpleasant to most pleasant. It is independent of odour descriptors. (d) Odour descriptor: odour panellists’ descriptions for characterizing odours assessing the intensity (1–5) of eight primary odour wheel descriptors (McGinley and McGinley, 2006): vegetable, fruity, floral, medicinal, chemical, fishy, offensive and earthy. Primary descriptors were complemented by secondary descriptors. In another series of tests, the presence of odorous gases was examined in the headspaces above samples by Dräger short-term gas detection tubes for MT, DMS, amines and ammonia (Dräger Safety Inc., Pittsburg, PA). The third series of tests, based on the results of the first two series, which suggested focusing on VOSCs, monitored the concentrations of MT, DMS and DMDS in the headspaces above biosolids by GC–MS (Trace Ultra equipped with an ITQ 1100 External Ion Trap MS, Thermo Scientific, Milan, Italy). For this quantitative study, two sets of experiments lasting 14 d each were performed six months apart. These experiments were conducted in duplicate. Using a gas-tight syringe, headspace gas samples (1 mL) were injected directly into the GC. Chromatographic separation was performed (Rtx-5MSi column, Restek, Bellefonte, PA) with the oven temperature following the program: 45 °C for 0.5 min, increase to 100 °C at 45 °C min 1 and hold for 0.5 min, increase to 300 °C at 30 °C min 1 and hold for 1 min. Detection was performed by electron ionization (70 eV) with analysis in full scan and selected ionmonitoring modes. MT (2000 ppmv in nitrogen; Linde Canada Ltd., Mississauga, Ontario, Canada), DMS and DMDS (P99%; Sigma– Aldrich, St. Louis, MO) standards were used to construct quantitative calibration curves. Example chromatograms of samples and standard gases are presented in Fig. S1. 2.8. Readily extractable protein The method of Higgins et al. (2006) was modified to measure the readily extractable protein. Biosolids were mixed on a stir plate for 30 min (10 wet g of biosolids in 100 mL of 50 mM phosphate buffer solution, pH 8), then 2 mL of the slurry were centrifuged at maximum speed (14 800 rpm) in a micro-centrifuge for 5 min. Supernatants were then collected and analysed for protein concentration (DC Protein Assay kit, Bio-Rad, Hercules, CA). 3. Results and discussion 3.1. Biosolids electro-dewatering Results for a typical ED and heat-treatment experiment are presented in Figs. 1 and 2. During ED, energy was consumed at a constant rate, resulting in a nearly linear filtrate removal rate after a short lag time (Fig. 1). The temperature of the dewatering biosolids cake increased from room temperature to above 80 °C over 10 min due to Joule heating (Fig. 1). The ED increased the total solids from 17.9 ± 0.3 wt% (±standard deviation) to 32.9 ± 2.3 wt% (Fig. 2). On
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average, the pH of 7.1 ± 0.2 in the untreated biosolids decreased to 4.6 ± 0.1 after ED (Fig. 2). Heat-treatment did not affect the pH or the total solids of the samples (Fig. 2). Finally, ED and heat-treatment decreased total coliform counts by more than 5 logs (Fig. 2). This result is similar to that obtained previously for ED, in which disinfection was attributed to the high temperatures (Navab-Daneshmand et al., 2012). 3.2. Regrowth of bacterial pathogen indicators During the 7-d incubation period, the pH of all samples remained stable (data not shown) and the total solids concentration only changed slightly (Figs. 3a and b). For the ED and heat-treated samples, total coliform counts remained below the detection limit (Figs. 3c and d). To mimic possible contamination of biosolids during storage or transport, ED and heat-treated samples were seeded with untreated biosolids. This inoculation increased the counts to 2.0 103 and 3.2 103 MPN g 1 TS in the ED and heat-treated samples, respectively. Total coliform counts in the inoculated ED biosolids returned to the detection limit or below on Day 4 of incubation under both aerobic and anaerobic conditions (Figs. 3c and d). In the inoculated heattreated samples, however, total coliform counts increased after inoculation both with and without oxygen. It would appear that heat inactivation (Navab-Daneshmand et al., 2012) during heat-treatment or ED was sufficient to control the observable regrowth of total coliforms over 7 d, but that other factors which developed during ED prevented the multiplication of the external bacterial contamination. Although there was about a 20 wt% difference in total solids between ED and untreated or heat-treated biosolids (Figs. 3a and b), the water activity of all these samples was well above 0.95 (data not shown), which is the minimum limit for the growth of E. coli and other coliforms (Tapia et al., 2008). Therefore, it is unlikely that water scarcity had limited the regrowth of these bacteria. These data also suggest that the availability of oxygen (comparing aerobic and anaerobic incubations) does not affect coliforms regrowth in untreated, ED or heat-treated biosolids (Figs. 3c and d). It is possible that the low pH of ED biosolids (4.6 compared to 7.0 in the heat-treated case; Fig. 2) affected the regrowth of total coliforms. This explanation would correspond with that of Presser et al. (1997), who reported an increase of 4–5 times in the growth rate of E. coli over a similar pH range. Other possible explanations include the removal of essential nutrients with the filtrate from the biosolids matrix during ED, and the production of inhibitory compounds.
Fig. 2. Total solids, pH and total coliform counts in untreated, electro-dewatered and heat-treated biosolids. Bars represent the standard errors of four experiments, performed several weeks apart. The asterisks (*) for total coliforms indicate the detection limits as these counts were below detection.
Fig. 3. (a) and (b) Total solids, and (c) and (d) total coliform counts, in untreated (UT), electro-dewatered (ED), inoculated electro-dewatered (IED), heat-treated (HT), and inoculated heat-treated (IHT) biosolids during 7-d incubations under aerobic (on the left) and anaerobic (on the right) conditions. Bars represent the standard errors of four experiments for untreated, electro-dewatered and heattreated samples, and the range of two experiments for inoculated electrodewatered and inoculated heat-treated samples. Each set of experiments were performed several weeks apart. The asterisks () for total coliforms are the detection limits as these counts were below detection.
Further studies are needed to clarify the exact mechanism leading to the apparent lower regrowth potential of ED biosolids.
3.3. Odour production
Fig. 1. Treatment conditions and effects on biosolids: (a) consumed energy, cake temperature and removed filtrate during electro-dewatering. Values are averages of four experiments, performed several weeks apart with standard errors below 5.6%, 5.2%, and 16.5% after 4 min for consumed energy, cake temperature, and removed filtrate, respectively.
Olfactometric analysis of the bottles’ headspaces after 7 d anaerobic incubation revealed a reduction of 50% in odour detection and recognition thresholds of the ED, compared to the untreated biosolids (Table 1). When these thresholds were tested for a blank control sample (nitrogen gas incubated in similar bottles, but without biosolids), they were at least 26 times lower than those of the headspace above ED biosolids. Therefore, the nitrogen gas used did not influence the observed odour profile. Despite a reduction in odour detection and recognition thresholds by ED, the hedonic tone (measuring the pleasantness of the odours) for these biosolids remained similar to those for untreated and heattreated samples. Among the eight main descriptors of the odour characterization wheel (McGinley and McGinley, 2006), the odours from all three biosolids samples were described as offensive and earthy. The relative intensity of these descriptors was slightly
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T. Navab-Daneshmand et al. / Chemosphere 113 (2014) 109–115 Table 1 Odour characterization of headspaces above biosolids after 7-d incubations under anaerobic conditions.
a b c d e
Parameter
Blank N2 gasa
Biosolids samples Untreated
Heat-treated
Electro-dewatered
Detection threshold ( 103) Recognition threshold ( 103) Hedonic tone (range)b Main primary odour descriptor relative intensityc Offensive Secondary descriptorsd Earthy Secondary descriptors Fishy Secondary descriptors
0.4 0.2 5 ( 2 to
25.0 11.5 3 ( 5 to
17.9 8.3 3 ( 7 to
13.0 5.6 3 ( 4 to
9)
1.50 Raw meat, garbage 0.70 Musty 2.65 Dead fish
2)
1.50 Sewer, rancid, garbage 0.50 Musty 0.50 Nonee
2)
2.15 Sewer, manure, garbage 0.90 Musty 0 None
2)
1.35 Sewer, manure, garbage 1.15 Musty 0 None
The blank bottles were incubated in the same conditions as the biosolids’ bottles. Hedonic tone scale: unpleasant 10, neutral 0, pleasant + 10. Primary descriptor relative intensity scale: not utilized by panelists 0, mild odour 1, strong odour 5. Secondary descriptors mainly used by panelists in association with primary descriptor. Panelist did not use secondary descriptors for biosolids fishy odours.
different between samples, and ED biosolids exhibited the highest intensity of earthy odours. Furthermore, the sewer odour was used as the common secondary descriptor for the offensive nature of the odours from all three samples. A second series of experiments used Dräger tubes to characterize odorous compounds in the headspaces above biosolids. In these tests, amines, ammonia, MT and DMS were detected at fairly high concentrations above the untreated samples, whereas they were detected at relatively low concentrations above the ED biosolids (Table 2). The concentrations of amines and ammonia were consistent with the fishy odour reported only for the untreated biosolids by the olfactometric analysis (Table 1). In addition, the highest concentrations of the odorous compounds were detected for MT and DMS (Table 2). The VOSCs such as MT and DMS, are characteristic of sewer odours. Because MT and DMS were the highest concentrations detected by the Dräger tube tests (Table 2) and because sewer odour was the most common offensive secondary descriptor used for all three samples (Table 1), a third series of experiments were performed to quantify the time profile of VOSCs in the headspaces above the samples, and to ascertain the dynamics of odour production. There were large differences between the concentrations of VOSCs (sum of MT, DMS and DMDS) for the two sets of experiments that were performed six months apart (Fig. 4). In addition, while MT was detected in both cases, DMS and DMDS were detected only in Experiment 1. Nonetheless, common observations can be made from the two experiments: heat-treated biosolids produced the highest concentrations of VOSCs, but VOSCs were always below the GC–MS detection limits for ED biosolids (58, 8 and 78 ppmv for DMS, DMDS and MT, respectively). The higher concentrations of VOSCs for the heat-treated biosolids, compared to those for the untreated biosolids, correspond to the higher concentrations of readily extractable proteins in the heat-treated biosolids (Fig. 5). This is in agreement with Higgins et al.’s (2006) suggestion that readily extractable proteins are the precursors of VOSCs, hence their concentrations correlate with VOSCs
Fig. 4. Volatile organic sulphur compounds (VOSCs; sum of methanethiol, dimethyl sulphide and dimethyl disulphide as total sulphur) in untreated, electro-dewatered and heat-treated biosolids over 14 d of incubation under anaerobic conditions in (a) experiment 1, and (b) experiment 2. Experiments were performed several months apart. Bars represent the range of two replicates. The concentrations for electrodewatered samples are plotted at the detection limits (sum of 58, 8 and 78 ppmv for DMS, DMDS and MT, respectively). Note the different y-axis scales between experiments.
production. In contrast, the ED biosolids contained higher concentrations of readily extractable proteins than the untreated biosolids despite their relatively less VOSC production. Therefore, other
Table 2 Concentration of odorous gases detected by Dräger gas tubes after 7 d of incubation under anaerobic conditions. Biosolids sample
Amines ppm as ammonia
Ammonia ppm
Methanethiol ppm
Dimethyl sulphide ppm
Untreated Heat-treated Electro-dewatered
3.7 N/A 1.5
1.3 N/A 1.1
>100 >100 6.7
>300 >300 25.9
N/A: not available
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2014.04.012. References
Fig. 5. Readily extractable protein concentrations in untreated, electro-dewatered and heat-treated biosolids in Experiment 1. Bars represent the range of two replicates.
factors may have reduced VOSCs emissions for ED biosolids. As discussed for regrowth, the high (>0.95) water activity in all samples suggests that the difference in total solids is not the factor that affected microbial activity leading to differences in odour production. Alternatively, other factors such as lower pH or the production of inhibitory compounds could have been involved in the reduction mechanisms.
4. Conclusions Odour production and regrowth potential of total coliforms in ED biosolids were studied using a laboratory-scale ED unit with 10 min cycles. Heat treatment at 80 °C for 10 min was chosen as the control. Both ED and heat-treatment decreased total coliform counts by more than 5 logs to below the detection limits, and no regrowth was observed after 7 d of aerobic or anaerobic incubation. When treated biosolids were inoculated with untreated biosolids to mimic possible field contamination, total coliform levels in the ED biosolids returned back to the detection limit within 4 d, whereas they increased by 4–5 logs in the heat-treated samples. The ED treatment reduced the odour detection and recognition thresholds in the headspace of biosolids compared to those of untreated and heat-treated biosolids. However, the hedonic tones and the odour descriptions remained similar across all samples. The differences in odour thresholds were correlated with the presence of MT, DMS and DMDS (quantified by GC–MS) above the untreated and heat-treated samples, while these compounds remained at trace levels in the ED biosolids. The 2.5 units lower pH of the ED samples may be a factor contributing to the decrease in coliforms regrowth and odour production. Other factors may include the removal of essential nutrients and precursors, or the production of inhibitory compounds. Although further studies are necessary to clarify the mechanisms, our results show the benefits of ED for producing biosolids with lower levels of coliforms and odours. Acknowledgements The Natural Sciences and Engineering Research Council of Canada’s Collaborative Research and Development program and Ovivo provided funding for this study. The authors thank Frederic Biton (Ovivo), Bruno Desmarais (Ovivo), Alain Silverwood (Ovivo), Céline Gagnon (Aquatech), and Gilbert Samson (Régie d’Assainissement des Eaux du Bassin LaPrairie) for their technical support and useful comments.
APHA, 2012. Standard Methods for the Examination of Water and Wastewater, twenty second ed. American Water Works Association, Washington, DC, USA. ASTM, 2011. Standard Practice for Determination of Odor and Taste Thresholds by a Forced-Choice Ascending Concentration Series Method of Limits. ASTM International, West Conshohocken, PA. http://dx.doi.org/10.1520/E0679-04R11. BSI, 2003. Air Quality: Determination of Odour Concentration by Dynamic Olfactometry, BS EN 13725;2003. The British Standard Institution, London, UK. Bureau, M.A., Drogui, P., Sellamuthu, B., Blais, J.F., Mercier, G., 2012. Municipal wastewater sludge stabilization and treatment using electrochemical oxidation technique. J. Environ. Eng. 138, 743–751. CCME, 2010. A Review of the Current Canadian Legislative Framework for Wastewater Biosolids, PN 1446. Canadian Council of Ministers of the Environment, Winnipeg, Canada. Chang, J.S., Abu-Orf, M., Dentel, S.K., 2005. Alkylamine odors from degradation of flocculant polymers in sludges. Water Res. 39, 3369–3375. Curiale, M., 2004. MPN Calculator, Build 24.. Eschborn, R., Abu-Orf, M., Sarrazin-Sullivan, D., 2011. Integrating electrodewatering into advanced biosolids processing where does it make sense? In: Proceedings of the Water Environment Federation: Adapting Residuals Management to a Changing Climate, Sacramento, CA, USA. pp. 141–153. Esmaeily, A., Elektorowicz, M., Habibi, S., Oleszkiewicz, J.A., 2006. Dewatering and coliform inactivation in biosolids using electrokinetic phenomena. J. Environ. Eng. Sci. 5, 197–202. Forbes, B., Adams, G., Hargreaves, R., Witherspoon, J., McEwen, D., Erdal, Z., Hentz, L., Murthy, S., Card, T., Glindemann, D., Higgins, M., 2004. Impacts of the in-plant operational parameters on biosolids odor quality–final results of WERF odor project phase 2 field and laboratory study. In: Proceedings of the Water Environment Federation: WEF/A&WMA Odors and Air Emissions, Bellevue, WA, USA, pp. 487–500. Hébert, M., 2008. Guidelines for the Beneficial Use of Fertilising Residuals: Reference Criteria and Regulatory Standards. Développement durable, Environnement et Parcs, Québec, Canada. Higgins, M.J., Chen, Y.C., Murthy, S.N., Hendrickson, D., Farrel, J., Schafer, P., 2007. Reactivation and growth of non-culturable indicator bacteria in anaerobically digested biosolids after centrifuge dewatering. Water Res. 41, 665–673. Higgins, M.J., Chen, Y.C., Yarosz, D.P., Murthy, S.N., Maas, N.A., Glindemann, D., Novak, J.T., 2006. Cycling of volatile organic sulfur compounds in anaerobically digested biosolids and its implications for odors. Water Environ. Res. 78, 243– 252. Huang, J., Elektorowicz, M., Oleszkiewicz, J.A., 2008. Dewatering and disinfection of aerobic and anaerobic sludge using an electrokinetic (EK) system. Water Sci. Technol. 57 (2), 231–236. Kim, H., Murthy, S., Peot, C., Ramirez, M., Strawn, M., Park, C.H., McConnell, L.L., 2003. Examination of mechanisms for odor compound generation during lime stabilization. Water Environ. Res. 75, 121–125. Krach, K.R., Li, B., Burns, B.R., Mangus, J., Butler, H.G., Cole, C., 2008. Bench and fullscale studies for odor control from lime stabilized biosolids: the effect of mixing on odor generation. Bioresour. Technol. 99, 6446–6455. McGinley, M.A., McGinley, C.M., 2006. Performance Verification of Air Freshener Products and other Odour Control Devices for Indoor Air Quality Malodours. St. Croix Sensory Inc., Lake Elmo, MN, USA. Murthy, S., Higgins, M., Chen, Y.C., Peot, C., Toffey, W., 2006. High-solids centrifuge is a boon and a curse for managing anaerobically digested biosolids. Water Sci. Technol. 53 (3), 245–253. Murthy, S., Kim, H., Peot, C., McConnell, L.L., Strawn, M., Sadick, T., Dolak, I., 2003. Evaluation of odor characteristics of heat-dried biosolids product. Water Environ. Res. 75, 523–531. Navab-Daneshmand, T., Beton, R., Hill, R.J., Gehr, R., Frigon, D., 2012. Inactivation mechanisms of bacterial pathogen indicators during electro-dewatering of activated sludge biosolids. Water Res. 46, 3999–4008. Presser, K.A., Ratkowsky, D.A., Ross, T., 1997. Modelling the growth rate of Escherichia coli as a function of pH and lactic acid concentration. Appl. Environ. Microbiol. 63, 2355–2360. Qi, Y.N., Dentel, S.K., Herson, D.S., 2008. Effect of total solids on fecal coliform regrowth in anaerobically digested biosolids. Water Res. 42, 3817–3825. Rosenfeld, P.E., Henry, C.L., Dills, R.L., Harrison, R.B., 2001. Comparison of odor emissions from three different biosolids applied to forest soil. Water Air Soil Pollut. 127, 173–191. Saveyn, H., Van der Meeren, P., Pauwels, G., Timmerman, R., 2006. Bench- and pilotscale sludge electrodewatering in a diaphragm filter press. Water Sci. Technol. 54 (9), 53–60. Sidhu, J., Gibbs, R.A., Ho, G.E., Unkovich, I., 2001. The role of indigenous microorganisms in suppression of Salmonella regrowth in composted biosolids. Water Res. 35, 913–920.
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Bacterial pathogen indicators regrowth and reduced sulphur compounds’ emissions during storage of electro-dewatered biosolids Tala Navab-Daneshmand, Samia Enayet, Ronald Gehr, Dominic Frigon ⇑ Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, Quebec H3A 0C3, Canada
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Regrowth of coliforms not observed
in all electro-dewatered biosolids after 7 d. Regrowth of total coliforms observed in inoculated heat-treated biosolids. Lower odour detection and recognition thresholds for electrodewatered biosolids. Little volatile organic sulphur compounds above electro-dewatered biosolids. pH decrease, nutrient removal and inhibitor formation may explain observed effects.
a r t i c l e
i n f o
Article history: Received 12 August 2013 Received in revised form 3 April 2014 Accepted 4 April 2014
Handling Editor: Hyunook Kim Keywords: Bacterial pathogen indicators Biosolids Electro-dewatering Odour production Reduced sulphur compounds Regrowth
a b s t r a c t Electro-dewatering (ED) increases biosolids dryness from 10–15 to 30–50%, which helps wastewater treatment facilities control disposal costs. Previous work showed that high temperatures due to Joule heating during ED inactivate total coliforms to meet USEPA Class A biosolids requirements. This allows biosolids land application if the requirements are still met after the storage period between production and application. In this study, we examined bacterial regrowth and odour emissions during the storage of ED biosolids. No regrowth of total coliforms was observed in ED biosolids over 7 d under aerobic or anaerobic incubations. To mimic on-site contamination during storage or transport, ED samples were seeded with untreated sludge. Total coliform counts decreased to detection limits after 4 d in inoculated samples. Olfactometric analysis of ED biosolids odours showed that odour concentrations were lower compared to the untreated and heat-treated control biosolids. Furthermore, under anaerobic conditions, odorous reduced sulphur compounds (methanethiol, dimethyl sulphide and dimethyl disulphide) were produced by untreated and heat-treated biosolids, but were not detected in the headspaces above ED samples. The data demonstrate that ED provides advantages not only as a dewatering technique, but also for producing biosolids with lower microbial counts and odour levels. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Production of biosolids and their handling and disposal costs have increased substantially over the past few decades (Higgins ⇑ Corresponding author. Tel.: +1 (514) 398 2476; fax: +1 (514) 398 7361. E-mail address: [email protected] (D. Frigon). http://dx.doi.org/10.1016/j.chemosphere.2014.04.012 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
et al., 2007). One of the most common methods for biosolids disposal is land application. According to the United States Environmental Protection Agency (USEPA) regulations, biosolids can be land applied if they fall under Class A or Class B, which are defined based on the levels of specific bacterial, viral and eukaryotic indicators (USEPA, 2003). In Canada, regulations follow similar principles and approaches (CCME, 2010). Another obstacle for biosolids land
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application is nuisance odours. While the USEPA does not regulate biosolids odour production for land application, certain Canadian jurisdictions categorize fertilizing biomaterials into four groups from less odorous than cow manure to more odorous than pig manure. Biosolids in the latter category cannot be land applied. Municipal biosolids are typically classified between cow and pig manures (Hébert, 2008). In addition to specific regulatory restrictions, emissions of odours during biosolids storage may irritate populations living close to treatment plants or land application sites. This could reduce public support for land application and limit the flexibility which operators have for biosolids management. Electro-dewatering (ED) is a relatively new technology that can increase the solids content of biosolids up to 70 wt%, while using less energy than heat drying. It has been shown to reduce total coliforms and Escherichia coli to below detection limits, producing Class A biosolids (Esmaeily et al., 2006; Saveyn et al., 2006; Navab-Daneshmand et al., 2012), however, the USEPA regulations on the microbiological requirements for biosolids, mentioned above, are to be met at the time of application, and not immediately after the process. Low bacterial counts after treatment processes do not guarantee that inactivation is irreversible, and regrowth can occur during storage. For example, centrifuged anaerobically digested biosolids have shown increases of 2–4 logs of faecal coliforms and E. coli when incubated at 25–37 °C for 24 h (Higgins et al., 2007; Qi et al., 2008). Regrowth of bacterial pathogen indicators by several orders of magnitude during storage can be affected and controlled by several environmental parameters. One such factor is biosolids moisture content, as lower bacterial pathogen growth rates are expected in drier environments. A second factor that could impact bacterial regrowth is the availability of oxygen as an electron acceptor. Typically, biosolids are stored in deep containers or in large piles. The surface layer of the pile is aerobic due to atmospheric exposure. Oxygen diffusion, however, is limited downwards and it penetrates only a few centimeters from the surface, resulting in anoxic or anaerobic conditions lower down (Yamada and Kawase, 2006). A third factor affecting bacterial regrowth is the pH of the sludge matrix. The ambient pH affects nutrient availability and changes the solubility of substances consumed by or inhibitory to bacteria; it directly impacts microbial metabolism and enzyme activities (Sidhu et al., 2001). There is an optimal pH for microbial growth that is specific for each species; for example, E. coli growth rate increases by 4–5 times from pH 4 to pH above 6 (Presser et al., 1997). During ED, a pH gradient is developed in the sludge cake from low pH near the anode (as low as 2.2; Huang et al., 2008), to high pH near the cathode (as high as 7.7; Navab-Daneshmand et al., 2012). This pH gradient is generated by electrolysis on the electrode surfaces that produces hydrogen ions at the anode and hydroxide ions at the cathode. We are not aware of regrowth studies for ED biosolids, but regrowth has been shown to be substantial for biosolids from other sources such as anaerobic digestion. Thus, the first objective of this work is to determine the effect of ED treatment on the regrowth of bacterial pathogen indicators during storage. It has been noted that ED produces biosolids with less objectionable odours (Eschborn et al., 2011; Bureau et al., 2012), but detailed information was not reported. Several classes of compounds have been identified as the causes of odours from biosolids; the main class is volatile organic sulphur compounds (VOSCs) including methanethiol (MT), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) (Murthy et al., 2003; Forbes et al., 2004; Krach et al., 2008). These compounds are commonly reminiscent of sewer odours, and they have a very low odour threshold. The VOSCs are formed from precursors released during the breakdown of readily extractable proteins in biosolids, including cysteine and methionine, that are subsequently degraded to MT and hydrogen sulphide (Higgins et al., 2006). While some authors have argued that DMDS formation was the result of abiotic oxidation of MT in the presence
of molecular oxygen (Higgins et al., 2006), the presence of DMDS under strict anaerobic conditions has also been reported (Turkmen et al., 2004). Another important class of odorants is amines and ammonia that cause fishy odours. They were reported to be the main contributors to the odour profile of lime-stabilized biosolids (Kim et al., 2003). The high acid dissociation constant (typically pKa > 9) of protonated amine groups means that a high pH is required for the efficient volatilization of these compounds, as it is the non-ionized form that is volatile (Chang et al., 2005). Finally, a third group of compounds responsible for biosolids odours is volatile fatty acids, which contribute to rancid or vinegary odours (Rosenfeld et al., 2001; Murthy et al., 2003). They are, however, typically considered to be minor contributors. In addition, the source, dewatering process and conditioning of biosolids impact the availability of precursors to the formation of odorants and the levels of oxygen that modulate biotic and abiotic pathways of odorant formation (Forbes et al., 2004; Murthy et al., 2006). There have been few – if any – comprehensive descriptive or quantitative data on odours or odour classes from ED biosolids in the published literature. Thus, the second objective of the current study is to establish the principal differences in odour generation during storage between untreated and ED biosolids. In this study, ED biosolids were compared to untreated biosolids, and to heat-treated biosolids because it had been shown that heat was the main inactivation mechanism during ED (NavabDaneshmand et al., 2012). Regrowth was studied under both aerobic and anaerobic storage conditions. Additionally, to mimic possible contamination with bacterial pathogen indicators from raw biosolids at treatment plants or during transport, some ED and heat-treated samples were inoculated with untreated biosolids before incubation. Odour production was investigated only under anaerobic conditions because they typically favor odour production. Odour profiles after 7 d of incubation were assessed by olfactometric tests, and the dynamics of selected VOSCs concentrations were determined by a GC–MS assay. 2. Materials and methods 2.1. Biosolids Biosolids were sampled from an activated sludge wastewater treatment plant without primary clarification near Montréal (Québec, Canada). The plant treats a flow of 60 000 m3 d 1 on average, with a hydraulic retention time (HRT) of 12 h, and a solids retention time of 6 d. At the time of the study, a cationic polymer (PAM C-65 L; Jes-Chem, Guelph, Ontario, Canada) was added at a concentration of 10–15 kg t 1 of solids before the centrifuge dewatering units. Biosolids samples were taken immediately after centrifugation, brought to the laboratory on ice and stored at 4 °C for up to 4 d. 2.2. Electro-dewatering and heat treatment of biosolids The laboratory ED unit was a CINETIK CK-lab model (Ovivo, Boucherville, Québec, Canada), which used a direct current electrical field. Similarly to our previous study (Navab-Daneshmand et al., 2012), the maximum voltage and current were set at 60 V and 5.5 A, respectively. For each ED experiment, 165 wet g biosolids were placed on a filter medium (100% PPS Ryton, woven) over a stainless steel perforated cathode. The ceramic-coated titanium anode applied 140 kPa constant pressure. The 10 min ED cycle reduced total coliforms to below detection limits. Heat-treatment was the control process as it had been shown that the inactivation of bacterial pathogen indicators during biosolids ED is due to high temperatures (>75 °C; Navab-Daneshmand
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et al., 2012). For heat-treatment, approximately 40 wet g of biosolids were placed in a 50 mL glass tube, and incubated in a water bath at 80 °C. It took 6 min for the biosolids samples to reach 80 °C; hence to have 10 min heat-treatment, the tubes were incubated in the water bath for a total of 16 min. 2.3. Seeding To mimic possible field contamination of treated biosolids, some ED and heat-treated samples were inoculated with untreated biosolids. For this purpose, 2 wet g of untreated biosolids were added to 40 mL distilled water and homogenized with a disperser (ULTRA-TURAX S10N-10G, IKA Works Inc., Wilmington, NC). 2 mL of the slurry were added to 100 wet g of ED or heat-treated biosolids in 50 mL sterilized glass tubes and mixed thoroughly by hand. 2.4. Biosolids incubation About 50 wet g of sample was placed in a 1 L media bottle that was closed by a silicon septum. The bottles were placed on a roller apparatus (Wheaton Industries Inc., Millville, NJ, USA) to maintain sample uniformity and covered to avoid light exposure. Incubation proceeded at room temperature (22 ± 0.5 °C). For regrowth experiments, aerobically incubated bottles were left open vertically on the bench for 10 min daily, while the anaerobically incubated ones were flushed daily for 10 min with pure nitrogen. Four sets of experiments lasting 7 d each were performed 3–8 weeks apart. For the odour production experiments, anaerobically incubated bottles were flushed with nitrogen at the beginning of the experiments (Day 0) and were kept closed for their entire duration (14 d). Anaerobic indicator strips were placed in each bottle (BD GasPak, Franklin Lakes, NJ); the blue color of the humid strips turned white to confirm the absence of oxygen. Headspace samples were taken through the silicon septa for odour analysis. 2.5. Physical–chemical parameters Standard Method No 2540-B (APHA, 2012) was used for total solids measurement. Following Navab-Daneshmand et al. (2012), pH of biosolids was measured by creating a slurry (0.8 wet g in 9.2 mL of distilled water). Water activity was measured using a manometric method (HygroLab Set 2 equipped with an AW-DIO probe, ROTRONIC, Switzerland). Water activity of samples was measured in sealed containers every 15 s and the value was recorded when the vapour pressure was stable for at least 10 min. If condensation was present on the container walls before the test, water activity was recorded as 1.0 without taking measurements. 2.6. Total coliforms enumeration A modified microplate MPN method (Navab-Daneshmand et al., 2012) was used to enumerate total coliforms using Standard Method No. 9223 (APHA, 2012; Colilert reagent, IDEXX Laboratories, Westbrooke, ME) with a microplate reader (SpectraMax M5, Molecular Devices, Sunnyvale, CA). An online MPN calculator was used to calculate MPN g 1 total solids (Curiale, 2004). The detection limit was determined to be 3 positive wells in the first row corresponding to 75–750 MPN g 1 TS, depending on the dryness. 2.7. Odour analyses The impact of ED on odour production was analysed in several ways. In one series of tests, olfactometric analyses of the headspaces above samples were done following ASTM E679-04 standard
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protocol and British Standard 13725, using 5 panellists (BSI, 2003; ASTM, 2011). Samples were sent overnight to Pinchin Environmental Ltd. (Mississauga, Ontario, Canada) for analysis of the following parameters: (a) Detection threshold: dilution ratio at which the sample was detected by at least 50% of the panellists. (b) Recognition threshold: the dilution ratio at which the sample was correctly recognized by at least 50% of the panellists. (c) Hedonic tone: a reference scale ( 10 to + 10) ranking odours from most unpleasant to most pleasant. It is independent of odour descriptors. (d) Odour descriptor: odour panellists’ descriptions for characterizing odours assessing the intensity (1–5) of eight primary odour wheel descriptors (McGinley and McGinley, 2006): vegetable, fruity, floral, medicinal, chemical, fishy, offensive and earthy. Primary descriptors were complemented by secondary descriptors. In another series of tests, the presence of odorous gases was examined in the headspaces above samples by Dräger short-term gas detection tubes for MT, DMS, amines and ammonia (Dräger Safety Inc., Pittsburg, PA). The third series of tests, based on the results of the first two series, which suggested focusing on VOSCs, monitored the concentrations of MT, DMS and DMDS in the headspaces above biosolids by GC–MS (Trace Ultra equipped with an ITQ 1100 External Ion Trap MS, Thermo Scientific, Milan, Italy). For this quantitative study, two sets of experiments lasting 14 d each were performed six months apart. These experiments were conducted in duplicate. Using a gas-tight syringe, headspace gas samples (1 mL) were injected directly into the GC. Chromatographic separation was performed (Rtx-5MSi column, Restek, Bellefonte, PA) with the oven temperature following the program: 45 °C for 0.5 min, increase to 100 °C at 45 °C min 1 and hold for 0.5 min, increase to 300 °C at 30 °C min 1 and hold for 1 min. Detection was performed by electron ionization (70 eV) with analysis in full scan and selected ionmonitoring modes. MT (2000 ppmv in nitrogen; Linde Canada Ltd., Mississauga, Ontario, Canada), DMS and DMDS (P99%; Sigma– Aldrich, St. Louis, MO) standards were used to construct quantitative calibration curves. Example chromatograms of samples and standard gases are presented in Fig. S1. 2.8. Readily extractable protein The method of Higgins et al. (2006) was modified to measure the readily extractable protein. Biosolids were mixed on a stir plate for 30 min (10 wet g of biosolids in 100 mL of 50 mM phosphate buffer solution, pH 8), then 2 mL of the slurry were centrifuged at maximum speed (14 800 rpm) in a micro-centrifuge for 5 min. Supernatants were then collected and analysed for protein concentration (DC Protein Assay kit, Bio-Rad, Hercules, CA). 3. Results and discussion 3.1. Biosolids electro-dewatering Results for a typical ED and heat-treatment experiment are presented in Figs. 1 and 2. During ED, energy was consumed at a constant rate, resulting in a nearly linear filtrate removal rate after a short lag time (Fig. 1). The temperature of the dewatering biosolids cake increased from room temperature to above 80 °C over 10 min due to Joule heating (Fig. 1). The ED increased the total solids from 17.9 ± 0.3 wt% (±standard deviation) to 32.9 ± 2.3 wt% (Fig. 2). On
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average, the pH of 7.1 ± 0.2 in the untreated biosolids decreased to 4.6 ± 0.1 after ED (Fig. 2). Heat-treatment did not affect the pH or the total solids of the samples (Fig. 2). Finally, ED and heat-treatment decreased total coliform counts by more than 5 logs (Fig. 2). This result is similar to that obtained previously for ED, in which disinfection was attributed to the high temperatures (Navab-Daneshmand et al., 2012). 3.2. Regrowth of bacterial pathogen indicators During the 7-d incubation period, the pH of all samples remained stable (data not shown) and the total solids concentration only changed slightly (Figs. 3a and b). For the ED and heat-treated samples, total coliform counts remained below the detection limit (Figs. 3c and d). To mimic possible contamination of biosolids during storage or transport, ED and heat-treated samples were seeded with untreated biosolids. This inoculation increased the counts to 2.0 103 and 3.2 103 MPN g 1 TS in the ED and heat-treated samples, respectively. Total coliform counts in the inoculated ED biosolids returned to the detection limit or below on Day 4 of incubation under both aerobic and anaerobic conditions (Figs. 3c and d). In the inoculated heattreated samples, however, total coliform counts increased after inoculation both with and without oxygen. It would appear that heat inactivation (Navab-Daneshmand et al., 2012) during heat-treatment or ED was sufficient to control the observable regrowth of total coliforms over 7 d, but that other factors which developed during ED prevented the multiplication of the external bacterial contamination. Although there was about a 20 wt% difference in total solids between ED and untreated or heat-treated biosolids (Figs. 3a and b), the water activity of all these samples was well above 0.95 (data not shown), which is the minimum limit for the growth of E. coli and other coliforms (Tapia et al., 2008). Therefore, it is unlikely that water scarcity had limited the regrowth of these bacteria. These data also suggest that the availability of oxygen (comparing aerobic and anaerobic incubations) does not affect coliforms regrowth in untreated, ED or heat-treated biosolids (Figs. 3c and d). It is possible that the low pH of ED biosolids (4.6 compared to 7.0 in the heat-treated case; Fig. 2) affected the regrowth of total coliforms. This explanation would correspond with that of Presser et al. (1997), who reported an increase of 4–5 times in the growth rate of E. coli over a similar pH range. Other possible explanations include the removal of essential nutrients with the filtrate from the biosolids matrix during ED, and the production of inhibitory compounds.
Fig. 2. Total solids, pH and total coliform counts in untreated, electro-dewatered and heat-treated biosolids. Bars represent the standard errors of four experiments, performed several weeks apart. The asterisks (*) for total coliforms indicate the detection limits as these counts were below detection.
Fig. 3. (a) and (b) Total solids, and (c) and (d) total coliform counts, in untreated (UT), electro-dewatered (ED), inoculated electro-dewatered (IED), heat-treated (HT), and inoculated heat-treated (IHT) biosolids during 7-d incubations under aerobic (on the left) and anaerobic (on the right) conditions. Bars represent the standard errors of four experiments for untreated, electro-dewatered and heattreated samples, and the range of two experiments for inoculated electrodewatered and inoculated heat-treated samples. Each set of experiments were performed several weeks apart. The asterisks () for total coliforms are the detection limits as these counts were below detection.
Further studies are needed to clarify the exact mechanism leading to the apparent lower regrowth potential of ED biosolids.
3.3. Odour production
Fig. 1. Treatment conditions and effects on biosolids: (a) consumed energy, cake temperature and removed filtrate during electro-dewatering. Values are averages of four experiments, performed several weeks apart with standard errors below 5.6%, 5.2%, and 16.5% after 4 min for consumed energy, cake temperature, and removed filtrate, respectively.
Olfactometric analysis of the bottles’ headspaces after 7 d anaerobic incubation revealed a reduction of 50% in odour detection and recognition thresholds of the ED, compared to the untreated biosolids (Table 1). When these thresholds were tested for a blank control sample (nitrogen gas incubated in similar bottles, but without biosolids), they were at least 26 times lower than those of the headspace above ED biosolids. Therefore, the nitrogen gas used did not influence the observed odour profile. Despite a reduction in odour detection and recognition thresholds by ED, the hedonic tone (measuring the pleasantness of the odours) for these biosolids remained similar to those for untreated and heattreated samples. Among the eight main descriptors of the odour characterization wheel (McGinley and McGinley, 2006), the odours from all three biosolids samples were described as offensive and earthy. The relative intensity of these descriptors was slightly
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T. Navab-Daneshmand et al. / Chemosphere 113 (2014) 109–115 Table 1 Odour characterization of headspaces above biosolids after 7-d incubations under anaerobic conditions.
a b c d e
Parameter
Blank N2 gasa
Biosolids samples Untreated
Heat-treated
Electro-dewatered
Detection threshold ( 103) Recognition threshold ( 103) Hedonic tone (range)b Main primary odour descriptor relative intensityc Offensive Secondary descriptorsd Earthy Secondary descriptors Fishy Secondary descriptors
0.4 0.2 5 ( 2 to
25.0 11.5 3 ( 5 to
17.9 8.3 3 ( 7 to
13.0 5.6 3 ( 4 to
9)
1.50 Raw meat, garbage 0.70 Musty 2.65 Dead fish
2)
1.50 Sewer, rancid, garbage 0.50 Musty 0.50 Nonee
2)
2.15 Sewer, manure, garbage 0.90 Musty 0 None
2)
1.35 Sewer, manure, garbage 1.15 Musty 0 None
The blank bottles were incubated in the same conditions as the biosolids’ bottles. Hedonic tone scale: unpleasant 10, neutral 0, pleasant + 10. Primary descriptor relative intensity scale: not utilized by panelists 0, mild odour 1, strong odour 5. Secondary descriptors mainly used by panelists in association with primary descriptor. Panelist did not use secondary descriptors for biosolids fishy odours.
different between samples, and ED biosolids exhibited the highest intensity of earthy odours. Furthermore, the sewer odour was used as the common secondary descriptor for the offensive nature of the odours from all three samples. A second series of experiments used Dräger tubes to characterize odorous compounds in the headspaces above biosolids. In these tests, amines, ammonia, MT and DMS were detected at fairly high concentrations above the untreated samples, whereas they were detected at relatively low concentrations above the ED biosolids (Table 2). The concentrations of amines and ammonia were consistent with the fishy odour reported only for the untreated biosolids by the olfactometric analysis (Table 1). In addition, the highest concentrations of the odorous compounds were detected for MT and DMS (Table 2). The VOSCs such as MT and DMS, are characteristic of sewer odours. Because MT and DMS were the highest concentrations detected by the Dräger tube tests (Table 2) and because sewer odour was the most common offensive secondary descriptor used for all three samples (Table 1), a third series of experiments were performed to quantify the time profile of VOSCs in the headspaces above the samples, and to ascertain the dynamics of odour production. There were large differences between the concentrations of VOSCs (sum of MT, DMS and DMDS) for the two sets of experiments that were performed six months apart (Fig. 4). In addition, while MT was detected in both cases, DMS and DMDS were detected only in Experiment 1. Nonetheless, common observations can be made from the two experiments: heat-treated biosolids produced the highest concentrations of VOSCs, but VOSCs were always below the GC–MS detection limits for ED biosolids (58, 8 and 78 ppmv for DMS, DMDS and MT, respectively). The higher concentrations of VOSCs for the heat-treated biosolids, compared to those for the untreated biosolids, correspond to the higher concentrations of readily extractable proteins in the heat-treated biosolids (Fig. 5). This is in agreement with Higgins et al.’s (2006) suggestion that readily extractable proteins are the precursors of VOSCs, hence their concentrations correlate with VOSCs
Fig. 4. Volatile organic sulphur compounds (VOSCs; sum of methanethiol, dimethyl sulphide and dimethyl disulphide as total sulphur) in untreated, electro-dewatered and heat-treated biosolids over 14 d of incubation under anaerobic conditions in (a) experiment 1, and (b) experiment 2. Experiments were performed several months apart. Bars represent the range of two replicates. The concentrations for electrodewatered samples are plotted at the detection limits (sum of 58, 8 and 78 ppmv for DMS, DMDS and MT, respectively). Note the different y-axis scales between experiments.
production. In contrast, the ED biosolids contained higher concentrations of readily extractable proteins than the untreated biosolids despite their relatively less VOSC production. Therefore, other
Table 2 Concentration of odorous gases detected by Dräger gas tubes after 7 d of incubation under anaerobic conditions. Biosolids sample
Amines ppm as ammonia
Ammonia ppm
Methanethiol ppm
Dimethyl sulphide ppm
Untreated Heat-treated Electro-dewatered
3.7 N/A 1.5
1.3 N/A 1.1
>100 >100 6.7
>300 >300 25.9
N/A: not available
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2014.04.012. References
Fig. 5. Readily extractable protein concentrations in untreated, electro-dewatered and heat-treated biosolids in Experiment 1. Bars represent the range of two replicates.
factors may have reduced VOSCs emissions for ED biosolids. As discussed for regrowth, the high (>0.95) water activity in all samples suggests that the difference in total solids is not the factor that affected microbial activity leading to differences in odour production. Alternatively, other factors such as lower pH or the production of inhibitory compounds could have been involved in the reduction mechanisms.
4. Conclusions Odour production and regrowth potential of total coliforms in ED biosolids were studied using a laboratory-scale ED unit with 10 min cycles. Heat treatment at 80 °C for 10 min was chosen as the control. Both ED and heat-treatment decreased total coliform counts by more than 5 logs to below the detection limits, and no regrowth was observed after 7 d of aerobic or anaerobic incubation. When treated biosolids were inoculated with untreated biosolids to mimic possible field contamination, total coliform levels in the ED biosolids returned back to the detection limit within 4 d, whereas they increased by 4–5 logs in the heat-treated samples. The ED treatment reduced the odour detection and recognition thresholds in the headspace of biosolids compared to those of untreated and heat-treated biosolids. However, the hedonic tones and the odour descriptions remained similar across all samples. The differences in odour thresholds were correlated with the presence of MT, DMS and DMDS (quantified by GC–MS) above the untreated and heat-treated samples, while these compounds remained at trace levels in the ED biosolids. The 2.5 units lower pH of the ED samples may be a factor contributing to the decrease in coliforms regrowth and odour production. Other factors may include the removal of essential nutrients and precursors, or the production of inhibitory compounds. Although further studies are necessary to clarify the mechanisms, our results show the benefits of ED for producing biosolids with lower levels of coliforms and odours. Acknowledgements The Natural Sciences and Engineering Research Council of Canada’s Collaborative Research and Development program and Ovivo provided funding for this study. The authors thank Frederic Biton (Ovivo), Bruno Desmarais (Ovivo), Alain Silverwood (Ovivo), Céline Gagnon (Aquatech), and Gilbert Samson (Régie d’Assainissement des Eaux du Bassin LaPrairie) for their technical support and useful comments.
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