Eric C. Sivret

Monitoring techniques for odour abatement assessment

Odorous emissions from sewers and wastewater treatment plants are a complex mixture of volatile chemicals that can cause annoyance to local populations, resulting in complaints to wastewater operators. Due to the variability in hedonic tone and chemical character of odorous emissions, no analytical technique can be applied universally for the assessment of odour abatement performance. Recent developments in analytical methodologies, specifically gas chromatography, odour assessment approaches (odour wheels, the odour profile method and dynamic olfactometry), and more recently combined gas chromatography-sensory analysis, have contributed to improvements in our ability to assesses odorous emissions in terms of odorant concentration and composition. This review collates existing knowledge with the aim of providing new insight into the effectiveness of sensorial and characterisation approaches to improve our understanding of the fate of odorous emissions during odour abatement. While research in non-specific sensor array (e-nose) technology has resulted in progress in the field of continuous odour monitoring, more successful long term case-studies are still needed to overcome the early overoptimistic performance expectations. Knowledge gaps still remain with regards to the decomposition of thermally unstable volatile compounds (especially sulfur compounds), the inability to predict synergistic, antagonistic, or additive interactions among odorants in combined chemical/sensorial analysis techniques, and the long term stability of chemical sensors due to sensor drift, aging, temperature/relative humidity effects, and temporal variations. Future odour abatement monitoring will require the identification of key odorants to facilitate improved process selection, design and management.

Determination of VOSCs in sewer headspace air using TD–GC–SCD

The management of odorous emissions from sewer networks has become an important issue for sewer operators resulting in the need to better understand the composition of volatile organic sulfur compounds (VOSCs). In order to characterise the composition of such malodorous emissions, a method based on thermal desorption (TD) and gas chromatography coupled to sulfur chemiluminescence detector (GC-SCD) has been developed to determine a broader range of VOSCs, hydrogen sulfide (H2S), methanethiol (MeSH), ethanethiol (EtSH), dimethyl sulfide (DMS), carbon disulfide (CS2), ethylmethyl sulfide (EMS), 1-butanethiol (1-BuSH), dimethyl disulfide (DMDS), diethyl disulfide (DEDS), and dimethyl trisulfide (DMTS). Parameters affecting the chromatographic behaviour of the target compounds were studied (e.g., temperature program, carrier gas velocity) as well as the experimental conditions affecting the adsorption/desorption process (temperature, flow and time). Optimised extraction of VOSCs samples was achieved under adsorption temperatures of less than -20°C, and a desorption flow rate of ~6 ml/min. Active collection on the cold trap enabled a small gas volume of 50-100ml to be sampled for all analytes without breakthrough. Calibration curves were derived at different TD loading volumes with determined linearity ranging between 0.09 ng and 60.1 ng. The method detection limits (MDLs) were in the range of 0.10-5.26 μg/m(3) with TD recoveries higher than 66% and reproducibility (relative standard deviation values) between 1.8% and 6.1% being obtained for all compounds. The VOSCs characterisation at different sewerage collection sites in Sydney, Australia (for seasonal, weekly and diurnal) showed that six of the ten targeted compounds were consistently detected at all sample events. Diurnal patterns of VOSCs investigated were clearly observed with the highest concentration occurring after 12 pm (noon) for H2S and MeSH. The consecutive 5 day analysis showed no significant difference in the targeted VOSCs concentrations and demonstrated the suitability of the method for routine sewer VOSCs emission measurements.

Nitrous oxide monitoring for nitrifying activated sludge aeration control: A simulation study

An activated sludge aeration control concept was developed utilizing off‐gas nitrous oxide concentrations as a surrogate for autotrophic nitrifying bacterial inhibition and aeration air as a master control variable. The control concept was evaluated using a simulated pilot scale bioreactor (mathematically calibrated liquid phase process model and a model to link off‐gas nitrous oxide generation to liquid phase conditions) as a data generator. When applied to the simulated system, the process controller was successful at maintaining the process at the desired operating setpoint and promoting stable operation by minimizing periods of significant inhibition. Furthermore, it provided a more efficient use of the air supplied to the bioreactor during periods of varying feed loading by matching the air supply to the metabolic demands, substantially reducing periods of over and under‐aeration. The findings of this research demonstrate the potential for off‐gas nitrous oxide monitoring as a completely non‐invasive alternative to liquid phase monitoring used in conventional dissolved oxygen control. Investigations are currently underway at the laboratory scale to evaluate the benefits and limitations associated with this control concept, with particular emphasis on implementation issues and the quantification of potential aeration and cost savings. Biotechnol. Biotechnol. Bioeng. 2008;101: 109–118.

Is H2S a suitable process indicator for odour abatement performance of sewer odours

Odour abatement units are typically designed and maintained on H(2)S concentrations, but operational failures are reported in terms of overall odour removal, suggesting a wide range of malodorous compounds emitted from sewers that may not be efficiently removed by existing odour abatement processes. Towards providing greater insight into this issue, several activated carbon filters and biofilters treating odorous emissions from sewer systems in Sydney (Australia) were monitored by collecting and analysing gas samples before and after treatment. The monitoring studies were conducted by both olfactometric measurements and gas-chromatography-based chemical analysis. Single H(2)S assessment often failed to indicate the odour abatement performance for treatment systems in the abatement units studied, particularly when the incoming H(2)S concentrations were in the sub-ppm range (i.e. below H(2)S odour threshold). Chemical analysis indicated that some non-H(2)S odorous compounds were not removed efficiently during odour treatment. Additionally, when odour eliminations were correlated with the removal of individual compounds (Pearsons correlations) it was observed that the correlation (with a coefficient of 0.79) was best when the overall removal of all the measured odorous compounds that exceeded their odour threshold values was used for the analysis. These findings may help to further advance the design and operation of odour abatement processes to address the treatment of sewer odour emissions.

Odour emission ability (OEA) and its application in assessing odour removal efficiency

Odourous emissions from sewer networks and wastewater treatment plants (WWTPs) can significantly impact a local population. Sampling techniques such as wind tunnels and flux hood chambers are traditionally used to collect area source samples for subsequent quantification of odour emission rates using dilution olfactometry, however these methods are unsuitable for assessing liquid samples from point sources due to the large liquid volumes required. To overcome this limitation, a gas phase sample preparation method was developed for assessing the total Odour Emission Ability (OEA) from a liquid sample. The method was validated using two volatile organic sulphur compounds (VOSCs), dimethyl-trisulphide (DMTS) and bismethylthiomethane (BMTM) that are frequently detected from sewers and WWTPs and are relatively stable compared with common VOSCs like mercaptan or methyl mercaptan. The recovery rates of DMTS and BMTM were quantified by injecting a known volume of a standard liquid sample into Tedlar bags using a static injection and a dynamic injection methodology. It was confirmed that both dynamic and static injection methods at ambient condition achieved high recovery rates with no need to consider increasing evaporation by elevating the temperature. This method can also be used to assess odour removal effectiveness of liquids by comparing the OEA before and after the treatment tests. Two application examples were presented.

Stability of Volatile Sulfur Compounds (VSCs) in sampling bags – impact of temperature

Volatile sulfur compounds (VSCs) are a major component of odorous emissions that can cause annoyance to local populations surrounding wastewater, waste management and agricultural practices. Odour collection and storage using sample bags can result in VSC losses due to sorption and leakage. Stability within 72 hour storage of VSC samples in three sampling bag materials (Tedlar, Mylar, Nalophan) was studied at three temperatures: 5, 20, and 30 °C. The VSC samples consisted of hydrogen sulfide (H2S), methanethiol (MeSH), ethanethiol (EtSH), dimethyl sulfide (DMS), tert-butanethiol (t-BuSH), ethylmethyl sulfide (EMS), 1-butanethiol (1-BuSH), dimethyl disulfide (DMDS), diethyl disulfide (DEDS), and dimethyl trisulfide (DMTS). The results for H2S showed that higher loss trend was clearly observed (46-50% at 24 hours) at 30 °C compared to the loss at 5 °C or 20 °C (of up to 27% at 24 hours) in all three bag materials. The same phenomenon was obtained for other thiols with the relative recoveries after a 24 hour period of 76-78% at 30 °C and 80-93% at 5 and 20 °C for MeSH; 77-80% at 30 °C and 79-95% at 5 and 20 °C for EtSH; 87-89% at 30 °C and 82-98% at 5 and 20 °C for t-BuSH; 61-73% at 30 °C and 76-98% at 5 and 20 °C for 1-BuSH. Results for other sulfides and disulfides (DMS, EMS, DMDS, DEDS) indicated stable relative recoveries with little dependency on temperature (83-103% after 24 hours). DMTS had clear loss trends (with relative recoveries of 74-87% in the three bag types after 24 hours) but showed minor differences in relative recoveries at 5, 20, and 30 °C.

Distribution and sensorial relevance of volatile organic compounds emitted throughout wastewater biosolids processing

A diverse range of volatile organic compounds (VOCs) are emitted from wastewater biosolids processing. Odorous emissions are predominately made up of volatile sulfur compounds (VSCs) which are typically the only odorants measured. However, a range of VOCs are known to contribute to malodours yet previous studies often overlook the contribution of VOCs in comparison with VSCs. This study aims to evaluate how emissions are affected by different biosolids processing configurations, and if any non-sulfur VOCs should be included in odour measurement and management. Non-sulfur VOCs emitted from biosolids throughout six wastewater treatment plants in the Sydney, Australia region were measured at six locations on average twice each week over 2-3weeks at each site. Variations in types of VOCs emitted throughout and between the sites were assigned to differences in WWTP processing configurations, plant operation and variations in industrial and municipal flows to the sewer network, referred to as sewer catchments. The presence of VOCs is likely due to biotic generation as well as industrial or residential additions to the sewer network. The dewatered and stored biosolids samples had the highest levels of VOC emissions. Sensorially important odorants were p-cresol and butanoic acid, based on the frequency of detection and odour activity values. Other compounds with a high risk of nuisance impacts were trimethylamine, indole and phenol emitted from the dewatered and stored biosolids, and volatile fatty acids from the anaerobic digester inlet and outlet at one particular site. The findings show that non-sulfur VOCs should be added to odorant monitoring campaigns at WWTPs. Identification of VOCs as sensorially important odorants opens opportunities for the more efficient management of nuisance odours, through targeted odour control or process improvement.

Sewer odour abatement monitoring – an Australian survey

Odourous emissions from sewer networks can significantly impact a local population causing odour annoyance. A survey of nine Australian wastewater utilities that serve over 8.4 million people and operate over 59,000 km of sewer networks was undertaken to summarise the current monitoring practices in Australia with the view to assist the water industry to further improve their practices in operating and monitoring sewer odour abatement systems. Results indicated that most odour abatement systems were monitored through complaints from the surrounding community, H(2)S is the dominant online and offline monitoring parameter and that a variety of different H(2)S instruments are used across the industry but the reported use is dominated by two manufacturers. The monitoring data were primarily used for decision making and diagnosis, and there was limited use of non-H(2)S odourant analysis. The water industry had several significant limitations in terms of its inability to provide gas flow data, process monitoring and complaint data as well as being able to link process monitoring data with maintenance information for instrumentation. The improved collection and management of this data would yield benefits to the water industry in terms of odour abatement design, performance and management.

Reduced sulfur compounds in the atmosphere of sewer networks in Australia: geographic (and seasonal) variations.

The management of odorous emissions from sewer networks has become an important issue for sewer system operators resulting in the need to better understand the composition of reduced sulfur compounds (RSCs). Gaseous RSCs including hydrogen sulfide (H2S), methanethiol (MeSH), dimethyl sulfide (DMS), carbon disulfide (CS2), dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) were measured in the atmosphere of selected sewer networks in two major Australian cities (Sydney and Melbourne) during 2011-2012. The RSC concentrations in the sewer air were detected in a highly variable range. H2S and MeSH were found at the highest concentrations, followed by DMS (39.2-94.0 μg/m(3)), CS2 (18.3-19.6 μg/m(3)), DMDS (7.8-49.6 μg/m(3)) and DMTS (10.4-35.3 μg/m(3)). Temporal trends in the occurrence of targeted RSCs were observed and the highest sulfur concentration occurred either in summer or spring, which are typically regarded as the warmer seasons. Statistical significant difference in the magnitude of targeted RSCs was found between samples collected in Sydney and Melbourne.

Impact of Storage Conditions on the Stability of Volatile Sulfur Compounds in Sampling Bags

Odorous emissions from agricultural and waste management operations can cause annoyance to local populations. Volatile sulfur compounds (VSCs) are dominant odorants that are often lost during collection using sample bags. The degree of VSC losses depends on factors such as storage time, bag materials, temperature, sample relative humidity (RH), light exposure, and the presence of volatile organic compounds (VOCs). To assess the impact of those factors on the stability of 10 VSCs (hydrogen sulfide, methanethiol, ethanethiol, dimethyl sulfide, tert-butanethiol, ethyl methyl sulfide, 1-butanethiol, dimethyl disulfide, diethyl disulfide, and dimethyl trisulfide), laboratory-based experiments were conducted according to a factorial experimental design. Linear mixed-effects models were constructed for loss predictions. The estimated recovery of HS in Tedlar bag was 8 to 10% higher than in Mylar and Nalophan between 6 and 30 h. At ≤20°C and without being exposed to light, at least 75% relative recovery of the 10 VSCs in Tedlar bags can be achieved after 18 h, whereas, a maximum of 12 h of storage should not be exceeded to ensure a minimum of 74% relative recovery of the VSCs in Mylar and Nalophan bags.