During the last years, there is a need to plan strategical investments to reduce Wastewater Treatment Plant (WWTP) odour impact, which it is generally carried out by covering and deodourizing the main process units involved.

   Odour dispersion modelling software combined to dynamic olfactometry is the standard technique conducted to evaluate the odour impact studies. However, the results obtained by this methodology strongly depend on the emission sources set in the software. Thus, this study is focused on providing a dynamic odour impact assessment using CALPUFF software simulations, calculated under different “what-if” scenarios, providing a new approach to define the calibration of the emission sources.

   Competing interests: The author has declared that no competing interests exist.

   Academic editor:  Carloz N. Díaz

   Content quality: This paper has been peer-reviewed by at least two reviewers. See scientific committee here

   Citation:  J. Vilarroig, D. Miguel, M. García, A. Macias, I. Beltrán, J. Climent, 2021, Innovative odour impact assesment for WWTP using experimental and dispersion modelling techniques, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, Olores.org.

   Copyright:  2021 Olores.org. Open Content  Creative Commons license. It is allowed to download, reuse, reprint, modify, distribute, and / or copy articles in olores.org website, as long as the original authors and source are cited. No permission is required from the authors or the publishers.

   ISBN: 978-84-09-37032-0

   Keyword: Methodology; Primary Settling; CALPUFF; Odour Reduction; Computational Simulation

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Abstract

   During the last years, there is a need to plan strategical investments to reduce Wastewater Treatment Plant (WWTP) odour impact, which it is generally carried out by covering and deodourizing the main process units involved. Odour dispersion modelling software combined to dynamic olfactometry is the standard technique conducted to evaluate the odour impact studies. However, the results obtained by this methodology strongly depend on the emission sources set in the software. Thus, this study is focused on providing a dynamic odour impact assessment using CALPUFF software simulations, calculated under different “what-if” scenarios, providing a new approach to define the calibration of the emission sources. One of the main process units of WWTP studied in this work was the Primary Settling stage. As a difference of previous works reported in the literature, an exhaustive olfactometric campaign was carried out measuring separately: (i) the passive areas with low emission, due to its low turbulence, and (ii) the active area, according to the perimeter overflow with high desorption. Once the sources were measured separately, they were set in the software individual odour sources. The main outcome of this approach has been a more realistic diagnosis by means of a new source calibration approach. It has provided useful information for the decision-making process to reach a solution that reduces the odour impact in the most cost-effective way.

1. Introduction

   Odour emission is already considered as a specific form of air pollution (Gostelow, Parsons, & Stutetz, 2001). It starts in the sewage and spreads throughout the different stages of the WWTP (Sówka et al., 2017). The cause of this pollution lies in the release into the atmosphere of certain compounds that, even in very low concentrations, could cause olfactory nuisance in certain points far away from the emission area.

   There are currently different methodologies for the sampling and quantification of odorous emissions (Capelli et al., 2013; Diallo et al., 2018) . However, the most widespread and standardised technique is still being the dynamic olfactometry. Thus, a numerical value in OU (odour units) is obtained to characterise an odorous source, which is carried out under the UNE-EN 13725 standard. This technique is used to quantify the odour emission of the different odorous sources, differentiating whether it is an active or passive emission source (Frechen, 2004). Subsequently, these values are set into Gaussian-Lagrangian dispersion models that are able to calculate the dispersion of a large amount of pollutants and particles transported in the atmosphere both in stationary and dynamic simulations. In these models, meteorology’s evolution is the key to determine the pollutant’s dispersion, considering changes in wind speed and direction, humidity, rain and the kind of atmosphere over time (Sówka et al., 2017). The correct characterisation of the emission sources is also critical, especially those with high odour concentrations such as primary settling, grit chambers or sludge treatment buildings. The main advantage of these models lies in their simplicity and their ability to simulate large areas at a very low computational cost. Accordingly, the level of affection by means of odour values can be obtained in the study area around the emitting source.

   This article develops a new methodology for the study and correction of odour sources in WWTPs based on experimental characterisation and computational simulation using atmospheric dispersion models. Firstly, a detailed characterisation of the WWTP emission sources has been carried out using olfactometry techniques in order to identify the critical and most significant sources. It is known that there can be significant differences in flow measurement depending on the method used to characterise the emission sources, with flow chambers, Lindvall fans and wind tunnels (Nicolas, et al., 2013). In this case, a dynamic olfactometry campaign has been designed according to the UNE-EN-13725 standard, analysing different process units and paying special attention to primary settlers. Due to the great importance of this process, it has been divided into two independent focus: the top passive surface and the perimeter channel. The difference in emissions between these two sources has been quantified since the H2S volatilization varies for different flow conditions (Santos et al. 2012). Subsequently these values have been introduced with the rest of the emission values of the other WWTP processes. Thus, the difference between both emission sources enables the application of separated correction measures. Thus, it has been possible to develop new strategies for on-site correction of possible odour emissions at the WWTP, acting more effectively on the most significant sources.

2. Materials and methods

   The study was carried out at the Castelló de la Plana WWTP (Fig. 1), which has a current treatment capacity of 45,000 m3 / day receive the wastewater from the municipalities of Castelló and Benicasim. Currently, it covers the treatment needs for a total population of 170,000 inhabitants. This WWTP has 2 different water lines with 2 primary settling tanks, 2 secondary settling tanks and 1 biological reactor for each line. The water passes an ultraviolet treatment while sludge is digested and obtained biogas is recovered through cogeneration. In addition to the WWTP process units, an homogenisation tank located at the entrance to the WWTP and a wastewater pumping plant (WWPP) located 1.5 km from the plant, have been also considered.

   On the one hand, 22 different points have been sampled and their odour concentration has been estimated by means of dynamic olfactometry according to the UNE-EN-13725 standard. On the other hand, 18 immission points were considered to analyse the odour intensity reached according to the simulated scenarios (Fig. 2). During the sampling campaign, a new technique has been used to measure the primary settlers perimetral channel in which the Lindvall chamber was used (Fig. 3).

Fig. 1.: Castellón de la Plana WWTP	Fig. 2.: Immision points considered in the study

   The simulations were carried out using CALPUFF software. Firstly, the olfactometric campaign provided the emission coefficients for each process unit and they were set-up in the model. The meteorological information has been introduced through files obtained with the WRF (Weather Research Forecast) meteorological model for a full year with an hourly time period. A 50x50 km grid with a resolution of 3 km and 11 vertical levels was used in the model.

Table 1: Simulation Scenarios considered in the study.
Scenario	Process improvement
#1	PCh1
#2	PCh1 + PCh2
#3	PCh 1+ PCh2 + D + SS + DCh
#4	PCh1 + PCh2 + HR
#5	PCh1 + PCh2 + HR + D + SS
#6	PCh1 + PCh2 + HR + D + SS + DCh
PCh#: Perimeter Channel line # D: Degreaser SS: Sludge Silo DCh: Distribution Chamber HR: Homogenization Reservoir		Fig 3.: Lindvall chamber over the perimetral channel

   To analyse the improvement achieved in each scenario, the OUe/m3 are obtained for the different hourly percentiles obtained throughout the year at each of the immission points. The percentiles analysed are 50, 75, 80, 85, 90 and 98. For each percentile of each immission point, the percentage difference is obtained with respect to the same value corresponding to the scenario without improvements. This way, the average percentage reduction in each of the percentiles can be obtained.

   Both the odour concentrations and emissions of each of the processes were obtained after the olfactometric campaigns. A computational model was set up in CALPUFF to simulate the current situation of the plant, as well as the improvement scenarios proposed in Table 1. To design the scenarios, the sources with the highest odour flux value have been chosen and its emissivity has been reduced to values near zero. Using this methodology, for each scenario simulated in CALPUFF, it has been possible to determine the total odour emission and its corresponding reduction percentage depending on the improvement actions. As can be seen in Table 1, 6 different scenarios has been proposed. Corrective actions have been taken into different processes to gradually reduce de odour affection.

3. Results and discussion

   The total emission of the WWTP has been estimated at 59667 OUe/s. In Fig. 4. the highest emission values of the 22 sampled points are shown. The homogenization tank presents the highest value with 20434 OUe/s but it is important to highlight the great relevance of the emission in the perimeter channels of the primary settling decanters. In the case of line 1, the emission from the perimeter channel is 4600 OU_E/s while on the passive surface of the same decanter, the registered value is 8035 OU_E/s. On the other hand, in the perimeter channel of line 2 the registered value is 7313 OU_E/s while this value is 3998 OU_E/s for the passive surface of the same settling. Since the emission value through the perimeter channels can represent between 36% (line 1) and 65% (line 2) of the total odour emission value, it is confirmed the great relevance of making separate measures in the primary settling process.

Fig. 4.:Odour emission of the WWTP processes.	Fig. 5.:Odour values for C981hour with no improvements.

   In Fig. 5, different odour concentration values obtained in the current situation (without improvements) of the WWTP can be observed. There is a central zone (red) with values higher than 20 OU_E/m³, an intermediate zone (yellow) with values between 10 and 20 OU_E/m³ and an external zone and further away from the WWTP (green) with values between 5 and 10 OU_E/m³.

Fig. 6.: Odour emission of the WWTP processes.	Fig. 7.: Odour values for C981hour for scenario #6

   In the graph of Fig. 6, the odour reduction percentages achieved in each of the scenarios can be seen. In the case of scenario 2, where only the coverage of the perimeter channels is considered, the odour emission is reduced at 22.3%. It can be seen how the greatest reduction gap occurs in scenario 3, in which the coverage of the homogenization pool is proposed. In this case, the reduction increases to 56.9% and reaches its maximum value in scenario 6 with a value of 59.6%. When comparing the odour affected areas in the existing case without improvements (Fig. 5) and in the case of scenario 6 (Fig. 7), it can be observed a great reduction of the red zone which is practically restricted to the limits of the WWTP.

4. Conclusions

   The most relevant conclusions as well as the most important results, are clearly exposed below.

   In this study, a new odour measurement methodology has been presented using a “what if” scenario diagnosis. This methodology allows to obtain odour reductions achieved based on the improvements adopted.

   The emission sources have been rigorously measured. A new technique has been used to measure the perimeter channels of the primary decanters. This way, it has been possible to divide the emission of these processes into two differentiated sources.

   The results show that the primary settling perimeter channels odour emission, varies among 36% and 65% of the total process emission. This highlights the need for separate measurements on the perimeter channels and the main passive surfaces.

   For this particular project, the coverage of the primary settling perimeter channels represents a reduction of 22.3% in the odour emission produced in the entire WWTP.

   The source that produces the greatest impact is the homogenization reservoir. Although the odour concentration is low, its area is large and therefore its emission is substantially increased. Action on this focus raises the reduction achieved to values close to 60%.

5. References

Capelli, L., Sironi, S. and del Rosso, R. 2013. Odor sampling: Techniques and strategies for the estimation of odor emission rates from different source types. Sensors. (Switzerland), 13(1), pp. 938–955.

Diallo, M., Maalem, F. and Piet-Sarnet, H. 2018. Implementation of a new protocol of odour field investigations for the Paris wastewater treatment plants. Chemical Engineering Transactions, 68, pp. 145–150.

Frechen, F. B. 2004. Odour emission inventory of German wastewater treatment plants - Odour flow rates and odour emission capacity. Water Science and Technology, 50(4), pp. 139–146.

Gostelow, P., Parsons, S. A. and Stutetz, R. M. 2001. Odour measurements for sewage treatment works. Water Research, 35(3), pp. 579–597.

Sówka, I. et al. 2017. Seasonal odor impact range of selected wastewater treatment plants – Modeling studies using Polish reference model. Water Science and Technology, 2017(2), pp. 422–429.

Nicolas, J., Guillot, J.M., Sironi, S. 2013. Comparison of hood techniques to sample odour from passive area sources. Paper prepared for the CEN TC264 WG2, document N302.

Santos, J. M., Kreim, V., Guillot, J. M., Reis Jr, N. C., Melo de Sá, L., Horan, N.J. An experimental determination of the H2S overall mass transfer coefficient from quiescent surfaces at wastewater treatment plants. Atmospheric Environment. 60, pp 18-24