Odour impact is generally low around Waste Water Treatment Plants (WWTP) when conventional Odour Control Unit (OCU) such as chemical scrubbers or biofilters are implemented. However, for some specific WWTP processes such as sludge thermal drying releasing odorous Volatile Organic Compounds (VOC) in particular aldehydes and ketones, these conventional OCU are not effective enough to avoid odour nuisances in the environment.

   To fix this issue, we have proposed a simple and relevant two stages treatment line to treat odorous VOCs from WWTP. The first step of the treatment line consists of an enhanced water scrubber (absorption) and the second one, an Activated Carbon (AC) filter (adsorption). The enhanced water scrubber, patented by Suez and named Azurair^TM Cool, uses a chiller to cool the inlet scrubber water in order to increase VOCs removal in scrubber and dehumidifie air before the second stage of AC filtration.

Catherine Gracian ^1,*, Valérie Nastasi ¹

¹ Suez International, 183 avenue du 18 Juin 1940, 92500 Rueil Malmaison, France

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

   Academic editor: Carlos N Díaz.

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

   Citation: Catherine Gracian, Valérie Nastasi, Odorous VOC removal by an advanced water scrubber in waste water treatment plant, IWA2021 Conference, Bilbao, Spain, www.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

   Keywords: Deodorization, non condensable, sludge dryer, enhanced absorption

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Abstract

   Odour impact is generally low around Waste Water Treatment Plants (WWTP) when conventional Odour Control Unit (OCU) such as chemical scrubbers or biofilters are implemented. However, for some specific WWTP processes such as sludge thermal drying releasing odorous Volatile Organic Compounds (VOC) in particular aldehydes and ketones, these conventional OCU are not effective enough to avoid odour nuisances in the environment.

   To fix this issue, we have proposed a simple and relevant two stages treatment line to treat odorous VOCs from WWTP. The first step of the treatment line consists of an enhanced water scrubber (absorption) and the second one, an Activated Carbon (AC) filter (adsorption). The enhanced water scrubber, patented by Suez and named Azurair^TM Cool, uses a chiller to cool the inlet scrubber water in order to increase VOCs removal in scrubber and dehumidifie air before the second stage of AC filtration.

   This paper summarizes firstly the results of an Azurair^TM Cool pilot tests at 1/10 scale achieved in 2018 on a digested sludge drying facility and secondly presents the feedbacks of a one year operation for the first industrial Azurair^TM Cool implemented on a biological sludge drying facility of a WWTP in the South of France.

   The one-year operation of the industrial Azurair^TM Cool confirms the results of the pilot tests. In both cases, the treatment line, including an Azurair^TM Cool and an AC filter, removes more than 97% of odours released by the sludge dryers whereas usual chemical scrubbers remove only around 60% of these odours. The use of cooled water for scrubbing induces a rise in VOCs removal efficiency in the water scrubber of more than 30%. Furthermore, after one year of operation of the industrial unit, we confirmed that the Return On Investment (ROI) of the Azurair^TM Cool was les than 5 years.

   From the feedbacks of the WWTP operator, the implementation of the Azurair^TM Cool is a success thanks to a significative improvement of the olfactive impact around the WWTP, low consumption of AC, low production of waste and ease of operation.

1. Introduction

   1.1 Context

   Odour impact is generally well managed around Waste Water Treatment Plant (WWTP) when conventional Odour Control Unit (OCU) such as chemical scrubbers or biofilters, are implemented. However, for some specific WWTP processes releasing odorous Volatile Organic Compounds (VOC) such as aldehydes and ketones, these conventional OCU are not effective enough to avoid odour nuisances in environment (Mohamed et al., 2014).

   Sludge drying process generates very odorous foul air containing odorous VOC. For belt dryers it is common to reach 100,000 ouE/m3, depending on the air flow rate used in the drying process.

   From the state of the art, currently, there is no single process suitable for the treatment of this gaseous flow. Usual WWTP chemical scrubber, with acid and alkaline scrubbers are not efficient to remove VOC (Biard et al., 2017, Dauthuile et al., 2008). Biofiltration and other biotreatment are not adapted to discontinuous operation of sludge drying facilities with several weeks per year of shut down (Weber et al., 1995). Adsorption on activated carbon (AC) leads to huge adsorbent consumption to ensure sufficient odour and VOC removal (Ren et al., 2019). Thermal oxidation requires high investment cost and is sensitive to siloxane contained in sludge dryer foul air. Water scrubbing remove some VOC but not sufficiently to enable odour removal. Advanced Oxidation Process (Andersen et al., 2012, Vega et al., 2014) or oil emulsion scrubbing (Tatin, 2008) are, at the moment, non mature technologies, with high investment costs.

   From these observations, a combination of 2 processes has been suggested to remove odorous VOC from foul air of sludge drying facilities using a water scrubber (absorption) as a first stage and an AC filter as second stage (adsorption). The aim of the water scrubber is to remove the maximum of VOC, odorous or not, before the adsorption filter to decrease adsorbent consumption.

   To improve the first stage (water scrubber), it was suggested to use water at low temperature for scrubbing with two expected beneficial impacts: increase of gas solubility in water for absorption stage (Abdurrakhman et al., 2018) and decrease of air humidity for adsorption stage. Considering that a low temperature also induces adverse impacts for absorption such as a lower compound diffusion in air and water, higher viscosities and higher surface tension, the interest of low temperature water scrubber have been experimentally checked by pilot tests in 2018 (Gracian and Nastasi, 2019).

   1.2 Previous work (Gracian et Nastasi, 2019)

   Pilot test at 1/10 scale had been carried out in a digested sludge drying facility in 2018. Pilot equipment included (Figure 1) 1. a water scrubber fed with water at controlled temperature; 2. a chiller to cool scrubbing water at required temperature; 3. a heater to warm air at 20°C and decrease relative humidity; 4. a fan; 5. two AC filters in parallel 6. Pilot plant instrumentation includes flow rate sensors (FT) and temperature sensors (TT).

Figure 1. Pilot plant process flow diagram

   The performances of the cooled water scrubber to remove Total Non Methane VOC (VOC_T,NM) had been checked by a Design Of Experiment (DOE) method (Durakovic, 2017), using a factorial design at 2 factors: temperature of scrubbing water (T) varying from 10 to 17°C and flow rate of scrubbing water (L) varying from 1.7 to 2.9 m3/h for 500 Nm3/h of non condensable flow. Studied output response is the Removal Efficiency of VOC_T,NM (RE) which can be expressed as   (1)

   The first conclusion of the DOE was that water scrubber is, as expected, an efficient process to remove VOC released by sludge dryers, with measured removal efficiency between 19% and 61%. Variance analysis (ANOVA) performed had shown (Table 1) that the effects of temperature (T) and liquid flow rate (L) on VOC_T,NM removal efficiency (RE), are statistically significant at 90 % level of confidence whereas the effect of interaction between temperature and liquid flow rate isn’t. The negative value of T effect (-0.10), means that the VOC_T,NM removal efficiency drops when temperature increases, and the positive value of L effect (0,11) means that the VOC_T,NM removal efficiency rises when liquid flow rate increases.

Table 1: Factors effects and significance

   The DOE confirmed the initial assumption of an increase in the VOC removal for the water scrubber fed with cooled water treating VOC mixture generated by sludge dryers. It means that higher gas solubility in water has more impact than the adverse effects of lower diffusions, higher viscosities, higher surface tension.

   Removal efficiency of the overall pilot plant (water scrubber and AC filter) remained very high on odour during the test while it dropped from 99% to 97% indicating a progressive saturation of activated carbon in the adsorbent filters. At the end of the experimentation, odour concentration was 1500 ou_E/m³ (and was always below the goal of 6000 ou_E/m³).

   Owing to this pilot test, an industrial product of the Suez Azurair^TM range dedicated to air treatment, has been industrialized and patented: Azurair^TM Cool (patent WO 2019/162383).

   1.3 Objectives

   This paper presents the feedbacks of a one-year operation for the first industrial Azurair^TM Cool unit implemented. This first unit treats the non condensable of a biological sludge drying facility in the South of France.

   The key performance indicators of this odour treatment line are 1. Odour concentration on the ground around the WWTP linked to the release of these treatment line always less than 1 ou_E/m³ in hour average (leading to a threshold emission value of 12 000 ouE/m3), 2. Controlled consumption and management of AC, 3. Return on investment (ROI) lower than 5 years.

2. Materials and methods

   The unit implemented to treat the non condensable of a biological sludge drying facility includes 1. an Azurair^TM Cool (a water scrubber fed with industrial water cooled- 2 seconds residence time); 2. a heater to warm air to decrease its relative humidity; 3. an AC filter containing 5 m³ of media. This treatment line is similar to the pilot line presented on Figure 1. The air flow rate to be treated is 2500 Nm3/h (dry basis) at a temperature of about 50°C and 100% of relative humidity (RH).

   In order to assess the performances of the unit under different operating conditions, 2 measurement campaigns were carried out: one in summer (July 2020) with higher water temperature and a recent charge of AC in the polishing filter and one in winter (February 2021) with lower water temperature and a saturated AC in polishing filter. The measurements were achieved at nominal operating conditions of the drying facility. The Table 2 summarizes the method of the measurement campaigns.

Table 2: Measurement campaigns

* Due to high air humidity and high temperature and lack of dilution system, the measurement of odour concentration at the plant inlet is underestimated.

3. Results

   As presented on Table 3, the odour removal of the overall line is high (>70%) for both measurement campaigns. During the summer campaign, the odour concentration at the carbon filter outlet is low (256 ou_E/m³) due to a recent replacement of activated carbon. During the winter campaign, despite good efficiency (>70%), the odour concentration at the AC filter outlet (19 760 ou_E/m³) is higher than the target of 12 000 ou_E/m³ set to ensure at all time the absence of odour nuisance in the environment (1 ou_E/m³ in one hour average on the ground). This odour leakage is related to the age of activated carbon in the filter (3 months) and indicates that the adsorbent media needs to be replaced.

   The VOC removal in Azurair^TM Cool are 55 % and 68% for summer and winter campaigns respectively. As expected, the efficiency in the summer is lower than in the winter due to higher water temperature.

   In addition to this quantified results, the feedbacks of the WWTP operator is that the implementation of the Azurair^TM Cool is a success thanks to a significative improvement of the olfactive impact around the WWTP, a controlled consumption of activated carbon and production of waste, and also an ease of operation.

4. Discussion

   Using the model elaborated by the DOE method on the pilot unit (equation 1), the VOC removal efficiency of the industrial Azurair^TM Cool is estimated at 21% and 41% for summer and winter campaigns respectively. These calculated removal efficiencies are largely lower than those measured (respectively 56% and 68% for summer and winter campaigns). These differences between the measurements and the model can be due 1. to higher temperature of foul air in the industrial unit (50°C) than in the pilot test (37°C), inducing a higher abatement potential due to the condensation of pollutants with the cooling of the air flow (this argument is supported by the higher VOC concentration at the inlet of the industrial unit than at the inlet of the pilot unit); 2. to different nature of VOC released by biological and digested sludge, suggesting that compounds generated by biological sludge drying are more soluble in water than those generated by digested sludge as observed for sludge composting (Maulini-Duran et al, 2013).

   Achieving a comparison of the industrial unit operating cost with and without cooling of the water for scrubbing, including the consumption of activated carbon (AC adsorption capacity of 10% in weight and 3€/kg AC) and the consumption of electrical energy for the water cooling (0,07 €/kWh), the saving on the industrial unit is 3.2 €/h for the operation of the dryer. Assuming 7000 hours per year of dryer operation, the saving on 5 years of operation exceeds 100k€ which is higher than the total cost of the Azurair^TM Cool. Therefore, the return on investment of the implemented Azurair^TM Cool versus a classical water scrubber is under 5 years.

   Moreover, the controlled consumption of AC with an Azurair^TM Cool leads to waste reduction generated by the treatment line and then to a better environmental footprint of the solution.

5. Conclusions

   The feedback after one year of operation of the first industrial Azurair^TM Cool has led to the following conclusions:

   1. Air treatment line composed by a water scrubber as a first stage and an AC filter as a second stage is a relevant solution to treat odours from sludge dryers with high efficiency. With odour removal higher than 97% this solution is largely more effective than chemical scrubbers currently implemented on sludge dryer facilities achieving around 60% odour removal.

   2. The use of cold water in scrubber is an economical way to enhance VOC removal and then AC consumption.

   3. The solution with an Azurair^TM Cool is appreciated by operator due to ease of operation, simplicity, efficiency to control offensive odour.

   The Azurair^TM Cool is an efficient process associated with AC filter to treat non condensable of sludge dryer facilities. Due to similarities in the VOC composition (Bouchy et al, 2008) between the non condensable of dryer and foul air from sludge treatment like Digelis^TM TH (Thermal Hydrolysis Process) or Dehydris Ultra^TM (Hydro Thermal Carbonization), the Azurair^TM Cool could also be a good solution for those applications.

6. References

   Abdurrakhman, A., Kurniawan, D., Machrus Adhim, M. 2018. The Effect of Temperature Variation on Water Scrubber System to Optimize Biogas Purification. ASTECHNOVA International Energy Conference. Yogyakarta (Indonesia) 2-3th November 2016.

   Andersen, K. B., A., Feilberg, A., Beukes, J. A. 2012. Use of non-thermal plasma and UV-light for removal of odour from sludge treatment. Water Science & Technology, 66(8), 1656-1662.

   Biard, P.F., Couvert, A., Renner, C. 2017. Intensification of volatile organic compound absorption in a compact wet scrubber at co-current flow. Chemosphere, 173, 612 - 621.

   Bouchy, L., Senante, E., Dauthuille, P., Aupetitgendre, M., Harry, J.P., Venot, S., Rougé, P. 2008. Odour creation potential of sludge during composting and drying. 3rd IWA Odour &VOC. Barcelone 8-10th October 2008.

   Dauthuille, P., Senante, E., Guimet, V., Bouchy, L., Budka, A., Venot, S., Aupetitgendre, M., Harry, J.P. 2008. The Human Nose: An Essential Tool In Controlling Olfactory Nuisance. 3rd IWA Odour &VOC. Barcelone 8-10th October 2008.

   Durakovic, B. 2017. Design of Experiments Application, Concepts, Examples: State of the Art. Periodicals of Engineering and Natural Sciences. 5 (3), 421‒439.

   Gracian, C., Nastasi, V. 2019. Advanced water scrubber for odorous VOC treatment in WWTP. 8th IWA Odour &VOC/Air Emissions Conference. Hangzhou 14-17th October 2019.

   Maulini-Duran, C., Artola, A., Font, X., Sánchez, A. 2013. A systematic study of the gaseous emissions from biosolids composting: Raw sludge versus anaerobically digested sludge. Bioresource Technology.147, 43-51.

   Mohamed, F., Kim, J., Huang, R., Nu, H., Lorenzo, V. 2014. Efficient Control of Odors and VOC Emissions via Activated Carbon Technology in Wastewater Treatment Plant. Water Environ. Research. 86(7), 594-605.

   Ren, B., Zhao, Y., Lyczko, N., Nzihou. 2019. Current Status and Outlook of Odor Removal Technologies. Waste and Biomass Valorization. 10, 1443-1458.

   Tatin R. 2008 Absorption Physique de Composés Organiques Volatils par Pulvérisation d'Emulsion d'Huile dans l'Eau - Etude thermodynamique et hydrodynamique - Application au calcul des efficacités d'abattement de COV sur effluents synthétiques et réels (Physical Absorption of Volatile Organic Compounds by Spraying Oil Emulsion Spray in Water - Thermodynamic and hydrodynamic study - Application to the calculation of VOC abatement efficiencies on synthetic and real effluents). PhD thesis, INSA, Université de Toulouse, Toulouse, France.

   Vega, E., Martin, M. J., Gonzalez-Olmos, R. 2014. Integration of advanced oxidation processes at mild conditions in wet scrubbers for odourous sulphur compounds treatment. Chemosphere. 109, 113–119.