The wastewater treatment plant (WWTP) of Torredembarra, located in the province of Tarragona, Spain, has suffered for years from noticeable odor emission problems, mostly during the summer season. During past years, the odor treatment in the WWTP relied on a series of biological and physico-chemical systems, which led to significant operating costs and which had fluctuating efficiency, mostly during summer season.
In order to mitigate these odor emission events, a comprehensive restructuring plan was developed for this whole treatment system. The emission rates of the main odorous compounds in all the critical points of the WWTP were assessed during different times of the year. The most relevant odour emission areas were then confined, and the headspace air treated in a series of new and retrofitted biotrickling filters.
2. Depuración de Aguas del Mediterráneo S.L. Avda. Benjamin Franklin 21. Parque Tecnológico Paterna. 46980 Paterna, Valencia
3. Agència Catalana de l'Aigua. C/ Provença 260. 08008 Barcelona
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: Josep Torà, Elvira César, Lidia Saúco, Maria Pignatelli, Jordi Robusté, Óscar Prado, 2021, Technical and economic optimization of the deodorization of a coastal WWTP through biological processes, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, Olores.org.
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Keyword: Biotrickling filter, hydrogen sulfide, minimization, wastewater management
The wastewater treatment plant (WWTP) of Torredembarra, located in the province of Tarragona, Spain, has suffered for years from noticeable odor emission problems, mostly during the summer season. During past years, the odor treatment in the WWTP relied on a series of biological and physico-chemical systems, which led to significant operating costs and which had fluctuating efficiency, mostly during summer season. In order to mitigate these odor emission events, a comprehensive restructuring plan was developed for this whole treatment system. The emission rates of the main odorous compounds in all the critical points of the WWTP were assessed during different times of the year. The most relevant odour emission areas were then confined, and the headspace air treated in a series of new and retrofitted biotrickling filters. After these operations, consistent H2S removal efficiencies above 98% have been obtained in all the biological systems, even though inlet H2S concentration often exceeded 500 ppmv in the one treating air from the sludge management area. No citizen complaints have been received ever since. Moreover, operating costs are dramatically lower than those of traditional, physico-chemical technologies.
The generation and emission of odorous compounds such as hydrogen sulfide (H2S), ammonia (NH3) and various volatile organic compounds (VOCs) is a common problem in WWTP. Among these, the case of coastal WWTPs in areas with high population seasonality, such as tourist localities, is especially problematic. The deodorization systems of these WWTPs must be capable of working under highly variable operating conditions, because of the increase in the load to be treated during holiday seasons due to higher sewage production and infiltration of marine sulphate.
The WWTP of Torredembarra is located about 300 m away from an important tourist town and around 1 km away from the Mediterranean Sea. For years, the WWTP has suffered from odor issues, leading to complaints from the nearby population, mostly during the summer season. These problems usually manifested in the form of accumulations of several hundred of ppmv of H2S in certain areas of the WWTP, as in the sludge drying building.
Before 2017, the WWTP had a combination of biological and physico-chemical odor treating systems (traditional biofilters, chemical scrubbers and activated carbon filters), which had variable, but commonly sub-optimal, performances. The Catalan Administration proposed to carry out an assessment study of critical points of generation of odor in the WWTP, also aimed at finding the best technology for deodorization under these particular conditions, both from technical and economic viewpoints. The most relevant results of both works are presented here.
2. Materials and methods
To establish the critical points in terms of odor generation in the WWTP, during 2016 and 2017 experts from the Autonomous University of Barcelona (UAB) and the company Aeris Tecnologías Ambientales S.L. developed various analysis campaigns. In these, the emission rates (M·L-2·t-1) of H2S, NH3 and total VOCs were determined in various elements of the WWTP under different meteorological conditions. H2S and NH3 concentrations were measured by means of a Hybrid MX-6 electrochemical sensor (Industrial Scientific, Spain). A PID Mini RAE 3000 (RAE Systems, Spain) was used to quantify the concentration of VOCs.
These analyses allowed prioritizing the critical areas, which turned out to be (Figure 1) two sludge thickeners (a), a sludge storage tank (b), two primary settlers (c), a series of distribution tanks prior to the secondary reactors (d) and various elements in the pretreatment (e) and sludge dewatering (f) buildings. Amongst these areas, only the sludge thickener and the primary settlers were connected to some form of deodorization systems (chemical scrubbers, in both cases). However, the efficiency of these systems was, in most cases, insufficient to prevent odor emission events.
Figure. 1. Critical elements in terms of odor emission in the WWTP.
For the design of the pipes and the treatment system, the aim was to maximize the deodorization efficiency at a minimum cost, by treating exclusively the air extracted from localized odor generation points. The ambient air of the buildings, containing minimal concentrations of odorous compounds, was then vented inside the WWTP. The extraction flow rate at each area was calculated taking into account the compositional characteristics, volume, flow rate, pH, temperature and turbulence of the water present within the area to be deodorized, as well as hazard concerns and the possibility of corrosion of plant elements. Each of the critical areas was categorized in terms of odor emission and grouped for joint treatment. Namely, three relevant areas were stablished: Area 1, the most critical one,comprising the two sludge thickeners and various elements in the sludge dewatering building; Area 2, comprising the sludge storage tank and various elements in the pretreatment building and Area 3, comprising the two primary settlers and several distribution tanks prior to the secondary reactors. Specific solutions for each of these areas were developed and commissioned between 2017 (Area 1) and 2019 (Areas 2 and 3).
3. Results and discussion
3.1. Area 1 (dewatering building + thickener) deodorization
The total air flow rate was set at 3.500 m3/h, enough to efficiently extract the polluted air from the different critical points. About 85% of the air comes from the sludge dewatering room, while the remaining 15% comes from the thickener. No evident corrosion has been found in these elements so far, proving that the extraction flow rate is adequate. A 2,4 m (diameter) x 7,1 m (height) biotrickling filter (Figure 2, left) was designed and installed herein in August 2017. The packing material selected was a mixture (1:1, v:v) of 2” plastic Pall rings and PU foam cubes. Previous experiences have proved that this mixture of materials can host a sufficient amount of biomass while keeping pressure drop relatively low. Also, their mechanical and chemical resistance under strongly acidic conditions is very high. This same mixture was also used in all the bioreactors described below.
Just a few days after the start up, consistent H2S removal efficiencies above 98% were reached, even under inlet H2S concentrations up to 500 ppmv (Figure 2, right). Only a faint VOCs smell remains at the outlet of the bioreactor, imperceptible a few meters away from the emission point. After around 4 years of operation, pressure drop remains below 3 mbar.
Figure 2. Biotrickling filter picture (left) and performance (right, data from September 11 th -30 th 2020). Concentration values above 500 ppmv (maximum detection imit of the sensor) have been assumed to be 500 ppmv.
It must be taken into account that the biological oxidation of sulfide to sulfate leads to a substantial pH drop in the recirculating liquid. In order to reduce the consumption of process water, the biotrickling filter was operated at pH values around 2. This led to rapid damage to the drain submerged extraction pumps, pipes and concrete distribution box, which had to be readily substituted with similar devices made with plastic materials. Overall, the main operating cost is electricity, as no potable water or any sort of chemicals are required. Main pump electrical consumption adds up to around 1,1 kWh/h, totaling around 750 €/year. It has been estimated that replacing the entire packing material, which will be needed eventually, upon compaction, would cost around 6.000 €. It is likely that this action will not be required at least for the first five years of operation. It has been estimated that the cost of the NaOH required to treat this stream through chemical scrubbing would be in the range of 8.000-12.000 €/year, while the electricity costs would be around 3.000 €/year. Overall, the selected option is, at least, five times cheaper than traditional chemical scrubbing.
3.2. Area 2 (sludge storage tank + pretreatment) deodorization
A second, 2,4 m (diameter) x 7,6 m (height), biotrickling filter (Figure 3) was specifically designed and installed for this area in 2019. In this case, the overall air flow rate was 4.000 m3/h, extracted from the water inlet chamber (35% of the total flow rate), two grit chambers (25% each), a Parshall flume (5%) and a sludge storage tank (10%). All the emission points in the pretreatment building, initially exposed, were confined. Also, the discharge area of the sludge storage tank was confined and a specific extraction, which is automatically activated by opening the silo, was installed.
Figure 3. Biotrickling filter picture (left) and performance (right, data from June 1st to August 31st 2020).
As Figure 3 shows, the performance of this system was even more stable, even though the inlet H2S concentration was highly variable. The inlet H2S concentration fed during the summer period of 2020 was 78,3 ± 39,4 ppmv, corresponding to an H2S load of 23,9±12,0 g/m3h. The outlet H2S concentration was always below the detection limit of the sensor (0,1 ppmv). The operating cost of this biotrickling filter is comparable to that of the system described in the previous section. Also, the total pressure drop of the system is commonly between 2,5 and 3,5 mbar.
3.3. Area 3 (primary settlers + distribution tanks) deodorization
The primary settlers, originally entirely confined, already had an air treatment system comprising four chemical scrubbers and two conventional biofilters. Each settler had a 45.000 m 3 /h fan for air extraction. The approach followed here consisted in retrofitting two of the scrubbers, which were connected in series, into biotrickling filters (Figure 4, left), which required decreasing the air flow rate around 6-fold. To do that, the outlet ring of each of the settlers was confined (Figure 4, right). Around 40 % of the total air flow rate is extracted from each of the settlers, while the remaining 20 % comes from the distribution tanks. The current system allows for the deodorization of both settlers, though only one at a time. Retrofittings were performed mostly following the strategy reported by Gabriel and Deshusses (2003). The main actions undertaken involved the modification of the control processes, the substitution of the packing material and of the recirculation pumps and the removal of the chemicals dosing devices. EBRT was set at 4,4 s for each of the retrofitted reactors.
Again, the performance of this two-stage system was considered optimal after the retrofitting. An H2S concentration of 63,2±48,3 ppmv was fed between July and September 2020, which corresponds to an H2S load of 35,6±27,1 g/m3h. Again, the outlet H2S concentration was always below 0,1 ppmv. As in the previously mentioned systems, very low pressure drop values (<4,5 mbar in the two-stage system) have been noticed.
Figure 4. State of the reactors after being retrofitted (left) and close-up of the closure of the settlers (right).
To date, no biomass growth has been seen in any of the biotrickling filters that justifies the need to clean or to replace the biomass support material. Previous experiences show that the operating conditions favor that the new biomass generated in the bioreactors is compensated with that which is purged. Thus, it is not expected that these actions will be necessary at least during the first five years of operation.
Deodorization systems must not only be effective in removing odor-causing compounds, but also efficient from the point of view of resource consumption. Traditional, physico-chemical, odor-removal technologies usually involve considerable operating costs, and in many cases lead to the production of contaminated media. As an alternative, biological technologies have demonstrated high deodorization efficiencies without consuming drinking water or reagents. The systems described herein can cope with sudden variations in odor loads without suffering loss of efficiency. On the other hand, the economic cost of their operation is minimal.
Gabriel, D., Deshusses, M.A. 2003. Retrofitting existing chemical scrubbers to biotrickling filters for H2S emission control. Proc. Natl. Acad. Sci. USA 100, 6308–6312.