CET-vol95 DOI: 10.3303/CET2295043 Paper Received: 13 April 2022; Revised: 26 May 2022; Accepted: 23 June 2022 Please cite this article as: Toledo M., Munoz R., 2022, Odour Control Strategies in Wastewater Treatment Plants: Activated Sludge Recycling and Oxidized Nitrogen Recycling, Chemical Engineering Transactions, 95, 253-258 DOI:10.3303/CET2295043 CHEMICAL ENGINEERING TRANSACTIONS VOL. 95, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Selena Sironi, Laura Capelli Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-94-5; ISSN 2283-9216 Odour Control Strategies in Wastewater Treatment Plants: Activated Sludge Recycling and Oxidized Nitrogen Recycling Manuel Toledo, Raúl Muñoz* Institute of Sustainable Processes, University of Valladolid, Dr. Mergelina, s/n, 47011, Valladolid, Spain mutora@iq.uva.es One of the main concerns associated with wastewater management is the emission of unpleasant odours. The odorous impact derived from wastewater treatment plants (WWTPs) is a source of public environmental complaints in residential areas near these facilities, since odour pollution can cause significant negative effects on the quality of life and the environment. This odorous contamination typically derives from the presence of volatile sulphur compounds (VSCs) and volatile organic compounds (VOCs), some of which have very low odour threshold values, as is the case of H2S. For this reason, odour prevention strategies in wastewater treatment facilities need further research to find effective and low-cost technologies for the control of malodorous emissions. The purpose of this study was the reduction of the emission of malodorous compounds in WWTPs based on the optimization of the use of by-products derived from wastewater treatment such as the oxidized nitrogen (N-NOx) from residual streams rich in nitrate (N-NO3−) or nitrite (N-NO2−) and activated sludge (AS) from the mixed liquor of the nitrification tank or secondary settler. In the experimental tests, gas-tight 2.1 L glass bottles with synthetic septic wastewater were used to evaluate the potential of N-NOx and AS at different concentrations to biodegrade H2S, acetic acid and α-pinene as model odorants. Among the most remarkable results, odorant adsorption losses were observed during preliminary abiotic tests (4 h) with concentration losses of 25 % for H2S and α-pinene, and 7 % for acetic acid. The experiments carried out at different concentrations of AS (0, 10, 25, 50, 100 mg VSS/L) and oxidized nitrogen concentrations (1.5, 5, 7.5 and 10 mg N-NOx/L) showed an efficient H2S removal at 7.5–10 mg N-NOx/L and 50–100 mg VSS/L. However, NO3− supported a more effective H2S abatement than NO2−. The concentration of acetic acid showed a slight decrease due to its degradation by microorganisms (from 27 to 23 ppmv in 4.5 h), concomitantly with the complete biological oxidation of H2S. Conversely, α-pinene concentrations experienced a similar gradual decrease than in the abiotic tests, with a low influence of NO3−, NO2− and AS concentrations. Finally, a marked reduction of NO2− was observed when increasing AS concentration, suggesting that higher concentrations of NO2− compared to NO3− are required for complete biological oxidation of odorants during wastewater treatment. 1. Introduction One of the main concerns associated with Wastewater Treatment Plants (WWTPs) is the emissions of malodours (Lewkowska et al., 2016). Volatile sulphur compounds (VSCs) and volatile organic compounds (VOCs) rank among the most typical compounds that contribute to the wastewater management odour footprint (Hu and Liu, 2018; Lebrero et al., 2011). In particular, hydrogen sulphide (H2S) and volatile fatty acids (VFAs) emissions are the main responsible of odour nuisance in WWTPs (Beghi et al., 2012; Dinçer et al., 2020). Physical-chemical treatment have been previously tested in literature to mitigate the emission of odorants from wastewater treatment. However, these conventional solutions present important disadvantages such as the use of chemicals and energy consumption. For this reason, biological odour prevention methods have emerged as an economically and environmentally sustainable alternative for the minimization of odour pollution in WWTPs. In this context, the recirculation to the headworks of nitrates derived from centrate oxidation and activated sludge (AS) from the secondary settler could foster the adsorption and further oxidation of VSCs and VOCs in the raw wastewater. More specifically, activated sludge recycling (ASR) is based on the recirculation of waste activated sludge from the secondary settler or mixed liquor of the nitrification tank to the WWTP headworks. 253 These AS streams can adsorb and biologically oxidize most biogenic dissolved odorous compounds (e.g., sulphide, volatile fatty acids) in the influent (Kiesewetter et al., 2014). Similarly, oxidized ammonium recycling (OAR) consists of the recycling of residual streams rich in nitrate (N-NO3-) or nitrite (N-NO2-) to the WWTP headworks. The addition of nitrate or nitrite as electron acceptors to the influent wastewater promotes an in-situ anoxic odorant oxidation (Estrada et al., 2015). The present work evaluates the influence of the concentrations of fresh activated sludge, nitrate and nitrite on the removal of H2S, acetic acid and α-pinene, here selected as model odorants. 2. Materials and methods 2.1 Synthetic wastewater In order to mimic the physico-chemical characteristics and composition of a model urban wastewater, a modified synthetic wastewater (SW) was prepared according to Bajaj et al. (2008) as follows (mg/L in tap water): 250 of glucose, 110 of meat extract, 160 of casein peptone, 30 of NH2COH2, 7 of NaCl, 4 of CaCl2·2H2O, 2 of MgSO4·7H2O, 112 of K2HPO4·3H2O, 0.5 of CuCl2·2H2O, and 1100 of NaHCO3. 2.2 Chemical odorants In this study, hydrogen sulphide (H2S), acetic acid (C2H4O2) and α-pinene (C10H16) were used as model odorous compounds. Table 1 shows the chemical formula, odour perception, Henry solubility (Hcc), molecular weight and structure of each volatile compound. Table 1: Main characteristics of the model odorous compounds evaluated. Compound Chemical formula Odour perception Odour threshold value (ppm, v/v) (Nagata, 2003) Hcc Molecular weight (g/mole) Molecular structure Hydrogen sulphide H2S Rotten egg 0.00041 0.00091 (Rinker and Sandall, 2000) 34.10 Acetic Acid C2H4O2 Vinegar 0.0060 14 (Hartungen et al., 2004) 60.05 α-pinene C10H16 Pine, tupentine 0.018 0.00029 (Leng et al., 2013) 136.23 2.3 Experimental set-up Aliquots of 300 mL of SW supplemented with different concentrations of nitrate or nitrite (Table 2) by stock solutions of NaNO3 or NaNO2 (2 g/L) were added into glass bottles of 2.1 L, which were then closed with butyl septa and aluminium caps. The headspace was subsequently flushed with helium in order to provide anaerobic conditions. In order to mimic a septic wastewater, the target odorous compounds were injected into the bottle headspace through the butyl septum with a total mass of 2.10, 0.42 and 0.07 mg of acetic acid, α-pinene and H2S, respectively (corresponding to 2 µL of liquid acetic acid, 0.5 µL of liquid α-pinene and 250 µL of gaseous H2S at 22 %). After injection, each bottle was pressurized with helium at 500 mbar in order to fulfil the technical requirements of the mass spectrometer used to quantify the odorants in the headspace. Subsequently, each bottle was vigorously shaken for 2 min to facilitate gas-liquid equilibrium and the initial concentrations of each target compound was analysed in the headspace by mass spectrometry (MS). Finally, different concentrations of fresh activated sludge were injected into the aqueous phase (Table 2) and the monitoring of odorant concentration was carried out at 30, 60, 180 and 270 min. The glass bottles were gently incubated in a horizontal rotary incubator at 7 rpm and at ambient temperature (21 ºC). It is important to highlight that control experiments (i.e., without the supplementation of oxidized nitrogen and activated sludge) were also performed following the procedure previously described.It is important to highlight that ammonia concentration in the centrates from anaerobic digestion is ranged between 900 - 1500 mg/L, which could be transformed into N-NO3 or N-NO2, considering the centrate flowrate (QC) and the influent raw wastewater flowrate (QO), with maximum N-NO3 or N-NO2 concentrations of 6 - 10 mg N/L. Likewise, the volatile suspended solids (VSS) of waste activated sludge from secondary settlers is ranged between 4000 - 12000 mg VSS/L, from which a maximum AS concentration of 100 mg VSS/L could be recirculated if the waste activated sludge flowrate (QW) and the influent raw wastewater flowrate (QO) are considered (Toledo and Muñoz, 2022). 254 Table 2: Experimental design of the OAR and ASR assays Bottle identification AS concentration (mg VSS/L) N-NO3 and N-NO2 concentration (mg/L) Number of experiments B1 0 1.5 8 5 7.5 10 B2 10 1.5 8 5 7.5 10 B3 25 1.5 8 5 7.5 10 B4 50 1.5 8 5 7.5 10 B5 100 1.5 8 5 7.5 10 3. Results and discussion 3.1 Evaluation of odorant fate under abiotic conditions The initial headspace concentrations of acetic acid, α-pinene and H2S under abiotic conditions accounted for 26, 16 and 11 ppmv, respectively (Figure 1). The concentration of acetic acid, α-pinene and H2S showed a slight decrease after 270 min of experiment, which represented a ≈ 25 % loss for α-pinene and H2S and > 7 % for acetic acid. Taking into account that the gas-liquid equilibrium was reached at time 0, abiotic losses were associated to odorant adsorption onto the glass surface or butyl septum. Adsorption phenomena of odorants in glass is related to an elicit ionic adsorption of molecules to the silanol groups (positive ion exchange mode) and a hydrophobic adsorption mediated by the siloxane groups (Costa et al., 2020). Figure 1: Time course of acetic acid, α-pinene and H2S headspace concentrations under abiotic conditions 3.2 Influence of N-NOx and activated sludge concentration on odorant fate In this study, the biological oxidation of H2S under anoxic conditions was confirmed (Figure 2). The headspace concentration of H2S progressively decreased as the concentration of NO3-, NO2- and AS increased. More specifically, a sharp decrease in H2S headspace concentration was recorded for the first 60 min of assay, followed by a gradual decrease in H2S fate along with the occurrence of lower gas-liquid concentrations gradients. The fastest depletion of H2S was observed at 10 mg NOx/L and 100 mg VSS/L. In this context, the supplementation of septic wastewater with 100 mg VSS/L reduced H2S concentration to 0 ppmv in 4.5 h even at the lowest NO3- and NO2- concentrations tested. The addition of NO3- or NO2- to the influent wastewater promotes anoxic conditions, where oxidized nitrogen is used as an electron acceptor by microorganisms (e.g., Time (min) 0 50 100 150 200 250 300 C o n c e n tr a ti o n ( p p m v ) 0 5 10 15 20 25 30 Acetic Acid Pinene H2S 255 chemolithotrophic bacterial species or Sulphur Oxidizing Bacteria “SOB”) which are capable of oxidizing dissolved sulphide and other biodegradable odorants, thus preventing their further release as malodorous emissions (Kiesewetter et al., 2014). Overall, NO3- (Figure 2a) was more effective than NO2- (Figure 2b) to biodegrade H2S regardless of the AS and N-NOx concentrations. The anoxic oxidation of H2S by OAR and ASR can be described by Eq (1) and Eq (2) (Yang et al., 2005): 3𝐻2𝑆+2𝑁𝑂2 − +2𝐻+ 𝑆𝑂𝐵 → 3𝑆0 +𝑁2 +4𝐻2𝑂 (1) 5𝐻2𝑆+2𝑁𝑂3 − +2𝐻+ 𝑆𝑂𝐵 → 5𝑆0 +𝑁2 +6𝐻2𝑂 (2) Figure 2: Time course of H2S headspace concentration under different concentrations of N-NO3- (a), N-NO2- (b) and activated sludge (100 mg VSS/L) However, the addition of NO3- under sulphur limiting conditions could lead to a potential reduction of NO3- to NO2- during biological H2S oxidation according to Eq (3). This fact could cause toxicity problems in the environment or river ecosystems, in particular at low pH values (Alleman, 1985). 𝐻2𝑆+𝑁𝑂3 − +2𝐻+ 𝑆𝑂𝐵 → 𝑆0 +𝑁𝑂2 − +𝐻2𝑂 (3) In the case of acetic acid (Figure 3a), the experiments conducted at 0, 10 and 25 mg VSS/L showed a slight increase in acetic acid concentration regardless to the concentration of NO3-, NO2- and AS supplemented, reaching a maximum value of 30 - 31 ppmv. In the assays carried out at 50 and 100 mg VSS/L, acetic acid concentration remained almost constant for the first 180 min and a gradual decrease in concentration was observed from 27 ± 1 to 23 ± 1 ppmv due to the anoxic biodegradation of this odorant. In fact, most microorganisms present in AS can use the electron acceptor capacity of NO3- or NO2- to metabolize readily biodegradable compounds such as VFAs, which are key substrates in the biological removal of phosphate and nitrogen in AS processes (Janssen et al., 2002). On the other hand, the initial concentration of α-pinene of 13 ± 2 ppmv gradually decreased to a final concentration of 8 ± 1.5 ppmv, which accounted for a removal of 20 - 25 % of this odorant compound. Based on the low aqueous solubility of α-pinene (Table 1) and the low influence of NO3-, NO2- and AS concentrations on α-pinene fate (Figure 3b), the observed decrease in concentration could be attributed to adsorption phenomena on solid surfaces (Hale et al., 2015). Figure 3: Initial and final concentrations of acetic acid (a) and α-pinene (b) at the different activated sludge and N-NOx (NO3- or NO2-) concentrations Time (min) 0 50 100 150 200 250 300 H 2 S ( % ) 0 20 40 60 80 100 B5 (1.5 mg NO2 -/L) B5 (5 mg NO2 -/L) B5 (7.5 mg NO2 -/L) B5 (10 mg NO2 -/L) b B 1 ( In it ia l) B 1 ( F in a l) B 2 ( In it ia l) B 2 ( F in a l) B 3 ( In it ia l) B 3 ( F in a l) B 4 ( In it ia l) B 4 ( F in a l) B 5 ( In it ia l) B 5 ( F in a l) A c e ti c A c id ( p p m v ) 0 5 10 15 20 25 30 35 40 45 1.5 mg NOx/L 5 mg NOx/L 7.5 mg NOx/L 10 mg NOx/L 0 mgVSS/L 10 mgVSS/L 25 mgVSS/L 50 mgVSS/L 100 mgVSS/L B 1 ( In it ia l) B 1 ( F in a l) B 2 ( In it ia l) B 2 ( F in a l) B 3 ( In it ia l) B 3 ( F in a l) B 4 ( In it ia l) B 4 ( F in a l) B 5 ( In it ia l) B 5 ( F in a l) P in e n e ( p p m v ) 0 5 10 15 20 25 1.5 mg NOx/L 5 mg NOx/L 7.5 mg NOx/L 10 mg NOx/L 0 mgVSS/L 10 mgVSS/L 25 mgVSS/L 50 mgVSS/L 100 mgVSS/L a b Time (min) 0 50 100 150 200 250 300 H 2 S ( % ) 0 20 40 60 80 100 B5 (1.5 mg NO3 -/L) B5 (5 mg NO3 -/L) B5 (7.5 mg NO3 -/L) B5 (10 mg NO3 -/L) a 256 3.3 Evaluation of N-NOx fate during odorant oxidation Test carried out at 1.5 and 5 mg NO3-/L supported a complete NO3- depletion at AS concentrations higher than 25 mg VSS/L (Figure 4a), which was likely responsible for the incomplete biological oxidation of H2S. However, the reduction of NO2- was considerably more pronounced than that of NO3- regardless of the concentration of AS (Figure 4b). This fact explains the lower effectiveness of NO2- during odorant oxidation (see section 3.1) and suggests that higher concentrations of NO2- are required to carry out the complete biological oxidation of H2S. Figure 3: Final concentration of N-NO3- (a) and N-NO2- (b) as a function of the initial N-NOx (N-NO3- or N-NO2-) supplemented at different concentration of activated sludge In addition, the reduction of NO3- to NO2- during OAR and ASR simulations was also evaluated (Table 2). The tests carried out at concentrations ≥ 5 mg NO3-/L and 25 mg VSS/L experienced a partial reduction of NO3- to NO2-. Indeed, final values of 2.40, 2.86 and 4.44 mg NO2-/L were recorded in the tests conducted with initial concentrations of nitrate of 5, 7.5 and 10 mg NO3-/L, respectively, when 100 mg VSS/L were supplemented. Table 2: Reduction of NO3- to NO2- during OAR and ASR simulations AS concentration (mg VSS/L) Initial NO3- concentration (mg N/L) Final NO3- concentration (mg N/L) Final NO2- concentration (mg N/L) 0 1.5 1.55 - 10 - - 25 - - 50 - - 100 - - 0 5 5.56 - 10 4.97 - 25 4.36 - 50 - 2.40 100 - 2.40 0 7.5 6.26 - 10 5.58 - 25 4.90 1.77 50 3.31 2.96 100 - 2.86 0 10 9.46 - 10 8.86 - 25 7.76 1.98 50 5.93 4.14 100 1.77 4.44 4. Conclusions ASR and OAR were confirmed as an effective and low-cost odour control strategy. According to the results obtained, the most effective and rapid H2S abatement was found at oxidized nitrogen (N-NO3- or N-NO2-) activated sludge concentrations of 7.5 - 10 mg/L and 50 - 100 mg VSS/L, respectively. Acetic acid was partially 1 .5 m g N -N O x /L 5 m g N -N O x /L 7 .5 m g N -N O x /L 1 0 m g N -N O x /L N -N O 3 - ( m g /L ) 0 2 4 6 8 10 B1 (0 mg VSS/L) B2 (10 mg VSS/L) B3 (25 mg VSS/L) B4 (50 mg VSS/L) B5 (100 mg VSS/L) 1 .5 m g N -N O x /L 5 m g N -N O x /L 7 .5 m g N -N O x /L 1 0 m g N -N O x /L N -N O 2 - ( m g /L ) 0 2 4 6 8 10 B1 (0 mg VSS/L) B2 (10 mg VSS/L) B3 (25 mg VSS/L) B4 (50 mg VSS/L) B5 (100 mg VSS/L) a b 257 metabolized by microorganisms (from 27 to 23 ppmv in 4.5 h) when complete biological oxidation of H2S was reached and a slight decrease in α-pinene concentration was observed (from 13 to 8 ppmv in 4.5 h) by adsorption phenomena. Moreover, NO3- supported a more effective odorant abatement than NO2-. Finally, a marked reduction of NO2- was observed when increasing AS concentration, suggesting that higher concentrations of NO2- compared to NO3- are required to carry out the complete biological oxidation of odorants during wastewater treatment. 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