The Impact of Technological Processes on Odorant Emissions at Municipal Waste Biogas Plants

sustainability

Article The Impact of Technological Processes on Odorant Emissions at Municipal Waste Biogas Plants

Marta Wi´sniewska*, Andrzej Kulig and Krystyna Lelici ´nska-Seraﬁn

Faculty of Building Services, Hydro and Environmental Engineering, Warsaw University of Technology, 20 Nowowiejska Street, 00-653 Warsaw, Poland; [email protected] (A.K.); [email protected] (K.L.-S.) * Correspondence: [email protected]

Received: 29 May 2020; Accepted: 3 July 2020; Published: 7 July 2020

Abstract: Municipal waste treatment is inherently associated with odour emissions. The compounds characteristic of the processes used for this purpose, and at the same time causing a negative olfactory sensation, are organic and inorganic sulphur and nitrogen compounds. The tests were carried out at the waste management plant, which in the biological part, uses the methane fermentation process and is also equipped with an installation for the collection, treatment, and energetic use of biogas. The tests include measurements of the four odorant concentrations and emissions, i.e., volatile organic compounds (VOCs), ammonia (NH3), hydrogen sulphide (H2S), and methanethiol (CH3SH). Measurements were made using a MultiRae Pro portable gas detector sensor. The tests were carried out in ten series for twenty measurement points in each series. The results show a signiﬁcant impact of technological processes on odorant emissions. The types of waste going to the plant are also important in shaping this emission. On the one hand, it relates to the waste collection system and, on the other hand, the season of year. In addition, it has been proved that the detector used during the research is a valuable tool enabling the control of technological processes in municipal waste processing plants.

Keywords: ammonia; biogas plant; hydrogen sulphide; methanethiol; municipal waste treatment; VOCs

1. Introduction The use of fossil fuels, as well as the impact of greenhouse gases on the environment, have helped to initiate research related to the production of alternative fuels. In Europe and the world, there is an increase in greenhouse gas emissions in the atmosphere, the main source of which is carbon dioxide (CO2). Over 80% of global energy demand is covered by fossil fuels [1]. Biogas obtained as a result of biological treatment of biodegradable waste may play a key role in the energy industry in the future. Biogas as a renewable energy source can replace conventional fuels to produce heat and electricity and can also be used as gas fuel in the automotive industry. Research carried out so far indicates that biogas produced in the methane fermentation process provides significant benefits compared to other forms of bioenergy, because this technology is characterized by energy efficiency and environmental friendliness [2,3]. A modern waste management strategy should aim mainly at minimising waste generation (among others, by designing and manufacturing products that promote reuse and facilitate recycling and recovery), waste source separation and then reuse and recovery of energy and material resources from unavoidable waste [4]. The concept of “Zero Waste” is becoming more and more relevant by reducing the amount of waste, recycling, recovery, and minimisation of waste going to landfills [5,6]. However, literature data indicate that, still, unfortunately approximately 50% of municipal solid waste produced is sent to landfills [4]. Therefore, both the reduction of landfills volumes and the proper operation and reclamation of existing landfills are very important in terms of emission control (including

Sustainability 2020, 12, 5457; doi:10.3390/su12135457 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 5457 2 of 18 the minimisation of uncontrolled biogas emissions). In this context, the mechanical-biological waste treatment in both aerobic and anaerobic conditions (at biogas plants) has its full justification [7]. The biological process of methane fermentation is very complex and multiphase. In the last decade, devices for controlling individual process phases, as well as analytical tools, have been developed. These changes have contributed to increasing the energy efficiency of the process. The goal of the control system is to optimize the entire biogas production process and provide early warning to prevent failure of the entire process [8]. Thanks to the development of control and analysis systems, biogas plants can operate smoothly despite these extremely complex steps [9]. Until now, mainly biogas plants from agricultural, landfill, or sewage treatment plants were subjected to testing of odorous compounds [10–12]. Biogas plants processing municipal waste are not yet well understood in this respect and worldwide there are fewer of them compared to the previously mentioned installations [13]. However, due to energy (biogas energy production) and environmental (waste management) benefits, as well as publicly available source material, much more will probably be created in the future. Odorants, chemical compounds that cause an olfactory effect, are probably one of the most demanding environmental challenges for the emerging environmental policy [14,15]. Odorants that cause a negative olfactory effect generally contain nitrogen or sulphur, i.e., amines, phenolic compounds, aldehydes, thiols, ketones, and alcohols. Each of these components is produced mainly as a result of the activity of microorganisms that break down complex organic compounds present in the organic matter [16–20]. One of the main odorants emitted during the decomposition of biodegradable waste are volatile organic compounds (VOCs) [15,21]. The World Health Organization (WHO) has recognized volatile organic compounds as the most significant indoor air pollution. So far, about 500 volatile compounds and those present in indoor air have been identified. Only a few were considered pathogenic. Nevertheless, it is believed that many of them contribute to such health problems as: allergies, headaches, loss of concentration, drying and irritation of the nasal mucosa, throat, and eyes, etc. [21–25]. The term VOCs refers to a wide group of chemical compounds whose vapour pressure is at least 0.01 kPa at 20 ◦C[26,27]. They are also characterized by low aqueous solubility. VOCs in the atmosphere participate in photochemical reactions, producing photochemical oxidants. According to Eitzer [28], who undertook pioneering research on the exhaustive characteristics of VOCs emitted at various stages of the biodegradation process, most VOCs in composting plants are emitted at early stages of the process. For example, Delgado-Rodríguez et al. [29] found that emissions of volatile compounds are closely related to the phases of the composting process: aldehydes, alcohols, carboxylic acids, esters, ketones, sulphides, and terpenes are emitted mainly in the initial acid phase, while in the thermophilic phase ketones, organosulfur compounds, terpenes, and ammonia dominate. In the cooling phase, the main volatile compounds emitted are sulphides, terpenes, and ammonia. These authors also studied the impact of process control parameters (humidity, aeration, and C/N ratio) on the emission of volatile compounds in municipal solid waste composting. The C/N ratio had the greatest impact on VOCs emission, followed by aeration and moisture content. There are many odorous testing methods, which include sensory, sensor, and analytical methods. Table1 shows examples of the uses of these methods.

Table 1. Methods of assessing odour and odorants concentration [29–32].

Sensory Methods Sensor Methods Analytical Methods sensory evaluation method gas chromatography (GC); electronic nose gas chromatography coupled with mass spectrometry static olfactometry (e-nose) (GC–MS); dynamic olfactometry, portable detectors gas chromatography coupled with olfactometry (GC–O) ﬁeld olfactometry Sustainability 2020, 12, 5457 3 of 18

Sensory methods are used to determine the qualitative (method of sensory evaluation) or quantitative (olfactometry) smell. Using only sensory methods, it is not possible to obtain information on the types of compounds as well as their concentrations contained in the odorant mixture. For this purpose, analytical or sensor methods are used [33–35]. Portable detectors, classified as sensor methods, compared to most other methods, are characterized by uncomplicated service, as well as relatively low purchase costs [16]. The detectors use various types of sensors, which include: photoionization sensors—PID; • nondispersive infrared sensors—NDIR; • electrochemical sensors—EC; • thermal sensors—PELLISTOR [36,37]. • For the analysis of chemical compounds that cause an unpleasant olfactory effect, emitted during the treatment of municipal waste, photoionization and electrochemical sensors are most commonly used. In the case of the photoionization sensor, the operating principle is the ionization of neutral molecules of chemical compounds. When diffusing particles of VOCs encounter the UV lamp, they are ionized by photons. Then, the ions formed are directed between two polarized electrodes. The ions move towards the electrodes in the electric field generated by the electrometer. In this way, a current flow is generated, which is then converted into a voltage signal. This signal is proportional to the concentration of compounds subjected to ionization. Compounds having higher ionization energies than the maximum energies of the UV lamp photons are not detected. This type of sensor is most often used to measure the total concentration of VOCs [14,34,38–40]. The electrochemical sensor uses absorption of infrared radiation to identify the compounds. The principle of this type of sensor is to place the source of infrared radiation along the optical line with the detector. When the analysed gas appears in the measuring chamber, it absorbs radiation of a certain wavelength, and according to the Lambert–Beer law, there is a decrease in radiation reaching the detector which is converted into an electrical signal. This reduction in light intensity is proportional to the concentration of the gases or flammable vapours being detected [16,40]. This study undertakes intensive research aimed at analysing the impact of technological processes carried out at biogas plants (constituting waste treatment plants) on the emission of odorous compounds, using municipal waste as input material. So far, there have been few scientific studies related to the odour nuisance of this type of project or they have been carried out in a short period of time. Technological processes at municipal waste treatment plants are characterised by high variability and therefore require detailed analyses. The paper presents the results of almost a year-long research, which perfectly illustrates the complexity of the problem of odorant emissions, showing the relations between measured odorants and technological factors. The research results are potentially very valuable from the point of view of implementing new technologies in the field of both waste treatment and deodorization of process gases. Currently, there are in Poland eight biogas plants of this type but, in the future, it can be assumed that many more will be created due to the drive to obtain energy from raw materials available throughout the year.

2. Materials and Methods

2.1. Characteristic of the Analysed Plant The plant, being the subject of research, is located in the southern part of Poland. The plant is equipped with installations for mechanical and biological treatment of municipal waste. The input material for the fermentation process is the biodegradable fraction mechanically separated from the mixed waste stream. The fermentation process is carried out in two separate chambers. Each of them is equipped with four mixers and a digestate dewatering line. The fermentation process is carried out in semi-dry, mesophilic conditions. A ﬂow-chart of processes at the mechanical-biological waste treatment installation is shown in Figure1. Sustainability 2020, 12, x FOR PEER REVIEW 4 of 20

2. Materials and Methods

2.1. Characteristic of the Analysed Plant The plant, being the subject of research, is located in the southern part of Poland. The plant is equipped with installations for mechanical and biological treatment of municipal waste. The input material for the fermentation process is the biodegradable fraction mechanically separated from the mixed waste stream. The fermentation process is carried out in two separate chambers. Each of them is equipped with four mixers and a digestate dewatering line. The fermentation process is carried out in semi-dry, mesophilic conditions. A flow-chart of processes at the mechanical-biological waste Sustainabilitytreatment2020 ,installation12, 5457 is shown in Figure 1. 4 of 18

Fermentation Waste storage Mechanical part preparation Fermentation (in buffers)

Oxygen Oxygen Digestete Landfilling stabilisation (the stabilisation dewatering second stage) (the first stage)

Figure 1. Process ﬂowchart of mechanical-biological treatment of waste at the analysed plant.

Waste storageFigure 1. isProcess conducted flowchart in of a mechanical-biological closed hall. The hall treatment is equipped of waste at with the analysed three ventilators plant. which intakes are located: above the mixed waste and above the conveyors transporting waste to the machiningWaste hall. storage The processes is conducted of waste in a screeningclosed hall. and The sorting hall is equipped (mechanical with treatment) three ventilators are carried which out in intakes are located: above the mixed waste and above the conveyors transporting waste to the a hall equipped with a system of screens and separators connected by conveyors. The biodegradable machining hall. The processes of waste screening and sorting (mechanical treatment) are carried out fraction is transported to two buffers, located in a closed feedstock preparation hall for the fermentation in a hall equipped with a system of screens and separators connected by conveyors. The process.biodegradable The fermentation fraction is process transpor itselfted to takes two buffers, place in located two closed in a closed fermentation feedstock chambers.preparationAfter hall the fermentationfor the fermentation process is completed,process. The the fermentation input material process is subjecteditself takes toplace a dewatering in two closed process fermentation (via a press and achambers. centrifuge) After in athe closed fermentation dewatering process hall. is Dehydrated completed, the digestate input material is directed is subjected to special to tunnels, a locateddewatering in a closed process hall for(via the a press first stageand a ofcentrifuge) oxygen stabilisationin a closed dewatering (28 days) hall. and Dehydrated then to the opendigestate field for the secondis directed stage to of special oxygen tunnels, stabilization located in (14 a days).closed hall for the first stage of oxygen stabilisation (28 days) and then to the open field for the second stage of oxygen stabilization (14 days). 2.2. Study Methodology 2.2. Study Methodology

The studySustainability reported 2020, 12,here x FOR PEER includes REVIEW the determination of levels of ammonia, hydrogen6 of 20 sulphide, methanethiol,The study and VOCsreported at twentyhere includes measurement the determinat pointsion identiﬁed of levels of during ammonia, inventory hydrogen and sulphide, pilot tests as methanethiol, and VOCs at twenty measurement points identified during inventory and pilot tests sources of emissions of odorous compoundsTable 4. Characteristics (Table of2)[ the41 gas]. detector The tests sensors. were carried out in ten measurement as sources of emissions of odorous compounds (Table 2) [41]. The tests were carried out in ten Average series, from JulyKind to December of Sensor 2019 Type (Table of Sensor3). The Resolution sensor method Range was Accuracy used to determine the indicated measurement series, from July to December 2019 (Table 3). The sensor method wasFlow used Rate to determine compounds—thethe indicated compounds—the MultiRae Pro portable MultiRae multi-gas Pro portable gasdetector multi-gas0–100 (RAE gas detector Systems, (RAE Inc.; Systems, San Jose, Inc.; CA, San USA). ammonia (NH3) 1 ppm MeasurementsJose, CA, USA). were madeMeasurements with ﬁve were one-minute made with replicates five one-minute atppm each point,replicates and at the each obtained point, resultsand the were hydrogen sulphide Electrochemical 0–100 averaged.obtained The results characteristics were averaged. of individual The characteristics sensors0.1 ppm thatof individual the detector sensors is equipped that the detector with areis equipped presented in (H2S) (EC) ppm Table4with. At are the presented same time, in Table measurements 4. At the same (T) andtime, relative measurements humidity (T) (RH)and relative of air at humidity a height (RH) of 1.5 of mair were methanethiol 0–10 ±10% 250 cm3/min at a height of 1.5 m were carried out using a portable0.1 ppm Kestrel 4500 NV weather meter. The minimum, carried out using a(CH portable3SH) Kestrel 4500 NV weather meter.ppm The minimum, average, and maximum average, and maximum values of measurements during each0– of the series are presented in Figure values of measurementsvolatile organic during Photoionzsation each of the series are presented in Figure2a,b. Emission levels were 2a,b. Emission levels were calculated according0.01 to theppm methodology 100,000 presented in reference [42]. calculated accordingcompounds to the(VOCs) methodology (PID) presented in reference [42]. ppm

35 32 29.2 30 27.8 28.3 25.7 25.4 24.6 23.7 23.8 25 23.7 21.9 21.8 22.2 21.3 20.3 20.6 20.4 20.4 19.6 20 17.4 15.8 16.1 14.8 14.8 13.7 15 10.7 9.5

Temperature, T Temperature, [ºC] 10 8 7.5

(minimum,maximum) average, 5 0.6 0 012345678910 Measurement series

(a)

Figure 2. Cont. 100 92.6 85.6 90 81.8 80.8 77 80 73.3 71.2 69.4 67.5 68.9 68 70 65.4 62.7 65.3 59.9 62.4 60.5 60.8 56.4 60 57.2 55 55 53.5 54.3 49.46 51.5 47.2 46.3 50 46.2 40 32.3 30

Relative humidity, RH [%] humidity, Relative 20 (minimum, average, maximum) (minimum, average, 10 0 012345678910 Measurement series

(b) Sustainability 2020, 12, x FOR PEER REVIEW 6 of 20

Table 4. Characteristics of the gas detector sensors.

Average Kind of Sensor Type of Sensor Resolution Range Accuracy Flow Rate 0–100 ammonia (NH3) 1 ppm ppm hydrogen sulphide Electrochemical 0–100 0.1 ppm (H2S) (EC) ppm methanethiol 0–10 ±10% 250 cm3/min 0.1 ppm (CH3SH) ppm 0– volatile organic Photoionzsation 0.01 ppm 100,000 compounds (VOCs) (PID) ppm

35 32 29.2 30 27.8 28.3 25.7 25.4 24.6 23.7 23.8 25 23.7 21.9 21.8 22.2 21.3 20.3 20.6 20.4 20.4 19.6 20 17.4 15.8 16.1 14.8 14.8 13.7 15 10.7 9.5

Temperature, T Temperature, [ºC] 10 8 7.5

(minimum,maximum) average, 5 0.6 0 012345678910 Measurement series

Sustainability 2020, 12, 5457 5 of 18 (a)

100 92.6 85.6 90 81.8 80.8 77 80 73.3 71.2 69.4 67.5 68.9 68 70 65.4 62.7 65.3 59.9 62.4 60.5 60.8 56.4 60 57.2 55 55 53.5 54.3 49.46 51.5 47.2 46.3 50 46.2 40 32.3 30

Relative humidity, RH [%] humidity, Relative 20 (minimum, average, maximum) (minimum, average, 10 0 012345678910 Measurement series

(b)

Figure 2. Minimum, average, and maximum temperature (T) (a) and relative humidity (RH); (b) at individual measurement series.

Table 2. Measurement points at the examined biogas plants.

Mark of Odour Source Name of Odour Source Name of the Measurement Point a inside the hall-centre b waste storage plant mixed waste * c selectively collected waste * d in front of the hall entering e mechanical treatment plant inside the hall—at 1.5 m f inside the hall—at 4.0 m shredded preRDF fraction (pre refuse g storage shelter derived fuel) * h fermentation preparation plant inside the hall-centre i inside the hall-centre j over the wastewater tank (after the press) digestate dewatering plant over the wastewater tank k (after the centrifuge) l inside the hall oxygen stabilisation plant waste subjected to an oxygen m (1. stage) stabilization process * the technological wastewater n over the wastewater tank pumping station oxygen stabilisation shelter waste subjected to an oxygen o (2. stage) stabilization process * ventilator 1—process gases captured from p over-mixed waste roof ventilators from waste ventilator 2—process gases captured from r storage plant the overhead conveyor transporting waste to the sorting plant ventilator 3—process gases captured from s over selectively collected waste t roof ventilators from digestate ventilator 4-inside the hall u dewatering plant ventilator 5 * surface sources for which the gas sample was taken from under cover. Sustainability 2020, 12, 5457 6 of 18

Table 3. Dates of measurement series at a biogas plant.

Series Date Series Date 1 11 July 2019 6 03 October 2019 2 25 July 2019 7 17 October 2019 3 08 August 2019 8 07 November 2019 4 22 August 2019 9 21 November 2019 5 05 September 2019 10 30 December 2019

Table 4. Characteristics of the gas detector sensors.

Average Kind of Sensor Type of Sensor Resolution Range Accuracy Flow Rate

ammonia (NH3) 1 ppm 0–100 ppm hydrogen sulphide Electrochemical 0.1 ppm 0–100 ppm (H2S) (EC) 10% 250 cm3/min methanethiol (CH3SH) 0.1 ppm 0–10 ppm ± volatile organic Photoionzsation 0.01 ppm 0–100,000 ppm compounds (VOCs) (PID)

3. Results and Discussion The distribution of concentrations of tested odorants for the waste storage plant in individual measurement series is presented in Figure3a,b, while the distribution of emission levels from exhaust ventilators, which are at the waste storage plant, is presented in Figure4a–c. The highest level of VOCs emissions was recorded from the roof ventilator, which has its air intake located above the mixed waste stored p. This is consistent with the high concentrations of VOCs observed at the place where mixed municipal waste was stored b. Increased VOCs emissions were also recorded in the gases discharged from the ventilators, which have their air intakes located inside the hall r and aboveSustainability the 2020 selectively, 12, x FOR PEER collected REVIEW waste storage (10 measurement series). Periodically7 of 20 increased

VOCs concentrationsFigure 2. Minimum, were average, also recordedand maximum at temperature other measurement (T) (a) and relative points—in humidity the(RH); place (b) at of selectively collected wasteindividual storage measurementc–measurement series. series 7, inside the hall a–measurement series 2. The highest levels of NH3 (ammonia) emissions were also recorded from roof ventilators discharging air from 3. Results and Discussion waste storage plant p and r. A similar relationship was also observed for hydrogen sulphide and methanethiol.The The distribution presence of of concentrations these compounds of tested od wasorants only for notedthe waste in storage the gases plant discharging in individual from the measurement series is presented in Figure 3a,b, while the distribution of emission levels from exhaust waste storageventilators, plant which through are at roof the waste ventilators storage plant,p, r, and is presenteds. in Figure 4a–c.

25 a b c 20

VOCs concentration [ppm] VOCs concentration 5

0 12345678910 Measurement series

(a)

Figure 3. Cont. 10 a b c 9

3 Ammonia concentration [ppm] Ammonia concentration 2

0 12345678910 Measurement series

(b)

Figure 3. Distributions of volatile organic compounds (VOCs) (a) and ammonia (NH3) concentration; (b) at the waste storage plant in particular measurement series (a-inside the hall; b-mixed waste; c- selectively collected waste). Sustainability 2020, 12, x FOR PEER REVIEW 7 of 20

Figure 2. Minimum, average, and maximum temperature (T) (a) and relative humidity (RH); (b) at individual measurement series.

25 a b c 20

VOCs concentration [ppm] VOCs concentration 5

0 12345678910 Measurement series

Sustainability 2020, 12, 5457 7 of 18 (a)

10 a b c 9

3 Ammonia concentration [ppm] Ammonia concentration 2

0 12345678910 Measurement series

(b)

Figure 3. DistributionsFigure 3. Distributions of volatile of volatile organic organic compounds compounds (VOCs) (VOCs) (a) (anda) and ammonia ammonia (NH3) concentration; (NH3) concentration; (b) at the( wasteb) at the storage waste storage plant plant in particularin particular measurement series series (a-inside (a-inside the hall; theb-mixed hall; waste;b-mixed c- waste; Sustainability 2020, 12, x FOR PEER REVIEW 8 of 20 c-selectivelyselectively collected collected waste). waste).

0.45 p r s 0.4

0.35

0.3

0.25

0.2

0.15 VOCs emission [kg/h] 0.1

0.05

0 12345678910 Measurement series

(a)

0.5 p r s 0.45 0.4 0.35 0.3 0.25 0.2 0.15

Ammonia emission [kg/h] 0.1 0.05 0 12345678910 Measurement series

(b)

Figure 4. Cont. Sustainability 2020, 12, 5457 8 of 18 Sustainability 2020, 12, x FOR PEER REVIEW 9 of 20

0.45 p r s 0.4 0.35 0.3 0.25 0.2 methanethiol emission methanethiol 0.15 [kg/h] 0.1 0.05 0 H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S Hydrogen sulphide and Hydrogen CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH 12345678910 Measurement series

(c)

Figure 4. DistributionsFigure 4. Distributions of volatile of volatile organic organic compounds compounds (VOCs) (VOCs) (a), ( aammonia), ammonia (NH3); (NH (b) and3); (hydrogenb) and hydrogen sulphide (Hsulphide2S) and (H2S) methanethiol and methanethiol (CH (CH3SH);3SH); (c(c)) emissionemission at atthe the waste waste storage storage plant in plant particular in particular measurementmeasurement series (p -roofseries ventilator(p-roof ventilator 1-process 1-process gases gases captured captured from from over-mixed over-mixed waste; waste;r -roofr-roof ventilator ventilator 2-process gases captured from the overhead conveyor transporting waste to the sorting 2-process gases captured from the overhead conveyor transporting waste to the sorting plant; s-roof plant; s-roof ventilator 3-process gases captured from over selectively collected waste). ventilator 3-process gases captured from over selectively collected waste). The highest level of VOCs emissions was recorded from the roof ventilator, which has its air The analysisintake located of Figure above3 athe shows mixed that, waste in stored the case p. This of is a mixedconsistent waste with storagethe high site,concentrations the largest of increase in VOCs concentration,VOCs observed at tothe aplace level where of aboutmixed municipal 20 ppm, waste was was observed stored b. Increased during VOCs measurement emissions series 6. were also recorded in the gases discharged from the ventilators, which have their air intakes located During theinside same the series, hall ra and significant above the increase selectively in collected ammonia wa emissionsste storage (10 from measurement roof ventilators series). was also observed,Periodically the air intake increased of which VOCs concentrations is located inside were also the recorded hall, above at other the measurement conveyor points—in transporting the waste r and aboveplace the of selectively selectively collected collected waste waste storage storage–Figure c–measurement series4b. During 7, inside thethe hall tests, a–measurement an odour similar to the solventseries smell 2. The was highest perceptible levels of in NH the3 (ammonia) hall. The emissions sensing were substance also recorded was from probably roof ventilators the source of the discharging air from waste storage plant p and r. A similar relationship was also observed for increased VOCshydrogen emissions sulphide and compared methanethiol. to theThe resultspresence obtainedof these compounds in the other was only series. noted In in the gases case of waste collected selectively,discharging from increased the waste VOCs storage concentration plant through roof was ventilators observed p, r, and only s. during measurement series 7–at the place ofThe storage analysis of Figure this waste 3a showsc (Figure that, in the3a), case and of a during mixed waste measurement storage site, the series largest 10–increased increase level of emissions–inin VOCs the concentration, air discharged to a level by the of about ventilator 20 ppm, from was theobserved selectively during collectedmeasurement waste seriess (Figure6. 4a). During the same series, a significant increase in ammonia emissions from roof ventilators was also This probablyobserved, resulted the air from intake improper of which is and located ine insideffective the separatehall, abovewaste the conveyor collection. transporting waste r In theand case above of the the selectively roof ventilator collected wastep, which storage–Figure has its 4b. air During intake the locatedtests, an odour above similar the to mixed the waste, during eachsolvent of thesmell measurement was perceptible seriesin the hall. the The levels sensing of volatilesubstance compoundswas probably the emission source of were the similar; while the levelsincreased of VOCs ammonia emissions emission compared increased to the results significantly obtained in the in measurementother series. In the series case of 4–9. waste Analysing collected selectively, increased VOCs concentration was observed only during measurement series the results7–at of the the place levels of storage of hydrogen of this waste sulphide c (Figure and 3a), methanethioland during measur emissionsement series in 10–increased Figure4c, level a significant increase inof hydrogen emissions–in sulphide the air discharged emissions by canthe ventilator be observed from the during selectively series collected 6 and waste 9 (p ventilator). s (Figure 4a). Figure4c shows the resultsThis probably only resulted from ventilators from improperp, r andand ineffectives because, separate at the waste waste collection. storage plant, the values of H2S In the case of the roof ventilator p, which has its air intake located above the mixed waste, during (hydrogen sulphide) and CH3SH (methanethiol) concentrations were greater than 0.1 ppm only at these each of the measurement series the levels of volatile compounds emission were similar; while the measurementlevels points of ammonia (the emission threshold increased level significantly of determination in measurement of the series device 4–9. forAnalysing the measured the results chemical compounds).of the In levels each of of hydrogen the measurement sulphide and methanethiol series, high emissions levels of in emissionsFigure 4c, a significant of both hydrogen increase in sulphide and ammonia from p and r ventilators were observed. The composition of mixed waste and storage time probably had a significant impact on the results obtained. The long storage time of waste containing biodegradable fractions contributes to their compaction and uncontrolled anaerobic processes. Figure5a,b shows the results of VOCs concentration and ammonia concentration for the mechanical treatment plant, storage shelter for the shredded preRDF fraction (pre refuse derived fuel), and the fermentation preparation plant. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 20

hydrogen sulphide emissions can be observed during series 6 and 9 (p ventilator). Figure 4c shows the results only from ventilators p, r and s because, at the waste storage plant, the values of H2S (hydrogen sulphide) and CH3SH (methanethiol) concentrations were greater than 0.1 ppm only at these measurement points (the threshold level of determination of the device for the measured chemical compounds). In each of the measurement series, high levels of emissions of both hydrogen sulphide and ammonia from p and r ventilators were observed. The composition of mixed waste and storage time probably had a significant impact on the results obtained. The long storage time of waste containing biodegradable fractions contributes to their compaction and uncontrolled anaerobic processes. Figure 5a,b shows the results of VOCs concentration and ammonia concentration for the Sustainabilitymechanical2020, 12treatment, 5457 plant, storage shelter for the shredded preRDF fraction (pre refuse derived 9 of 18 fuel), and the fermentation preparation plant.

9 d e f g h 8 7 6 5 4 3 2 VOCs[ppm] concentration 1 0 12345678910 Measurement series

(a)

40 d e f g h 35

10 Ammonia concentration [ppm] Ammonia concentration 5

0 12345678910 Measurement series

(b)

Figure 5. Distribution of VOCs (a) and NH ;(b) concentration at measurement points related to Figure 5. Distribution of VOCs (a) and NH3; (b) concentration at measurement points related to mechanicalmechanical treatment treatment of of waste waste and and fermentation fermentation preparation preparation in inpa particularrticular measurement measurement series series (d-in (d-in hall front entrance; e-inside the hall, at 1.5 m; f -inside the hall, at 4.0 m; g-shredded preRDF fraction (pre refuse derived fuel); h-inside the fermentation preparation hall).

In individual measurement series, the highest VOCs concentration accompanied the storage of fuel from preRDF g waste and mechanical waste treatment operations—inside mechanical treatment plant f and inside the fermentation preparation plant h. The preRDF fraction mechanically separated from the mixed waste stream and stored under a covered shelter was also the source of the largest ammonia emission. The results obtained at point g are characterized by a large variation in the concentration of volatile organic compounds and a relatively constant level of ammonia (2–5 ppm), not including the result obtained during series 9 (35 ppm). Figure6a–c shows the distribution of measuring odorant concentrations for the digestate dewatering plant, while Figure7a,b presents the distribution of emission levels in individual measurement series from roof ventilators, which are at the digestate dewatering plant. Sustainability 2020, 12, x FOR PEER REVIEW 11 of 20

hall front entrance; e-inside the hall, at 1.5 m; f-inside the hall, at 4.0 m; g-shredded preRDF fraction (pre refuse derived fuel); h-inside the fermentation preparation hall).

In individual measurement series, the highest VOCs concentration accompanied the storage of fuel from preRDF g waste and mechanical waste treatment operations—inside mechanical treatment plant f and inside the fermentation preparation plant h. The preRDF fraction mechanically separated from the mixed waste stream and stored under a covered shelter was also the source of the largest ammonia emission. The results obtained at point g are characterized by a large variation in the concentration of volatile organic compounds and a relatively constant level of ammonia (2–5 ppm), not including the result obtained during series 9 (35 ppm). Figure 6a–c shows the distribution of measuring odorant concentrations for the digestate Sustainabilitydewatering2020, 12 plant,, 5457 while Figure 7a,b presents the distribution of emission levels in individual 10 of 18 measurement series from roof ventilators, which are at the digestate dewatering plant.

12 i j k 10

4 VOCs[ppm] concentration 2

0 12345678910 Measurement series

(a)

100 i j k 90 80 70 60 50 40 30 20 Ammonia concentration [ppm] Ammonia concentration 10 0 12345678910 Measurement series

Sustainability 2020, 12, x FOR PEER REVIEW 12 of 20 (b) 20 j k 18 16 14 12 10 8 6 4

concentration [ppm] concentration 2 0 Hydrogen sulphide and methanethiol Hydrogen H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH 12345678910 Measurement series

(c)

FigureFigure 6. Distribution 6. Distribution of VOCsof VOCs (a ),(a), NH NH3;(3; (bb)) andand H 22SS and and CH CH3SH;3SH; (c) (concentrationc) concentration at digestate at digestate dewateringdewatering plant plant in particular in particular measurement measurement series series (i-inside-inside the the hall; hall; j-overj-over wastewater wastewater tank tank(after (afterthe the press);press);k-over k-over wastewater wastewater tank tank (after (after the the centrifuge). centrifuge).

0.18 t u 0.16

0.14

0.12

0.1

0.08

0.06 VOCs emission [kg/h] 0.04

0.02

0 12345678910 Measurement series

(a) Sustainability 2020, 12, x FOR PEER REVIEW 12 of 20

20 j k 18 16 14 12 10 8 6 4

(c)

Figure 6. Distribution of VOCs (a), NH3; (b) and H2S and CH3SH; (c) concentration at digestate dewatering plant in particular measurement series (i-inside the hall; j-over wastewater tank (after the Sustainabilitypress);2020 ,k12-over, 5457 wastewater tank (after the centrifuge). 11 of 18

0.18 t u 0.16

0.14

0.12

0.1

0.08

0.06 VOCs emission [kg/h] 0.04

0.02

0 12345678910 Measurement series

Sustainability 2020, 12, x FOR PEER REVIEW 13 of 20 (a) 0.045 t u 0.04 0.035 0.03 0.025 0.02 0.015 0.01 Ammonia emission [kg/h] 0.005 0 12345678910 Measurement series

(b)

Figure 7. Distribution of VOCs (a), NH ;(b) emission at digestate dewatering plant in particular Figure 7. Distribution of VOCs (a), NH33; (b) emission at digestate dewatering plant in particular measurementmeasurement series series (t-roof (t-roof ventilator ventilator 4–inside 4–inside thethe hall; uu-roof-roof ventilator ventilator 5–inside 5–inside the thehall). hall).

AnalysisAnalysis of theof the results results obtained obtained forfor thethe digestatedigestate dewatering dewatering plant plant shows shows that thatthe highest the highest concentrationsconcentrations of both of both volatile volatile organic organic compounds compounds and and ammonia ammonia occur occur above above the theprocess process wastewater wastewater tankstanks after after the press the pressj and afterj and the after centrifuge the centrifugek, except k, forexcept series for 1 series and 10, 1 whereand 10, the where VOCs the concentration VOCs was atconcentration a similar level was inat a all similar measurement level in all points measurem relatedent points to digestate related dewatering.to digestate dewatering. Figure7b showsFigure that, in series7b shows 6, there that, was in aseries clear 6, increase there was in ammoniaa clear increase emissions in ammonia from roof emissi exhaustons from ventilators, roof exhaust whose air intakesventilators, are located whose at theair intakes digestate are located dewatering at the plant.digestate The dewatering increase inplant. ammonia The increase emissions in ammonia was due to emissions was due to the failure of one of the technological lines intended for digestate dewatering. the failure of one of the technological lines intended for digestate dewatering. In the case of tanks In the case of tanks (points j and k), the odorant concentration is associated to the greatest extent with (pointsoperationsj and k), carried the odorant out in relation concentration to these tanks, is associated including to the the maintenance greatest extent of their with filling operations levels and carried out incleaning. relation The to thesetanks are tanks, subjected including to weekly the maintenance cleaning, and ofthe their results filling of odorant levels concentrations and cleaning. above The tanks are subjectedthe tank after to weeklyits cleaning cleaning, are at a andlow thelevel, results compared of odorant to the results concentrations obtained before above cleaning. the tank after its cleaning areThe atconcentration a low level, of compared hydrogen sulphide to the results and meth obtainedanethiol before at the cleaning.measurement points related to digestateThe concentration dewatering ofvaried hydrogen during sulphide individual and measurement methanethiol series, at thereaching measurement maximum pointsvalues of related to digestate19.4 ppm dewatering (in series 9) variedand 10 ppm during (in series individual 5, 9 and measurement 10), respectively. series, reaching maximum values of 19.4 ppmAnalysing (in series Figures 9) and 8 10 and ppm 9, it (in can series be observed 5, 9 and that 10), therespectively. sources of the largest emission of the tested odorantsAnalysing in Figuresrelation 8to and the9 ,oxygen it can stabilization be observed process that the are sources technological of the wastewater largest emission from this of the process, which is directed to the tank of the pumping station n and wastes subjected to the first- tested odorants in relation to the oxygen stabilization process are technological wastewater from this degree oxygen stabilization process. In the case of digestate subject to stabilization, the concentration levels of both VOCs and ammonia are variable, although the measurements were carried out on the same day of the technological process. This indicates the different quality of digestate sent to the oxygen stabilization process, which is most likely the result of different compliance with the technological regime during digestate drainage or the quality differentiation of waste going to the plant. On the other hand, significantly lower concentrations of VOCs and NH3 in all measurement series accompanying the 2nd phase of aerobic processing testify to a properly conducted oxygen stabilization process of the 1st degree. Sustainability 2020, 12, 5457 12 of 18 process, which is directed to the tank of the pumping station n and wastes subjected to the first-degree oxygen stabilization process. In the case of digestate subject to stabilization, the concentration levels of both VOCs and ammonia are variable, although the measurements were carried out on the same day of the technological process. This indicates the different quality of digestate sent to the oxygen stabilization process, which is most likely the result of different compliance with the technological regime during digestate drainage or the quality differentiation of waste going to the plant. On the other hand, significantly lower concentrations of VOCs and NH3 in all measurement series accompanying the 2nd phase of aerobic processing testify to a properly conducted oxygen stabilization process of the 1stSustainability degree. 2020, 12, x FOR PEER REVIEW 14 of 20

25 l m n o

VOCs[ppm] concentration 5

0 12345678910 Measurement series

(a)

100 l m n o 90 80 70 60 50 40 30

Ammonia concentration [ppm] Ammonia concentration 20 10 0 12345678910 Measurement series

(b)

Figure 8. Distribution of VOCs (a) and NH ;(b) concentration at measurement points related to oxygen Figure 8. Distribution of VOCs (a) and NH3 3; (b) concentration at measurement points related to stabilisationoxygen stabilisation of digestate of digestate in particular in particular measurement measurement series series (l-inside (l-inside the hall; the hall;m-waste m-waste subjected subjected to an oxygento an oxygen stabilization stabilization process process (1st stage); (1st stage);n-over n the-over wastewater the wastewater tank; otank;-waste o-waste subjected subjected to an to oxygen an stabilizationoxygen stabilization process (2nd process stage). (2nd stage). Sustainability 2020, 12, 5457 13 of 18 SustainabilitySustainability 2020 2020, 12, ,12 x ,FOR x FOR PEER PEER REVIEW REVIEW 15 15of of20 20

4545 n n 4040 3535 3030 2525 2020 1515 concentration [ppm] concentration concentration [ppm] concentration 1010 5 5 Hydrohen sulphide and methanethiol Hydrohen Hydrohen sulphide and methanethiol Hydrohen 0 0 H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S H2S CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH CH3SH 1234567891012345678910 MeasurementMeasurement series series

Figure 9. Distribution of hydrogen sulphide and methanethiol related to wastewater storage from oxygen stabilisationFigureFigure 9. 9.Distribution Distribution of digestate of ofhydrogen (pointhydrogen nsulphide )sulphide in particular and and methan methan measurementethiolethiol related related to series. towastewater wastewater storage storage from from oxygenoxygen stabilisation stabilisation of ofdigestate digestate (point (point n) nin) inparticular particular measurement measurement series. series. Figure 10a,b shows the distribution of concentrations of VOCs and ammonia for all measurement FigureFigure 10a,b 10a,b shows shows the the distri distributionbution of ofconcentrations concentrations of ofVOCs VOCs and and ammonia ammonia for for all all points in themeasurementmeasurement ten measurement points points in inthe the series. ten ten measurement measurement series. series.

Sustainability 2020, 12, x FOR PEER REVIEW 16 of 20

(a)( a)

(b)

Figure 10.FigureValues 10. Values of minimum, of minimum, average, average, median, and and maximum maximum results resultsof VOCs; of(a) VOCs;and NH3 (; a(b)) and NH3; (b) concentrationconcentration at particular at particular measurement measurement points points during during all measurement all measurement series. series.

The above analysis indicates which places in the technological sequence are most exposed to the variability of odorant emissions as a result of changes in the quality of processed waste and implemented technological processes. The largest differences in VOCs and NH3 concentrations occur at measurement points related to the storage of mixed municipal waste (a, b, p) and preRDF (g), with the collection of process wastewater—both from the digestate dewatering process and its oxygen stabilization (j, k, n) and with waste directed to the aerobic treatment process (m, o). It is at these points in the technological sequence that the type of processed waste and the type of technological and operational measures taken have the greatest impact on odorant emissions. Figure 11 shows the correlation matrices for the odorants tested. Along the diagonal of the matrices, there are histograms representing the distribution of values of each variable. Table 5 shows the correlation coefficients based on Figure 11.

Figure 11. Correlation matrices for the tested compounds corresponding to all measurements. Sustainability 2020, 12, x FOR PEER REVIEW 16 of 20

(b)

SustainabilityFigure2020 10. ,Values12, 5457 of minimum, average, median, and maximum results of VOCs; (a) and NH3; (b14) of 18 concentration at particular measurement points during all measurement series.

The above analysis analysis indicates indicates which which places places in in the the technological technological sequence sequence are are most most exposed exposed to the to thevariability variability of odorant of odorant emissions emissions as asa result a result of ofchanges changes in in the the quality quality of of processed processed waste and implemented technological processes.processes. The largest didifferencesﬀerences inin VOCs and NH33 concentrations occur at measurement measurement points points related related to to th thee storage storage of ofmixed mixed municipal municipal waste waste (a, b (, ap,) band, p) preRDF and preRDF (g), with (g), withthe collection the collection of process of process wastewater—both wastewater—both from from the the digestate digestate dewatering dewatering process process and and its its oxygen stabilization ( j, k, n) and with waste directed to thethe aerobicaerobic treatmenttreatment processprocess ((mm,, oo).). It is at these points inin thethe technologicaltechnological sequence sequence that that the the type type of of processed processed waste waste and and the the type type of technological of technological and operationaland operational measures measures taken taken have have the greatest the greatest impact impact on odorant on odorant emissions. emissions. Figure 11 shows the correlationcorrelation matricesmatrices forfor thethe odorantsodorants tested.tested. Along the diagonal of the matrices, there are histograms representingrepresenting the distribution of values of each variable. Table 5 5 shows shows the correlation coecoefficienﬃcientsts basedbased onon FigureFigure 1111..

Figure 11. CorrelationCorrelation matrices for the tested comp compoundsounds corresponding to all measurements.

Table 5. Correlation table of examined parameters.

Odorant VOCs NH3 H2S CH3SH VOCs 1.00 0.54 0.34 0.39 NH3 0.54 1.00 0.60 0.73 H2S 0.34 0.60 1.00 0.76 CH3SH 0.39 0.73 0.76 1.00

Analysis of the correlations between the tested odorants (Figure 11 and Table5) indicates the largest relationship between hydrogen sulphide and methanethiol at 0.76 and between ammonia and methanethiol at 0.73. Larger relations between the above odorants were observed at measurement points related to waste storage (a, b, c, p, r, s) and digestate dewatering (i, j, k, t, u)—at the level of 0.76–0.81. The smallest correlation was observed between hydrogen sulphide and volatile organic compounds. Analysis of the correlation between points of subsequent stages in the technological processes showed relationships at a similar level. The diﬀerences were observed for points related to mechanical treatment and fermentation preparation—for these correlation points they were lower.

4. Conclusions In this study, the results of several months of research conducted at a biogas plant processing municipal waste in Poland have been presented. They show the relations between particular measured odorants and between odorants and technological factors. The novelty and scientific contribution Sustainability 2020, 12, 5457 15 of 18 presented in this work are related to the impact of technological aspects on odorant emissions at the municipal waste biogas plant. The literature review shows that, so far, such analyses have been conducted mainly at agricultural biogas plants, biogas plants on landfills or biogas plants related to sewage treatment. The impact of technological factors was identified by measuring odorant concentration (volatile organic compounds, ammonia, hydrogen sulphide, and methanethiol) and observing their changes between individual measurement series. The main conclusions and contributions of this research can be summarised as follows:

1. Odorant sources can be divided into the following ﬁve categories related to technological processes conducted at analysed biogas plant: waste storage, preRDF storage, waste mechanical treatment and fermentation preparation, digestate dewatering, and oxygen stabilization. 2. The biggest odorant concentrations accompany such unit operations as: storage of mixed municipal waste, digestate dewatering, digestate oxygen stabilization of the 1st-degree, and technological wastewater storage (both from digestate dewatering and its oxygen stabilization). The largest organized emissions are related to the evacuation of gases by means of roof ventilators. 3. The biggest VOCs concentrations are associated with mixed-waste storage (19.79 ppm) and aerobic stabilization of 1st-degree digestate (23.56 ppm). In turn, the highest NH3 concentrations accompany such technological processes as digestate dewatering (technological wastewater storage: 100 ppm) and 1st-stage oxygen digestate stabilization (100 ppm). The highest CH3SH concentrations also accompany the storage of mixed municipal waste, as well as digestate dewatering and 1st-stage oxygen stabilization (10 ppm). The biggest concentration of hydrogen sulphide is associated with the storage of wastewater from the digestate aerobic stabilization process (40 ppm), which indicates too long storage time and is the result of operational irregularities.

4. The highest emissions of odorants tested—to 0.42 kg/h (VOCs), 0.44 kg/h (NH3), 0.41 kg/h (CH3SH), and to 0.25 kg/h (H2S)–are emissions from a roof ventilator which has its air intake located above the mixed-waste storage. 5. The following factors aﬀect the concentration of the odorants tested, and thus the volume of emissions:

a municipal waste collection system in the service area (clearly higher odorant concentrations • accompany storage and mechanical treatment of mixed municipal waste in relation to selectively collected waste); trouble-free and continuous work of the technological line in the waste processing plant • (the sources of uncontrolled and increased odorant emissions are periodically occurring technological line failures); technological operations related to the unloading of transported waste and internal transport • of waste in the processing plant (especially with loaders and conveyors); keeping equipment and storage places clean at the waste treatment plant; • compliance with the technological regime and operational correctness. • 6. The largest diﬀerences in VOCs and NH3 concentrations occur at measurement points related to the storage of mixed waste and preRDF, with the collection of technological wastewater (both from digestate dewatering and its oxygen stabilization) and waste directed to the aerobic process. In this case, the type of waste processed and the type of technological and operational measures taken are of fundamental importance. 7. The odour nuisance of waste management plants, including municipal waste biogas plants, should be minimised by adapting the processes carried out to the best available techniques BAT conclusions [43]. 8. The detector used during the research is a valuable tool enabling control of technological processes in such facilities. Sustainability 2020, 12, 5457 16 of 18

9. Further research should combine olfactometric and meteorological tests in addition to odorants.

Author Contributions: M.W.: data collection, investigation, writing—original draft preparation; A.K.: conceptualization, methodology, supervision; K.L.-S.: writing—review and editing, visualisation, validation. All authors have read and agreed to the published version of the manuscript. Funding: Work was co-financed by the Dean’s grant (No. 504/04464): “The impact of technological processes and meteorological conditions on odour emission at municipal waste biogas plants”. Conflicts of Interest: The authors declare no conflict of interest.

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