Anaerobic Digestion Performance: Separate Collected Vs

energies

Article Anaerobic Digestion Performance: Separate Collected vs. Mechanical Segregated Organic Fractions of Municipal Solid Waste as Feedstock

Przemysław Seruga 1,* , Małgorzata Krzywonos 1 , Anna Seruga 2, Łukasz Nied´zwiecki 3 , Halina Pawlak-Kruczek 3 and Agnieszka Urbanowska 4

1 Department of Bioprocess Engineering, Wrocław University of Economics and Business, Komandorska 118/120, 53-345 Wrocław, Poland; [email protected] 2 Wrocław University of Economics and Business, Komandorska 118/120, 53-345 Wrocław, Poland; [email protected] 3 Faculty of Mechanical and Power Engineering, Wroclaw University of Science and Technology, Department of Boilers, Combustion and Energy Processes, Wyb. Wyspia´nskiego27, 50-370 Wrocław, Poland; [email protected] (Ł.N.); [email protected] (H.P.-K.) 4 Faculty of Environmental Engineering, Chair in Water and Wastewater Treatment Technology, Wroclaw University of Science and Technology, Wyb. Wyspia´nskiego27, 50-370 Wrocław, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-713-680-872

Received: 19 June 2020; Accepted: 20 July 2020; Published: 22 July 2020

Abstract: The replacement of fossil fuel with renewable energy sources seems as though it will be crucial in the future. On the other hand, waste generation increases year by year. Thus, waste-to-energy technologies ﬁt in with the actual trends, such as the circular economy. The crucial type of generated waste is municipal solid waste, which is in the research area. Regarding the organic fraction of municipal solid waste (OFMSW), anaerobic digestion (AD) allows the recovery of biogas and energy. Furthermore, if it is supported by source segregation, it should allow the recovery of material as fertilizer. The AD process performance (biogas yield and stability) comparison of source-segregated OFMSW (ss-OFMWS) and mechanically sorted OFMSW (ms-OFMSW) as feedstocks was performed in full-scale conditions. The daily biogas volume and methane content were measured to assess AD eﬃciency. To verify the process stability, the volatile fatty acid (VFA) content, pH value, acidity, alkalinity, and dry matter were determined. The obtained biogas yield per ton was slightly higher in the case of ss-OFMSW (111.1 m3/ton), compared to ms-OFMSW (105.3 m3/ton), together with a higher methane concentration: 58–60% and 51–53%, respectively, and followed by a higher electricity production capacity of almost 700 MWh for ss-OFMSW digestion. The obtained VFA concentrations, at levels around 1.1 g/kg, pH values (slightly above 8.0), acidity, and alkalinity indicate the possibilities of the digester feeding and no-risk exploitation of either as feedstock.

Keywords: energy recovery; biogas; organic waste; food waste; green waste; process control

1. Introduction Climate change, followed by the energy crisis, is one of the most critical issues of our world. Further research devoted to sources of renewable energy may allow us to protect the environment, along with natural resources. It will also enable us to reduce the consumption of fossil fuel [1]. This should lead to sustainability and support the reduction of human impact on other present day challenges, i.e., water table challenges, soil, space, carbon emissions, etc. [2,3] Additionally, waste generation is a major contributor, causing a negative environmental impact [3]. Municipal solid waste

Energies 2020, 13, 3768; doi:10.3390/en13153768 www.mdpi.com/journal/energies Energies 2020, 13, 3768 2 of 14

(MSW) is found to be the most complicated waste stream [2]; thus, its management system has to be environmentally, economically, and socially acceptable [4]. In the last few decades, waste-to-energy (WtE) technology has been considered as a solution for MSW handling, where energy production has been identified as a potential source to replace fossil fuels in the future [5]. It fits within zero waste and the circular economy (CE). The current policy utilizes circular management to reduce the exploitation of primary resources and to ensure long-term sustainability [6]. An essential part of the CE is the life cycle of products [7]. The circular economy is a model of production and consumption that involves sharing, reusing, repairing, and recycling existing materials and products, extending their life cycle. In practice, the circular economy implies reducing waste to a minimum [6,8]. Regarding WtE and CE, the primary concern remains MSW management. A substantial amount of MSW is generated globally. Only a tiny part is turned into useful resources [9]. About 2.01 billion tons of MSW were produced globally in 2017, and a rising trend is observed. It has been estimated that the yearly generation may rise to 3.40 billion tons by 2050 [10]. Landfilling continues to be the predominant disposal method, irrespective of a country’s income [10,11]. Only in high-income countries do recycling, incineration, and other advanced waste disposal methods account for slightly more than 50% by mass [10]. It is characterized by an 84% collection efficiency and 15% recycling efficiency of collected MSW [9,12]. MSW management efficiency still has to be improved [13]. The main fraction of MSW, representing from 42 to 75% (globally 46%) of total content, is the organic fraction of municipal solid waste (OFMSW) [14,15]. During the last few years, the OFMSW has been widely studied, considering the possibility of energy recovery, including various technologies [16,17]. These technologies can be divided into thermal-based treatments, such as gasification or pyrolysis, and biological processes, such as anaerobic digestion (AD) or composting [5]. Biological treatment is used with higher proportions of the OFMSW, which makes thermal treatment inadequate [5]. Energy recovery via AD is a sustainable process, which could be successfully applied to treating organic waste. It is also an environmentally friendly manner of managing MSW [18,19]. Nonetheless, researchers are facing the optimization of MSW management to improve biogas yield mostly at the laboratory scale, while investigations of existing facilities are limited. The use of the digestate of MSW, for energy purposes, i.e., in hydrothermal carbonization (HTC), could be much more profitable, due to landfilling cost reduction [20], especially when agricultural usage is limited. Another essential matter facing CE is source segregation. The organic fraction of municipal solid waste segregated based on its source (ss-OFMWS) is thought of as an answer to the non-landfill waste management system [12]. When source-segregated waste is processed, i.e., via AD, not only is biogas recovered, but so is additional value material, in the form of fertilizer, which can be used on crops. If the waste is not segregated by source so that the organic fraction is recovered by the mechanical–biological treatment (MBT) plant, the usage of the product fermented in this manner is limited on soil [21,22]. Therefore, the interest of government and industry in establishing separate collections of MSW should appear. However, changes in the political and management systems, as well as the environmental, economic, and social factors, influence the final effect [23–25]. As a result, there has been an increase in the number of industrial-scale facilities geared towards recovering valued materials and energy from non-segregated MSW [23]. MBT includes composting or anaerobic digestion to stabilize the biodegradable fractions as a stage within a sorting and separation process [12]. Source-segregated biowaste has more significant recovery potential, compared to the mechanical sorted organic fraction of municipal solid waste generated from mixed MSW (ms-OFMSW) [26,27]. The segregation at source ensures in the greater safety of treatment process performance, as well as emerging product reuse [26,27]. However, the AD performance with ss-OFMWS still has to be verified, mainly at a full scale with existing regional facilities. The literature suggests that AD is the best of the many options for the biological treatment of OFMSW with regard to the environment and the economy. However, it should be mentioned that attention should be paid to process efficiency (i.e., biogas yield), together with performance (stability), which is crucial for the full-scale operation. Moreover, the comparison of ss-OFMWS and ms-OFMSW Energies 2020, 13, 3768 3 of 14 should be made to define the benefits of source segregation, which is more effort for people and generates additional costs for the collection system [11]. This study aims to make a comparison between the AD of separately segregated biowaste and the AD of the mechanically sorted OFMSW, considering process efficiency and performance. The research was performed at full scale, using an operating plant.

2. Materials and Methods

2.1. The ms-OFMWS Characterization The ms-OFMWS was separated mechanically using screening (drum screen 60 mm) for glass, stone, and ceramic ballistic separation, as well as ferric and non-ferric metal separation. The separation was carried out at an MBT plant (ZGO Ga´c,Oława, Poland), located in Lower Silesia, in the southwestern part of Poland, and kept in a 300 m3 buﬀer before feeding the AD. The composition of the feedstock is presented in Table1.

Table 1. The composition of the mechanically sorted organic fraction of municipal solid waste (ms-OFMSW).

Fraction Mass Share Organic (incl. green waste) (%) 48.3 2.7 ± Wood (%) 7.2 0.8 ± Paper (%) 3.6 0.8 ± Plastics (%) 3.9 0.7 ± Glass (%) 3.6 1.1 ± Inert waste (%) 2.9 1.0 ± Textiles (%) 0.3 0.5 ± Metals (%) 0.1 0.1 ± Hazardous (%) 0.1 0.1 ± Tetra Pak (%) 0.7 0.2 ± Others (%) 0.6 0.2 ± Fine fraction 0–15 mm (%) 28.7 3.8 ±

2.2. The ss-OFMWS Characterization The ss-OFMWS was manually sorted (plastic bags, stones, metals, etc., were sorted out) and shredded (TR2A200, O.M.A.R. S. r.l., Italy) to a fraction of about 60 mm. The composition of the feedstock is presented in Table2.

Table 2. The composition of the segregated at source organic fraction of municipal solid waste (ss-OFMSW).

Fraction Mass Share Organic (incl. green waste) (%) 68.1 5.2 ± Wood (%) 8.1 0.5 ± Paper (%) 2.4 0.7 ± Plastics (%) 1.1 0.4 ± Glass (%) 0.8 0.4 ± Inert waste (%) 1. 4 0.9 ± Textiles (%) 0.1 0.4 ± Metals (%) 0.1 0.1 ± Hazardous (%) 0.1 0.1 ± Tetra Pak (%) 0.3 0.1 ± Others (%) 0.4 0.1 ± Fine fraction 0–15 mm (%) 17.1 2.3 ± Energies 2020, 13, 3768 4 of 14

2.3. The AD Full-Scale Research Energies 2020, 13, x FOR PEER REVIEW 4 of 16 The study was conducted at an MBT plant (ZGO Ga´c,Oława, Poland). The plant is located in The study was conducted at an MBT plant (ZGO Gać, Oława, Poland). The plant is located in Lower Silesia, in the southwestern part of Poland. A simpliﬁed diagram of the installation is presented Lower Silesia, in the southwestern part of Poland. A simplified diagram of the installation is in Figure1. presented in Figure 1.

Figure 1. 1. SimplifiedSimpliﬁed diagram diagram of ofthe the anaerobic anaerobic digestion digestion (AD) (AD) facility: facility: 1: municipal 1: municipal solid waste solid (MSW) waste (MSW) mechanical treatment; treatment; 2: 2:mechanically mechanically sorted sorted orga organicnic fraction fraction of municipal of municipal solid waste solid (ms-OFMSW) waste (ms-OFMSW) loading;loading; 3: 3: digester digester operating operating on ms-OFMWS; on ms-OFMWS; 4: biowaste 4: biowaste sorting sorting and shredding; and shredding; 5: segregated 5: segregated at source organic fraction of municipal solid waste (ss-OFMSW) loading; 6: digester operating on ss- at source organic fraction of municipal solid waste (ss-OFMSW) loading; 6: digester operating on OFMSW; 7: combined heat and power (CHP) units; A: MSW delivered to mechanical-biological ss-OFMSW; 7: combined heat and power (CHP) units; A: MSW delivered to mechanical-biological treatment (MBT) plant; B: ms-OFMSW feedstock for digester; C: separate collected biowaste delivered treatment (MBT) plant; B: ms-OFMSW feedstock for digester; C: separate collected biowaste delivered to MBT plant; D: ss-OFMSW feedstock for digester; E: biogas. to MBT plant; D: ss-OFMSW feedstock for digester; E: biogas. The anaerobic digestion (AD) processes were conducted in 1500 m3, full-scale Kompogas® The anaerobic digestion (AD) processes were conducted in 1500 m3, full-scale Kompogas® separate separate digester chambers using ms-OFMSW and ss-OFMWS as the feedstocks. The process digester chambers using ms-OFMSW and ss-OFMWS as the feedstocks. The process temperature was temperature was maintained at 54 °C. The agitation speed was set at 0.45 rpm, in 300 s operation and maintained120 s break cycles, at 54 ◦ withC. The alternating agitation direct speedions. was The set operating at 0.45 rpm, volume in 300 (about s operation 1100 m3 and) in the 120 digester s break cycles, 3 withwas determined alternating through directions. setting The up operating the extraction volume pump (about operation 1100 time. m ) in the digester was determined throughThe settingAD process up the parameters, extraction such pump as operation temperature, time. digester filling level, biogas yield, input weight, electricity production, etc., were archived every day by the central control system. Moreover, samples from the digester inlet, outlet, and middle section were collected on a weekly basis. To verify the performance and stability of the process, the pH value, dry matter content, acidity, alkalinity, and

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The AD process parameters, such as temperature, digester filling level, biogas yield, input weight, electricity production, etc., were archived every day by the central control system. Moreover, samples from the digester inlet, outlet, and middle section were collected on a weekly basis. To verify the performance and stability of the process, the pH value, dry matter content, acidity, alkalinity, and concentrations of fatty acids (acetic (HAC), propionic (HPR), butyric (HB) with isobutyric (HIB), and valeric (HV) with isovaleric (HIV)) were determined. The flowmeter was used to measure the biogas stream. A biogas analyzer (ADOS Biogas 401) (ADOS GmbH, Aachen, Germany) allowed for determining its composition (the content of methane, hydrogen sulfide, and oxygen), before combustion in the combined heat and power (CHP) units. Average values are reported.

2.4. Analytical Methods The fatty acid content was determined using gas chromatography (Varian GC 450) (Varian BV, Middelburg, the Netherlands) with a flame ionization detector (FID) detector (H2: 30 mL/min, air: 300 mL/min, He: 30 mL/min) (Varian BV, Middelburg, the Netherlands). Helium (constant flow through a 1 mL/min column) was used as the carrier gas with a 1:30 split. The pH-metric titration, which was used in compliance with the standard methods, was utilized to determine acidity and alkalinity [28]. The acidity/alkalinity ratio (index R) was also determined. The standard methods were used to ascertain the suspended solids (SS), dry organic mass, and pH [29]. To determine the composition of the OFMSW, the waste samples were collected over a month. Each sampling day, at about 60 min intervals, 10–15 kg samples were collected to prepare the daily sample. In total, two samples of OFMWS of approx. 100 kg were collected. Before testing, the daily waste samples collected over a week were mixed and averaged. Then the pooled samples were divided by quartering and a representative sample of about 100 kg was selected. The determination of the composition was made in 11 main material fractions: organic waste, paper, plastics, textiles, wood, glass, metals, multi-material waste, hazardous waste, inert waste, other waste. A fine fraction was also isolated. The average values based on the weekly samples are reported.

2.5. Calculation Methods

The average monthly biogas yield was determined using the formula: Y = (Vav/Mav), where Y 3 3 stands for biogas yield (m /ton), Vav for the biogas daily volume average over a month (m ), and Mav for daily feeding amount average over a month (tons).

3. Results and Discussion The research was performed in the Lower Silesia region in Poland. The MSW separate collection standard, based on five fractions: paper, plastics, glass, biowaste, and residual waste, was implemented in Poland in 2018. However, due to a transitional period, mixed MSW is still collected. Thus, the MBT plant receives waste streams which have to be pretreated. In the case of biowaste, the amount of the collected waste is still less than that obtained from mixed MSW in the analyzed area. The AD was evaluated at the full-scale MBT plant (ZGO Gać,Oława, Poland) with the ms-OFMSW and ss-OFMSW digestions, and the results were compared. The composition of both feedstocks is presented in Tables1 and2. It was observed that organic fraction content is higher in ss-OFMSW (68.1%), compared to ms-OFMSW (48.3%), which was as expected. The wood quantity was at a similar level for both OFMSWs (7.2–8.1%). The fine fraction content was over 10% higher in ms-OFMSW, which can be explained by the coal ashes, which is typical of Polish conditions. Furthermore, over a 7.5% content of the “others” fraction (Table2) was observed in ss-OFMSW, despite the manual sorting. It may indicate the poor quality of source segregation or a poor sorting process. Due to ms-OFMSW, the “others” fraction content was almost 16% (Table1), which may confirm that the source segregation in the analyzed MBT plant area still has to be improved regarding biowaste, as well as for packaging fractions (plastic, paper, and glass). Energies 2020, 13, 3768 6 of 14

The feedstocks were analyzed before the digester feeding, and the results of the analysis can be found in Table3. As can be seen, the SS and the fatty acid content remain at similar levels. However, ss-OFMSW is characterized by a higher pH value, alkalinity, and ammonium content compared to ms-OFMSW (Table3). This can be explained by a higher organic matter content (Table2), mainly green waste content.

Table 3. The characterization of the mechanically sorted and segregated at source organic fractions of municipal solid waste (ms-OFMSW and ss-OFMSW).

Parameter ms-OFMSW ss-OFMSW pH ( ) 7.1 0.2 8.0 0.2 − ± ± Suspended solids (SS) (%) 49.8 0.3 46.2 0.3 ± ± Ammonium nitrogen (g/L) 1453 65 3879 87 ± ± Acidity (mg CH COOH/kg) 655 50 463 44 3 ± ± Alkalinity (mg CaCO /kg) 4210 145 6758 163 3 ± ± Acetic acid (mg/kg) 98 8 107 6 ± ± Propionic acid (mg/kg) 6.0 0.5 3.0 0.5 ± ± Butyric acid (mg/kg) 1.0 1.0 1.0 1.0 ± ± Isobutyric acid (mg/kg) 1.0 1.0 1.0 1.0 ± ± Valeric acid (mg/kg) 1.0 1.0 1.0 1.0 ± ± Isovaleric acid (mg/kg) 1.0 1.0 1.0 1.0 ± ±

The process of anaerobic digestion of both feedstocks conducted in the MBT Plant was stable. The exploitation of historical trends was similar (Figures2 and3). For both digesters, the average filling was kept at about 1110 m3 (Figure2a,b). The recorded hydraulic retention time (HRT) was about 28 and 31 days for ms-OFMSW and ss-OFMSW AD, respectively. The total volumes of biogas produced were 131,390.7 m3 and 119,478.9 m3 for ms-OFMSW and ss-OFMSW, respectively, followed by total input of 1256.6 tons and 1076.8 tons (Figures2 and3). Based on that, it can be found that biogas yield per ton was slightly higher in ss-OFMSW (111.1 m3/ton) than for ms-OFMSW (105.3 m3/ton). The obtained biogas productivity was similar to the results reported in our previous study [7] and by other researchers [21,30]. Nonetheless, owing to a variety of OFMSWs, it is not easy to make a comparison of the collected results with those facilities described in Europe or other places in the world. Campuzano and González-Martínez [31] analyzed the composition of the OFMSW and methane yield in 43 cities in 22 countries worldwide. The methane yield ranged from 61 L/kg up to 580 L/kg with an average value of 415 138 L CH /kg. The average yield of methane in Italy totaled 400 L/kg; ± 4 meanwhile, in Denmark, it amounted to 499 L [31]. Furthermore, it should be noted that a more significant variability of daily biogas yield was observed during ss-OFMSW anaerobic digestion. The difference between minimal and maximal daily biogas yield was 45.9 m3/ton, while the difference in the case of ms-OFMSW was 27.1 m3/ton (Figure2a,b). This di fference might be a consequence of the variation in feedstock composition. The composition of ms-OFMSW was averaged during the sorting process, while that of ss-OFMSW was characterized by changes in green waste and kitchen waste content. This waste has different biogas potential, which impacts AD efficiency [31]. The small difference in biogas yield for OFMSWs might also indicate the similar quantities of the digestible organic matter, which proves the necessity of source-segregation improvement. The recirculation of the digestate was maintained at a constant level to ensure organic matter degradation and no compensation of biogas productivity while increasing its proportion was observed. Energies 2020, 13, x FOR PEER REVIEW 7 of 16 EnergiesEnergies2020 2020, 13, 13,, 3768 x FOR PEER REVIEW 77 of of 16 14

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(b) (b) Figure 2. Biogas yield and digester filling changes during organic fraction of municipal solid waste FigureFigure 2. 2.Biogas Biogas yieldyield andand digesterdigester ﬁllingfilling changeschanges during organic fraction of of municipal municipal solid solid waste waste (OFMSW) digestions: (a) mechanically sorted; (b) segregated at source. (OFMSW)(OFMSW) digestions: digestions: ((aa)) mechanicallymechanically sorted;sorted; (b) segregated at source.

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) 70 3 60 60 50 50 40 40 30 30 20 20 Output volume (m volume Output Input material (ton), material Input 10 Output volume (m volume Output Input material (ton), material Input 10 0 0 1 6 11 16 21 26 31 1 6 11 16 21 26 31 Process time (d) Process time (d) Input material Output volume Input material Output volume (a) (a) Figure 3. Cont.

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80 )

3 70 60 50 40 30 20

Output volume (m volume Output 10 Input material (ton), Input 0 1 6 11 16 21 26 31 Process time (d)

Input material Output volume

(b)

FigureFigure 3. 3.Input Input andand output changes changes during during organic organic fraction fraction of municipal of municipal solid solid waste waste (OFMSW) (OFMSW) digestions: (a) mechanicallydigestions: sorted; (a) mechanically (b) segregated sorted; at source. (b) segregated at source.

Methane is is the the most most desired desired component component of biogas of biogas.. It is deemed It is deemed a renewable a renewable energy, energy,which might which bemight transformed be transformed into usable into energy usable (heat energy or (heatelectricity). or electricity). The potential The potentialenergy recovery energy of recovery various of feedstocksvarious feedstocks varies [32]. varies Therefore, [32]. Therefore, increasing increasing the efficiency the efficiency of methane of methane production production from from various various substrates is is tantamount tantamount to to greater greater energy energy recove recoveryry in in AD AD [32]. [32]. The The measurement measurement of ofthe the biogas biogas component contentcontent mightmight indicate indicate feedstock feedstock quality. quality. In thisIn this study, study, methane, methane, hydrogen hydrogen sulfide, sulfide, oxygen, oxygen,and carbon and monoxidecarbon monoxide concentrations concentrations were monitored. were monitored. During ss-OFMSW ss-OFMSW AD, AD, a a higher higher methane methane concen concentrationtration was was observed observed (58–60%) (58–60%) compared compared to to ms-OFMSW (51–53%),(51–53%), whichwhich aaffectedffected the the production production of of electricity electricity in combinedin combined heat heat and and power power (CHP) (CHP)units. Theunits. second The second important important biogas biogas quality quality parameter parameter is the is hydrogen the hydrogen sulfide sulfide concentration. concentration. It was Itmuch was much lower lower in biogas in biogas from from ss-OFMSW ss-OFMSW AD AD (200–250 (200–250 ppm), ppm), than than in in biogas biogas obtained obtained from from AD AD of ofms-OFMSW ms-OFMSW (800–1000 (800–1000 ppm), ppm), which which can can confirm confirm the the presence presence of of coal coal ash ash in in this this feedstock. feedstock. TheThe ADAD of ofms-OFMSW ms-OFMSW required required the the addition addition of more of more desulfurizing desulfurizing agent agent (in this (in study, this study, ferric chlorideferric chloride solution solution(PIX 116, (PIX Kemipol, 116, Kemipol, Poland)), Poland)), which which generates genera highertes higher process process costs costs and hasand ahas higher a higher risk risk of CHP of CHPunit damage.unit damage. The The oxygen oxygen concentration concentration was was lower lowe thanr than 0.2%, 0.2%, which which proves proves there there were were no no leaks leaks in inthe the facility. facility. The differences differences in in the the electricity electricity production production efficiency efficiency regarding regarding the theAD ADof ms-OFMSW of ms-OFMSW and ss- and OFMSWss-OFMSW are arepresented presented in Table in Table 4. 4It. can It can be befound found that that even even a small a small differen di fferencece in biogas in biogas yield yield (6 3 3 m(6/ton) m3/ton) results results in a in significant a significant disparity disparity in total in total production production capacity capacity (87,000 (87,000 m ). m Furthermore,3). Furthermore, a highera higher methane methane content content promotes promotes electricity electricity yield yield an andd an an increase increase in inyearly yearly production production capacity capacity by by almost 650 650 MWh, MWh, when when considering considering ss-OFMSW ss-OFMSW AD AD (Table (Table 4).4).

TableTable 4. 4. ElectricityElectricity production production efficiency. efficiency. Yearly Yearly Yearly Biogas Electrical Yearly Electricity Biogas Methane Electrical CHP Units Electricity Input Biogas BiogasProduction Methane Production CHP Units Electricity ElectricityProduction Input Yield Content Production Efficiency Yield Material Yield ProductionCapacity Content Possibility Efficiency Yield ProductionCapacity Material 3 3 Possibility3 m /tonCapacity m % kWh/m CH4 % kWh/tonCapacity MWh ms-OFMSWm3 105.3/ton 1,579,500m3 % 52kWh/m 103CH4 % 40 kWh/ton 219.0 MWh 3285.4 ms-OFMSWss-OFMSW 105.3 111.1 1,579,500 1,666,500 52 59 10 10 40 40 219.0 262.2 3285.4 3932.9 ss-OFMSW* Quantities calculated111.1 based 1,666,500 on mechanical-biological 59 treatment 10 (MBT) plant 40 capacity: 262.2 15,000 ton/year 3932.9/digester; Electricity yield calculated using the formula: biogas yield methane content electrical production possibility *Quantities3 calculated based on mechanical-biological× treatment (MBT× ) plant capacity: 15,000 (10 kWh/m CH4) combined heat and power (CHP) unit efficiency (40%); Annual production of electricity ton/year/digester;based on the formula:× Electricity annual biogas yield productioncalculated capacityusing themethane formula: content biogas electricalyield × methane production content possibility × 3 × × (10 kWh/m CH ) CHP unit efficiency (40%). 3 electrical production4 × possibility (10 kWh/m CH4) × combined heat and power (CHP) unit efficiency (40%); Annual production of electricity based on the formula: annual biogas production capacity × methane content × electrical production possibility (10 kWh/m3CH4) × CHP unit efficiency (40%).

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ExceptExcept for for the the biogas biogas yield, yield, ensuring ensuring process process stability stability is is also also crucial, crucial, especially especially atat aa full full scale.scale. Thus, it isThus, crucial it tois crucial monitor to themonitor values th ofe values the process of the indicatorsprocess indicators [31]. The [31]. changes The changes in the processin the process parameters parameters during the ms-OFMSW AD are presented in Figures 4 and 5. The variations of ss-OFMSW during the ms-OFMSW AD are presented in Figures4 and5. The variations of ss-OFMSW AD are AD are presented in Figures 6 and 7. presented in Figures6 and7.

8.65 36 8.6 35 8.55 34 pH (-) pH 8.5 (%) SS 33 8.45 32 1234 1234 Process time (week) Process time (week) Inlet Middle Outlet Inlet Middle Outlet

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(e)

FigureFigure 4. Parameter4. Parameter changes changes inin thethe coursecourse of of the the mechan mechanicallyically sorted sorted organic organic fraction fraction of municipal of municipal solidsolid waste waste anaerobic anaerobic digestion digestion process process at full at scale:full scale: (a) pH (a) value; pH value; (b) suspended (b) suspended solids; solids; (c) ammonium (c) ionammonium concentration; ion concentration; (d) acidity; (e ()d alkalinity.) acidity; (e) alkalinity.

The pH value is considered to be the factor stabilizing anaerobic digestion [5,31]. Generally, the pH value ought to be between 7.2 and 7.8. It might, however, be inﬂuenced by temperature and sampling, and it might be unique to the particular facility [7,33]. During testing the ms-OFMSW AD, the pH-value was 8.5 in the inlet section, with slight growth to 8.6 in the outlet (Figure4a), which was typical for the examined MBT plant. Regarding the ss-OFMSW AD, the pH value was 8.3–8.4 in the inlet section, with slight growth to 8.6 in the outlet (Figure6a). The pH value decrease is usually caused by the organic overload of the digester and might result in reducing the activity of microorganisms, causing the decomposition of volatile fatty acids [5,31]. An alkalinity increase (pH values above 8.0) + aﬀects the NH3 and NH4 dissociation balance. Considering the higher ammonium ion concentration Energies 2020, 13, 3768 10 of 14

(of about 1g/kg) during the AD of ss-OFMSW, compared to ms-OFMSW AD (Figures4c and6c), it was expected to achieve higher pH values. The higher acidity, together with lower acidity in ms-OFMSW AD (Figure4d,e) than in ss-OFMSW AD (Figure6d,e), may explain the observations. Nevertheless, mostly because of this digester buﬀer capacity, pH changes are normally detectable only at the point when the process is already unstable. Hence, the determination of low fatty acid (acetic, propionic, butyric, and valeric) concentrations (with ratios) is the source of the most reliable information regarding the stability for digestion [34]. The process of producing and accumulating volatile fatty acids (VFAs) could inhibit anaerobic digestion, which could result in slowing the production of biogas [28]. Energies 2020, 13, x FOR PEER REVIEW 10 of 16 The obtained VFA concentrations, at levels around 1.2 g/kg, indicate the possibilities of digester feedingEnergies and 2020 no-risk, 13, x FOR exploitation. PEER REVIEW There is an optimum loading rate for a given size of the10 digestion of 16 chamber, which cannot be exceeded. Otherwise, the loading rate increment could stop being eﬀective, because it might lead to an accumulation of excess VFAs and subsequently to reactor collapse [28]. 1050 100 15 1000 80 1050 60100 10 950 15 1000 4080 5 900 2060 10 950 0 Propionic acid

Inlet (mg/kg) 850 0 40 5 900 1234 123420 concentration (mg/kg) Process time (week) 0 Process time (week) Propionic acid Acetic acid concentration- Acetic acid concentration- Inlet (mg/kg)

850 0 Middle and Outlet (mg/kg) Inlet Middle Outlet 1234Inlet Middle Outlet

1234 concentration (mg/kg) Process time (week) Process time (week) Acetic acid concentration- Acetic acid concentration- Inlet Middle Outlet Middle and Outlet (mg/kg) Inlet Middle Outlet (a) (b)

(a) 10 (b) 8 610 4 8 2 6

Butyric acid Butyric 0 4 21234

concentration (mg/kg) concentration Process time (week) Butyric acid Butyric 0 Inlet Middle Outlet 1234

concentration (mg/kg) concentration Process time (week) Inlet Middle Outlet (c)

Figure 5. The changes in the concentration of fatty(c )acid s in the course of the mechanically sorted organic fraction of municipal solid waste anaerobic digestion at full scale: (a) acetic acid; (b) propionic Figure 5. The changes in the concentration of fatty acids in the course of the mechanically sorted acid;Figure (c) butyric5. The acid.changes in the concentration of fatty acids in the course of the mechanically sorted organic fraction of municipal solid waste anaerobic digestion at full scale: (a) acetic acid; (b) propionic organic fraction of municipal solid waste anaerobic digestion at full scale: (a) acetic acid; (b) propionic acid;acid; (c) butyric (c) butyric acid. acid.

8.7 36

8.6 8.7 35 8.5 36 8.6 34

pH (-) pH 8.4 35 8.5 (%) SS 8.3 33 34

pH (-) pH 8.4

8.2 (%) SS 32 8.3123433 1234Process time (week) 8.2 Process time (week) 32 1234 Inlet Middle Outlet Inlet Middle Outlet 1234 Process time (week) Process time (week) Inlet (a) Middle Outlet Inlet (b) Middle Outlet

(a) Figure 6. Cont. (b)

EnergiesEnergies2020 2020, 13, 13, 3768, x FOR PEER REVIEW 11 of 16 11 of 14

5500 1100 5450 1050 5400 1000 5350

COOH/kg) 950

5300 3 5250 900 5200 850 5150 800 5100 1234

Ammonium ions (mg/kg) ions Ammonium 1234

Process time (week) (mg CH Acidity Process time (week) Inlet Middle Outlet Inlet Middle Outlet

9500 /kg)

3 9000

8500

8000

7500 1234

Alkalinity (mg CaCO Alkalinity (mg Process time (week) Inlet Middle Outlet

(e)

FigureFigure 6. 6.Parameter Parameter changes in in the the course course of ofthe the segregat segregateded at source at source organic organic fraction fraction of municipal of municipal solidsolid waste waste anaerobic anaerobic digestion digestion process process at fullat full scale: scale: (a) pH(a) value;pH value; (b) suspended(b) suspended solids; solids; (c) ammonium (c) ionammonium concentration; ion concentration; (d) acidity; (ed)) alkalinity.acidity; (e) alkalinity.

The ms-OFMSW AD was stable. The acetic acid content amounted to 0.87–1.0 g/kg, with a propionic acid concentration of between 5 and 10 mg/kg in the inlet section (Figure5a,b). In the outlet 1200 40 section, the acids were nearly zero, which may confirm full organic40 matter decomposition (Figure5a,b). Owusu-Agyeman et al. [35] also reported30 that acetic acid is30 the most abundant VFA in the anaerobic 1000 20 digestion of organic waste. However, the share20 of acetic acid decreased with time [35] and its relative 10 abundance was800 much lower than in this study. Differences in the composition could be attributed to 10 0 the fact that the AD temperature of the batch was much lower (35 ◦C) [35], as well as to differences Propionic acid 1234 in the compositionInlet (mg/kg) 600 of the feedstock. The0 organic waste in the study of Owusu-Agyeman et al. [35] concentration (mg/kg) consisted of alcohol1234 and soda beverage, food, dairy, fruit, fat, and oilProcess waste. time Tampio (week) et al. [36] reported Process time (week) Inlet Middle Outlet Acetic acid concentration- acid Acetic Acetic acid concentration- acid Acetic that the acetic acidInlet fractionMiddle dominated onlyOutlet in the(mg/kg) Outlet and Middle first period of the anaerobic digestion of food waste, (b) which was then surpassed by butyric acid. Additionally, in this case, mesophilic conditions (37 ◦C) [36] could be considered as a plausible(a) explanation for the obtained difference. The butyric and valeric acid contents were at deficient levels during AD (Figure4c) and did not go beyond 10 mg /kg even in the inlet section. Similar concentrations of butyric acid were reported by Kor-Bicakci et al. [37] for the anaerobic digestion of mixed sludge in thermophilic conditions (55 ◦C). The ss-OFMSW AD was also stable. The content of acetic acid ranged from 1.0 to 1.2 g/kg; the concentration of propionic acid in the inlet section was between 15 and 32 mg/kg (Figure7a,b). Both acid levels were higher than in ms-OFMSW, which confirms the higher organic matter content (Tables1 and2). Furthermore, in the outlet section, the acetic acid content reached about 10 mg /kg (Figure7a), which may confirm that the organic matter did not fully decompose. Thus, the inoculation volume should be increased. The propionic acid content was nearly zero, similar to ms-OFMSW AD (Figures 7b and5b). In the inlet section, the butyric acid content was about four to five times higher than in the digestion of ms-OFMSW (Figures5c and7c). A higher amount of organic acids may also confirm the higher content of green waste in ss-OFMSW and organic matter in general (Tables1 and2). The valeric and isovaleric acid contents were nearly zero during AD. Energies 2020, 13, x FOR PEER REVIEW 11 of 16

5500 1100 5450 1050 5400 1000 5350

COOH/kg) 950

5300 3 5250 900 5200 850 5150 800 5100 1234

Ammonium ions (mg/kg) ions Ammonium 1234

Process time (week) (mg CH Acidity Process time (week) Inlet Middle Outlet Inlet Middle Outlet

9500 /kg)

3 9000

8500

8000

7500 1234

Alkalinity (mg CaCO Alkalinity (mg Process time (week) Inlet Middle Outlet

(e)

Figure 6. Parameter changes in the course of the segregated at source organic fraction of municipal solid waste anaerobic digestion process at full scale: (a) pH value; (b) suspended solids; (c) ammonium ion concentration; (d) acidity; (e) alkalinity. Energies 2020, 13, 3768 12 of 14

1200 40 40 30 30 1000 20 20 10 800 10 0

Propionic acid 1234

Inlet (mg/kg) 600 0 concentration (mg/kg) 1234 Process time (week) Process time (week) Inlet Middle Outlet Acetic acid concentration- acid Acetic Acetic acid concentration- acid Acetic Inlet Middle Outlet (mg/kg) Outlet and Middle Energies 2020, 13, x FOR PEER REVIEW (b) 12 of 16 (a)

25 20 15 10 5 0 Butyric acid Butyric 1234

concentration (mg/kg) concentration Process time (week) Inlet Middle Outlet

(c)

FigureFigure 7.7. TheThe changes in in the the concentration concentration of offatty fatty acids acids in inthe the course course of the of thesegregated segregated at source at source organicorganic fraction fraction ofof municipal solid solid waste waste anaerobic anaerobic digestion digestion at atfull full scale: scale: (a) acetic (a) acetic acid; acid; (b) propionic (b) propionic acid;acid; ( c(c)) butyric butyric acid.acid.

4. ConclusionsThe pH value is considered to be the factor stabilizing anaerobic digestion [5,31]. Generally, the pH Thevalue performed ought to researchbe between confirmed 7.2 and that7.8. It AD mi isght, an ehowever,ffective processbe influenced of ms-OFMSW by temperature and ss-OFMSW and sampling, and it might be unique to the particular facility [7,33]. During testing the ms-OFMSW AD, treatment. It allows for recovering energy from waste. The obtained results confirmed that the the pH-value was 8.5 in the inlet section, with slight growth to 8.6 in the outlet (Figure 4a), which was performance of AD with ss-OFMWS, in a full-scale AD plant, is better in comparison to ms-OFMSW, typical for the examined MBT plant. Regarding the ss-OFMSW AD, the pH value was 8.3–8.4 in the from the perspective of renewable energy generation. The obtained biogas yield per ton was slightly inlet section, with slight growth to 8.6 in3 the outlet (Figure 6a). The pH value decrease3 is usually highercaused in by the the case organic of ss-OFMSW overload (111.1 of the m /digesterton), compared and might to ms-OFMSW result in reducing (105.3 m the/ton), activity followed of by themicroorganisms, higher methane causing concentration: the decomposition 58–60% of and volatile 51–53%, fatty respectively. acids [5,31]. An Yearly alkalinity electricity increase production (pH capacityvalues above was almost8.0) affects 700 the MWh NH higher3 and NH (3285.44+ dissociation vs. 3932.9 bala MWh)nce. Considering for ss-OFMSW the higher digestion. ammonium However, theion AD concentration performance (of withabout ss-OFMWS 1g/kg) during still the has AD to beof verified,ss-OFMSW, mainly compared at full to scale, ms-OFMSW considering AD the agricultural(Figures 4c useand of6c), by-products. it was expected to achieve higher pH values. The higher acidity, together with lowerThe acidity obtained in ms-OFMSW VFA concentrations, AD (Figure at 4d,e) levels than around in ss-OFMSW 1.2 g/kg, pHAD values(Figure (slightly 6d,e), may above explain 8.0), the acidity, andobservations. alkalinity, Nevertheless, indicate the possibilities mostly because of the of this digester digester feeding buffer and capacity, no-risk pH exploitation changes are of normally using either asdetectable feedstock. only at the point when the process is already unstable. Hence, the determination of low fattyAD acid is (acetic, a suitable propionic, treatment butyric, method and valeric) for both concentrations OFMSWs. (with It canratios) be is found the source that, of comparedthe most to ms-OFMSW,reliable information ss-OFMSW regarding brings morethe stability benefits: for higher digestion biogas [34]. yield The and electricityprocess of productionproducing potential,and asaccumulating well as lower volatile pre-treatment fatty acids costs. (VFAs) Moreover, could theinhibit by-products anaerobic of digestion, ss-OFMSW which AD could seem toresult have in more organicslowing recycling the production possibilities. of biogas The [28]. digestate and the effluents from ss-OFMSW AD might have an agriculturalThe obtained application, VFA concentrations, while the ms-OFMSW at levels around digestion 1.2 mightg/kg, indicate generate the almost possibilities the same of digester quantity of feeding and no-risk exploitation. There is an optimum loading rate for a given size of the digestion waste. Further research on the use of the digestate from ss-OFMSW AD systems is recommended, as it chamber, which cannot be exceeded. Otherwise, the loading rate increment could stop being effective, might be possible to recover the nutrients for agricultural purposes. Nonetheless, the environmental because it might lead to an accumulation of excess VFAs and subsequently to reactor collapse [28]. safety of such solutions should be assessed prior to wide application. The ms-OFMSW AD was stable. The acetic acid content amounted to 0.87–1.0 g/kg, with a propionic acid concentration of between 5 and 10 mg/kg in the inlet section (Figure 5a,b). In the outlet section, the acids were nearly zero, which may confirm full organic matter decomposition (Figure 5a,b). Owusu-Agyeman et al. [35] also reported that acetic acid is the most abundant VFA in the anaerobic digestion of organic waste. However, the share of acetic acid decreased with time [35] and its relative abundance was much lower than in this study. Differences in the composition could be attributed to the fact that the AD temperature of the batch was much lower (35 °C) [35], as well as to differences in the composition of the feedstock. The organic waste in the study of Owusu-Agyeman et al. [35] consisted of alcohol and soda beverage, food, dairy, fruit, fat, and oil waste. Tampio et al. [36] reported that the acetic acid fraction dominated only in the first period of the anaerobic digestion of food waste, which was then surpassed by butyric acid. Additionally, in this case, mesophilic conditions (37 °C) [36] could be considered as a plausible explanation for the obtained difference. The butyric and valeric acid contents were at deficient levels during AD (Figure 4c) and did not go beyond

Energies 2020, 13, 3768 13 of 14

Author Contributions: P.S. and M.K. conceived and designed the experiments; P.S., A.S., and Ł.N. performed the experiments; P.S., H.P.-K., and A.U. analyzed the data; H.P.-K. and A.U. contributed reagents/materials/analysis tools; P.S., M.K., A.S., and Ł.N. wrote and revised the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by Zakład Gospodarki Odpadami GAC´ in Poland. The proofreading for English language was supported by David Harrold. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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