energies

Article The Enhancement of Energy Efficiency in a Plant through Sustainable Biogas Use: Case Study from Poland

Adam Masło ´n 1,* , Joanna Czarnota 1 , Aleksandra Szaja 2, Joanna Szul˙zyk-Cieplak 3 and Grzegorz Łagód 2,* 1 Department of Environmental Engineering and Chemistry, Rzeszow University of , Powsta´nców Warszawy 6, 35-959 Rzeszów, Poland; [email protected] 2 Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka 40B, 20-618 Lublin, Poland; [email protected] 3 Fundamentals of Technology Faculty, Lublin University of Technology, Nadbystrzycka 38, 20-618 Lublin, Poland; [email protected] * Correspondence: [email protected] (A.M.); [email protected] (G.Ł.); Tel.: +48-17-865-1278 (A.M.); +48-81-538-4322 (G.Ł.)  Received: 30 September 2020; Accepted: 13 November 2020; Published: 19 November 2020 

Abstract: The improvement of energy efficiency ensuring high nutrients removal is a great concern for many wastewater treatment plants (WWTPs). The energy balance of a WWTP can be improved through the application of highly efficient digestion or its intensification, e.g., through the introduction of the co-substrates with relatively high energy potential to the sludge (SS). In the present study, the overview of the energetic aspect of the Polish WWTPs was presented. The evaluation of energy consumption at individual stages of wastewater treatment along with the possibilities of its increasing was performed. Additionally, the influence of co-digestion process implementation on the energy efficiency of a selected WWTP in Poland was investigated. The evaluation was carried out for a WWTP located in Iława. Both energetic and treatment efficiency were analyzed. The energy balance evaluation of this WWTP was also performed. The obtained results indicated that the WWTP in Iława produced on average 2.54 GWh per year (7.63 GWh of electricity in total) as a result of the co-digestion of with poultry processing . A single cubic meter of co-substrates fed to the digesters yielded an average of 25.6 4.3 Nm3 of biogas (between 18.3 and 32.2 Nm3/m3). ± This enabled covering the energy demand of the plant to a very high degree, ranging from 93.0% to 99.8% (98.2% on average). Importantly, in the presence of the co-substrate, the removal efficiency of organic compounds was enhanced from 64% (mono-digestion) to 69–70%.

Keywords: energy efficiency and security; sustainable energy policy; biogas; co-digestion; sewage sludge

1. Introduction Currently, a sustainable energy policy is the main strategy that has contributed to increasing the global energy and environmental security. One of its pillars is the improvement of the energy efficiency of the existing facilities. The greatest recipient of electricity in is the water and sewerage , which is responsible for even up to 40% of total consumption. The entire water management sector corresponds to approximately 4% of global electricity consumption. The water demand, and thus the amount of wastewater produced are on an increase; therefore, the actions aimed at improving the energy efficiency in the sector are necessary. Importantly, electricity constitutes even up to 40% of the operating budget of water companies and about 20% of the costs related to the supply

Energies 2020, 13, 6056; doi:10.3390/en13226056 www.mdpi.com/journal/energies Energies 2020, 13, 6056 2 of 21 and treatment of potable water [1]. Additionally, a further increase in the energy consumption in municipal water supply systems is predicted over the next 15 years, amounting to 60–100% [2]. Wastewater treatment plants (WWTPs) are an indispensable element of a municipal system. These facilities require the supply of substantial amounts of electricity, which is necessary for wastewater transport, conducting technological processes and the operation of administrative objects [1]. In particular, maintaining the stability of technological processes represents a significant share of the operational costs of an object [3]. Therefore, there is still a need for the solutions improving the efficiency of recovery and production of the electrical and thermal energy at WWTPs. For this reason, innovative wastewater treatment based on nitrogen removal using the Anammox process [4] or granulated [5] are being developed. These systems are characterized by lower energy consumption, as compared to the conventional technological solutions. Additionally, the application of various operational and control strategies of the aeration system as well as optimization of energy consumption involving a complex automatic system for data collection, simulation, and prediction, are practiced [6–8]. Another possibility may be the application of waste such as sewage sludge to produce an alternative source of energy (biogas). It is a mixture containing methane (CH4) and carbon dioxide (CO2) that is generated in the process (AD) [9,10]. Biogas may be easily used as fuel to generate heat or electricity at WWTPs [11,12]. It should also be noted that the treatment and disposal of sewage sludge is related with a significant financial and energy expenditure for WWTPs. The sludge processing accounts for 40–60% of the total operational costs [13]. Therefore, over the recent years, the sewage sludge management systems have changed significantly, especially in relation to the circular economy [14]. There are various methods to increase the biogas production in WWTPs. One of the most promising is the implementation of an anaerobic co-digestion process (AcoD). In this strategy, an adequate selected substance is added to the main substrate to improve the AD process efficiency and stability. Thus far, several examples of the AD process application have been reported [15–17]. However, in Poland, the adoption of the co-digestion aimed at balancing the C/N ratio to increase the biogas production is not widespread mainly due to numerous operational difficulties. Commonly, the results of modeling and lab-scale investigations do not comply with a technical scale. Operators must take into account the financial aspect, process efficiency, as well as legal regulations. The novel aspect of the study involved presenting the complex and multi-threaded analysis of implementation the co-digestion strategy of a selected WWTP. The evaluation includes both the energetic and technological aspects. The presented information might contribute to increasing the knowledge in this field and might allow for more common use of this strategy under Polish conditions. Thus far, such evaluation has not been considered in the case of the Polish WWTPs. The presented results and experiences constitute an innovative material in the field of renewable energy sources in a wastewater industry. The successful application of this strategy depends on a variety of factors, including the type and dose of co-substrate as well as the appropriately selected operating conditions. Importantly, supplementation of the feedstock with an improper substrate might bring many negative effects, contributing to the breakdown of the process and the decreasing of biogas production [18]. Importantly, the AD process might be implemented at the WWTPs with the daily flow greater than 20,000 m3/d. However, for smaller WWTPs, this technology might be unprofitable. For such facilities, the cost related with construction and maintenance of anaerobic digesters is significant. Importantly, in this case, sewage sludge might be dehydrated (total solids content (TS) > 15%) and then transported to a local High-Solid Anaerobic Digestion Reactor, where it could be treated/digested with other biodegradable . This strategy might be considered as a cost-effective solution for small WWTPs [19]. Thus, the actions aimed at implementing the low-energy or passive WWTPs are being taken [20]. These result in a reduction of the operational costs, improvement of stability and reliability of the wastewater treatment processes, as well as mitigation of the WWTP impact on the aquatic environment [21–23]. Energies 2020, 13, 6056 3 of 21

EnergiesIn the2020, present13, x FOR PEER study, REVIEW the overview of energetic aspect of WWTP functioning in3 Polandof 22 was shown. The authors indicated the energy consumption at individual stages of WWTP along with the possibilitiesIn the presentof increasing study, the overview energy e ffi of ciency. energetic Moreover, aspect of WWTP the current functioning situation in Poland in terms was of biogas productionshown. The and authors its utilizationindicated the at energy Polish consumption WWTPs was at individual evaluated. stages Additionally, of WWTP along the influencewith the of the implementationpossibilities of increasing of a co-digestion the energy process efficiency. on Moreover the energy, the e fficurrentciency situation of a selected in terms WWTP of biogas in Poland wasproduction investigated and its in utilization this study. at Polish For this WWTPs purpose, was evaluated. the energy Additionally, consumption the atinfluence individual of the stages of implementation of a co-digestion process on the energy efficiency of a selected WWTP in Poland was treatment was analyzed. An energy balance evaluation of WWTP in Iława was performed as well. investigated in this study. For this purpose, the energy consumption at individual stages of treatment The justifiability of using the biogas from the co-digestion of sewage sludge in terms of improving the was analyzed. An energy balance evaluation of WWTP in Iława was performed as well. The energyjustifiability efficiency of using was the analyzed. biogas from By investigatingthe co-digestion the of energy sewage intensitysludge in ofterms processes of improving and biogas the usage withinenergy WWTPs, efficiency conducting was analyzed the. By research investigating on the the topic energy was intensity justified. of processes Attention and was biogas also usage drawn to the energywithin potential WWTPs, ofconducting the sewage the sludgeresearch generated on the topic in thewas Polish justified. WWTPs, Attention taking was intoalso drawn account to thethe possible increaseenergy of potential the biogas of the production. sewage sludge generated in the Polish WWTPs, taking into account the possible increase of the biogas production. 2. Energy Intensity and Biogas Energy Technology of WWTPs 2. Energy Intensity and Biogas Energy Technology of WWTPs 2.1. Energy Consumption in the Technological Wastewater Treatment Processes 2.1. Energy Consumption in the Technological Wastewater Treatment Processes Electricity is used in all stages of treatment that involve electric drive systems in the form of electric engines.Electricity Electric is enginesused in all are stages used of for treatment compressing, that involve pumping, electric and drive transporting systems in the liquids form andof gases electric engines. Electric engines are used for compressing, pumping, and transporting liquids and using pumps, fans, and compressors; moreover, they are used in mixers, presses, and other devices gases using pumps, fans, and compressors; moreover, they are used in mixers, presses, and other for processing the wastes separated from wastewater (screenings, sand, and sewage sludge) [24]. devices for processing the wastes separated from wastewater (screenings, sand, and sewage sludge) Electric[24]. Electric drive systems drive systems are employed are employed in each in stage each of stage wastewater of wastewater treatment; treatment; thus, all thus, technological all operationstechnological such operations as mixing, such aeration, as mixing, and aeration, pumping, and being pumping, part of being the wastewater part of the wa treatmentstewater system, afftreatmentect the consumption system, affect the of electricityconsumption [7 ].of Inelectricity turn, the [7]. thermalIn turn, the energy thermal is usedenergy in is the used sewage in the sludge processing—insewage sludge processing digesters,— sludgein digesters, dryers, sludge etc. dryers, In a wastewater etc. In a wastewater treatment treatment plant, electricalplant, electrical and thermal energiesand thermal are alsoenerg usedies are for also the used social for needs the social of employees, needs of employees, heating of heating technical of technical and administrative and buildings,administrative lighting buildings, the premises, lighting the etc. premises, Therefore, etc. the Therefore, wastewater the wastewater treatment treatment system should system also be consideredshould also as be a thermal–energetic considered as a thermal system–energetic [1,2,21]. An system exemplary [1,2,21]. structure An exemplary of electricity structure consumption of electricity consumption in a WWTP located in southern Poland is presented in Figure 1. The analysis in a WWTP located in southern Poland is presented in Figure1. The analysis of electricity consumption of electricity consumption in particular technological elements of the WWTP enables to determine in particular technological elements of the WWTP enables to determine the energy consumption the energy consumption structure and thus its rationalization [2]. structure and thus its rationalization [2].

Mechanical treatment 74.0 % Pumping

Biological treatment

Sewage sludge treatment 6.5 % 8.0 % 10.9% Other

0.6%

FigureFigure 1. 1. ElectricityElectricity consumption consumption in inparticular particular parts parts of a wastewater of a wastewater treatment treatment plant (WWTP plant (WWTP)) located located inin southern southern PolandPoland [based [based on on authors’ authors’ own own work work].].

TheThe municipal municipal wastewater wastewater treatment treatment process process involves involves two two stages—mechanical stages—mechanical (removal (removal of floating, andfloating, suspended and suspended solids) and solids) biological and biological treatment treatment (removal (removal of organic of organic pollutants, pollutants, as well as aswell nitrogen as and phosphorusnitrogen and compounds). phosphorus compounds). Additionally, Additionally, each stage each includes stage includes wastewater wastewater pumping. pumping. The The mechanical treatmentmechanical involves treatment the removalinvolves of the screenings, removal of mineral, screenings, and organic mineral, solids. and organic These processes solids. These are realized inprocesses screens, grit are realizedchambers, in asscreens, well as grit primary chambers settling, as well tanks. as primary They are settling characterized tanks. They by aare relatively low consumption of electricity and usually consume less than 1% of the energy used by the entire Energies 2020, 13, x FOR PEER REVIEW 4 of 22 Energies 2020, 13, 6056 4 of 21 characterized by a relatively low consumption of electricity and usually consume less than 1% of the WWTPenergy [used25]. Itby mainly the entire depends WWTP on [2 the5]. It employed mainly depends devices on and the characteristicemployed devices of influent and characteristic wastewater asof wellinfluent as the wastewater adopted operatingas well as the conditions. adopted Theoperating consumption conditions of. energyThe consumption for the pumping of energy of rawfor wastewaterthe pumping is determinedof raw wastewater by the is pumping determine heightd by andthe p rangesumping from height 0.02 and to 0.37ranges kWh from/m3 0.02[26]. to In 0.37 the 3 biologicalkWh/m [ treatment26]. In the ofbiological wastewater, treatment the aeration of wastewater of nitrification, the aeration chambers of nitrification is the most energy-intensivechambers is the 3 process,most energy requiring-intensive between process, 0.18 requiring and 0.8 kWh between/m3; thus,0.18 and it consumes 0.8 kWh/m of; up thus, to 75%it consum of energyes of withinup to this75% stage of energy of treatment within [this7,21 stage]. The of consumption treatment [7, of21 energy]. The c foronsumption aeration canof energy be reduced for aeration through can smart be controlreduced [7 through]. Apart smart from control the aeration, [7]. Apart the from mixing the aeration, of activated the mixing sludge of and activated sludge sludge recirculation and sludge also requirerecirculation substantial also quantitiesrequire substantial of energy. quantities A literature of review energy. indicates A literature that thereview conventional indicates activated that the conventional activated sludge process consumes between 0.27 and 0.6 kWh/m3 [24]. sludge process consumes between 0.27 and 0.6 kWh/m3 [24]. An inherent part of a WWTP is the processing and disposal of sewage sludge. The energy An inherent part of a WWTP is the processing and disposal of sewage sludge. The energy demand demand of sludge processing is related with the WWTP type of employed technology as well as the of sludge processing is related with the WWTP type of employed technology as well as the type type of devices. The energy demand of sludge management in a WWTP ranges from 5 to 30% of the of devices. The energy demand of sludge management in a WWTP ranges from 5 to 30% of the energy consumed in a plant [20]. Depending on the WWTP size, the thickening, aerobic or anaerobic energy consumed in a plant [20]. Depending on the WWTP size, the thickening, aerobic or anaerobic stabilization (digestion) and sludge dewatering processes are involved. At large WWTPs, the stabilization (digestion) and sludge dewatering processes are involved. At large WWTPs, the anaerobic anaerobic digestion is the most widely applied process, and additional drying or combustion of digestion is the most widely applied process, and additional drying or combustion of sewage sludge sewage sludge are also performed. The biogas produced in the AD process may be used as a are also performed. The biogas produced in the AD process may be used as a replacement for a fossil replacement for a fossil fuel [9,27,28]. Its generation from sewage sludge for energy production is fuel [9,27,28]. Its generation from sewage sludge for energy production is profitable at the WWTPs profitable at the WWTPs receiving 8000–10,000 m3 of wastewater per day. Usually, the sludge receiving 8000–10,000 m3 of wastewater per day. Usually, the sludge management operates on a cyclic management operates on a cyclic basic; thus, it can be easily optimized in terms of the electrical or basic; thus, it can be easily optimized in terms of the electrical or thermal energy consumption. thermal energy consumption. FigureFigure2 2presents presents a a scheme scheme of of a a conventional conventional WWTP WWTP >> 100100,000,000 PE PE size size.. Therein, Therein, the the objects objects with with highhigh electricityelectricity consumptionconsumption asas wellwell asas thethe devicesdevices potentially generated and and recovered recovered electrical electrical andand/or/or thermal thermal energyenergy areare illustrated.illustrated. The most significantsignificant energy consumption occurs occurs during during the the activatedactivated sludgesludge aerationaeration (A),(A), pumpingpumping (B and D D),), and mixing mixing of of activated activated sludge sludge (C) (C).. Moreover, Moreover, energyenergy consumption consumption is is related related with with maintaining maintaining optimum optimum temperature temperature in digesters in digesters (E) and (E) the and drying the ofdrying sewage of sludgesewage (F).sludge In turn, (F). In the turn, energy the gainenergy in gain a WWTP in a WWTP involves involves the application the applicat of biogasion of biogas for the productionfor the production of electricity of electricity in Combined in Combined Heat andHeat Power and Power (CHP)—(Ga) (CHP)—(Ga) generators generators/co/co-generators-generators and thermaland thermal energy energy generation generation in boilers in (Gb)boilers as well(Gb) as as production well as production of thermal of energy thermal from energy the combustion from the ofcombustion dried sludge of dried (H). sludge (H).

FigureFigure 2. 2. SchemeScheme ofof a a WWTP WWTP showingshowing the objects contributing to to the the consumption consumption and and gains gains of of electricalelectrical and and thermal thermal energyenergy (authors’(authors’ ownown elaborationelaboration based on Dymaczewski and Szulc Szulc [2 [299]]).).

Energies 2020, 13, 6056 5 of 21

However, sewage sludge has the greatest potential of energy recovery that can positively influence the energy balance of the entire plant [30]. The recovery of energy from sewage sludge can be performed in two ways. The first involves the application of the biogas produced in the anaerobic digestion process, whereas in the second, sewage sludge is combusted in thermal installations. In some cases, sewage sludge has to be dried beforehand, and external fuel has to be supplied, which negatively impacts the process economy. It is also possible to employ a solar dryer. The energy savings in WWTPs are mainly connected with the improvement or replacement of the energy-intensive devices (pumps, mixers, air blowers) as well as the implementation of smart control and monitoring systems for the wastewater treatment processes [7]. Furthermore, the possibility of electrical and thermal energy production from sewage sludge (e.g., thermal hydrolysis, digestion, co-digestion, sludge combustion), as well as the recovery of energy from wastewater (heat pumps), should be also considered. The application of innovative wastewater treatment and sewage sludge processing methods with a simultaneous rationalization of energy consumption enables achieving the self-sufficiency of a facility in terms of the energy demand. However, in order to meet the objectives of sustainable development and the circular economy, the production of electrical and thermal energy exceeding the needs of the object is required. The surplus energy can be transmitted to a distribution network. Additionally, intensification of the electricity production can be achieved through the application of photovoltaic panels, as well as wind and water turbines localized at the area of the WWTP [2]. The most common potential energy savings in a WWTP are presented in Table1.

Table 1. Distribution of energy consumption and potential for energy saving in a selected WWTP with an activated sludge system [31].

Share in Energy Treatment Stage Potential for Energy Saving Comments Consumption [%] 5–10% through the modification depending on the Wastewater Pumping of the existing pumps; up to 30% 10 topography where () through better conservation and WWTP is located adjustment to the throughput 20–50% through the optimization of technological parameters, mainly pertains to Biological Treatment of 55 optimization of aeration and aeration and mixing in Wastewater mixing in biological reactors the reactor implementation of online control 30% of energy efficiency can be Processing and Disposal achieved through the application determined by operating of Sewage Sludge 35 of mesophilic anaerobic digestion conditions and the type (Dewatering and with implementation of of selected substrate Transport of Sludge) additional substrate

In addition to the above-mentioned examples, some WWTPs also use advanced technologies to enhance the energy efficiency. The pre-processing of sludge (conditioning and disintegration) or the anaerobic stabilization process may improve the energy efficiency of WWTPs even by up to 50%. Further implementation of integrated co-digestion processes and highly efficient co-generation may increase the efficiency by up to 80% [31]. However, due to significant investment costs, these technologies are not implemented in many Polish WWTPs.

2.2. Biogas Production from Sewage Sludge in a Wastewater Treatment Plant Taking into account the energy balance of a wastewater treatment plant, the anaerobic digestion of sewage sludge is the most beneficial solution. The biogas from sewage sludge can be employed for the production of heat, as well as mechanical and electrical energy. The typical methods of biogas utilization include the following: (1) combustion in heating boilers producing the heat for the heating, (2) combustion in gas engines coupled with the generators producing mechanical energy Energies 2020, 13, 6056 6 of 21 powering pumps, and (3) combustion in gas engines or gas turbines coupled with power generators, producing electricity powering the devices in a WWTP. Each of the above-mentioned methods also generates thermal energy, which may be used for technological purposes (e.g., maintaining appropriate temperature in digesters) or hot water preparation (CHP process) [9,10,27,32,33]. However, the biogas application depends mainly on its characteristics. It consists of 60–67% methane, 30–33% carbon dioxide, 1–2% hydrogen, and 0–3% nitrogen (v/v). Moreover, biogas can additionally contain small amounts of hydrogen sulfide, ammonia, and hydrocarbons [34]. Nevertheless, the detailed composition is determined by the properties of sewage sludge and operational parameters. Depending on the methane content in the biogas, its calorific value approximates 23.4 MJ/m3, while the removal of carbon dioxide and other pollutants further improves this value [9]. Currently, one of the priorities in anaerobic digestion technology is the intensification of biogas production as well as improvement of the process stability. Those might be achieved through the application of a properly selected substrate to the sewage sludge [21,35–37]. Moreover, the utilization of various pre-treatment methods including mechanical (ultrasonic, microwave, electro-kinetic and high-pressure homogenization), thermal, chemical (acidic, alkali, ozonation, Fenton and Fe(II)-activated persulfate oxidation) as well as biological approaches (enzyme application) has been reported [38–43]. However, the co-digestion of sewage sludge with other wastes is considered as the most promising solution. It is defined as an anaerobic simultaneous degradation of two or more appropriately selected substrates [15,36]. As compared to the previously mentioned pre-treatment methods, it is characterized by relatively low cost of implementation. Moreover, it should be noted that in the co-digestion process, various organic wastes are used as co-substrates. In this way, an increase in biogas production as well as management of various groups of waste may be achieved. Importantly, this strategy involves the untapped biogas potential of the existing digesters [44]. Therefore, the AcoD process strategy is consistent with the principles of sustainable development [45–47]. As opposed to mono-digestion, co-digestion presents numerous advantages, such as improvement of the biogas and methane production, amelioration digestion efficiency, as well as enhancement of the process stability. However, a crucial element is the selection of substrates with complementary composition to the main feedstock. These components should ensure the required nutrient balance in feedstock [48] and improve the C/N ratio as well as dilute inhibitory or toxic compounds in the main feedstock [35,48,49]. Additionally, wastes with high biogas potential and significant micro and macro elements content are preferred. Sewage sludge is characterized by an unfavorable C/N ratio, low dry matter content, as well as relatively low biogas potential [15,50,51]. Therefore, it is should be co-digested with the substrates containing significant amounts of easily biodegradable organic matter and characterized by high buffer capacity. The presence of various micro and macro elements, deficient in sewage sludge, is also highly demanded. Moreover, the cost of transport and possible pre-treatment of substrates should be considered before the implementation of this strategy. Taking into consideration the above-mentioned facts, the most widespread is the application of the following substrates: organic fraction of (OFMSW); fat, oil, and grease wastes (FOG); various agro-industrial by-products (e.g., fruit and vegetable waste, glycerol and slaughterhouse wastes) [52,53]. Among the mentioned substrates, the use of wastes characterized by significant fat content is of great interest. These by-products originate from the food industry, including slaughterhouses, meat processing plants, and dairies, as well as facilities producing fats and oils. Moreover, this waste can also be sourced from restaurants as well as municipal and industrial WWTPs, in which flotates are separated [35,36,54,55]. Their widespread use in co-digestion with sewage sludge is due to the high biogas potential (0.7 to 1 m3/kg VS), significant content of biodegradable organic matter, and a beneficial C/N ratio [50,55,56]. However, the application of this co-substrate may cause several operational problems related to the clogging of digesters and pipes as well as the appearance of foam in the anaerobic reactor [57,58]. Additionally, the inhibition of long-chain fatty acids (LCFA) may be observed in the FOG presence [56]. However, there are many successful examples of use of such wastes in co-digestion with sewage sludge in terms of biogas and energy production [35,50,56,59,60]. Energies 2020, 13, 6056 7 of 21

Table2 presents certain biogas properties with the operational conditions of various organic substances characterized by significant fat content that can be co-digested with sewage sludge. These data pertain not only to the laboratory studies but also to the full-scale implementation of co-digestion [32,61].

Table 2. The maximum biogas production and methane content in biogas for different organic co-substrates [60].

Maximum Biogas Production Methane Content Content of Substrate in Organic Co-Substrates (mL/g VS) in Biogas (%) the Feedstock (v/v %) Waste meat 170.3 76 3 Soapstock 117.17 73 10 By-products after 99.91 72 10 biodiesel production Molasses 95.69 73 0.5 Waste from fat separator 88.68 70 3

3. Biogas Utilization at WWTPs in Poland In recent years, a dynamic development and expansion of sewerage networks and municipal WWTPs has been observed in Poland. This leads to the generation of substantial amounts of municipal sewage sludge. According to the statistical data, 447,000 Mg TS of municipal sewage sludge was produced in Poland in 2003, whereas in 2018, it increased to 583,000 Mg TS [62]. The principles for the processing and final management of municipal sewage sludge are regulated both by the EU and Polish law [63–66]. The current energy demand of WWTPs in Poland is approximately 1 TWh/year [24]. Compared to other countries, it constitutes a relatively low value. The consumption of electricity by all WWTPs in Europe was calculated as 27 TWh/year [67]. A constant development of the municipal infrastructure indicates an increase in the energy consumption for WWTP in Poland, even up to 5.5 TWh/year, which will comprise ca. 2.5–3.5% of the total country electricity demand [2]. Currently, there are almost 2500 WWTPs in Poland; however, only 140 are equipped with the installations to conduct the anaerobic digestion of sewage sludge (Figure3). This means that only 6% of the WWTPs operating in Poland are capable of producing biogas. The total volume of digesters in all WWTPs is estimated at 800,000–900,000 m3. Nevertheless, not every plant utilizes the produced biogas for the generation of thermal and/or electrical energy. Only 112 plants are equipped with biogas-powered generators, a single plant boasts a steam turbine, and another is equipped with a thermoelectric generator for power generation from the exhaust gases [68]. The total output of the power generators in Polish WWTPs amounts to 72 MWel. The production of electricity in WWTPs equals 336.5 GWh (Figure4). In 2018, the total electricity demand in Poland amounted to 172 TWh /year [62]. On the basis of the total amount of processed sludge in the AD processes, it can be stated that the energy potential of sewage sludge is used in less than 40% [69]. For comparison, in Croatia, approximately 30% of sewage sludge is converted into biogas through the AD process [70]. Nevertheless, there is still a potential for the construction of anaerobic digesters in Poland. It is estimated that as many as 170 installations for biogas production may be built in the Polish WWTPs [69]. This potential can be increased further through the application of new technologies, leading to the enhancement of the biogas production (e.g., thermal hydrolysis). That may constitute over 1% of energy production in Poland, which would cover the total energy demand of all the currently operated WWTPs. It is estimated that even up to 0.5–0.7 billion Nm3 of the biogas can be generated at WWTPs [68]. Comparing these data to the natural gas production in Poland (4.6 billion Nm3 in 2018) [71], it constitutes a substantial amount of generated fuel. In turn, the total natural gas consumption in Poland amounts to 19.7 billion Nm3 (data for 2018) [72]. In addition, the biogas installations in WWTPs can act as storage power stations due to the ease of biogas storage; in addition, its production is stable and related to the economic activity in urbanized areas. Energies 2020, 13, x FOR PEER REVIEW 8 of 22

Energies 2020, 13, x FOR PEER REVIEW 8 of 22 operated WWTPs. It is estimated that even up to 0.5–0.7 billion Nm3 of the biogas can be generated at WWTPs [68]. Comparing these data to the natural gas production in Poland (4.6 billion Nm3 in operated WWTPs. It is estimated that even up to 0.5–0.7 billion Nm3 of the biogas can be generated 2018) [71], it constitutes a substantial amount of generated fuel. In turn, the total natural gas at WWTPs [68]. Comparing these data to the natural gas production in Poland (4.6 billion Nm3 in consumption in Poland amounts to 19.7 billion Nm3 (data for 2018) [72]. In addition, the biogas 2018) [71], it constitutes a substantial amount of generated fuel. In turn, the total natural gas installations in WWTPs can act as storage power stations due to the ease of biogas storage; in consumption in Poland amounts to 19.7 billion Nm3 (data for 2018) [72]. In addition, the biogas addition, its production is stable and related to the economic activity in urbanized areas. Energiesinstallations2020, 13, 6056 in WWTPs can act as storage power stations due to the ease of biogas storage8 of 21; in addition, its production is stable and related to the economic activity in urbanized areas.

Figure 3. WWTPs in Poland utilizing the anaerobic digestion of sewage sludge, data from [ 69].

FigureFigure 3. 3.WWTPs WWTPs in in Poland Poland utilizing utilizing the the anaerobic anaerobic digestion digestion of of sewage sewage sludge, sludge data, data from from [69 [].69].

450 400 450 350 400 300 350 250 300 200 250 150 200 100 Electricity [GWh] Electricity 150 50 100 Electricity [GWh] Electricity 0 50 2014 2015 2016 2017 2018 0 Year 2014 2015 2016 2017 2018 Figure 4. Production of electricity from biogas in WWPSYear in 2014–2018, authors’ own elaboration based Figure 4. Production of electricity from biogas in WWPS in 2014–2018, authors’ own elaboration based on statistical data [62]. on statistical data [62]. Figure 4. Production of electricity from biogas in WWPS in 2014–2018, authors’ own elaboration based 4. Materialson statistical and Methods data [62].

4.1. Description of the Plant

4.1.1. General Characteristics of the Wastewater Treatment Plant in Iława The WWTP in Iława involves mechanical and biological treatments. It is located in the warmi´nsko-mazurskievoivodeship (northeast of Poland). This facility serves approximately 32.5 thousand inhabitants. It was designed for an average daily wastewater flow of 26,940 m3/d (maximum daily flow—31,115 m3/d) and population equivalent equal to 154,117. The WWTP in Iława was constructed in 1991 and has been modernized several times. The mechanical part of the WWTP includes screens, grit chambers, and grease traps as well as primary clarifiers. In turn, the biological treatment is conducted in nitrification and denitrification chambers as well as the chambers for biological Energies 2020, 13, x FOR PEER REVIEW 9 of 22

4. Materials and Methods

4.1. Description of the Plant

4.1.1. General Characteristics of the Wastewater Treatment Plant in Iława The WWTP in Iława involves mechanical and biological treatments. It is located in the warmińsko-mazurskie voivodeship (northeast of Poland). This facility serves approximately 32.5 thousand inhabitants. It was designed for an average daily wastewater flow of 26,940 m3/d (maximum daily flow—31,115 m3/d) and population equivalent equal to 154,117. The WWTP in Iława was constructed in 1991 and has been modernized several times. The mechanical part of the WWTP Energies 2020, 13, 6056 9 of 21 includes screens, grit chambers, and grease traps as well as primary . In turn, the biological treatment is conducted in nitrification and denitrification chambers as well as the chambers for phosphatebiological removal.phosphate Moreover, removal. this Moreover, part includes this part radial includes secondary radial clarifiers. secondary The clarifiers. sludge management The sludge linemanagement comprises li rawne comprises sludge pumping raw sludge stations, pumping two gravitational stations, two thickeners gravitational for the thickeners primary for sludge, the aprimary mechanical sludge, thickening a mechanical station thickening for excess station sludge for from excess secondary sludge clarifiers from secondary (two drum clarifiers thickeners) (two 3 3 3 3 withdrum the thickeners) output of with Qv = the12 output m /h, of two Qv closed = 12 m digesters/h, two closed with digesters the capacity with of the 1885 capacity m each, of 1885 sludge m decantereach, sludge centrifuge decanter for dewateringcentrifuge for the dewatering digested sludge, the digested and a solarsludge sludge, and dryer.a solar The sludge treated dryer. effl uentThe istreated discharged to theis discharged Iławka river, to the which Iławka is the river, right-hand which is tributary the right of-hand the Drw˛ecarivertributary of the (Baltic Drwęca Sea catchment)river (Baltic (Figure Sea catchment)5). (Figure 5).

FigureFigure 5.5. WWTPWWTP inin IławaIława (authors’(authors’ own elaboration).elaboration).

TheThe treatment treatment plant plant receives receives municipal municipal wastewater, wastewater, which which is a mixture is a mixture of domestic of domestic and industrial and 3 wastewater.industrial wastewater The wastewater. The wastewater from a poultry from slaughterhouse a poultry slaughterhouse and processing and processing plant (about plant 900 (about m /d), 3 cellulose900 m3/d), processing cellulose factory processing (about factory 250 m (about/d) as well 250 m as3/d) a potato as well processing as a potato facility processing (approximately facility 3 42,000(approx m imatelyover a 42 two-month,000 m3 over campaign) a two-month are discharged campaign) to are the discharged Iława WWTP. to the Iława WWTP. AsAs is is shown shown in in Table Table3, 3 it, wasit was observed observed that that the the wastewater wastewater characteristics characteristics significantly significantly di differedffered in 2017–2019.in 2017–2019. This This was caused was caused by the by seasonal the seasonal discharge discharge of industrial of industrial wastewater. wastewater. Moreover, Moreover, it should beit noticedshould thatbe noticed Iława is that a tourist Iława .is a tourist This fact city may. This also fact influence may also significant influence fluctuations significant fluctuations in the quantity in andthe qualityquantity of and the quality influent of wastewater. the influent wastewater.

Energies 2020, 13, 6056 10 of 21

Energies 2020, 13, x FOR PEER REVIEW 10 of 22 Table 3. Characteristic of the influent wastewater in 2017–2019.

Table 3. CharacteristicHydraulic of the influen Loadt wastewater in 2017–2019. ParameterHydraulic Minimum Load Maximum Average SD Total wastewater,Parameter m3/d Minimum4886.0 8816.0Maximum 6176.0 Average 837.0 SD Total wastewater, mPhysicochemical3/d Parameters4886.0 8816.0 6176.0 837.0 ParameterPhysicochemical Minimum ParametersMaximum Average SD Parameter Minimum Maximum Average SD mg/L 519.5 1220.0 752.4 165.4 Biochemical oxygen demand (BOD5) mg/L 519.5 1220.0 752.4 165.4 Biochemical oxygen demand (BOD5) kg/d 2908.7 6736.0 4613.9 947.1 kg/d 2908.7 6736.0 4613.9 947.1 mg/L 1057.0 2301.0 1559.4 320.8 (COD) mg/L 1057.0 2301.0 1559.4 320.8 Chemical oxygen demand (COD) kgkg/d/d 6116.96116.9 15,108.415,108.4 9595.59595.5 2032.5 2032.5 mgmg/L/L 182.5182.5 815.0815.0 542.8542.8 139.5139.5 TotalTotal suspended suspended solids solids (TSS) (TSS) kgkg/d/d 1119.31119.3 5220.45220.4 3353.93353.9 926.4 926.4 Total nitrogen mgmg/L/L 68.268.2 128.5128.5 101.4101.4 14.8 14.8 Total nitrogen (TN) (TN) kgkg/d/d 453.2453.2 992.3992.3 623.6623.6 101.1101.1 mg/L 8.9 69.3 16.0 10.0 Total phosphorous (TP) mg/L 8.9 69.3 16.0 10.0 Total phosphorous (TP) kg/d 62.4 338.6 96.5 48.9 kg/d 62.4 338.6 96.5 48.9 4.1.2. Sludge-Energy System in the WWTP 4.1.2. Sludge-Energy System in the WWTP In the Iława WWTP, the anaerobic digestion process is conducted in two anaerobic digesters underIn the mesophilic Iława WWTP, conditions the anaerobic (37–39 digestion °C) at hydraulic process is retention conducted time in two (HRT) anaerobic of 37d digesters. The generated under mesophilicbiogas after conditions scrubbing (37–39 is stored◦C) at in hydraulic a biogas retentionholder with time the (HRT) capacity of 37d. of 1040 The generatedm3. It is used biogas for the after co- 3 scrubbinggeneration isstored of electrical in a biogas and holder thermal with energy. the capacity The ofWWTP 1040 m comprises. It is used three for the co co-generation-generators (CHP of electricalsystem) and—two thermal with energy.the output The of WWTP 253 kW comprises of electricity three co-generatorsand 318 kW (CHPof thermal system)—two energy (Petra with the 300, outputElteco) of as 253 well kW as of a electricitysingle generator and 318 producing kW of thermal 190 kW energy of elec (Petratricity 300, and Elteco) 250 kW as of well thermal as a single energy generator(Gesco), producingwith the combined 190 kW of output electricity of 0.696 and 250MW kW (electricity) of thermal and energy 0.881 (Gesco), MW (thermal with the energy). combined The outputproduced of 0.696 electricity MW (electricity) is used to andconduct 0.881 the MW treatment (thermal processes energy). The(powering produced pumps, electricity air blowers, is used etc.). to conductIn turn, the the treatment produced processes thermal energy (powering is used pumps, for maintaining air blowers, temperature etc.). In turn, in the digesters produced as well thermal as for energyheating is usedthe social for maintaining buildings. Moreover, temperature the in surplus digesters of thermal as well asenergy for heating is employed the social for buildings.heating the Moreover,floor (with the surplusthe tota ofl surface thermal area energy of is 2800 employed m2) in for the heating solar dryer the floor that (with comprises the total two surface buildings. area 2 ofAdditionally, 2800 m ) in the the solar heat dryer generated that comprises by the heat two pump buildings. integrated Additionally, with secondary the heat clarifiers generated is used by the for heatdrying pump sewage integrated sludge with (Figure secondary 6). clarifiers is used for drying sewage sludge (Figure6).

FigureFigure 6. 6.Scheme Scheme of of energy energy usage usage in in the the WWTP WWTP in i Iława.n Iława.

Energies 2020, 13, 6056 11 of 21

4.2. Methods The analysis of sludge and biogas-energy management was carried out on the basis of the data obtained from a technical scale (the Iława WWTP) for the period from January 1st, 2017 to December 31st, 2019. In this study, the amount of generated sewage sludge and the produced biogas in relation to the volume of wastewater and the load of pollutants flowing into the wastewater treatment plant were analyzed. In order to perform this evaluation, the basic parameters pertaining to the production of sludge, biogas, and electricity were used. The following parameters were examined: biogas yield, VS removal, OLR and energy consumption at individual stages. The influence of the BOD5 load in the influent and OLR of the digester on the amount of biogas yield from sewage sludge was examined. Moreover, the analysis of the impact of the co-substrates on biogas in digestion process production was performed. The facility’s energy consumption and an energy balance were also carried out in the biogas and energy system of the wastewater treatment plant in Iława. Statistical analyses were performed to identify the major cause and effect relationships. Univariate linear correlations were used to simplify the analyses (Pearson’s coefficient r). A distribution-free statistical method was used, because a normal distribution was not obtained for many of the parameters examined.

5. Results and Discussion In the Iława WWTP, anaerobic digesters are supplied by the mixture of primary and waste sludge with the additive of co-substrates. The poultry processing wastes are used as additional components. The amount of generated sewage sludge depends on the composition and volume of influent wastewater as well as the adopted treatment technology [73]. Due to the significant share of industrial wastewater (poultry, cellulose, and potato processing wastewater), the amount of sewage sludge was varied notably. It amounted to 0.42–1.27 kg TS/m3 for primary sludge (PS) and 0.19–1.08 kg TS/m3 for excess sludge (ES). In the analyzed period of 2017–2019, the mean annual amount of produced sewage sludge amounted to 23,834 m3 and 15,725 m3 for PS and ES, respectively. In turn, the total amount of sludge directed to digesters reached 1510.8 Mg TS/year and 1467.0 Mg TS/year for PS and ES, respectively. The total solids content (TS) in the primary and waste sludge amounted to 6.5 2.0% and 9.4 1.8%, ± ± respectively. The poultry fat waste was added to the feedstock digester as an emulsion with dry mass content of 11.5%. The daily volume of this component ranged from 9.8 to 22.6 m3/d, with the average value of 14.7 3.4 m3/d. The detailed data of the co-substrate introduced into digesters is presented in ± EnergiesFigure 20207, 1. 3, x FOR PEER REVIEW 12 of 22

Raw sludge Excess sludge Fat waste 9000 8000

7000 substrates

- 6000 co /d] 5000

4000 kg TS kg [ 3000 2000 1000

0 Daily amount of of amount Daily

Figure 7. The Figuredaily flow 7. The of substrates daily flow supplied of substrates to the supplied digester to in the the digester Iława WWTP in the. Iława WWTP.

Raw sludge Excess sludge Fat waste 100% 90% 80%

% 70% 60% 50% 40%

Percentage Percentage 30% 20% 10% 0%

Figure 8. The percentage share of particular co-substrates in the feedstock supplied digester in the Iława WWTP.

The monthly biogas yield in digesters ranged from 78,110 to 103,895 Nm3 (93,204 Nm3/month, on average). In the autumn months, a greater amount of biogas was produced than in the remaining period. However, no seasonal trend was observed over the previous three years. The daily amount of the produced biogas ranged from 2774.4–3353.5 Nm3/d, with the mean value of 3063.5 ± 168.9 Nm3/d (Figure 9). In the considered period, 0.220 to 0.451 Nm3 was obtained from 1 kg TS of all substrates introduced to digesters. The mean unit yield of biogas amounted to 0.321 Nm3/kg TS. In relation to the volume of co-substrates introduced to digesters, a single cubic meter yielded 25.6 ± 4.3 Nm3 of biogas, on average (18.3–32.2 Nm3/m3). It should be also noticed that dried distillery spent wash was periodically added to digesters with the average amount of 15–20 m3/d. However, mainly due to its lower biogas potential, as compared to fat waste, its application as a co-substrate was discontinued. This confirms the findings presented in the literature that the co-digestion process is usually conducted using a two-substrate mixture. Nevertheless, Grosser et al. [55] reported a beneficial effect of anaerobic co-digestion of sewage sludge, fat waste as well as OFMSW. In the presence of fat waste, the VS removal was increased from 50% to 56.4%. Moreover, the methane yield was enhanced by 52%, as compared to mono-digestion. Therein, the average methane yield reached the level of 0.45 Nm3/kg VS. Introducing an additional co-substrate resulted in a further improvement of the digestion process. In relation to the mono-digestion of sewage sludge, the biogas yield increased by 82% (from 0.300 to 0.547 Nm3/kg VS), while the VS removal improved by 29% (from 50% to 64.7%). However, the application of co-

Energies 2020, 13, x FOR PEER REVIEW 12 of 22 Energies 2020, 13, 6056 12 of 21

Raw sludge Excess sludge Fat waste In the presence9000 of a co-substrate, the removal efficiency of organic compounds increased from approximately8000 64% (mono-digestion of sewage sludge) to 69–70%. Simultaneously, the sludge

dewatering level7000 decreased from 21% to 18%, while the polymer consumption remained unchanged. substrates

The amount- 6000 of introduced fat waste was continuously monitored and modified in order not to co exceed the level/d] 5000 of 4.8 kg VS/(m3 d) [15,74]. The share of co-substrate in the feedstock was maintained 4000 · at the level fromTS kg 6.2 to 21.2% and it depended on the amount of sewage sludge (Figure8). The dosing of [ 3000 co-substrate should be constantly controlled due to the possibility of biogas composition deterioration 2000 (increase the CO10002 content) and operational problems, especially during the dewatering of digested sludge [55,75]. It is0 assumed that the share of additional substrates for co-digestion with sewage sludge should not of amount Daily exceed 30% [76]. During operation, OLR ranged from 1.74 to 3.64 kg VS/(m3 d), while · HRT was maintained at 37 days. For comparison, in the Millbrae WPCP (USA) plant with similar 3 3 parameters (Qdav = 7570 m /d), sewage sludge and fat waste were processed in two digesters (1900 m each) atFigure HRT 7. of The 38–58 daily d flow [32]. of substrates supplied to the digester in the Iława WWTP.

Raw sludge Excess sludge Fat waste 100% 90% 80%

% 70% 60% 50% 40%

Percentage Percentage 30% 20% 10% 0%

FigureFigure 8. 8.The The percentage percentage share share of of particular particular co-substrates co-substrates in in the the feedstock feedstock supplied supplied digester digester in in the the IławaIława WWTP. WWTP.

TheThe monthly monthly biogas biogas yield yield in in digesters digesters ranged ranged from from 78,110 78,110 to to 103,895 103,895 Nm Nm3 (93,2043 (93,204 Nm Nm3/3month,/month, onon average). average). In In the the autumn autumn months, months, a a greater greater amount amount of of biogas biogas was was produced produced than than in in the the remaining remaining period.period. However, However, no no seasonal seasonal trend trend was was observed observed over over the the previous previous three three years. years The. The daily daily amount amount of theof producedthe produced biogas biogas ranged ranged from 2774.4–3353.5from 2774.4–3353.5 Nm3/d, Nm with3/d, the with mean the value mean of value 3063.5 of 3063.5168.9Nm ± 168.93/d ± (FigureNm3/d9 ).(Figure In the 9). considered In the considered period, 0.220 period, to 0.451 0.220 Nm to 30.451was obtainedNm3 was from obtained 1 kg TSfrom of all1 kg substrates TS of all introducedsubstrates tointroduced digesters. to The digesters. mean unit The yield mean of biogasunit yield amounted of biogas to 0.321 amounted Nm3/kg toTS. 0.321 In relationNm3/kgto TS the. In volumerelation of to co-substrates the volume o introducedf co-substrates to digesters, introduced a single to digesters, cubic meter a single yielded cubic 25.6 meter4.3 yielded Nm3 of 25.6 biogas, ± 4.3 ± onNm average3 of biogas, (18.3–32.2 on average Nm3/ m(18.33). –32.2 Nm3/m3). ItIt should should be be also also noticed noticed that that dried dried distillery distillery spent spent wash wash was periodicallywas periodically added added to digesters to digesters with thewith average the average amount amount of 15–20 ofm 3 15/d.–20 However, m3/d. However, mainly due mainly to its lower due tobiogas its lower potential, biogas as compared potential, toas fatcompared waste, its to application fat waste, its as application a co-substrate as a wasco-substrate discontinued. was discontinued. This confirms This the confirms findings the presented findings inpresented the literature in the that literature the co-digestion that the co process-digestion is usuallyprocess conductedis usually conducted using a two-substrate using a two- mixture.substrate Nevertheless,mixture. Nevertheless, Grosser et al. Grosser [55] reported et al. [5 a5 beneficial] reported e ff aect beneficial of anaerobic effect co-digestion of anaerobic of sewage co-digestion sludge, of fatsewage waste assludge well, as fat OFMSW. waste as In well the presence as OFMS ofW. fat In waste, the presence the VS removal of fat waste was increased, the VS from removal 50% was to 56.4%.increased Moreover, from 50 the% methaneto 56.4%. yield Moreover, was enhanced the methane by 52%, yield as comparedwas enhanced to mono-digestion. by 52%, as compared Therein, to themono average-digestion. methane Therein, yield reachedthe average the levelmethane of 0.45 yield Nm reached3/kg VS. the Introducing level of 0.45 an Nm additional3/kg VS. co-substrate Introducing resultedan additional in a further co-substrate improvement resulted of thein a digestion further improvement process. In relation of the to digestion the mono-digestion process. In ofrelation sewage to sludge,the mono the-digestion biogas yield of sewage increased sludge by, 82% the biogas (from 0.300yield toincreased 0.547 Nm by3 8/kg2% VS),(from while 0.300 the to 0.547 VS removal Nm3/kg improvedVS), while by the 29% VS (from removal 50% improved to 64.7%). by However, 29% (from the 50 application% to 64.7% of). co-substrates However, the had application no significant of co-

Energies 20202020,, 1313,, 6056x FOR PEER REVIEW 1313 of of 2122 Energies 2020, 13, x FOR PEER REVIEW 13 of 22 substrates had no significant influence on the methane content in biogas, which amounted to 67–69%, influence on the methane content in biogas, which amounted to 67–69%, regardless of the mixture substratesregardless hadof the no mixture significant composition influence on[55 the]. methane content in biogas, which amounted to 67–69%, compositionregardless of [the55]. mixture composition [55].

Qd av. Biogas

9000 Qd av. Biogas 3500 /d]

9000 3500 3

3000 /d] m

7500 3

3000 N m 7500 [

/d] 2500

N 3 6000 [

/d] 2500

3 6000

[m 2000

4500 biogas

[m 2000 1500

4500 biogas of of

d av. d 1500 Q

3000 of d av. d 1000

Q 3000 1000 1500 500 1500 500 volume 0 0 volume

0 0 Daily Daily

Figure 9. Daily biogas production vs. the wastewater flow in the WWTP. Figure 9. Daily biogas production vs. the the wastewater wastewater flow flow in the WWTP WWTP.. The poultry industry wastes are often used as components in co-digestion with sewage sludge. The poultry industry wastes are often used as components in co co-digestion-digestion wit withh sewage sewage sludge sludge.. Their biogas potential is highly diversified and may reach 0.57 Nm3/kg VS [77], 0.61 Nm3/kg VS [78], Their biogas potential is highly diversifieddiversified and may reach 0.57 Nm3/kg/kg V VSS [7 [777], 0.61 Nm 3/kg/kg VS VS [ [7788],], 0.631 Nm3/ kg VS [79], even up to 1.24 Nm3/kg VS [80]. At the WWTP in Koziegłowy (Poland) 0.631 Nm3/ kg VS [79], even up to 1.24 Nm33/kg VS [80]. At the WWTP in Koziegłowy (Poland) 0.631(1,200 , Nm000 PE),/ kg theVS poultry[79], even industry up to waste 1.24 Nmaddition/kg VS increased [80]. At the the yield WWTP of biogas in Koziegłowy by up to 30 (Poland)% (from (1,200,000(1,200,000 PE), the poultry industry waste addition increased the yield of biogas by up to 30%30% (from 0.38 to 0.49 Nm3/kg VS) at OLR = 1.66 kg VS/(m3·d ). The maximum share of this waste amounted to 0.38 to 0.49 Nm 33//kgkg VS VS)) at at OLR OLR == 1.661.66 kg kg VS VS/(m/(m3·d3 ).d). The The maximum maximum share share of ofthis this waste waste amounted amounted to 13%, in relation to the total mass of the digested mixture· [80]. The comparison of biogas yield and the to 13%, in relation to the total mass of the digested mixture [80]. The comparison of biogas yield 13amount%, in relation of added to waste the total fat massconfirm of the the digested beneficial mixture effect of [8 this0]. The substrate comparison on the of course biogas of yield digestion. and the A and the amount of added waste fat confirm the beneficial effect of this substrate on the course of amountstatistically of added significant waste dependence fat confirm wasthe beneficial noted between effect theof this share substrate of waste on fat the and course biogas of yield digestion. (Figure A digestion. A statistically significant dependence was noted between the share of waste fat and biogas statistically10). significant dependence was noted between the share of waste fat and biogas yield (Figure yield10). (Figure 10).

0.5 35 0.5 35 0.5 30 0.4 30 0.4 25 25 0.40.3

subtrates] 20

/kg TS] /kg - 3 0.3

0.3 subtrates] 20

/kg TS] /kg

- co 3 r = 0.43

0.3 3 15 r=0.79

Biogas yield yield Biogas [Nm 0.2 r = 0.43 co

p < 0.01 3 15 r=0.79 Biogas yield Biogas

/m p<0.001 Biogas yield yield Biogas [Nm 0.2

0.2 p < 0.01 3 10 Biogas yield Biogas

/m p<0.001 0.20.1 3 10 0.1 [Nm 5 0.1 [Nm 5 0.10.0 0 0.0 0 5 10 15 20 25 0 0 5 10 15 20 25 0 5 10 15 20 25 0Share of5 fat waste10 in co15-substrates20 [%]25 Share of fat waste in co-substrates [%] Share of fat waste in co-substrates [%] Share of fat waste in co-substrates [%]

(a) (b) (a) (b) Figure 10. InfluenceInfluence of waste fat share in the digesteddigested mass on the biogas yield.yield. Dependence between Figure 10. Influence of waste fat share in the digested mass on the biogas yield. Dependence between the share ofof wastewaste fatfat andand ((aa)) biogasbiogas yield,yield, ((bb)) volumetricvolumetric biogasbiogas yield.yield. the share of waste fat and (a) biogas yield, (b) volumetric biogas yield. The obtained obtained biogas biogas yield yield data data can be can compared be compared with those with from those other from Polish other WWTPs Polish employing WWTPs The obtained biogas yield data can be compared with those from other Polish WWTPs theemploying co-digestion the co- ofdigestion sewage of sludge sewage and sludge various and substrates.various substrates. In the In Rzesz the óRzeszóww WWTP WWTP (398,000 (398 PE),,000 employing the co-digestion of sewage sludge and various substrates. In the Rzeszów WWTP (398,000 thePE), feedstock the feedstock was supplemented was supplemented by waste by fat waste (edible fat fat, (edible spent fat, cooking spent oils)cooking and distillery oils) and decoction.distillery PE), the feedstock was supplemented by waste fat (edible fat, spent cooking oils) and distillery Therein,decoction the. Therein, biogas yield the biogasranged yield from 0.252 ranged to 0.519 from Nm 0.2523/kg to TS 0.519 of sludge Nm3/kg [81 TS]. In of the sludge WWTP [81 in]. MielecIn the decoction. Therein, the biogas yield ranged from 0.252 to 0.519 Nm3/kg TS of sludge [81]. In the (85,750WWTP PE),in Mielec the ice-cream (85,750 PE) production, the ice-cream wastes production were added wastes to digesters. were added In this to digesters case, in. the In presencethis case, WWTPin the presence in Mielec of (85a co,750-substrate, PE), the a ice significan-cream productiont increase of wastes 80–100 were% in biogasadded yieldto digesters was observed. In this . caseThis, in the presence of a co-substrate, a significant increase of 80–100% in biogas yield was observed. This

Energies 2020, 13, 6056 14 of 21

Energies 2020, 13, x FOR PEER REVIEW 14 of 22 of a co-substrate, a significant increase of 80–100% in biogas yield was observed. This parameter 3 3 parameterreached 19.9 reached Nm3/m 19.93 during Nm /m co-digestion, during co- whiledigestion for the, while mono-digestion for the mono of-digestion sewage sludge,of sewage it was sludge only, 3 3 it9.6 was Nm only3/m3 9.6[82 Nm]. /m [82]. Another example of a successful implementation of the co co-fermentation-fermentation strategy concerned the use of fatfat wastewaste from from fish fish processing processing at at the the Swarzewo Swarzew WWTPo WWTP (90,000 (90,000 PE). PE). In this In case,this case, the biogas the biogas yield yieldwas increased was increased up to up 80%. to 80%. The fishfish waste, addedadded inin thethe amountamount of of 1000 1000 kg kg as as a a co-substrate, co-substrate, contributed contributed to to the the generation generation of 3 ofan additionalan additional 107 Nm 1073 Nmof biogasof biogas [83]. In [8 turn,3]. In in turn, the Krosno in the WWTP Krosno (117,000 WWTP PE), (117 the,000 co-fermentation PE), the co- fermentationof sewage sludge of sewage with sludge an organic with an fraction organic of fraction municipal of municipal wastes was wastes performed. was performed Therein,. Therein, biogas 3 3 biogasproduction production was achieved was achieved at the level at the of level 11.2 Nmof 113.2of Nm biogasof biogas per 1 m per3 of 1 them feedstockof the feedstock [84]. [84]. The statistical statistical analysis analysis of of the the results results indicated indicated that that the daily the daily biogas biogas yield was yield significantly was significantly related related with the BOD5 load in the influent and OLR (Figure 11). A strong correlation was observed with the BOD5 load in the influent and OLR (Figure 11). A strong correlation was observed between between BOD5 in the influent (r = 0.948, p < 0.001) and the biogas yield (Figure 11a) as well as between BOD5 in the influent (r = 0.948, p < 0.001) and the biogas yield (Figure 11a) as well as between the theamount amount of co-substrates of co-substrates and and the volumetricthe volumetric biogas biogas yield yield (Figure (Figure 11b). 11b). The biogasThe biogas yield yield in the in plant the plant ranged from 0.465 to 0.9763 Nm3/kg BOD5 removed (mean 0.694 Nm3 3/kg BOD5 removed); the highest ranged from 0.465 to 0.976 Nm /kg BOD5 removed (mean 0.694 Nm /kg BOD5 removed); the highest co-digestion efficiency was obtained at the BOD load of 2900–3300 kg O2/d. The yield of biogas co-digestion efficiency was obtained at the BOD load of 2900–3300 kg O2/d. The yield of biogas depended on the OLR of the digester (r = 0.953, p < 0.001) ( (FigureFigure 11c).11c). The The highest highest biogas yield was observed at OLR = 1.7–2.0 kg TS/(m3·d). In turn, the dependence between the daily biogas yield and observed at OLR = 1.7–2.0 kg TS/(m3 d). In turn, the dependence between the daily biogas yield and digester OLR was moderate (r = 0.43,· p < 0.01) (Figure 11d). digester OLR was moderate (r = 0.43, p < 0.01) (Figure 11d).

] 1.2 35 1.0 30

5 removal 5 0.8 25

subtrates] -

BOD 20

0.6 co

3 15 r = 0.952

/kg 3

Biogas yield Biogas 0.4 Biogas yield yield Biogas /m p < 0.001 r = 0.948 3 10

[Nm 0.2 p < 0.001 5 [Nm 0.0 0 2500 3250 4000 4750 5500 6250 7000 80 90 100 110 120 130 140 150 160 170 BOD5-load [kg O2/d] Total volume of co-substrates [m3/d]

(a) (b)

0.5 4000 /d] 3 3500 0.4 3000 0.3 2500

r = 0.430 /kg TS] /kg

3 2000 p < 0.01

0.2 1500 Biogas yield yield Biogas [Nm r = 0.953 1000 0.1 p < 0.001 500 0.0 [Nm yield biogas Daily 0 1.5 2.0 2.5 3.0 3.5 4.0 1.5 2.0 2.5 3.0 3.5 4.0 OLR [kg TS/(m3·d)] OLR [kg TS/(m3·d)] (c) (d)

Figure 11. Influence of technological parameters on the daily biogas yield. Dependence between: (a) Figure 11. Influence of technological parameters on the daily biogas yield. Dependence between: BOD5 in the influent and the biogas yield; (b) total volume of co-substrates and the volumetric biogas (a) BOD in the influent and the biogas yield; (b) total volume of co-substrates and the volumetric biogas yield; (c5) OLR of the digester and the biogas yield; (d) OLR of the digester and the daily biogas yield. yield; (c) OLR of the digester and the biogas yield; (d) OLR of the digester and the daily biogas yield.

The assessment assessment of of the the biogas-energy biogas-energy system system of a WWTP of a WWTP usually usually involves involvesthe analysis the of analysis production of productionand demand and of demand energy of of theenergy WWTP. of the That WWTP allows. That for consideringallows for considering such facility such as afacility combined as a combinedtechnological technological and energetic and system. energetic In thesystem. analyzed In the WWTP, analyzed the monthlyWWTP, electricitythe monthly consumption electricity consumption ranged from 172.7 to 229.83 MWh/month. The energy intensity of the WWTP in Iława was estimated on the basis of the power consumption as well as the flow of wastewater and BOD5 load removed in technological processes. It amounted to 1.06 kWh/m3 (0.812–1.37 kWh/m3) and 1.46

Energies 2020, 13, 6056 15 of 21

Energies 2020, 13, x FOR PEER REVIEW 15 of 22 ranged from 172.7 to 229.83 MWh/month. The energy intensity of the WWTP in Iława was estimated kWh/kgon the BOD basis5 of(0.92 the– power2.13 kWh/ consumptionBOD5) (Figure as well 12), ason the average. flow of While wastewater comparing and the BOD results5 load with removed those in 3 3 reportedtechnological in literature, processes. the It energy amounted intensity to 1.06 of kWh the / Iławam (0.812–1.37 WWTP can kWh be/m considered) and 1.46 as kWh moderate./kg BOD 5 Accord(0.92–2.13ing to kWh Maktabifard/BOD5) (Figure et al. [1220),], onthe average. unit energy While consumption comparing of the the results plant with ranges those from reported 0.26 to in 1.11literature, kWh/m3 the. In energyturn, the intensity report of the IławaPolish WWTPWaterworks can be Chamber considered of Commerce as moderate. establish Accordinged the to energyMaktabifard-intensity et index al. [20 for], the the unit Polish energy WWTPs consumption at 0.71–1.04 of kWh/m the plant3 (data ranges for 2017) from [ 0.2669]. toIn 1.11the selected kWh/m 3. PolishIn turn, WWTPs the report, the energy of the Polishintensity Waterworks is as follows: Chamber the Rzeszów of Commerce WWTP established –from 1.03 theto 1.57 energy-intensity kWh/BOD5 andindex 0.865 for the kWh/m Polish3 [ WWTPs85], the atKrosno 0.71–1.04 WWTP kWh/—m30.61(data–1.11 for 2017) kWh/m [693 ].[8 In4], the the selected Opole Polish WWTP WWTPs,—0.723 the 3 kWh/menergy3and intensity 1.66 kWh/BOD is as follows:5 [8 the6]. Rzeszów WWTP –from 1.03 to 1.57 kWh/BOD5 and 0.865 kWh/m [85], 3 3 the Krosno WWTP—0.61–1.11 kWh/m [84], the Opole WWTP—0.723 kWh/m and 1.66 kWh/BOD5 [86].

kWh/m3 kWh/kg BOD5 rem 1.6 2.5 1.4 2.0 ]

1.2

] 3 1.0 1.5

0.8 BOD5 removal BOD5

[kWh/m 0.6 1.0 0.4

0.5 Electricity consumptionElectricity

0.2 [kWh/kg Electricity consumtionElectricity 0.0 0.0

Figure 12. Energy intensity of the WWTP in Iława. Figure 12. Energy intensity of the WWTP in Iława. In the Iława WWTP, a significant amount of the generated biogas was used for the production In the Iława WWTP, a significant amount of the generated biogas was used for the production of electricity. From January 2017 to September 2019, three CHP generators produced 7.63 GWh of of electricity. From January 2017 to September 2019, three CHP generators produced 7.63 GWh of electricity, with the mean annual value of 2.54 GWh. However, the monthly electricity production electricity, with the mean annual value of 2.54 GWh. However, the monthly electricity production was diversified, ranging from 177.7 MWh (February 2018) to 236.1 MWh (October 2017), which also was diversified, ranging from 177.7 MWh (February 2018) to 236.1 MWh (October 2017), which also affected the operation of the three CHP generators. The greatest amount of electricity was produced affected the operation of the three CHP generators. The greatest amount of electricity was produced in the autumn months (September–October). The unit electricity production ranged from 2.15 to in the autumn months (September–October). The unit electricity production ranged from 2.15 to 2.41 2.41 kWh/Nm3 biogas, with a mean value of 2.27 0.07 kWh/Nm3. As compared to other Polish WWTPs, kWh/Nm3 biogas, with a mean value of 2.27 ± 0.07± kWh/Nm3. As compared to other Polish WWTPs, the efficiency of electricity production in the Iława WWTP can be considered as significant. For example, the efficiency of electricity production in the Iława WWTP can be considered as significant. For this parameter amounted to 1.9–4.8 kWh/Nm3 in the Mielec WWTP [82], 3.82–4.51 kWh/Nm3 in the example, this parameter amounted to 1.9–4.8 kWh/Nm3 in the Mielec WWTP [82], 3.82–4.51 kWh/Nm3 Krosno WWTP) [84], 2.02–2.48 in the Rzeszów WWTP [83], 0.36–2.13 in the D˛ebicaWWTP [87], in the Krosno WWTP) [84], 2.02–2.48 in the Rzeszów WWTP [83], 0.36–2.13 in the Dębica WWTP [87], 1.16–2.21 kWh/Nm3 biogas in the Zamo´s´cWWTP [88], and 0.95–2.18 kWh/Nm3 biogas in the Opole 1.16–2.21 kWh/Nm3 biogas in the Zamość WWTP [88], and 0.95–2.18 kWh/Nm3 biogas in the Opole WWTP [84]. In turn, the estimated electricity recovery from wastewater amounted to 1.15 kWh/m3. WWTP [84]. In turn, the estimated electricity recovery from wastewater amounted to 1.15 kWh/m3. In the analyzed period, the operation of CHP generators enabled covering the electricity demand 93.0% In the analyzed period, the operation of CHP generators enabled covering the electricity demand to 99.8%. The self-sufficiency level of the Iława WWTP in 2017–2019 amounted to 98.2% (Figure 13). 93.0% to 99.8%. The self-sufficiency level of the Iława WWTP in 2017–2019 amounted to 98.2% (Figure Compared to other Polish WWTPs, the analyzed plant indicated a significant potential for energy 13). Compared to other Polish WWTPs, the analyzed plant indicated a significant potential for energy production. The electricity demand in other Polish WWTPs achieved the following values: 35%—the production. The electricity demand in other Polish WWTPs achieved the following values: 35%—the Opole WWTP [86], 46.5%—the D˛ebicaWWTP [87], 52.2%—the Zamo´s´cWWTP [88], 74.3%—the Opole WWTP [86], 46.5%—the Dębica WWTP [87], 52.2%—the Zamość WWTP [88], 74.3%—the Rzeszów WWTP [85]. In Poland, there are several energy neutral WWTPs, i.e., the Tychy-Urbanowice Rzeszów WWTP [85]. In Poland, there are several energy neutral WWTPs, i.e., the Tychy-Urbanowice WWTP, the Swarzewo WWTP, and the Słupsk WWTP, in which the electricity production from biogas WWTP, the Swarzewo WWTP, and the Słupsk WWTP, in which the electricity production from exceeds 100% of the total electricity consumption in the plant [69]. biogas exceeds 100% of the total electricity consumption in the plant [69]. Along with an increase of biogas production as well as the related electricity and thermal energy generation, a growth in the plant self-sufficiency—in relation to the power demand—was noted. This resulted from the modernization of the Iława WWTP, which has been performed since 2006 and contributed to achieving the self-sufficiency of the facility. The application of the co-digestion process was one of the directions, which resulted in a systematic increase in the production of electricity

Energies 2020, 13, x FOR PEER REVIEW 16 of 22

Consumption Production Self-sufficiency

250 100 Energies 2020, 13, 6056 16 of 21 99 200 98 and thermal150 energy. The application of co-substrates led to an increase in the energy production97 by 50% (Figure 14). The total cost of the consecutive modernization that contributed to achieving96

self-suffi100ciency was approximated as 5.5 million euro. Importantly, the application of renewable95 sufficiency [ %] [ sufficiency energy produced based on sludge also resulted in a reduced emission of pollutants to the environment,-

[MWh/month] 94 50 in relation to a conventional coal-fired plant. The sustainable production of electricity in 2017–201993 Self enabled avoiding0 the emission of 5836 Mg CO2 and 274.7 PM (estimated on the basis of the92 data of National Center for Emissions Management [89], which is a significant ecological effect. Energies 2020, 13, x FOR PEER REVIEW 16 of 22

Consumption Production Self-sufficiency Figure 13. Energy balance at the Iława WWTP. 250 100 Along with an increase of biogas production as well as the related electricity and thermal99 energy 200 generation, a growth in the plant self-sufficiency—in relation to the power demand—was noted.98 This resulted150 from the modernization of the Iława WWTP, which has been performed since 972006 and contributed to achieving the self-sufficiency of the facility. The application of the co-digestion96 process

was one100 of the directions, which resulted in a systematic increase in the production of electricity95 and sufficiency [ %] [ sufficiency thermal energy. The application of co-substrates led to an increase in the energy production by- 50%

[MWh/month] 94 50 (Figure 14). The total cost of the consecutive modernization that contributed to achieving93 Self self- sufficiency0 was approximated as 5.5 million euro. Importantly, the application of renewable92 energy produced based on sludge also resulted in a reduced emission of pollutants to the environment, in relation to a conventional coal-fired plant. The sustainable production of electricity in 2017–2019 enabled avoiding the emission of 5836 Mg CO2 and 274.7 PM (estimated on the basis of the data of National Center for Emissions Management [89], which is a significant ecological effect. Figure 13. Energy balance at the Iława WWTP.WWTP.

Along with an increase of biogas production as well as the related electricity and thermal energy Production Self-sufficiency generation, a3000 growth in the plant self-sufficiency—in relation to the power demand—100was noted. This resulted from the modernization of the Iława WWTP, which has been performed since 2006 and 2500 contributed to achieving the self-sufficiency of the facility. The application of the co-digestion80 process 2000 was one of the directions, which resulted in a systematic increase in the production of60 electricity and thermal energy.1500 The application of co-substrates led to an increase in the energy production% by 50% 40

(Figure 14). 1000 The total cost of the consecutive modernization that contributed to achieving self- [MWh/year] sufficiency was approximated as 5.5 million euro. Importantly, the application of renewable energy 500 20 produced based on sludge also resulted in a reduced emission of pollutants to the environment, in 0 0 relation to a conventional coal-fired plant. The sustainable production of electricity in 2017–2019 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 enabled avoiding the emission of 5836 Mg CO2 andYear 274.7 PM (estimated on the basis of the data of National Center for Emissions Management [89], which is a significant ecological effect.

Figure 14. Energy balance for the Iława WWTP.WWTP. 6. Conclusions Production Self-sufficiency 6. Conclusions3000 100 The biogas generated during the anaerobic digestion of sewage sludge is a valuable source of The biogas2500 generated during the anaerobic digestion of sewage sludge is a valuable80 source of renewable energy, which may improve the energy balance of a facility. Simultaneously, the mentioned renewable energy,2000 which may improve the energy balance of a facility. Simultaneously, the process reduced the operational costs and the emission of pollutants compared to the60 combustion

mentioned process reduced the operational costs and the emission of pollutants compar% ed to the of conventional1500 fuels, e.g., hard coal. Achieving a full self-sufficiency in terms of energy demand in combustion of conventional fuels, e.g., hard coal. Achieving a full self-sufficiency in 40terms of energy WWTPs is possible1000 through the implementation of a highly efficient anaerobic digestion. One of the demand in [MWh/year] WWTPs is possible through the implementation of a highly efficient anaerobic digestion. possibilities is the addition of co-substrates characterized by a significant fat content20 with relatively One of the possibilities500 is the addition of co-substrates characterized by a significant fat content with high biogas potential as well as favorable composition of sewage sludge. An example of an effective relatively high biogas0 potential as well as favorable composition of sewage sludge. An0 example of an application of this strategy is the WWTP in Iława. On the basis of the analysis, the authors showed effective application2006 of 2007this strategy2008 2009 is2010 the2011 WWTP2012 in2013 Iława.2014 On2015 the2016 basis2017 of2018 the2019 analysis, the authors that as a result of the co-digestion of sewage sludgeYear and poultry processing wastes, the production of a total of 7.63 GWh of electricity, with a mean annual amount of 2.54 GWh, was recorded. That allowed covering the energy demandFigure in a significant14. Energy balance degree, for ranging the Iława from WWTP 93.0%. to 99.8% (mean value of

6. Conclusions The biogas generated during the anaerobic digestion of sewage sludge is a valuable source of renewable energy, which may improve the energy balance of a facility. Simultaneously, the mentioned process reduced the operational costs and the emission of pollutants compared to the combustion of conventional fuels, e.g., hard coal. Achieving a full self-sufficiency in terms of energy demand in WWTPs is possible through the implementation of a highly efficient anaerobic digestion. One of the possibilities is the addition of co-substrates characterized by a significant fat content with relatively high biogas potential as well as favorable composition of sewage sludge. An example of an effective application of this strategy is the WWTP in Iława. On the basis of the analysis, the authors

Energies 2020, 13, 6056 17 of 21

98.2%). The actions taken by the authors have shown that the strategy in the Iława WWTP allows utilizing the unused potential of fermentation chambers for the production of clean energy. Importantly, in the presence of a co-substrate, the removal efficiency of AD was improved. An enhancement from 64% (mono-digestion) to 69–70% (co-digestion) was found. Additionally, the AcoD technology contributed to successful , thus bringing benefits for both the WWTP and the poultry company. The considerations undertaken by the authors are of a multi-faceted analysis of the sludge management and the biogas and energy system in the wastewater treatment plant. This publication provides innovative information pertaining to renewable energy in the wastewater industry so that wastewater treatment plants can use their own resources, in this case sewage sludge, and obtain savings related to their own energy production.

Author Contributions: Conceptualization and methodology, A.M., J.C. and G.Ł.; formal analysis, G.Ł.; investigation, resources, data curation and visualization, A.M., J.C.; writing—original draft preparation, A.M., J.C.; writing—review and editing, A.S., J.S.-C. and G.Ł.; All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported within the statutory research of particular scientific units under subvention for a science program. Acknowledgments: The kind help from Iława WWTP staffs gratefully acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

AcoD anaerobic co-digestion process AD anaerobic digestion BOD5 Biochemical Oxygen Demand CHP combined heat and power ES excess sludge FOG fat, oil, and grease wastes HRT hydraulic retention time LCFA long-chain fatty acids MWel megawatts electric OFMSW organic fraction of municipal solid waste OLR organic loading rate PE population equivalent PS population equivalent TS total solids content VS volatile solids WWTP wastewater treatment plant

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