CHEMICAL ENGINEERING TRANSACTIONS VOL. 70, 2018 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Timothy G. Walmsley, Petar S. Varbanov, Rongxin Su, Jiří J. Klemeš Copyright © 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-67-9; ISSN 2283-9216 A Review of Hygienization Technology of Biowastes for Anaerobic Digestion: Effect on Pathogen Inactivation and Methane Production Xiaojun Liu, Thomas Lendormi*, Jean-Louis Lanoisellé Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56300 Pontivy, France thomas.lendormi@univ-ubs.fr This work reviews the application of the hygienization-related technology to the inactivation of pathogens in biowastes prior to anaerobic digestion. First, the paper focused on the existing thermal pasteurization from the perspectives of the worldwide regulations and its energy consumption in biogas plants. In addition, the study attention was attributed to the emerging alternative pasteurization technologies (i.e. Pulsed Electric Fields, High Hydrostatic Pressure, Power Ultrasound, Microwaves and chemical pretreatment). These technologies have been developed and well-studied for food-processing industries. Finally, the effect of these technologies, either traditional thermal treatment or emerging non-thermal treatment, on the methane potential of biowastes was discussed. 1. Introduction Thermal pasteurization has been a major solution to the pathogen inactivation in the food-processing industries (Brennan and Grandison, 2012). This thermal technology has also been borrowed by numerous countries for the purpose of the hygienization (HYG) of certain biowastes that present a potential source of biological risks to public and animal health, which covers a variety of municipal solid wastes (MSW), agricultural waste (AW), animal by-products (ABP) and waste activated sludge (WAS). It is a common practice that the biowastes, rich in organic matters, are treated via biological transformation like composting and anaerobic digestion (AD). Anaerobic digestion is a transformation of organic matters, under anaerobic conditions and by means of fermentative microbiological activities, into biogas that serves as a source of renewable energy in a biogas plant (BGP) (Guarino et al., 2016). The thermal hygienization of bio-wastes before entering AD units remains unclear concerning its treatment efficiency, energy consumption and the impact on the biogas yield of the biowastes treated. The major interest of BGPs is the maximization of the energy recovery of biogas from the wastes to make financial profits and to reduce carbon footprint. Therefore, the heat dedicated to the thermal hygienization may bring negative effect when realizing these economic and environmental goals. However, many non-thermal pathogen inactivation technologies other than thermal pasteurization have been developed for food-processing industries in recent decades, chiefly for the conservation of food quality and the reduction of energy consumption during the treatment. Electrical pretreatment causes the electroporation on the cellular membrane of microorganisms that are inactivated after a number of pulses during very short time (Zhang et al., 1995). High Hydrostatic Pressure (HHP) can induce a phase transition of the lipid bilayers of pathogens under the intensive pressurization (Martín et al., 2002). Power Ultrasound (PUS) creates cavitation to shock and kill bacteria (Piyasena et al., 2003). Microwaves (MW) give rise to a volume heating of the products and bring about the thermal pretreatment (Tyagi and Lo, 2013). Acid or alkali pretreatment stresses the pathogens by modifying their living conditions. These pretreatment technologies could be a future research focus for the hygienization of biowastes (van Fana et al., 2017). This paper contributes to a comprehensive review of the existing thermal pasteurization from the perspectives of the worldwide regulations and its energy consumption. The work also reviews the application of the emerging alternative pasteurization technologies on the biowastes. Besides, the effect of these technologies on the methane potential of the biowastes was discussed. DOI: 10.3303/CET1870089 Please cite this article as: Liu X., Lendormi T., Lanoiselle J.-L., 2018, A review of hygienization technology of biowastes for anaerobic digestion: effect on pathogen inactivation and methane production , Chemical Engineering Transactions, 70, 529-534 DOI:10.3303/CET1870089 529 2. Thermal hygienization 2.1 Worldwide regulations on hygienization of biowastes Many countries and authorities impose imperatively, prior to the transformation units, a thermal pasteurisation unit of biowastes. The European Union (EU) Regulation No 142/2011 demands that the ABP and the derived products (particle size < 12 mm) should be maintained at 70 °C for one hour without interruption before the introduction into anaerobic digesters (European Commission, 2011). Alternatively, the EU member states are authorized to propose their own hygienization process on condition that the treatment efficiency meet the criteria set by the relevant regulation (i.e. a reduction of 5 log10 of Enterococcus faecalis or Salmonella senftenberg in ABP). The hygienization parameters are usually regulated depending on the AD operational temperature, namely the mesophilic AD (35 - 40 °C, MAD) or thermophilic AD (> 50 °C, TAD). Austria, Germany proposed a hygienization of wastes at 70 °C for 1 h and 0.5 h prior to MAD and TAD respectively (Colleran, 2000) while in Denmark it was modified as 1.5 h and 1 h at 65 °C for MAD and TAD (Bendixen, 1999). Sweden uses a hygienization step combined in a TAD digester with a minimum retention time for 10 h at 52 °C (Grim et al., 2015). In the United States, instead of the federal government, it is the member states who act to manage the hygienization of ABP. However, the hygienization of WAS (bio-solids) is regulated on a federal level by US EPA who proposes a categorization of sewage sludge into four groups and different Time-Temperature regime applies depending on the category of WAS (US EPA, 2003). In China, the hygienization of biowastes is integrated into the major transformation units (like AD or composting facilities) with the minimum temperature and retention time specified (Ministry of Health of China, 2013). A summary of the operational parameters of the hygienization of biowastes regulated by different countries is presented in Figure 1. In order to compare the efficiency of these processes with various parameters, a normalised parameter, Pasteurisation value (F-value), was calculated for each process. F-value represents the treatment time required to obtain, at a reference temperature, the same pasteurisation performance as that obtained at a given temperature. The formula to calculate F-value is available in Eq(1). F - value= ∫ 10 T - Tref z dt t 0 (1) where F is the Pasteurisation value (min); t is the treatment time (min); T is the given temperature (°C); Tref is the reference temperature (here Tref = 70 °C); z is the temperature rise required for one log10 reduction of pathogen’s decimal reduction time (here z = 7 °C for Enterococcus faecalis). The F-value permits the comparison of the sanitation efficiency at one same reference temperature (e.g., a hygienization of biowaste at 65 °C for 1.5 h followed by a MAD treatment in Denmark is equivalent to an effect of pathogen inactivation at 70 °C for 17.4 min). It can be concluded from the figure that the EU-level hygienization takes more caution than those proposed by its own members and other countries in the world. Some researchers questioned this prudence in terms of its efficiency and validity of the pathogen dynamics of the treatment (Goden et al., 2017). Figure 1: F-values of the regulation of hygienization for biowastes (1Amon and Boxberger, 1999; 2Bendixen, 1999; 3Grim et al., 2015; 4US EPA, 2003; 5Ministry of Health of China, 2013; 6European Commission, 2011) 530 2.2 Heat demand for thermal hygienization process Table 1 gives a summary of the representation of the heat consumption of the hygienization process in the total heat generation of European BGP. The hygienization of EU scenario (70 °C, 1 h) represents around 6 - 19 % of the total heat production in the BGP in Finland, Germany, Sweden and UK. It required much more in one Irish BGP (30 - 57 %) and this could be explained by the fact that the main feedstock of this BGP was the animal slurry that contained more water content (Coultry et al., 2013). This resulted in a stronger thermal inertia of the substrates, thus requiring more heat than usual. Therefore, the efficiency improvement of the hygienization process can favor the energy production, enlarge the eco-benefit and increase the financial interests for BGP. Table 1: Summary of representation of the heat demand of hygienization process in the total heat production of BGP (* considering the heat consumption of whole AD units) (7Berglund and Börjesson, 2006; 8Coultry et al., 2013; 9 Pöschl et al., 2010; 10Whiting and Azapagic, 2014; 11 Grim et al., 2015) Country of the BGP Treatment capacity (kt‧y-1) Substrates Operational Parameters of HYG AD process Q required by HYG Q generated by biogas Sweden *,7 20 - 60 AW, crops, WAS, MSW 70 °C, 1 h MAD 6 - 17 % Germany 8 10 - 20 AW, crops, MSW 70 °C, 1 h MAD 10 - 15 % Ireland 9 10.7 Slurry 70 %, AW 30 % 70 °C, 1 h MAD 30 - 57 % UK *,10 5.1 Slurry 50 %, AW 50 % 70 °C, 1 h MAD 17 % Sweden 11 25.2 MSW 83 %, ABP 18 % 52 °C, 10 h TAD 9 % 2.3 Enhancement of methane potential by thermal hygienization Thermal hygienization is often performed prior to the AD process. To some extent, it serves as a mild thermal pretreatment. There have been many studies reporting that this thermal pretreatment enhanced the bio-methane potential (BMP) of the biowastes like WAS and ABP. Nevertheless, the results are not always positive when it comes to the digestive tract content, the grease trap sludge (Luste et al., 2009), the primary sludge and the digested WAS (Nazari et al., 2017), the slaughterhouse by-products (Hejnfelt and Angelidaki, 2009), the codigestion of slaughterhouse waste with food waste (Grim et al., 2015), and the dewatered manure (Rafique et al., 2010). Table 2: Literature review of mild thermal pretreatment related to methane yield enhancement of biowastes Authors, year Operational condition Origin of the substrates AD process BMP enhancement Edström et al., 2003 70 °C, 1 h ABP MAD +400 % Gavala et al., 2003 70 °C, 4 - 7 d Primary sludge TAD from +80 % to +86 % 70 °C, 1 - 7 d Secondary WAS MAD from +20 % to +26 % Climent et al., 2007 70 °C, 0.3 - 3 d Secondary WAS TAD +50 % Luste et al., 2009 70 °C, 1 h Digestive tract content MAD −23 % 70 °C, 1 h Drumsieve waste MAD +48 % 70 °C, 1 h Air flotation ABP sludge MAD +5.9 % 70 °C, 1 h Grease trap sludge MAD −6.7 % Hejnfelt and Angelidaki, 2009 70 °C, 1 h Slaughterhouse ABP TAD/MAD no sig. difference Luste and Luostarinen, 2010 70 °C, 1 h ABP and WAS MAD +13 % Rafique et al., 2010 50 - 70 °C, 1 h Dewatered pig manure MAD no sig. difference Luste and Luostarinen, 2011 70 °C, 1 h Cattle slurry MAD +33 % Rodríguez-Abalde et al., 2011 70 °C, 1 h Piggery ABP MAD +52 % Poultry ABP MAD +4.3 % Luste et al., 2012 70 °C, 1 h Cattle slurry MAD +20 % Yan et al. 2013 50 - 100 °C, 0.5 h Dewatered WAS MAD from +272 % to +684 % Grim et al., 2015 70 °C, 1 h Slaughterhouse waste TAD no sig. difference Nazari et al., 2017 80 °C, 5 h Various sewage sludge MAD from −33 % to +4.6 % 531 Table 2 presents a literature review of the impact of mild thermal pretreatment (whose treatment temperatures were inferior to 100 °C, serving as a hygienization step) on the methane production of various biowastes. It is interesting to note that besides the short-term thermal pretreatment (for hours), longer pretreatment time was also studied in order to have a more comprehensive knowledge about its effect on AD methane production (Climent et al., 2007). This long-term mild thermal pretreatment (for days) was reported to promote cell wall breaking of the substrates (Yao et al., 2016). 3. Alternative pasteurization technology Many alternative non-thermal pasteurization technologies are available nowadays in food-processing industries, for example Pulsed Electric Fields (PEF), High Hydrostatic Pressure (HHP), Power Ultrasound (PUS), Microwaves (MW) and chemical pretreatment by acid and alkali. Numerous studies concerning the positive impact of these technologies on the dewaterbility and on the methane potential enhancement of sewage sludge were conducted (Zhen et al., 2017). A methane production surplus of 33 - 250 %, 10 - 115 %, 9 - 138 %, 20 - 106 % and 1.5 - 145 % was achieved for electro-technology, HHP, PUS, MW and chemical treatment respectively (Carrère et al., 2010). In addition, several studies payed attention to the pathogen inactivation and BMP enhancement of biowastes, as resumed in Table 3. The specific energy input or the chemical dose of the pretreatment was extracted or calculated from the original papers if available. Table 3 Literature review of the application of the emerging hygienization technology to various biowastes for pathogen inactivation and BMP enhancement Method Authors, year Operational parameters Type of biowaste Reduction of the pathogens and BMP enhancement PEF Keles et al., 2010 50 Hz, 0.6 - 1.2kV∙cm-1 WAS 1.4 log10 of Salmonella spp. MW Pino-Jelcic et al., 2006 1 kW, 2450 MHz, 110 s Primary sludge WAS 4.2 log10 of Fecal coliforms 2.0 log10 of Salmonella spp. Hong et al., 2004 7.27 kJ∙g TS-1, 85 °C Primary sludge 6.8 log10 of Fecal coliform 11.4 kJ∙g TS-1, 85 °C WAS 6.5 log10 of Fecal coliform 10.1 kJ∙g TS-1, 65 °C AD sludge 5.6 log10 of Fecal coliform Hong et al., 2006 4.86 kJ∙g TS-1, 65 °C 7.60 kJ∙g TS-1, 65 °C Primary sludge WAS ~5.6 log10 of Fecal coliform ~5.4 log10 of Fecal coliform 10.1 kJ∙g TS-1, 65 °C AD sludge ~3.5 log10 of Fecal coliform PUS Chu et al., 2001 20 kHz, 0.1 - 0.3 W∙mL-1 120 min WAS 3 log10 of Total coliform 2.4 log10 of heterotrophs Ruiz-Hernando et al., 2014 5 - 27 kJ∙g TS-1 WAS 4 log10 of Escherichia coli +10 % CH4 production Alkali Ruiz-Hernando et al., 2014 35 - 157 g NaOH∙kg TS-1 24 h WAS 4 log of Escherichia coli +30 % CH4 production Coupled Jin, 2010 70 g NaOH∙kg TS-1 + MW, 120 °C, 30 min Dairy manure ~+50 % CH4 production 70 g CaO∙kg TS-1 + MW, 120 °C, 30 min Dairy manure ~+50 % CH4 production 2 % (v/v) H2SO4 + MW, 120 °C, 30 min Dairy manure no sig. CH4 enhancement 0.74 % (v/v) HCl + MW, 120 °C, 30 min Dairy manure ~+50 % CH4 production While increasing research focus was given to the WAS, scarce studies are available concerning the effect of these novel technologies on either the effect on pathogen reduction or methane potential enhancement of the ABP in a systematic way. 4. Conclusions Worldwide regulations on the hygienization of biowastes focus on a mild thermal pretreatment prior to the AD process. European Union proposes the strictest sanitary rules (70 °C for 1 h) as compared to other countries. This thermal hygienization accounts for around 6 - 19 % of the total energy production in European biogas plants. The hygienization, serving as a mild thermal pretreatment, might give rise to a positive impact on the 532 methane yield of the majority of the AD feedstock, while no effect or negative effect could also be seen for slaughterhouse by-products and several primary WAS. The application of emerging technologies on the WAS has been systematically studied in terms of its effects on the dewaterbility, on the methane potential enhancement and on the pathogen inactivation. Nevertheless, these effects on the ABP and other biowastes are insufficient. The energy consumption (considering the effect of BMP enhancement) and the related environmental impact (life cycle assessment) are not well studied either. These could be future research focuses and improve a cleaner energy production of anaerobic digestion. Acknowledgments This study was conducted in the framework of the PhD thesis funded by the French Regional Council of Brittany and the Departmental Council of Morbihan. The thanks are extended to the mixed economy company LIGER (Locminé, Morbihan, France) for its financial and technical support. 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