Thermo-chemical pre-treatment consists of thermal disruption with the simultaneous addition of acid, alkaline or oxidative compounds. There are two main pre-treatment groups: a) High temperature thermo- chemical processes (115–170 °C), and b) Low tem- perature thermo-chemical processes (50–90 °C). In both cases, chemical agents such as NaOH, KOH, O3, Na2CO3, HCl or H2O2 are added to the pre-treatment separately or in combination. An important advantage of thermo-chemical processes is the reduced reagent consumption, four to six times lower than single-stage chemical pre-treatments (Park et al. 2014). Kim et al. (2003) reported that adding NaOH at 121 °C resulted in 87 % COD solubilization, which is higher than most results reported for either thermal or chemical pre-treatments. Under these conditions, VS reduction was twice as high as that of the control. Similarly, Valo et al. (2004) found that compared to single thermal and chemical pre-treatments, the most efficient process in terms of COD solubilization was a combined oxidant and thermal process, with a value of 8 % 3 at 170 °C and pH 12 Biogas production and volatile solids removal improved by around 72 % using the thermo-chemical process. Park et al. (2014) concluded that thermo- chemical pre-treatment with NaOH (pH 12, 121 °C, 1 h) achieved higher COD solubilization and removal during a subsequent anaerobic/aerobic stabilization process. Moreover, methane yield increased by 154 %. While alkaline reagents have received more attention, the use of acid media during thermo-oxidative pre-treatment has been reported to improve sludge dewaterability, color generation, and methane yield compared to thermo-oxidative alkaline processes (Takashima and Tanaka 2008). However, maximum VSS reduction was achieved under alkaline conditions. Depending on oxidant strength, either methane production or solids reduction were favored.

the first full-scale lysate-centrifuge was BSC-4-2 (Baker Hughes) with input sludge flow 100–110 m3/h, installed in the Central Wastewater Treatment Plant in Prague (1,200,000 PE). Daily excess activated sludge input flow during normal operation was about 4,600 m3 (7 g/l TS), and the thickened sludge production was about 650 m3/d (50 g/l TS). The improvement of methane yield and biodegradability is influenced by the quality of input excess activated sludge and the parameters and efficiency of the thickening centrifuge. The improvement of methane yield from thickened activated sludge measured in batch laboratory tests was on average 11.5 –31.3 % dependent on the sludge quality. From 2002 there is in operation a further full-scale lysate-centrifuge in WWTP in Liberec (150,000 PE), where the specific biogas production increased from 0.443 Nm3/kg to 0.560 Nm3/kg, which represents an increment of 26% (Dohanyos et al.)

Highlights The advantages of ultrasonic technique for municipal sludge treatment are studied. Ultrasound can improve the final biogas production. Ultrasound can improve the efficiency of anaerobic digestion process. Ultrasound Applications in Wastewater Sludge Pretreatment: A Review Samir Kumar Khanal,David Grewell,Shihwu Sung &J. (Hans) van Leeuwen Abstract Municipal wastewater sludge, particularly waste activated sludge (WAS), is more difficult to digest than primary solids due to a rate-limiting cell lysis step. The cell wall and the membrane of prokaryotes are composed of complex organic materials such as peptidoglycan, teichoic acids, and complex polysaccharides, which are not readily biodegradable. Physical pretreatment, particularly ultrasonics, is emerging as a popular method for WAS disintegration. The exposure of the microbial cells to ultrasound energy ruptures the cell wall and membrane and releases the intracellular organics in the bulk solution, which enhances the overall digestibility. This review article summarizes the major findings of ultrasonic application in WAS disintegration, and elucidates the impacts of sonic treatment on both aerobic and anaerobic digestion. This review also touches on some basics of ultrasonics, different methods of quantifying ultrasonic efficacy, and some engineering aspects of ultrasonics as applied to biological sludge disintegration. The review aims to advance the understanding of ultrasound sludge disintegration and outlines the future research direction. There is general agreement that ultrasonic density is more important than sonication time for efficient sludge disintegration. Published studies showed as much as 40% improvement in solubilization of WAS following ultrasonic pretreatment. Based on kinetic models, ultrasonic disintegration was impacted in the order: sludge pH > sludge concentration > ultrasonic intensity > ultrasonic density. Both laboratory and full-scale studies showed that the integration of an ultrasonic system to the anaerobic digester improved the anaerobic digestibility significantly. The liquid in rarefaction zones becomes gaseous due to pressure, forming microbub- bles. The bubbles migrate to high-pressure zones and grow to a critical size before violently collapsing. The process of bubble formation and collapse is known as cavitation (Khanal et al. 2007), and can generate strong hydromechanical shear forces that disrupt sludge structure and increase local temperature and pressure to values up to 1000 °C and 500 bar respec- tively (Chu et al. 2002). Ultrasound can exert physical, chemical and biological effects on sludge, including particle size reduction, organic matter solubilization, enzyme release and stimulation of biological activity (Yu et al. 2009; Yan et al. 2010; Pilli et al. 2011; Guo et al. 2013). Braguglia et al. (2011) also found that biogas production increases were directly related to the disintegration degree and the specific energy applied. However, Erden and Filibeli (2010a) found that energy inputs over *9700 kJ/kgTS can decrease sludge disintegration, probably related to the oxidation of soluble organic matter due to the radicals generated by ultrasound. Reports show sludge solubilization results between 1 and 40 % for COD and 2–47 % for disintegration degree (DD) after sonication (Table 3). Solubilization during ultrasonic pre-treatment is mainly related to the effects on EPS, and even at high specific energy levels it does not promote cell disruption (Salsabil et al. 2009). Biogas production has been reported to increase up to 83 % after ultrasound (most reports in the range of 20–40 %), with increases of 6–47 % in volatile solids reduction (Table 3) Most ultrasound pre-treatments use low frequency waves (20–35 kHz) because they favor cavitation (Chu et al. 2002) One of the principal drawbacks of ultrasound pre- treatment is its high energy cost, which is not always compensated by increased biogas production. Thick- ening is considered fundamental to achieve a positive energy balance during ultrasound pre-treatment (Braguglia et al. 2011). Perez-Elvira et al. (2009), who reported that with a level of energy consumption corresponding to that of industrial equipment (6 kWh/m3), the ultrasound energy balance is positive at any sludge concentration

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High-pressure homogenization pre-treatment (HPH) subjects sludge to pressures of up to 600 bar and subsequent depressurization as it passes through a valve. Depressurized sludge impacts on a solid surface as it comes out of the valve, releasing cell contents due to the action of the pressure gradient, shear stress, turbulence and cavitation. The high pressure and consequent pressure drop along the valve are the main disruptive factors of the process (Zhang et al. 2012). it allows high lysis efficiency without the use of chemicals. Zhang et al. (2012) studied HPH to improve mesophilic AD of secondary sludge (20–50 MPa, 1–2 cycles). Solubilization after pre-treatment was low (3–8 %), with a proportional increase in methane yield when solubilization was [4 %. Removal of solids improved up to 138 % during AD, while accumulated biogas production improved up to 64–115 %. Methane content of biogas increased from 47 to 64 % due to pre-treatment. AD performance after homogenization was related to the intensity of the pre-treatment in terms of pressure and number of pressurization cycles. Specific energy needed for the process was 3380 kJ/kgTS, lower than the energy needed for sonication or microwave pre-treatment shown increased net energy production from mesophilic AD at short SRT (Wahidunnabi and Eskicioglu 2014). However, these results cannot be extrapolated to thermophilic AD, as the HPH energy balance is lower under those conditions.

avitation is a process of cavity bubble formation which burst within the liquid to create intense pressure spots and shock waves. These factors create localized energy and turbulence which causes an impact on adjacent particles and also mixing of insoluble substances like oil and water to form emulsion [26]. This mechanism is favorable in cases where AD of sludge is hindered due to the presence of lipid-containing substances which are insoluble in water. Their insolubility causes adversity in their interaction with hydrolytic bacteria which decreases the efficiency of the overall hydrolysis process. Applying localized energy supplies insignificant amounts to small elements of the liquid volume resulting in an increase of internal energy of the liquid elements to that point which causes phase change from liquid to gas and the formation of bubbles filled with vapor and gases. Following, when the bubbles leave the high energy zones, they violently implode and disappear. The localized energy could be provided by a laser beam or a stream of heavy elementary particles such as protons by molecular or optical cavitation process based on the source of applied energy [27]. Hydrodynamic cavitation was frequently been proved as a more energy-efficient method compared to other cavitation techniques [28]. Hydrodynamic cavitation for pre-treatment of sludge where cavitation was generated by using a venturi cavitation system in which bubbles are created in venturi throat (constriction) has been used. The system achieved better energy efficiency than high-speed homogenizer in terms of soluble COD/kJ WAS and also the authors observed linear relationship between total solid concentration and the increased insoluble COD for WAS indicating towards better cavitation formation at high concentration of total solids [29]. In another study, the degradation of WAS was analyzed using a novel rotation generator of hydrodynamic cavitation at pilot scale [30]. Cavitation (as a pre-treatment) of WAS resulting in an increment in soluble COD from 45 to 602 mg/L along with a 12.7% increase in biogas production due to improved AD of the pretreated WAS [30].

Nanobubbles are spherical liquid structures containing gas which are stable and efficient when possessing typical overall diameters in the nanometer range (less than 103 nm). The presence of negative charge on nanobubbles is observed when present in pure water over a wide pH range. Nanobubbles stabilized their structures because of the same charge repulsion that occurred between adjacent nanobubbles [18]. However, some reports suggest hydrophobic attraction between negatively charged surfaces of nanobubbles and these contradictory reports could be attributed to the differences in nanobubble generation techniques, surface tension, or varying molecular arrangement at the gas-liquid interface [19]. Nanobubbles with diameters of approximately 13 nm have been well-engineered as spherical water packages with gas for food safety applications whose efficiency is well established based on bubble surface stability and the electrostatic charges present on the bubble surface [20]. Besides possessing high stability, nanobubbles in liquid systems also show a high mass transfer rate and enhanced solubility in gas [21]. Nanobubbles with a varying range of diameters have been engineered by different methods such as constant purging of octafluoropropane gas into an ultrasonicated solution of mixed surfactant which creates bubbles ranging in 400–700 nm mean diameter [22]. Palladium electrode with ultrasonication has been used to form nanobubbles of 300–500 nm diameter [23]. Nanobubbles form reactive free radicals as they collapse due to the presence of ions in groups at the gas-liquid interface [24]. The ability of nanobubbles to form reactive free radicals makes them potent applicants in the field of pre-treatment of wastewater components. In submerged systems, nanobubbles formed by the use of air or nitrogen are known to enhance the activity of aerobic and anaerobic microorganisms that improve the waste degradation efficiency and overall water quality [25]. According to the studies the higher negative charges were observed on sludge components on the addition of nitrogen gas nanobubble water the degradation of carbohydrates and proteins get increased along with methane production, that is, 29% more than that of control [25].

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2 - 6 days SRT @ 50 - 70oC to complete acidogenisis. Followed by 14-30 days Thermophillic AD. Reported 18.5% increase in biomethane. The purpose of dual digestion is to improve AD by adding an anaerobic pre-digestion step oriented to hydrolysis. Anaerobic pre-treatment can be conducted under mesophilic or thermophilic conditions, but higher hydrolysis kinetics has been reported at high tempera- tures, prompting more interest in thermophilic pre- treatment. The most common method of anaerobic pre- treatment is temperature phased AD (TPAD), which uses thermophilic (~55 °C) or hyperthermophilic (60–70 °C) conditions for hydrolysis (Carrere et al. 2010). The principal aim of TPAD systems is to separate hydrolysis and methanogenesis into two steps. Yu et al. (2013a) recently proposed separating hydrolytic/ acidogenic and methanogenic microorganisms based on both SRT and temperature criteria, in a process they called temperature-staged biologically-phased AD (TSPB-AD). The optimum conditions for the pre- treatment were a 4-day SRT at 45 °C, allowing a significant improvement of 85 % in daily methane production compared to a single-phase system. The system was energy self-sufficient under the studied conditions. Thermophilic-thermophilic systems have also been studied, with interesting results. Bolzonella et al. (2012) evaluated the performance of a hyper-ther- mophilic (65 °C) hydrolytic step followed by thermophilic digestion (55 °C). Working with 2 days of SRT, the hydrolytic step improved SV removal by 53 % and biogas yield by 48 % compared to conventional digestion. Unlike aerobic pre-treatment, thermophilic anaerobic pre-treatment does not increase methane concentration. Under hyperthermophilic con- ditions (70 °C), Bolzonella et al. (2007) achieved COD solubilization of 23 % during pre-treatment, with improvements of 20–50 % in specific biogas production with mesophilic and thermophilic systems without pre-treatment. The thermophilic step removed around 25 % of volatile solids. They found that the thermophilic-mesophilic system produced around 24–28 % more methane per kg of removed VS than the thermophilic–thermophilic system. Volatile solids reduction of the systems was 30 %, with 14–17 % during pre-treatment. The inclusion of pre-treatment allowed reaching 5-log destruction of fecal coliforms, 97 % inactivation of helminth eggs and no detectable Salmonella in the sludge after digestion

Enzyme Addition Hydrolytic enzymes have an important role during sludge stabilization, transforming polymeric sub- stances into more biodegradable compounds (Yang et al. 2010). The addition of external enzymes to AD can therefore generate important benefits during AD, such as improved dewaterability and methane pro- duction (Davidsson and Jansen 2006; Dursun et al. 2006). Barjenbruch and Kopplow (2003) also reported improvements in sludge dewaterability by applying enzymes, with a reduction of 25 % in CST using a carbohydrase pre-treatment. Moreover, a 12 % increase in biogas yield was achieved during the subsequent AD. . Davidsson and Jansen (2006) also reported a small increase in methane potential during mesophilic batch digestion of secondary sludge (*3 %) using a mixture of polysaccharide-degrading enzymes, lipase and protease. The integration of enzymatic pre-treatment with ultrasound and thermal processes further increased methane yield by 18 and 14 %, respectively. The highest VSS reduction (68 %) was observed with the mixture ratio of 1:3 for protease and amylase at 50 °C. Under those conditions, concentration of reducing sugar and ammonia increased by around 150 and 78 %, respectively. Rashed et al. (2010) studied the effect of six commercial enzyme mixtures on different sludge combinations (with primary, secondary and digested sludge). They found that VS reduction during batch assays depends on both enzyme and sludge combina- tions. Optimal enzyme doses were around 0.1 % (TS basis), and VS reductions improved by 16.3 %. The synergistic effect of using enzyme mixes was observed, with consistently good results.

The presence of proteolytic bacteria like Geobacillus stearothermophilus in the activated sludge makes aerobic thermophilic pre-treatment possible without bio-augmentation, providing impor- tant benefits to the overall stabilization process (Dumas et al. 2010). Moreover, aerobic processes can degrade materials that do not degrade under anaerobic conditions, further improving stabilization (Carrere et al. 2010). Different configurations have been proposed to improve AD using aerobic microorganisms. Pre- treatment before conventional mesophilic digestion (Pagilla et al. 1996; Borowski and Szopa 2007; Jang et al. 2014), co-treatment in sludge recirculation (Dumas et al. 2010), aerobic post-treatment after digestion (Novak et al. 2011; Tomei et al. 2011a, b) and the addition of aerobic bacteria to anaerobic digesters (Miah et al. 2005) have all been reported. Jang et al. (2014) studied the effect of thermophilic aerobic digestion at 55 °C to improve meshophilic AD of mixed sludge. The methane production rate improved 42 %, while methane yield improved 12 % due to the pre-treatment. Aerobic pre-treatment has also been reported to positively affect the centrifugability of digested sludge (Pagilla et al. 1996). Dumas et al. (2010) studied the effect of incorpo- rating a thermophilic digester at 65 °C in the recircu- lation of a meshophilic AD system. Their results indicate that the aerobic-anaerobic system improved the reduction of volatile suspended solids by 39–83 %. However, there was no improvement in methane production, and improvement in solids reduction was related to aerobic oxidation of organic matter during pre-treatment. Miah et al. (2005) studied the effect of adding aerobic thermophilic bacteria to an anaerobic digester. Methane yield improved by 21–112 % and VS reduction by 4–44 %.

The use of the inherent hydrolytic potential of the sludge can reduce the energy costs of sludge pre-treatment, while avoiding the addition of chemical compounds or external enzymes. there are commercial applications of auto-hydrolytic processes, with the Enzymic Hydrolizer marketed by Monsal being one of the most extended processes (Mayhew et al. 2002). Pre-treatment reduced resistance to sludge flow, and methane production during AD improved by 23 % after 12 h of pre-treatment. The energy balance of the process was positive with 8 % TS concentrated sludge and 12-h treatment time.

Activated carbon enhanced anaerobic digestion of food waste – Laboratory-scale and Pilot-scale operation Author links open overlay panelLeZhangaJingxinZhangbKai-CheeLoha Highlights from paper Highlights Optimum adding dose of activated carbon (AC) was 15 g/L working volume to enhance AD. Adding AC significantly improved methane yield and process stability in 8 L digester. Color removal of liquid phase of digestate and sludge granulation were improved. Abundance of phyla Firmicutes and Elusimicrobia were enriched by 1.7–2.9 times by AC. Methane yield in 1000 L pilot-scale digester increased 41% by AC supplementation. https://www.sciencedirect.com/science/article/abs/pii/S0956053X18300862?via%3Dihub

Based on a Dry Solids concentration 1600 to 8000 tpa I have converted this to work out daily flow rate and assumed COD. Biogas production will depend on the effluent makeup and BOD reduction Typical performances are BOD removal : 80-90 % COD removal :1.5xBOD Biogas production 0.5m3/kg COD removed Methane production 0.35m3/kg COD removed HRT up to 30% reduction dependant on effluent chemistry Initial dose rate is initially much higher for slug dosing at 10 - 20kg per day for 3 days.. This is to establish the PAC within the digester Insight into sludge anaerobic digestion with granular activated carbon addition: Methanogenic acceleration and methane reduction relief Qian Jiang 1, He Liu 2, Yan Zhang 3, Min-Hua Cui 3, Bo Fu 3, Hong-Bo Liu 3 Affiliations expand PMID: 33002784 DOI: 10.1016/j.biortech.2020.124131 Abstract In this study, the multiple effects of granular activated carbon (GAC) on sludge anaerobic digestion at ambient (16-24 °C), mesophilic (35 °C) and thermophilic (55 °C) temperature were investigated. After GAC addition, although the methane yields of raw sludge were reduced by 6.5%-36.9%, the lag phases of methanogenesis were shortened by 19.3%-30.6% and the reductions of methane yields were declined to only 5.9%-8.1% simultaneously for pretreated sludge. The inhibitory substances like phenols that generated by thermal pretreatment were reduced after GAC addition, which were demonstrated to be responsible for the methanogenic acceleration. Meanwhile, the methane reduction due to the non-selective adsorption by GAC could be mitigated by pretreatment and elevated temperature. Thus, a strategy coupling thermal pretreatment with detoxification by GAC was proposed to improve the methane production rate and avoid the negative effects during sludge anaerobic digestion with GAC addition. Keywords: Anaerobic digestion; Granular activated carbon; Methane reduction relief; Methanogenic acceleration; Waste activated sludge.

Conditioning of wastewater sludge using freezing and thawing: role of curing Kai Hu 1, Jun-Qiu Jiang, Qing-Liang Zhao, Duu-Jong Lee, Kun Wang, Wei Qiu While this method is very energy-intensive, in regions where freezing occurs naturally it could be a sustainable alternative to other technologies (Montusiewicz et al. 2010). The process compacts the floc structure and reduces the sludge- bound water content, thus improving dewateration (Jean et al. 2001). Solubilization of total nitrogen (TN), ammonia and orthophosphates was also observed, with increases of 79, 39 and 114 % in their respective soluble concentrations. Biogas pro- duction increased *52 %, with no significant effects on methane concentration.

Solubilisation of OM & release of hydrolytic enzymes Low temperature pre-treatment consists of applying moderately high temperatures to sludge, normally under 100 °C. While the main mechanism of high temperature lysis is physical disruption and solubilization of organic matter, low temperature hydrolysis can include thermal solubilization, stimulation of thermophilic bacteria and solubilization by hydrolytic enzymes released from the sludge (Climent et al. 2007; Ferrer et al. 2008; Carvajal et al. 2013). Low temperature pre-treatment has resulted in solubilization levels ranging from negligible values to 20 % in terms of COD. Biogas production during AD after low temperature pre-treatment can be significantly enhanced, as reports show improvements that vary from 10 to 984 % (Table 1). While the range is very wide, it is important to note that most of the reports are in the range between 10 and 40 %. Higher values could be related to low biodegradability of raw sludge, as Appels et al. (2010) reported an improvement of 984 % for a sample with a biogas yield of only 34.83 mL/g ODS. This important improvement for secondary sludge reflects the very low biodegradability of raw sludge (*6 % of VSS destruction), and reaffirms the suit- ability of this process to improve the digestion of recalcitrant sludge. Improvements in VS and TS reduction are not necessarily proportional to improve- ments in biogas production. Moreover, low tempera- ture pre-treatment has also been reported to improve sludge rheology and increase methane concentration in biogas (Ruffino et al. 2015). Higher temperatures may be necessary to degrade complex organic pollutants.Gavala et al. (2004)

Disintegration of solids removal of pathogen May create new chemical bonds Hydrolysis at pressures of 600 to 2500kPa for 30 mins to one hour HX or steam injections. Above 190C recalcitrant compounds can form Ammonia and metals released Reduces viscosity high temperature pretreatment increases charge in sludge compounds and colloids and Extra Cellular Polymeric substances (EPS) release Divergent results have been reported regarding the influence of high temperature lysis on digestion performance, with some authors reporting no increase in biogas production due to thermal pre-treatment (Climent et al. 2007) and others describing increases of up to 150 % (Carre`re et al. 2008). Similarly, improvements in solids reduction ranging from 7 to 105 % have been reported (Table 2). These differences could be related to process conditions and sludge characteristics. Since thermal pre-treatment increases digestion kinetics, the most significant improvements in methane production are normally achieved with short solid retention times (SRT) (Carrere et al. 2008). Nevertheless, Wett et al. (2010) showed that the combined effect of solids reduction and better centrifugability caused by hydrolysis at 160–180 °C can represent a 25 % reduction in sludge volume, highlighting the positive influence that high temperature hydrolysis can have on disposal costs. Finally, energy balances for thermal hydrolysis/AD systems have been evaluated with positive results when high sludge concentrations and proper energy integration are achieved (Pickworth et al. 2006; Bougrier et al. 2007; Fdz-Polanco et al. 2008). Pickworth et al. (2006) and Fdz-Polanco et al. (2008) have shown that it is possible to establish a self-sufficient process with increased electricity gen- eration of around 40 % due to the introduction of thermal pre-treatment.

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The principle of acid pre-treatment is similar to alkaline, as the acid breaks down polymers into monomers or oligomers, which increases the digestion rate as partial hydrolysis has been carried out (Devlin et al. 2011). However, current results show that alkaline pre-treatment is more effective than acid in terms of hydrolysis and solubilization of organic matter (Chen et al. 2007). Devlin et al. (2011) reported an increase of 12–32 % in biogas production after acidic pre- treatment with HCl compared to untreated WAS, while VS removal increased by 5 %. No effects were observed over methane concentration in biogas. Others effects on digestion include a 40 % lower polymer requirement prior to dewatering, effective destruction of Salmonella and close to a 3-log reduction in E. coli.

Ozonation

Fenton Reaction

POMS/DMDO peroxidation

Caustic may inhibit AD if used to excess. The chemical dosage is one of the key operational parameters of alkaline pre-treatment. While high doses are normally associated with higher levels of solubi- lization, excessively high doses can reduce the activity of anaerobic consortia due to the pH of pre-treated sludge or the presence of inhibitors (Li et al. 2012; Fang et al. 2014). Saponified EPS Alkaline pre-treatment is relatively simple and with low energy requirements (Neyens and Baeyens 2003). It is very effective for solubilizing COD. A ‘‘maximum hydrolysable’’ COD protocol has been proposed based on 24-h alkaline disintegration (Mu¨ller 2003). NaOH, KOH, Mg(OH)2, Ca(OH)2 increases in sCOD of 39.8, 36.6, 15.3 and 10.8 resp. Regarding methane production, improvements of up to 120 % were observed, related to higher NaOH doses and sludge concentration and shorter SRT during AD. Contrary to the report of Kim et al. (2003), methane concentration in biogas improved from 72 to 85 % due to pre-treatment, even while doses were significantly lower. The main disadvantages of alkaline pre-treatment are associated with the presence of residual chemicals in the pre-treated sludge that can destroy the bicar- bonate buffer system in anaerobic digesters and inhibit anaerobic microorganisms (Li et al. 2012; Kim et al. 2013b), as well as the high consumption of reagents.Excessive alkali dosages have also been reported to promote the formation of refractory compounds through the Maillard reaction (Penaud et al. 1999), similar to the effect observed when thermal lysis is conducted at temperatures exceeding 180 °C.

High solids affects penetration of microwaves High energy consumption Microwave moves dipoles to generate heat Application of microwaves has both thermal and non-thermal effects on sludge properties. Thermal effects occur due to the interaction of the electric field of the radiation with dipolar molecules, increasing temperature due to molecular rotation and friction. Non-thermal effects are associated with a rapid change in the dipole orientation of the macromolecules making up the cell membrane, breaking the hydrogen bonds between molecules and therefore disrupting the membrane (Appels et al. 2013). At lower temperatures, microwave has shown better pre-treatment results than conventional heating, probably related to the aforementioned non-thermal effect. Kuglarz et al. (2013) studied the effect of both microwave and thermal lysis in the range of 30–100 °C. They found that the application of microwave until 70 °C was the most effective method to improve biogas production and energy gain. Coelho et al. (2011) studied the effects of microwave pre-treatment on mesophilic and thermophilic digestion of secondary sludge, achieving biogas yield improvements of up to 32 and 35 % for mesophilic and thermophilic digesters, respectively. Additionally, solids reduction improved 48 % for thermophilic and 38 % for mesophilic systems. Eskicioglu et al. (2009) studied the effect of microwave pre-treatment over batch mesophilic AD of secondary sludge, achieving a maximum solubilization of 31 % and biogas production improvements on the same order. At 336 kJ/kg sludge specific energy (800 W, 80 °C), Appels et al. (2013) achieved 6.6 % COD solubilization and a more than threefold increase in VFA concentration. Methane production increased around 50 % and organic solids reduction increased 37.5 %. However, microwave generation requires higher energy consumption when compared to conventional thermal pre-treatment. The increase in temperature is associated with an increase in biodegradability while a higher concentration of solids present in WAS inversely affects the degree of penetration of microwaves to the sample .

Electric pulse technology uses high voltage pulsing electric fields (20–30 kV) to disrupt bacterial cell membranes. At full scale, focused pulse pre-treatment has increased biogas pro- duction by 15–40 % and sludge reduction by 2–30 %, depending on the fraction of the sludge stream that is subjected to pre-treatment (Rittmann et al. 2008; Zhang et al. 2009). Another electrical-based method consists in the use of electrolysis. Charles et al. (2013) used a 12 V two- chamber electrolysis process with an anion exchange membrane to alter WAS pH as an alternative to chemical addition. The pH level in the anode decreased from 6.9 to 2.5, while in the cathode pH increased to 10.1, achieving COD solubilization of 9.5 and 6.4 % respectively. Mixing the two pre-treated currents neutralized pH to 6.5, and therefore chemical neutralization was not necessary. Biogas yield during the following mesophilic batch AD increased 31 %, which was attributed to the effects of the pH level and not to cell disruption by electrolysis.

Yang et al. (2011) reported that pre- treatment using light irradiation at 100 ± 10 lmol/ m2 s improved thermophilic (55 °C) digestion due to cell disruption and increased organic carbon dissolu- tion (9 % after pre-treatment). In this study, methane yield improved up to 12 % and VS removal up to 53 % with 24 h treatment time, with partial VS removal occurring during pre-treatment.

Implementation of thermophilic anaerobic digestion (55°C) and excess sludge disintegration by means of lysate-thickening centrifuge can improve the raw sludge biodegradation and biogas production to such an extent, that the WWTP can be energetically self-sufficient. The energy balance is calculated for daily loading of 80,000 kg of organic solids and for biogas specific production rate 0.61 Nm3/kg of input organic solids. Total biogas production can be utilized in cogeneration units and corresponds to 108.1 MWh/d of electric energy and 167 MWh/d of heat energy. In combination with a heat recuperation of sludge output it could cover the total energy demand even in wintertime