pretreatment methods to enhance anaerobic digestion of organic solid waste

Applied Energy 123 (2014) 143–156

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Pretreatment methods to enhance anaerobic digestion of organic solidwaste

http://dx.doi.org/10.1016/j.apenergy.2014.02.0350306-2619/� 2014 Elsevier Ltd. All rights reserved.

Abbreviations: AD, anaerobic digestion; COD, chemical oxygen demand; CSTR, continuously stirred reactor; FW, food waste; HHW, household wasthydroxymethylfurfural; HPH, high pressure homogenizer; HRT, hydraulic retention time; MSW, municipal solid waste; OFMSW, organic fraction of municipal soliOLR, organic loading rate; OM, operational and maintenance; OSW, organic solid waste; PEF, pulsed electric field; SRT, solid retention time; SS, sewage sludge; THPhydrolysis process; TPAD, temperature phased anaerobic digestion; WWTP, wastewater treatment plant; TS, total solid; UASB, upflow anaerobic sludge blanket; VSsolid; VFA, volatile fatty acid; WAS, waste activated sludge.⇑ Corresponding author at: Department of Civil and Mechanical Engineering, University of Cassino and the Southern Lazio, Via Di Biasio, 43, 03043 Cassino, FR, It

+39 07762993989.E-mail addresses: [email protected], [email protected] (J. Ariunbaatar).

Javkhlan Ariunbaatar a,c,⇑, Antonio Panico b, Giovanni Esposito a, Francesco Pirozzi b, Piet N.L. Lens c

a Department of Civil and Mechanical Engineering, University of Cassino and the Southern Lazio, Via Di Biasio, 43, 03043 Cassino, FR, Italyb Department of Civil, Architectural and Environmental Engineering, University of Naples Federico II, Via Claudio, 21, 80125 Naples, Italyc UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX Delft, The Netherlands

h i g h l i g h t s

� Pretreating organic solid wastes leads to an enhanced anaerobic digestion process.� Pretreatments may also reduce the cost for post treatment of digestates.� Efficiency of pretreatment methods depends on the substrates’ characteristics.� Only few pretreatment methods are successfully applied at full-scale to date.

a r t i c l e i n f o

Article history:Received 1 August 2013Received in revised form 24 January 2014Accepted 9 February 2014

Keywords:Anaerobic digestionPretreatmentOrganic solid wasteSustainabilityEconomic cost

a b s t r a c t

This paper reviews pretreatment techniques to enhance the anaerobic digestion of organic solid waste,including mechanical, thermal, chemical and biological methods. The effects of various pretreatmentmethods are discussed independently and in combination. Pretreatment methods are compared in termsof their efficiency, energy balance, environmental sustainability as well as capital, operational and main-tenance costs. Based on the comparison, thermal pretreatment at low (<110 �C) temperatures and two-stage anaerobic digestion methods result in a more cost-effective process performance as compared toother pretreatment methods.

� 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1442. Mechanical pretreatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

2.1. Process description and mode of action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1442.2. Mechanical pretreatment of OFMSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1452.3. Mechanical pretreatment of miscellaneous OSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

3. Thermal pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
3.1. Process description and mode of action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1463.2. Thermal pretreatment at lower temperatures (<110 �C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1463.3. Thermal pretreatment at higher temperature (>110 �C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
e; HMF,d waste;, thermal, volatile

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144 J. Ariunbaatar et al. / Applied Energy 123 (2014) 143–156

4. Chemical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
4.1. Process description and mode of action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
4.1.1. Alkali pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.1.2. Acid pretreatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1464.1.3. Effects of accompanying cations present in the acid/alkaline reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1474.1.4. Ozonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

4.2. Chemical pretreatments of OSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5. Biological pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
5.1. Conventional biological pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.2. Two-stage AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5.2.1. Temperature phased anaerobic digestion (TPAD). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1485.2.2. Biohythane production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

6. Combination of various pretreatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.1. Thermo-chemical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.2. Thermo-mechanical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1486.3. Various pretreatments combined with a two-stage AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
7. Comparison of pretreatment methods to enhance anaerobic digestion of OFMSW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1498. Feasibility of a full scale application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

8.1. Energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1498.2. Economic feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1518.3. Environmental aspects and sustainability of pretreatment methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

1. Introduction

Anaerobic digestion (AD) is one of the oldest and well-studiedtechnologies for stabilizing organic wastes [1]. Among the treat-ment technologies available for treating organic solid wastes(OSW), AD is very suitable because of its limited environmentalimpacts [2–5] and high potential for energy recovery [2,3,6]. Suchpositive aspects coupled with the recent concerns on rapid popula-tion growth, increasing energy demand, and global warming havepromoted further research on the AD process development andimprovement in order to enhance biogas production, achieve fasterdegradation rates and reduce the amount of final residue to be dis-posed [3,4,7].

AD is a biological process that converts complex substrates intobiogas and digestate by microbial action in the absence of oxygenthrough four main steps, namely hydrolysis, acidogenesis, aceto-genesis and methanogenesis. Most researchers report that therate-limiting step for complex organic substrates is the hydrolysisstep [8–26], due to the formation of toxic by-products (complexheterocyclic compounds) or non desirable volatile fatty acids(VFA) formed during the hydrolysis step [27,28]; whereas metha-nogenesis is the rate-limiting step for easily biodegradable sub-strates [24,27,29,30]. Extensive research has been conducted onpretreatment methods to accelerate the hydrolysis step [31,32]and to obtain suitable by-products from this step [28], as well asto improve the quality of useful components like nitrogen andphosphorus to be recycled [33].

According to European Union Regulation EC1772/2002, sub-strates such as municipal solid waste (MSW), food waste (FW),and slaughterhouse wastes need to be pasteurized or sterilized be-fore and/or after AD. Taking this regulation into account, pretreat-ment methods could be applied, thus obtaining a higher energyrecovery and eliminating the extra cost for pasteurization and/orsterilization [34,35]. Pretreatment methods could nevertheless beunsustainable in terms of environmental footprints, even if theyenhance the AD process performance [36]. The effects of variouspretreatment methods are highly different depending on the char-acteristics of the substrates and the pretreatment type. Hence, it isdifficult to compare and systematically assess the applicability andsustainability of such methods at a full scale.

In the recent past a number of reviews have been publishedwith a common aim to assess the pretreatment effects. Table 1shows that most of the research on pretreatment methods hasbeen conducted on wastewater treatment plant (WWTP) sludgeand/or lignocellulosic substrates; whereas there is a limited num-ber of reviews on the recently growing interest of pretreatmentmethods to enhance AD of OSW, specifically the organic fractionof municipal solid waste (OFMSW). Therefore, this paper aims toreview the most recently studied pretreatment methods includingmechanical, thermal, chemical and biological methods to enhanceAD of OSW, with an emphasis on OFMSW. The pretreatment meth-ods will be compared in terms of efficiency, energy balance, costand process sustainability.

2. Mechanical pretreatment

2.1. Process description and mode of action

Mechanical pretreatment disintegrates and/or grinds solid par-ticles of the substrates, thus releasing cell compounds and increas-ing the specific surface area. An increased surface area providesbetter contact between substrate and anaerobic bacteria, thusenhancing the AD process [3,24,25]. Esposito et al. suggested thata larger particle radius results in lower chemical oxygen demand(COD) degradation and a lower methane production rate [37]. Like-wise, Kim et al. showed that particle size is inversely proportionalto the maximum substrate utilization rate of the anaerobic mi-crobes [38]. Therefore, mechanical pretreatments such as sonica-tion, lysis-centrifuge, liquid shear, collision, high-pressurehomogenizer, maceration, and liquefaction are conducted in orderto reduce the substrate particle size.

In addition to size reduction, some methods result in other ef-fects depending on the pretreatment. Hartmann et al. reported thatthe effect of maceration is more due to shearing than cutting of thefibers [39]. Sonication pretreatment generated by a vibrating probemechanically disrupts the cell structure and floc matrix [40]. Themain effect of ultrasonic pretreatment is particle size reductionat low frequency (20–40 kHz) sound waves [41]. High-frequencysound waves also cause the formation of radicals such as OH�,HO�2, H�, which results in oxidation of solid substances [42].

Table 1Reviews on different pretreatment methods to enhance AD using various substrates.

Substrate Pretreatment methods Important findings Reference

OFMSW All pretreatment methods � Physical pretreatments are widely applied for OFMSW, whereas other methods are not spread atindustrial level

� Further research on pretreatment should focus more on the modelling as well as mass and energybalance of the pretreatment effect and the whole AD process

[61]

All organicsubstrates

All pretreatment methods � The most popular pretreatment methods are thermal and ultrasonic for WWTP sludge, chemicalfor lignocellulosic substrates, and mechanical for OFMSW

� Systematic studies on energy balance and economic feasibility are necessary� Further development of descriptive and predictive variables is required

[39]

Lignocellulosicsubstrates

Thermal, thermo-chemical,chemical

� Pretreatments could improve the digestibility of lignocellulosic substrates� Pretreatments could result in more efficient process as compared to the conventional process

[73]

Lignocellulosicsubstrates

Thermal, thermo-chemical,and chemical

� Thermal pretreatments as well as lime and ammonia based chemical methods are more effectivein improving the digestibility of lignocellulosic substrates

[34]

Pulp & papersludge

Thermal, thermo-chemical,chemical

� Pretreatments could result in reduced HRT, increased methane production, and reduced sludgesize

[40]

WWTP sludge Ultrasound, chemical, thermal,and microwave

� Pretreatments result in enhanced biogas production (30–50%)� Comprehensive model for evaluating the economic feasibility was developed

[22]

WWTP sludge Thermal, thermo-chemical,and chemical

� The effect of pretreatment methods depends on the characteristics of sludge and the intensity ofthe method

� Pretreatments could yield a better digestate with high recoverable nutrients

[3]

WWTP sludge Thermal and thermo-chemical � Thermal pretreatment at high temperature (>175 �C) as well as thermo-chemical methods aremore effective in improving sludge dewaterability

[60]

J. Ariunbaatar et al. / Applied Energy 123 (2014) 143–156 145

A high pressure homogenizer (HPH) increases the pressure upto several hundred bar, then homogenizes substrates under strongdepressurization [43]. The formed cavitation induces internal en-ergy, which disrupts the cell membranes [44]. These pretreatmentmethods are not common for OFMSW, but they are more popularwith other substrates such as lignocellulosic materials, manureand WWTP sludge. Size reduction through beads mill, electropora-tion and liquefaction pretreatments of OFMSW has been studied atlab scale, whereas rotary drum, screw press, disc screen shredder,FW disposer and piston press treatment are successfully applied atfull scale. Both electroporation and liquefaction pretreatmentscause cellular structure damage, thus the effect on the AD processis similar to maceration [45,46].

The advantages of mechanical pretreatment include no odourgeneration, an easy implementation, better dewaterability of thefinal anaerobic residue and a moderate energy consumption. Dis-advantages include no significant effect on pathogen removal andthe possibility of equipment clogging or scaling [47,48].

2.2. Mechanical pretreatment of OFMSW

Mechanical pretreatments such as rotary drum were used as aneffective technology for OFMSW separation and pretreatment priorto AD, which could enhance the biogas production by 18–36%[49,50]. Davidson et al. found small variations in both methaneyields per gVS (gram volatile solids) and content of methane in bio-gas while studying the biomethane potential of source-sortedOFMSW pretreated with different mechanical methods includingscrew press, disc screen shredder, FW disposer and piston press[51]. Similarly, Zhang and Banks found no significant enhancementwith such pretreatment methods [52]. Hansen et al. studied the ef-fects of the same pre-treatment technologies on the quantity andquality of source-sorted OFMSW. They found that screw press pre-treatment resulted in a smaller substrate particle size, while ashredder with magnetic separation yielded a higher (5.6–13.8%as compared to the other methods) methane production [53]. Incontrast, Bernstad et al. reported that the screw press pretreatmentmethod also result in a loss of biodegradable materials and nutri-ents, even though it enhances the biogas production in general[54].

Izumi et al. studied the effect of the particle size on FW bio-methanation. Size reduction through a beads mill resulted in a40% higher COD solubilization, which led to a 28% higher biogas

production yield. However, excess size reduction to particles smal-ler than 0.7 mm caused an accumulation of VFA [8]. As the meth-anogens are sensitive to acidic intermediates [55], excessive sizereduction may result in a decreased AD process performance.Few research on electroporation, liquefaction, and high frequencysonication pretreatment methods to enhance OFMSW has beenconducted. Electroporation pretreatment of OFMSW resulted in20–40% higher biogas production [46], and liquefaction resultedin 15–26% higher biogas production [3], whereas sonication re-sulted in 16% higher cumulative biogas production as comparedto untreated substrates [56]. The higher biogas production wasmostly due to the more extensive solubilization of the particulates.

2.3. Mechanical pretreatment of miscellaneous OSW

Maceration, sonication and HPH are the simplest mechanicalpretreatments for OSW such as WWTP sludge and lignocellulosicsubstrates. Size reduction of lignocellulosic substrates results in a5–25% increased hydrolysis yield, depending on the mechanicalmethods used [34], whereas for WWTP sludge and manure, the ef-fects of pretreatments significantly differ. Generally, applying mac-eration pretreatment enhances biogas production by 10–60% [3].For instance, maceration of fibers in manure up to 2 mm resultedin a 16% increase of the biogas production, while size reductionup to 0.35 mm resulted in a 20% increase, and no significant differ-ence was observed with further size reduction [57].

Barjebruch and Kopplow treated surplus sludge with HPH at600 bar, and showed that the filaments were completely disinte-grated [44]. Engelhart et al. studied the effect of HPH on the ADof sewage sludge (SS), and achieved a 25% increased VS reduction[58]. This improvement was induced by the increased soluble pro-tein, lipid, and carbohydrate concentration. The HPH of WWTPsludge has been applied at full scale, resulting in a 30% biogasenhancement, thus the working volume of digesters could be de-creased by 23% [3].

Sonication prior to the AD process resulted in an enhancementof the biogas production of 24–140% in batch systems, and 10–45%in continuous or semi-continuous systems [3]. However, not allstudies confirm the enhancement of VS destruction or higher bio-gas production. Sandino et al. studied sonication of waste activatedsludge (WAS) and obtained only a negligible increase in both VSdestruction and mesophilic methane production [59].


3. Thermal pretreatment


Thermal treatment is one of the most studied pretreatmentmethods, and has been successfully applied at industrial scale[3,31,61]. Thermal pretreatment also leads to pathogen removal,improves dewatering performance and reduces viscosity of thedigestate, with subsequent enhancement of digestate handling[2,31,32,62]. Various temperatures (50–250 �C) to enhance theAD of different OSW (mainly WWTP sludge and lignocellulosicsubstrates) have been studied. However, to the best of ourknowledge, no systematic research on various temperature andtreatment times to enhance AD of OFMSW has been conducted.

The main effect of thermal pre-treatment is the disintegrationof cell membranes, thus resulting in solubilization of organic com-pounds [17,63–65]. COD solubilization and temperature have a di-rect correlation. Higher solubilization can also be achieved withlower temperatures, but longer treatment times. Mottet et al. com-pared different thermal pretreatment methods and found no sig-nificant difference between steam and electric heating, whereasmicrowave heating solubilized more biopolymers [66]. The higherrate of solubilization with microwave pretreatment can be causedby the polarization of macromolecules [47,63]. Concerning the lig-nocellulosic substrates, temperatures exceeding 160 �C cause notonly the solubilization of hemicellulose but also solubilization oflignin. The released compounds are mostly phenolic compoundsthat are usually inhibitory to anaerobic microbial populations [34].

Bougrier et al. suggested that thermal pretreatment at hightemperatures (>170 �C) might lead to the creation of chemicalbonds and result in the agglomeration of the particles [42]. Oneof the most known phenomena is the Mallaird reaction, which oc-curs between carbohydrates and amino acids, resulting in the for-mation of complex substrates that are difficult to be biodegraded.This reaction can occur at extreme thermal treatment at tempera-tures exceeding 150 �C, or longer treatment time at lower temper-atures (<100 �C) [3,25,34,67,68].

In addition to these chemical reactions, thermal pretreatmentcan also result in loss of volatile organics and/or potential biome-thane production from easily biodegradable substrates. Therefore,the effects of thermal pretreatment depend on the substrate typeand temperature range.

3.2. Thermal pretreatment at lower temperatures (<110 �C)

Protot et al. suggested that thermal pretreatment at tempera-tures below 100 �C did not result in degradation of complex mole-cules, but it simply induces the deflocculation of macromolecules[64]. Barjenbruch and Kopplow obtained a similar conclusion withpretreatment at 90 �C. Their results showed that the filaments arenot disintegrated, but they were only attacked with thermal pre-treatment [44]. Neyens and Bayens reported that thermal pretreat-ment resulted in the solubilization of proteins and increased theremoval of particulate carbohydrates [60].

Thermal pretreatment of sludge even at lower temperature(70 �C) has a decisive effect on pathogen removal [24]. Probablybased on such results, the EU Regulation EC1772/2002 requiresOSW to be pretreated at least an hour at 70 �C. In this regard,numerous studies on thermal pretreatment at 70 �C were con-ducted. For instance, pretreating household waste and algal bio-mass at 70 �C for 60 min and 8 h, respectively, did not result inenhancement of the biogas production [18,69]. Appels et al. ob-tained a negligible increase of biogas production from sludge pre-treated at 70 �C for 60 min, whereas the biogas production wasimproved 20 times when applying a 60 min pretreatment at

90 �C [19]. Rafique et al. achieved a maximal enhancement of78% higher biogas production with a 60% methane content by pre-treatment at 70 �C [10]. Ferrer et al. obtained a 30% higher biogasproduction with a 69% methane content [17], whereas Climentet al. obtained a 50% biogas volume increase with pretreatmentat 70 �C prior to thermophilic AD [70]. Gavala et al. reported thatpretreatment of primary and secondary WWTP sludge at 70 �Chas a different effect on the thermophilic and mesophilic methanepotential. Thermal pretreatment at 70 �C was shown to have a po-sitive effect on mesophilic AD of primary sludge, but not on itsthermophilic AD; whereas it enhanced both the thermophilic andmesophilic methane production of secondary sludge. This can beexplained by the chemical composition of the OSW substrates: pri-mary sludge contains higher amounts of carbohydrates, whereassecondary sludge contains higher amounts of proteins and lipids[26].

3.3. Thermal pretreatment at higher temperature (>110 �C)

Liu et al. studied the thermal pre-treatment of FW and fruit andvegetable waste at 175 �C; they obtained a 7.9% and 11.7% decreaseof the biomethane production, respectively, due to the formation ofmelanoidins [62]. Ma et al. obtained a 24% increase of the biome-thane production with FW pretreated at 120 �C [9]. Rafique et al.studied pretreatment of pig manure at temperatures higher than110 �C [10]. They observed hardening and darkening of manure,which resulted in a low biogas yield. Hardening and the darkbrownish color development of the substrate indicated the occur-rence of Mallaird reactions.

4. Chemical pretreatment


Chemical pretreatment is used to achieve the destruction of theorganic compounds by means of strong acids, alkalis or oxidants.AD generally requires an adjustment of the pH by increasing alka-linity, thus alkali pretreatment is the preferred chemical method[71]. Acidic pretreatments and oxidative methods such as ozona-tion are also used to enhance the biogas production and improvethe hydrolysis rate. The effect of chemical pretreatment dependson the type of method applied and the characteristics of the sub-strates. Chemical pretreatment is not suitable for easily biodegrad-able substrates containing high amounts of carbohydrates, due totheir accelerated degradation and subsequent accumulation ofVFA, which leads to failure of the methanogenesis step [72]. In con-trast, it can have a clear positive effect on substrates rich in lignin[13].

4.1.1. Alkali pretreatmentDuring alkali pretreatment, the first reactions that occur are sol-

vation and saphonication, which induce the swelling of solids [31].As a result, the specific surface area is increased and the substratesare easily accessible to anaerobic microbes [34,73,74]. Then, CODsolubilization is increased through various simultaneous reactionssuch as saponification of uronic acids and acetyl esters, as well asneutralization of various acids formed by the degradation of theparticulates [75]. When substrates are pretreated with alkali meth-ods, an important aspect is that the biomass itself consumes someof the alkali [34], thus higher alkali reagents might be required forobtaining the desired AD enhancement.

4.1.2. Acid pretreatmentAcid pretreatment is more desirable for lignocellulosic sub-

strates, not only because it breaks down the lignin, but also


because the hydrolytic microbes are capable of acclimating toacidic conditions [76]. The main reaction that occurs during acidpretreatment is the hydrolysis of hemicellulose into perspectivemonosaccharides, while the lignin condensates and precipitates[34,77]. Strong acidic pretreatment may result in the productionof inhibitory by-products, such as furfural and hydroxymethylfurf-ural (HMF) [73,76]. Hence, strong acidic pretreatment is avoidedand pretreatment with dilute acids is coupled with thermal meth-ods (see also Section 2.5). Other disadvantages associated with theacid pretreatment include the loss of fermentable sugar due to theincreased degradation of complex substrates, a high cost of acidsand the additional cost for neutralizing the acidic conditions priorto the AD process [73,78,79].

4.1.3. Effects of accompanying cations present in the acid/alkalinereagents

In addition to the effects of the alkali and acid themselves, theAD might be affected by the accompanying cations present in thesereagents including sodium, potassium, magnesium, calcium, sincethe chemicals are added mostly as salts or hydroxides of these cat-ions. Therefore, the inhibitory concentrations of these cationsshould be considered [3,80].

Kim et al. studied the inhibition of the sodium ion concentrationon the thermophilic AD of FW, and reported that more than 5 g/L ofsodium resulted in lower biogas production [38]. Sodium is moretoxic to propionic acid utilizing bacteria as compared to otherVFA degrading bacteria [81]. The inhibitory level of the potassiumion starts at 400 mg/L, though anaerobic microbes are able to tol-erate up to 8 g/L potassium [82]. The potassium ion is more toxicto thermophilic anaerobes as compared to mesophilic or psychro-philic anaerobes [83].

The optimum concentrations of calcium and magnesium ionshave been reported to be 200 mg/L and 720 mg/L, respectively[84,85]. Excessive amounts of calcium ions can cause precipitationof carbonates and phosphates, which results in scaling of the reac-tors, pipes, and biomass; thus it reduces the specific methanogenicactivity and results in a loss of buffer capacity [113]. Also high con-centrations (>100 mM) of the magnesium ion can cause disaggre-gation of methanogens, thus the conversion of acetate isinhibited [85].

Furthermore, AD could also be enhanced indirectly due to thesupplementation of trace metals such as cobalt (Co), molybdenum(Mo), selenium (Se), iron (Fe), tungsten (W), copper (Cu) and nickel(Ni), which play a role in many biochemical reactions of the anaer-obic food web. For instance Zhang and Jahng used supplements oftrace metals such as Fe, Co, Mo and Ni to stabilize a single-stagereactor treating FW, and concluded that Fe was the most effectivemetal for stabilization of the AD process [86]. Facchin et al.achieved a 45–65% higher methane production yield from FW withsupplementation of a trace metals (Co, Mo, Ni, Se, and W) cocktail[87]. Nevertheless, supplementing trace metals to solid waste ADplants should not be considered as a pretreatment method, thoughit could be an effective method for achieving higher biogas produc-tion rates with a higher methane content.

4.1.4. OzonationAnother chemical pretreatment method is ozonation [3], which

does not cause an increase of the salt concentration and no chem-ical residues remain as compared to other chemical pretreatmentmethods. Moreover, it also disinfects the pathogens [88,89]. Hence,ozonation has gained great interest for sludge pretreatment [3,90],and to a lesser extend OFMSW. Ozone is a strong oxidant, whichdecomposes itself into radicals and reacts with organic substrates[90] in two ways: directly and indirectly. The direct reaction de-pends on the structure of the reactant, whereas the indirect reac-tion is based on the hydroxyl radicals. As a result, the

recalcitrant compounds become more biodegradable and accessi-ble to the anaerobic bacteria [91].

4.2. Chemical pretreatments of OSW

Chemical pretreatments are widely applied on wastewatersludge and lignocellulosic substrates [3,34,73], while very limitedresearch has been conducted on OFMSW. Ozonation pretreatmenthas only been conducted on wastewater or sludge from WWTP. Ingeneral, the optimal ozone dose for enhancing AD of WWTP sludgeranges between 0.05 to 0.5 gO3/gTS [3,91–93]. Cesaro and Belgi-orno reported that the optimum ozone dose for source-sortedOFMSW is 0.16 gO3/gTS, which resulted in a 37% higher cumulativemethane production [56]. Lopez-Torres and Llorens obtained a11.5% increased methane production with alkaline pretreatmentof OFMSW [74]. Neves et al. achieved 100% of the potential produc-tion with alkaline (0.3 gNaOH/gTS) pretreated barley waste [28].Patil et al. studied the effect of alkaline pretreatment of water hya-cinth, which has a lower lignin content as compared to other plants[94]. They found that the alkaline pretreatment had a smaller effectthan mechanical pretreatments. Therefore, acidic and alkaline pre-treatment are not suitable for substrates with a low lignin content.

5. Biological pretreatment

Biological pretreatment includes both anaerobic and aerobicmethods, as well as the addition of specific enzymes such as pep-tidase, carbohydrolase and lipase to the AD system. Such conven-tional pretreatment methods are not very popular with OFMSW,but have been applied widely on other types of OSW such asWWTP sludge and pulp and paper industries.

The hydrolytic-acidogenic step (first step) of a two-phase ADprocess is considered as a biological pretreatment method by someresearchers [3,95–97], while others consider it as a process config-uration of AD, but not a pretreatment method [31]. Physically sep-arating the acidogens from the methanogens can result in a highermethane production and COD removal efficiency at a shorterhydraulic retention time (HRT) as to conventional single-stagedigesters [98]. Parawira et al. reported that optimizing the firsthydrolysis stage could stimulate the acidogenic microbes to pro-duce more specific enzymes, thus resulting in more extended deg-radation of substrates [99]. Therefore, in this review paper the firststep of the two-phase AD systems are considered as a pretreatmentmethod.

5.1. Conventional biological pretreatment

Aerobic pretreatment such as composting or micro-aerationprior to AD can be an effective method to obtain a higher hydroly-sis of complex substrates due to the higher production of hydro-lytic enzymes, which is induced by the increased specificmicrobial growth [100]. Fdez-Guelfo et al. reported that pretreat-ment by composting resulted in a higher specific microbial growthrate (160–205% as compared to untreated OFMSW) than by ther-mochemical pretreatment [14]. Lim and Wang also affirmed thatthe aerobic pretreatment yielded a greater VFA formation due tothe enhanced activities of the hydrolytic and acidogenic bacteria[100]. However, according to the results obtained by Brummelerand Koster, a pre-composting treatment of OFMSW resulted in a19.5% VS loss [101]. Mshandate et al. also observed a loss of poten-tial methane production with a longer aerobic pretreatment of sisalpulp waste [102].

Miah et al. investigated the biogas production of SS pretreatedwith aerobic thermophilic bacteria closely related to Geobacillusthermodenitrificans [15]. According to their results, the highest


amount of biogas (70 ml/gVS) with a 80–90% methane content wasachieved at 65 �C. Melamane et al. studied the AD of wine distillerywastewater pretreated with the fungus Trametes pubescens. Thisfungal pretreatment obtained a 53.3% COD removal efficiency,which increased the total COD removal efficiency of the AD systemup to 99.5% [103]. Muthangya et al. used pure cultures of the fun-gus Trichoderma reseei to aerobically pretreat sisal leaf decortica-tion residues. Their results showed that aerobic incubation for4 days resulted in a 30–40% cumulative biogas increase with ahigher (50–66%) methane content [104]. Romano et al. studiedtwo types of enzymes capable to hydrolyze plant cell walls to en-hance the biomethanation of Jose Tall wheat grass [16]. They didnot obtain a significant biogas enhancement or VS reduction,though the hydrolysis step was accelerated [15].

5.2. Two-stage AD

A two-stage AD system consists of a hydrolytic-acidogenic stagefollowed by the methanogenic stage. The advantages of such sys-tems include: (i) increased stability with better pH control; (ii)higher loading rate; (iii) increased specific activity of methanogensresulting in a higher methane yield; (iv) increased VS reductionand (v) high potential for removing pathogens [6,105–109]. Thedisadvantages include: (i) hydrogen built-up resulting in inhibitionof acid-forming bacteria; (ii) elimination of possible interdepen-dent nutrient requirements for the methane forming bacteria;(iii) technical complexity and (iv) higher costs [110,111].

Verrier et al. compared two-stage methanization of vegetablewastes with mesophilic and thermophilic single stage continu-ously stirred tank reactors (CSTR). They found that for easily biode-gradable wastes, a two-stage reactor converted 90% of the wastesto biogas, which outperformed both the mesophilic and thermo-philic single stage CSTR and could withstand higher organic loads[112]. Zhang et al. investigated the effect of pH on two-phase ADof FW, and suggested that adjusting the pH to 7 in the hydrolysisstage can improve both the total solids (TS) loading rate and biogasproduction yield [113].

5.2.1. Temperature phased anaerobic digestion (TPAD)Recently more research is being conducted on temperature

phased anaerobic digestion (TPAD). This method usually consistsof a primary digester at thermophilic (or hyper-thermophilic) tem-perature followed by a mesophilic secondary digester. The advan-tages of TPAD include not only higher methane production yields,but also a pathogen free high nutrient digestate [114]. Riau et al.suggested that TPAD is preferred if the purpose is to achieve path-ogen free digestate, which can be directly used as soil conditioner[115].

Schmit and Ellis reported that TPAD outperformed conventionalAD processes including dry digestion of source separated OFMSW[116]. Lee et al. investigated TPAD of FW and excess sludge at70 �C in the primary reactor, followed by a secondary reactor withtemperatures of 35 �C, 55 �C and 65 �C [117]. The best result wasachieved at a solid retention time (SRT) of 4 days and 70 �C inthe primary reactor, followed by a secondary reactor at 55 �C.Wang et al. compared the conventional thermophilic digestionwith TPAD (hyper-thermophilic (80 �C) and thermophilic (55 �C)primary reactor followed by a mesophilic reactor), treating FWwith polylactide. They obtained a COD solubilization of 82%,85.2%, 63.5% with TPAD with a hyper-thermophilic first stage,TPAD with a thermophilic first stage, and a conventional thermo-philic digester, respectively. Moreover, 82.9%, 80.8%, 70.1% of theorganics were converted into methane with TPAD with a hyper-thermophilic first stage, TPAD with a thermophilic first stage anda conventional thermophilic digester, respectively [118]. Songet al. compared the biogas production and pathogen removal of

WAS with TPAD compared to a single stage mesophilic and ther-mophilic digester [119]. TPAD yielded a 12–15% higher VS reduc-tion and it was as stable as the single stage mesophilic reactor,whereas pathogen removal was as high as in the single stage ther-mophilic reactor.

5.2.2. Biohythane productionOptimizing two-stage conditions may result in the production

of bio-hydrogen from the primary reactor and biomethane fromthe second reactor, making it a very attractive biohythane produc-ing system. Numerous studies have been conducted on the optimi-zation of such systems with reactors both at mesophilic and/orthermophilic temperatures. Liu et al. obtained 43 mlH2/gVS and500 mlCH4/gVS from household waste (HHW) [120], whereasWang et al. obtained 65 mlH2/gVS and 546 mlCH4/gVS from FW[110]. Chu et al. reported that the optimum hydrogen productionfrom FW is achieved at pH 5.5–6 in thermophilic AD. The bio-hydrogen content was 52% with no methane in the first stage,whereas the methane content was 70–80% in the secondary reac-tor. Based on a mass balance, 9.3% of the COD was converted tohydrogen and 76.5% converted to methane [121]. Escamilla-Alva-rado et al. studied the optimization of two-stage AD of OFMSW,and obtained an overall biogas production of 661 ± 2.5 and703 ± 2.9 ml/gVS for mesophilic and thermophilic operations,respectively. The biogas produced from the primary reactor con-tained 3–10% hydrogen, whereas the biogas from the secondaryreactor contained 25–61% methane [122].

6. Combination of various pretreatments

6.1. Thermo-chemical pretreatment

Different pretreatment methods rely on various mechanisms tosolubilize particulate organic matter [11,27]. Hence pretreatmentmethods in combination have also been studied to obtain a furtherenhancement of biogas production and faster AD process kinetics.

Shahriari et al. investigated the AD of OFMSW pretreated with acombination of high temperature microwaves and hydrogen per-oxide pretreatment [123]. The combination of microwaves withchemical pretreatments as well as the microwave irradiation attemperatures higher than 145 �C resulted in a larger componentof refractory material per gCOD, causing a decrease of the biogasproduction. A similar trend was observed with pig manure pre-treated with lime and heated at temperatures higher than 110 �C[10,124]. This could be explained by the increased hydrolyses ofproteins and carbohydrates due to the chemical pretreatment,and in the presence of heat the produced amino acids and sugarsreacted together forming complex polymers such as melanoidins.However, alkaline pretreatment coupled with thermal methodsat a lower temperature (70 �C) could result in a higher (78%) biogasproduction with a higher (60%) methane content as compared tothe best results (28% increase of biogas production with 50% meth-ane content) obtained by thermal pretreatment at higher temper-atures (>100 �C) [10]. This enhancement of the AD process is due tothe reduction of the hemicellulosic fraction [10,124].

6.2. Thermo-mechanical pretreatment

Mechanical pretreatment combined with thermal treatmenthave also been studied to enhance the AD of OFMSW, though thiscombination is not popular for OFMSW. Zhang et al. obtained thehighest enhancement of biogas production (17%) by grinding (upto 10 mm) rice straw and heating it to 110 �C [103]. Chiu et al.compared the hydrolysis yield of sludge pretreated with a combi-nation of ultrasonic and alkaline pretreatment. Simultaneous


ultrasonic and alkaline pretreatment of sludge resulted in the high-est hydrolysis rate of 211 mg/l min [25]. Wett et al. (2010) studiedthe disintegration of sludge pretreated at 19–21 bar pressure and160–180 �C for 1 h. The combined pretreatment resulted in a 75%increased biogas production at steady state, and the dewateringcharacteristics of the sludge were also improved, thus the disposalcost was reduced by 25%. However, the increased hydrolysis ofprotein caused a 64% increase of the ammonia concentration inthe reactor [125], which may lead to process instability. Schiederet al. studied the temperature and pressure catalyzed (160–200 �C at 40 bar for 60 min) hydrolysis to improve the AD of SS,and achieved a 70% higher biogas recovery at 5 days shorter diges-tion as compared to AD of untreated SS [126].

6.3. Various pretreatments combined with a two-stage AD

Considering the first stage of two-stage AD as biological pre-treatment, three-stage processes can be classified as a combinedpretreatment system. Kim et al. studied semi-anaerobic CSTRs fol-lowed by two-stage upflow anaerobic sludge blanket (UASB) reac-tors treating FW, and obtained a 95% COD removal and a biogasproduction of 500 mg/gVS at HRT of 16 days [127]. The same re-search group also reported that the same amount of biogas witha higher methane content (67.4%) could be obtained at a lowerHRT (10–12 days) by increasing the temperature of the acidogenicstage from mesophilic to thermophilic [128]. Kvesitadze et al. stud-ied the two-stage thermophilic co-digestion of OFMSW and pre-treated corn stalk by freeze explosion. The best results of104 mlH2/gVS and 520 mlCH4/gVS were obtained with alkaline(pH = 9) pre-hydrolysis, which could increase the heat and electric-ity production by 23% and 26%, respectively, as compared to thesingle stage process design [129]. Kim et al. investigated the hydro-gen and methane production by a two-phase AD system fed withthermally pretreated FW; they found at least 3.4 days were neces-sary to produce hydrogen from FW [130]. Moreover, recycling themethanogenic effluent to the hydrogenesis step was applied to re-duce water usage, which further increased the hydrogen produc-tion by 48% [130,131].

7. Comparison of pretreatment methods to enhance anaerobicdigestion of OFMSW

A systematic comparison of pretreatment methods in terms oftheir efficiencies, economic feasibility and environmental impactsare necessary for choosing the desired pretreatment method. Tothe best of our knowledge, no comparison of pretreatment efficien-cies to enhance the AD of OFMSW has been conducted so far. Theefficiency of the AD process can be evaluated through the methaneyield per amount of removed or initial feed of TS, VS, and COD. Thesubstrate solubilization rate and anaerobic biodegradation is alsoused to evaluate the AD process performance. Table 2 comparesthe efficiency of pretreatment methods including mechanical, ther-mal, biological and a combination of them for enhancing the AD ofOFMSW in terms of biogas production enhancement per amount ofinitial feed VS.

In general, OFMSW results in 280–557 ml/gVS biogas produc-tion, which is 70–95% of the organic matter in the feed. The pre-treatment effects vary depending on the substrate characteristicsand the type of AD system. The most commonly used mechanicalpretreatment methods are size reduction by beads mills, electro-poration, pressurization, disc screen, screw press and shredderwith magnetic separation. Mechanical pretreatments result in a20–40% increased biogas yield as compared to the untreated sub-strates. Both chemical and thermochemical methods could yield

up to 11.5–48% higher biogas yield depending on the pretreatmentconditions and substrate characteristics.

In thermal pretreatment, temperature plays a major role in theenhancement of biogas production. Low temperature (70 �C) pre-treatment can result in a 2.69% higher biogas production for FW,whereas it does not have any significant biogas productionenhancement for HHW or commingled OFMSW. Pretreating FWat high temperature results in 24% and 11.7% increased biogas pro-duction at 120 �C and 150 �C, respectively. Higher temperatures(175 �C) result in a decreased biogas production, due to formationcomplex polymers such as melanoidins.

Conventional biological pretreatments are not very popular forOFMSW, whereas two-stage AD systems with hydrogen recoveryhave become an interesting research field among the scientificcommunity. Pretreatments such as composting could result inhigher microbial activities [14]; but also result in a loss of volatileorganics, and thus a potential methane production [101]. More-over, for easily biodegradable wastes such as FW, hydrolysis isnot necessarily the rate-limiting step, thus the increased hydrolysisdue to pretreatment may lead to VFA accumulation, which subse-quently inhibits the methanogens. Therefore, a two-stage AD ispreferred for easily biodegradable OFMSW, as compared to con-ventional single-stage digesters coupled with other pretreatmentmethods [132,122].

8. Feasibility of a full scale application

This review showed pretreatment methods can enhance the ADperformance. Nevertheless, the high capital cost, high consumptionof energy, required chemicals and sophisticated operating condi-tions (maintenance, odor control etc.) are the major factor hinder-ing their full-scale application [77,124,133,134]. There are only afew examples of the thermal hydrolysis process (THP) that havebeen applied at a full-scale such as the Cambi, Porteous, and Zim-pro process, and thermochemical pretreatment methods such asSynox, Protox, and Krepro. It should be noted that these methodsare all applied for WWTP sludge. Concerning OFMSW, only a fewmechanical pretreatment methods (Fig. 1), Cambi THP (Fig. 2),and an AD with a pre-hydrolysis stage (two-stage AD, Fig. 3) havebeen applied at a full scale.

8.1. Energy balance

The required energy depends on the desired pretreatment tem-perature. If it is above 100 �C, most of the energy is utilized inwater vaporization, thus making it less desirable [138]. Someresearchers report that microwave heating has advantages overconventional heating due to the direct internal heating with noheat loss [22]. However, according to Mottet et al., neither micro-wave nor ultrasound was energy incentive for pretreating mixedsludge, as the enhanced methane yields were not enough to com-pensate the required energy [66]. Yang et al. reported that thermalpretreatment significantly improves the total amount of biogasproduced, and the extra biogas produced can be utilized to reducethe costs through an efficient heat exchanger [135].

Escamilla-Alvarado et al. obtained a better energy balance withtwo-stage AD systems treating OFMSW [122]. However, the highergross energetic potential was due to the higher performance in themethanogenic reactor rather than the hydrogen production fromthe first stage. Nasr et al. also estimated the energy balance oftwo-stage AD of thin stillage, and concluded that optimizing thetwo-stage AD process can increase the energy balance by 18.5%[136]. Lu et al. reported that a two-stage reactor showed a betterenergy balance with a surplus of 2.17 kJ/day, as compared to a sin-gle stage system for treating SS [27].

Table 2Comparison of pretreatment methods to enhance AD of OFMSW.

Substrate Pretreatment condition Type of AD system Results Reference

OFMSW (source-sorted) Disc screen Thermophilic batch 80.63% VS reduction with 338 mlCH4/gVS [51]Screw press 63.2% VS reduction with 354 mlCH4/gVSShredder with magnetic separation 63% VS reduction with 289 mlCH4/gVS

OFMSW (source-sorted) Screw press Thermophilic batch 461 mlCH4/gVS [53]Disc Screen 428 mlCH4/gVSShredder with magnetic separation 487 mlCH4/gVS

OFMSW Rotary drum Thermophilic batch 457–557 mlCH4/gVS with 57.3–60.6%methane

[49]

OFMSW Shear shredder, shredded and chopped,rotary drum, and wet macerator

Batch (wet and dry) Semi-continuous

Negligible effect on the enhancement ofbiogas production was achieved. Howeverthe kinetics of the process was faster atsemi-continuous experiments

[52]

OFMSW (source-sorted) Semi-composting with Rotary drum Mesophilic batch 5–11.5% VS higher reduction, and 18–36%higher biogas production

[50]

OFMSW Composting Thermophilic dry batch 160–205% Higher specific microbialgrowth rate

[14]

OFMSW Composting Mesophilic dry batch 19.5% VS loss, which is 40% loss ofmethane

[101]

Food waste 4 days microaeration with 37.5 mlO2/Ld Mesophilic wet batch 21% Higher methane yield for inoculatedsubstrate, and 10% higher methane yieldfor non-inoculated substrate

[100]

OFMSW (synthetic) Thermophilic pre-hydrolysis Thermophilic(continuous 2-stage)

81.5% COD removal with 95.7% VSSdestruction and 2 times higher biogasproduction

[111]

OFMSW (synthetic) Mesophilic and thermophilic pre-hydrolysis

Mesophilic and thermophilic(continuous two-stage)

Mesophilic pre-hydrolysis performedbetter in producing hydrogen, whereasthermophilic resulted in bettersolubilization. The highest methaneproduction from second stage was 341mlCH4/gVS

[122]

OFMSW and corn stalk Freeze explosion followed by thermophilicpre-hydrolysis

Thermophilic 104 ml-H2/g-VS and 520 mlCH4/gVS [129]

OFMSW Alkaline NAa 11.5% Higher COD solubilization, methaneyield of 0.15 m3 CH4/kgVS (172% higherthan untreated)

[74]

OFMSW Microwave pretreatment at 115–145 �C for40 min

Mesophilic batch 4–7% Higher biogas produced thanuntreated

[123]

OFMSW Pre-hydrolysis at 55 �C (TPAD) Mesophilic continuous 47.5–71.6% VS destruction and methaneyield of 299–418 ml/g-VS

[85]

OFMSW Sonication at 20 kHz for 30–60 min Mesophilic batch 60% Increased COD resulted in 24% highermethane yield

[56]

Household waste 70 �C for 60 min Thermophilic batch Methane yield of 500 mlCH4/gVS, noenhancement due to pretreatment

[69]KOH until pH = 10 at 70 �C, 60 min

Household waste 160–200 �C, 40 bar for 60 min Mesophilic continuous 55–70% COD solubilization, and 3% higherbiogas production

[126]

Household waste Mesophilic pre-hydrolysis(hydrogenogenic)

Mesophilic continuous 43 mlH2/gVS from first stage, 500 mlCH4/gVS from second stage which is 21%higher than single stage system

[120]

Food waste Microwave with intensity of 7.8 �C/min Mesophilic batch 24% Higher COD solubilization and 6%higher biogas production

[63]

Food waste Size reduction by beads mill Mesophilic batch 40% Higher COD solubilization and 28%higher biogas production

[8]

Food waste Addition of HCl until pH = 2 Thermophilic batch 13 ± 7% Higher COD solubilization and48% higher biogas production

[9]

120 �C, 1 bar for 30 min 19 ± 3% Higher COD solubilization and24% higher biogas production

HCl until pH = 2 at 120 �C 32 ± 8% Higher COD solubilization and 40%higher biogas production

Pressurized until 10 bar and depressurized 12 ± 7% Higher COD solubilization and48% higher biogas production

Frozen at �80 �C for 6 h, and thawed for30 min

16 ± 4% Higher COD solubilization and56% higher biogas production

Food waste with polylactide Hyper-thermophilic/thermophilic pre-hydrolysis

Thermophilic (TPAD) 15–18% Higher methane conversion ratiosthan conventional thermophilic digester

[118]

Food waste Semi-aerobic and anaerobic pre-hydrolysis Mesophilic continuous 95% COD destruction which resulted inmethane yield of 500 ml/gVS

[127]

Food waste Thermophilic pre-hydrolysis Thermophilic HRT can be reduced to 10 days [128]Food waste Thermophilic pre-hydrolysis Mesophilic 61.3% VS destruction, methane yield of

280 ml/gVS[148]

Food waste Mesophilic pre-hydrolysis Mesophilic continuous(2 stage system)

9% and 13% Higher biogas production thanmesophilic and thermophilic AD,respectively

[112]

Food waste Mesophilic pre-hydrolysis Mesophilic Best results of 520 mlCH4/gTS wasachieved at pH = 7

[113]


Table 2 (continued)

Substrate Pretreatment condition Type of AD system Results Reference

Food waste Mesophilic pre-hydrolysis Mesophilic continuous 65 mlH2/gVS and 546 mlCH4/gVS [110]Food waste Thermophilic pre-hydrolysis Mesophilic continuous 205 mlH2/gVS and 464 mlCH4/gVS [121]Food waste 400 pulses with electroporation Mesophilic continuous 20–40% Higher biogas production due to

substrate cell breakage[46]

Food waste 70 �C for 2 h Mesophilic continuous 2.69% Higher methane production [21]150 �C for 1 h 11.9% Higher methane production

Food waste Frozen/thawed and pre-hydrolysis for7 days

Mesophilic continuous 10% Higher COD solubilization, 23.7%higher biogas production

[149]

Frozen/thawed and pre-hydrolysis for12 days

4% Higher COD solubilization, 8.5% higherbiogas production

Food waste 70 �C thermal and mesophilic pre-hydrolysis

Mesophilic continuous 91% of FW was converted to biohythanewith 8% hydrogen and 83% methane

[127]

Food waste 175 �C, 60 min Mesophilic batch 7.9% Decrease in biogas production [62]Fruits and vegetables waste 11.7% Decrease in biogas production

a NA – Not available.

Fig. 1. Mechanical pretreatment methods to enhance AD of OFMSW: (A) screw press; (B) disc screen; (C) shredder with magnet [53].


8.2. Economic feasibility

As the pretreatment of OFMSW is relatively new, its cost esti-mation is still based on lab-scale level data. For instance, Maet al. estimated the net profit of various pretreatments to enhance

the biogas production of FW, and obtained the best result(10–15 euro/ton FW) with less energy intensive methods (acidand freeze–thaw) [9]. However, they have not considered thermalpretreatment at lower temperatures, which could have been moreeconomic.

Fig. 2. A simplified scheme of Cambi THP: (A) cambi whole process; (B) detailed THP process. Source: Cambi official website http://www.cambi.no.


The estimation of the economic feasibility of pretreatmentmethods based on a full-scale application has only been reportedfor WWTP sludge. Rittman et al. estimated the operational andmaintenance (OM) cost of a full-scale AD (3300 m3) treating380 m3 sludge per day based on the application of focused-pulsedpretreatment technology, which could generate a benefit of540,000 USD per year [137]. Muller reported that a rough costestimate of pretreatment methods is between 70 and 150 US$/tonTS for capital and OM cost [33]. Bordeleau and Droste esti-mated the cost of pretreatment methods to enhance the AD ofsludge, based on the existing literature. They concluded thatthe microwave (0.0162 US$/m3) and conventional thermal(0.0187 US$/m3) pretreatments were cheaper than ultrasound(0.0264 US$/m3) and chemical (0.0358 US$/m3) methods [22].

However, the amount of sludge is an important factor to considerwhen estimating the pretreatment cost. Ultrasound pretreatmentcould be energetically feasible if a typical value of 6 kWh/m3

sludge for a full-scale application is considered [138]. If a higherenergy is required, biological pretreatment such as adding hydro-lytic bacteria could be a cheaper option [57,139].

The cost estimation of conventional biological pretreatment hasnot been reported to date. The economical feasibility of a two-stageAD were estimated by Bolzonella et al., who reported that the payback time for a full-scale two-stage AD system with hyper-thermo-philic pre-stage followed by mesophilic reactor is 2–6 yearsdepending on the method of sludge disposal [140].

In addition to the calculation of net benefits, local circum-stances such as labor, treatment capacity, transport, collection cost,

http://www.cambi.no

Fig. 3. A simplified scheme of two-stage AD [83]. Source: Adapted from Ge et al. [95]

Table 3Sustainability evaluation of pretreatment methods to enhance OFMSW.

Pretreatment Method Efficiency Energy requirement and economic cost Environmental impact

Mechanical +++ ++ +Thermal at high temperatures (>110 �C) + + +++Thermal at low temperatures (<110 �C) +++ +++ +++Conventional biological methods (enzyme addition, composting etc.) + +++ +++Two-stage AD (anaerobic pre-hydrolysis) +++ +++ +++Chemical +++ + +Thermochemical +++ + +


energy prices, tax, purchase tariffs, land price, market, price of di-gested material, disposal of residue, additional mixing and pump-ing should be considered as well [4,5,61,132,141–146].

8.3. Environmental aspects and sustainability of pretreatmentmethods

In addition to the energy balance and economic analysis, envi-ronmental consideration such as pathogen removal, use of chemi-cals, and the possibility for a sustainable use of the residues,impacts on human health and the environment should be consid-ered as well when choosing a pretreatment method[4,22,133,141–149]. Moreover, the anaerobic residues have thepossibility to be used as soil fertilizers. Thus, the soil type as wellas the potential gaseous emissions such as N2O should be consid-ered [147]. Carballa et al. evaluated the environmental aspects ofdifferent pretreatment methods including chemical (acidic andalkaline), pressurize-depressurize, ozonation and thermal treat-ment in terms of Abiotic Resources Depletion Potential, Eutrophi-cation Potential, Global Warming Potential, Human andTerrestrial Toxicity Potential through a life cycle assessment. Theyconcluded that the pressurize-depressurize and chemical pretreat-ment methods outperformed ozonation, freeze–thaw and thermalmethods [36].

Table 3 gives a simple sustainability assessment of pretreat-ment methods to enhance OFMSW was carried out based on exist-ing literature. Pretreatment methods with higher efficiencies, andthat are more economically as well as environmental friendlymethods obtained more plus points. The pretreatment methodswith the most number of plus points were evaluated as the mostsustainable. Table 3 shows that the thermal pretreatment at lowtemperature and the two-stage AD system were assessed as the

most sustainable methods to enhance the AD of OFMSW, followedby conventional biological methods and mechanical pretreatment.Chemical, thermochemical or thermal pretreatment methods athigh temperatures could result in a higher enhancement of theAD process as compared to untreated substrates. However, thecosts of the methods as well as the environmental considerationsmake it less desirable.

9. Conclusion

The growing global concerns on the increasing amount of waste,energy demand, and global warming have stimulated research onthe acceleration and enhancement of the AD process. Pretreatmentmethods can be categorized as mechanical, thermal, chemical, bio-logical or a combination of them. Among the widely reported pre-treatment methods tested at lab scale, only few mechanical,thermal and thermochemical methods were successfully appliedat full scale. Based on a simple sustainability assessment, thermalpretreatment (at low temperatures) and two-stage AD systems of-fer more advantages as compared to the other pretreatment meth-ods. These include: (i) higher biogas yield; (ii) decisive effect onpathogen removal; (iii) reduction of digestate amount; (iv) reduc-tion of the retention time; (v) better energy balance and (vi) bettereconomical feasibility.

Acknowledgements

This research was financially supported by Erasmus MundusJoint Doctoral Program on ETECOS3 (Environmental Technologiesfor Contaminated Solids, Soils and Sediments) under the EU Grantagreement FPA No. 2010-0009. This research was also conducted in


the framework of the project ‘‘Integrated system to treat buffaloslurry, aimed to recover water and safe energy – STABULUM’’funded in agreement with the Decision of the European Commis-sion No. C (2010) 1261, 2nd March 2010, by the AgricultureDepartment of the Campania Region in the context of the Pro-gramme of Rural Development 2007–2013, Measure 124 Coopera-tion for development of new products, processes and technologiesin the agriculture and food sectors.

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pretreatment methods to enhance anaerobic digestion of organic solid waste

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