Environ. Eng. Res.

2020; 25(1): 1-17 pISSN 1226-1025

https://doi.org/10.4491/eer.2018.334 eISSN 2005-968X

A review of anaerobic digestion systems for biodegradable

waste: Configurations, operating parameters, and current trends
Dinh Pham Van1†, Takeshi Fujiwara1, Bach Leu Tho2, Pham Phu Song Toan1,3, Giang Hoang Minh2
1
Okayama University, 3-1-1 Tsushima, Kita Ward, Okayama 700-8530, Japan
2
National University of Civil Engineering, 55 Giai Phong Road, Hai Ba Trung District, Ha Noi, Vietnam
3
The University of Da Nang - University of Technology and Education, 48 Cao Thang, Hai Chau District, Da Nang City, Vietnam

ABSTRACT
With benefits to the human health, environment, economy, and energy, anaerobic digestion (AD) systems have attracted remarkable attention
within the scientific community. Anaerobic digestion system is created from (bio)reactors to perform a series of bi-metabolism steps including
hydrolysis/acidogenesis, acetogenesis, and methanogenesis. By considering the physical separation of the digestion steps above, AD systems
can be classified into single-stage (all digestion steps in one reactor) and multi-stage (digestion steps in various reactors). Operation of the
AD systems does not only depend on the type of digestion system but also relies on the interaction among growth factors (temperature, pH,
and nutrients), the type of reactor, and operating parameters (retention time, organic loading rate). However, these interactions were often
reviewed inadequately for the single-stage digestion systems. Therefore, this paper aims to provide a comprehensive review of both single-stage
and multi-stage systems as well as the influence of the growth factors, operating conditions, and the type of reactor on them. From those
points, the advantages, disadvantages, and application range of each system are well understood.

Keywords: Anaerobic digester, Anaerobic digestion systems, Single-stage, Solid waste treatment, Three-stage, Two-stage

1. Introduction sugar, pulp/paper, etc.), and sewage sludge [6, 7]. Landfilling bio-de-
gradable waste leads to various issues that threaten the environment
Solid waste generation is an inevitable consequence of human and public health [1, 8]. Also, the fact that the world population
activities and its rapid increase in recent years has caused sig- continues to grow together with the progress of human civilization
nificant problems that humankind has to deal with. In 2010, nearly leads to the increase of global energy demand. The traditional
1.3 billion metric tons of municipal solid waste (MSW) were pro- energy sources such as fossil fuel are exhausting, and nuclear
duced worldwide, and the annual generation is estimated to in- energy is a potential risk to the environment and human health
crease up to 2.2 billion metric tons by 2025 [1, 2]. The organic [9]. Thus, the increasing energy demand is becoming a global
fraction of MSW (OFMSW) was often reported around 50-60% challenge, and this has promoted the interest in search of alternative
of total MSW, which had been cast off in landfills for many years energy sources. Meanwhile, biodegradable waste can be converted
[2-5]. According to Hoornweg and Bhada-Tata [1], an annual to methane gas (renewable energy source) by using anaerobic diges-
amount of 250.05 million tons MSW was dumped into landfills tion (AD) systems. Therefore, with benefits to the human health,
in high-income countries; that of low-income and middle-income environment, economy, and energy conservation, the AD systems
countries were 2.67 and 157.1 million tons, respectively. Besides have attracted remarkable attention within the scientific commun-
OFMSW, major sources of biodegradable waste also originate from ity [10, 11].
agriculture (animal manures, energy crops, algal biomass, harvest The AD systems are constituted from reactors to perform a
remains), food industry (food/beverage processing, dairy, starch, series of bi-metabolism steps including hydrolysis/acidogenesis,

This is an Open Access article distributed under the terms Received September 20, 2018 Accepted January 30, 2019
of the Creative Commons Attribution Non-Commercial License
†
(http://creativecommons.org/licenses/by-nc/3.0/) which per- Corresponding author
mits unrestricted non-commercial use, distribution, and reproduction in any Email: dinh88.nuce@gmail.com
medium, provided the original work is properly cited. Tel: +81-86-251-8994 Fax: +81-86-251-8994
Copyright © 2020 Korean Society of Environmental Engineers ORCID: 0000-0003-1867-0478

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Dinh Pham Van et al.

acetogenesis, and methanogenesis [11-13]. By considering the phys- The first step (hydrolysis, also called solubilisation) breaks down
ical separation of the digestion steps in various reactors, the AD high molecular weight constituents (e.g., lipids, carbohydrate, and
digestion systems include single-stage, two-stage, and three-stage. protein) into smaller soluble organic matter (e.g., fatty acid, glucose,
In which, reactors are the place where growth factors (temperature and amino acid) by exo-enzymes, and is represented by reaction
(T), pH, nutrients) and operating parameters (retention time (RT) (1) [15, 21]. The hydrolytic microorganism has ability to strongly
and organic loading rate (OLR)) are controlled to allow one or resist the environmental fluctuations and the toxins which may
several digestion steps occur [14, 15]. The growth factors affect be present in the feedstock [22]. They can work in a wide range
the living conditions of anaerobes and determine the success or of pH (4-11) [23]. However, pH-values in the range of 6-8 were
failure of the reactors [11]. Meanwhile, the RT determines the often reported to provide an optimum working condition for hydrol-
contact time between microorganism and substrate so that it is ysis [3, 24, 25]. The hydrolysis of lignocellulosic materials is a
long enough to complete the transformation. A too long RT will relatively slow process. Thus, hydrolysis is considered as a rate-lim-
lead to such a big reactor that will increase investment and operating iting process in the digestion of lignocellulose [13, 18, 26].
costs. The OLR determines a measure of the amount of daily organic Fortunately, this issue can be accelerated significantly by using
matter treated by a certain volume of the reactor. Both RT and pre-treatment processes including physical, chemical, and bio-
OLR rely on the growth factors and the type of reactor. So the logical methods. The details of these methods are presented by
performance of the AD systems is determined by the complex Ariunbaatar et al. [27].
relationships among the growth factors, the operating parameters,
Hydrolysis
system type, type of reactor, and also coordination of reactors           (1)
in the system. These relationships were often reviewed in-
adequately in the single-stage digestion system [6, 11, 12]. The second step (acidogenesis) transforms the products of
Meanwhile, for the multi-stage digestion systems, the information the hydrolytic process into volatile fatty acids (VFAs) such as
is still limited. Demirel and Yenigün [16] are two of few authors propionic acid, butyric acid, acetic acid and ethanol by the action
who did a review on the operation of the two-stage digestion system. of the acidogenic bacteria [28-30]. They have characteristics of
However, they only investigated one configuration of the con- strong and fast growth with a minimum doubling time of 30 min
tinuous stirred tank reactor (CSTR) for hydrolysis/acidogenesis [31]. Acidogenesis can be described by Eq. (2)-(5) [15, 18, 21,
and the upflow anaerobic sludge blanket (UASB) reactor for the 26]. The pH-conditions significantly influence VFA products. By
methanogenesis. Moreover, the three-stage digestion system has stepwise shifting pH from 4 to 8, the main products changed
limited information. So far there have been no studies providing from butyric and acetic acids to acetic and propionic acids [32,
a systematic and full review of the AD systems, including the 33]. Moreover, the VFAs formation was strongly inhibited with
coordination between the functional reactors in a system as well pH below 4.0 [34]. The pH in the range of 5.5-6.5 was often reported
as the operation of each reactor. as the optimal range [11].
This paper aims to provide a comprehensive review of the AD
   Acid forming bacteria   (2)
systems for biodegradable waste including the single-stage,
two-stage, and three-stage systems. Each type of them is discussed
about classification, configuration, and operation. Moreover, ad-      →           (3)
vantages, disadvantages, and application ranges for each system
are also evaluated. The factors influencing the AD systems are    →             (4)
identified and discussed based on the existing literature.
Therefore, this study is as a guideline for the AD system design.    →      (5)
This paper is organized into five following sections. Section 1
is an introduction. Section 2 presents mechanism of AD processes The third step (acetogenesis) transforms most products of acido-
and anaerobic reactors which are responsible for the AD process. genesis into acetic acid (CH3COOH), hydrogen (H2), and carbon
In section 3, the operation of the AD systems is systematized dioxide (CO2) as shown in Eq. (6)-(9) [14, 15, 18, 35]. The growth
according to the biological phase separation, and the newest ach- kinetic of acetogenesis is slower than which of acidogenesis, with
ievements are reviewed within the literature. Section 4 presents a minimal doubling time in the range of 1.5-4 d [31]. Acetogens
the current applications. Finally, section 5 is conclusions and are strict anaerobes, the present of oxidants like oxygen or nitrate
recommendations. is toxic [26], and they work better in acid-weak environment (pH
from 6.0 to 6.2) [31]. Remarkably, high partial pressure of hydrogen
product (≥ 10-4 atm) inhibits Eq. (6) and (7), hence hydrogen product
2. Basics of Anaerobic Digestion should be released [34, 36].

2.1. Biochemical Mechanism of Biogas Production        →        (6)
The anaerobic digestion is a series of bio-metabolism steps includ-
ing hydrolysis, acidogenesis, acetogenesis, and methanogenesis,         →         (7)
respectively [11-13, 17-20] (see Fig. S1). In which, every digestion
step has different growth characteristics as presented below.       →         (8)

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   →          (9) tration has a great impact on the cost, performance, and technique
of the AD process [47, 48]. Categorization of anaerobic reactors
The fourth step (methanogenesis) plays the most important role is summarized in Fig. 1(a). The dry digesters have been designed
in generating methane gas by methanogens. There are two basic to serve the feedstock having TS ≥ 20%, hence solid concentration
mechanisms for methane generation including acetoclastic and within the reactor is also high (TS ≥ 15%) [3, 47, 49]. In the
hydrogenotrophic methanogenesis. The first way, acetotrophic bac- dry digester, the substrate particles play the roles of nutrient source
teria ferment acetic acid to CH4 and CO2 as Eq. (10) [26, 37]. and also supporting media. Especially, the microorganisms not
The second one, hydrogenotrophic methanogens use CO2 and H2 only attach to the surface but also penetrate through substrate
as a food source, as given in Eq. (11) [20, 26, 37]. While the minimum particles [50]. The dry type can be classified into three groups
doubling time of hydrogenotrophic bacteria is in the range of 4-12 including the horizontal plug-flow, vertical plug-flow, and
h, acetotrophic bacteria have a much lower maximum growth non-flow (batch type). The brief descriptions of these digesters
rate with doubling times of 2-3 d [31, 37]. Generally, methanogens are shown in Table S1, and the discussion about them is presented
are extremely sensitive to pH condition, the presence of oxygen, in the next section. The wet digesters are defined to serve the
and other factors such as free ammonia (FAN), H2S, and VFAs feedstock having TS ≤ 15% and well known in the wastewater
[15]. They cannot work at pH condition lower than 6.2, even treatment field [5, 51]. They can often be classified based on the
die in the pH condition under 6.0 [15, 18], and can be inhibited growth of bacteria inside including suspended growth and attached
by releasing free ammonia (FAN) when pH-value is over > 7.8 growth [52]. In the suspended growth digester, the microorganisms
[31]. The optimal pH-value was often reported at neutral environ- are maintained in suspension within the liquid. Meanwhile, they
ment (7.0-7.2) [11, 18, 38, 39]. Methanogenic process lost stable attach and grow on the surface of supporting media in the attached
stage when FAN concentration reached 0.6-0.69 g/L [40, 41], re- growth reactor. Detail descriptions of their representatives are
duced 50% performance when FAN concentration was 1.45 g/L shown in Table S2.
[42], and failed by FAN concentration of 1.7-1.8 g/L [43]. The The design and operation of the anaerobic reactor are charac-
optimum concentrations of sulfur for the growth of methanogens terized by two parameters including RT and OLR. In which, the
were reported in the literature to vary from 16 to 160 mg-S/L RT is defined as the average time that the substrate maintained
[44]. The methanogenesis was often stable with a low VFA concen- in a reactor and quantified by the equation: RT = reactor vol-
tration (< 200 mg/L), and inhibition occurred when acetate concen- ume/daily flow. The RT has to be long enough to ensure the
tration exceeded 3 g/L [36]. The oscillation of temperature in the completion of one or several digestion steps that the reactor is
reactor should be as small as possible, that is, < 1°C/d for thermo- in charge. For the wet reactor, the substrate materials are often
philes and 2-3°C/d for mesophiles [15]. separated into liquid and solid materials thus the RT is divided
into hydraulic RT (HRT) and solid RT (SRT). Meanwhile, the
  acetociastic bacteria    (10)
OLR gives a measure of the amount of daily organic matter treated
by a certain volume of the reactor and calculated indirectly via
   hydrogenotrophic bacteria     (11)
RT by equation OLR = Organic-concentration/RT. Both RT and
OLR depend on the process parameters that influence the anaero-
2.2. Anaerobic Reactors bic bacterial growth such as temperature, pH, and waste
Anaerobic reactor (digester) is considered as the heart of digestion characteristics.
systems, which encourages anaerobic microorganisms to thrive
inside for responding to the digestion steps. While the success
of the digester depends very much on pH condition as described 3. Anaerobic Digestion Systems
in section 2.1, its digestion rate is significantly dependent on tem-
perature condition. Temperature is not only one of the most im- The simplest anaerobic digestion system contains one reactor
portant in the selection of microbial group inside reactors, but which is responsible for doing all four digestion steps. It is called
it also influences the state of substrates such as the solubility, the single-stage digestion system. However, along with the devel-
metabolic rate, and ionization equilibria. According to temperature opment of science and biotechnology, scientists have found that
conditions, there are three microorganism groups including psy- each digestion step has different optimal thriving conditions [3,
chrophiles, mesophiles and thermophiles, respectively living in 11, 31]. Therefore, the idea of the physical separation of the
the temperature range of 4-15oC, 20-40oC and 45-70oC [45]. The digestion steps (multi-stage digestion) has been given the aim
influence of temperature on the growth rate of each group is shown to optimize each of them to reach the highest performance of
in Fig. S2. In general, the increase in temperature has benefits transformation. Currently, the multi-stage digestion systems only
of enhancing reaction rates significantly [17]. However, the meso- include two-stage and three-stage systems. The two-stage system
philes are less sensitive to environmental changes than the thermo- performs hydrolysis and methanogenesis in two different reactors.
philes, hence operation reactor at thermophilic temperature re- More complex, the three-stage system performs the hydrolysis,
quires higher technic than mesophilic condition [14, 46]. acidogenesis/acetogenesis, and methanogenesis in various
The anaerobic digesters can be categorized in several different reactors. This paper classifies the subcategories of the AD systems
ways, but the main classification can be set between wet and based on type (wet or dry) of reactor employed as shown in
dry types relied on total solid (TS) contents because solid concen- Fig. 1(b).

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Dinh Pham Van et al.

a b

Fig. 1. a) Classification of digesters; b) classification of AD systems.

3.1. The Single-Stage Systems of 1-8 m3, the 1-3 m3 one is the most commonly used. In this
In principle, the environmental conditions are not necessarily opti- system, the daily feedstock is about 1/40-1/50 of the reactor volume
mal for any digestion step, but they satisfy all four steps [18]. [21]. The farm-scale one (lagoon digester or anaerobic pond, see
However as shown in section 2.1, the range of growth conditions Table S2(c) is operated with an HRT in the range of 30-60 d,
for the methanogen is covered by the other process steps. Moreover, SRT of 50-100 d, and TS of the feedstock of 0.5 to 5% [52].
methanogens are the most sensitive to the environment and also 3.1.1.2. The wet high-rate systems
have the slowest growth among microorganism consortium. The wet high-rate systems have been developed from the low-rate
Therefore, the environmental conditions in the single-stage diges- digestion for improving performance. The feedstock can be heated
tion systems should be optimized for methanogenesis. Commonly, and mixed to make a uniform environment, which leads to the
the single-stage system operates with some requirements: Carbon fact that the reactor is less volume, more stable and more efficient
to nitrogen (C/N) of the substrate in the range of 15-30 [6, 11, [54]. The first generation of the wet high-rate systems uses the
13]; pH ranging between 6.8 and 7.4 [6, 11]; HRT 30 d at mesophilic wet plug flow (WPF) reactor (see Table S2(d)), which is supported
temperature, 20 d at thermophilic, and 50 d at psychrophilic. by a heat source. This system has been applied the most for manure
with solid content in the range of 11-14% and HRT of 15-20 d
3.1.1. The wet single-stage systems
[21, 55]. A more complicated reactor called completely mixed
They are the single-stage systems employing the wet reactor, and reactor (CMR) or continuous stirred tank reactor (CSTR) requires
they can be classified into two groups including low-rate (OLR internal mixing activities, see Table S2(e). This system often oper-
= 0.5-1.6 kg-VS/m3.d-1) and high-rate (OLR = 1.6-4.8 kg-VS/m3.d-1) ates at mesophilic temperature with OLR in the range of 1.5-5
[31]. A number of studies have shown that they are sensitive kg-VS/m3.d-1 and RT from 15-20 d [21]. In the CSTR, the bacteria
to inhibitor such as FAN. According to the study of Duan et al. get washed away together with the effluent out of reactor [21].
[41], FAN level > 0.6 mg/L was the key factor influencing system Meanwhile, to reach higher OLR, the reactor need a higher concen-
stability. Nakakubo et al. [42] showed that methane yield was tration of bacteria maintained inside. Therefore, the biomass of
reduced 50% when FAN concentration was up to 1.45 g/L. And the effluent is separated in a settling tank and pumped back to
Yen and Brune [53] reported that the system could fail when the reactor where the biomass concentration is maintained in the
FAN increased within to the range of 1.7-1.8 g/L. range of 5-10% VS [21, 34, 36]. It is called anaerobic contact
3.1.1.1. The wet low-rate system (AC) system as shown in Table S2(f) Waasa, a famous brand name
The wet low-rate system is the oldest and simplest type, has its of AC system, is used for treating OFMSW. In Waasa system,
key characteristics such as long RT (30-60 d), poor mixing process, fresh materials are mixed with effluent water to attain 10-15%
and non-heating process [39, 54]. It is a very simple operation, TS. The OLR could reach 4-8 kg-VS/m3.d-1 with the efficiency
but very low performance; hence the low-rate systems have only of 100-150 m3-biogas/ton-waste and TS reduction of 50-60% [5].
been used within household-scale and farm-scale in developing When the substrate of the feedstock is mainly in the soluble
countries such as China, India, and Vietnam [21]. The diagram state (or TS < 3-5%), the wet reactors with high biomass concen-
principle of these systems is shown in Table S2(a). It doesn’t trations such as upflow anaerobic sludge blanket (UASB), expanded
matter if it is small or large-scale, there is always a stratification granular sludge bed (ESGB), expanded bed (EB), fluidized bed
inside reactors with four zones: (i) a scum layer, (ii) a liquid layer (FB), internal circulation (IC), and anaerobic fixed bed (AFB) can
(or supernatant), (iii) a layer of digesting solids, and (iv) a layer be employed. The principles and application range of these reactors
of digested solids [17, 39]. Among these systems, the house- are presented in Table. S2. They are well known in the industrial
hold-scale one (Table S2(b)) is also known as floating-dome digester wastewater treatment sector. In the United States, application allo-
or fixed dome digester. Although it has capacity in the range cation of anaerobic technologies in the industrial wastewater treatment

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for period 2002-2007 included 34% UASB, 33% IC, 22% EGSB, will quickly settle down to the bottom and clog the recirculated-gas
1% AFB, and 10% others [37]. pores [5]. The system produces about 140 m3-biogas/ton-waste
with VS removal rate of 50% [7]. For the substrate of OFMSW,
3.1.2. The dry single-stage systems
the pH inside is often in the range of 7.8-8, hence alkali additive
The high solid waste can be treated by the wet digestion systems, is not required. When pH drops, the feeding pump should stop
but it needs to be added a large amount of water. Unfortunately, until process regulates itself [3]. A part of the biogas product is
dilution of the waste stream not only demands higher costs of injected back at the bottom of the reactor with high pressure (5
water and energy consumption but also requires a reactor with bar) for mixing purpose and keeping materials suspended [3, 21].
higher volume. Moreover, dilution of waste can drag on reducing The system is often operated in the mesophilic temperature with
biogas yield and make more wastewater [47]. Therefore, the dry RT of 18-23 d and produces biogas yield of 220-270 L/kg-VS. After
digesters have been developed to deal with high solid waste (see
digestion, the digestate is composted within 2-3 weeks [5].
Table. S1). The main issues with the operation of dry systems
The dry-batch digestion system (see Table S1(c)): This system
compared to which of wet systems are the mixing and pumping
originates to mimic the landfill process. However, unlike natural
of the substrate with high viscosity [51]. Along with the evolution
biodegradation in the landfill, the reaction in the dry-batch system
of technologies, these issues gradually reduced. Nowadays Dranco,
is accelerated by two basic factors. The first one is the continuous
Valorga, Kompogas, and Biocell are the most commonly applied
recirculation of the leachate. It permits formation of a high humidity
systems of pilot scale plans in Europe [5, 51]. Generally, the dry
environment and diffuseness of the inoculum and the substrates.
systems are stronger than the wet ones because of stability even
The second one is that temperature in the systems is controlled
with ammonia concentration up to 2.5-3 g/L and VFAs in the
at the optimum range. This resulted in many fold higher biogas
range of 23-24 g/L [3, 56].
production rates and lower RT than observed in landfills [49].
The Dranco system is such a vertical plug-flow digester (see
The digester is loaded with fresh materials (TS of 30-40%), dis-
Table. S1(a)). In this system, a part of the digestate is taken back
charged, and then filled with a new batch. The digestion time
to the mixing pump where it is blended with fresh materials
finishes when biogas production ceases. After more than 20 d
(separated sources, particle size < 4 cm) for inoculation with
digesting at the mesophilic temperature, the digestate is aerated
a ratio of 6-8:1. After that, the mixture is introduced into the
for 1-3 weeks for composting [5, 51]. For each ton of waste, the
top of the reactor and moves downward to the conical bottom
system produces 90 kg of biogas (58% CH4), 455 kg wastewater,
where a screw conveyor removes the digestate. The rest of digestate
is dewatered before being composted. There is no active mixing and 310 kg of compost [39]. Despite saving 40% equipment cost
within the reactor, hence 20-30% of biogas can be lost because compared to continuous-dry systems, the batch-dry systems have
of the incomplete digestion [3]. The system has been proved suc- much lower applied ratio because they need much more space
cessful to treat solid wastes with TS ranging from 20 to 50% [47]. for construction [5].
When OFMSW (TS of 30-45%) is processed in the mesophilic 3.1.3. Assessments
condition with HRT 20 d, VS decreases 55% and 5-8 kg-VS/m3.d-1
The single-stage digestion systems have a wide application range
OLR is produced [3, 7]. In the thermophilic temperature with
and can cover the most type of biodegradable waste. The evaluation
RT 14 d, the amount of OLR could be up to 15 kg-VS/m3.d-1 and
of the systems based on some technical targets is shown in Table 1;
65% of VS could be destroyed [5]. For digestion of OFMSW, alkali
it also includes their advantages and disadvantages. In the sin-
additive is often unnecessary, and pH inside the reactor is nearly
gle-stage digestion systems, the strong group bacteria (facultative
8. Each ton of waste can produce 120-170 m3-biogas (equivalent
microorganisms) can easily repulse the weak groups (methanogens)
to 200 kWh) and the plant consumes 30-40% of it and exports
when living in the same reactor. Thus, the fluctuations of the
the rest [3].
load, pH, and solid concentration of the feedstock could harm
The Kompogas system employs a horizontal plug-flow digester
the stability of the system. If the rate of acid formation is more
with internal rotors to assist in degassing and homogenizing waste
than the rate of methane formation, it means there is an accumu-
(see Table. S1(b)). The incoming waste stream (particle size <
lation of VFAs, the system must be stopped and waited for turning
50 mm) is mixed with the liquid of the digestate to attain TS
stable status. Therefore, the two-stage system can be considered
in the range of 23-28% for the system to flow properly. At higher
as an optimal solution to deal with the mentioned issues above.
TS values, the mixture is hard to flow because of too high viscosity
while lower TS values lead to accumulating sand and glass inside
3.2. The Two-Stage Systems
the reactor [5, 47]. The system often operates in the thermophilic
condition (55-60oC) with HRT of 14-20 d. The digestate is dewatered 3.2.1. Configuration of two-stage systems
by using a screw press then the solid part is composted by aeration The growth characteristics of the hydrolysis/acidogenesis and the
within 2-3 weeks [3, 5]. For one ton of waste, the system can methanogenesis are very different. Therefore, the idea for two-stage
produce 130-150 m3-biogas, 500 kg-compost, and 300 L liquid digestion is given to optimizing every digestion step. In the present,
fertilizer. there are two different viewpoints about separating digestion-steps
The Valorga system uses a vertical plug-flow reactor (see Table. into two reactors. The first perspective supposes that the first
S1(a)). The feedstock is the mixture of the incoming waste stream tank allows the hydrolysis/acidogenesis to occur, and the second
(size < 30 mm) and the digestate with TS adjusted in the range tank optimizes the acetogenesis/methanogenesis [49, 76].
of 25-30%. If TS concentration is less than 20%, the grit particles Meanwhile, the second perspective proposes that the acetogenesis

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Dinh Pham Van et al.

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as fast as possible while avoiding being inhibited and overloaded.

Therefore, a series of operating conditions including pH, temper-
ature, acid concentration, nutrients, and substrate concentration
(TS) must be controlled. For the high-solid waste such as OFMSW,
the waste stream should be shredded to reduce particle size (<
15 mm) before being fed to the reactor [3]. The hydrolysis/aceto-
genesis generally proceeds sufficiently fast in the mixing tanks,
hence no further reactor has been developed for the hydrolysis/ace-
togenesis [37]. Among mixing tanks, the CSTR is used the most,
the wet plug-flow reactor sometimes [3, 49].
Total solids: Hydrolyzing the high-solids feedstock allows the
reactor to operate larger capacity, requires less energy for heating
and less water consumption. However, a too high solid content
can cause high viscosity of the mixtures, which leads to the fact
that insufficient mixing or mixing can be too energy-consuming
[18, 78, 79]. Moreover, the increase of solid content (in the range
Fig. 2. The configuration of the two-stage digestion system. of 5-40%) causes the increase of inhibitors and insoluble solids
content, which leads to the decrease of the hydrolytic conversion
should be in the first reactor and the second reactor only optimizes rate [79]. Also, a high solid concentration (TS > 15%) of feedstock
methanogenesis [18, 54]. The second point of view might like requires much longer RT (10-15 d) than usual [18]. Thus, TS
to maximize hydrogen product from acetogenesis. However, there of 15-20% in the feedstock is often considered as the upper limit
are several reasons why this idea is not reasonable. Firstly, hydrogen for hydrolysis/acidogenesis when using the mixed reactor [78].
production from AD is currently not economically viable because pH value: Moestedt et al. [80] reported that the pH below 4.5
of the high cost required to enrich the hydrogen gas to meet the led to requiring HRT up to 15 d in the hydrolytic/acidogenic reactor.
commercial quality standards [38]. Secondly, as described in sec- Yu and Fang [81] lifted up pH from 4 to 5.5, which resulted solubility
tion 2.1, methanogens (hydrogen consumer) and acetogens and acidification of substrates increased significantly [81]. Whereas
(hydrogen producer) should work in close cooperation. Thirdly, under alkaline conditions, Zhang et al. [25] reported that VFAs
acetogens are also sensitive and strict anaerobes; they should not formation was significantly decreased when pH increased from
work together with facultative bacteria (hydrolytic/acidogenic mi- 7 to 11. Deublein and Steinhauser [35] showed that pH > 10
croorganism). In fact, the true separation of the digestion-steps caused an irreversible loss of the activity of the microorganisms.
is very difficult to achieve [21], hence the acetogenesis can occur The pH values between 5.5 and 6.5 were often reported as an
in both reactors but mostly in the second one. Diagram of two-stage optimal range and the best pH values was 6.0 [54, 82-84]. These
systems is shown in Fig. 2. results are reasonable for each condition of hydrolysis and acido-
Because of the different environmental conditions between two genesis as presented in section 2.1.
Temperature: Most studies have agreed that the hydro-
reactors, a buffer tank is often set between two reactors as shown
lytic/acidogenic rate is proportional to the increase of temperature
in Fig. 2 for many purposes such as removing non-hydrolysable
[23, 85, 86]. In addition, compared to the mesophilic conditions,
materials, controlling pH, and even controlling the organic concen-
the thermophilic regimes have been increased destruction of patho-
tration [38, 77]. The two-stage system can use one, two, or all
gens which might have a severe impact on reactor and environment
three water recirculation loops (R1, R2, and R3, shown in Fig. 2)
[87]. Therefore, thermophilic temperature seems better than meso-
in case of need. Using these recirculation loops brings many advan-
philic one. However, Kozuchowska and Evison [88] reported that
tages, such as further controlling of pH (reduction of acidity due
the mesophilic operations provided a more stable condition than
to using the high alkalinity effluent from the second reactor); mix-
the thermophilic operation in acidification of coffee waste.
ing/diluting of the high solid feedstock; and improving activities
Komemoto et al. [89] even notified that the solubilization rate
of bacteria [38, 76]. However, if the inhibitors are accumulated
of food waste was significantly higher in mesophilic conditions
inside reactors and cause the unstable condition of the system
than others. Thus, regarding energy and efficiency, the mesophilic
due to long time being used, the incoming substrate should be
temperature in the range of 35-37oC is still preferred to use for
diluted by fresh water. After hydrolysis, the solid content of feed-
the hydrolysis/acidogenesis of organic waste, see Table 3.
stock decreases significantly. Therefore, the two-stage system may
Retention time: RT of this reactor depends on not only the
be a dry-wet configuration (high solid in the first digester and
environmental conditions but also waste characteristics. For hy-
low solid in the second digester) or wet-wet configuration (both
drolysis/acidogenesis of the source-sorted OFMSW (TS 8.2%),
stages are low solid) [5, 49]. The hydrolysis/acidogenesis usually
Pavan et al. [90] recommended that RT would be safe in the range
employs a CSTR [37]. Meanwhile, the methanogenesis uses the
of 2-3 d at the mesophilic temperature. At the same temperature
wet high-rate reactors including CSTR, UASB, AFB, FB, EB, IC,
above but for food waste, Paudel et al. [91] optimized RTs and
and EGSB [14, 36].
reported that one d was the best. Meanwhile, for agro-industrial
3.2.2. The hydrolytic/acidogenic reactor wastewater, Dareioti and Kornaros [92] even got the highest effi-
The mission of this reactor is to convert the substrate to the VFAs ciency of acidogenesis achieved at lower RT (0.75 d). In general,

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Dinh Pham Van et al.

RT in the range of 1 to 3 d is preferred to use to deal with high-solid technology.

waste in the mesophilic condition [49, 54]. In case the reactor RT, OLR: Because of using the wet reactor then the term RT
is operated in batch mode, RT should be maintained within 7-12 includes SRT and HRT. The SRT values greater than 20 d are
d [38]. needed for effective performance at the mesophilic condition, and
Inhibitions: Despite strong and resilient characteristics the hy- it should be 7-15 d at the thermophilic temperature [36]. Meanwhile,
drolytic/acidogenic microorganism can still be inhibited by such HRT is shorter than SRT, varies from several h to nearly 20 d
high concentrations of organic acids (VFAs) and free ammonia. depending on the hydrolytic products, is often coupled with tem-
It has been reported that VFA up to 30 g-COD/L did not inhibit perature and OLR values (see Table 3). Turovskiy and Mathai
the hydrolysis process at neutral pH (6-7) [93], but VFA levels [54] recommended that HRT should be about 10 d at the mesophilic
of 40-50 g-COD/L at low pHs (5.0-5.5) caused stopping the hydrol- temperature for sludge treatment. Also at mesophilic temperature,
ysis [94]. Koster and Lettinga [95] reported that acidogenesis were Paudel et al. [91] optimized the methanogenic reactor with HRT
severely affected by ammonia concentration in the range of 20 d, ORL of 1.24 g VS/L/d for co-digestion of FW and brown
4,051-5,734 mg NH3-N/L. water. Rincón et al. [98] announced that 17 d was the best HRT
Nutrients: Deublein and Steinhauser [35] are two of the few with OLR of 9.2 g-COD/L/d when processing olive mill solid waste.
authors who mentioned about nutrient demands for hydrol- To deal with MSW at mesophilic temperature, Li et al. [97] reported
ysis/acidogenesis and noted that C/N ratio should be in the range that OLR up to 3.8 kg-VS/m3.d-1 with HRT of 15 d resulted in
of 10-40 while trace elements are not a special requirement. biogas yield of 540 mL/g-VS. Meanwhile, also for MSW but at
the thermophilic condition, Pavan et al. [90] needed a shorter
3.2.3. The methanogenic/acetogenesis reactor HRT (7.7 d) to get higher OLR (5.7 kg-VS/m3.d-1) with the same
This phase often employs wet high-rate reactors such as CSTR, biogas yield. Also, Pavan et al. [90] recommended that HRT should
UASB, AFB, FB, EB, IC, and EGSB [14, 36]. These reactors have be in the range of 8-9 d to treat OFMSW at the thermophilic
to be maintained with the methanogen-rich anaerobic environment condition.
which is obligate anaerobic and sensitive to the variation of temper- Nutrients and Inhibitions: It was often reported that the optimal
ature, pH, and also RT [11, 15]. Therefore, operating conditions C:N:P ratio in the range of 75-150:5:1 [36]. However, Lissens et
in the second reactor have to be complied strictly. al. [47] showed that methanogenic reactor run well with C/N below
Solid concentrations: TS of the substrate coming to the methane 10. Methanogenic inhibition occurred when acetate concentration
reactor depends on the type of reactor employed. In case of using exceeded 3 g/L, even though there was sufficient alkalinity to
CSTR or CMR, TS of the substrate is allowed up to 10%. Meanwhile, maintain pH above 7.0 [36]. The optimum concentrations of sulfur
AFB reactor requires lower solid content of the influent (TS ≤ for the growth of methanogens were reported in the literature
5%) [21, 55]. UASB, EB, FB, EGSB, and IC reactors even require to vary from 16 to 160 mg-S/L [44]. Total ammonia of 3-7 g/L
lower solid content of the influent (TS ≤ 3%) with biomass concen- may be tolerated in this reactor, but threshold values of FAN
tration inside be maintained in the range of 3.5-4% [21, 36]. toxicity were reported to be in the range of 100-250 mg/L [36].
The pH value: As shown in Section 2.1, the optimal range
of pH condition for methanogens and acetogens are very close 3.2.4. Assessments
to each other. Moreover, acetogens are stronger than methanogens. The idea of the two-stage digestion not only optimizes each of
Therefore, optimizing pH condition for methanogens does not affect hydrolysis/acidogenesis and methanogenesis to accelerate overall
the growth of acetogens. In fact, pH in the range of 7.0-8.0 was digestion progress but also brings many other advantages. The
often used (see Table 3). Moreover, this phase is an alkalinization methanogens grow in their optimal environment, which leads to
process (converting acids to biogas), hence pH of the substrate the fact that it is more difficult for them to be invaded by other
to this reactor should be lower than the range above. Remarkably, groups compared to the single-stage digestion systems. Therefore,
when pH inside reactor drops below 6.5, the process should be the two-stage digestion system is much more robust than the sin-
stopped to adjust pH value. gle-stage one and can run well with the fluctuation of the waste
Temperature: The optimal temperature for methanogenic bac- stream. Moreover, biogas produced from the hydrolysis/acido-
teria under the mesophilic and thermophilic conditions are 35-37oC genesis is not economical, can be removed out of the gas collection
and 55oC, respectively [15, 96]. Li et al. [97] and Pavan et al. system easily without any more process. Thus, by this way, the
[90] performed the digestion of MSW, used the same condition costs of methane enrichment could be saved significantly. The
in hydrolysis but the different temperatures in methanogenesis. recent results in applying the two-stage digestion systems are shown
While at a temperature of 37oC, Li et al. [97] had to operate the in Table 3. Most studies indicated that the two-stage system was
reactor with HRT 15 d to get 710 mL-CH4/g-VS, at a higher temper- better than the single-stage one because of shorter RTs or higher
ature (55oC) Pavan et al. [90] only needed to operate the reactor values of VS destruction [59, 66, 70, 71] (more details see Table 2
with HRT 12.5 d to get the same methane yield. In general, the and Table 3). However, with the low solid contents in the feedstock
biogas production rates under the thermophilic condition are sharp- (TS < 3%), using the two-stage system does not bring more efficient
ly higher than those under mesophilic condition [15, 46]. Therefore, than the single-stage system. For examples, Schievano et al. [57]
using thermophilic condition leads to increasing OLR and saving used both single-stage and two-stage digestion systems to treat
the construction cost significantly. However, thermophilic metha- OFMSW at low TS condition (1.75-3.4%) with the same conditions
nogens are more sensitive to environmental change than mesophilic (HRT of 25 d, temperature of 55oC) and reported that the methane
anaerobes [15]. Thus, thermophilic reactor requires higher yield and VS destruction were observed almost the same in both

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systems. Halalsheh et al. [69] who experimented digestion of sewage phase for each digestion step make a complex matrix configuration
with SS in the range of 350-430 mg/L, also confirmed that the of the three-stage digestion systems (see Table S3).
two-stage digestion system was not better than the single-stage.
3.3.2. Operations
3.3. The Three-Stage Systems The first reactor: It is optimized for hydrolysis accompany with
mechanical size reduction of the incoming feedstock. Biogas pro-
3.3.1. Configuration of three-stage systems
duced in this phase is mostly CO2. This reactor allows the prevailing
The second reactor of the two-stage digestion systems is responsible acidic conditions to increase the rate of particle disruption and
for both steps of acetogenesis and methanogenesis. Unfortunately, hydrolysis [25, 29]. Thus, the buffering agent is not often required
acetogens and methanogens have substantially different physio- in this reactor [22]. However, sometimes pH dropping below four
logical properties and nutrient requirements. The growth balance leads to inhibiting hydrolysis immediately [80]. In fact, whether
between them can be lost by the environmental changes leading or not using a buffering agent depends on characteristics of the
to severe inhibition [101]. Therefore, the idea of three-stage diges- waste and the ways to operate the system. High temperature accel-
tion has come out to separate the hydrolysis/acidogenesis, aceto- erates solubilization of the feedstock. He et al. [85] performed
genesis, and methanogenesis to various reactors [46, 101, 102]. hydrolysis of food waste and reported that RT was three day to
However, as the reasons discussed in the previous section, the maximize COD-soluble concentration (46.76 g/L) at 70ºC, but it
separation of both processes is not a good idea. From another took a longer time (5 d) at 55ºC (43.33 g/L) and 35ºC (36.25 g/L).
viewpoint, Zhang et al. [103] and Stewart [22] proposed the However, mesophilic temperature (30-35°C) is still often used along
three-stage digestion model with sequential steps: incomplete hy- with RT of 2-3 d. Operation of this reactor could be referenced
drolysis, hydrolysis/acidogenesis, and acetogenesis/methanogenesis. to the hydrolytic/acidogenic reactor discussed in the previous
This proposal would increase the hydrolytic rate of feedstock in section. Also, the inhibitors in the feedstock can be retained in
the first reactor significantly. Therefore, the second proposal is this reactor caused by absorption into sediment, and thus they
more valuable. In fact, due to non-homogeneous metabolism in reduce adverse impact on the next reactors [22].
reality then the distribution of digestion steps in three-stage diges- The second reactor: It is optimized for acidogenesis and
tion is, respectively hydrolysis/acidogenesis, acidogenesis/aceto- acetogenesis. Biogas produced in this phase mainly includes CO2
genesis, and acetogenesis/methanogenesis. Hydrogen can be ob- and H2 [22]. As shown in section 2.1, acidogenesis and acetogenesis
tained in the first and second reactors, however, as being discussed thrive optimally in the pH condition in the range of 5.5-6.5 and
in the previous section, the collection of hydrogen is not 6.0-6.2, respectively. Therefore, the best condition for the second
economical. Diagram of the three-stage digestion system is shown reactor is around pH 6.0. The mesophilic temperature (35-37ºC)
in Fig. 3. is preferred to use in these processes with HRT of 2 d to deal
Also, like the two-stage systems, the waste stream coming to with OFMSW (see Table 4). The completion of the second stage
the three-stage system has to undergo a mechanical pretreatment is marked by the conversion of almost VFAs into acetic acid.
(particle size < 15 mm), and water is sometimes added to the The third reactor: The operation of this reactor is similar to
feedstock to adjust TS (max. 15%). CSTR is still often used in the second reactor of two-stage digestion systems, such as optimum
the hydrolysis reactor. After hydrolysis, the non-hydrolysable mate- pH between 7.0 and 7.2 [18].
rials are removed in the first buffer tank. Here, pH and VFAs
concentration of the substrate can also be corrected for the next 3.3.3. Assessments
step. The second reactor operates such an anaerobic suspended As reported by Salsali et al. [66], three-stage digestion system
growth process. Therefore, the second reactor is often a CSTR had a higher performance of VS reduction and biogas yield com-
or UASB reactor. The effluent of the second reactor coming to pared to the two-stage system in the digestion of waste activated
the third one has to pass through a buffer tank as shown in Fig. 3 sludge. In fact, using three-stage digestion systems brings many
for purposes of controlling pH, and concentration of acetic acid. disadvantages such as the high cost of investment, operations,
Selections of the type of reactor, feeding mode, and temperature and maintenance. Therefore, it is difficult to find out the application
of the three-stage anaerobic digestion system for pilot scale. Pile
[104] gave little information about the Inland Empire Regional
Water Recycling Plant I, California, US. Before 2001, the plant
had been operated as a single-stage system but not efficiently.
Therefore, since 2001, the plant has been operated in sepa-
rated-phases including the first reactor with HRT 2.5-3.5 d at
32-40°C; the second reactor with HRT 18-20 d at 56-58°C; and
the third reactor with HRT 13-17 d at 42-48°C. VS reduction has
been improved from approximately 55% to 60-65%. However, from
Tables 3 and 4, the three-stage systems did not produce more
methane yield than the two-stage systems. The three-stage digestion
is obviously a good idea for optimizing all digestion steps in princi-
ple, but it has not been proved persuasively to be better than
Fig. 3. Diagram of the three-stage anaerobic digestion system. the two-stage digestion system even in the lab scale.

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4. Current Application The discovering a significant increase of OLR in the thermophilic

condition compared to the mesophilic condition has brought many
advantages such as reducing the volume of the reactor in a new
The achievements of the AD not only solve the waste problems
design and raising the capacity of the old reactor. Although the
but also bring a new approach to energy production. For two
thermophilic technology requires a high stability of waste stream
recent decades, the total installed AD capacity has been increased
because of the sensitivity of the thermophilic bacteria, this is no
five-fold worldwide from 2 million tons to 11 million tons a year
longer an obstacle to the development of current technology.
[106]. Group of European countries has contributed the most pro-
Thermophilic AD systems have only been studied and applied
portion to the total biogas power generation with the increase
in recent years, it takes a long time to become popular. Thus,
of big biogas plants from 18 in 2001 to 418 in 2015 [10, 107].
mesophilic AD systems are still more dominant in the present.
In which, Germany has been a leader in using energy from biogas
The dry single-stage digestion systems are more stable, more robust,
with a power capacity of 12,500 GWh by 2014 [10]. Beside European
have a smaller reactor, and produce less residual wastewater than
countries, the United States also contributes a significant pro-
the wet systems. It has been most applied in Europe to deal with
portion in using bio-waste for power generation with 181 AD facili-
OFMSW which accounts for the highest proportion of the bio-
ties processing OFMSW (total capacity of 780,000 tons/y, 2015)
degradable waste sectors. Thus, based on cumulative installed
[108]. In contrast, although Japan is also a developed country
capacity, the dry single-stage digestion systems are still over-
with high potential of technologies, the application of biogas tech-
whelming applicated as shown in Fig. 4. The batch process often
nologies is still slow: from one large biogas by 2004 (150 Nm3-gas/h)
accompanies with disadvantages such as variable biogas pro-
up to six plants by 2009 (total 2,400 Nm3-gas/h) [107]. Meanwhile,
duction following time, long HRT, big reactor, and uncompleted
for developing countries, low capacity digestion systems are com-
degradation. However, it brings many advantages including simple
monly used that China is a typical example with the largest biogas
material handling, less pre-processing requirements, easily control-
program in the world. By the end of 2009, there had been over
ling, and especially easy separation of hydrolysis and
30.5 million household biogas digesters (12.4 billion m3-gas/y)
methanogenesis. Therefore, the batch process which has been soon
and 25,012 large-scale biogas projects (7,225 million m3-gas/y)
being applied since 2009 for digestion of OFMSW, is estimated
[10, 109]. Another smaller program, SNV (a Netherland
continuously to increase in use due to their simplicity and low
Development Organization) supported the domestic biogas pro-
cost [5, 111]. It is easy to understand the lack of three-stage system
grams for 17 countries in Asia and Africa. By the end of 2012,
in Fig. 4 because the three-stage system has not been proved to
SNV installed total 479,609 household digesters for Asian countries
have more methane yield than the two-stage system meanwhile
and 24,990 small digesters for African countries [110]. The biogas
it is much more complex in operating and also consumes higher
program in Nepal and Vietnam counted for over 80% of this total.
energy.
In general, because of lacking technologies and the financial burden
then the wet low rate digesters are commonly used in the developing
countries with a large quantity but small-capacity (mainly house-
hold scale and farm scale). Meanwhile, the European countries
5. Conclusions and Recommendations
with technical potential have advantage conditions to build
large-scale plants which have much higher power capacity than The single-stage wet low-rate digestion systems are characterized
by producing low biogas yield and low OLR (0.5-1.6 kg-VS/m3.d-1),
China and other developing countries [5, 111], also see Fig. S3.
requiring long RT (30-60 d) of the feedstock and big reactors,
So based on current installed capacity, the contribution of AD
and depending on the outdoor temperature during the operation.
systems in the European can represent a panorama in applying
However, they bring many advantages such as easy operation and
AD systems over the world. Fig. 4 shows the current application
cheap investment. In perspective economic benefits, they should
of various AD systems in Europe by 2014.
only be applied for fine biodegradable waste sources (such as
animal manures) without requiring any pretreatment step.
Therefore, the wet low-rate system is a good solution for warm-cli-
mate rural areas, where agricultural land is available, and digestive
products can turn back to serve for agricultural activities.
The single-stage wet high-rate digestion systems are much more
effective than the wet low-rate systems with OLR up to 4-8
kg-VS/m3.d-1. However, the operation is more complex than the
low-rate one because of the attached equipment and pre-treatment
process sometimes. The feedstock with low solid contents (< 3-5%)
is the best source for these systems with many selections of reactors
such as UASB, FBR, EBR, EGBR, and CSTR. When TS > 5%,
the CSTR is often used. In fact, these systems have been applied
widely to deal with sewage sludge, agricultural products, and in-
dustrial wastewater. They can also be used to treat high solid
Fig. 4. Applications of AD systems in Europe based on cumulative capacity feedstock (TS > 20%) such as OFMSW, but it requires to be diluted.
installed in 2014 [111]. At that time, water consumption and energy expense significantly

13
Dinh Pham Van et al.

reduce the energy benefit of the system. FD, Hiligsmann S, eds. Microbial fuels. USA: CRC Press; 2017.
The single-stage dry digestion systems have brought a milestone p. 77-151.
for the AD technology, which can deal with high solid feedstock 4. Zhang W, Zhang L, Li A. Anaerobic co-digestion of food waste
(TS = 30-35%) and high OLR (up to 15 kg-VS/m3.d-1). In case with MSW incineration plant fresh leachate: Process perform-
of too high solid contents in the feedstock (> 35%), it should ance and synergistic effects. Chem. Eng. J. 2015;259:795-805.
be diluted. They are very suitable for handling OFMSW which 5. Rapport J, Zhang R, Jenkins BM, Williams RB. Current anaerobic
accounts for a large amount of biodegradable waste sector. These digestion technologies used for treatment of municipal organic
systems can be distinguished by continuous operation mode or solid waste. In: California Environmental Protection Agency.
batch operation mode. For continuous mode, there are many advan- California: California Integrated Waste Management Board;
tages attached such as low water consumption, small reactor, very 2008.
little wastewater production, cheap pre-treatment, waste residue 6. Kothari R, Pandey A, Kumar S, Tyagi V, Tyagi S. Different
well applied for composting, and less heat requirement. Thus, aspects of dry anaerobic digestion for bio-energy: An overview.
the continuous mode systems should be applied for urban areas. Renew. Sust. Energ. Rev. 2014;39:174-195.
For batch mode systems, it costs lower for equipment invest and 7. Kayhanian M, Tchobanoglous G, Brown RC. Biomass con-
operation. However, it needs much larger space than the continuous version processes for energy recovery. In: Kreith F, Goswami
mode systems and gas generation is variable. Therefore, the batch DY, eds. Handbook of energy efficiency and renewable energy.
mode systems is a great solution for suburban areas. Florida: CRC Press; 2007. p. 22.1-22.67.
The two-stage AD systems with separation of hydrolysis and 8. Han D, Tong X, Currell MJ, Cao G, Jin M, Tong C. Evaluation
methanogenesis in various reactors could have shorter HRT and of the impact of an uncontrolled landfill on surrounding ground-
higher OLR (up to 50 kg-COD/m3/d) than the single-stage systems. water quality, Zhoukou, China. J. Geochem. Explor. 2014;136:
They bring many advantages such as lower construction cost, 24-39.
much more operational flexibility, more robust, highest methane 9. Chen HH, Lee AH. Comprehensive overview of renewable en-
content, higher digestion efficiency, and especially well running ergy development in Taiwan. Renew. Sust. Energ. Rev.
with low C/N (< 10). The best application range for solid content 2014;37:215-228.
10. Deng Y, Xu J, Liu Y, Mancl K. Biogas as a sustainable energy
of the feedstock is in the range of 5-15%. When TS < 5%, using
source in China: Regional development strategy application
the single-stage wet systems might better. When TS > 20%, the
and decision making. Renew. Sust. Energ. Rev. 2014;35:294-303.
dilution of the waste stream causes a significant increase in energy
11. Mao C, Feng Y, Wang X, Ren G. Review on research achieve-
needed for heating, pumping and expanding reactors, hence it
ments of biogas from anaerobic digestion. Renew. Sust. Energ.
is better to use the dry single-stage digestion system. The two-stage
Rev. 2015;45:540-555.
AD systems are suitable to deal with industrial wastewater.
12. Chiu S, Lo I. Reviewing the anaerobic digestion and co-digestion
The performance of the three-stage digestion systems has not
process of food waste from the perspectives on biogas pro-
been improved compared to the two-stage systems. Moreover, be-
duction performance and environmental impacts. Environ. Sci.
cause it has more complex operation, more expensive investment,
Pollut. Res. 2016;23:24435-24450.
more energy for maintenance and operation compared to two-stage
13. Zhang C, Su H, Baeyens J, Tan T. Reviewing the anaerobic
AD systems, application of the three-stage system for large scale digestion of food waste for biogas production. Renew. Sust.
is not a good selection at present. However, three-stage systems Energ. Rev. 2014;38:383-392.
are still a good idea; its operation should be studied further to 14. Chernicharo L, Augusto C. Anaerobic reactors. In: Biological
reduce the cost involved. Wastewater Treatment Series, London: IWA publishing; 2007.
15. Gerardi MH. The microbiology of anaerobic digesters. In:
Wastewater Microbiology Series. New Jersey: Wiley-Interscience;
Ackowledgments 2003.
16. Demirel B, Yenigün O. Two-phase anaerobic digestion proc-
The authors would like to acknowledge the Graduate School of esses: A review. J. Chem. Technol. Biotechnol. 2002;77:743-755.
Environmental and Life Science (Okayama Univ., Japan) for their 17. Appels L, Baeyens J, Degrève J, Dewil R. Principles and potential
financial support. of the anaerobic digestion of waste-activated sludge. Prog.
Energ. Combust. Sci. 2008;34:755-781.
18. Ostrem K. Greening waste: Anaerobic digestion for treating
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