Heat and Mass Limitations in an Anaerobic Digestion Process

DELİA TERESA SPONZA*, RUKİYE ÖZTEKİN

Department of Environmental Engineering,

Dokuz Eylül University,

Tınaztepe Campus, 35160 Buca/Izmir,

TURKEY

*Corresponding Author

Abstract: - In this study, heat and mass limitations in an anaerobic reactor containing domestic solids were

researched in batch reactors. The dynamic and static anaerobic data for 365 days showed that the methane

production for the dynamic digestion reactor was measured as 176.86 m3 which is extremely high for static

anaerobic one (102.78 m3). As the heat transfer data increased with elevated temperature the methane

productions also were highlighted. The external mass transfer was observed for easily degradable solids. In the

calculation of external mass transfer during the degradation of organics dissolved with difficulty some

semiempirical regressions were used. In the calculation of internal mass transfer the microorganisms in the

solids were taken into consideration and the diffusion was defined with Fick's law. The diffusion coefficient D,

was found to be constant. Generally, the diffusion coefficient of solids in water (Dw) was < 1.0. The effect of

the total solid (TS) concentration in anaerobic batch reactors (TS between 12% and 39%) was investigated. The

methane gas production decreased minorly when the TS levels elevated to 30%. At a TS percentage of 39%, the

methane generation decreased significantly. At high TS, the mass transfer was inhibited and ended with

lowered methane generations while the hydrolysis process did not affect significantly at high TS

concentrations.

Key-Words: - Anaerobic treatment; Methane (CH4) gas generation; Diffusion coefficient; Dynamic and static

anaerobic processes; Fick’s law; Heat and mass limitations; Solid anaerobic batch treatment;

Hydrolysis; Liquid/gas mass transfer; Domestic solid wastes.

Received: April 8, 2023. Revised: September 16, 2023. Accepted: November 11, 2023. Published: December 20, 2023.

1 Introduction

The solid wastes can be removed with different

processes like biochemical incineration,

gasification, composting, landfill, and anaerobic

digestion, [1], [2], [3]. Anaerobic treatment is a

complicated process and has three steps: (1)

microbial digestion, (2) physicochemical processes

and (3) mass transfer of some gaseous end

substances. Anaerobic digestion is an excellent

effective technology with low cost for energy reuse

from organic wastes with elevated moisture ratio

ending with gasification and pyrolysis, [4]. Based

on solid ratio; anaerobic treatment can be classified

as wet (< 19% TS) and dry (≥19% TS) processes,

[5].

Dry remediation technologies, solid-state or

high-solids anaerobic processes were very

significant since the ratio of water put to the raw

waste is impostantly lowered since the volume of

the digester is lowered. This process has succees

with minor waste volume and the waste transport

emissions decreased. As a result, small digester

capacities with elevated organic loading rates were

processed, [6]. Some problem was detected at

anaerobic digesters with high total solids. The

utilization of anaerobic digester with high solids is

limited due to low heat and mass transfer rates

during long times and inhibitions due to presence of

toxic compounds such as volatile fatty acids and

ammonia, [7]. The anaerobic processes with high

solids were slow processes for releasing of

hydrolysed organics and microbial degradations,

[8]. In order to industrialization of this process the

heat and mass transfer should be improved. The

anaerobic digestion processes with high solid ratios

was studied in of crop waste, [9], and animal

manures, [10]. The effects of operational conditions,

[11], feedstock properties, [12], and pre-treatment

steps, [13], the benefits of co-digestion, [12], the

stability of the process, [14], and the lowcost

benefits of anaerobic digestion with high solid were

investigated, [7].

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Anaerobic digestion is very sensitive to increases

in temperature by lowering the steady-state

condition in the process ending with low biogas

productions. The temperature in the anaerobic

reactor is related on the heat exchange between the

anaerobic digester and environment. During biogas

generation anaerobic digestion produce clean power

and provide sanitation by degrading the organic

compounds, [15]. During anaerobic digestion the

dominated microorganisms degrade ultimately the

organic compounds to methane and carbondioxide

gas under anaerobic conditions, [16]. This

phenomenon is used to degrade the domestic sludge

and the municipal and industrial solid wastes in the

sludges, [17].

Mixing has a big importance for gas generation

in the anaerobic digesters. Without mixing, the

organic substances were not distributed, the fluidity

was pure, and the mass transfer was not cccurred

during static digestion process, [18], [19], [20], [21].

Mixing in the dynamic anaerobic digestion cause to

excellent methane generations. The mixing process

were as follows: mechanical mixing, [22], mixing

by slurry recirculation, [23], and mixing with gas

recirculation, [24], [25]. The importance of

mechanical mixing were elevated heat and mass

transfer percentages, with low hydraulic dead space,

and high metabolism rate of anaerobic bacteria.

Mass transfer of gaseous products affect

significantly the transportation of total and methane

gases from the liquid phase. Furhermore,

transportation of gases from the liquid media affects

the pH value of the reactor and the buffer capacity

of the carbonate system since the thermodynamics

and kinetics of microbiological reactions governed

by pH value and CO2, CH4 and H2 gases. Mixing in

the liquid phase governed the substrate mass

transfer and sludge retention, [26], and gas volume

percentages in high rate degradation processes, [27].

A lot of data containing gas generation in anaerobic

digesters were based on the liquid-gas steady-state

conditions. This has a pozitive effect for poorly

soluble gases such as CH4, and to CO2 and H2S gas

solubilities.

In bad mixed anaerobic digesters, the gas void

ratio and the dose of substrates were studied, [28],

[29]. The hydrodynamic mass transfer of different

digesters using a viscous Newtonian and non-

Newtonian fluid was studied, [30]. The studies

showed that mechanical mixing has elevated mass

transfer rates and have good homogeneity of the

anaerobic reactors, [30]. Three mixing procedures

were compared and it was noted that the mixing

energy level was highest in mechanical mixing,

[31]. Mechanical mixing improves the homogeneity

and quick mass transfer at a reduced mixing

duration, for a non-Newtonian fluid with excess

organic loadings. Limitations in slurry and gas

recovering were elevated apparatus costs at

digesters with high solid wastes, [32]. A slurry

recirculation digester for anaerobic digestion of

straw granules was studied, [33]. It was found

blockage to transfer the straw particles to the liquid

surface even thougth the particle size was smaller

than 3 mm, [33]. Reuse of gas in hay digestion; It

has been noted that it tends to bring straw particles

to the liquid surface and accumulate crusts on the

liquid surface that are not effective for anaerobic

digestion, [34]. Therefore, these processs has

limited improvements in biogas generations at high

capacity digestion systems, [35].

Among the currently used anaerobic digestion

processes; The most perfect model has been

designed, [36]. To ensure anaerobic two-stage

digestion of sewage sludge; It is intended to

simulate the dynamic activity of a semi-continuous

pilot scale reactor, [37]. The modified anaerobic

digestion model for olive mill solid waste and olive

mill wastewater at thermophilic temperature;

Anaerobic semi-continuous tubular digesters have

been found to provide steady-state behaviour, [38].

The data indicated that the modified anaerobic

model could predict the steady-state results of gas

flows, total gas and methane percentages, pH, and

volatile fatty acids extensively. In another model,

degradation of grass silage occurred in two semi-

continuous digesters under mesophilic conditions,

[39]. This model was calibrated by a Genetic

Algorithm in MATLAB/ SIMULINK, [39].

In this study, heat and mass variations in the

anaerobic processes with high solid content process

with anaerobiv model for municipal solid wastes in

anaerobic batch reactors were reaserched. Anaerobic

batch reactor experiments were carried out with

solid dose ranging from 19% to 45%. CH4(g)

production performances were evaluated. Anaerobic

digestion model No.1 was used to compare the

experimental results to detect the impact of water

ratio on the performance of anaerobic degradation.

The effect of total solids on the yields of anaerobic

digestion was investigated. The impact of the

hydrolysis and transportation of liquid/gas mass

transfer on anaerobic digestion was resaearched.

The significance of the experimental data was

correlated with ANOVA statistical analysis method.

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2 Materials and Methods

2.1 Properties of Compounds

Cellulose, hemicellulose and lignin ratios of

domestic solid wastes were also measured by the

method specified in the literature, [40]. The

hemicellulose type substrates were solved by an

acid (36 g/l C19H42NBr and 32.9 ml/lH2SO4

(99.99%)) for 80 min at 130oC. The soluble organics

were measured by extraction with a detergent (40 g/l

C12H25NaO4S, 23.88 g/l C10H14N2Na2O8.2H2O, 9.23

g/l Na2B4O7.10H2O, 6.88 g/l Na2HPO4 and 16 ml/l

C6H14O4) at 130oC for 90 min and the lignin type

organics were measured by discarting the cellulose

type organics for 4 h with H2SO4 (99.99%),

respectively. At the last step, the extracted samples

were washed with deionized water and dried at

130oC before to the next step.

2.2 Studies Performed During Tests

A cardboard with a density of 1.58 kg/m3, was used

as an organic compound with a ratio of 35% in

municipal solid waste. The cardboard was cutted

and sieved at 1.2 mm. The studies were performed

in 1000 ml batch glass reactors with a working

volume of 400 ml. A mixture of cardboard, water,

microorganisms and trace metals was prepared to

have different total solid percentage contents from

‘‘wet’’ to ‘‘dry’’ anaerobic conditions as given in

follows: 12%, 19%, 25%, 32%, 39%, 45%. An

organic substrate /microorganism ratio of 50 (w/w)

was used to control the negative effects of starter

microorganisms. The microorganisms were taken

from a leachate of municipale solid wastes in an

industrial treatment plant degrading municipal solid

wastes. 2.5 ml trace element solution was added to

the mixture. This trace element solution comprised

of: 3 g/l FeCl2.4H2O, 0.9 g/l CoCl2.6H2O, 0.5 g/l

NiCl2.6H2O, 0.5 g/l MnCl2.4H2O, 0.09 g/l H3BO3,

0.09 g/l ZnCl2, 0.09 g/l Na2SeO3, 0.09 g/l

CuCl2.2H2O and 0.09 g/l Na2MoO4.2H2O,

respectively. The analyses were performed during

450 days under mesophilic conditions (35oC)

without mixing.

2.3 Analytical Procedures

The total gas generations and the gas composition

were measured in the strat-up period while the

metnane generation was measured under staedy

state conditions in the digester. The total gas

generation was detected by the water displacement

method The biogas composition was analysed by a

gas chromatography–mass spectrometry (GC-MS);

a gas chromatograph (GC) (Agilent Technology

Model 8890N GC equipped with a mass selective

detector (Agilent Technology Model 5989 inert

MSD) by injecting a sample volume of 2 ml. Mass

spectra were recorded using a VGTS 250

spectrometer equipped with a capillary SE 52

column (HP5-MS 30 m, 0.25 mm ID, 0.25 μm) at

220°C with an isothermal program for 10 min. The

initial oven temperature was kept at 55oC for 1 min

during 2 min, then the time was increased to 5.5

min. Helium (He) was used as the carrier gas at

constant flow mode (1.7 ml/min, 49 cm/s linear

velocity). The calibration was carried out with a

standard gas composed of 29% CO2(g), 3% O2(g),

8% N2(g) and 60% CH4(g), respectively.

Volatile fatty acid concentrations were measured

after centrifugation of samples at 14000 rpm for 40

min. in a GC-MS (Agilent 8890N GC – Agilent

5989 inert MSD). As carrier gas nitrogen [N2(g)]

was used. All other pollutant assays were performed

according to the Standard Methods (2022), [41].

2.4 Measurement of Methane Productions

Biochemical methane potential was measured

according to the method stated in the literature, [42].

The tests were performed in 600 ml glass serum

bottles at 35oC. The glass bottles were filled with

synthetic medium containing nutrients and trace

elements, and granular sludge from a mesophilic

anaerobic digester treating yeast industry

wastewater in İzmir, Turkey. The final sludge

concentration in the bottles was 45 gVS/l. The

bottles were loaded with 1 g of cardboard (0.95

gVS). The calculated biochemical methane gas

productions were accounted daily.

2.5 Anaerobic Degradation

The steps of anaerobic degradation consist from

hydrolysis, acidogenesis, acetogenesis, and

methanogenesis as shown in Figure 1 (Appendix).

In hydrolysis, big molecular weigth compounds

were break down too much smaller compounds. In

the acidogenesis step, long-chain fatty acids

transformed to short-chain volatile fatty acids, while

in the acetogenesis step, acetate, CO2, and/or

hydrogen (H2) are formed via the fermentation of

the volatile fatty acids. The produced acetate,

CO2(g) and/or H2(g) were transformed to methane

via methanogen bacteria.

Anaerobic degradation process is affected by the

temperature variations yielding with lowering of

methane gas productions. The temperature in the

anaerobic reactor affects the heat exchange between

anaerobic reactor and anaerobic environment and a

constant and appropriate temperature is required to

sustain digesti. In the anaerobic digesters high

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methane generations was detected as the

temperature was elevated, [43], [44]. In order to

increase the energy demand of the anaerobic

Archae; an elevated heat transfer ratio is necessary.

The static heating was performed by transporting of

thermal energy to the anaerobic reactor with hot

water in the reactor. For this a heat exchange coil

device is necessary in the reactor. The negative

points of static heating were low thermal yields

cause to lowering of steady-state conditions during

methane generation. When studying the static

heating process during anaerobic digestion,

polyethylene coiled tubing was used and a large

temperature change was observed when operating in

the vertical direction after heating, [45]. The

dynamic heating process contains the releasing of

thermal energy to the anaerobic reactor during direct

heating of the recycled slurry. This cause to rapid

heat transfer and homogenous increase of

temperature. The dynamic digestion heating

methods exhibited limitations during the pipeline

transport, [46].

A lot of anaerobic digestion process was

extensively used in the literature according to

anaerobic digestion model No.1 model. This model

is an effective for methane productions, [47], [48],

[49], and includes a lot of steps. The growth and

decay of different types of microorganisms were

also taken into considerations, [50]. To simulate

digestion in the presence of high solids during the

digestion of organic matter of domestic solid waste;

A new model was created based on the anaerobic

digestion model No.1, [51]. In the comparison of

conventional ‘wet’ anaerobic digestion model No.1,

simulates the reactor mass to volume variation

depending of high degradation of total solids. The

model contains the relationships between total solid

yields and methane generations. Municipal waste

sizes, organic rate and organic matter load; A

dynamic mathematical model was developed that

includes the effects of anaerobic co-digestion of

organic matter in municipal waste and domestic

sludge on methane production and COD yield, [52].

The anaerobic degradation of the sewage sludge is

simulated using the anaerobic digestion model No.1,

while the degradation step of the organic substances

was modelled by a surface-based kinetic, depending

on the diameter of the composite substance.

2.6 Anaerobic Digestion Model No.1

A lot of mathematical models were present to

determine the anaerobic digestion. These models

were usefull to determine the anaerobic reactor

dynamics. Anaerobic digestion model No.1 model

four steps were present during degradation of

organics: hydrolysis, acidogenesis, acetogenesis,

and methanogenesis with generation and decay of

microorganisms present in the anaerobic reactor,

[36].

Physicochemical steps such as acid–base and

notral pH variations and gas–liquid emissions. Due

to high numbers of processes, and high 100 value of

parameters, the calibration of the anaerobic reactor

is difficult during methane productions. The

anaerobic digestion model No.1 was utilized for

optimization of the anaerobic methane gas

productions A standardized anaerobic digestion

model No.1 can be effective to solve the inhibitions

during settling of volatile fatty acids. Since

anaerobic digestion process is very sensitive due to

anaerobic Archae bacteria the mathematical

equlibrium can not be solve the problems. The

efficiency of the model was related to the data used

for fitting of the model data. It was a relathionship

between the inlet and output data. Each inlet value

introducing to the methane reactor at a duration t is

correlated with the output and leaves the digester at

a time t + ∆t. This time can be defined as retention

duration. A lot of operational conditions such as

flows and mixing regime, composition of the

organics and substrate concentrations affects the

yield. Therefore, complex ploblems originated from

the parameters given above should be solved by

expert researchers.

A lot of intelligence model techniques like

artificial networks, fuzzy logic and expert models

can be used to monitore and predict the anaerobic

digestion. Nature-inspired methods were produced

for specific biological systems relevant to biomass

properties affects significantly the methane gas

production in the anaerobic processes. It is

important to note that which suitable models can be

used to simulate the methane productions in

anaerobic reactors.

2.7 Intelligence Models

Artificial intelligence models can be used in

environmental, agricultural applications, in

anthropological experiences, in medical fields, and

in different types of engineering applications. These

models can be utilized for modeling, predicting, and

simulating to find meaningful solutions. Modeling is

very difficult due to the complexity of the processes

which were nonlinear, [53]. Therefore, with

artificial intelligence models it is easy to predict

nonlinear equations. Last studies in computing

results decreased the time which is necessary to

develop the equations, and to combine the new

results based on new modifications.

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Artificial intelligence can be applied to the new

data developed in micro-scale. In the anaerobic

digestion, when the relathionships is complicated it

is not possible to generate new mechanistic process

models. Contrarily, some researchers were used

artificial intelligence models to study under steady-

state conditions during digestion process, [53]. By

utilization of these model methane gas generation

should be improved. Nature-inspired computing is a

recently developed model for artificial intelligence

techniques. Living and nonliving natural models can

be able to study with the same or different data.

When a central administration was not present, the

processing was distributed. Steady-state conditions

should be maintained during solving of the

problems in engineering applications. Algorithms

are iterative procedures to solve the problems step

by step for specific aims. With computational

optimization the algorithms can be designed,

implemented and improved by solving the

optimization problems, [54].

Optimization steps includes the improvement of

the yields, of the efficiency, and decrease the energy

and cost spent for the processs. If enuogh time was

present, the problems can be solved under

laboratory conditions while it is difficult to solve the

problems in real applications such as anaerobic

digestion. It is necessary some computer

simulations.

2.8 Performed Statistical Data

ANOVA test were applied to the experimental data

to determine F and P values and to show the

significance between dependent and independent

variables, [55]. Variation of the experimental data

mean and standard deviation values was indicated

by F ratio. F explain the variation of the data

averages/expected variation of the date averages. P

indicates the significance data, and d.f showes the

freedom degrees. Regression analysis was applied to

the experimental data to detect the regression

coefficient R2, [56]. These data were calculated

using Microsoft Excel Program.

All experiments were carried out three times and

the results are given as the means of triplicate

samplings. The data relevant to the individual

pollutant parameters are given as the mean with

standard deviation (SD) values.

3 Results and Discussions

3.1 Why Heat Transfer is Important?

Heat transfer shows the flow of thermal energy

managed by a non-equilibrium and non-uniform

temperature in the processes. Factors affecting the

heat transfer in an anaerobic digester with high solid

content were as follows: microorganisms in starter

biomass (temperature, physiology, maximum

growth rates, catabolic heat generation), substrate

type (particle diameter, type, porosity), equipment

diameter (length and apparatus type) and operational

conditions (biomass percentage in the starter

culture, pH, temperature and humidity of the gas),

[57], [58], [59]. Interphase mass transfer of methane

gas from a liquid substrate was illustrated in Figure

2 (Appendix).

The heat transfer in an anaerobic digester with

high solid content was relevant with the activity of

metabolic rates. The substrates with low thermal

conductivities cause to small heat transfer rates.

Mechanical mixing and air supply with gas are two

efficient methods to improve substrate agitation and

provides suitable heat and mass transfer during the

anaerobic digestion, [60]. During mechanical

mixing, the size of the impeller and mixing

properties affect significantly the, impeller, [61],

[62]. Flow rates were affected by the speed and have

not an important effect on the mixing rates. No more

studies were found researching the specific impacts

of the mixing methods on the performance of

anaerobic digesters with high solid content, [61],

[62].

3.2 Why Mass Transfer is Important?

Many efforts have been made due to the important

A lot of studies were performed to detect the advice

of mass transfer on the reaction yields. Mass

transfer yields is relevant with concentration

gradient of gases and substrates during advection or

diffusion. In anaerobic processes the yields depend

to mass transfers, to grafting, to the types of the

organics, to temperature, to moisture ratio, and to

mixing yields. In the compairison with wet

anaerobic digestion reactors with high solid wate

ratio exhibited some problems due to mass transfer

was inhibited. To improve the relathionship between

biodegradable substrates and anaerobic Archae

bacteria it is important to provide suitable mass

transfer, [63].

An anaerobic digester with high solid wastes has

solids, liquids, and gas steps (Figure 2, Appendix)

relevant to a solid organic compound, starter

anaerobic culture, water, and gases namely methane,

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CO2, NH3, and H2S). The organic solid compound

should have a porous structure. Furthermore, the the

shape and the diameter of the organic compounds,

the void space for gas in tranferring were important

parameters. The surface area of the substrates for

bacterial methabolism and solid substrate uptaken

yields are affected by the porosity of the substrates,

[64].

The start-up of anaerobic digester having high

solid content performed by a starter bacterial mass

followed by the generation of a complex reaction

and ultimately advances u up to stabilization of the

solid wastes, [65]. In these studies, a minimum size

of seed should be used, and the anaerobic yields are

related with the activity of the of reactions. Volatile

fatty acid was released and diffused to

methanogenic Archae bacteria. In order to effective

metabolism of Archae bacteria the presence of an

alkaline buffer solution protect these bacteria from

the volatile fatty acid accumulation and inhibition,

[66], [67].

The volumetric mass transfer coefficient kLa is

correlated with kL (mass transfer coefficient), and

with specific surface area. In dry and semi-dry

anaerobic digesters, the volumetric mass transfer

coefficient kLa is significantly lowered depending

to two items: (1) The solid–liquid/gas interface is

small, since low biogas bubble and inadequate

mixing. (2) The mass transfer coefficient was

relevant with low moisture percentage. Eq. 1

indicates the difference between the diffusivity

coefficients of readily degradable organics in the Di

digester and in the water Di (Eq. 1):

󰇛󰇜󰇛󰇜󰇧󰇛󰇜

󰇛󰇜󰇨

(1)

Diffusivity coefficients significantly lowered

with lowering of porosity depending to water

percentage and the elevated viscosity which is

significantly depend to the solid content, [68], [69],

[70].

Based on these assumptions, the maximum yield

of the volumetric mass transfer coefficient is

dependent to the total solids. A kT = 0.5 d-1 was

taken into consideration for the alalysis at TS =

10%.

3.2.1 Operational Conditions Internal Mass

Transfer

Mass transfer in the anaerobic digester was modeled

by taken into consideration the diffusion alone and

utilizint the Fick's law. The diffusion coefficient D

in Fick's law is relevant to a constant or is not

dependent to substrate dose. D is not measurable

parameter and it can be calculated by multiplying

the diffusion coefficient (Dw) of the solids in

aqueous digester by a constant (< 1). No extensive

knowledge was obtained about diffusion coefficient

of solides in anaerobic digester utilizing Fick's law

as showed in in Eq. (2), [71]:



 (2)

where, J: is the diffusion flux with an expression of

amount of substrate per unit space per unit time. J

indicates the dose of the substrate through a unit

space during a unit duration. D: is the diffusion

coefficient or diffusivity. Its dimension is space per

unit duration. dφ/dx: is the concentration gradient,

φ: is the dose of the concentration of substrate per

unit volume. x: is the size of the length.

Internal mass transport inhibits the biofilm

growth and bacteria types and affects the porosity

and the cohesiveness, [72]. Dense bacterial growth

can be defined by slow diffusion percentages. The

porous media in bacterial growth increase the

diffusion, [73]. Furthermore, extracellular polymeric

material in bacterial growth impede the diffusion,

and improved the resistance to antimicrobial

chemicals, [74].

3.2.2 Operational Conditions in External Mass

Transporting

External mass transfer in granulated bacteria c can

be defined with small dimensions, while

complicated biomass negatively affects the bacteria.

During the transferring of organic materials from

the bulk liquid to the surface of a gigester, separated

transport ways should be taken into consideration

based on the diameter of the organic compounds.

Since the complex hydrodynamic parameters

generated problems some semiempirical

relathionships should be taken into consideration to

detecte rate of external mass transfer, [75]. An

important problem was detected during operation of

the anaerobic processes. In the absence of gaseous

products shows the failing of the biologic process If

the diffusion from the mass to the liquid phase is

slow; the products are in the gas phase and the

external mass transfer resistance is elevated and

affects the whole reaction rate. The external mass

transfer resistance is significantly affected by flow

conditions; temperature, pressure, superficial

velocity and particle size of substrate in the digester,

[76]. By variation of the parameters mentioned

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above external mass transfer resistance can be

decreased.

As a result, with the information of new data

some fundamentel points led to improve mass

transfer in anaerobic microorganisms. Under these

conditions it is important to understand the effective

paramaters to maintain elevated biogas generation in

anaerobic digesters.

3.3 Impact of Some Factors Affecting of Heat

and Mass Transfer

In order to excellent optimize the anaerobic

digesters with high solid wastes the influences of

some inhibitory compounds should be taken into

consideration to elevate the mass and heat transfer

ratios, the biogas efficiency and substrate

degradation yields. On the other hand, some factors

in effluent like chemical and rheological, total solid

levels, the ratio of organic compound to biological

mass, adsorbents and surfactants concentrations, and

mixing ratios affected the mass and heat transfer in

anaerobic digester with high solid wastes. Different

kind of waste inoculums can be used in anaerobic

digester with high solid wastes such as organic

substrates in domestic solid, food wastes,

agricultural wastes, animal manures or organic

industrial wastes. Various types of wastes cause to

change of physicochemical properties and the

anaerobic yields. The composition of waste bacteria

varies with particle dimentions, structure and

moisture percentages. Similarly, substrate type,

crystal shape porosity, particle dimention, surface

area and homogeneity also affect the yield of

anaerobic digestion.

3.3.1 Factors Affecting the Particle Dimention of

Feedstock

The particle size is significant for biodegradation of

substrate and growth of microorganisms. The

surface area, density, heat/mass transfer, and

microbial metabolites affect the particle dimention

of feedstock. In a wet anaerobic digestion, lowering

of particle diameter improves significantly the bio-

degradation kinetics and treatment yields, [77]. The

degradation of bacteria with substrates and

decreasing of particle size exhibited the

pretreatment of organics to water and chemical

substances. This cause to homogeneity and not

clogging was detected. The impact of particle size

changes on biogas generation for anaerobic

digesters containing high solids should be

researched very well, [6]. The lowering of particle

size highligted the specific surface space and

accelerates the microbial contact and improve the

methane generation for the organic compounds with

an elevated fiber ratio and degraded with difficulty,

[6]. The big specific surface space affect

significantly the growth and activity of bacteria. The

bacterial metabolizing starts from the surface,

penetrating the particle’s interior and the porosity of

the microorganisms is important for aquous flow

versus solid organics in anaerobic disgester with

high solid ratio.

Despite small grinding elevated the organic

compound substrate contacting and conversion ratio,

it elevated the problem originated from volatile fatty

acids due to decanting of volatile fatty acids, [78].

In order to take some measures, the decanting of the

inhibitory compounds on the asurface area should

be prevented. The steady-state size affects the

biodegradability and seed/organic material ratio,

[79], [80]. Different suggestions were reported in

recent literature about particle size affecting the

methane generations, [81], the particle size

distribution of input wastes for mechanistic and

data-driven models, the paricle size is relevant with

gas production for Biot numbers for heat

transferring as mentioned in Eq. (3):

 

 (3)

and, Biot numbers for mass transfer was given in

Eq. (4):

 

 (4)

Biot numbers provide to obtain the competitive

importance of internal resistance versus to external

resistance. The length scale L in the expression for

Bi is was accepted as the particle size, λ and D, are

the thermal and mass diffusivities of solid wastes,

and h and km are the heat and mass transfer

coefficients of solid wastess, respectively.

3.3.2 The Effect of Total Solid Content in

Feedstock

The solid content significantly contributes to the

diffusion and reaction kinetics, for quick diffusion

and biodegradaion efficiencies, [82]. When the total

solids elevated, the viscosity increased extremely,

[83], while when the diffusivity coefficient showed

a significant decrease, [84], the reduced mass

transfer led to the accumulation of acids leading to

the fail anaerobic digester having high solid ratio.

Under semi-dry conditions, the volatile fatty acids

elevated and the volatile solid ratio lowered. This

limits the methane generations, [85]. When pH was

between 4.0 and 6.9 the volatile acids not

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dissociated since were inhibitory to the bacterial

Archae, [86].

The diffusion coefficient, affecting the mass

transfer, was investigated by a lot of studies

containing wet conditions. Contarily, this coefficient

in dry anaerobic digesters lowered immediately in

anaerobic digesters with high solid ratio since the

the diffusion phenomenon affect significantly the

microorganism’s rates, [87], [88]. Diffusion of

liquides in in anaerobic digesters with high solid

ratio was operated slowly when mixing was not

used. When the total solids are greater than 19% and

mixing is low, organic compounds originated from

waste biota react and further degraded. The bacteria

should be dispersed before the reactor start-up and

for a suitable metabolic kinetic the seed and bacteria

should be homogenized. The limitation of the

transfer cause accumulation of acids during

biodegradation, ending with the limitations in whole

microbial activity, [89].

High total solid ratios contribute to the physical

properties of the bacteria and waste seeds. The

rheological properties of the digestere containing

viscoelastic compounds elevated the problems

relevant with yields especially at high total solids

reactors. This situation affects the yield of mass

transfer and sludge viscosity increased extremely

when the diffusive coefficient decreased at digeters

containing high solid content. Decreasing of mass

transfer cause to the decanting of fatty acids and

ammonia. This cause to fail of the process stabiliy

resulting in variations of dominated bacteria

metabolic reactions.

3.3.3 Variation of Reactor Temperature

The temperature of an anaerobic digested reactor

with high solid content affect the microbial

reactions in the feedstock and cause to heat losses.

The anaerobic reactors were operated in 3

temperature types;

1. Psychrophilic (8oC - 33oC),

2. Mesophilic (35oC - 42oC),

3.Thermophilic (55oC - 65oC).

Among them mesophilic and thermophilic

conditions are extensively used, [90]. It is important

to choose an operating temperature versus to type of

feedstock such as food waste, straw derivative,

domestic sludge and woody waste. This should

improve thereaction kinetic, reactor stability, and

biogas efficiencies. The feedstock composition is

important to provide effective conditions for

anaerobic digestion processes. In digester with high

solid content the feedstock exhibited poor mass

transfer, while a thermophilic temperature can be

improving the anaerobic degradation yields

resulting in high methane generations, [91]. Heat

losses in thermophilic digesters is importantly

sensitive to the disturbances in environmental

conditions. In a thermophilic digester, the hydrolysis

process decreased significantly with elevated fatty

acid decantation ending with low stability, [92].

3.3.4 Hydrodynamics Properties of the Digester

In anaerobic digesters with high solid ratio, the

homogenation of the mechanical stirring reactor was

attributed to long digestion time and poor substrates

removal rates due to mass transfer inhibitions. These

negative effects can be overcomed by leachate or

digestate reuse, [12]. The mass transfer for digesters

can be characterized by the Sherwood number as

shown in Eq. (5):

󰇛󰇜 (5)

which presents the ratio of advective mass

transport to diffusive mass transport, [93]. Since Sh

is significantly affect the Reynolds number Re and

Schmidt number Sc (the ratio of kinematic viscosity

to biomass diffusivity) variation of mixing rate the

an advective mass transfer should be made for a

good diffusivity. The influent waste with a high

solid content can be characterized by its rheological

structure likes high viscosity, shear-thinning, and

thixotropy, [83]. The value of Sc for digesters with a

high solid content was importantly elevated than

wet anaerobic digesters. This phenomenon, inhibits

the mass diffusion. For a constant ratio of total

solids in the digesters with high solid content the Sc

will be fixed since increasing of Sh cause to elevate

of Re. The Re for mechanical strirring for high solid

content digesters with non-Newtonian fluid is

illustrated in Eq. (6), [61]:



 (6)

where, ρ: is the density of the fluid (kg/m3), N: is the

rotational frequency (1/s), D: is the diameter of the

mixing apparatus (m), K: is the consistency (Pa sn),

and n: is the rheological flow index.

The digesters with high solid ratio have more

unfermented metabolites. Due to inhibited heat and

mass transfer compared to normal anaerobic

digesters, recirculation of organic matter, nitrogen,

phosphorus and anaerobic bacteria the digesters

with high solid content digestated more efficiently

ending with high organic compound degradation

and extended digestion and more methane gas

productions.

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3.3.5 Implementation of the Adsorbents or

Surfactants

Non-ionic surfactants can be changed the organic

material structure and make it hydrophilic and more

accessible to enzymes with low-toxicity and more

H2 generation. This improve the yield of hydrolysis

of bacteria, [94]. In order to minimize the treatment

cost in anaerobic digesters without pretreatment iron

can be added to the digester. Fe0 powder can be used

to increase the methane efficiency by 17%.

Compared with Fe0 powder, Fe is more effective in

improving the reaction rate by 23% due to its

elevated mass transfer yields with aquous and

biomass. Fe in powder form can cause microbial

Fe3+ depletion. This ultimately led to the highest

methane production yields like 34.80% and the

biggest reduction in volatile suspended solids

(52.90%). The methane generation percentage can

be elevated by implementation of limonite. This

elevated the removal of soluble organic metabolites

and increasing the distributions of bacteria

contacting with protein and amino acids, [95].

Implementation of Fe and Ni can elevate the

methane generation by highlihting the

concentrations of archaeal bacteria, [96]. Porous

adsorbents like biochar and activated carbon can be

utilized to vary the types of bacteria and improve

the biodegradation of substrates and buffering ratio

and mass transfer and increasing the electron

transfer ratio ending with high diffusion yields, [97],

[98], [99], [100]. The presence of biochar also

increases the numbers of Syntrophomonas and

methanogens Methanosarcina and Methanocelleus,

and acetoclastic/hydrogenotrophic bacteria.

3.4 Modelling of the Anaerobic Digesters

Anaerobic digestion is a mixture of biochemical

reaction between feedstock hydrodynamics and

bacteria to produce methane gas as effluent. The

modelling of these phenomenon is difficult. When

anerobic digestion was modelled in order to elevate

the reactor performance some suitable scenarios

should be performed for a good optimization, design

control by taken some measures by preventing the

falling of operation of anaerobic reactors. Three

types of models can be utilized to operate the

anaerobic digesters: (a) biochemical kinetic models,

[101], (b) computational fluid flow models, [102],

and (c) data-driven models, [103].

3.5 Maximum Methane Generation

Percentages

The maximum methane generation percentages

were measured at 12% TS, 19% TS, 25% TS, 32%

TS, 39% TS and 45% TS, respectively, after 365

days operation duration (Table 1, Appendix). The

methane yields were relevant to TS ratio in the

digesters. Analyses of variance were performed. The

role of TS content on anaerobic digestion was

investigated in anaerobic batch reactors. A range of

TS contents from 12% to 45% was evaluated. 4.9,

3.2, 2.9, 1.9 and 0.9 ml/g VS/d maximum methane

gas generation percentages were detected after 19%

TS, 25% TS, 32% TS, 39% TS and 45% TS,

respectively, after 365 days. The total methane

generations slightly lowered with increasing TS

concentrations as the TS ratio was increased from

12% to 32% TS. Two different explanations can be

performed at 39% TS while methane percentage

was inhibited. These limits can be explained by the

inhibition of anaerobic digestion at high solids ratio

since the accumulation of volatile acids. 7.16 ml/g

VS/d maximum methane production was detected at

12% TS ratio after 365 days. The methane

generation was inversely correlated with total solid

doses (Table 1, Appendix).

In batch tests, higher methane production was

found in digesters containing 22% TS compared to

33% TS for mesophilic conditions, [104]. Similarly,

increased methane production was mentioned in

thermophilic batch tests at 22% TS compared to

29% TS and even higher than 34% TS, [105]. Low

methane yields of ≈ 21 ml/g VS were obtained at

39% TS.

3.6 Measured pH and Volatile Fatty Acid

Levels

pH values and VFA concentrations were observed

after 365 days at anaerobic digesters containing

12% TS, 19% TS, 25% TS, 32% TS, 39% TS and

45% TS, respectively (Table 1, Appendix).

7.2, 7.4, 7.9 7.8 and 6.6 pH values were

measured at digesters containing 12% TS, 19% TS,

25% TS, 39% TS and 45% TS, respectively, after

365 days of operation time. The pH=8.0 value was

found at digesters containing 32% TS after 365 days

(Table 1, Appendix).

27, 29, 33, 39 and 16 g/l volatile fatty acid

concentrations were obtained at digesters containing

12% TS, 19% TS, 32% TS, 39% TS and 45% TS

contents, respectively, after 350 days of operation

time (Table 1, Appendix). The maximum 43 g/l

VFA concentration was obtained at digesters

containing 25% TS after 365 days operation time

(Table 1, Appendix).

3.7 Cumulative Methane Generations

The cumulative methane levels at digesters

containing 32% TS and 39% TS, were 149 and 142

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ml/g VS/d, respectively, after 365 days operation

time (Figure 3, Appendix). The maximum

cumulative methane production was measured as

55, 129, 152, 159 and 186 ml/g VS/d at digesters

containing 12% TS after 53 days, 120 days, 159

days, 230 days and 365 days operation times,

respectively. The cumulative methane generations

data slightly decreased at digesters containing 12%

TS and 39% TS (Figure 3, Appendix).

3.8 Properties of Dynamic Digestion

The resaerchers studied with anaerobic digesters

containing high solid ratio at anaerobic digestion

model No.1 mentioned that two gas generation

maximums were detected at anaerobic digestion

with medium temperature (30oC – 38oC) and

elevated temperature (50oC – 55oC), [92], [106]. For

minor and medium-types biogas processes, the

medium temperature digestion process was found to

be the most economical one with a temperature of

35o C. As a result, 35oC was selected as the

digestion temperature for determine the impacts of

dinamic digestion researchers for anaerobic

digestion containing high solid wastes process at

anaerobic digestion model No.1.

The cumulative biogas generation ratio of the

dinamic digestion researchers with anaerobic

digestion containing high solid wastes process at

anaerobic digestion model No.1 during 365-day

assays was operated at 35oC (Figure 4, Appendix).

0.19, 0.37, 0.46, 0.55, 0.56, 0.49, 0.48, 0.38 and

0.32 m3/m3.d maximum biogas production rates

were found for dinamic digester researchers for

anaerobic digestion model No.1 after 29, 55, 85,

121, 159, 179, 210, 230 and 259 days operation

times, respectively, at 35oC (Figure 4, Appendix).

0.69 m3/m3.d maximum biogas production rate of

dinamic digester for anaerobic digestion containing

high solid wastes process at anaerobic digestion

model No.1 was detected after 145 days operation

time, at 35oC (Figure 4, Appendix).

Figure 4 (Appendix) shows the biogas generation

ratio of dinamic digester researchers: 0.134

m3/m3.d) and standard digestion researchers: 0.165

m3/m3.d). The biogas generation ratio of dinamic

digester researchers was importantly bigger than

that of static digestion researchers (Figure 4,

Appendix).

3.9 Data Analysis of Static Digestion

Researchers for Anaerobic Digestion

Containing High Solid Wastes Process

at Anaerobic Digestion Model No.1.

The cumulative biogas production rate of static

digestion researchers for anaerobic digestion

containing high solid wastes process at anaerobic

digestion model No.1 during the 365-day

experiment at 35oC digestion temperature was

determined and is summarized in Figure 5

(Appendix). 0.04, 0.26, 0.20, 0.17, 0.25, 0.24, 0.21,

0.20 and 0.19 m3/m3.d maximum biogas generation

ratios were detected for static digestion researchers

for high solid wastes process at anaerobic digestion

model No.1 model after 25, 50, 75, 100, 150, 175,

200, 225 and 250 days operation times, respectively,

at 35oC (Figure 5, Appendix).

0.36 m3/m3.d maximum biogas production rate of

static digestion researchers for anaerobic digestion

containing high solid wastes process at anaerobic

digestion model No.1 was detected after 160 days

duration at 35oC (Figure 5, Appendix).

Mixing is an important point affecting

significantly the gas generation ratio of anaerobic

digestion. With no mixing, several problems like

uneven material distribution, poor fluidity, and

difficulties in the heat and mass transportations in

the static digestion process were detected, [107],

[108], [109], [110]. Contrarily, mixing by dinamic

digester significantly accelerate the digestion yield

and biogas generation ratio.

Static digestion models can be utilized as a

limiting method, when a lot of tests were necessary

for complicated anaerobic systems. In order to

explain in detail, the digestive process, son

unravelling mechanisms should be taken into

consideration in molecular scale. For example,

phospholipids dissolved by the gastric mucosa

mentioned that to contact with globular proteins like

β-lactoglobulin to streigtenentgh its shape and

inhibits to the role of pepsin, [111].

A wide variety of static digestion models have

been investigated by utilizing some parameters like

pH, ionic strength, time of each step and enzymatic

studies and the data obtained was not extensively

explained and compared with other references. In

order to solve the problem, the data should be

compared with other data. It is necessary to mention

a digestion menu for all scientific communities.

Furthermore, the international researchers group

namely INFOGEST consisting of all scientists

working about digestion was declared a consensus

about static digestion for public health, [112], [113].

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3.10 Differences between Dinamic Digestion

and Static Digestion Resaerchers for

Anaerobic Digester Containing High

Solid Compounds for Anaerobic

Digestion Method No.1.

The dinamic digestion and static digestion

resaerchers for anaerobic digester resaerchers high

solid compounds process with anaerobic digestion

method No.1 was tabulated in Table 2 (Appendix).

The digesters of both researchers were operated

under a temperature of 35oC, by different

temperatures. A high static-curve fitting interaction

was found between the cumulative biogas

generation and duration. The cumulative biogas

generation of dinamic digester researchers (standard

deviation of 74.24 m3) and static digestion

researchers (standard deviation of 33.86 m3) process

were 222.88 m3 and 88.56 m3, respectively. This

showed that cumulative biogas generation of

dinamic digester researchers was importantly

elevated than that of static digester researchers

Table 2 (Appendix). After 550 days of the assays,

dinamic digester reaserchers elevated the the outlet

of digestion. Contrarily, the cumulative biogas

generation provided the quick growth in static

digestion researchers. This showed that the

anaerobic digestion with high solid content for

dinamic digester researchers was not long compared

to anaerobic digestion with high solid content

process for static digestion researchers. In this last

process, a lot of organics were not transformed

ultimately due to lower mass release in static

digestion researchers for anaerobic digestion with

high solid content at anaerobic digestion model

No.1 Table 2 (Appendix).

The energy results of dinamic digestion and

static digester researchers for anaerobic digestion

containing high solids ratio process at anaerobic

digestion model No.1 are illustrated in Table 2

(Appendix). Assuming that the calorific value of the

fresh liquid was calculated as 4267 kJ/kg, [114].

The total energy of liquid for dinamic digestion and

static digestion researchers for anaerobic reactor

having high solid ratio process at anaerobic

digestion model No.1 were calculated as 28645.8

MJ (Table 2, Appendix). It can be concluded that

the ingredients of dinamic and static digestion

researchers was calculated as 60%. Total biogas

generation of dinamic digestion and static digestion

reserchers for anaerobic digestion containing high

solids ratio process at anaerobic digestion

containing high solid ratio model were calculated as

223.88 and 95.33 m3, respectively (Table 2,

Appendix). Based on these data, the biomass energy

transformation yields of the liquid calculated for the

dinamic digestion and static digestion researchers

for process at anaerobic digestion containing high

solid ratio model were 19.89% and 8.56%,

respectively (Table 2, Appendix).

4 Conclusions

When we compare the data obtained from dynamic

digestion and static digestion researchers after 365

days showed that the biogas generation for dynamic

digestion researchers was 134.32 m3 or 151.67%

bigger than that of static digestion researchers with

the same digestion temperature for municipal solid

wastes with the anaerobic digestion model No.1 for

anaerobic digestion containing high solid ratio in

anaerobic digestion process in anaerobic batch

reactors. The role of total solid content on anaerobic

digestion with high solid waste was researched in

anaerobic batch reactors. A ratio of total solids

varying from 12% to 45% was evaluated.

7.16 ml/g VS/d maximum methane generation

yield was detected at 12% TS content after 365 days

duration. Methane generation was not correlated to

the Total Solid dose. The maximum pH of 8.0 was

found at 32% TS content after 365 days duration.

The maximum 43 g/l VFA dose was measured at

25% TS content after 365 days duration. The

maximum cumulative methane generation data were

detected at 12% TS content for 57, 129, 152, 159,

and 186 ml/gVS cumulative methane generation

after 53 days, 120 days, 159 days, 230 days, and 365

days durations, respectively.

The cumulative methane generation data slightly

lowered with the TS ratio increasing from 12% TS

to 39% TS. The total methane generation slightly

lowered as the TS doses were increased from 12%

to 32% TS. Two different items happened at 39%

TS: Methane generation was decreased as found at

39% TS. This can be explained as follows: In the

hydrolysis scale and liquid/gas mass transportation

was researched and it was found that the mass

transfer inhibition could be attributed to low

methane generation at elevated TS, and that

hydrolysis yield constant slightly lowered by

elevating TS.

A simple anaerobic digestion model is useful to

assume the methane generation. To lower the

addition of fossil fuels to energy requirements some

solar thermal collectors can be utilized to provide

thermal energy in anaerobic digesters. Photovoltaic

(PV) panels can be used as electricity source.

Different approaches, like the development of a

sustainable mobility scenario on the utilization of

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methane on vehicles, will provide to detect elevated

yields from anaerobic digesters.

For elevated yields from the generated anaerobic

digestion model No.1 integrations to the vehicles, to

thermal tanks, to solar thermal collectors, to PV

panels, to amplification devices, and high-pressure

devices liquefied methane can be collected.

Acknowledgement:

This research study was undertaken in the

Environmental Microbiology Laboratories at Dokuz

Eylül University Engineering Faculty

Environmental Engineering Department, İzmir,

Turkey. The authors would like to thank this body

for providing financial support.

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APPENDIX

Fig. 1: Generalized representation of the steps involved in anaerobic digestion; (LCFA: Long chain fatty acids,

VFAs; Volatile fatty acids. Reproduced with permission from Ref., [92].

Fig. 2: Interphase mass transfer of biogas from a liquid substrate (e.g., waste stream), Gj and GD,j are the jth

undissolved and dissolved biogas species. Reproduced with permission from Ref., [92].

Table 1. Effect of TS content on HSAD performance for maximum CH4(g) production rate, CH4(g) production

yield, pH and VFA concentrations, respectively.

TS Content

(%)

Maximum CH4(g) Production Yield

(ml/g VS/d)

Time

(days)

Values

VFA Concentrations

(g/l)

12%

7.16

365

7.2

19%

4.9

365

7.4

25%

3.2

365

7.9

32%

2.9

365

8.0

39%

1.9

365

7.8

45%

0.9

365

6.6

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DOI: 10.37394/232012.2023.18.11

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E-ISSN: 2224-3461

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Fig. 3: The values of cumulative CH4(g) production for different TS contents after 365 days and at 35oC.

Fig. 4: The values of biogas production rates of dynamic digestion (DD) process after 365 days and at 35oC.

Fig. 5: The values of biogas production rates of static digestion (SD) process after 365 days and at 35oC.

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Table 2. The comparison of anaerobic digestion model No.1 model with high solid ratio model for dynamic

digestion (DD) and static digestion (SD) processes.

Digestion

Processes

Energy

Slurry

(MJ)

Anaerobic Digestion Model No.1

High Solid Ratio Model

Cumulativ

e Biogas

Generation

(m3)

Energy

Biogas

(MJ)

Biogass Energy

Transformatio

n Yields (%)

Cumulativ

e Biogass

Generation

(m3)

Energy

Biogas

(MJ)

Biogass Energy

Transformatio

n Yields (%)

Dynamic

digestion

(DD)

28645.

222.88

17192.8

29.84

223.88

11458.3

19.89

Static

digestion

(SD)

28645.

88.56

11452.9

12.84

95.33

4927.08

8.56

Differenc

e between

DD and

process

(Δ = DD

– SD)

134.32

5739.96

17.00

128.55

6531.24

11.33

Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

Prof. Dr. Delia Teresa Sponza and Post-Dr. Rukiye

Öztekin took an active role in every stage of the

preparation of this article.

The authors equally contributed in the present

research, at all stages from the formulation of the

problem to the final findings and solution.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

This research study was undertaken in the

Environmental Microbiology Laboratories at Dokuz

Eylül University Engineering Faculty

Environmental Engineering Department, İzmir,

Turkey. The authors would like to thank this body

for providing financial support.

Conflict of Interest

The authors have no conflicts of interest to declare

that are relevant to the content of this article.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

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WSEAS TRANSACTIONS on HEAT and MASS TRANSFER

DOI: 10.37394/232012.2023.18.11

Deli

a Teresa Sponza, Ruki

ye Özteki

n

E-ISSN: 2224-3461

139

Volume 18, 2023