\makeatletter
\@ifundefined{HCode}
{\documentclass[screen, CRCHIM, Unicode, Thematic, HideArticleType, published]{cedram}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\def\ndash{\mbox{--}}
\def\hyphen{\mbox{-}}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\def\jobid{crchim20221137}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\def\refinput#1{}
\def\back#1{}
\def\botline{\\\hline}
\newenvironment{inftab}{}{}
\def\0{\phantom{0}}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\DOI{10.5802/crchim.229}
\datereceived{2022-11-18}
\daterevised{2023-03-11}
\datererevised{2023-03-31}
\dateaccepted{2023-04-11}
\ItHasTeXPublished

}
{\documentclass[crchim]{article}
\def\CDRdoi{10.5802/crchim.229}
\let\newline\break
\makeatletter
}
%\cdr@showtranslationtrue
\makeatother

%\copyeditor{RK}

\usepackage{upgreek}

\dateposted{2023-06-12}
\begin{document}

\begin{noXML}

%\TopicEF{Materials and Clean Processes for Sustainable Energy and
%Environmental Applications}{Mat\'eriaux et proc\'ed\'es propres pour
%des applications \'energ\'etiques et environnementales}

\title{Waste cooking oil pretreatment using microwave and ultrasound methods}

\alttitle{Pr\'{e}traitement des huiles de cuisson usag\'{e}es \`{a} l'aide de m\'{e}thodes par micro-ondes et par ultrasons}

\author{\firstname{Halima} \lastname{Kerras}}
\address{Environmental Process Engineering Laboratory (LIPE), Process Engineering Faculty, Constantine3 University, Algeria}
\email[H. Kerras]{kerras.halima@gmail.com}

\author{\firstname{Nawel} \lastname{Outili}\IsCorresp}
\addressSameAs{1}{Environmental Process Engineering Laboratory (LIPE), Process Engineering Faculty, Constantine3 University, Algeria}
\email{out.nawel@gmail.com}
\email[N. Outili]{nawel.outili@univ-constantine3.dz}

\author{\firstname{Abdeslam-Hassen} \lastname{Meniai}\CDRorcid{0000-0002-0357-3512}\IsCorresp}
\addressSameAs{1}{Environmental Process Engineering Laboratory (LIPE), Process Engineering Faculty, Constantine3 University, Algeria}
\email[A.-H. Meniai]{meniai@yahoo.fr}

\shortrunauthors

\keywords{\kwd{Waste cooking oil (WCO)}
\kwd{Microwave} 
\kwd{Ultrasonic bath}
\kwd{Pretreatment}
\kwd{Zero waste}}

\altkeywords{\kwd{Huile de cuisson usag\'{e}e}
\kwd{Four micro onde}
\kwd{Ultrason}
\kwd{Pretraitement}
\kwd{Zero d\'{e}chet}}

\begin{abstract}
The present study focused on the comparison of two intensified
pretreatment techniques for waste cooking oil (WCO) collected from
university restaurants. The cooking oil was 100\% soybean and
had an acidity value (AV) of 4.48~mg~KOH/g oil. Ultrasound (US) and
microwave (MW) irradiations were applied as non-chemical pretreatment
methods with zero generated waste. The considered factors for
the MW pretreatment were power and
time ranging from 100 to 800~W and from 20 to 120~s, respectively. 
The US pretreatment was carried out at a
temperature ranging from 20 to 40~\textdegree C 
for a residence time ranging from 5 to 50~min.
For the two non-chemical pretreatment methods, optimal conditions were
as follows: for microwave irradiation, a power of 100~W, a residence
time of 50~s, and an acid value reduced to 1.056~mg~KOH/g; whereas, the
US pretreatment reduced the AV to 1.014~mg~KOH/g at 
40~\textdegree C, 30~min and 100~W. Finally, the pretreated WCO under
optimal conditions was used for biodiesel production using conventional
transesterification. Results showed that the pretreatment methods led
to good results where the best biodiesel yield was 98\%, obtained with
US pretreatment, while MW led to a yield of 96\%. Both yields were higher than
94\%, obtained for biodiesel production without any pretreatment.
\end{abstract}

\begin{altabstract} 
La pr\'{e}sente \'{e}tude s'est concentr\'{e}e sur la comparaison de
deux techniques de pr\'{e}traitement intensifi\'{e} des huiles de
cuisson usag\'{e}es (OMD), collect\'{e}es dans les restaurants des
campus universitaires. L'huile de cuisson est compos\'{e}e \`{a} 100~\%
de soja et a un indice d'acidit\'{e} (AV) de 4,48~mg~KOH/g d'huile. Les
irradiations par ultrasons (US) et micro-ondes (MW) ont \'{e}t\'{e}
appliqu\'{e}es comme m\'{e}thodes de pr\'{e}traitement non chimiques
qui ne g\'{e}n\`{e}rent aucun d\'{e}chet. Les facteurs
consid\'{e}r\'{e}s \'{e}taient une puissance entre 100 et 800 watts et
un temps entre 20 et 120~s pour le pr\'{e}traitement MW. Le
pr\'{e}traitement US a \'{e}t\'{e} r\'{e}alis\'{e} pour une
temp\'{e}rature comprise entre 20 et 40~\textdegree C et une dur\'{e}e
comprise entre 5 et 50~min. Pour les deux m\'{e}thodes de
pr\'{e}traitement non chimique, les conditions optimales \'{e}taient
les suivantes : pour l'irradiation par micro-ondes, une puissance de
100~W, un temps de s\'{e}jour de 50~s et l'indice d'acide r\'{e}duit
\`{a} 1,056~mg~KOH/g, alors que le pr\'{e}traitement am\'{e}ricain a
r\'{e}duit l'AV \`{a} 1,014~mg~KOH/g avec les conditions de
40~\textdegree C et 30~min \`{a} 100~Watts. Enfin, le WCO
pr\'{e}trait\'{e} dans des conditions optimales a \'{e}t\'{e}
utilis\'{e} pour la production de biodiesel par transest\'{e}rification
conventionnelle et les r\'{e}sultats ont montr\'{e} que les
pr\'{e}traitements avaient donn\'{e} de bons r\'{e}sultats et le
meilleur rendement de biodiesel de 98~\% a \'{e}t\'{e} obtenu avec le
pr\'{e}traitement am\'{e}ricain tandis que le MW a donn\'{e} un
rendement de 96~\%.
%\vspace*{-2pt}
\end{altabstract}


\maketitle

%\vspace*{-2pt}

\twocolumngrid

\end{noXML}


\section{Introduction}

Effective waste management offers a variety of advantages, including
the creation of a cleaner, greener environment, energy conservation,
job opportunities, etc. Waste biomass has significant value-adding
potential, but adopting these solutions in developing countries will
require significant efforts~\cite{1}.

Waste cooking oil (WCO) could be produced from domestic and industrial sources.
This type of waste is generated by food premises, food industry,
and households~\cite{2}. It is harmful
to human health and environmentally unfriendly when it is improperly
disposed in nature without undergoing any treatment. Oil and grease
management is a significant challenge due to their unsustainable
disposal issues, which can cause contamination and pollution of water
and land resources \cite{3}. However, due to careless and
irresponsible practices and the absence of adequate legislation and its
enforcement, majority of household WCO is inappropriately disposed. 
Inappropriate management of the WCO creates several socioeconomic
and environmental problems. Open burning of used cooking oil and direct
discharge into soil and water streams are the main inappropriate
management methods for used cooking oil. According to the Environmental
Protection Agency (EPA), WCO can cover and suffocate animals, plants,
and their environment by causing oxygen depletion. WCO causes rancid
odors, fouls shorelines, and clogs water treatment plants as well as
household kitchen\break pipes \cite{1,2}.

One of the solutions for recycling used cooking oil is its conversion
into bioenergy, preventing competition from the use of edible oil. WCO
has also recently become a leading product as a raw material for a
variety of bio-based materials. On the other hand, WCO can also produce
a wide range of added-value green chemicals such as lubricants,
epoxies, soap, surfactants, polymers, plasticizers, etc \cite{4,5,6}.


The demand for energy has greatly increased, but the sources are
limited, with most of the energy coming from fossil fuels \cite{7}. The
demand for energy is unpredictable due to the future development of
technologies and demography. Environmental degradation
is another problem associated with the burning of fossil
fuels~\cite{8}. Thus, for the last decades, researchers had been
conducting investigations looking for alternative fuels \cite{9}.
{Previously, many researchers have suggested that biodiesel can become
a promising fuel alternative to substitute petroleum-based
diesel}. Biodiesel is a fatty acid methyl ester (FAME) or a fatty acid
ethyl ester (FAEE). It can be synthesized by transesterification of
triglycerides with a monohydric alcohol in the presence of a catalyst
in either an alkaline or an acidic environment~\cite{10}.\looseness=-1

Biodiesel is used due to its oxygenated, renewable, biodegradable,
economically viable, environmentally friendly origin, high combustion
efficiency, and reduced emissions \cite{11}. According to
researchers, biodiesel can reduce emissions of non-burnt hydrocarbons,
carbon monoxide and particulates. In addition, this fuel contains no
sulfur-containing elements, avoiding sulfur oxide emissions and sulfide
production, and contains only 11\% oxygen by weight \cite{12}. It
plays an important role in the reduction of environmental degradation,
global warming, and climate change \cite{13}.

Compared to new oil, WCO has a high free fatty acid (FFA)
content, which could retard the transesterification process and thus
require more advanced transesterification conditions \cite{14}.
As this involves high material and energy intensity, mass transfer
restrictions, high losses, waste generation, and water use, acid
removal have generally the greatest economic impact during WCO
pretreatment~\cite{15}.

For example, when pretreatment by esterification is carried out by the
addition of acids such as sulfuric acid, special precautions must be
taken to avoid risks and corrosion. In addition, the reaction is
carried out at high temperatures and generates a large amount of salt.
Also, glycerolysis requires a high temperature of up to 
200~\textdegree C, a long reaction time of 2 to 6~h, and has a slow
reaction speed. In addition to the formation of process residues,
adsorption pretreatment requires additional steps for the 
adsorbent preparation,
such as high temperature calcinations \cite{16,17}.

The transesterification reaction is the most used method to produce
biodiesel from the WCO (Figure~\ref{fig1}). It consists of reacting triacylglycerol from
vegetable oils or animal fats with alcohol, usually methanol or
ethanol, to produce methyl or ethyl esters known as biodiesel. It takes
three moles of alcohol to react with a triglyceride (oil) to produce a
fatty acid methyl or ethyl ester and glycerol as a by-product
\cite{18,19,20}. 


\begin{figure*}
\includegraphics{fig01}
\vspace*{-4pt}
\caption{Transesterification reaction for biodiesel production from WCO.\label{fig1}}
\vspace*{-5pt}
\end{figure*}

Some of the important parameters affecting the ester yield during the
transesterification reaction are reaction time, alcohol to vegetable
oil molar ratio, stirring rate, catalyst concentration, and reaction
temperature \cite{6,9,19,21,22}.

The effect of alcohol type on the transesterification process revealed
that methanol was the most suitable, with a faster reaction and good
ester isolation~\cite{23}. The feedstock quality,
mainly the FFA content and fatty acid composition, influences the
properties of the biodiesel, The high amount of FFA in the oil results
in a very low yield of biodiesel through the conventional base catalyst
transesterification process~\cite{24}.

Hence, the high FFA content of more than 2\% tends to generate soap
when it reacts with the catalyst for an alkaline transesterification
reaction, so a pre-treatment process is necessary to reduce the FFA
content present in waste cooking oils \cite{3}. 

On the other hand, attractive and productive biodiesel sources are oil
palm, beauty leaf tree, pongamia, Jatropha, coconut, sunflower, soy,
rapeseed, jojoba, neem, moringa, cotton, rice bran, castor, and
microalgae. The main raw materials are lipids of vegetable or animal
origin \cite{25}.

During the frying process, the oil is heated in the range of 160 to
200~\textdegree C, and the transfer between the food and the oil
reacts with moisture and oxygen repeatedly. It undergoes several
degradation reactions, such as hydrolysis, oxidation, and
polymerization, as well as many physicochemical changes~\cite{20}.
The degradation of oil could be affected by the heat and mass transfer
that occurred due to the nature of the fried food. Frying food in the
presence of air and water triggers a series of interrelated reactions.
It has been shown that the moisture and fat content, as well as the
thermal conductivity of fried foods, are important factors causing the
deterioration of oil~\cite{26}.

Several types of pretreatment processes for waste cooking oil are
available, such as steam injection, neutralization, vacuum
evaporation, alcohol extraction, steam distillation, esterification
reaction, and adsorption. Neutralization for deacidification has many
limitations because glycerides can also be saponified by alkali, and
separation of glycerides from the soap paste formed is then very
difficult, resulting in a significant loss of glycerides. In addition,
alcohol extraction for deacidification requires a large amount of
solvent and several extraction stages due to the limited solubility of
FFAs in alcohol. Steam distillation for deacidification requires high
temperatures and consumes a large amount of energy with low efficiency.
Studies have indicated that other pretreatment methods were also used
to reduce FFA and water content in WCO, such as filtration and the use
of a magnesium sulfate drying agent followed by vacuum filtration to
remove any suspended matter and magnesium crystals \cite{16}.
Adsorption can be an alternative procedure to reduce or remove FFA from
oils. Adsorbent treatment has the advantage of reducing oil loss and
soap contamination \cite{27}. There are several types of
adsorbents, such as activated carbon, bleaching earth, montmorillonite,
sepiolite, kaolinite, cristobalite, and bentonite~\cite{28}.
Recently, there have been studies on the adsorption of FFA
using anionic resins \cite{20}. In addition, the mixture of
activated carbon and silica gel can reduce the acid number by up to
53.9\%. However, the most widely used method to remove FFA content from
the oil is the esterification process with acid as a homogeneous
catalyst, converting FFA to free fatty acid\break ester~\cite{29}. 

Several investigations have examined the treatment of waste oils in order to
reduce their FFA content for their recovery in biodiesel. Few 
studies considering parametric studies and optimization of
operating conditions for various processes of pretreatment, are
presented.

Mueanmasa~\cite{30} studied the optimization of esterification of
extracted oil from waste coffee grounds (WCGs) using response surface
methodology (RSM), with an initial FFA of the WCGO of 16.5\%. The
results showed that the FFA percentage was 0.83 when the methanol to
FFA ratio was 10, the amount of catalyst was 15~wt\%, the reaction
time was 115~min, and the reaction temperature was 65~\textdegree C.
The FFA of WCGO was reduced by 94.97\%.

Alptekin and Canakci~\cite{31} used as factors: sulfuric acid,
hydrochloric acid, and sulfamic acid (amidosulphonic acid) as
catalysts; methanol as the alcohol; and reaction temperature at 
60~\textdegree C for the pretreatment of chicken fat. The used chicken fat
had an acid value of 26.89~mg~KOH/g oil, which corresponds to an
FFA level of about 13.45\%. The results showed that the FFA level of
the chicken fat with 15\% FFA may be reduced to below 1\% when using
20\% sulfuric acid and a methanol molar ratio of 40:1 during 60, 70, and
80~min at 60~\textdegree C.

Siow \etal~\cite{32} performed an optimization
study of sulfuric acid-catalyzed esterification using response surface
methodology (RSM) in order to reduce the high level of FFA content 
below to 1\%. Hence, it could be suitably used for biodiesel production via
alkali-catalyzed transesterification, when the acid value of mealworm
oil was 21.57~mg~KOH/g, which corresponded to a FFA level of about
10.84\%.

Asri~\etal~\cite{27} studied waste
frying oil collected from a fast food restaurant serving
California fried chicken (CFC) with a FFA \% of 2.82. To reduce the FFA
and water content of waste frying oil, various types of adsorbents were
used, such as activated carbon, bleaching earth, and coconut coir 
(\textit{Cocos nucifera} L.). 
Coconut coir was found to be the best one for reducing
the FFA content of waste frying oil compared to the others (activated
carbon, bleaching earth, or their mixture), and FFA \% decreased to
0.31\%.

D\'{i}az and Brito~\cite{16} performed the FFA adsorption
using an anion-exchange resin for reducing or removing FFA from oils.
Oils with different acid values (AV) or FFA contents were used as raw
materials; waste oil (AV $=$ 2.13~mg~KOH/g oil) from frying in a canteen
and Jatropha curcas oil (AV $=$ 6.62~mg~KOH/g oil). After adsorption, the
AV were 0.25 and 0.28, respectively. FFA adsorption is an easier method
comparing to esterification since it can be carried out at a low temperature
and does not require alcohol, reducing the costs of raw materials.

Miroud \etal~\cite{17} used two pretreatment
methods: acid esterification (neutralization) and adsorption onto
activated charcoal. The results showed that the two methods reduced the
FFA to almost 50\% of its initial value.

On the other hand, the transesterification process is used in
biodiesel synthesis under the influence of many parameters, such as
temperature, reaction time, catalyst concentration, and oil to alcohol
molar ratio. Various researchers investigated this synthesis with and
without pretreatment.

Siow \etal~\cite{32} applied esterification pretreatment in
mealworm oil for reducing FFA\% in order to increase biodiesel yield to
92\%. 

D\'{i}az and Brito~\cite{16} studied adsorption as a treatment for
reducing or removing FFA from oils prior to the transesterification
reaction for biodiesel production, and they obtained biodiesel yields
of 95 and 96\% for waste oil from frying in a canteen and Jatropha
curcas oil, respectively.

Alptekin and Canakci~\cite{33} studied the acid pretreatment (sulfuric,
hydrochloric, and sulfamic acid) of chicken fat and
transesterification reaction for producing biodiesel. The pretreatment
was achieved in two steps. The results showed that the FFA level of the
chicken fat with 15\% FFA can be reduced to less than 1\% when using 20\%
sulfuric acid and a methanol molar ratio of 40:1 for 60, 70, and 80~min
at 60~\textdegree C, with a high ester yield obtained after the
transesterification reaction with KOH. The methyl ester yield was
87.4\%.

These methods used additional materials and chemicals and were
often time-consuming, which affected the pretreatment cost and its
impact on the environment, hence the total cost of the biodiesel
production process from the WCO. The disregard for green chemistry's
guiding principles was the cause of all these drawbacks.

The principle of green chemistry recommends minimizing or eliminating
hazardous materials, producing degradable products, preventing
pollution and detecting problems before major emissions or accidents
occur, and consuming less energy~\cite{34}.

Recently, several researchers have proposed process
intensification techniques to improve mixing, heat and mass transfer,
and product separation. Examples are co-solvent, ultrasonic, and
microwave heating methods;  membrane reactors; reactive distillation;
static mixers; and in situ biodiesel production~\cite{24}.

Microwave ovens use a mechanism that involves the generation of
electromagnetic radiation, which is absorbed by water molecules,
sugars, and fats in food, causing their vibration and therefore
generating heat. This mechanism is based on the interaction between
electromagnetic radiation and the polar molecules in the food, which
converts electrical energy into high-frequency electromagnetic
waves~\cite{35}.

Considerable attention has been paid to microwave heating
techniques in biodiesel production from a variety of renewable
feedstocks in batch mode. The interaction of microwaves with the
reactants (triglycerides and methanol) to accelerate the
transesterification reaction is already well-established, with a 95\%
yield obtained in a short reaction time~\cite{28}. 

\break
The term ``sonochemistry'' is frequently used to
describe the application of ultrasound to chemical reactions and
processes. Ultrasonication is the application of sound waves to produce
energy; this frequency is above the range of human hearing, 20~kHz.
Low-frequency ultrasound is typically used in process intensification
since it helps to maximize the physical effects (such as acoustic
microstreaming, microturbulence, shock waves, and shear forces) by
improving the mass transfer~\cite{36}. Ultrasonic irradiation
produces cavitation in the sample liquid. The effect of cavitation produces
bubbles in consecutive cycles of compression and rarefaction. Then, the
bubbles collapse creates spatially resolved regions of extreme
excitation as well as concomitant shock waves that may lead to
nucleation in regions of supersaturated solution~\cite{37}.

{However, to the best of the authors' knowledge, MW and US have not been
used as pretreatment without any chemical additives, to reduce the FFA
content of WCO in order to be used for biodiesel production.}

{In this study, a physical method is proposed and optimized with
respect to conditions that allow reducing FFA without chemicals or the
generation of residues. The goal of this work is the application of an
intensification process to reduce the FFA content of frying
oil and to improve the transesterification reaction in order to produce
biodiesel with a good yield and minimal impact on the environment.}

\section{Methodology}


The processes involved in this experiment were intensification
pretreatment by microwave and ultrasonic irradiation, indices
determination, analysis and transesterification reaction for biodiesel
production. All chemical reagents used were of analytical grade and
were used without any purification or treatments. The chemicals
involved were methanol and ethanol (purchased from Sigma Aldrich),
phenolphthalein, sodium hydroxide and HCl (purchased from Biochem),
including collected waste cooking oil (100\% soybean). 

\subsection{WCO collection and characterization}

A sample of WCO used for frying was collected from the
university restaurant at Constantine 3. The recovered oil was
filtered through Whatman filter paper in order to remove food residues
prior to its use for the experiments.

{The WCO was first characterized in
terms of acid and saponification values with titration methods and   
physical properties such as density measured by a Mettler Toledo
hydrometer, the refractive index measured by a refractometer, pH
measured by a Hanna Instruments pH meter, and humidity measured by a
Denver Instrument.}

The used cooking oil sample was evaluated using the standard titration
method to determine the characteristics before and after pretreatment.

\subsubsection{Acid value (AV)}

The acid {value} represents the percentage of FFA in the
oil after conversion by an appropriate factor \cite{32}. It is
defined as the amount of potassium hydroxide (KOH) needed to neutralize
the FFA in 1~g of fat. It is used to determine the content
of fatty acids in the oil. 




A volume of 10~ml of methanol alcohol and phenolphthalein were used as
solvent and indicator, respectively and mixed with 1~g of oil sample.
The solution was titrated with a 0.1~M KOH solution~\cite{19}. The
amount of FFA present in the sample defines the acid
value (AV) in mg KOH/g~\cite{38}. The acid value and the FFA
percentage can be calculated using the equations below \cite{3}:
{\begin{equation}\label{eqn1}
\mathrm{AV}=\frac{\mathrm{MW}_{\mathrm{KOH}}\cdot 
V_{\mathrm{KOH}} \cdot C_{\mathrm{KOH}}}
{W_{\mathrm{real}}}
\end{equation}}\unskip\noindent

\noindent
$\mathrm{MW}_{\mathrm{KOH}}$: Molecular weight of potassium hydroxide (g/mol).

\noindent
$V_{\mathrm{KOH}}$: The volume of KOH needed to neutralize the FFA (ml).

\noindent
$C_{\mathrm{KOH}}$: Normality of the KOH solution (0.1~M). 

\noindent
$W_{\mathrm{real}}$: Weight of the sample (g)~\cite{39}.

Where FFA\%~\cite{39} is calculated using:
{\begin{equation}\label{eqn2}
\mathrm{FFA}(\%)\approx \frac{\mathrm{Acid}~\mathrm{value}}{2}
\end{equation}}\unskip\noindent


\subsubsection{Saponification value (SV)}

The saponification value (SV) represents the mass (in mg) of KOH required to 
saponify 1 g of fat~\cite{32}. It describes the total amount of
saponifiable oil units per unit weight of oil. A relatively high SV
indicates the existence of a large amount of low molecular weight fatty
acid chain. A higher SV of crude oil shows the existence of a high
proportion of fatty acids that can guide soap creation throughout the
transesterification process \cite{38}. A volume of 10~ml of 0.5~M
ethanolic KOH solution and a few drops of phenolphthalein were used as
solvent and indicator, respectively. The mixture of oil and solvent was
heated up to boiling for 30~min. After cooling, the mixture was
titrated with a 0.5~M HCl solution until the pink color disappeared.
The saponification value can be calculated using the following
equations:
{\begin{equation}\label{eqn3}
\mathrm{SV}=\frac{(V'-V)\cdot \mathrm{MW}_{\mathrm{KOH}}\cdot 
C_{\mathrm{HCl}}}{W_{\mathrm{real}}}
\end{equation}}\unskip\noindent

\noindent
$V'$: Volume of acid required for blank test (with acid and base only) (mL).

\noindent
$V$: Volume of acid required to neutralize sample (mL).

\noindent
$\mathrm{MW}_{\mathrm{KOH}}$: The molecular weight of KOH (g/mol).

\noindent
$C_{\mathrm{HCl}}$: The concentration of hydrochloric solution (0.5~M).

\noindent
$W_{\mathrm{real}}$: Weight of the sample (g).

\subsubsection{Determination of FFA recovery (\%)}

{The efficiency of FFA recovery (\%) was determined as an increase in
FFA content in the WCO sample. The equation can be used to calculate
the FFA recovery (\%):}
{\begin{equation}\label{eqn4}
\mathrm{FA}~\mathrm{Recovery}~(\%)=\frac{A_{1}-A_{2}}{A_{1}}\times 100
\end{equation}}\unskip\noindent
With $A_{1}$ and $A_{2}$ the FFA contents of the WCO sample were determined 
at the initial time and after the pretreatment process, respectively.


\subsubsection{Environmental factor (E-Factor)}


The first principle of green chemistry, known as the prevention
principle, can be quantified by the E-Factor (or environmental factor
of Sheldon) \cite{34}. The E-factor is a measure of the actual waste
generated during a process, which is defined as everything other than
the desired product. It includes reagents, solvent losses, process
aids, fuel, and the chemical yield (energy input). Since it includes
all processes and water, it can frequently result in exceptionally high
E-factors and makes meaningful process comparisons challenging.
However, water is typically removed from the E-factor \cite{40}. In
simple terms, it is the total mass of raw materials minus the total
mass of the product, all divided by the total mass of the product,
defined as early as 1992 as follows \cite{34}: 
{\begin{equation}\label{eqn5}
\mathrm{E}\hyphen\mathrm{Factor} = \mathrm{mass}~\mathrm{of}~\mathrm{waste}~(\mathrm{kg})/
\mathrm{mass}~\mathrm{of}~\mathrm{product}~(\mathrm{kg})
\end{equation}}\unskip\noindent
Therefore, any output materials, excluding recyclable ones, from the
reaction other than the main product are considered waste. Its ideal
value is zero corresponding to a production free of waste.

\subsubsection{Energy consumption}

In order to compare the energy consumption of each pretreatment method,
the energy corresponding to each method was calculated as follows
\cite{40,41}:
{\begin{equation}\label{eqn6}
Q=P\times t
\end{equation}}\unskip\noindent
where $P$ is the power of the used apparatus (Watts) and $t$ is the time of
residence of the sample in (s) and $Q$ is the energy consumed during the
pretreatment method (J).


\subsection{Sample analysis}
\subsubsection{FTIR}

Infrared spectroscopy is a rapid method for characterizing the
functional groups and major components of different samples. Fourier
transform infrared spectroscopy (FTIR) is used to predict the
structural and chemical binding (targeting functional groups) between
different molecules in the samples. The principle of FTIR spectroscopy
is to send IR radiation onto a sample to be tested. Certain wavelengths
are then absorbed by the chemical bonds of the molecules in the sample.
We then generate an FTIR spectrum, which permits determining these
chemical bonds by detecting the transmission according to their
wavelengths. The FTIR spectra were recorded using the infrared
spectrophotometer JASCO, in the range of 
$0\ndash 4500~\mathrm{cm}^{-1}$ at a resolution of \cite{42}. 


\subsection{Intensification pretreatment methods}
\subsubsection{Microwave pretreatment}

The microwave used in this work is of the Condor type with a frequency
of 2450~MHz, a maximum power of 800~W, and a range of 10 to 100\%. A
mass of 50~g of frying oil (100\% soybean) at an initial temperature of
18~\textdegree C was put in a closed Erlenmeyer in the microwave for a
pretreatment time of 20~s to 2~min, without agitation or adding
chemicals. After the set time, sample temperature, acid value and
saponification value were measured.

\subsubsection{Ultrasonic pretreatment}

In this method, the laboratory ultrasonic bath has a 24~MHz frequency
and varies in time and temperature. A 50~g sample of frying oil (100\%
soybean) with a temperature of 18~\textdegree C was placed in a closed
Erlenmeyer in an ultrasonic bath for temperatures ranging from 20 to 
40~\textdegree C and a pretreatment time of 5 to 40~min with no
agitation or chemical addition. After the residence time, the sample
temperature, acid number and saponification number were measured for
each sample.

\subsection{Production\,of\,Biodiesel\,from\,pretreated\,WCO}

In this study, biodiesel was made from pretreated WCO via
a transesterification reaction, with 50~g of pretreated oil used at the
optimal conditions obtained by the conventional method mentioned in a
previous study \cite{6}. In brief, the conditions of the reaction were;
KOH catalyst concentration of 2~wt\%, a methanol to oil molar ratio of
about 4.73 and a minimum temperature of 45~\textdegree C. The mixture
was carried out in a reflux assembly for 30~min. The reacting
mixture was stirred during the heating to ensure a good mixture. After
the reaction, the content was transferred to a separating funnel for
separation and allowed to settle in two phases. From the separating
funnel, the glycerin layer was removed, fatty methyl ester was
collected, and then FAME (fatty acid methyl ester) was washed with
distillate water to remove excess residual FFA. Finally, the sample was
dried to remove excess methanol and catalyst from the reaction mixture.
The yield was calculated using Equation~(\ref{eqn7})~\cite{28} and the
transesterification reaction stages are shown in Figure~\ref{fig2}.
{\begin{eqnarray}
\mathrm{Yield}~(\%)&=&{\mathop{(}}\mbox{mass of obtained 
biodiesel~(g)}/\nonumber\\
&&\mbox{~~mass of sample oil~(g)}{\mathop{)}} \times 100
\label{eqn7}
\end{eqnarray}}\unskip\noindent


\begin{figure*}
\includegraphics{fig02}
\vspace*{-9pt}
\caption{Biodiesel synthesis.\label{fig2}}
%\vspace*{9pt}
\end{figure*}

\section{Results and discussion}
\subsection{Oil characterization}

Table~\ref{tab1} shows the measured characteristic parameters of WCO 
after drying as well as the range of Codex norms for soybean oil.



\begin{table*}
\caption{Characterization of WCO and dried WCO and the CODEX norms\label{tab1}}
\vspace*{-7pt}
\begin{tabular}{cccc}
\thead
Parameters & WCO  & Dried WCO & Codex norms for Soybean oil \\
\endthead
AV (mg~KOH/g) & 4.48 & 3.268 & 0.6 max \\ 
FAA\% & 2.24 & 1.634 & - \\ 
SV (mg~KOH/g) & 188.5 & 186.618 & 189--195 \\ 
RI at 20~\textdegree C & 1.473 & 1.47 & 1.466--1.470  (40~\textdegree C) \\ 
pH & 5--6 & 5--6 & - \\ 
Density (g/cm\tsup{3})  at 20~\textdegree C & 0.920 & 0.920 & 0.919--0.925 \\ 
Humidity (\%) & 0.02 & - & - 
\botline
\end{tabular}
%\vspace*{5pt}
\end{table*}


The waste and dried cooking oil densities were similar at 
0.92~g/cm\tsup{3} and were within the Codex norms range. Due to the
heating process during cooking and the contact with food as well as
cooling to which the oil was subjected, the FFA content generally
increased with the used quantity of WCO~\cite{43}. The acid value
of the WCO was 4.48~mg~KOH/g oil, over seven times the maximum
established limit of 0.6~mg~KOH/g so that the oil could be consumed.
On the other hand, it was necessary to decrease the FFA to less than 
2\% to be considered an acceptable feedstock for biodiesel production
with important yields, in which, if the percentage of FFA is more than
2\%, the WCO needs to undergo a pretreatment process for a reduced
acidity value \cite{3}.


As the saponification value for all the analyzed oils was lower than
the ones reported in the Codex range, The literature mentioned that the
higher the saponification value, the higher the number of carboxyl
groups present in the oil or fat~\cite{32}. 

Note that the drying of the oil reduces the acidity value from 4.48 to
3.27 and saponification value from 188.5 to 186.6 before the
application of the pre-treatment, i.e.\ a reduction of 
27\% of FFA and 0.99\% for saponifiable compounds in the WCO. 


\subsection{Pretreatment of waste oil by microwave} 

The experiments are investigated in a Condor type laboratory microwave
and the various pre-treatment steps are as follows:

\subsubsection{Effect of power and temperature} 

In this part the power and time of the microwave were varied and the
indices for each case were measured. The results are presented in   
Table~\ref{tab2}.



\begin{table*}
\caption{Final temperature, acid and saponification values at
different powers and residence time in the microwave oven\label{tab2}}
\begin{tabular}{ccccccccccccc}
\thead
$P$ (W) & \multicolumn{3}{c}{100} & \multicolumn{3}{c}{240} 
& \multicolumn{3}{c}{400} & \multicolumn{3}{c}{800} \\ 
\cline{2-4}\cline{5-7}\cline{8-10}\cline{11-13}
$t$ (s) & $T_{\mathrm{fin}}$ & AV & SV & $T_{\mathrm{fin}}$ & AV & SV 
& $T_{\mathrm{fin}}$ & AV & SV & $T_{\mathrm{fin}}$ & AV & SV \\ 
\endthead
\020 & 22 & 2.239 & 184.71 & 24 & 1.878 & 171.60 & 31 & 2.71 & 187.8\0 & 36 & 1.93 & 179.70 \\ 
\030 & 22 & 1.679 & 182.09 & 27 & 1.568 & 169.60 & 31 & 2.20 & 185.84 & 40 & 1.36 & 176.82 \\ 
\040 & 22 & 1.387 & 179 & 32 & 1.342 & 172.12 & 32 & 1.65 & 185.12 & 51 & 1.06 & 176.47 \\ 
\050 & 22 & \textbf{1.056} & 178.6\0 & 32 & 1.394 & 171.96 & 36 & 1.39 & 173.17 & 63 & 1.11 & 171.8\0 \\ 
\060 & 24 & 1.853 & 179.85 & 34 & 1.116 & 169.66 & 39 & 1.96 & 159.54 & 68 & 1.68 & 162.57 \\ 
120 & 30 & 1.903 & 198.81 & 44 & 1.669 & 146.92 & 50 & 1.93 & 206.50 & 90 & 1.95 & 167.43 
\botline
\end{tabular}
\vspace*{-8pt}
\end{table*}

According to the obtained results, the acid value decreased with the
microwave pretreatment time and then increased indicating an optimum
for each power (Figure~\ref{fig3}). The best values were obtained for
times of 50~s at 100~W and 40~s at 800~W while the sample temperature
rose from 22 to 51~\textdegree C from the first to the second
conditions. Emma Chiavaro \etal~\cite{44}, showed that for high-oleic
sunflower oil with an initial FFA percentage of 0.17, decreasing to
0.14\% at 3~min. On the other hand, the FFA \% of other oils
increased in proportion to the microwave heated duration. Hence, the
effect of heated microwave treatment depends not only on their
specifications (power and time) but also on the type of oil and their
chemical composition. 


\begin{figure}
\vspace*{4pt}
\includegraphics{fig03}
\vspace*{-7pt}
\caption{Effect of microwave power for the acidity 
value of WCO.\label{fig3}}
\vspace*{-10pt}
\end{figure}

Borges \etal~\cite{45} demonstrated that soybean oil~was not regularly
affected by the increasing exposition time, even at 15~min. The 0.75\%
FFA of the~treated oil decreased to 0.70\% at 1~min, then, increased to
0.72 and 0.73\% at 3 and 5~min, respectively, then decreased back to
0.72\% at 10~min and finally increased to 0.75\% at 15~min. For soybean
oil, no correlation was observed between FFA \% values and the exposure
heating time.

The saponification index also decreased and remained within the normal
values of the Codex Alimentarius. Saponification value represents
all the saponifiable compounds present in the oil, so the reduction of
FFA by reducing the acidity value leads to a reduction in
the saponification value.
%

The results show that for each power there was an optimal residence
time for maximum AV reduction.
\vspace*{10pt}

\break
For microwave pretreatment at a power of 800~W for a duration of 40~s,
the acidity value went from a value of 4.48 to values of 1.06 (without
drying) and 1.12 (with drying) corresponding to 76.34 and 75\%
reductions of the free fatty acid content, respectively. 
%\vspace*{12pt}

\subsection{Ultrasonic pretreatment of WCO}

The pre-treatment process in this case was carried out in an ultrasonic
bath containing water. The results are shown in Table~\ref{tab3}.


\begin{table*}
\caption{Final temperature, acid and saponification values at different
initial temperature and residence time in the ultrasonic
bath\label{tab3}}
\begin{tabular}{cccccccccc}
\thead
$T$ (\textdegree C) & \multicolumn{3}{c}{20} & \multicolumn{3}{c}{30}
& \multicolumn{3}{c}{40} \\ 
\cline{2-4}\cline{5-7}\cline{8-10}
t (min) & $T_{\mathrm{fin}}$ (\textdegree C) & AV & SV & $T_{\mathrm{fin}}$ & AV & SV & $T_{\mathrm{fin}}$ & AV & SV \\ 
\endthead
\05 & 20 & 2.756 & 176.57 & 31 & 2.185 & 182.98 & 43 & 2.175 & 192.14 \\ 
10 & 21 & 2.767 & 176.41 & 36 & 1.868 & 175.97 & 43 & 1.884 & 185.88 \\ 
20 & 21 & 2.713 & 182.1\0 & 39 & 1.339 & 175.81 & 44 & 1.104 & 185.27 \\ 
30 & 21 & 2.468 & 172.51 & 39 & 1.399 & 139.79 & 46 & 1.014 & 185.36 \\ 
40 & 40 & 1.619 & 204.27 & 45 & 1.605 & 212.23 & 50 & 1.354 & 204.97 \\ 
50 & 41 & 1.542 & 192.02 & 48 & 1.625 & 191.7\0 & 58 & 1.392 & 205.99 
\botline
\end{tabular}
\end{table*}

From Figure~\ref{fig4}, it can be seen that the acid {value} decreased
with the time of pretreatment with the ultrasonic bath at different
temperatures of 20, 30 and 40~\textdegree C. It was observed that at
20~\textdegree C the acidity value decreased down to 1.542~mg~KOH/g
during 50~min. On the other hand, for the other temperatures of 30 and
40~\textdegree C there was another increase, indicating the presence of
an optimum at 20~min with an acidity value of 1.399~mg~KOH/g and at
30~min with an acidity value of 1.014~mg~KOH/g respectively.
Fluctuations in the values of the saponification index for the
different temperature levels could be noted.

\begin{figure}
\includegraphics{fig04}
\caption{Effect of ultrasonic on the acidity value of WCO.\label{fig4}}
\end{figure}

In comparison with previous pretreatment work using conventional
methods (esterification and adsorption), the pretreatment of the
filtered WCO by activated coal adsorption was carried out by adding 
10\% by mass of activated carbon and stirring at 300~rpm for 30~min at
a temperature of 55~\textdegree C. The acid number was reduced by 17\%.
The pretreatment by esterification was carried out by adding more than
50\% by volume of methanol and 1\% sulfuric acid at 55~\textdegree C
for about an hour. The content of FFA was reduced by 
26\%. The optimum of our study for microwave pre-treatment was
achieved with a power of 800~W/40~s. The acidity value was reduced by
76.34\%. On the other hand, the ultrasonic pre-treatment was applied
with a power of 120~W for 30~min and the acidity value was reduced by
74.55\%.
%\vspace*{22pt}


\subsection{Fourier transform infrared spectroscopy\newline (FTIR) analysis of samples}

The fresh soybean oil, the WCO and pretreated cooking oil with
microwave and ultrasonic bath were analyzed. Figure~\ref{fig5} shows 
the spectra of four oils, the various functional groups relating to the peaks 
in the spectrum (cm\tsup{-1}) at corresponding wave numbers are identified and 
tabulated in Table~\ref{tab4}.


\begin{figure}
\includegraphics{fig05}
\vspace*{4pt}
\caption{Fourier transform infrared spectroscopy (FTIR) analysis of
samples.\label{fig5}}
\end{figure}

\begin{table*}
\caption{Descriptions of wave number observed from FTIR analysis
between current and literature values~\cite{42,46,47}\label{tab4}}
\begin{tabular}{cccc}
\thead
\parbox[t]{3.3cm}{\centering Wave number (cm\tsup{-1}) (current study)}
& ID  & \parbox[t]{3.3cm}{\centering Ref. wave number (cm\tsup{-1})}\vspace*{2pt} 
& Description  \\ 
\endthead
3003.6--3010.5  & \01  & 2989--3029  & C--H Stretching vibration of cis-double bond (${=}$CH)  \\ 
2919.7--2926.5  & \02  & 2850--2970  & C--H Stretching (asym)--C--H (CH\tsub{2}) \\
2850.3--2853.2  & \03  &             & C--H Stretching (sym)--C--H (CH\tsub{2}) \\ 
1743.3--1746.2  & \04  & 1690--1760  & C${=}$O Carbonyl group Stretching \\ 
1455--1463      & \05  & 1340--1470  & C--H Alkane Bending (scissoring)--C--H (CH\tsub{2}) \\
1373.1--1375.9  & \06  &             & C--H Alkane Bending (sym)--C--H (CH\tsub{3}) \\ 
1232--1240      & \07  & 1050--1300  & C--O Stretching \\
1154-- 1163.8   & \08  &  & \\
1086.7--1100    & \09  &  &  \\ 
719.3--723.2    & 10 & 690--900    & C--H Aromatic Rocking --(CH\tsub{2})n-- 
\botline
\end{tabular}
\vspace*{8pt}
\end{table*}

Significant information on the oxidative status of oils can be obtained
by studying the intensity and absorbance values of many bands of the
infrared spectrum~\cite{48}. An increase or decrease in some of the
wave number regions was observed in this study for the comparison
between the fresh soybean oil, waste soybean oil and treated oils with
intensified methods. Table~\ref{tab4} shows the most significant bands:
band near 3009~cm\tsup{-1} due to the cis double-bond stretching
vibration; bands near 2926~cm\tsup{-1} due to the asymmetric and
symmetric stretching vibration of the aliphatic CH\tsub{2} functional
group; band at 1743 to 1746~cm\tsup{-1} due to the ester carbonyl
functional group of the triglycerides; band near 1455~cm\tsup{-1} due
to the bending vibrations of CH\tsub{2} aliphatic groups; band at
1373 to 1376~cm\tsup{-1} due to bending vibrations of CH\tsub{3}
groups; band at 1050--1300~cm\tsup{-1} associated with the
stretching vibration of the C--O ester groups. Finally, the band near
720~cm\tsup{-1} due to the C--H Aromatic Rocking --(CH\tsub{2})n--.

The FTIR spectra of the different oil samples show that there is a
remarkable decrease in the band 3010~cm\tsup{-1} for fresh oil to
3004~cm\tsup{-1} for the used oil. Thus, the value of this frequency in
the non-oxidised soybean oil samples is shown in \cite{49} with an
absorbance of 3007~cm\tsup{-1}. The intensity of this band is increased
to around 3005~cm\tsup{-1} for oils pretreated by microwave and
ultrasound. Then, a small increase in the band at 1745~cm\tsup{-1} to
1746~cm\tsup{-1} for fresh soybean oil and waste oil respectively
corresponds to the carbonylic compounds (Table~\ref{tab4}) generated
from the hydroperoxide decompositions during the heating~\cite{48}; the
intensity of it tended to increase with oxidative treatment and
formation of secondary oxidation products~\cite{50}. This band is
decreased to 1741~cm\tsup{-1} for oil treated by microwave and
ultrasonic bath. 

It should be noted that these pretreatment methods are significant for
oil used for frying with a low acidity assay. On the other hand, as the
previous work mentioned, there is a change through the heating of oil
for 10~min in a microwave.

\subsection{Comparison of conventional biodiesel with pretreated cooking oil}

The key response in terms of biodiesel economics is the yield of
biodiesel production. Figure~\ref{fig6} shows the yield obtained for
the transesterification reaction without pretreatment is 94\%. We
notice an increase in yield for the microwave pretreatment to 96\% and
it is increased to 98\% for the frying oil pretreatment with
ultrasound, which is a better result compared to some previous work
that applied methods using additional components and generated
waste (D\'{i}az  and Brito~\cite{16} studied the adsorption
pretreatment and they obtained biodiesel yield with 95 and 96\% for
WCO in a canteen and \textit{Jatropha curcas} oil respectively).

\begin{figure}
\includegraphics{fig06}
\caption{Biodiesel yield for different process: (a) no pretreatment;
(b) MW pretreatment; (c)~US pretreatment.\label{fig6}}
\end{figure}


\subsection{Quality of produced biodiesel}

The biodiesel produced by using pretreated oil was tested for various
physicochemical properties and compared with ASTM standards as shown in\unskip\break
Table~\ref{tab5}.


\begin{table*}
\caption{A comparison of the physicochemical properties of the
biodiesel produced to international standards\label{tab5}}
\tabcolsep=4pt
\begin{tabular}{ccccc}
\thead
Properties  
& \parbox[t]{2.5cm}{\centering Conventional biodiesel}
& \parbox[t]{2.5cm}{\centering Microwave pret biodiesel}
& \parbox[t]{2.5cm}{\centering Ultrasonic pret  biodiesel}
& \parbox[t]{2.5cm}{\centering ASTM norms of biodiesel \cite{51}}
\vspace*{2pt} \\ 
\endthead
Density at 15~\textdegree C (g/cm\tsup{3})  & 0.8884 & 0.8865 & 0.8856 & 0.875--0.900 \\ 
Kinematic viscosity (mm\tsup{2}/s) & 6.89 & 3.98 & 4.28 & 1.9--6.0 \\ 
Cloud point \textdegree C & ${-}$1 & ${-}$1 & ${-}$1 & ${-}$3--${-}$12 \\ 
Flash point \textdegree C & $>$130 & ${>}$130 & 166 & 100--170 \\ 
Pour point \textdegree C & ${-}$4 & ${-}$3 & ${-}$3 & ${-}$15--${-}$10 \\ 
Cetane number & 51 & 53.95 & 56 & 48--65 \\ 
Refractive index  & 1.45679 & 1.45626 & 1.45348 & - 
\botline
\end{tabular}
\end{table*}

\begin{itemize}
\item It can be seen from these results that the majority of the
properties of the biodiesel produced from the oil pre-treated with
microwave and ultrasonic methods are within the ASTM standard, such as
density, viscosity, flash point, and cetane number.
\item The cloud point and the pour point are not within the standards.
Such value is due to the presence of small droplets of wash water
influencing these two points.
\item The refractive index also changes according to the
pre-treatments, and the lowest refractive index is obtained for biodiesel
produced from oil pre-treated with ultrasonic methods.
\end{itemize}

\subsection{Green chemistry}

Green chemistry is the conception of chemical products and processes
that use and generate less hazardous substances, reduce pollution and
waste, encourage the use of renewable feedstock, lead to safer designs,
optimize materials and energy usage, and minimize costs \cite{52}.
The importance of generating as little waste as possible has emphasized
the application of sustainable development and green chemistry
principles~\cite{53}. In this study, the environmental factor was
calculated since there was no reaction in the pretreatment process.
The calculated environmental factor for microwave and ultrasonic bath
methods was compared to the conventional pretreatment method used in
the previous work (esterification and adsorption pretreatment)
\cite{17}, and the results are shown in Figure~\ref{fig7}.

\begin{figure}
{\vspace*{6pt}}
\includegraphics{fig07}
\caption{E-Factor of pretreatment of WCO for conventional and non
conventional methods: (a) US; (b) MW; (c) Adsorption; (d)
Esterification.\label{fig7}}
\vspace*{-10pt}
\end{figure}

In addition, the time and the consumed energy are important factors for
an economic consideration of the process and were also calculated in
order to \mbox{compare} the present pretreatment with a previous adsorption
and esterification \cite{17} that used additional material to
perform AV reduction \cite{13}. 

As shown in Figures~\ref{fig8} and~\ref{fig9}, increased times of
pretreatment led to an increase in energy consumption for each method.
Conventional heating on a laboratory hot plate for esterification
pretreatment required about 1800~kJ of energy for 3600~s whereas the
adsorption pretreatment required about 900~kJ. While microwave and
ultrasonic processes required 32 and 216~kJ of energy with residence
time of 40~s and 1800~s respectively. These results showed that
non-conventional heating and mixing \mbox{(process intensification)}
techniques had the potential to reduce the process energy requirements
significantly.

\begin{figure}
\includegraphics{fig08}
\caption{Time of pretreatment of WCO for conventional and intensified
methods: (a) US; (b) MW; (c) Adsorption; (d) Esterification.\label{fig8}}
\end{figure}

\begin{figure}
\includegraphics{fig09}
\caption{Consumed energies of pretreatment of WCO for conventional and
non conventional methods: (a) US; (b) MW; (c) Adsorption; 
(d)~Esterification.\label{fig9}}
\end{figure}

Another important gain with microwave process was that the treatment
was carried out in few seconds compared to other two methods of
biodiesel conversion.

Figure~\ref{fig9} shows that intensified pretreatment were more
respectful for the environment compared with conventional
pretreatment. Indeed, both adsorption and esterification consumed
materials and generated waste. The adsorbent could be reused several
times but its efficiency decreased with the number of uses.

For the esterification pre-treatment, it generated waste: the excess
alcohol, the catalyst and the free fatty acid phase recovered after
decantation. The MW and US pretreatments were respectful of green
chemistry principles as they did not generate any waste.


\section{Conclusion}

A study on the effect of microwave irradiation and
ultrasound for the pretreatment of frying oil recovered from the
university campus restaurants (100\% soybeans) was carried out? 
Results showed that the two processes, when applied under optimal
conditions, allowed a reduction of FFA in reasonable time
and without the use of products or generation of waste. These processes
could be used in the chemical industries to refine crude oils in order
to reduce their FFA content, as well as a pretreatment
process in the case of using oils rich in free fatty acids in
transesterification for the production of biodiesel or any other
product with added value derived from used and recycled oils with the
goal of valorizing. The efficiency of the two processes to effectively
remove the FFA without deteriorating the quality of the oils with the
objective of their recycling, in particular in biodiesel is
confirmed.

For~future work, a pretreatment synergy will be proposed to decrease
the FFA in frying oils to a value within the range of
the norm.

\section*{Conflicts of interest}
Authors have no conflict of interest to declare.

\back{}


\bibliographystyle{crchim}
\bibliography{crchim20221137}
\refinput{crchim20221137-reference.tex}

\end{document}
