Petroleum  is  one  of  the  most  important  energy  resources  and  a  raw  material  of  the  chemical   industry. Nevertheless oil spills during exploration, transportation, and refining, can cause some environmental problems to soil, groundwater and air.

There are various remediation techniques available for environmental restoration activities and can be classified in several ways such as in situ or ex situ, those for soil , surface or groundwater remediation, or basing on the physics or chemistry of the treatment.

The majority of techniques are categorized as physical treatments with only one process, biodegradation, falling under the biological treatment category. Chemical treatments involve the application of agents to promote extraction of the hazardous substance, and physical treatments involve removal of the hazard through physical means. The relative benefit of the various remediation methods has dependence in large-scale applicability as well as overall cost.


2. Current Technologies


Fig. 1 Remediation methods


Three primary strategies used separately or in conjunction to remediate most sites are:

  • Elimination or alteration of contaminants.
  • Extraction or separation of contaminants from environmental media.
  • Immobilization of contaminants.

Treatment technologies capable of contaminant elimination by altering their chemical/physical structure are:

  • thermal
  • biological
  • chemical

These elimination technologies can be applied in situ or ex situ to contaminated media.

Treatment technologies commonly used for extraction and separation of contaminants from environmental media include

  • soil treatment by thermal desorption
  • soil washing
  • solvent extraction
  • oil vapor extraction (SVE)
  • ground water treatment by either phase separation
  • carbon adsorption
  • air stripping
  • ion exchange


or some combination of these technologies.


Immobilization technologies include stabilization, solidification, and containment technologies, such as placement in a secure landfill or construction of slurry walls. No immobilization technology is permanently effective, so some type of maintenance is desired. Stabilization technologies are often proposed for remediating sites contaminated by metals or other inorganic species.

Selection and integration of technologies should use the most effective contaminant transport mechanisms to arrive at the most effective treatment scheme. For example, more air than water can be moved through soil. Therefore, for a volatile contaminant in soil that is relatively insoluble in water, SVE would be a more efficient separation technology than soil flushing or washing.

These concepts about site remediation strategies and representative technologies associated with them are summarized in Figure 2.[1]


Remedial by function

Fig. 2 Classification of remedial technologies by function


Generally, no single technology can remediate an entire site. Several treatment technologies are usually combined at a single site to form what is known as a treatment train.

Remediation of  Soils

Used and abandoned sites contain soils contaminated by petroleum hydrocarbons. These would have resulted from spills and leaks of gasoline, kerosene, fuels, diesels, lubricating oils and similar products during the years of industrial activities at the site. Some soils at old sites also get contaminated by PCB, and metals such as Zinc, lead, cadmium.

Excavation and Disposal (“Dig and Dump”)

Earlier practices of excavation and disposal involved digging out contaminated soil from the location of contamination and dumping it off in a landfill or other locations (e.g. a depression) where the contaminated soil is not considered a hazard to human and ecological health. No treatment might have been carried out on the contaminated soil prior to dumping at the target site; hence the approach is nick-named “Dig and Dump”. Regulatory restrictions on land disposals have brought some changes to this practice. In North America and most countries in Europe, it is now illegal to co-dispose hazardous and non-hazardous wastes in landfills. There are only designated or licensed landfill sites that accept PHC contaminated soils. Furthermore, disposal in landfill sites without pre-treatment is now outlawed in several jurisdictions. Wastes for disposal in landfills should be characterized and the parameters must meet certain pre-specified waste acceptance criteria. Dig and Dump approach is very simple to implement. However, its main disadvantages are cost of transporting the contaminated soil to its final destination and the contamination (if soil is dumped untreated) of the final destination (in case of future re-use).

Chemical Treatments

Oxidation, in waste remediation, refers to the movement of a contaminant to a more oxidized or more environmentally benign state. It involves the use of chemicals for the destruction of organic contaminant constituents. The chemical oxidants most commonly employed to date include peroxide, ozone, chlorine dioxide and permanganate. They can oxidize hazardous materials that are either organic or inorganic compounds.


Fig. 3 Oxidation Treatment



Depending on site and contamination characteristics, in situ chemical treatment can be achieved  by  either  injection  of  liquid,  gaseous,  and  colloidal  reactive  chemicals  into subsurface  soils  and/or  aquifers,  or  by  installation  of  a  permeable  chemical  treatment wall across the ground-water flow path.

The successful  application  of  a particular  in situ chemical treatment technique requires

  • effective chemical reaction of the introduced chemical with contaminants at a proper rate and extent
  • effective delivery of the reagents throughout the zone to be treated.

A   primary   obstacle   for   in   situ   chemical   treatment   techniques   involves   delivery, distribution  and  mass  transfer  of  chemicals  in  subsurface  environments. A major concern for in situ chemical flushing is the uncertainty of the fate and effects of washing reagents  in  the  subsurface  environment.[2]

The table below  provides  some  features  for  application  of  oxidation  for  in  situ  treatment.

Site-specific information is needed for field application.




Chemical Injection –  Oxidation


Hydrogen Peroxide



 Treatable Compounds  Chlorinated solvents, polyaromatic hydrocarbons, and petroleum products. Not effective for chlorinated   alkanes and saturated aliphatic hydrocarbons.
 Influencing Factors   
 pH  Prefer low pH of 2 to 4, but   feasible up to near neutral   pH  Prefer neutral pH of 7 to 8,   but effective over a wide range.  Effective at natural soil pH
 Permeability  Prefer high permeability, but feasible for low permeability with use of advanced oxidant  delivery   techniques, such as deep soil mixing and soil fracturing.  Fenton’s reagent and ozone rely on free radical   generation and thus transport away from point of injection is constrained.
 Temperature  All oxidants are affected by temperature to varying degrees.
 Depth  With use of the advanced delivery techniques, depth is generally not a limitation.
 Oxidant Degradation  Easily degraded in contact with   soil/groundwater.  The oxidant is very stable  Ozone degradation in soils is   limited.
 Other factors  May need to supply iron   (FeSO4)   to form Fentonís   reagent.
 Potential Detrimental   Effects

 Particulates can be generated and permeability loss is possible.

 Potential side effects include gas evolution with peroxide and ozone and generation of    fugitive emissions,   potentially toxic byproducts, potential effects on/of metals and   reduction of biomass.

 Tab. 1 Summary  of  In  Situ  Chemical  Treatment  Technologies  by  Chemical Injection of Oxidants


Another chemical method is the chemical extraction. It does not destroy wastes but is a means of separating hazardous contaminants from soils, sludges, and sediments, thereby reducing the volume of the hazardous waste that must be treated. They vary in regard to the chemical employed, type of equipment used, and mode of operation. There are:

  • Acid Extraction: uses hydrochloric acid to extract heavy metal contaminants from soils.
  • Solvent extraction: uses organic solvent as the extractant. It is commonly used in combination with other technologies, such as solidification/stabilization, incineration, or soil washing, depending upon site-specific conditions. Acid extraction is suitable to treat sediments, sludges, and soils contaminated by heavy metals. Solvent extraction has been shown to be effective in treating sediments, sludges, and soils containing primarily organic contaminants such as PCBs, VOCs, halogenated solvents, and petroleum waste.

Factors that may limit the applicability and effectiveness of the process include:

  • Some soil types and moisture content levels will adversely impact process performance.
  • Higher clay content may reduce extraction efficiency and require longer contact times.
  • Organically bound metals can be extracted along with the target organic pollutants, which restrict handling of the residuals.
  • The presence of detergents and emulsifiers can unfavorably influence the extraction performance.
  • Traces of solvent may remain in the treated solids; the toxicity of the solvent is an important consideration.
  • Solvent extraction is generally least effective on very high molecular weight organic and very hydrophilic substances.
  • After acid extraction, any residual acid in treated soil needs to be neutralized.
  • Capital costs can be relatively high and the technology may be more economical at larger sites.


Physical Treatments

Several physical treatments are available for soil remediation.


Cover systems provide a stabilization mechanism by reducing the amount of water through-put in a burial trench or retention basin. The cover typically is composed of a surface layer that supports vegetation, a drainage layer, a low-permeability layer, and a gas-venting layer. The requirements of the site dictate which layers are necessary, and some sites will not need all layers. The decomposition of organic matter results in the generation of methane gas. If uncontrolled, gas that migrates within the cover system can balloon and possibly combust.

Cementitious Waste Forms

Sulfur polymer cement (SPC) has been used to stabilize high loadings of volatilized toxic metals. SPC is a sulfur composite material with a melting point of 110-120 °C,that resists attack by most acids and salts. The advantages of SPC are:

  • it has a greater waste-to-agent ratio than concrete;
  • it has the ability to be remelted and reformed;
  • it is less permeable than concrete;
  • in its final waste form it is devoid of water;
  • it can be processed at relatively low temperatures

Others methods which require more technological processes and installations are summarized in the table below with their advantages and disadvantages.

tab a

tab b

tab c

tab d

Tab. 2  Remediation Methods



Water Treatments

Water is by far the largest byproduct of the fossil fuel industry, thus a special effort requires the water treatment. Early treatment remedies for groundwater contamination were primarily pump-and-treat operations. This method involves extracting contaminated groundwater via wells or trenches and treating the groundwater above ground (ex situ) using processes such as air stripping, carbon adsorption, biological reactors, or chemical precipitation. Many of these processes produce highly contaminated wastes that then have to be disposed.

Some of the techniques for ground and surface water are summarized in the table below.[3]




Degree of


Cost Technological


Research Needs



  • Specific environment governs success of process
  • Aerobic fluidized-bed had high chlorophenol loading rates
  • Technique  is expensive
  • Heating of water is a prerequisite which radically increases costs
  • Little experience with in-situ bioremediation of groundwater
  • Little is known about fungal adaptation and processes in degradation of anthropogenic substances




  • Low dose rates of electrons more efficient
  • E-beam technology has shown removal efficiencies up to 99.99% in full-scale operation
  • Promising technology
  • Shown to be effective and economical for removal or hazardous organic compounds through lab and pilot plant studies


  • In-situ colloid remediation process using polyelectrolyte capture successful in laboratory column tests
 Further work necessary to   demonstrate at actual site



 Removal by

Sorption to


  • Can be regenerated in-situ
  • Selective removal achieved if specific surfactant that sorbs contaminant selectively is used
  • Solute removed can be recovered
 Batch and column experiments   done to demonstrate use of organo   -oxide for treatment of nonionic   organic pollutants


 Air Sparging/

Air Stripping


In-Situ Air Sparging   (IAS)

Pump & Treat (P&T)

  • Efficiency with which O2 is transferred to groundwater must be addressed if IAS to be proven effective
  • Air and steam stripping technologies most effective with VOCs and ammonia
In-well aeration (IAS) has potential to be more cost-effective and efficient than conventional P&T


  • Much to be learned about in-well aeration
  • Because of potential for loss of control, air sparging systems should never by installed without a pilot test
Tab. 3  Water Remediation Techniques



3.Research & Development

Others remediation techniques summarized in this part are still in the investigational stages, have been applied only in the laboratory or on a small scale or in few projects..  

Remediation Using Actinide Chelators

Research groups are working to design and synthesize organic chelators for selective binding of actinide ions from soils and waste streams. Their studies show that multidentate oxoligands such as hydroxamate, iminodiacetate, and hydroxypyridinones are potential candidates for binding actinide ions present in acidic, aqueous solutions. They have also identified and synthesized chelating agents for plutonium. Specific application under the complex conditions relevant to environmental remediation strategies for actinides is in an early stage of development. Potential solubility problems, stability, and pH requirements exist that could limit the use of chelators. [4]

Critical Fluid Extraction

Organic compounds, primarily PCBs and PAHs, appear to be the most amenable to extraction from soils and sludges with a technique utilizing liquefied gas, typically carbon dioxide, propane, butane, and sometimes alcohol. High pressure and moderate temperatures are used to compress the gas to fluid state. At the critical temperature and pressure, where the fluid state occurs, the mass transfer capabilities of the “critical fluid” are at their best. Volatile and semivolatile organics in liquid and semi-solid wastes have been removed with 99.9 percent efficiencies in the laboratory.


Many organic compounds absorb light energy at visible or ultraviolet wavelengths. This energy promotes the decomposition of the chemical. Ultraviolet radiation is sufficient to cleave many types of covalent bonds. It has been shown to degrade PCBs, dioxins, PAHs, and several aromatic constituents of gasoline, including benzene, toluene, ethylbenzene, and xylene. Liquid, gaseous, and solid media are suitable to such treatment. Ultraviolet light is sometimes used to intensify the oxidation process in the presence of the principal oxidizing chemicals, such as ozone or hydrogen peroxide.

Electrokinetic Oxidation

Oxidation of soil using low voltage pulsed current to achieve electrokinetic and Electrochemical Redox reactions. The EKOGRID™ technology utilizes low voltage DC power source and optimized pulsed voltage patterns distributed to steel rod electrodes that are installed around and in the area to be treated, referred to as “The Grid.” Installed electrodes should reach the depth of polluted soil or ground water.

Description of Process and Scientific Principles

The technology works best in capillary soil conditions, in which the effective oxidizing agents (i.e. radicals and oxygen) are formed at the surface of each grain of soil and evenly spread. The finer the soil grain size, the more effectively the system operates (faster and more complete remediation). Electrokinetic oxidation systems based on electrodes and low voltage control unit can be installed on dry land or underwater to treat bottom sediments. It also works in fractured porous bed rock. The system utilizes the soil matrix and reactions that occur at the surface of each grain, precisely where the pollution is.

Electrokinetic oxidation utilizes reaction products in electrochemical and electro kinetic processes to oxidize organic pollutants in humid porous matrices. In these reactions, free radicals and oxygen are generated which enhance the remediation process. Electrokinetic oxidation is a remediation method that provides enhanced bioremediation; microbiological activity is boosted with oxygen formed in the reactions. The EKOGRID patented pulsing voltage output creates redox reactions (e.g. electrolysis) on the pore surfaces and the electro-osmotic forces move the charged contents of the pore water quickly back and forwards. Freshly formed oxygen and hydroxyl radicals will oxidise and split hydrocarbon chains to lighter fractions, finally forming carbon dioxide and water, which are the final end products. The formation of oxygen will activate the microbes able to decompose various types of organic contaminants. As a result EKOGRID™  is not only a technology to crack hydrocarbons, but also an enhanced bioremediation method. Optimizing the environmental conditions for the microorganisms, for example the availability of nutrients, and follow-up of the microbe activity, for example through soil respiration, is an important part of the EKOGRID™  process.


Fig. 4 EKOGRID™ Control unit on a site in Ecuador [5]


Technology Differentiation and Sustainability

Although the reactions described in the above section are typical for all treatment methods that are based on oxidizing and advanced oxidation processes, the results when utilizing EKOGRIDTM are achieved without the use of harmful chemicals. As the pollutants are remediated in-situ (not transported or relocated) and no chemicals are used in the process, there is also no secondary pollution as a result of utilizing the EKOGRID™ system.

Because of the extremely low operation voltage and very low current densities applied, EKOGRID™  causes no hazard or danger to humans, flora, or wildlife. In fact, after installation, normal daily activities can continue, allowing remediation to be carried out simultaneously with construction or daily operation activities on site. This low amount of energy consumption also allows the use of alternative energy sources, such as solar panels or wind turbines, to power the entire remediation process as long as a backup power source is available.

One single treatment is sufficient to obtain the decontamination of both groundwater and soil. Since this can take place in-situ, the expenses due to excavation, storage, final processing or removal of polluted soil, or acquisition of clean replacements will be greatly reduced or become not necessary at all.



Nanotechnologies    could    offer    a    step-change    in    remediation    capabilities:  treating  persistent  contaminants  which  have  limited  remediation  alternatives,  avoiding  degradation-related  intermediates  and  increasing  the  speed  at  which  degradation  or  stabilisation  can  take place. Between   2013-2017,   the   NanoRem[6] project carried  out  an  intensive  development  and  optimization  program  for  different  nanoparticles  (NPs),  along  with  analysis  and  testing  methods,  investigations  of  fate  and  transport  of  the  NPs  and  their  environmental impact.

Several media, as showed in Fig. 6 can be treated.


nanoparticles uses


Fig. 5 Type of media treated with nanoparticles


Because many of the remediation projects using nanoparticles are just beginning or are ongoing, cost and performance data are limited. However, as the technology is applied at an increasing number of sites with varying geologies, more data will become available on performance, cost, and environmental aspects, thereby providing site managers and other stakeholders with additional information to determine whether the technology might be applicable to their specific sites.

Although the technology is likely a beneficial replacement of current practices for site remediation, potential risks are poorly understood. The factors and processes affecting ecotoxicity are complex, and knowledge of the potential impacts of manufactured nanoparticles in the environment on human health is still limited.

Nanomaterials have highly desired properties for in situ applications. Because of their minute size and innovative surface coatings, nanoparticles may be able to pervade very small spaces in the subsurface and remain suspended in groundwater, allowing the particles to travel farther than larger, macro-sized particles and achieve wider distribution. Many different nanoscale materials have been explored for remediation, such as nanoscale zeolites, metal oxides, carbon nanotubes and fibers, enzymes, various noble metals, titanium dioxide, zerovalentiron (nZVI)

Their active lifespan in situ is limited by a number of processes:

  • Agglomeration – where nZVI particles are attracted to each other and aggregate into larger particles.  In almost all cases this reduces their effective surface area and their mobility in water.
  • Passivation – where nanoparticle surfaces are chemically inactivated (although activity may remain within particles).
  • Immobilisation in the aquifer solid matrix (e.g. through the processes of sorption).

These processes place limitations on treatment effectiveness by restricting the ability of nanoparticles to reach and treat contaminants in situ. To overcome these limitations, a number of modifications have been developed to improve the effectiveness of nZVI by reducing the scale of agglomeration and the immediacy of passivation. These include:

  • Stabilisation – using a range of coatings, including biopolymers such as starch, chitosan, and carboxymethyl cellulose.
  • Emulsification – where aqueous nZVI is surrounded by an oil-liquid.
  • Anchoring nZVI onto carbon, cellulose acetate, polymeric resin or silica to prevent agglomeration and aid dispersion of the nZVI.


Fig. 6 Nanoparticles type


NanoRem has developed a range of nanoparticle types, some based on zerovalent iron and some not.


Fig. 7 Catalyst made with Nanoparticles


A catalyst made of nanoparticles of gold on aluminium atoms successfully treated chloroform-contaminated groundwater and is effective in treating other water contaminats such as nitrites and nitrophenol. [7]


Biosurfactants in the petroleum industry can be used in the cleanup of oils spills, the removal of oil residue from storage tanks, microbial-enhanced oil recovery, and the bioremediation of soil and water.

Biosurfactants are mainly produced by aerobic microorganisms in aqueous media with a carbon source feedstock, such as carbohydrates, hydrocarbons, fats, and oils.

These compounds have amphipathic molecules with hydrophobic and hydrophilic portions that act between fluids of different polarities (oil/water and water/oil), allowing access to hydrophobic substrates and causing a reduction in surface tension, an increase in the area of contact of insoluble compounds (such as hydrocarbons) and the enhancement of the mobility, bioavailability, and biodegradation of such compounds.

Biosurfactants offer a number of advantages over chemical surfactants, such as biodegradability due to their simple chemical structure, environmental compatibility, low toxicity. Table 3 offers a list of biosurfactant applications in the oil industry.[8]

Step In Petroleum Production Chain Applications
  • Reservoir wettability modification
  • Oil viscosity reduction
  • Drilling mud
  • Paraffin/asphalt deposition control
  • Enhanced oil displacement
  • Oil viscosity reduction
  • Oil viscosity reduction
  • Oil emulsion stabilization
  • Paraffin/asphalt deposition
Oil tank/container cleaning
  • Oil viscosity reduction
  • Oil sludge emulsification
  • Hydrocarbon dispersion
Tab. 4 Common Application of biosurfactants in the petroleum industry


Since biosurfactants are not yet competitive with chemical surfactants from the economic standpoint, a more thorough investigation of biosurfactant production from agro-industrial waste is needed to reduce the production cost and allow the large-scale production of these natural compounds. The versatility and efficiency demonstrated in the application of biosurfactants in the oil production chain and the removal of hydrophobic contaminants make these compounds promising biomolecules.


Phytoremediation is a set of processes that uses plants to remove, transfer, stabilize and destroy organic/inorganic contamination in ground water, surface water, and leachate. There are several ways plants can be used for the phytoremediation. These mechanisms include enhanced rhizosphere biodegradation, hydraulic control, phyto-degradation and phyto-volatilization.


Fig. 8 Phyremediadion forms International [9]


This process can take several forms:

  • Phytoextraction – uptake and concentration of substances from the environment into the plant biomass.
  • Phytostabilization – reducing the mobility of substances in the environment, for example by limiting the leaching of substances from the soil.
  • Phytotransformation – chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation) or immobilization (phytostabilization).
  • Phytostimulation – enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known as rhizosphere degradation.
  • Phytovolatilization – removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and / or less polluting substances.
  • Rhizofiltration – filtering water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots.

There are a number of limitations to phytoremediation, such as:

  • It is limited to shallow soils, streams, and ground water.
  • High concentrations of hazardous materials can be toxic to plants.
  • It involves the same mass transfer limitations as other biotreatments.
  • Climatic or seasonal conditions may interfere or inhibit plant growth, slow remediation efforts, or increase the length of the treatment period.
  • It can transfer contamination across media, e.g., from soil to air.
  • It is not effective for strongly sorbed (e.g., PCBs) and weakly sorbed contaminants.
  • Phytoremediation will likely require a large surface area of land for remediation.
  • The toxicity and bioavailability of biodegradation products is not always known. Products may be mobilized into ground water or bioaccumulated in animals.

More research is needed to determine the fate of various compounds in the plant metabolic cycle to ensure that plant droppings and products manufactured by plants do not contribute toxic or harmful chemicals into the food chain or increase risk exposure to the general public.

Radio-Frequency Heating

Radio-frequency heating (RFH) uses a high frequency alternating electric field for in situ heating of soils. The technique depends on the presence of dielectric materials with unevenly distributed electrical charges. The application of an electric field produces movement of the “polar” molecules, and this vibration creates mechanical heat. A spread of radio frequencies (e.g., 6.78 MHz, 13.56 MHz, 27.12 MHz, and 40.68 MHz along with seven higher frequencies) regulated and assigned by the Federal Communications Commission (FCC) can be used in industrial, scientific, or medical applications.

A radio-frequency heating system usually consists of [10]:

  • A three-phase power supply.
  • A radio-frequency source with an oscillator that generates a low-power current at the desired radio frequency, several serial amplifiers that increase the strength of the oscillator current, and a final amplifier that delivers the current at the prescribed output level.
  • An applicator system consisting of electrodes or antennae.
  • A monitoring control system.
  • A grounded metal shield over the treatment area.
  • A vapor collection and treatment system

radio freq

Fig. 9 Radio-Frequency Heating with Electrode


Early   lab-scale   experimentations  demonstrated  the  possibility  of  heating  up  reservoir  materials  well  above  water  boiling  temperature  by  applying  RF  irradiation,  and  studied the effect on the temperature increase rate and on the oil   recovery   of   factors   such   as   temperature,   pressure, connate water   salinity, and solven injection. Lab-scale experimentations suffer from the impossibility to scale down geometrically  the  irradiation  process,  since  electromagnetic  irradiation  at  RF  frequency  is  characterized  by  very  long  wavelengths, incompatible with small volume tests, while by changing   the   frequency,   both   reservoir   permittivity   and   temperature profile change abruptly. Thus,  field  experiments  are  necessary  for  the  experimental  determination  of  the  temperature  profiles  arising  from  RF  downhole irradiation, but really few data are available, due to the  high  costs  associated.

With respect to other thermally enhanced  remediation  techniques  such  as  steam  injection,  resistive heating or conductive heating, ISRFH has two main advantages:

  • the heating rate is much higher and the temperature  profiles  more  uniform,  independently  from  soil  geology;
  • there is  no  fluid  injected  into  the  polluted  soil,  and  thus  the  risk  to  enlarge  the  contamination  to  surrounding areas is minimized.[11]








[8] Applications of Biosurfactants in the Petroleum Industry and the Remediation of Oil Spills, Int. J. Mol. Sci. 2014, 15, 12523-12542; doi:10.3390/ijms150712523

[9] Journal of Environmental Bioremediation & Biodegradation. 2014, 2(4), 178-191 doi:10.12691/ijebb-2-4-5


[11] Thermal Treatment of Hydrocarbon-Impacted Soils:A Review of Technology Innovation for Sustainable Remediation; Engineering 2 (2016) 426–437