Membranes and reforming technology for OMW treatment

S.Tosti

a, M.Incelli

b, S.Cordiner

b, S.Schiavone

c, A.Santucci

a, G.Buceti

a

aENEA, FSN Department, C.R. ENEA Frascati, Via E. Fermi 45, I-00044 Frascati (RM), Italy

bUniversity of Roma Tor Vergata, Department of Industrial Engineering cUniversity of Roma Tre

Olive oil production: figures and methods

In recent decades, olive oil industry has increasingly become one of the most important sector in

agro-industrial market among Mediterranean countries. In fact, the present world production of

olive oil is around 2,5 million tons for year, with a share of 95% for the Mediterranean area (77%

EU), in particular Spain, Italy and Greece [1]. The olive mill wastewater (OMW) generated from

olive oil extraction is in a range from 0,5 to 1,5 m3 per ton of treated olives and this implies an

annual OMW world production of around 30 million tons [2]. The OMW are the by-products of oil

production, in particular of the three possible olives treatments: traditional; three phases; two phases

(see figure 1).

Process

Traditional Three phases Two phases

Figure 1 The three methods of olives oil production

In the traditional treatment the liquid component is separated from the olives pasta through

compression by hydraulic presses while centrifugation manages the liquid phase to separate the oil.

This is a batch process and the OMW results to be on average at higher concentration in organic

substances compared to the other two approaches, which are continuous cycles, based on the

addition of water. In particular, the three phases treatment needs the addition of great quantities of

water (around 1 m3 per ton of milled olives) while the two phases treatment needs smaller additions

of water but it produces oil, a “wet” sansa and little or no OMW. Which method is most used

depends on the country (see figure 2) [3] as well as concern and legislation on OMW.

Olives

Washing

Milling

Pressing

Oil and OMW

Separation (Decantation)

Oil OMW

Pomace

Olives

Washing

Milling and beating ( + water)

Horizontal centrifugation

Oil and OMW

Vertical centrifugation

Oil OMW

Pomace

Treatments

Exhaust pomace

Pomace oil

Olives

Washing

Milling and beating

Horizontal cenrifugation

Oil Wet pomace (Alperujo)

Treatments

Exhaust pomace

Pomace oil

Figure 2 Distribution by countries of oil production methods

OMW, the critical issue

OMW usually have a water content exceeding 80% with a sour pH (4,0-6.7). The main organic

compounds are oils and fats (5-10 gL-1

), polyphenols (up to 12 gL-1

) and sugars (up to 20-30 gL-1

).

Because polyphenols and other organic mixtures, OMW have high values of COD (chemical-

oxygen-demand) and BOD5 (bio-chemical-oxygen-demand). That makes the environmental impact

produced by 1 m3 of OMW equivalent to 200 m

3 of urban waste water and the OMW a critical issue

of the oil industry sector [4].

The difficulties of OMW treatment are mainly related to:

a. its high organic loading

b. seasonal operation

c. high territorial scattering

d. the presence of non-biodegradable organic compounds like long-chain fatty acids and

phenols with anti-bacterial effect

Different countries adopt different approaches in regulating the disposal of OMW and at the

moment there is no European Union directive. In Italy, there is a limitation in land disposal of 50 m3

ha-1

y-1

for OMW coming from traditional treatment and 80 m3 ha

-1 y

-1 for OMW coming from

continuous cycles. The land disposal has two effects conflicting among them and at different

timescale. First is the herbicide and phytotoxic effect, detected immediately after the disposal,

which inhibits the microbial activity of the land. The second, detected only months after the

disposal, is the fertilizing effect due to the presence of organic substances. In any case, land

spreading, which has a cost of around 5-10 € m-3

of OMW, imply the alteration of the land

microbial and bacterial composition and the risk of aquatic groundwater pollution. This is matter of

concern because, due to the difficulty in the law enforcement, the illegal OMW dumping is far

from being a rare practice.

Options in treatments

Alternatives to land spreading are not easy and several approaches have been proposed in the past.

The following are the most used:

1. Lagooning

2. Biological process

aerobic pre-treatment (aerobic bacteria digest biological wastes), often used to

improve the operation of anaerobic digestions

anaerobic co-digestion (use of microorganisms to break down biodegradable

material in the absence of oxygen) with other sewage

3. Advanced oxidation process (AOP), a set of chemical treatment method to remove organic

material in water and wastewater, consisting in reactions with hydroxyl radicals, OH (active

and unstable species), which cause destruction of organic pollutants, toxic and chlorinated

compounds. Fenton’s reagent, photocatalysis, UV irradiation, wet air oxidation,

electrochemical oxidation are part of these processes.

4. Membrane technologies. Membranes have been used in water and wastewater applications

since the 1960's but initially membrane processes were too costly and applied in niche

applications or special circumstances. This changed during the 1990's due to the emergence

of several drivers, including legislation to achieve improved treatment standards, and

resource scarcity, which created the need to use membranes on saline or wastewater sources.

The rapid uptake of membranes since 2000 has led to a dramatic fall in costs, to the extent

that membranes now often compete with conventional processes, while achieving much

better quality standards.

5. Chemical and electrochemical treatments, most consisting in the addition of chemicals,

and/or the use of an electrolytic cell, that produce the coagulation, precipitation or

destruction of dissolved organic compounds

6. Reforming. All the previous treatments imply financial investments, some of them, like the

biological process, unsustainable for SMEs. A new approach proposed by ENEA Frascati is

based on the OMW reforming carried out by a dedicated reactor. This approach is

particularly suitable in view of waste valorisation.

Membranes, the technology

There are different types of membrane process used in the water and wastewater field, depending of

the level of purification to be reached. Membrane filtration, in which a micro-porous separating

layer (semipermeable or iono-selective filters obtains the molecular-physical and ionic-chemical

separation), provides a barrier to the finest particles (retentate) present in the feed source but allows

dissolved components to pass through (permeate). Depending on the specific needs, ultrafiltration

(UF) and microfiltration (MF) are the main options for this purpose. UF has pores of 0.01–0.02 μm,

while MF for water treatment has pores of 0.04-0.10 μm. In wastewater applications, coarser MF

pore sizes of 0.2 and 0.4 μm can be used, but finer MF membranes for water treatments are also

suitable. Reverse Osmosis (RO) and NanoFiltration (NF) membranes have a dense non porous

separating layer cast onto a porous support, and they are used for the removal of dissolved

substances. The separation spectrum illustrated in Figure 3 shows the particle size that the different

filtration technologies are designed to separate, together with some examples of common

challenges. The figure shows that membrane particle filtration is two to three orders of magnitude

coarser than RO. MF removes common particles found in water including bacteria and other

microbial organisms, while UF removes viruses in addition, thereby providing a physical

disinfection barrier. For RO pre-treatment of wastewater, membrane filtration is normally used in

combination with coagulation to control fouling, ensure operational stability and improve removals

of dissolved organics.

However, still the main concern about the technical implementation of membrane technologies

membrane in OMW treatment can be the high fouling potential.

Figure 3 Particle size separation by different filtration technologies

OMW valorisation and the economy of membranes

Beyond the concern on reduction of the polluting load, OMW could become, similarly to other

biomass and wastes, source of energy or high value products. In this direction, it goes the study [5]

done by ENEA Frascati laboratories on OMW reforming which can produce a gas stream rich in

hydrogen, methane and CO2. In particular, more than 3 kg of hydrogen per m3 of OMW can be

produced with reforming reactions using a noble metal catalyst (Pt, Pd and Rh), on CeO2-ZrO2

layered alumina pellets. In addition, the process is able to reduce the potential pollutant of OMW by

more than 90%, calculated as total organic content.

The main reactions could be figured out as in the following:

steam reforming (SR) of a generic alcohol

CnH2n+1OH + (n-1) H2O = nCO + 2nH2 ΔH> 0 (1)

SR of a Generic Hydrocarbon

CnHm + nH2O ↔ nCO + (n + m / 2) H2 ΔH> 0 (2)

partial oxidation of a generic hydrocarbon

CnHm + n / 2O2 ↔ nCO + m / 2H2 ΔH <0 (3)

These reactions are complemented by the Water Gas Shift (WGS) reaction that converts CO to CO2

in order to produce additional hydrogen:

CO + H2O = CO2 + H2 ΔH = -41 kJ mol-1 (4)

Actually OMW are too diluted to undergo any reforming process and they need to be concentrated.

In this direction, a further evolution has been a simulation study [6], carried out by ENEA in

collaboration with the University of Tor Vergata, by adopting multistage tangential membranes to

concentrate OMW and make the retentate available for the thermochemical treatment. Different

process solutions are envisaged, suitable for SMEs in terms of complexity and, therefore, of cost.

The first solution (MF+UF+NF) is aimed at obtaining a permeate with a very low content in organic

substances such as a seepage spill. The second solution (MF+NF) is oriented to produce a permeate

usable as raw product for the extraction of polyphenols by the pharmaceutical and cosmetic

industries. The third solution (MF+UF+NF+OI) generates a highly diluted permeate (which can be

spilled into the sewer) and a retentate consisting of ready to use polyphenols (refined product) for

pharmaceutical purposes. Figure 4 show the schematic layout of the third solution.

Figure 4 Schematic layout of OMW filtration + reforming process

The concentration of phenols (12.78 gL-1

) is significantly reduced in the NF and OI permeates,

reaching 0.13×10-3

12.78 gL-1

and 0.14×10-3

gL-1

, respectively. Viceversa, the concentration of the

phenols occurs in the retentate of the different tangential filtration stages: in particular, the MF and

UF retentate have a concentration of polyphenols of 13.59 gL-1

and 26.00 gL-1

, respectively.

In the same study, the economic analysis assessed the main economic indicators of an investment

for the construction of a small-medium size OMW treatment plant (milling capacity up 2 t/h of

olives). The total cost of the plant (membranes, heat exchangers, reactor,…) is calculated to be

around 80 k€. Table 5 shows the values of the economic parameters1 taken into consideration to

assess the economic soundness of the proposal. The VAN values obtained are significantly positive

(from 35 k€ to about 60 k€), indicating the economic viability of the investments. The calculated

TIR (from 15% to 22%) is about 2-3 times the value of the interest rate considered (7% per annum):

in practice, this rate should more than double to make the investment ineffective. Finally, ROI

values from 20% to 26% indicate that in a few years the initial investment is totally recovered.

1VAN (Current Net Value) is the greatest benefit that a business investment can generate compared to a reference

financial investment. The calculation was considered as a reference investment of 10 years with an average market yield

rate of 7%. Internal Rate of Return (TIR) indicates the interest rate that should be considered to render the VAN null.

Return on Investments (ROI) indicates the rate of financial return

Figure 5 Return on Investments (ROI), VAN (Current Net Value), Internal Rate of Return (TIR) for the three solutions

It appears that all plant solutions are appealing compared to the practice of soil shedding. In

particular, solutions 2 and 3 (MF+UF and MF+UF+NF+OI) provide greater net gain.

Membranes, the LCA in a case study

Following the economic assessment, a LCA [7] has been carried out by ENEA Frascati on a

specific case study, the Fontana Laura Mill, a medium size mill located 30 km out of Rome. This is

a family run enterprise working in the olive mill sector since 1928. In the 4 weeks peak season, the

mill is 24 h open with up 25 employees. The company produces oil with both the traditional olive

processing system based on granite mills and cold squeezing, and with a modern continuous

extraction system based on centrifugation. In the study, only continuous treatment, being the most

used, has been considered. In order to quantify environmental impacts, in the SimaPro 8 software,

the base for most of the analysis, the following impacts/category indicators have been considered:

1. Acidification. Emissions of compounds resulting from the combustion of fossil fuels,

particularly sulfur oxides and nitrogen oxides. Category indicator: sulfur dioxide (SO2)

2. Eutrophication. Excess of nutrients in a given environmental like nitrates and phosphates.

Category Indicator: Phosphates (PO4 2-)

3. Greenhouse Effect. Category Indicator: Carbon Dioxide (CO2)

4. Impact of human toxicity. Toxic substances present in the environment. Category Indicator:

1,4 dichlorobenzene (1,4 DCB)

5. Abiotic Depletion. Depletion of non-resources renewable. Category indicator: Antimony (kg

Sb eq.)

6. Photochemical Smog, like ozone and other oxidizing chemicals and fine dust. Category

indicator: ethylene (C2H4)

7. Ecotoxicity, inhibitory action towards the microorganisms depressing it and slowing its

activity and causing it as a result of imbalances in natural ecosystems. Category Indicator:

Cresol (CH3C6H4OH)

Fig. 6 shows the contribution to each category indicator of each step in the production chain.

Fig. 7 and table 1 show the LCA results and the significant improvement in sustainability from all

the seven parameters, eutrophication in particular.

Figure 6 Oil milling steps vs environmental indicators

Figure 7 Change in environmental indicators before and after the OMW treatment

Impact Category

Unit Total Washing Milling Beating Decantation Centrifugatio

n OMW

disposal Pomace disposal

No

treat Trea

t No

treat Trea

t No

treat Trea

t No

treat Trea

t No

treat Trea

t No

treat Trea

t No

treat Trea

t No

treat Trea

t

Abiotic depletion

Kg Sb eq

1,27E-05

7,44E-06

7,36E-07

7,36E-07

1,35E-06

1,35E-06

5,89E-07

5,89E-07

1,39E-06

1,35E-06

6,76E-07

6,76E-07

5,28E-06

8,02E-08

2,66E-06

2,66E-06

Global warming (GWP 100a)

kg co2 eq

1,78E+01

1,58E+01

1,84E+00

1,84E+00

3,37E+00

3,37E+00

1,47E+00

1,47E+00

3,37E+00

3,37E+00

2,05E+00

2,05E+00

3,02E+00

1,06E+00

2,64E+00

2,64E+00

Human toxicity

kg 1,4-DB eq

3,03E+00

2,64E+00

3,12E-01

3,12E-01

5,72E-01

5,72E-01

2,49E-01

2,49E-01

5,72E-01

5,72E-01

3,29E-01

3,29E-01

5,59E-01

1,66E-01

4,40E-01

4,40E-01

Terrestrial ecotoxicity

kg 1,4-DB eq

2,78E-02

2,53E-02

3,60E-03

3,60E-03

6,59E-03

6,59E-03

2,88E-03

2,88E-03

6,59E-03

6,59E-03

3,36E-03

3,36E-03

2,64E-03

2,00E-04

2,09E-03

2,09E-03

Photochemical oxydation

kg CH2H4e

q

3,88E-03

3,38E-03

3,80E-04

3,80E-04

6,97E-04

6,97E-04

3,04E-04

3,04E-04

6,97E-04

6,97E-04

4,00E-04

4,00E-04

7,38E-04

2,41E-04

6,65E-04

6,65E-04

Acidification

kg SO2 eq

7,80E-02

5,24E-02

8,01E-03

8,01E-03

1,47E-02

1,47E-02

6,41E-03

6,41E-03

1,47E-02

1,47E-02

8,60E-03

8,60E-03

1,37E-02

4,21E-03

1,19E-02

1,19E-02

Eutrophication

kg PO4---eq

1,09E+01

4,05E-02

6,51E-03

6,51E-03

1,19E-02

1,19E-02

5,21E-03

5,21E-03

1,19E-02

1,19E-02

1,51E-03

1,51E-03

1,09E+01

6,63E-04

2,76E-03

2,76E-03

Table 1 Before and after the OMW treatment

Conclusions

The treatment of the OMW today finds the most widespread practice in the land disposal, especially

by small and medium-sized enterprises. However, this practice is characterized by uncertain

environmental sustainability. In the present study, three plant solutions for the treatment of OMW

have been presented, based on the use of tangential membranes coupled to reforming.

Although the proposed technologies (tangential membranes and reforming) are mature and already

available on the market, a pilot plant is planned to be built in order to test the proposed process in

different operating conditions, in particular regarding the high variability of the OMW composition

and its effect on the optimization of the operating parameters and of the other process

characteristics (membrane fouling, reforming catalysts, etc.). Energy efficiency analysis will be also

considered with the main aim to balance the syngas production in the electrical energy

consumption.

Bigliography

[1] «International Olive Oil Organization, “World Olive Oil Figures”,» 2014. [Online]. Available:

http://www.internationaloliveoil.org/estaticos/view/131-world-olive-oil-figures?lang=it_IT.

[2] C. M. S.-M. M. Roig A, «An overview on olive mill wastes and their valorisation methods,» Waste

Management, vol. 26, n. 9, pp. 960-9, 2006.

[3] N. P. M. J. M. Gholamzadeh, «Study on Olive Oil Wastewater Treatment: Nanotechnology Impact,»

Journal of Water and Environmental Nanotechnology, vol. 1, n. 2, pp. 145-161, 2016.

[4] H. L. K. P. E. Tsagaraki, «Olive mill wastewater,» in Utilisation of By-Products and Treatment of Waste in

the Food Industry, New York, Springer, 2007, p. 133–157.

[5] S. T. e. al., «Reforming of olive mill wastewater through a Pd-membrane reactor,» international journal

of hydrogen energy, pp. 10252-10259, 2013.

[6] M. Incelli, «Trattamento Di Acque Di Vegetazione Di Oleifici: Studio Di Fattibilità Di Impianti Con

Separatori A Membrana”,» Tesi Laurea Magistrale, Università di Roma Tor Vergata, Rome, 2014.

[7] S. S., «Trattamento di acque di vegetazione di olifici: Analisi del Ciclo di Vita di impianto con separatori a

membrana,» Tesi Laurea Magistrale, Università di Roma Toma Tre, Rome, Rome, 2016.