Abstract

Large volumes of process water and steam are required for extraction and refining of edible seed oils. Water usage in most industries has decreased over the years as increasing emphasis is being placed on water re-use in modern facilities. However, given the size of the edible seed oil industry globally, there is a lack of current quantitative data about water use and wastewater generation. As the world moves towards a circular economy and water becomes more scarce, it is imperative that the industry players provide meaningful input/output data in order to benchmark and identify areas for waste valorization. This review provides data currently available in the public domain on the specific wastewater intake and wastewater generation by the edible seed oil industry, highlighting the need for further data collection. In addition, wastewater quality and current and future wastewater treatment technologies are discussed, as well as the potential for valorization of solid waste and effluent from the industry, and potential avenues for future research.

SELECTED ABBREVIATIONS

     
  • AWW

    acid wastewater

  •  
  • EFO

    edible fruit oils

  •  
  • ESO

    edible seed oil

  •  
  • EVO

    edible vegetable oil

  •  
  • SED

    specific effluent discharge

  •  
  • SWI

    specific water intake

INTRODUCTION

According to statistics published by the United States Department of Agriculture (USDA) Foreign Agricultural Service, the production of edible vegetable oil (EVO) has increased year-on-year over the past decade. The total volume is set to reach around 208 million metric tons (MT) in 2019 (USDA 2019). Palm oil, a fruit oil from Indonesia and Malaysia, constitutes more than one third of this amount (USDA 2019). Deforestation to make way for palm oil plantations in these countries is environmentally contentious because this practice results in biodiversity losses and greenhouse gas emission increases (Malahayati & Masui 2018). Similarly, the diversion of arable land away from growing food crops to growing oilseeds for first generation biofuel production has sparked the ‘food vs fuel’ debate (Hassan et al. 2019). In a number of countries/regions, a significant percentage of seeds oils are cultivated specifically for biodiesel (Argentina (∼70%), the European Union (∼40%), Thailand (∼70%) and Brazil (∼35%), (USDA 2019)).

Edible seed oil (ESO) and edible fruit oil (EFO) make up the bulk of vegetable oils produced world-wide, but more research has been dedicated to remediation and valorization of palm oil and olive oil industry wastes than ESO wastes (Gibon et al. 2009; Lai et al. 2012). In some instances, including in Europe and South Africa (SA), extracted palm oil is imported and refined locally with other EVO, so some refineries cannot be strictly delineated into EVO or EFO facilities (Gibon et al. 2009; Schneider & Finkbeiner 2013; Welz et al. 2017). Nevertheless, because seeds have hard husks and fruits are relatively soft, overall processing for EFOs differs substantially from that for ESOs (Durnford 2008; Hamm et al. 2013; Savoire et al. 2013).

The ESO industry is resource, water, energy and waste intensive and, in many instances, wastes and by-products are not utilized to their full potential (Welz et al. 2017). In addition, there is a lack of data available from the ESO industry at large in the public domain (Schneider & Finkbeiner 2013). In order for the industry to set benchmarks, and for researchers to define realistic new avenues for investigation, it is imperative that reliable qualitative and quantitative data are available (Schneider & Finkbeiner 2013; Welz et al. 2017). To address the lack of primary data from the ESO industry, FEDOIL commissioned the establishment of a valid database of primary data from six companies belonging to the European vegetable oil and protein oil industry, and performed a life cycle assessment to determine the potential environmental impacts of the industry using a gate-to-gate approach (Schneider & Finkbeiner 2013). However, water usage was largely excluded, and wastewater treatment was not considered because insufficient data were collected to model the composition and emissions of an ‘average’ wastewater treatment process. A number of reports on the characteristics of ESO effluent are available in other reports, but those on specific water usage and effluent discharge are scanty, and, in many instances, outdated. This review provides the (limited) currently available information, and highlights the data gaps and potential avenues for research.

Processing of edible seed oils

Processing of seeds is divided into three phases: pre-treatment, separation, and refining (Figure 1). The steps that are applied are incumbent on site preferences and feedstock. After physical-mechanical pre-treatment to remove husks and debris, the oil is separated from the rest of the seed material by expelling (often referred to as pressing) and/or extraction. The residue obtained from expelling is known as an oilcake. Typically, expelling is followed by extraction of residual oil from the oilcake with high oil content seeds (e.g. sunflower, rapeseed/canola), while extraction alone (no expelling) is employed with low oil content seeds (e.g. soybeans). Extraction is accomplished using a solvent (usually commercial hexane, which is a mixture of hexane isomers). The solvent is recovered from the oil-solvent mixture (miscella) by distillation, and re-used (Durnford 2008; Hamm et al. 2013; Savoire et al. 2013).

Figure 1

Basic schematic of input water, and solid and liquid products, by-products and wastes from a typical edible seed oil processing facility (adapted from Welz et al. 2017).

Figure 1

Basic schematic of input water, and solid and liquid products, by-products and wastes from a typical edible seed oil processing facility (adapted from Welz et al. 2017).

Most crude ESO is then refined to prevent spoilage and to remove substances that may affect the taste, odour and appearance of the oils, including gums, waxes, free fatty acids (FFAs), pigments, aldehydes and ketones (Lai et al. 2012; Hamm et al. 2013). Most of the process water (direct and indirect) is used in these neutralization processes, the most common of which are shown in Figure 1. In alkali (chemical) refining, the oil is usually conditioned using phosphoric acid (H3PO4), after which sodium hydroxide (NaOH) is mixed with the oil to saponify FFAs, neutralize H3PO4 and hydrate and saponify phospholipids; after a specified reaction time, the oil is mixed with hot water and the unwanted soapstock is split from the oil. Bleaching removes pigmented molecules, primary and secondary oxidation products, soaps and polyaromatics via electrostatic adsorption onto the surface of bentonite clays; deodorizing removes most of the volatile compounds responsible for off-odours, and pesticide residues by steam stripping at 160–260 °C (Hamm et al. 2013). A similar steam stripping process, distillation, also removes FFAs from oils that have been physically refined (i.e. have not been chemically neutralized) (Hamm et al. 2013).

Potential for valorization and re-use of edible oil solid waste

Although the main focus of this review pertains to wastewater, other by-products and wastes from the ESO industry are briefly discussed here. This is because when industries look to apply circular economy principles at a local level, all processes need to be assessed holistically in order to evaluate the potential for valorization, resource reduction, and re-use (Corona et al. 2019).

In a recent national survey (NATSURV) of the EVO in SA, it was ascertained that seed hulls and fines were traditionally re-used as low grade animal fodder or bedding, AWW was often used to make soap, and waxes were used in candle-making (Welz et al. 2017, Figure 1). Among others, there is potential for valorization of the hulls/fines, for example, as a component in structural materials, or for extraction of value-added products. Indeed, it has recently been shown that it may be viable to extract levoglucosan and/or furfural from bio-oils obtained from pyrolysed sunflower seed hulls (Casoni et al. 2019).

Bleaching clay is an expensive natural resource that is typically landfilled when spent, which means that there are economic and environmental incentives for re-use (Boukerroui et al. 2018). At the very least, the spent earth can be used as fuel in boilers (Pandey et al. 2003). Successful clay regeneration has been achieved using solvents, acids or alkalis (e.g. Boukerroui et al. 2018), but these methods generate potentially toxic ancillary liquid waste streams, and can have high operational costs (Oladosu et al. 2017). Sub-critical water extraction and sub-critical fluid extraction are more advanced technologies that have been shown to recover a fraction of the oil from bleaching clays, but none of the studies to date have considered re-use of the clay (Oladosu et al. 2017). There is a real need for research pertaining to (i) regeneration of bleaching clays using ‘green’ chemical/physicochemical methods with minimal ancillary waste, and/or (ii) more sustainable methods of bleaching, as with the use of boiler ash as an alternative bleaching agent, as recently been described by Lau et al. (2019). Ways to re-use or valorize other forms of solid waste such as spent nickel catalyst and deodorizer distillate should also be considered (Pandey et al. 2003). For example, Sherazi et al. (2016) has found that saponified deodorizer distillates contain large fractions of valuable neutraceuticals, including tocopherols and sterols.

Specific water intake

Pre-treatment of edible seeds is energy demanding, but has no direct water requirements for ESO processing (Hamm et al. 2013). Extraction and refining consume large volumes of water and generate effluent that is difficult to remediate. Benchmarks for water use are available (e.g. in the 2006 EU ‘Best available technologies (BAT) in food, milk and drink industries’ document and similar documents (BMU 2002; CIAA-FEDOIL 2002; EC-BAT 2006; World bank group 2015; EC-BAT 2019)). However, when interrogating the references, these all emanate from the same data originally published in 1986 (BMU 1986), which is not widely accessible. Around the same time, Steffen et al. (1989) published specific water intake (SWI) data from 11 processing facilities in SA as part of an original National Survey (NATSURV) (Table 1). However, in a more recent NATSURV, oil facilities divulged robust data on SWI (Welz et al. 2017). FEDOIL, the umbrella body representing the European vegetable oil and protein oil industry commissioned TU Berlin to conduct a lifecycle assessment of oilseed crushing and refining by its members. Data were collected from six relevant European companies and a comprehensive document was published (Schneider & Finkbeiner 2013). The average SWI (water and steam) from three oilseed processing facilities were obtained and published (Table 1), but no details were provided in the document. Unfortunately, water use was not seen as a priority because the authors deemed the amount of process water to be relatively small when compared to other stages of the supply chain such as cultivation of oilseeds. They also motivated that there is currently no agreed standard on how to assess water use within the life cycle analysis method that they used (Schneider & Finkbeiner 2013). Although the authors stated that water consumption was ‘out of the scope’ of the study, they recommended that a future study be conducted on this aspect. While there is clearly a need to update industry benchmarks, it is difficult to extrapolate water usage goals from the limited data currently available in the public domain (Table 1).

Table 1

Literature values for water intake for edible seed oil processing

Oil typeProcessSpecific water intake (m3/MT oil)References
Seeda Crushing 2.6 (range: 2.1–3.1) (n = 11) Steffen et al. (1989)  
Seeda Refining 3.8 (range: 3.2–4.6) (n = 11) Steffen et al. (1989)  
Rapeseed Crushing 1.19 (n = 3) Schneider & Finkbeiner (2013)  
Rapeseed Refining 0.67 (n = 3) Schneider & Finkbeiner (2013)  
Soybean Crushing 2.60 (n = 3) Schneider & Finkbeiner (2013)  
Soybean Refining 0.77 (n = 3) Schneider & Finkbeiner (2013)  
aSeed Refining <1.5 Welz et al. (2017)  
aSeed Physical refining 0.29 Welz et al. (2017)  
Oil typeProcessSpecific water intake (m3/MT oil)References
Seeda Crushing 2.6 (range: 2.1–3.1) (n = 11) Steffen et al. (1989)  
Seeda Refining 3.8 (range: 3.2–4.6) (n = 11) Steffen et al. (1989)  
Rapeseed Crushing 1.19 (n = 3) Schneider & Finkbeiner (2013)  
Rapeseed Refining 0.67 (n = 3) Schneider & Finkbeiner (2013)  
Soybean Crushing 2.60 (n = 3) Schneider & Finkbeiner (2013)  
Soybean Refining 0.77 (n = 3) Schneider & Finkbeiner (2013)  
aSeed Refining <1.5 Welz et al. (2017)  
aSeed Physical refining 0.29 Welz et al. (2017)  

aTypes of seed not specified.

Specific wastewater generation

Refining typically generates larger volumes of effluent than pressing/extraction, provided cooling water is effectively recirculated (BMU 1986; Abdel-Gawad & Abdel-Shafy 2002; Hamm et al. 2013). Data on specific effluent discharge (SED) volumes from ESO processing are scant, and in most cases, dated (BMU 1986; Steffen et al. 1989). As with SWI, cited values can be traced back to data originally published in 1986 (BMU 1986). The total volume of effluent generated is provided in some more recent reports, but the amount of oil produced is not, so the SED cannot be calculated (e.g. Chipasa 2001; Dkhissi et al. 2018).

Of the limited data available, Steffen et al. (1989) reported an overall SED of 2.0 m3/MT oil for extraction and refining, but higher values (2.7 m3/MT oil) were reported more recently from an ESO factory in Egypt after the application of waste minimization measures (Abdel-Gawad & Abdel-Shafy 2002).

Only 1 of 17 vegetable processing facilities was willing to divulge data regarding SED volumes during a recent National Survey (NATSURV) in SA, despite offers to sign confidentially agreements (Welz et al. 2017). This general reluctance by the industry may be a factor for the lack of data available at a global level for both water use and wastewater generation. The ESO involved reported the highest SED volumes for solvent extraction (0.3 m3/MT oil), followed by chemical refining (0.15 m3/MT oil) and physical refining (0.15 m3/MT oil), while only 0.03–0.05 0.15 m3/MT oil was reported as being generated from soap-splitting (Welz et al. 2017). The latter is in stark contrast to the 1.0–1.5 m3/MT oil cited in the EU BAT document, and highlights the need for revision of the data to reflect the current status quo and allow the industry to benchmark realistically.

Wastewater quality

Cooling tower blow-down typically forms part of the general effluent, and is recognized as being relatively uncontaminated (Hamm et al. 2013). The refinery effluent (Figure 1) can be divided into two main categories: (i) acid wastewater (AWW) from soap splitting (chemical refining) and (ii) general process wastewater (GPWW) from either distilling (physical refining) or oil washing and deodorizing (chemical refining) that may or may not be combined with AWW. The AWW contains mainly FFAs and excess H2SO4 from soap splitting, while the GPWW typically contains large amounts of FFAs, as well as aromatics, residual gums, degumming acids (H3PO4 or citric acid), pesticides and soaps from chemical refining. General wash-water and cooling tower blowdown may be combined with the GPWW before treatment or discharge, while the AWW may or may not be added to the combined stream (Welz et al. 2017). If large volumes of EVO effluent are discharged via municipal reticulation systems into domestic wastewater treatment systems, system performance can be negatively affected. In most cases, pollutant loads for incoming wastewater are set by municipal wastewater treatment plants, so that some form of treatment is required before discharge (Welz et al. 2017). The effluent is also hazardous if released into the environment. For example, in August 2019, a spill from an EVO processing facility in SA into a nearby river system resulted in an environmental disaster (https://mg.co.za/article/2019-08-23-00-rivers-die-after-oil-chemical-spill).

Literature values for key parameters in GPWW and AWW are provided in Table 2. The pH of the GPWW exhibits large inter- and intra-site variability because both acids (e.g. H3PO4 for degumming and conditioning and alkalis (NaOH for neutralization during chemical refining) are typically added. The pH variability is challenging for most effluent treatment technologies, so it is adjusted prior to treatment (Chipasa 2001; Abdel-Gawad & Abdel-Shafy 2002; Pandey et al. 2003; Azbar & Yonar 2004). Both the GPWW and AWW contain high concentrations of organics, with chemical oxygen demand (COD) concentrations of up to 15.4 g/L and 350 g/L being reported in GPWW (Aslan et al. 2009) and AWW (Roux-van der Merwe et al. 2005), respectively. Apart from one anomalous result (1.0, Pintor et al. 2014), the reported biological oxygen demand (BOD) to COD ratios range from 0.2 to 0.6, demonstrating that the effluent is only slowly to moderately biodegradable. Good correlations between COD, and oil and grease (OG) concentrations in ESO effluents have been demonstrated (Saatchi et al. 2001). Discounting two anomalous results for the COD:OG ratio (13.9 and 34.4; Aslan et al. 2009), the ratios of 1.1–7.3 reflect (as expected), high residual oil content in both GPWW and AWW. High total phosphate (TP) concentrations are found in ESO effluent from residual oil phosphatides from H3PO4 (if used for degumming). It is important to monitor sulfate (SO42−) in the effluent from chemical refining because high concentrations of sulfuric acid (H2SO4) can be found from soap-splitting.

Table 2

Literature values for the most important parameters to measure in raw general process water and acid effluent from the edible seed oil industry (values are averages, ranges in brackets)

ProcessCountryOilpHCODt (gO2/L)CODs (gO2/L)BOD:CODt ratioOG (g/L)CODt:OG ratioTP (mg/L)SO42− (g/L)TSS (g/L)References
Raw general process wastewater (may contain acid wastewater) 
E, R SA Seed (1.8–10.5) 4.6
(1.1–9.0) 
– – 0.6
(0.1–1.4) 
7.3 – 1.8
(0.1–11.0) 
– Steffan et al. (1989)  
CR, PR, V Poland Seed, palm* (6.0–7.0) 1.2 ± 0.4
(1.0–1.7) 
– – – – 590 ± 94
(455–702) 
16.6 ± 4.8
(11.0–20.6) 
0.8 ± 2.6
(0.5–1.2) 
Chipasa (2001)  
– Turkey S/flower (7.4–9.6) 9.4
(5.6–15.3) 
– 0.6 3.0
(0.5–7.6) 
3.1 379
(216–556) 
0.6
(0.2–1.1) 
7.8
(3.7–12.8) 
Saatchi et al. (2001)  
CR, V India Seed, palm* 2.3 29.1 ± 4.5 – – 7.8 ± 0.2 3.8 57.4 ± 4 2.0 ± 0.2 7.9 ± 0.2 Pandey et al. (2003)  
E, CR Turkey Seed (6.3–7.2) (13.8–15.0) (6.5–7.0) 0.31 (3.6–3.9) – (61–63) NG (3.8–4.1) Azbar & Yonar (2004)  
Turkey S/flower (3.5–4.5) 9.2 ± 0.8
(8.3–9.7) 
5.4 ± 0.2
(5.2–5.6) 
0.2
0.3 (s) 
0.7 ± 0.1
(0.5–0.8) 
13.9 177 ± 210
(52–420) 
11.4 ± 0.5
(10.8–11.7) 
1.7 ± 0.2
(1.5–2.0) 
Aslan et al. (2009)  
Turkey Corn (2.7–2.8) 12.9 ± 2.2
(11.5–15.4) 
5.7 ± 0.9
(5.1–6.7) 
0.2
0.2 (s) 
0.4 ± 0.1
(0.3–0.5) 
34.4 583 ± 270
(275–775) 
12.0 ± 0.5
(11.9–12.3) 
2.9 ± 1.0
(1.1–3.0) 
Aslan et al. (2009)  
Portugal S/flower 6.3 4.1 ± 0.4 – 1.0 0.7 ± 0.3 5.8 900 ± 200 3.4 ± 0.1 2.4 ± 0.3 Pintor et al. (2014)  
E,CR SA Soya, S/flower (8.8–11.6) 6.2
(0.9–11.7) 
– – 5.5 1.1 88 ± 61
(6–152) 
0.3 ± 0.3
(0.02–0.8) 
0.8 ± 0.5
(0.2–1.3) 
Welz et al. (2017)  
Acid wastewater 
CR, PR, V Poland Seed, palm (2.5–6.0) 5.8 ± 2.0
(3.3–8.0) 
– – – – 5,694 ± 1,371
(4,599–6,982) 
1.6 ± 0.5
(1.1–2.3) 
4.0 ± 1.3
(2.5–5.4) 
Chipasa (2001)  
NG SA S/flower – 251.5 ± 98.6
(152.8–350.0) 
– – 201.4 ± 79.7
(122.2–281.5) 
1.2 – – 15.1 ± 14.3
(0.7–29.3) 
Roux-van der Merwe et al. (2005)  
Portugal S/flower 1.6 11 ± 4 – 0.5 3.0 ± 1.0 3.6 400 ± 200 0.2 ± 0.1 8.9 ± 0.5 Pintor et al. (2014)  
E,CR SA Soya, S/flower (1.8–5.8) 4.4 ± 3.3
(0.2–8.5) 
– – 1.6 2.8 402 ± 356
(186–602) 
4.0 ± 4.0
(0.04–9.3) 
1.6 ± 5.4
(0.2–6.1) 
Welz et al. (2017)  
ProcessCountryOilpHCODt (gO2/L)CODs (gO2/L)BOD:CODt ratioOG (g/L)CODt:OG ratioTP (mg/L)SO42− (g/L)TSS (g/L)References
Raw general process wastewater (may contain acid wastewater) 
E, R SA Seed (1.8–10.5) 4.6
(1.1–9.0) 
– – 0.6
(0.1–1.4) 
7.3 – 1.8
(0.1–11.0) 
– Steffan et al. (1989)  
CR, PR, V Poland Seed, palm* (6.0–7.0) 1.2 ± 0.4
(1.0–1.7) 
– – – – 590 ± 94
(455–702) 
16.6 ± 4.8
(11.0–20.6) 
0.8 ± 2.6
(0.5–1.2) 
Chipasa (2001)  
– Turkey S/flower (7.4–9.6) 9.4
(5.6–15.3) 
– 0.6 3.0
(0.5–7.6) 
3.1 379
(216–556) 
0.6
(0.2–1.1) 
7.8
(3.7–12.8) 
Saatchi et al. (2001)  
CR, V India Seed, palm* 2.3 29.1 ± 4.5 – – 7.8 ± 0.2 3.8 57.4 ± 4 2.0 ± 0.2 7.9 ± 0.2 Pandey et al. (2003)  
E, CR Turkey Seed (6.3–7.2) (13.8–15.0) (6.5–7.0) 0.31 (3.6–3.9) – (61–63) NG (3.8–4.1) Azbar & Yonar (2004)  
Turkey S/flower (3.5–4.5) 9.2 ± 0.8
(8.3–9.7) 
5.4 ± 0.2
(5.2–5.6) 
0.2
0.3 (s) 
0.7 ± 0.1
(0.5–0.8) 
13.9 177 ± 210
(52–420) 
11.4 ± 0.5
(10.8–11.7) 
1.7 ± 0.2
(1.5–2.0) 
Aslan et al. (2009)  
Turkey Corn (2.7–2.8) 12.9 ± 2.2
(11.5–15.4) 
5.7 ± 0.9
(5.1–6.7) 
0.2
0.2 (s) 
0.4 ± 0.1
(0.3–0.5) 
34.4 583 ± 270
(275–775) 
12.0 ± 0.5
(11.9–12.3) 
2.9 ± 1.0
(1.1–3.0) 
Aslan et al. (2009)  
Portugal S/flower 6.3 4.1 ± 0.4 – 1.0 0.7 ± 0.3 5.8 900 ± 200 3.4 ± 0.1 2.4 ± 0.3 Pintor et al. (2014)  
E,CR SA Soya, S/flower (8.8–11.6) 6.2
(0.9–11.7) 
– – 5.5 1.1 88 ± 61
(6–152) 
0.3 ± 0.3
(0.02–0.8) 
0.8 ± 0.5
(0.2–1.3) 
Welz et al. (2017)  
Acid wastewater 
CR, PR, V Poland Seed, palm (2.5–6.0) 5.8 ± 2.0
(3.3–8.0) 
– – – – 5,694 ± 1,371
(4,599–6,982) 
1.6 ± 0.5
(1.1–2.3) 
4.0 ± 1.3
(2.5–5.4) 
Chipasa (2001)  
NG SA S/flower – 251.5 ± 98.6
(152.8–350.0) 
– – 201.4 ± 79.7
(122.2–281.5) 
1.2 – – 15.1 ± 14.3
(0.7–29.3) 
Roux-van der Merwe et al. (2005)  
Portugal S/flower 1.6 11 ± 4 – 0.5 3.0 ± 1.0 3.6 400 ± 200 0.2 ± 0.1 8.9 ± 0.5 Pintor et al. (2014)  
E,CR SA Soya, S/flower (1.8–5.8) 4.4 ± 3.3
(0.2–8.5) 
– – 1.6 2.8 402 ± 356
(186–602) 
4.0 ± 4.0
(0.04–9.3) 
1.6 ± 5.4
(0.2–6.1) 
Welz et al. (2017)  

E = expelling/extraction R = refining (not specified) CR = chemical refining PR = physical refining V = value added processes (e.g. margarine) CODt = total COD CODs = soluble COD BOD = biological oxygen demand BOD:CODt = total BOD:COD OG = oil and grease TP = total phosphate TSS = total suspended solids Seed = seed type not stipulated S/flower = sunflower *palm (fruit) oil refined as well.

Wastewater treatment and valorization

Results of studies on the performance of full-scale conventional treatment systems, as well as laboratory scale studies are given in Table 3.

Table 3

Performance of conventional full scale plants and laboratory studies for the treatment of edible seed oil effluent

ProcessScaleOrganics
Inorganics
References
CODt
OG
TP
Influent (g/L)Removal (%)Influent (g/L)Removal (%)Influent (mg/L)Removal (%)
CC, DAF, ASP Full-scale 13.7–15.0 92–96 3.6–3.9 93–95 61–63 97 Azbar & Yonar (2004)  
CC, DAF, ASP Full-scale 11.3 99 7.7 99 – – Abdel-Gawad & Abdel-Shafy (2002)  
CC/floc, DAF Full-scale 2.1–3.6 37–83 – – 306–401 96–98 Chipasa (2001)  
CC, ASP Full-scale 24.6 ± 4.2 99.8 1.15 ± 0.1 >99 25.6 ± 0.01 39 Pandey et al. (2003)  
SMBR Lab 31 L 1.6 ± 0.1 86 ± 2 5.3 ± 0.7 95 ± 2 – – Ma et al. (2015)  
ASP Lab 3 L 1.5 84 – – – – Aslan et al. (2009)  
ASP Lab 3 L 1.8 92 – – – – Aslan et al. (2009)  
Gravity separation Lab 1 L 4.1 ± 0.4 91 0.7 ± 0.3 58 – – Pintor et al. (2014)  
Gravity separation Lab 1 L 11 ± 4 99 3.0 ± 1.0 90 – – Pintor et al. (2014)  
EC Lab 0.3 L 15.0 99 – – – – Un et al. (2009)  
EC, E-ox. Lab 1 L 8.8 99 – – – – Sharma & Simsek (2019)  
Fenton oxidation Lab 0.3 L 1.5 90 – – 108 99 Barbusiński & Fajkis (2011)  
Microfiltration Lab 3 L 0.5–18.0 67 – – – – Šereš et al. (2016)  
Reverse osmosis Lab 30 L 2.5 99 <0.05 – – – Shridar et al. (2002)  
Biosorption Lab jar 7.4 76 – – – – Dkhissi et al. (2018)  
UAF Lab 1.2 L 17.9–24.8 ∼88–99* 1.8–5.1 ∼53–99 – – Kalat D.G. Yüceer A. (2017)  
ProcessScaleOrganics
Inorganics
References
CODt
OG
TP
Influent (g/L)Removal (%)Influent (g/L)Removal (%)Influent (mg/L)Removal (%)
CC, DAF, ASP Full-scale 13.7–15.0 92–96 3.6–3.9 93–95 61–63 97 Azbar & Yonar (2004)  
CC, DAF, ASP Full-scale 11.3 99 7.7 99 – – Abdel-Gawad & Abdel-Shafy (2002)  
CC/floc, DAF Full-scale 2.1–3.6 37–83 – – 306–401 96–98 Chipasa (2001)  
CC, ASP Full-scale 24.6 ± 4.2 99.8 1.15 ± 0.1 >99 25.6 ± 0.01 39 Pandey et al. (2003)  
SMBR Lab 31 L 1.6 ± 0.1 86 ± 2 5.3 ± 0.7 95 ± 2 – – Ma et al. (2015)  
ASP Lab 3 L 1.5 84 – – – – Aslan et al. (2009)  
ASP Lab 3 L 1.8 92 – – – – Aslan et al. (2009)  
Gravity separation Lab 1 L 4.1 ± 0.4 91 0.7 ± 0.3 58 – – Pintor et al. (2014)  
Gravity separation Lab 1 L 11 ± 4 99 3.0 ± 1.0 90 – – Pintor et al. (2014)  
EC Lab 0.3 L 15.0 99 – – – – Un et al. (2009)  
EC, E-ox. Lab 1 L 8.8 99 – – – – Sharma & Simsek (2019)  
Fenton oxidation Lab 0.3 L 1.5 90 – – 108 99 Barbusiński & Fajkis (2011)  
Microfiltration Lab 3 L 0.5–18.0 67 – – – – Šereš et al. (2016)  
Reverse osmosis Lab 30 L 2.5 99 <0.05 – – – Shridar et al. (2002)  
Biosorption Lab jar 7.4 76 – – – – Dkhissi et al. (2018)  
UAF Lab 1.2 L 17.9–24.8 ∼88–99* 1.8–5.1 ∼53–99 – – Kalat D.G. Yüceer A. (2017)  

ASP = activated sludge process, DAF = dissolved air flotation, CC = chemical coagulation, EC = electrocoagulation, E-ox = electrooxidation, floc = flocculation.

SMBR = submerged membrane bioreactor, UAF = upflow anaerobic filter *extrapolated.

The high organic content in raw ESO effluent may suggest that biological treatment is an attractive option for organic removal, especially in cases where oils are emulsified and/or FFAs are saponified (Chipasa & Mędrzyca 2006). However, a number of researchers have shown that lipids remain in the treated effluent because they are both bio-degraded and bio-synthesized simultaneously, and are also released during endogenous microbial decay (Chipasa & Mędrzyca 2008). While the conventional activated sludge (CAS) system used as a stand-alone has shown some success at laboratory scale (Aslan et al. 2009), long retention times are required (Aslan et al. 2009), and process failures can occur due to poor settling by oil-microbe agglomerates, the formation of pin-point flocs and/or overgrowth of filamentous species (Reddy et al. 2003; Chipasa & Mędrzyca 2006).

Anaerobic digestion is generally deemed unsuitable for the treatment of wastewater with high fat and/or oil content because of mass transfer limitations and inhibition of the functional microbial consortia caused by the presence of excess long and medium chain fatty acids (e.g. Chipasa & Mędrzyca 2006; Valladão et al. 2011; Samarasiri et al. 2016). Indeed, potential reactor failure due to accumulation of fatty acids was demonstrated in 1.2 L mesophilic and thermophilic upflow anaerobic filters treating vegetable oil effluent when operated at organic loading rates of >5.7 g COD/day (Kalat & Yüceer 2017). Although promising COD removal rates were still achieved, results cannot be extrapolated to full scale, where it is more difficult to achieve optimal mixing of the reactor contents, mass transfer and diffusion rates, and release of gases (Lettinga et al. 2001).

Aerobic mesophilic and thermophilic treatment technologies have been researched and/or implemented for oily industrial wastewaters, including, oily petfood effluent (Kurian et al. 2005), swine wastewater (Juteau 2006), food processing wastewater (Chiang et al. 2001), and synthetic wastewater containing olive oil (Hung et al. 2009). In addition, a combined thermophilic anaerobic–aerobic system has been tested for treatment of palm oil mill effluent (POME), which is discharged at high temperatures of 80–90 °C (Chan et al. 2015). At this time, high temperature aerobic technologies have not been proposed in literature for the treatment of ESO effluent.

Provided the ESO effluent is pre-treated to reduce the OG concentration, the CAS process can be efficient, and is widely used in industry (Azbar & Yonar 2004). Some of the organic fraction can be removed by simple gravity separation or dissolved air flotation (DAF) alone, but emulsified oils remain in the effluent: Pintor et al. (2014) achieved 91–99% removal of COD (11 ± 4 g/L) and 58–90% removal of OG (3.0 ± 1.0 g/L) using gravity separation, while Saatchi et al. (2001) obtained 17–69% removal of COD (9,350 mg/L (average), 5,600–15,300 mg/L (range)) and 29–83% removal of OG (3,030 mg/L (average), 540–7,640 mg/L (range)) using DAF.

Conventional technology consists of a combination of primary physicochemical and secondary biological processes that are mostly effective in removing COD, OG, SO42− and TP (Table 2). This is accomplished by chemical coagulation or coagulation–flocculation with, for example, aluminium sulfate [Al2(SO4)3] or aluminium chloride [(AlCl3)] and poly ammonium chloride (PAC), followed by dissolved air flotation (DAF) (Chipasa 2001; Saatchi et al. 2001; Abdel-Gawad & Abdel-Shafy 2002; Pandey et al. 2003; Azbar & Yonar 2004). The CAS is still the secondary treatment of choice (Chipasa 2001; Abdel-Gawad & Abdel-Shafy 2002; Pandey et al. 2003; Azbar & Yonar 2004; Aslan et al. 2009).

The major problems with the conventional approach are that large volumes of chemicals are required, and copious amounts of potentially hazardous sludges are generated, which require specialized disposal; secondly, macronutrients are removed during the physicochemical process, and these may require replenishing in order to meet downstream carbon:nitrogen:phosphorus (C:N:P) ratio requirements for the CAS (Saatchi et al. 2001; Pandey et al. 2003; Reddy et al. 2003; Azbar & Yonar 2004; Un et al. 2009; Pintor et al. 2014; Sharma & Simsek 2019). Dkhissi et al. (2018) proposed the use of 10% cactus (Opuntiaficus indica) as bioflocculant instead of chemical coagulation. While this approach has merit from an environmental perspective, factors such as feedstock availability, cost of feedstock transport and preparation, shelf-life, and downstream processing, among others, are needed in order to assess ‘real-world’ feasibility of bioflocculants.

In order to reduce the amount of sludge, membrane technologies, electrocoagulation, and oxidation technologies have been explored for COD and OG reduction in laboratory experiments (Table 2). Researchers have achieved good removal efficiencies with hollow-fibre polyvinylidene membranes (0.1 mm pore) in a submerged membrane bioreactor (SMBR) (Ma et al. 2015), 200 nm pore size ceramic (aluminium oxide) microfiltration (MF) membranes operated with cross-flow (Šereš et al. 2016), and polyamide reverse osmosis (RO) membranes (Sridhar et al. 2002). The benefit of RO is that the water can be re-used in the process, an important factor to consider in water-scarce countries (Sridhar et al. 2002). However, for RO, the effluent needed to be pre-coagulated and pre-filtered, negating any perceived sludge-reduction benefits (Sridhar et al. 2002), while for MF the effluent needed to be pre-skimmed and filtered to remove particles >1 mm (Šereš et al. 2016). In the SMBR and MF systems, significant fouling occurred, which could only be removed by regular chemical cleaning of the membranes (Ma et al. 2015; Šereš et al. 2016), and the MF system required heating to 60 °C to maximize efficiency (Šereš et al. 2016). These studies suggest that membrane technologies do not offer better treatment options than conventional technologies at this point in time because of the high capex, operational costs, and in some instances, the environmental burden from ancillary waste streams.

Some research has been conducted on more advanced technologies for ESO wastewater treatment, including, electrocoagulation, electrooxidation and Fenton's oxidation. Electrocoagulation reduces the amount of sludge produced by removing suspended and colloidal particles, but cannot remove the soluble COD fraction, and is reliant on electrical energy (Un et al. 2009; Sharma & Simsek 2019). Un et al. (2009) managed to reduce electricity consumption from 131 to 42 kWh per kgCODremoved using Al sacrificial electrodes when PAC was added as a coagulant and sodium sulfate (Na2SO4) as a support electrolyte in small-scale (0.3 L) and pH-controlled reactors (pH = 7), and Sharma & Simsek (2019) increased the removal of soluble COD from 75% to 99% by combining electrocoagulation with electro-oxidation. The optimal use of Fenton reagent followed by precipitation with NaOH or calcium oxide (CaO) has been shown to effectively remove COD and TP from ESO, but has the same problem with sludge generation as conventional coagulation (Barbusiński & Fajkis 2011). This could possibly be overcome by utilizing a solid catalyst and/or photo-Fenton (Domingues et al. 2018). Some of these advanced technologies are promising and merit scale-up and techno-economic assessments, including environmental parameters.

Little has been reported in literature on valorization of ESO effluent. Roux-van der Merwe et al. (2005), showed that various strains of fungi isolated from soil were able to reduce the COD of acid ESO wastewater supplemented with N. The highest reductions were achieved by Cunninghamella echinulata and Aspergillus fumigatus (98.2% and 97.8%, respectively) when grown at 30 °C for 7 days. Similarly, Yu et al. (2018) showed that the oleaginous yeast, Trichosporon fermentans, was able to reduce the COD of refined soybean oil effluent by 94.7% within 40 h without the need for nutrient addition. Roux-van der Merwe et al. (2005) showed that some fungal strains were able to accumulate high fractions of gamma-linoleic acid (GLA), a valuable nutraceutical, in their biomass (maximum 45% (w/w) by Emericella nidulans with concurrent 95.3% COD reduction), and Yu et al. (2018), showed that their strain of T. fermentans was able to convert saturated to unsaturated fatty acids and accumulated 43% (w/w) oil in biomass under optimized conditions. The accumulated oils exhibited a favourable fatty acid profile for biodiesel production. Such biotechnologies that are able to achieve effluent remediation with concurrent production of valuable products feed into the global move toward circular economies and merit further pilot-scale research followed by techo-economic assessments.

Valorization of wastewater sludge

Wastewater sludge from the ESO industry is an indirect form of solid waste that is generally sent to landfill (Chipasa 2001; Welz et al. 2017). Pandey et al. (2003) characterized clarifier sludge from the CAS process at an ESO processing site. They found that the sludge contained 76% organic matter and a favourable nutrient content for composting as a low value product, but one that allowed the sludge to be directed away from landfill at this site. This is simple to institute and should be considered at other sites.

This clarifier sludge has also been shown to contain a large fraction of oil that is suitable for the production of second generation biofuel (Ngoie et al. 2019a, 2019b; Muanruksa & Kaewkannetra 2020). Although production of most next generation biofuels is currently not economically viable, it is only a matter of time until they gain traction as countries move towards circular economies and production technologies improve (Corea et al. 2019; Corona et al. 2019). Biodiesel generated from waste cooking oil is already being widely used, and a wealth of research manuscripts and reviews (e.g. Awogbeni et al. 2019; Moazeni et al. 2019; Tan et al. 2019) have been dedicated to evaluations of production processes and biodiesel quality assessments.

CONCLUSION

In line with the move to circular economies, waste from the edible oil industry has great potential for further valorization and water re-use. In order to provide realistic industry benchmarks, it is imperative that qualitative and quantitative water use and waste generation data are collected and collated.

REFERENCES

REFERENCES
Abdel-Gawad
A.
,
Abdel-Shafy
M.
2002
Pollution control of industrial wastewater from soap and oil industries: a case study
.
Water Science and Technology
46
,
77
82
.
Aslan
S.
,
Alyuz
B.
,
Bozkurt
Z.
,
Bakaoglu
M.
2009
Characterization and biological treatability of edible oil wastewaters
.
Polish Journal of Environmental Studies
18
,
533
538
.
Awogbeni
O.
,
Inambao
F. L.
,
Onuh
E. I.
2019
Prediction of properties, engine performance and emissions of compression ignition engines fuelled with waste cooking oil methyl ester – a review of numerical approaches
.
International Review of Mechanical Engineering
13
,
97
110
.
Barbusiński
K.
,
Fajkis
S.
2011
Oxidation of wastewater generated by rape oil soapstock splitting
.
Environmental Progress & Sustainable Energy
30
,
620
631
.
BMU Hannover University, Germany
2002
Best Available Techniques Reference Document on the Food and Milk Industry, fdm/tm/65, cited by the European Commission Integrated pollution prevention and control reference document on best technology in the food, drink and milk industries (2006)
.
BMU Hannover University, Germany and German Federal Ministry (GFM) for Environment, Nature Conservation and Reactor Safety
1986
Technische Anleitung zur Reinhaltung der Luft vom 27 Feb 1986, cited by the European Commission Integrated pollution prevention and control reference document on best available technology in the food, drink and milk industries (EC-BAT, 2006)
.
Boukerroui
A.
,
Belhocine
L.
,
Ferroudj
S.
2018
Regeneration and reuse waste from an edible oil refinery
.
Environmental Science and Pollution Control
19
,
18278
18285
.
Casoni
A. I.
,
Gutierrez
V. S.
,
Volpe
M. A.
2019
Conversion of sunflower seed hulls, waste from edible oil production, into valuable products
.
Journal of Environmental Chemical Engineering
7
,
102893
.
Chan
Y.
,
Chong
M.
,
Law
C.
2015
Optimization of thermophilic anaerobic-aerobic treatment system for palm oil mill effluent (POME)
.
Frontiers in Environmental Science and Engineering
9
,
334
351
.
Chipasa
K. B.
2001
Limits of physicochemical treatment of wastewater in the vegetable oil refining industry
.
Polish Journal of Environmental Studies
10
,
141
147
.
Chipasa
K. B.
,
Mędrzyca
K.
2006
Behaviour of lipids in biological wastewater treatment processes
.
Journal of Industrial Microbiology and Biotechnology
33
,
635
645
.
Chipasa
K. B.
,
Mędrzyca
K.
2008
Characterization of the fate of lipids in activated sludge
.
Journal of Environmental Sciences
20
,
536
542
.
CIAA-FEDIOL 109
2002
Confederation of the Food and Drinks Industry of the European Union – European Oil and Protein-meal Industry Federation – Comments to the first draft – vegetable oil, cited by the European Commission Integrated pollution prevention and control reference document on best available technology in the food, drink and milk industries (2006)
.
Corea
D. F.
,
Beyer
H. L.
,
Fargione
J. E.
,
Hill
J. D.
,
Possingham
H. P.
,
Thomas-Hall
S. R.
,
Schenk
P. M.
2019
Towards the implementation of sustainable biofuel production systems
.
Renewable and Sustainable Energy Reviews
107
,
250
263
.
Corona
B.
,
Shen
L.
,
Reike
D.
,
Carreón
J. R.
,
Worrel
E.
2019
Towards sustainable development through the circular economy – a review and critical assessment on current circularity metrics
.
Resources, Conservation and Recycling
151
,
104498
.
Dkhissi
O.
,
El Hakmaoui
A.
,
Souabi
S.
,
Chatoui
M.
,
Jada
A.
,
Akssira
M.
2018
Treatment of vegetable oil refinery wastewater by coaulation-flocculation using the cactus as a bio-flocculant
.
Material Environmental Science
9
,
18
25
.
Domingues
E.
,
Gomes
J.
,
Quina
M.
,
Quinta-Ferreira
R. M.
,
Martins
R. C.
2018
Detoxification of olive mill wastewaters by Fenton's process
.
Catalysts
8
,
662
.
Durnford
N.
2008
Oil and oilseed processing. Food technology fact sheet
.
Robert M. Kerr Food and Agricultural Products Centre
158 & 159
,
1
4
.
EC-BAT
2006
Integrated pollution prevention and control reference document on best available technology in the food, drink and milk industries. European IPCC Bureau
.
EC-BAT
2019
Integrated pollution prevention and control reference document for the food, drink and milk industries industrial emissions directive 2010/75/EU. European IPCC Bureau
.
Gibon
V.
,
Ayala
J. V.
,
Dijkmans
P.
,
Maes
J.
,
de Greyt
W.
2009
Future prospects for palm oil refinery and modifications
.
Innovation – Technologie
16
,
193
200
.
2012 12 – Palm oil and palm kernel oil refining and fractionation technology
.
Hamm
W.
,
Hamilton
R. J.
,
Calliauw
G.
2013
Edible oil Processing
, 2nd edn.
Markono Print Media
,
Chennai
,
India
.
Hassan
S. S.
,
Gwilym
A. W.
,
Jaiswal
A. K.
2019
Moving towards the second generation of lignocellulosic biorefineries in the EU: drivers, challenges, and opportunities
.
Renewable and Sustainable Energy Reviews
101
,
590
599
.
Hung
M. H.
,
Chang
Y. C.
,
Huang
F. S.
,
Lu
C. J.
2009
Biological heat potential and temperature effect of an autothermal thermophilic aerobic treatment (ATAT) system
.
Journal of Environmental Biology
30
,
615
619
.
Kalat
D. G.
,
Yüceer
A.
2017
Anaerobic mesophilic and thermophilic treatability of vegetable oil refining wastewater
.
Process Safety and Environmental Protection
109
,
151
157
.
Lai
O.-M.
,
Tan
C.-P.
,
Akoh
C. C.
2012
Palm oil: Production, Processing, Characterization and Uses
.
American Oil Chemists Society Press
,
Illinois
,
USA
.
Lau
S. Y.
,
Phuan
S. L.
,
Danquah
M. K.
,
Acquah
C.
2019
Sustainable palm oil refining using pelletized and surface-modified oil palm boiler ash (OPBA) biosorbent
.
Journal of Cleaner Production
230
,
527
535
.
Lettinga
G.
,
Rebac
S.
,
Zeeman
G.
2001
Challenge of psychrophilic anaerobic wastewater treatment
.
Trends in Biotechnology
19
,
363
370
.
Ma
A.
,
Lei
T.
,
Ji
X.
,
Gao
X.
,
Gao
C.
2015
Submerged membrane bioreactor for vegetable oil wastewater treatment
.
Chemical Engineering Technology
38
,
101
109
.
Moazeni
F.
,
Chen
Y.-C.
,
Zhang
G.
2019
Enzymatic transesterification for biodiesel production from used cooking oil, a review
.
Journal of Cleaner Production
216
,
117
128
.
Muanruksa
P.
,
Kaewkannetra
P.
2020
Combination of fatty acids extraction and enzymatic esterification for biodiesel production using sludge palm oil as a low cost substrate
.
Renewable Energy
146
,
901
906
.
Ngoie
W. I.
,
Oyekola
O. O.
,
Ikhu–Omoregbe
D.
,
Welz
P. J.
2019a
Valorisation of edible oil wastewater sludge: bioethanol and biodiesel production
.
Waste and Biomass Valorization
.
https://doi.org/10.1007/s12649-019-00633-w
.
Ngoie
W. I.
,
Oyekola
O. O.
,
Ikhu–Omoregbe
D.
,
Welz
P. J.
2019b
Qualitative assessment of biodiesel produced from primary edible oil wastewater sludge
.
Waste and Biomass Valorization
.
https://doi.org/10.1007/s12649-019-00772-0
.
Oladosu
W. A.
,
Manan
Z. A.
,
Wan Alwi
S. R.
2017
Recovery of vegetable oil from spent bleaching earth: state-of-the-art and prospect for process intensification
.
Chemical Engineering Transactions
56
,
133
138
.
Pandey
R. A.
,
Sanyal
P. B.
,
Chattopadhyay
N.
,
Kaul
S. N.
2003
Treatment and reuse of wastes of a vegetable oil refinery
.
Resources Conservation and Recycling
37 L
,
101
117
.
Pintor
A. M. A.
,
Vilar
V. J. P.
,
Botleho
C. M. S.
,
Boaventura
R. A. R.
2014
Optimzation of a primary gravity separation treatment for vegetable oil refinery wastewater
.
Clean Technology Environmental Policy
16
,
1725
1734
.
Reddy
K.
,
Drysdale
G. D.
,
Bux
F.
2003
Evaluation of activated sludge treatment and settleability in remediation of edible oil effluent
.
Water SA
29
,
245
250
.
Roux-Van der Merwe
M. P.
,
Badenhorst
J.
,
Britz
T. J.
2005
Fungal treatment of an edible-oil-containing industrial effluent
.
World Journal of Microbiology and Biotechnology
21
,
947
953
.
Saatchi
Y.
,
Hasar
H.
,
Cici
M.
2001
Treatability of vegetable oil industry effluents through physical-chemical methods
.
Fresenius Environmental Bulletin
10
,
854
858
.
Samarasiri
B. K. T.
,
Mihiranga
P. A. D.
,
Rathnasiri
P. G.
2016
Effect of lipid inhibition in anaerobic wastewater treatment: a case study using desiccated coconut wastewater
. In:
Annual Session of the Institution of Engineers
,
Sri Lanka
, pp.
1
10
.
Savoire
R.
,
Lanoisette
J.-L.
,
Voroblev
E.
2013
Mechanical continuous oil expression from oilseeds: a review
.
Food Bioprocess Technology
6
,
1
16
.
Schneider
L.
,
Finkbeiner
M.
2013
Lifecycle assessment of EU oilseed crushing and vegetable refining
. In:
Commissioned by FEDOIL
.
Sustainable Engineering
,
Berlin
,
Germany
.
Šereš
Z.
,
Maravič
N.
,
Takači
A.
,
Nicolič
I.
,
Soronja-Simovič
D.
,
Jokič
A.
,
Hodur
C.
2016
Treatment of vegetable oil refinery wastewater using alumina ceramic membrane: optimization using response surface methodology
.
Journal of Cleaner Production
112
,
3132
3137
.
Sherazi
S. T. H.
,
Mahesar
S. A.
,
Uddin
S.
2016
Vegetable oil deodoriser distillate: a rich source of the natural bioactive components
.
Journal of Oleo Science
65
(
12
),
957
966
.
Sridhar
S.
,
Kale
A.
,
Khan
A. A.
2002
Reverse osmosis of edible vegetable oil industry effluent
.
Journal of Membrane Science
205
,
83
90
.
Steffen
R.
&
Kirsten consulting engineers
1989
NATSURV 6: Water and Wastewater Management in the Edible Oil Industry
.
Water Research Commission report TT 40/89 ISBN 0947447377
.
Un
E. T.
,
Koparal
A. S.
,
Ogutveren
U. B.
2009
Electrocoagulation of vegetable oil refinery wastewater using aluminium electrodes
.
Journal of Environmental Management
90
,
428
433
.
USDA
2019
United States Department of Agriculture Foreign Agricultural Service Production Supply and Distribution Statistics
. .
Welz
P. J.
,
Le Roes-Hill
M.
,
Swartz
C. D.
2017
NATSURV 6: Water and Wastewater Management in the Edible Oil Industry (Edition 2)
.
WRC TT 702/16
.
World Bank Group
2015
Environmental Health and Safety Guidelines for Vegetable Oil Production and Processing
.
World Bank Group
,
Washington, DC, USA
.