Biodiesel wastewater treatment using nanofiltration membranes

Abstract

Water reclamation and reuse are economically and environmentally important issues for industries. Costly and ineffective treatments such as coagulation, flocculation, and/or biological treatment are the most widely used technologies for these processes. The aim of this paper is to study membrane separation to treat biodiesel wastewater as an alternative. To achieve this goal, two commercial nanofiltration membranes were used to treat medium-high strength wastewater. An Inopor Nano tubular ceramic membrane was used in a cross-flow module and a NF99HF Alfa Laval polymeric flat sheet membrane was used in a dead-end filtration cell. Wastewaters were prepared and treated with these membranes achieving maximum fluxes of permeate of 53.1 L h^-1 m^-2 for the ceramic membrane and 12.2 L h^-1 m^-2 for the polymeric membrane at 20 bar and 30 °C. The results shows a chemical oxygen demand retention between 80–85 % were obtained for both membranes, while the retention of ionic species due to changes in electrical conductivity was 76–89 %. Transmembrane pressure, organic load, hydraulic permeability, cross-flow velocity, wastewater characterization and fouling resistance-in-series analysis were also made and analyzed in detail in order to explore the actual use of nanofiltration.

Introduction

Sustainable development is a key paradigm when dealing with the global climatic change. One of the most important issues to study is wastewater management. Biodiesel industry has had a rapid development in the last decade, however, as higher amounts of renewable fuels are required, is necessary to treat more wastewaters. Biodiesel is the main product of a triglyceride transesterification reaction with short-chain alcohol catalyzed by homogenous basic compounds (Atadashi et al., 2011; Daud et al., 2015a). The industrial production of biodiesel has a refining stage that involves the elimination of by-products and the catalyst by washing with warm water (Vicente et al., 2004). These washes allow the removal of unreacted methanol, mono and diglycerides, glycerol, fatty acid soaps, and sodium methylate residues (Atadashi et al., 2011). Water washing continues to be the preferred refining alternative by engineers and manufacturers, despite the efforts made by various researchers to reduce water consumption (Alves et al., 2013; Atadashi et al., 2011; Gomes et al., 2010; Moreira et al., 2020; Noriega et al., 2018; Tajziehchi and Sadrameli, 2021; Torres et al., 2017a, b). A consumption from 20 to 130 L of water per 100 L of the biodiesel produced has been reported depending on the quality of the raw material used (Daud et al., 2015a; De Gisi et al., 2013; Veljković et al., 2014). Veljković et al. (2014) and later Daud et al. (2015a) collected similar values of physicochemical effluent parameters according to the types of processes, and it can be seen that the chemical oxygen demand (COD) covers a wide range from 10,000–600,000 ppm. Instead, reported values of biological oxygen demand (BOD₅) were ranged between 8000 to 200,000 ppm, pH values of 3.3–11.2, and fats and oils values of 500−20,000 ppm among other data. De Gisi and collaborators made a significant contribution when they studied the variation of the physicochemical parameters of biodiesel wastewater during different treatments. The variation of COD and pH published by the authors is remarkable: 11,000–44,000 ppm and 5.9 to 3.3 respectively. These parameters have an important effect on the type of process to be carried out (De Gisi et al., 2013).

Several treatments are currently used in industrial biodiesel plants. Since a portion of the organic matter is stable as an emulsion, the process frequently used consists of acidification, coagulation, and flocculation assisted by chemical additives. Finally, a biological treatment is used to reduce the final dissolved organic matter (Daud et al., 2015a; Kumjadpai et al., 2011). This strategy reduced the COD and BOD values between 93.0–99.0 % and 98.0–99.9 %, respectively (Daud et al., 2015a; Veljković et al., 2014). The drawbacks of such treatments are their operational slowness, the cost of additives, and the resulting difficulties in sludge management (Daud et al., 2015a, b; Rattanapan et al., 2011). In order to avoid chemical additives: electrocoagulation, photo-Fenton and/or biological treatments are performed (Veljković et al., 2014).

There is extensive experience with membrane filtration used to treat water effluents to be reclaimed or to be discharged in surface watercourses (Hube et al., 2019; Kim et al., 2018). The use of membranes provides a clean, sustainable and energy-efficient technology in comparison to the conventional methods. The wastewater membrane treatment includes reverse osmosis (RO) and forward osmosis (FO), nanofiltration (NF), ultrafiltration (UF), and in some cases microfiltration (MF) (Ahmad et al., 2018; Ochando-Pulido and Martinez-Ferez, 2018; Racar et al., 2017; Sahebi et al., 2020; Samaei et al., 2018; Sánchez-Arévalo et al., 2021; Sikdar and Criscuoli, 2017; Tibi et al., 2020).

Although the use of membranes for wastewater treatment has a considerable background, there are few contributions to treat biodiesel effluents. To the best of our knowledge, few works have been published in membrane treatment for biodiesel wastewaters. Treated effluent was fed to a reverse osmosis spiral wound polyamide membrane in the previous cited work of De Gisi et al. (2013). A reduction from 250 ppm to 18 ppm was achieved in the COD using a RO spiral membrane, i.e. a reduction of 92.8 %. The authors highlight this result but warn about fouling and recommend the use of flat membranes to facilitate membrane cleaning. Shirazi et al. (2013) claim a 75 % COD reduction using a superhydrophobic modified MF membrane fed with a wastewater containing 350 ppm of COD. The communication published by Jaber et al. (2015) showed a procedure for wastewater reuse in biodiesel washes making a combination of microfiltration, sand filter and dilution with fresh water. The results allow a 15 % reduction of freshwater use in the washing process. Mozaffarikhah and co-workers (2017) have published the results of three NF membranes used to treat biodiesel wastewater. With a single permeation experiment and after an optimization procedure the TW30 Dow-filmtec Co. NF membrane achieved almost 86 % reduction of an initial 20,000 ppm of COD.

This briefly discussed background shows that new skills are required in the knowledge of membrane technology applied to biodiesel wastewaters treatment.

In this study, we present the performance of two commercial NF membranes used to treat biodiesel wastewater. The literature consulted has no reports of ceramic NF membranes for this kind of effluent and in this regard, we have selected two membranes with a background in similar applications. The wastewater used in all the experiments was emulated from the characterization data of real effluent made by several authors. Also, a fouling analysis of both membranes was carried out.