Techno-economic assessment of minimal liquid discharge (MLD) treatment systems for saline wastewater (brine) management and treatment

Highlights

• Membrane-based technologies are used in Minimal Liquid Discharge (MLD) systems.

• Five MLD treatment schemes are analyzed to recover freshwater from seawater brine.

• MLD schemes with membrane distillation present the highest energy consumptions.

• The profit of MLD treatment and freshwater recovery can reach up to US$2.21/m³.

• All MLD treatment schemes are profitable from the sale of the freshwater produced.

Abstract

The management and treatment of brine (saline wastewater) are of great importance, as its discharge to the environment poses a significant threat. A new strategy called minimal liquid discharge (MLD) appears to be a promising and more cost-effective option than zero liquid discharge (ZLD) as it uses only membrane-based technologies, leading to up to 95 % freshwater recovery. This research study introduces and presents for the first time a techno-economic assessment of five MLD treatment schemes that can be implemented in the brine treatment. The technologies included are reverse osmosis (RO), high-pressure RO, forward osmosis (FO), osmotically assisted RO (OARO), and membrane distillation (MD). Results showed that the MLD schemes costs ranged from US$0.79/m³ to US$1.36/m³, while the freshwater recovery ranged from 78 % to 89 %. In schemes 2 and 5, the implementation of MD substantially increased the energy consumption (>20 kW h/m³), however, these schemes were more economical (<US$1/m³) than the other 3 schemes. If the produced freshwater is sold, then the profit from the MLD treatment can reach up to US$2.21/m³. Furthermore, the costs of MLD schemes are at the same level as the subsurface water supplies, so MLD schemes can be valuable in countries relying on subsurface water sources.

Introduction

Brine is a by-product of various industries such as desalination plants, oil and gas industries, petrochemical industries, textile industries, steel industries, etc. Generally, brine effluents have high concentrations of total dissolved solids (TDS) (up to 400 g/L), heavy metals, and organics (Table 1) (Lester et al., 2015; Blondes et al., 2016; Jiménez et al., 2018; Panagopoulos and Haralambous, 2020b; Sahebi et al., 2020; Bagheri et al., 2018). Recently, the increase in the production of brine effluents has become a reason for concern. As estimated by the author, the volume of brine generated only from desalination plants is approximately 128,652,000 m³/day in 2019 which is comparable to the water volume of 56,800 Olympic-size swimming pools (Panagopoulos and Haralambous, 2020b). In the previous decade, brine management practices included direct disposal to oceans and rivers, deep-well injection, disposal into sewage plants, evaporation ponds, and land use. Recently, these discharge practices have been deemed unsustainable due to harmful environmental impacts on the marine environment (e.g., eutrophication, alteration of the water's pH/salinity) (Panagopoulos et al., 2019a; Kress, 2019; Panagopoulos, 2020a; Zhuang et al., 2019; Panagopoulos and Haralambous, 2020a). Furthermore, due to increasing environmental issues and stricter regulations, different approaches for the management of the brine effluent are being considered (Panagopoulos et al., 2019a; Sadhwani Alonso and Melián-Martel, 2018; Roberts et al., 2012).

To this aim, research focuses on the implementation of a zero liquid discharge (ZLD) framework, in which both freshwater and salt(s) are recovered without wastewater effluent generation. Approximately 100 % of freshwater is recovered under a ZLD system and the solid salt(s) can be discarded in a more environmentally sustainable manner (DuPont, 2020; Hermsen, 2016). A ZLD system involves two or more desalination technologies combined into one hybrid system. In ZLD systems, both membrane-based and thermal-based technologies are commonly implemented (Panagopoulos and Haralambous, 2020b). A conventional ZLD system is one comprising of a thermal-based brine concentrator (BC) and a thermal-based brine crystallizer (BCr). The saline wastewater is processed initially into the BC and afterward into the BCr. As a result, the freshwater produced is collected, while the remaining solid salts are either utilized or disposed of. Another ZLD system variation is the one incorporating the membrane-based reverse osmosis (RO), before inserting the saline wastewater into the BC. This modification resulted in a decrease in energy and cost demands; however, due to osmotic pressure constraints, RO cannot be implemented in the treatment of effluents with significantly high salinity (Escobar and Schäfer, 2009; Sridhar, 2018). As a result, the disadvantages of high energy usage and high capital costs have limited the widespread adoption of ZLD systems (Mickley, 2008; Yusuf, 2018; Xiong and Wei, 2017). Recently, some studies have been carried out on the use of only membrane-based technologies for ZLD desalination where MD is applied instead of thermal-based BC, BCr, etc. The main reason behind this selection is that MD does not face state-of-the-art technology limitations (namely, high pressures in RO and material corrosion in thermal-based technologies) (Zhao et al., 2020; Schwantes et al., 2018).

A novel alternative option for the sustainable management of brine effluents is the minimal liquid discharge (MLD) framework. The MLD approach has recently gained attention as it has significantly lower energy and cost demands, while the freshwater recovery target is very high (up to 95 %). In MLD systems, two or more desalination systems are combined into one hybrid system, as in ZLD systems. However, the implementation of only membrane-based technologies in the MLD systems leads to significant advantages (Panagopoulos and Haralambous, 2020b). The lower energy and cost demands can be attributed to the fact that membrane-based technologies are primarily used in MLD treatment schemes in contrast to ZLD schemes that adopt both membrane-based and thermal-based technologies (Panagopoulos et al., 2019a). Thermal-based technologies such as multi-stage flash distillation (MSF), multi-effect distillation (MED), mechanical vapor compression (MVC), thermal vapor compression (TVC), etc. are phase-changing processes with high energy intensity due to the high enthalpy of water vaporization (40.65 kJ/mol) (Panagopoulos, 2020a; Jaffe and Taylor, 2018). Membrane-based technologies, on the other hand, have the advantages of low operational energy requirements and a relatively simplified operating process. This is why membrane-based systems are used to separate a broad range of fluids such as water, wastewater, oil, chemical mixtures (e.g., separation of ethyl acetate/ethanol/water mixture), etc. (Meng et al., 2020; Wu et al., 2020; Sridhar, 2018). Membrane-based technologies include several technologies such as RO, electrodialysis (ED), electrodialysis reversal (EDR) pervaporation (PV), forward osmosis (FO), membrane distillation (MD), osmotically assisted reverse osmosis (OARO), high-pressure reverse osmosis (HPRO), etc. (Nicolaisen, 2003; Padaki et al., 2015; Dyer et al., 2000; Panagopoulos et al., 2019a). MLD framework is a strategy that, like the ZLD framework, follows the circular economy model, a new concept of viable development recently supported by the European Union (Bonviu, 2014; Ismail and Matsuura, 2016). The General Motors Assembly Plant (San Luis Potosi - Mexico) is an example of the MLD strategy as 90 % of the plant’s wastewater is recovered as freshwater with a combination of membrane-based technologies, namely reverse osmosis (RO), ion exchange (IEX), etc. (Veolia Water Technologies, 2014).

To determine the feasibility and economic efficiency of ZLD schemes, numerous research studies have been carried out. Such studies have been carried out for desalination (Guo et al., 2016; Wyk et al., 2020; Panagopoulos, 2020c), textile (Mohan et al., 2020; Bahadur and Bhargava, 2019; Rajakumari and Kanmani, 2008), food (Tabassum et al., 2015), oil and gas industries (Li et al., 2014; Han et al., 2020), etc. Nonetheless, it should be emphasized that all results were exclusively linked to ZLD after a thorough literature review by the author. To my knowledge, this is the first time that MLD treatment schemes are introduced and assessed. This research article provides an assessment of five MLD treatment schemes that can be used in the treatment of desalination brine. All treatment schemes consist only of membrane-based technologies (RO, HPRO, FO, OARO and MD) and are evaluated for their overall performance. Furthermore, brine treatment through MLD schemes is compared with brine discharge practices and conventional water sources. The research article is structured as follows: Section 2 describes both the MLD treatment schemes and the membrane-based technologies used in these treatment schemes, while the findings of the five MLD schemes assessment as well as the current status and prospects are discussed in Section 3. Finally, conclusions are pointed out in Section 4.