Introduction

Salinity is an important parameter for wastewater treatment (Kartal et al. 2006). Wastewater is defined as “high-salinity” or brine when the inorganic salt contents range from 1 to 3.5% w/w, and seawater usually contains 3.5% w/w sodium chloride (Pernetti and Di Palma 2005). High-strength wastewater with variable salinity and nutrient loads was produced in a number of industrial processes (Jesus et al. 2017), such as aquaculture in coastal areas, the nuclear industry, agriculture and food-processing, petroleum and natural gas extraction and leather manufacturing (Calheiros et al. 2012; Lutz et al. 2013; Jesus et al. 2014; Lee et al. 2016; Zhang et al. 2019a). However, the discharge of untreated high-salinity wastewater may cause serious environmental pollution and affect the aquatic life, water potability, and agriculture (Lefebvre and Moletta 2006).

Therefore, it is necessary to use various methods and technologies to treat high salinity wastewater. Physiochemical techniques usually used to treat saline wastewater; however, they are energy-consuming with long startup time and high operating costs (Lefebvre and Moletta 2006). Nowadays, there are some documented cases where biotechnology has been used successfully for treating saline wastewater from multiple sources. However, a comprehensive assessment regarding the feasibility of using biotreatment for the treatment of high-salinity wastewater is not available. Therefore, the review aimed to (1) outline the major sources of high-salinity wastewaters and their environmental influences; (2) summarize the traditional treatment technologies for high-salinity wastewater; (3) emphasize the functions and mechanisms of major biotreatment technologies for high-salinity wastewater; and (4) put forward the future recommendations for biotreatment technologies for high-salinity wastewater.

Sources and environmental concerns of high-salinity wastewater

The composition and concentration of saline wastewater depend on their sources (Liang et al. 2017). There were several main sources of high-salinity wastewater: wastewater discharged during direct seawater use, aquaculture, agricultural runoff from saline-alkali lands, and industrial processes (Fig. 1).

Fig. 1
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Source and entry pathway of high salinity wastewater in the environment

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Due to the shortage of global freshwater resources, the direct use of seawater has become a feasible solution (Voutchkov 2018). Some coastal cities, such as Hong Kong and Qingdao, use seawater for fire control, road flushing, toilet flushing and other nondirected contacts with human beings, and it is considered an important option to relieve the pressure on fresh water (Chen et al. 2012; Li et al. 2018c). High-salinity wastewater discharged from the direct use of seawater drains into the sewer affects the subsequent wastewater treatment plant (WWTP) degradation process (Liu et al. 2016). In addition, China is the world's largest producer of aquaculture products, but the high level of mariculture is associated with many environmental issues (Xiang 2007). Inorganic nitrogen, active phosphate phosphorus, organics, and salts are the main pollutants in maricultural pollution areas (Liang et al. 2018). Seawater soda industries also produce a large quantity of high-salinity wastewater. For example, high concentrations of salts (NO3−: 188 g/L; total dissolved solids (TDS):1.63 × 105 mg/L) wastewater from some soda ash factories discharged into Arabian Sea were reported by Jadeja and Tewari (2007). Most desalination plants use reverse osmosis (RO) to desalinate seawater and brackish water; thus, these processes inevitably produce RO concentrate. The salinity of concentrate from seawater RO (SWRO) desalination facilities was up to 6.5–8.5 × 103 mg/L (Missimer and Maliva 2018).

The accumulation of soluble salts in the soil has caused land degradation, water quality deterioration and serious problems related to agricultural development (Cassel and Sharma 2018; Fang et al. 2005). More than 20% of agricultural land is threatened by salinization globally. The salinized soils covered approximately one-tenth of the Chinese total land area, accounting for 100 Mio. ha (Li 2010). However, the area of saline-alkali land is still expanding (Wei and Zhang 2018): saline-alkali soil area increased from 401.48 × 103 in 1954 to 1097.45 × 103 ha in 2005 in the western Songnen Plain, China (Yang et al. 2010), and continued desertification and salinization of the grasslands in the source regions of the Yangtze and Yellow Rivers was observed from 1968 to 2008 (Na et al. 2013). When the saline alkali soil area was over-irrigated or rainfall occurred, it became a source of saline wastewater (Liang et al. 2017). Approximately 1.3–2 × 108 m3 saline-alkaline farmland drainage flows into the south of Chagan Lake from the Qianguo irrigation area every year (Yang et al. 2015). Generally, the soluble salt concentration is characterized by the conductivity (EC) value or by measuring the TDS (Liang et al. 2017). In the Aksu Oasis area of Northwest China, the TDS in irrigation water increased from 1200 to 9.01 × 103 mg/L in drainage (Hu et al. 2019). In the Arys Turkestan Canal zone (Southern Kazakhstan), the TDS value of irrigated agricultural drainage exceeds 1200 mg/L, and Na+ and HCO3− are the main constituents of dissolving salts (Karimov et al. 2009). An average TDS of 1191.33 mg/L was detected in drainage water of the Fayoum watershed, Egypt (Abdel Wahed et al. 2015).

Industries such as printing, dyeing, refining, chemicals, mining, currieries, pharmaceuticals, and food processing may produce high saline wastewater. The wastewater from printing and dyeing practices had high pH, high turbidity, poor biodegradability, complex composition, and high chrominance and contained inorganic contaminants, such as chloride, heavy metals, sulfate, sulfide, and nitrogen (Xu et al. 2018). Hossain et al. (2018) reported that the TDS values of the knit dyeing and woven dyeing industries in Bangladesh were as high as 2000–3000 mg/L and 5000–6000 mg/L, respectively. In leather manufacturing, after tanning processes, approximately 40% of the chromium amount remains in the solid and liquid wastes, especially spent tanning solutions (Fabiani et al. 1996). In addition, saline wastewater is discharged during soaking, liming, deashing, pickling, chrome tanning, and finishing operations (Xiao and Roberts 2010). The TDS concentration in tanneries ranges from 65.4 to 1281.1 mg/L in Nigeria (Akan et al. 2007). In the Lokpaukwu-Ishiagu mining areas of southeastern Nigeria, it is conservatively estimated that nearly 3.3 × 104 m3 of untreated drainage of abandoned mines, including approximately 710,000 kg of dissolved solids and 586 kg of potentially toxic metals, ran out to the Ivo River watershed each year (Ezekwe et al. 2013). Pharmaceutical wastewater carries not only chemical oxygen demand (COD), ammonia, and suspended solids but also organic and inorganic constituents (e.g., spent solvents, catalysts, and reactants) (Fent et al. 2006; Lefebvre and Moletta 2006). For example, an average TDS of 22,168 mg/L with Cl−, PO43−, Na+, and K+ as the major salts were reported in pharmaceutical wastewater of a Singapore pharmaceutical factory (producing penicillin family antibiotics) (Ng et al. 2014).

A large amount of high-salinity untreated wastewater that is discharged directly is the source of high-strength wastewater, which causes great damage to the environment. High-salinity wastewater strongly reduces soil productivity, worsens the water environment, hinders economic development and threatens food production.

Treatment technologies for high-salinity wastewater

The treatment methods and technologies of high-salinity wastewater are mainly divided into physical, chemical, biological, ecological engineering and a combination of these technologies. Some examples of these methods are listed in Table 1. Physical and chemical technologies are widely used in high-salinity wastewater treatment, primarily including evaporation, membrane techniques, such as RO and nanofiltration (NF), ion exchange, advanced oxidation processes and electrochemical techniques (Hou et al. 2019; Liang et al. 2017). However, physical and chemical techniques are associated with some disadvantages, such as high operational costs and difficulty in achieving the expected treatment results. Biological treatment technologies have the advantages of low operational cost, obvious treatment effect and no secondary pollution. Researchers usually change the environment of bacteria to enhance their salt tolerance and halophilism to improve high-salinity wastewater treatment.

Table 1 Summary of high-salinity wastewater treatment methods
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Use of biological technologies for treating high-salinity wastewater

Biological technologies eliminate pollutants from wastewater through the reproduction and metabolism of microorganisms that are economical, highly effective, stable and environmentally friendly (Huang et al. 2019). Saline wastewater from industrial and aquaculture activities is frequently contaminated with nutrients and organics; thus, microorganisms are particularly important for the removal of COD and NH4+–N under high salinity conditions (Liang et al. 2017). However, high salinity of wastewater leads to osmotic pressure in microbial cells to exceed normal living conditions, and a massive die-off can change the sludge microbial communities (Pollice et al. 2010; Zhang et al. 2019b), which considerably reduces treatment efficiency and limits traditional biological treatment technologies. Biological treatment technologies of high-salinity wastewater generally include aerobic sludge plants (such as the traditional aerobic activated sludge process, aerobic granular sludge, the sequencing batch reaction (SBR) system, biofilms, and biofilters), anaerobic sludge plants, cultivation and domestication of salt-tolerant and halophilic bacteria from high salinity environment. Some examples of these methods are listed in Table 2. These reports have shown the potential use of biological technologies for saline wastewater treatment. For example, Ramaswami et al. (2019) found that fixed-bed reactor (FBR) with plastic media fed with NF permeate of landfill leachate yielded NH4+–N removals > 97% Muñoz Sierra et al. (2019). reported that anaerobic membrane bioreactors (AnMBR) exhibited a phenol removal of 96% at 26 g Na+/L. In this mini-review, we reviewed the use of biotechnology to treat high-salinity wastewater, as well as the function and purification mechanisms of halotolerant and halophilic microorganisms under salt stress.

Table 2 Use of biological technologies for high-salinity wastewater treatment
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Aerobic sludge plants

The traditional activated sludge process has been used in wastewater biological treatment for more than a century (Bengtsson et al. 2019) and can be directly used for treating wastewater with salinity below 10 g/L NaCl, regarding that most of microorganisms in activated sludge are non-halophilic which cannot survive with the salinity exceed 10 g/L (Tan et al. 2019). The properly domesticated microbes can adapt to high salt conditions (Hamoda and Al-Attar 1995). Aloui et al. (2009) described that under NaCl concentrations up to 4% (w/v) and organic loading rates up to 855 mg COD/L/d, the fish processing saline wastewater was efficiently treated with acclimatized activated sludge. At 2.0 wt% salinity, the SBR system inoculated with activated sludge achieved a 95% removal rate of COD, biochemical oxygen demand (BOD), NH4+–N and total phosphorus (TP) with Candidate_division_TM7 as the dominant bacteria genus (Zhao et al. 2016). Some studies also reported the removal of heavy metals under high-salinity conditions by the aerobic activated sludge process. Industrial saline wastewaters contain heavy metals, such as refinery effluents (Soda et al. 2011). Zhang et al. (2019b) studied the removal of selenite (Se4+) in artificial wastewater under high-salinity conditions of 70 g/L by using activated sludge in aerobic SBRs. The reactor removed soluble Se with relatively high efficiency in the beginning of the experiment. But the experiment failed because the activated sludge was not adapted to high salinity, and the removal efficiency was recovered from the 20th batch. di Biase et al. (2020) studied the removal of ammonia, thiocyanate, and cyanate in an aerobic up-flow submerged attached growth reactor to treat gold mine wastewater. The ammonia remove efficiency was achieved over 98% and the residual cyanate concentrations were below 2 mg-SCN-N/L.

Compared with the traditional activated sludge process, aerobic granular sludge (AGS) is denser, more compact and spherical, thereby occupying a lower footprint (Bengtsson et al. 2019; Beun et al. 1999). The effectiveness and stability of AGS in treating salt-containing wastewater has been evaluated in some studies. Pronk et al. (2014) evaluated the effect of stepwise increased salinity levels on nitrification, denitrification, nitrous oxide emissions, phosphate, and COD removal from synthetic wastewater. Ammonia oxidation was not affected at any salt concentration, but nitrite oxidation and phosphate removal were severely inhibited at 20 g Cl−/L. Hou et al. (2019) followed the performance and microbial characteristics of AGS under different salinities and alternating salinities. These researchers found that alternating salinity not only increased the COD removal efficiency but also generated a high concentration of granular biomass with good settling ability.

The effectiveness of biofilms and biofilters to treat high-salt wastewater has been discussed in several studies. Aslan and Simsek (2012) investigated the NO2–N/NOx–N ratio and NH4+–N removal efficiencies under various NaCl concentrations (0–40 g/L) under constant environmental conditions in a submerged biofilter reactor. These researchers found that the removal rate of NH4+–N (from 92 to 95%) increased with a small increase in salt concentration (1 g/L NaCl), and over this concentration, each NaCl addition induced NH4+–N oxidation. Navada et al. (2019) found that the ammonia oxidation capacity of moving bed biofilm reactors (MBBRs) was weakly influenced by the salinity increase rate, and the microbial community composition changed least for the largest salinity increment.

Anaerobic sludge plants

Anaerobic processes seem more attractive than aerobic processes because the processes consume little energy without aeration and can generate new energy in the form of methane or fuel alcohols (Xiao and Roberts 2010). Lefebvre et al. (2006) used an upflow sludge blanket reactor (UASB) reactor to study the anaerobic digestion of tannery soak liquor, and 78% COD removal efficiency was achieved at a TDS concentration of 71 g/L. Chung et al. (2009) observed approximately 100% removal efficiency of perchlorate and nitrate after 35 days of operation with denitrifying upflow packed-bed bioreactor (10% w/v NaCl concentration). Shi et al. (2014) treated pharmaceutical wastewater with high salinity through sequential anaerobic–aerobic treatment process and achieved 91.8–94.7% COD removal efficiency. Carrera et al. (2019) studied the development and stability of AGS in two SBR reactors treating fish canning wastewater. Their results demonstrated that the presence of anaerobic feeding/reaction phase increased the removal efficiency of organics (80–90%), which was higher than that using complete aerobic phase (75–85%).

In 1995, anaerobic ammonia oxidation (anammox) was first discovered unexpectedly in a denitrification fluidized bed reactor (Mulder et al. 1995). Anammox refers to the microbial anaerobic oxidation of ammonium by nitrite (NO2−), which forms N2; it is a chemoautotrophic process (Reitner and Thiel 2011; Strous et al. 1998). The anammox process has the potential to treat saline wastewater, as studies have been conducted in the Black Sea, Golfo Dulce, and the tropical east Pacific, which showed that anammox was the main pathway to remove active nitrogen from the ocean (Dalsgaard et al. 2003; Kuypers et al. 2003; Lam et al. 2009). Numerous studies have indicated that the processes related to anammox have been successfully applied to treat saline wastewater. We summarized two strategies to treat saline wastewater with high ammonia nitrogen through the anammox process: (1) acclimation of freshwater-derived anammox bacteria (FAB) and (2) enrichment of marine anammox bacteria (MAB) from salt lakes or marine sediments.

Acclimation of FAB to the saline environment is a practical strategy to treat saline wastewater with high ammonia nitrogen (Li et al. 2018b). Jin et al. (2011) demonstrated that the anammox upflow anaerobic sludge blanket (UASB) reactor performed well under 30 g NaCl /L in the continuous flow. Gonzalez-Silva et al. (2017) found that stepwise adaptation of FAB from 0 to 3 g NaCl /L took 153 days while taking only 40 days from 3 to 30 g NaCl /L, and the dominant genera were shifted from Candidatus Brocadia fulgida to Ca. Kuenenia stuttgartiensis was observed at 3 g NaCl /L. The nitritation-anammox process (SNAP) system was reported to be able to treat saline (3%) ammonium rich (185 mg/L) wastewater after gradual adaptation, and Kuenenia (anammox), Nitrosomonas (ammonia oxidizing bacteria, AOB) and Nitrosovibrio (AOB) bacteria were salt adaptable microbes (Ge et al. 2019).

Nakajima et al. (2008) first successfully established an enrichment culture of MAB in a column-type reactor. Li et al. (2018a, b, c) reported that the nitrogen removal performance of MAB in SBR was enhanced with Mn2+ and Ni2+ addition. Rios-Del Toro et al. (2017) raised a novel upflow anaerobic sediment trapped (UAST) reactor for enrichment of MAB and realized high nitrogen removal efficiencies (> 95%). The results obtained by these researchers indicated that Candidatus Kuenenia and Candidatus Anammoximicrobium had great potential for removing NH4+–N. These studies suggest that MAB can be applied to the treatment and recovery of high-salinity wastewater in the future.

Cultivation and domestication of salt-tolerant and halophilic bacteria

According to the microorganisms’ requirements for salt concentration classification, true halophilic organisms must grow with salt concentrations > 3 M NaCl, whereas salt-tolerant organisms do not depend on salt for growth but can tolerate appreciable salt concentrations (< 1 M NaCl) (Mokashe et al. 2018).

The physiological and biochemical mechanisms of salt-tolerant and halophilic bacteria to salt stress are complex. We are reviewed from three aspects: (1) maintain the balance of osmotic pressure. Mostly extremely halophilic archaea and bacteria accumulate compatible solutes (organic solutes: trehalose, alanine, proline, glycine betaine), maintain high inorganic solute concentrations (K+, Mg2+, Ca2+), or elevate the anionic phospholipid proportion of the cellular membrane to maintain the hydration state of the cytoplasmic membrane (Mokashe et al. 2018; Mukhtar et al. 2019; Nath 2016). Compatible solutes are mainly neutral but polar compounds that are easily soluble in water without interfere with cell metabolism (Sharma et al. 2016) and they also have protein-stabilizing properties that support the correct folding of peptides inside and outside the cell under deformed conditions (Street et al. 2006). Patel et al. (2018) reported that a halophilic bacteria strain Exiguobacterium profundum PHM11 accumulated L-proline to tolerate salinity. (2) Maintain enzyme activities by salinity. Most enzymes of halobacteria have robust activity and stability under hypersaline conditions and lose activity at salt concentrations lower than 2 M (Zhuang et al. 2010). Halophilic bacteria have been isolated from different marine and hypersaline environments by Setati (2010). These organisms have been shown to produce a wide array of hydrolytic enzymes, which can not only maintain activity at high NaCl concentrations but also have high resistance to denaturation. (3) Produce exopolysaccharides (EPS) appear to prevent injury from free radicals. The EPS have unique water holding and cementing properties, which can enhance cell water retention and protect bacteria from hydric stress and water potential fluctuations (Sharma et al. 2016). Boujida et al. (2018) isolated ten halophilic strains from different hypersaline environments, and all the halophilic bacteria EPS had high emulsifying and antioxidant activities. Singh et al. (2019) also reported the EPS obtained from a strain of halophilic bacterial has a potential as a natural antioxidant.

The treatment of high-salinity wastewater in a biological reactor acclimates common activated sludge by gradually increasing the salinity of wastewater, which undoubtedly increases the treatment time and cost. Halophiles prefer salt and thrive on saline or hypersaline environments. The cultivation and domestication of salt-tolerant and halophilic bacteria growing in extreme high-salinity environments (such as salt lakes, bay salt fields, saline-alkali soils, and pickled food factories) used directly to treat high-salinity wastewater is currently the research hotspot. A strain of Halomonas, isolated from coastal sediments, was able to anaerobically degrade 100% of Reactive Brilliant Red K-2BP in the presence of 10–15% NaCl (Guo et al. 2008). Abou-Elela et al. (2010) isolated a salt-tolerant microorganism (Staphylococcus xylosus) from a vegetable pickled plant wastewater and they found that the COD removal efficiency was up to 88% when S. xylosus was applied as solo inoculum for biodegradation in the case study to treat vegetable pickled acid wastewater with 7.2% salinity. Wu et al. (2013) adopted halophilic microorganisms (Bacillus sp. strain) in the aerobic process to treat pretreated wastewater in ethyl chloride production with 4% NaCl and achieved 58.3% COD removal. Maharja et al. (2017) used halophilic bacteria in sequential oxic-anoxic bioreactor to remove dissolved organics and suspended soilds from tannery saline soak liquor. Tan et al. (2017) found a good biodegradability of the domesticated marine activated sludge in treating the industrial phenolic waster, and the removal of phenol, COD and NH4+–N achieved 99%, 80% and 68%. Halophilic purple phototrophic bacteria (PPB) isolated from Brisbane river sediments were reported to be able to remove COD, nitrogen, and phosphorus from high salinity domestic wastewater (Hulsen et al. 2019). These observations indicate that biological treatment techniques inoculated with specific halophilic microorganisms provide a promising avenue for the treatment of saline wastewater.

Future recommendations

Published investigation on the biotreatment of high-salinity wastewater indicated that the cultivation and domestication of salt-tolerant and halophilic microorganisms could be a promising technique. However, unfortunately, study on the practical manufacturing wastewater in saline conditions is scarce. Synthetic wastewater has single pollutant component and low salt concentration compared to actual wastewater. Therefore, it is necessary to characterize salt-tolerant or halophilic strains able to degrade contaminant in actual high salinity wastewater and future efforts are required to transition from lab to on field applications.

Many researchers studied the removal efficiency of different industrial sectors by biotreatment technologies. Most of them focus on the traditional high salinity wastewater resources such as currieries and food processing industries, however, different target pollutants in different types of high-salt wastewater. High-salinity wastewater from many industries such as the nuclear industries, petroleum and natural gas extraction, and mining has not been studied, therefore shock loading from different industrial sectors is warranted to enrich biological treatment process for efficient removal of targeted contaminants under high salinity conditions.

Many studies involving biodegradation of specific compounds by the pure and mixed culture of microbes under high-salinity conditions has been conducted. But, the complete degradation pathway, microbial catabolic enzymes involved in the degradation process and the interactions between salt, specific contaminants and microbes are still unknown. Identification and development of bacterial metabolic enzymes and their corresponding degradation pathway under high-salinity conditions are strongly recommended for future study.

Conclusion

Biological technologies are resource-saving and environmentally friendly and have been widely employed in different types of wastewater treatment. Saline wastewaters from different sources usually contain complex pollutants, such as NaCl, and high concentrations of various salts, organic compounds, heavy metals, pesticides, antibiotics and so on. These pollutants can cause land degradation, water quality deterioration and serious problems related to agricultural development. The current status and potential of biological treatment of high-salinity wastewaters were reviewed in this paper. Although high salinity inhibits the growth of microbes, prior studies have shown that the acclimated microorganisms can gradually adapt to the environment of high salinity and can be used to treat saline wastewater. However, more studies are needed to investigate the different types of target contaminants and the interactions between salt and specific contaminants in activated sludge. In addition, it is suggested to carry out the purification culture and domestication of salt-tolerant and halophilic bacteria under practical conditions in future research.