Abstract

A unique aqueous silica removal process using naturally occurring diatoms for water reuse and desalination is described. Several strains of brackish water diatoms have been isolated and tested. Among them Pseudostaurosira and Nitzschia species showed promise. Reverse osmosis (RO) concentrate samples from two full-scale advanced water purification facilities and one brackish groundwater RO plant in Southern California have been successfully treated by this process. This new photobiological process could remove aqueous silica, as well as phosphate, ammonia, nitrate, calcium, iron and manganese very effectively. Under non-optimized conditions, 95% of 78 mg·L−1 reactive silica in an RO concentrate sample could be removed within 72 hours. In most cases, addition of nutrients was not necessary because the RO concentrate typically contains sufficient concentrations of macronutrients derived from the source water (i.e., treated wastewater and brackish groundwater). Preliminary characterization of organics indicated that there was no major generation of dissolved organics, which could potentially foul membranes in the subsequent RO process. This new algal process has a strong potential for its application in desalination and water reuse in the United States and around the world.

INTRODUCTION

The drought in California is an unprecedented crisis and has made the state's water supply more vulnerable than it has ever been. Not only California, but other arid and semi-arid states and countries are facing an urgent need for alternative water resources as well. In recent years, more and more water utilities in the southwestern United States and around the world have begun exploring water from unconventional water resources, such as reclaimed water and brackish groundwater, using reverse osmosis (RO) (Greenlee et al. 2009; Pérez-González et al. 2012). Brine (concentrate) management and minimization has become a critical issue in RO-based water reuse and desalination projects, especially in inland areas where the means of brine disposal are limited. In order to minimize the volume of RO concentrate further, many advanced water treatment facilities are considering adding an additional stage of RO process to recover another 10% to 15% of usable water, although serious scaling due to the presence of inorganic scalants, including silica, calcium, and phosphate, is a major obstacle (Asano et al. 2007). In order to solve this challenge, a unique photobiological process utilizing selectively cultured diatoms has been developed to efficiently remove these inorganic scalants from RO concentrate so that additional RO can be employed to recover more fresh water (Ikehata et al. 2017). This approach will help reduce the environmental impacts of water reuse and brackish water desalination by harnessing the natural power of microalgae that has been known for decades, but largely overlooked in water and wastewater treatment.

Previously, rapid removal of reactive silica and orthophosphate was observed in a silica-rich brackish agricultural drainage water and an RO concentrate sample from the groundwater replenishment system (GWRS), Orange county water district (OCWD) using a mixed diatom culture obtained from an evaporation pond in the Central Valley of California (Ikehata et al. 2017). Silica was likely utilized by the diatoms in the silicification process (Lewin 1954; Martin-Jézéquel et al. 2000). One strain of diatom, Pseudostaurosira trainorii PEWL001, was isolated from the mixed culture, and an additional three strains, including Nitzschia communis PEWL002, Anomoeoneis sphaerophora PEWL003, and Halamphora sydowii PEWL004, were isolated from another water-sediment sample from the evaporation pond. In this study, these isolated strains, in particular P. trainorii PEWL001 and N. communis PEWL002 (Figure 1), were used to treat RO concentrate samples from different full-scale RO facilities in Southern California. The impacts of this algal treatment on dissolved organic matter (DOM) in the selected ROC were also studied.

Figure 1

Photomicrograph of (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

Figure 1

Photomicrograph of (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

MATERIALS AND METHODS

A brackish water diatom P. trainorii E. Morales PEWL001 was isolated from agricultural drainage water collected in the Central Valley of California, USA, during the summer of 2010 as described earlier (Ikehata et al. 2017). First, the drainage water sample was incubated at room temperature (∼25 °C) under continuous illumination over a period of time (∼10 days) until algal colonies became visible. Strains were then isolated from the colonies by a combination of serial dilution, agar plate, and micropipette techniques (Andersen & Kawachi 2005). Another brackish water diatom N. communis Rabenhorst PEWL002 was isolated from a drainage water sample collected from the same area in November 2015. The diatom seed cultures were maintained in 15 mL or 50 mL VWR clear polypropylene centrifuge tubes containing 0.2 μm filtered diluted synthetic seawater containing Guillard's F/2 medium (Guillard 1975) or 0.2 μm filtered RO concentrate sample from the GWRS (see below). The concentration of total dissolved solids (TDS) in the F/2 medium was 7 g·L−1, which is similar to that of the RO concentrate samples treated in this study.

RO concentrate samples were obtained from three full-scale RO facilities, including the GWRS of the OCWD in Fountain Valley, CA, USA, the Leo J. Vander Lans Advanced Water Treatment Facility (LVL AWTF) of the Water Replenishment District of Southern California (WRD) in Long Beach, CA, USA, and the Chino I Desalter of Chino Basin Desalter Authority/Inland Empire Utilities Agency (IEUA) on April 22nd, 2016, November 21st, 2013, and August 25th, 2016, respectively. The collected RO concentrate samples were characterized for basic water quality (Table 1) and kept refrigerated until use. The analytical methods used are also listed in Table 1.

Table 1

Basic water quality of RO concentrate samples collected from three full-scale RO facilities in Southern California

ParameterAnalytical MethodOCWD GWRSWRD LVL AWTFaChino I Desalter
Sodium (mg·L−1HACH ISENA38101 1,167 667 337 
Potassium (mg·L−1HACH 8049 171 71 11 
Calcium (mg·L−1HACH 8204 456 416 1,264 
Magnesium (mg·L−1Calculated 139 99 118 
Iron (mg·L−1HACH 8008 <0.02 0.24 0.03 
Copper (μg·L−1HACH 8143 <1 
Manganese (mg·L−1HACH 8149 0.396 0.358 0.375 
Ammonia-N (mg·L−1HACH 10023/10031 5.2 4.1 <0.02 
Boron (mg·L−1EPA 200.7 Rev 4.4 0.9 Not tested Not tested 
Chloride (mg·L−1HACH 8207 1,900 810 760 
Sulfate (mg·L−1HACH 8051 980 800 570 
Bicarbonate (mg·L−1HACH 8203 1,077 1,318 1,732 
Nitrate-N (mg·L−1HACH 10206 25 23 248 
Reactive silica (mg·L−1HACH 8185 133 78 146 
Orthophosphate (mg·L−1HACH 8048 5.6 8.5 1.04 
TDSs (mg·L−1Oakton TDSTestr2 6,690 3,880 4,260 
Turbidity (NTU) EPA 180.1 1.16 2.07 0.623 
Total hardness (mg·L−1 as CaCO3HACH 8213 1,720 1,453 3,650 
Alkalinity (mg·L−1 as CaCO3HACH 8203 883 1,080 1,420 
Total chemical oxygen demand (mg·L−1HACH 8000 245 154 129 
Dissolved chemical oxygen demand (mg·L−1)b HACH 8000 217 104 53 
Temperature (°C) Oakton TDSTestr2 20.2 Not tested 30.6 
pH Oakton pHTestr 2 7.98 8.2 7.3 
Color at 455 nm (PtCo unit) HACH 8025 271 96 
ParameterAnalytical MethodOCWD GWRSWRD LVL AWTFaChino I Desalter
Sodium (mg·L−1HACH ISENA38101 1,167 667 337 
Potassium (mg·L−1HACH 8049 171 71 11 
Calcium (mg·L−1HACH 8204 456 416 1,264 
Magnesium (mg·L−1Calculated 139 99 118 
Iron (mg·L−1HACH 8008 <0.02 0.24 0.03 
Copper (μg·L−1HACH 8143 <1 
Manganese (mg·L−1HACH 8149 0.396 0.358 0.375 
Ammonia-N (mg·L−1HACH 10023/10031 5.2 4.1 <0.02 
Boron (mg·L−1EPA 200.7 Rev 4.4 0.9 Not tested Not tested 
Chloride (mg·L−1HACH 8207 1,900 810 760 
Sulfate (mg·L−1HACH 8051 980 800 570 
Bicarbonate (mg·L−1HACH 8203 1,077 1,318 1,732 
Nitrate-N (mg·L−1HACH 10206 25 23 248 
Reactive silica (mg·L−1HACH 8185 133 78 146 
Orthophosphate (mg·L−1HACH 8048 5.6 8.5 1.04 
TDSs (mg·L−1Oakton TDSTestr2 6,690 3,880 4,260 
Turbidity (NTU) EPA 180.1 1.16 2.07 0.623 
Total hardness (mg·L−1 as CaCO3HACH 8213 1,720 1,453 3,650 
Alkalinity (mg·L−1 as CaCO3HACH 8203 883 1,080 1,420 
Total chemical oxygen demand (mg·L−1HACH 8000 245 154 129 
Dissolved chemical oxygen demand (mg·L−1)b HACH 8000 217 104 53 
Temperature (°C) Oakton TDSTestr2 20.2 Not tested 30.6 
pH Oakton pHTestr 2 7.98 8.2 7.3 
Color at 455 nm (PtCo unit) HACH 8025 271 96 

aThis sample was collected before the recent facility expansion, which involved the addition of third stage RO and was completed in 2014.

bFiltered through a 0.2 μm membrane filter.

A HACH DR-2800 spectrophotometer and a HACH 2100N turbidimeter (Loveland, CO, USA) were used for the colorimetric and turbidity analyses, respectively. A HACH ISENA38101 combined with an HQ40d portable meter was used for sodium analysis. Boron analysis was performed by TestAmerica (Irvine, CA, USA). An Oakton pHTestr2 and a TDSTestr2 (Vernon Hills, IL, USA) were used for the pH, TDS, and temperature measurement. UV-Vis and fluorescence analyses were conducted with a Varian Cary 100 Bio UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) and a Horiba FluoroMax-4 spectrofluorometer (Horiba Scientific, Edison, NJ, USA) in the Urban Water Research Center at the University of California, Irvine, CA, USA.

A series of RO concentrate treatment experiments were conducted in a bench-scale semi-batch mode using 500 mL polyethylene terephthalate (PETE) bottles (Φ = 65 mm) and VWR SuperClear 50 mL polypropylene centrifuge tubes with screw caps (Φ = 29 mm, VWR International, USA). These containers were placed in an illuminating reflective incubator with 9 W light-emitting diode (LED) bulbs (light temperature 5,000 K, 800 lm each; Cree, Inc., Durham, NC, USA). The LED bulbs emitted visible light radiation ranging from 400 to 750 nm with a sharp peak at 450 nm and a broader peak at 550 nm. The relative radiant power was two times higher at the former peak than at the latter one. The photosynthetically active radiation was measured as 1.6 μE·s−1·m−2 using an International Light Technologies ILT1400 portable radiometer with an attenuated PAR sensor (Peabody, MA, USA). The incubation temperature was at 25 ± 2 °C. Prior to the diatom inoculation, RO concentrate samples were filtered through 0.2 μm membrane filters. No chloramine residual was detected in the RO concentrate samples at the time of the treatment experiment. Pre-cultured diatom suspension (500 μL or 5 mL, respectively) was added to the 50- or 500 mL containers to initiate the photobiological treatment. The seed culture was pre-grown in the GWRS ROC or Guillard's F/2 medium as described above. The initial biomass concentration in each container was about 0.15 g dry weight L−1. Aliquots of samples were withdrawn periodically from the containers to measure color, reactive silica and orthophosphate concentrations during the treatment. Once reactive silica concentration was reduced below 1 mg·L−1, supernatant was removed from the containers by decantation while a majority of algal biomass was kept in the container. Fresh RO concentrate was added to the container for another semi-batch cycle. The supernatant was further analyzed for water quality. At the end of the last cycle of the semi-batch experiment, the dry weight of biomass was determined using the method described earlier (Ikehata et al. 2017). In the case of brackish groundwater RO concentrate treatment, sodium phosphate monobasic (ACS reagent; Sigma-Aldrich, St Louis, MO) or F/2 medium concentrate (no silica, F/2 Algae Food; Fritz Aquatics, Mesquite, TX) was added to adjust the initial orthophosphate concentration.

RESULTS AND DISCUSSION

As shown in Figure 2 the photobiological treatment using isolated diatoms was very effective in removing reactive silica and orthophosphate from RO concentrate samples obtained from two full-scale advanced water purification facilities, namely LVL AWTF and GWRS. Three semi-batch cycles were successfully performed in both cases, although the silica removal was apparently faster in the former RO concentrate sample (up to 35 mg·L−1·day−1) than the latter (up to 8 mg·L−1·day−1). The diatom growth and silica uptake might be inhibited by certain dissolved inorganic constituents, such as ammonia (Natarajan 1970; Azov & Goldman 1982) and copper (Florence & Stauber 1986), as well as organics such as herbicides (Debenest et al. 2009). In addition, the color of the latter RO concentrate sample was almost three times higher than the former sample (Table 1) and might have reduced the light available for photosynthesis. The rate of silica removal by the purified N. communis PEWL002 from GWRS RO concentrate was similar to that observed during the RO concentrate treatment using a mixed diatom culture (Ikehata et al. 2017). The silica removal accelerated in the second and third cycles, which implies that the diatom biomass concentration is an important factor. At the end of the third cycle, the biomass concentration was 2.1 g dry weight L−1.

Figure 2

Removal of reactive silica and orthophosphate from (a) LVL AWTF and (b) GWRS RO concentrate samples by the photobiological treatment using P. trainorii PEWL001 and N. communis PEWL002.

Figure 2

Removal of reactive silica and orthophosphate from (a) LVL AWTF and (b) GWRS RO concentrate samples by the photobiological treatment using P. trainorii PEWL001 and N. communis PEWL002.

The rates of silica removal by the two diatom species were almost identical in LVL AWTF RO concentrate in the first and second cycles. However, the silica removal by P. trainorii PEWL001 slowed down significantly in the third cycle, likely due to contamination by green algal cells (Ikehata et al. 2017). No contamination was observed during the LVL AWTF RO concentrate treatment with N. communis PEWL002, whereas a very similar contamination issue occurred in the case of the GWRS RO concentrate treatment with P. trainorii PEWL001, which implied that further purification of the latter diatom strain would be required. At the end of the third cycle, the biomass concentrations of P. trainorii PEWL001 and N. communis PEWL002 were 0.61 and 1.5 g dry weight L−1, respectively.

Figure 3 shows the removal of nutrients and RO scaling constituents by the photobiological treatment of LVL AWTF and GWRS RO concentrate samples using N. communis PEWL002. A similar result was obtained with P. trainorii PEWL001 (data not shown). A majority (>70%) of iron and manganese were removed by the photobiological treatment. In addition, two other major RO scaling factors, calcium and bicarbonate, were removed by more than 60%. The precipitation of calcium carbonate as calcite or aragonite was speculated (Borowitzka & Larkum 1987).

Figure 3

Removal of nutrients and scaling constituents from (a) LVL AWTF and (b) GWRS RO concentrate samples by the photobiological treatment using N. communis PEWL002.

Figure 3

Removal of nutrients and scaling constituents from (a) LVL AWTF and (b) GWRS RO concentrate samples by the photobiological treatment using N. communis PEWL002.

In those RO concentrates from the advanced water reclamation facilities, phosphorus was apparently the limiting nutrient. While ammonia was the preferred nitrogen source and was completely removed in the case of GWRS RO concentrate treatment (Figure 3(b)), both nitrate and ammonia were consumed simultaneously in the case of LVL AWTF RO concentrate treatment (Figure 3(a)). The reason for this difference is unclear because these RO concentrate samples contained fairly similar levels of phosphorus and nitrogen compounds (Table 1). Additional experiments are currently being conducted to explore this issue.

In addition to the RO concentrate samples from the two advanced water reclamation facilities, another sample from Chino I Desalter, which is a brackish groundwater desalination facility, was treated by the photobiological treatment. It was found that phosphorus in the RO concentrate sample was not enough (1.0 mg·L−1 as orthophosphate) to complete the silica removal (Figure 4; blue diamonds – please refer to the online version of this paper to see Figure 4 in color: http://dx.doi.org/10.2166/ws.2017.142). Therefore, phosphate was added as sodium phosphate or F/2 medium component. It was found that 5 mg·L−1 of orthophosphate was enough to completely remove 146 mg·L−1 of silica. The silica removal rate was 18 mg·L−1·day−1, although it accelerated in the second and third cycles (data not shown). Also, pure sodium phosphate was less effective than F/2 medium at facilitating silica removal (Figure 4). Trace minerals and/or vitamins in the F/2 medium (Guillard 1975) might have enhanced the diatom growth and silica uptake.

Figure 4

Removal of reactive silica from Chino I Desalter RO concentrate sample by the photobiological treatment using P. trainorii PEWL001: (a) reactive silica removal and (b) orthophosphate uptake.

Figure 4

Removal of reactive silica from Chino I Desalter RO concentrate sample by the photobiological treatment using P. trainorii PEWL001: (a) reactive silica removal and (b) orthophosphate uptake.

As the goal of this photobiological RO concentrate treatment is to enable the secondary RO without fouling and scaling, it is very important to characterize the organic matter after the photobiological treatment. Besides, it is well known that phytoplankton, including diatoms, excrete dissolved and particulate organic matter (Bjørrisen 1988; Biddanda & Benner 1997) and that seawater RO desalination is often affected by harmful algal brooms and organic particulate matter called transparent exopolymer particles associated with them (Caron et al. 2010; Villacorte et al. 2013). The preliminary analysis appeared to be very encouraging.

After the photobiological treatment of LVL AWTF RO concentrate sample using brackish water diatoms P. trainorii PEWL001 and N. communis PEWL002, filtered color (not shown), UV absorbance at 254 nm (not shown), and chemical oxidation demand (COD; Figure 5) were not significantly increased. A similar result was obtained when GWRS RO concentrate was treated in the same way.

Figure 5

Changes in dissolved and particulate chemical oxygen demand (COD) before and after the photobiological treatment of LVL AWTF RO concentrate sample using (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

Figure 5

Changes in dissolved and particulate chemical oxygen demand (COD) before and after the photobiological treatment of LVL AWTF RO concentrate sample using (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

Preliminary analysis of DOM was attempted using fluorescence spectrometry. As shown in Figure 6, the strong fluorescence peak due to the UV humic-like component (A peak), as well as weaker peaks due to the visible humic-like component (C peak), marine humic-like component (M peak), and protein-like component (T peak), was present in the excitation–emission matrix (EEM) of the raw (untreated) LVL AWTF RO concentrate sample, which is similar to that of the raw GWRS RO concentrate sample (not shown), as well as the reported EEM of RO concentrate from another RO facility (Bagastyo et al. 2011). The appearance of the EEM of photobiologically treated LVL AWTF RO concentrate was very similar to that of untreated ROC even after three semi-batch cycles (Figure 2(a)). The peak integrals and fluorescence were compared before and after the treatment as shown in Figure 7. Overall peak integral was decreased by the photobiological treatment using both P. trainorii PEWL001 and N. communis PEWL002. Peak A intensity decreased significantly (about 21%), especially with P. trainorii, indicating the humic-like component was degraded by the photobiological treatment. While the intensities of peaks C and M were also slightly decreased (14% of peak C, 18% of peak M in the case of the treatment with P. trainorii), the intensity of peak T did not change significantly in the RO concentrate samples after the photobiological treatment with both diatom species. More detailed analysis of DOM with EEM and size exclusion chromatography is currently underway. The impact of the photobiological treatment on trace organic compounds, such as pharmaceuticals and personal care products, and disinfection byproducts, in the RO concentrate samples is also being investigated.

Figure 6

Excitation–emission matrix (EEM) spectra of untreated LVL AWTF RO concentrate sample.

Figure 6

Excitation–emission matrix (EEM) spectra of untreated LVL AWTF RO concentrate sample.

Figure 7

Impact of the photobiological treatment on the LVL AWTF RO concentrate EEM peak integrals: (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

Figure 7

Impact of the photobiological treatment on the LVL AWTF RO concentrate EEM peak integrals: (a) P. trainorii PEWL001 and (b) N. communis PEWL002.

CONCLUSIONS

Three RO concentrate samples from three full-scale RO facilities in Southern California have been successfully treated by the photobiological treatment using isolated brackish water diatoms, P. trainorii PEWL001 and N. communis PEWL002, in laboratory-scale photo-bioreactors. The photobiological treatment could be performed with at least three cycles in a semi-batch mode. The rate of silica removal varied in the different RO concentrate samples, which indicated the presence of some inhibitory components in certain samples. Nutrient addition was not needed when the RO concentrate samples from advanced water treatment facilities (LVL AWTF and GWRS) were treated. However, the brackish groundwater RO concentrate tested in this study (Chino I Desalter) did not contain enough phosphorus to complete silica removal and its supplementation was required. In addition to silica, orthophosphate, calcium, iron, manganese, bicarbonate, ammonia, and nitrate were effectively removed by the photobiological treatment. Since many of them are responsible for RO scaling, there is a potential to use this technology as a pretreatment of RO concentrate from the primary RO to make the secondary RO more feasible, cost effective and environmentally friendly. Preliminary analysis of DOM showed no significant increase in organic matter that could cause RO membrane fouling.

ACKNOWLEDGEMENTS

The authors would like to thank Dr Kenneth P. Ishida, Mr Donald W. Phipps, Dr Jana Safarik, and Dr Megan H. Plumlee from OCWD, Fountain Valley, CA, Dr Cathy Chang, Dr Paul Fu, and Mr Howard Salamanca from WRD, Lakewood, CA, and Dr Jeff Noelte, Mr Brian Noh, and Ms Joanne Chan from IEUA for providing RO concentrate samples and valuable information and suggestions. The assistance of Dr Eduardo Morales from Herbario Criptogámico, Universidad Católica Boliviana San Pablo, Cochabamba, Bolivia, with diatom identification is also gratefully acknowledged. The authors would also like to thank Dr Barbara A. Cottrell from the University of California, Irvine, for her help on UV-Vis and EEM analysis. The assistance of Ms Yuan (Abby) Li, Dr Harshad V. Kulkarni, and Ms Susie Harris from Pacific Advanced Civil Engineering, Inc., Fountain Valley, CA, is also gratefully acknowledged. This work was financially supported by the National Science Foundation under the Small Business Innovation Research Program (Award #: 1648495, PI: KI).

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