Optimizing Photobiological Treatment of Reverse Osmosis Concentrate




Gao, Han

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Water scarcity as an emerging concern worldwide highlights the importance of alternative water management and augmentation strategies. As one of the important water treatment technologies, reverse osmosis (RO) has been widely used in increasing number of brackish water desalination facilities (BWDFs) and advanced water purification facilities (AWPFs). However, lack of efficient and economical RO concentrate (ROC) managements, associated with RO scaling from elevated silica (SiO2) and calcium raised challenges like permeation reduction and membrane scaling. In recent years, a diatom- based photobiological treatment has been studied to remove dissolved silica, calcium, nutrients, and other constituents from ROC prior to the secondary RO. Although the technical feasibility of the photobiological treatment has been demonstrated, no research was done to systematically study the impacts of light and temperature on the new process. In this study, different parameters such as light temperatures, intensities and colors, illumination duration, and incubation temperatures were tested to optimize silica uptake rate in the photobiological treatment with brackish water diatom Gedaniella flavovirens Psetr3 and to produce more freshwater and reducing capital cost. ROC samples from Orange County Water District, Groundwater Replenishment System(OCWD GWRS, Fountain Valley, CA) and San Antonio Water System, H2Oaks Center (SAWS H2Oaks, San Antonio, TX) were used as model ROCs from AWPF and BWDF. Furthermore, the treatability of several other ROCs, applicability of using sunlight as alight source, and removal of N-nitrosamines, including N-nitrosodimethylamine (NDMA) and N-nitrosomorpholine (NMOR) from OCWD GWRS ROC, were investigated in this research. Light temperatures (2,700, 3,000, 4,000 and 5,000 K) did not impact silica uptake rate significantly. The difference between light colors (red, green, yellow, blue, and white)had no marked impact on SAWS H2Oaks ROC, but for OCWD GWRS ROC, the blue light resulted in a slightly higher (~28 mg/L/day) silica uptake rate than other colors (~23mg/L/day). However, blue light bulbs were not recommended to use as a light source because the light output was approximately six times weaker than other colored bulbs. The photosynthetically active radiation (PAR) of 200 µmol m-2 s-1 was found to be sufficient for the photobiological treatment as the uptake rate was around 40 mg/L/day. The silica uptake was slower (30 to 35 mg/L/day) at lower PAR values (50 and 100µmol m-2 s-1). Intermittent light with 12 hours light and 12 hours dark did not slow down the silica uptake. The optimum temperature for the photobiological treatment was found to be 23 to 30 °C. The silica uptake was much slower at 10 °C, while the diatoms could not survive at a higher temperature (40 °C), which exhibited no silica removal. Four additional ROCs were tested and confirmed the treatability with the diatom-based photobiological treatment, excluding the one from West Basin Municipal Water District, which has a high concentration of ammonia (310 mg/L as N) and known to be toxic for the diatom treatment in previous studies. The ROCs from the Closed Circuit RO at OCWD and Hamby Water Reclamation Facility (Abilene, TX) could be treated in three cycles with an average silica uptake rate of 33 mg/L/day (for both). Kay Bailey Hutchison Water Treatment Plant (El Paso, TX) ROC was treatable repeatedly with a silica uptake rate of 31 mg/L/day with supplementary nutrients of 10 mg/L of nitrate-N and 5.5 mg/L of orthophosphate. Unlike the experiments in the laboratory using LED as a light source at controlled temperature, the photobiological experiment carried out outdoors was affected by variable factors such as weather and temperatures. In the 1st run, strong UV radiation(~5.2 W m-2) had a negative impact on the diatoms and silica uptake rates. The high temperature (highest average of 42.5 °C) also killed/bleached the diatoms in the 2nd run. In the 3rd attempt, three cycles of repeating silica uptake were successful under average temperature of 21 ± 5 °C and UV radiation of <0.2 W m-2. NDMA and NMOR could be degraded simultaneously by the diatom-based photobiological treatment using sunlight as a light source. Based on the experimental results, the highest silica uptake for OCWD GWRS and SAWS H2Oaks ROCs were 62 and 44 mg/L/day, respectively. With a desired treated silica concentration of 60 mg/L, a 20-million gallon (LWH: 1,330’ × 1,000’ × 2’) photobioreactor would be needed on the site of OCWD GWRS to treat its 17.6 MGD of ROC, while the reactor size would be 1.8 million gallons (LWH: 600’ × 200’ ×2’) for SAWS H2Oaks Center to treat its 1.11 MGD of ROC. This research revealed the impacts of different factors on silica uptake rate using diatom-based photobiological treatment and treatability study of different ROCs. The results of the outdoor experiments verified many of my lab experimental results such as the impacts of temperature, light intensity, as well as the non-requirement of continuous light, which will help our future research with larger scale continuous flow pilot photobioreactors.



esalination, water reuse, diatom, Gedaniella flavovirens, brackish water, light, temperature


Gao, H. (2021). Optimizing photobiological treatment of reverse osmosis concentrate (Unpublished thesis). Texas State University, San Marcos, Texas.


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