Water shortage is becoming more common due to droughts and global population increases resulting in the increasing popularity of water reuse to create new water sources. Reverse osmosis (RO) membrane systems are popular in these applications since they can produce drinking water quality effluent. Unfortunately, RO systems have the drawback of generating concentrate streams that contain contaminants rejected by the membrane including chemicals of emerging concern (CECs). CECs are chemicals such as hormones, steroids, pesticides, pharmaceuticals, and personal care products that are used for their intended purpose and then released into wastewater. CECs are believed to be detrimental to aquatic wildlife health and pose an unknown human health risk. This research gathered the existing knowledge on CEC presence in concentrate, available proven concentrate treatment methods, their CEC removal abilities, and current CEC regulations. It was found that 127 CECs have been measured in RO concentrate with 100 being detected at least once. The most potent treatment process available is UV/H2O2 as it offers the highest removal rates for the widest range of chemicals. The less expensive process of ozone/biologically activated carbon offers slightly lower removal abilities. This comprehensive report will provide the groundwork for better understanding, regulating and treating concentrate stream CECs.

INTRODUCTION

Water reuse is becoming increasingly popular as an answer to water shortage throughout the world. Limited water supplies are over-exerted and further polluted by the growing population reliant upon them.

Reverse osmosis (RO) has become a popular treatment process in water reuse as it produces very high quality effluent. The membranes used in RO are capable of removing contaminants that pass through other treatment processes making it likely that their use will continue to increase. Unfortunately, the RO process produces a brine concentrate stream that contains elevated concentrations of the contaminants removed by the membrane.

A constituent of this concentrate is chemicals of emerging concern (CECs). These are chemicals such as hormones, steroids, pesticides, pharmaceuticals, and personal care products that are present in wastewater. They are used for their intended purpose and then released into wastewater. These compounds have been found to have a detrimental health impact on aquatic life and pose an unknown, although likely minimal, health risk to humans. (Snyder et al. 2008).

Concentrate from RO systems in coastal areas is often discharged into the ocean without treatment. The CECs that are present in concentrate are at elevated concentrations making its discharge most dangerous to organisms directly exposed to the flow. There is also concern about these CECs existing in the receiving waters further away from plant outfalls.

The purpose of this research was to gather the existing knowledge about which CECs are present in brine, the available brine treatment methods, their CEC removal abilities, and the current CEC regulations. The research began by compiling a list of the known CECs in RO brine. This database includes the minimum, maximum and average CEC concentrations given in a comprehensive list. The data presented were gathered from previous research as well as questionnaires sent to various treatment plants with RO systems used for water reuse applications. A study was then conducted to determine the treatment methods which are most effective at reducing the risk posed by the chemicals in this database. This includes the strengths and weaknesses of each system. Research concluded by determining the regulations in place for CECs around the world.

The information presented in this article is the current knowledge on the topic of CECs in RO concentrate. It will aid in understanding, regulating and treating the chemicals to help protect aquatic life and public health.

BACKGROUND

Knowledge on brine CEC removal is contained in many individual reports with no centralized database of knowledge. A literature review was conducted to gather this knowledge. The results are presented in four main sections: (1) the known health issues posed by CECs; (2) the regulations that have been imposed for CECs present in treatment plant effluent; (3) the CECs that have been found in RO brine; and (4) the treatment methods being researched for removing CECs.

ISSUES POSED BY EFFLUENT CECS

The presence of CECs was first discovered in surface water sources and wastewater in the United States and Europe in the 1960s. Concerns about potential environmental risks came about in 1999 when the presence of pharmaceuticals in a river was connected to the feminization of fish living downstream from a wastewater treatment plant. Around the same time, the non-steroidal anti-inflammatory drug diclofenac was attributed to the renal failure of vultures. This poisoning has been blamed for a decline of more than 95% of the bird's population in India since the 1990s. Public awareness became even more heightened when a study found that organic wastewater contaminants, including CECs, were present in at least 111 of 139 streams tested. While it has been found that CEC levels in the environment are usually only trace concentrations due to dilution effects, their chemical persistence, microbial resistance and synergistic effects are still unknown. There has also been evidence that even low concentrations can have adverse effects on aquatic life (Deegan et al. 2011).

Pharmaceutical use has become widespread as drug manufacturers continually develop new medicines to treat an increasing number of health issues. A large proportion of society is aging and relying more heavily on these new drugs. This is becoming an issue since the human body only absorbs a proportion of the medicine and then releases the unused proportion in waste that ends up in wastewater treatment plants. It is believed that 30–90% of medicine taken by the population enters unchanged into treatment plants (Watkinson et al. 2007). This flow is a problem as conventional treatment plants are not designed to remove these contaminants. It has been reported that a wastewater treatment plant in Rubí, Spain that serves 125,550 people (which produces an average daily flow rate of 22,000 m3 per day) discharges approximately 300 g (0.66 lbs) of pharmaceuticals every day (Radjenovic et al. 2006).

Testing has shown that typical removal rates of CECs in wastewater treatment plants are only 60–90% (Carballa et al. 2004). This incomplete elimination adds further reason for concern on the impact of plant effluent on the environment. The impact of effluent CECs was proven by a 2003 study that detected changes in the structure and function of algal communities that were in receiving streams (Watkinson et al. 2007).

Wastewater treatment plants in many coastal communities discharge their effluent into the ocean exposing sea life to CECs. A study conducted in 2006 found that marine flatfish living near treatment plant outfalls were being affected by endocrine disruptors in the flow. Male fish displayed elevated levels of plasma vitellogenin and estradiol. Other studies have also shown that fish exposed to ethinylestradiol levels measured in the environment exhibited altered reproduction abilities due to the estrogenic effects of the chemical (Vidal-Dorsch et al. 2012).

Tests on CEC levels at treatment plant outfalls show an environmental impact risk. Concentration levels between 0.0004 and 0.0009 μg/L of a wide range of contaminants have been detected. Several recalcitrant CECs including gemfibrozil, oxybenzone and sulfamethoxazole have even been detected well away from the outfalls (Vidal-Dorsch et al. 2012).

CECs have also been detected in drinking water sources in the United States. DEET has been found to occur most frequently and is not readily removed by standard drinking water treatment processes. Carbamazepine and dilantin are also frequently detected in raw water and have shown some persistence in treatment processes. Atrazine has repeatedly been found to have the highest concentration in raw waters with concentrations reaching 571 ng/L (Snyder et al. 2008).

No cases of human health impacts attributed to CECs in treatment plant effluent were discovered in the course of this study. The minimal exposure of humans to effluent CECs limits the potential danger they pose. Despite this, groundwater contamination from chromium VI in Hinkley, California in 1993 and water contamination warnings in several cities on the US East Coast in early 2014 show the concern that exists on this topic.

Regulations on CECs

No regulations specifically on RO concentrate CECs were found during the course of this study. Regulations on wastewater treatment plant CECs and drinking water CECs have begun to be established.

The United States does not have nationwide regulations on CECs in wastewater treatment plant effluent. Permits issued by the National Pollution Discharge Elimination System (NPDES) focuses on basic contaminants and not trace organic compounds (Audenaert et al. 2014).

The Environmental Protection Agency (EPA) has currently established primary drinking water regulations for approximately 90 contaminants ranging from coliform and viruses to heavy metals and disinfectants. Many of these have been found to be discharged from treatment plants. Even though this list is fairly large, there are still a significant number of contaminants for which regulations have not been implemented. These can be found in the EPA's Contaminant Candidate List. This list is currently on version 3 and contains 104 chemicals and 12 microbiological contaminants. The EPA uses this list to further its research on the listed constituents to determine whether they need to be regulated. Even with its large database and continued research, the EPA itself does not have the jurisdiction to impose regulations on the chemicals it deems hazardous. States are left with this role in determining which chemicals need to be regulated (EPA 2012).

California is the one state setting the standard in CEC regulation by using the EPA's 2012 Guidelines for Water Reuse to institute regulatory policies. Beginning in 2009, California's State Water Resource Control Board (SWRCB) implemented a new recycled water policy which allowed for extensive research on the risks posed by CECs and recommendations of how CECs should be monitored. The findings were published in the report Monitoring Strategies for Chemicals of Emerging Concern (CECs) in Recycled Water – Recommendations of a Scientific Advisory Panel. Standards were established in this report on which chemicals should be monitored and what acceptable concentrations of each should be. The influence of this publication can be seen in California as every California Regional Water Quality Control Board report on treatment plants consists of a requirement for a special study on plant effluent CECs including a plan for continual monitoring of CEC concentrations along with their frequency (Anderson et al. 2010).

International regulation of CECs varies with each country's environmental agencies. The European Union, Germany and Switzerland have established lists of chemicals known to cause environmental harm while pushing for their discontinued use. Switzerland has also proposed an indicator list consisting of carbamazepine, diclofenac, sulfamethoxazole, benzotriazole and mecoprop (Kazner 2011).

The regulations in Europe are leading to the installation of ozonation systems to protect the environment. A push is also underway to gather monitoring data in order to set a regulatory framework. Watch lists (similar to those by the EPA but focusing on plant discharge) are under development to aid in environmental protection. Switzerland is the leader in this work as they are the first country to set some nationwide regulations and indicator compounds (Audenaert et al. 2014).

Many other nations follow the World Health Organization's (WHO) guidelines on the maximum allowable concentrations of chemicals in drinking water. These recommendations are found in the chemical fact sheet in the WHO's ‘Guidelines for Drinking-Water Quality’ report. The fact sheet also provides the acceptable daily intake, recommended treatment processes, occurrence, and limit of detection concentrations.

CECs present in concentrate

There is limited knowledge on the CECs that are present in RO concentrate. Running tests to measure CEC concentrations is an expensive and time-consuming task meaning that even advanced research studies are limited in the number of CECs they can test for at one time. Cross-referencing multiple studies has helped build a more comprehensive list but more experiments need to be conducted to expand this knowledge.

Fewer CECs have been found in RO concentrate than in plant influent. Treatment systems commonly used before RO systems have been shown to remove CECs with varying degrees of success. It has been found that personal care products such as tonics and musks are well-removed in primary treatment with most of the remainder removed during biological treatment (Carballa et al. 2004). Several pharmaceuticals including ibuprofen, naproxen and sulfamethoxazole are largely removed during biological treatment (Carballa et al. 2004). Some plants also then employ filtration systems that offer some additional removal of certain CECs. Even with these multiple barriers, however, removal rates remain at unsatisfactory levels. The chemicals that remain after these treatment methods are then subjected to the RO membrane.

RO membranes are known to offer high removal rates of CECs while yielding very high quality water. Unfortunately, the process concentrates the removed contaminants into a brine concentrate stream. This concentrate flow has increased toxicity levels that require additional treatment to alleviate. Even the common practice of dilution through mixing the concentrate flow with other plant effluent provides no net improvements in environmental safety (Snyder et al. 2007).

A compilation of the CECs and their concentrations measured in RO concentrate by studies around the world was assembled into a single table as part of this study. It was found that a total of 127 CECs have been measured in RO concentrate with 100 CECs being present at detectable levels. Several of these CECs were detected in over ten separate measurements. These include atenolol, caffeine, carbamazepine, diclofenac, gemfibrozil, ibuprofen, naproxen, sulfamethoxazole, triclosan, trimethoprim, TCEP, and DEET. All are commonly used in society so it is not surprising to find them in the concentrate. A summary of these results is shown in Tables 1 and 2.

Table 1

Concentrations of pharmaceutical CECs measured in RO concentrate

Chemicals Chemical uses Number of tests Number of detects Number of unmeasurable detects Number of non-detects Minimum concentration (μg/L) Maximum concentration (μg/L) Average concentration (μg/L) Standard deviation 
Pharmaceuticals 
 4-AAA Analgesic metabolite 11.847 15.569 13.708 2.632 
 5,5-diphenylhydantoin Anticonvulsant 0.145 0.145 0.145 – 
 Acebutolol Beta blocker 0.760 0.760 0.760 – 
 Acetaminophen Analgesic ND 0.150 0.041 0.050 
 Amoxycillin Antibiotic ND 21.887 21.887 – 
 Atenolol Beta blocker 13 13 0.465 49.739 9.051 16.743 
 Atorvastatin Statin ND ND ND – 
 Azithromycin Antibiotic 1.400 343.244 115.432 139.508 
 Bacitracin Antibacterial ND ND ND – 
 Bezafibrate Fibrate 0.500 0.583 0.542 0.059 
 Bisoprolol Beta blocker 0.940 0.940 0.940 – 
 Butalbital Barbiturate 0.298 0.298 0.298 – 
 Caffeine Stimulant 14 14 0.025 50.000 6.949 15.275 
 Carbamazepine Anti-epileptic 26 26 0.112 7.266 1.666 1.807 
 Carisoprodol Muscle relaxant 1.905 1.905 1.905 – 
 Cefaclor Antibiotic ND ND ND – 
 Celiprolol Beta blocker 1.800 1.800 1.800 – 
 Cephalexin Antibiotic ND ND ND – 
 Chlortetracycline Antibiotic ND ND ND – 
 Cimetidine Anti-histamine ND 6.438 6.438 – 
 Ciprofloxacin Antibiotic ND 0.430 0.430 – 
 Clarithromycin Antibiotic 0.800 36.800 23.691 16.725 
 Clindamycin Antibiotic ND ND ND – 
 Clopidogrel Blood thinner 4.509 5.852 5.201 0.672 
 Codeine Opiate 0.673 2.031 1.362 0.605 
 Dehydronifedipine Human plasma metabolite 2.105 2.105 2.105 – 
 Diazepam Anti-epileptic 0.007 0.774 0.533 0.356 
 Diclofenac Anti-inflammatory 12 12 0.001 1.500 0.436 0.441 
 Dilantin Anti-epileptic 0.270 1.753 0.779 0.484 
 Doxycycline Antibiotic ND ND ND – 
 Enrofloxacin Antibiotic ND ND ND – 
 Erythromycin Antibiotic 14 11 7.984 1.332 2.255 
 Erythromycin-H2Antibiotic – 
 Famotidine Anti-histamine 0.000 3.958 2.500 1.881 
 Fenofibric acid Receptor agonist 0.800 1.480 1.140 0.481 
 Fluoxetine Antidepressant 0.017 1.143 0.202 0.382 
 Furosemide Diuretic ND 2.522 2.522 – 
 Gemfibrozil Cholesterol reducer 21 21 0.000 11.997 2.436 3.412 
 Glibenclamide Antidiabetic 0.016 0.016 0.016 – 
 Hydrochlorothiazide Diuretic 0.214 27.000 15.321 13.718 
 Hydrocodone Analgesic 0.015 0.541 0.270 0.174 
 Ibuprofen Anti-inflammatory 13 13 0.033 21.250 2.953 6.146 
 Indometacin Anti-inflammatory 0.895 0.895 0.895 – 
 Iohexol X-ray contrast media 2.400 71.300 31.236 29.021 
 Iomeprol X-ray contrast media 0.386 3.900 2.143 2.485 
 Iopamidol X-ray contrast media 2.626 2.626 2.626 – 
 Iopromide X-ray contrast media 11 10 ND 7.000 1.365 2.480 
 Ketoprofen Anti-inflammatory 0.429 0.429 0.429 – 
 Lidocaine Anesthetic 0.418 0.418 0.418 – 
 Lincomycin Antibiotic ND 0.057 0.057 – 
 Lorazepam Anti-epileptic 2.880 6.473 5.176 1.994 
 Lovastatin Statin ND ND ND – 
 Mefenamic acid Anti-inflammatory 0.001 0.001 0.001 – 
 Meprobamate Tranquilizer 10 10 0.780 7.972 2.457 2.283 
 Metoprolol Beta blocker 0.033 8.553 2.056 2.935 
 Metronidazole Antibacterial 7.447 8.247 7.847 0.566 
 Monensin Antibiotic ND ND ND – 
 Nadolol Beta blocker 0.000 1.014 0.667 0.578 
 Nalidixic acid Antibiotic 0.085 0.189 0.137 0.074 
 Naproxen Anti-inflammatory 18 18 0.000 9.223 1.374 2.223 
 Nicotine Receptor antagonist 0.912 5.683 3.298 3.374 
 Nifedipine Calcium channel blocker 0.273 0.273 0.273 – 
 Norfloxacin Antibiotic 0.120 0.120 0.120 – 
 Ofloxacin Antibacterial ND 25.309 9.526 9.793 
 Oleandomycin Antibiotic 0.010 0.010 0.010 – 
 Oxytetracycline Antibiotic ND ND ND – 
 Paroxetine Antidepressant 0.508 0.508 0.508 – 
 Penicillin G Antibiotic ND ND ND – 
 Penicillin V Antibiotic ND ND ND – 
 Pentoxifylline Blood thinner 0.000 0.000 0.000 – 
 Primidone Anti-epileptic 0.320 0.905 0.551 0.236 
 Propranolol Beta blocker 1.050 7.200 4.246 2.602 
 Propyphenazone Anti-inflammatory 0.156 0.258 0.207 0.072 
 Ranitidine Anti-histamine ND 8.240 5.825 2.216 
 Roxithromycin Antibiotic 0.150 0.150 0.150 – 
 Salbutamol Bronchodilator 0.198 0.889 0.588 0.310 
 Salinomycin Antibacterial ND ND ND – 
 Sotalol Beta blocker 0.022 6.196 3.409 2.818 
 Sulfadiazine Antibiotic 0.472 0.472 0.472 – 
 Sulfamerazine Antibiotic ND ND ND – 
 Sulfamethazine Antibacterial ND 0.635 0.635 – 
 Sulfamethizole Antibiotic ND ND ND – 
 Sulfamethoxazole Antibiotic 24 23 ND 8.638 3.085 2.640 
 Sulfasalazine Antibiotic ND 0.045 0.045 – 
 Sulfathiazole Antibiotic ND ND ND – 
 Tetracycline Antibiotic ND ND ND – 
 Theobromine Stimulant 1.010 1.010 1.010 – 
 Theophylline Bronchodilator 0.753 0.753 0.753 – 
 Timolol Beta blocker 0.018 0.018 0.018 – 
 Triclosan Antibacterial 14 14 0.008 3.371 0.923 1.249 
 Trimethoprim Antibacterial 14 14 0.000 1.172 0.512 0.394 
 Tylosin Antibiotic 0.005 0.005 0.005 – 
 Venlafaxine Antidepressant 0.333 0.333 0.333 – 
Chemicals Chemical uses Number of tests Number of detects Number of unmeasurable detects Number of non-detects Minimum concentration (μg/L) Maximum concentration (μg/L) Average concentration (μg/L) Standard deviation 
Pharmaceuticals 
 4-AAA Analgesic metabolite 11.847 15.569 13.708 2.632 
 5,5-diphenylhydantoin Anticonvulsant 0.145 0.145 0.145 – 
 Acebutolol Beta blocker 0.760 0.760 0.760 – 
 Acetaminophen Analgesic ND 0.150 0.041 0.050 
 Amoxycillin Antibiotic ND 21.887 21.887 – 
 Atenolol Beta blocker 13 13 0.465 49.739 9.051 16.743 
 Atorvastatin Statin ND ND ND – 
 Azithromycin Antibiotic 1.400 343.244 115.432 139.508 
 Bacitracin Antibacterial ND ND ND – 
 Bezafibrate Fibrate 0.500 0.583 0.542 0.059 
 Bisoprolol Beta blocker 0.940 0.940 0.940 – 
 Butalbital Barbiturate 0.298 0.298 0.298 – 
 Caffeine Stimulant 14 14 0.025 50.000 6.949 15.275 
 Carbamazepine Anti-epileptic 26 26 0.112 7.266 1.666 1.807 
 Carisoprodol Muscle relaxant 1.905 1.905 1.905 – 
 Cefaclor Antibiotic ND ND ND – 
 Celiprolol Beta blocker 1.800 1.800 1.800 – 
 Cephalexin Antibiotic ND ND ND – 
 Chlortetracycline Antibiotic ND ND ND – 
 Cimetidine Anti-histamine ND 6.438 6.438 – 
 Ciprofloxacin Antibiotic ND 0.430 0.430 – 
 Clarithromycin Antibiotic 0.800 36.800 23.691 16.725 
 Clindamycin Antibiotic ND ND ND – 
 Clopidogrel Blood thinner 4.509 5.852 5.201 0.672 
 Codeine Opiate 0.673 2.031 1.362 0.605 
 Dehydronifedipine Human plasma metabolite 2.105 2.105 2.105 – 
 Diazepam Anti-epileptic 0.007 0.774 0.533 0.356 
 Diclofenac Anti-inflammatory 12 12 0.001 1.500 0.436 0.441 
 Dilantin Anti-epileptic 0.270 1.753 0.779 0.484 
 Doxycycline Antibiotic ND ND ND – 
 Enrofloxacin Antibiotic ND ND ND – 
 Erythromycin Antibiotic 14 11 7.984 1.332 2.255 
 Erythromycin-H2Antibiotic – 
 Famotidine Anti-histamine 0.000 3.958 2.500 1.881 
 Fenofibric acid Receptor agonist 0.800 1.480 1.140 0.481 
 Fluoxetine Antidepressant 0.017 1.143 0.202 0.382 
 Furosemide Diuretic ND 2.522 2.522 – 
 Gemfibrozil Cholesterol reducer 21 21 0.000 11.997 2.436 3.412 
 Glibenclamide Antidiabetic 0.016 0.016 0.016 – 
 Hydrochlorothiazide Diuretic 0.214 27.000 15.321 13.718 
 Hydrocodone Analgesic 0.015 0.541 0.270 0.174 
 Ibuprofen Anti-inflammatory 13 13 0.033 21.250 2.953 6.146 
 Indometacin Anti-inflammatory 0.895 0.895 0.895 – 
 Iohexol X-ray contrast media 2.400 71.300 31.236 29.021 
 Iomeprol X-ray contrast media 0.386 3.900 2.143 2.485 
 Iopamidol X-ray contrast media 2.626 2.626 2.626 – 
 Iopromide X-ray contrast media 11 10 ND 7.000 1.365 2.480 
 Ketoprofen Anti-inflammatory 0.429 0.429 0.429 – 
 Lidocaine Anesthetic 0.418 0.418 0.418 – 
 Lincomycin Antibiotic ND 0.057 0.057 – 
 Lorazepam Anti-epileptic 2.880 6.473 5.176 1.994 
 Lovastatin Statin ND ND ND – 
 Mefenamic acid Anti-inflammatory 0.001 0.001 0.001 – 
 Meprobamate Tranquilizer 10 10 0.780 7.972 2.457 2.283 
 Metoprolol Beta blocker 0.033 8.553 2.056 2.935 
 Metronidazole Antibacterial 7.447 8.247 7.847 0.566 
 Monensin Antibiotic ND ND ND – 
 Nadolol Beta blocker 0.000 1.014 0.667 0.578 
 Nalidixic acid Antibiotic 0.085 0.189 0.137 0.074 
 Naproxen Anti-inflammatory 18 18 0.000 9.223 1.374 2.223 
 Nicotine Receptor antagonist 0.912 5.683 3.298 3.374 
 Nifedipine Calcium channel blocker 0.273 0.273 0.273 – 
 Norfloxacin Antibiotic 0.120 0.120 0.120 – 
 Ofloxacin Antibacterial ND 25.309 9.526 9.793 
 Oleandomycin Antibiotic 0.010 0.010 0.010 – 
 Oxytetracycline Antibiotic ND ND ND – 
 Paroxetine Antidepressant 0.508 0.508 0.508 – 
 Penicillin G Antibiotic ND ND ND – 
 Penicillin V Antibiotic ND ND ND – 
 Pentoxifylline Blood thinner 0.000 0.000 0.000 – 
 Primidone Anti-epileptic 0.320 0.905 0.551 0.236 
 Propranolol Beta blocker 1.050 7.200 4.246 2.602 
 Propyphenazone Anti-inflammatory 0.156 0.258 0.207 0.072 
 Ranitidine Anti-histamine ND 8.240 5.825 2.216 
 Roxithromycin Antibiotic 0.150 0.150 0.150 – 
 Salbutamol Bronchodilator 0.198 0.889 0.588 0.310 
 Salinomycin Antibacterial ND ND ND – 
 Sotalol Beta blocker 0.022 6.196 3.409 2.818 
 Sulfadiazine Antibiotic 0.472 0.472 0.472 – 
 Sulfamerazine Antibiotic ND ND ND – 
 Sulfamethazine Antibacterial ND 0.635 0.635 – 
 Sulfamethizole Antibiotic ND ND ND – 
 Sulfamethoxazole Antibiotic 24 23 ND 8.638 3.085 2.640 
 Sulfasalazine Antibiotic ND 0.045 0.045 – 
 Sulfathiazole Antibiotic ND ND ND – 
 Tetracycline Antibiotic ND ND ND – 
 Theobromine Stimulant 1.010 1.010 1.010 – 
 Theophylline Bronchodilator 0.753 0.753 0.753 – 
 Timolol Beta blocker 0.018 0.018 0.018 – 
 Triclosan Antibacterial 14 14 0.008 3.371 0.923 1.249 
 Trimethoprim Antibacterial 14 14 0.000 1.172 0.512 0.394 
 Tylosin Antibiotic 0.005 0.005 0.005 – 
 Venlafaxine Antidepressant 0.333 0.333 0.333 – 
Table 2

Concentrations of non-pharmaceutical CECs measured for in RO concentrate

Chemicals Chemical uses Number of tests Number of detects Number of unmeasurable detects Number of non-detects Minimum concentration (μg/L) Maximum concentration (μg/L) Average concentration (μg/L) Standard deviation 
Hormones 
 Diethylstilbestrol Estrogen ND ND ND – 
 Equilin Estrogen ND ND ND – 
 Estradiol Estrogen 11 10 ND 0.028 0.014 0.010 
 Estriol Estrogen ND 0.000 0.000 – 
 Estrone Estrogen 0.021 0.612 0.139 0.210 
 Ethynylestradiol Estrogen ND 0.017 0.006 0.010 
 Testosterone Androgen hormone ND 0.000 0.000 – 
Industrial chemicals 
 4-n-octylphenol Corrosion inhibitor ND ND ND – 
 4-tert-octylphenol Corrosion inhibitor ND 1.183 0.887 0.420 
 Bisphenol A Plasticizer ND 1.343 0.532 0.471 
 Nonylphenol Plasticizer ND 9.433 9.433 – 
 Para-chlorobenzene sulfonic acid Chemical producer ND ND ND – 
 Tris-2-chloroethyl phosphate Plasticizer 1.900 3.100 2.500 0.849 
Flame retardants 
 TCEP Hydrochloride salt 12 12 0.426 5.810 1.416 1.489 
 TCPP Flame retardant 3.033 3.033 3.033 – 
 TDCPP Flame retardant 1.233 1.233 1.233 – 
 Tetrabromobisphenol A Brominated flame retardant ND ND ND – 
Personal care products 
 Acesulfame potassium Artificial sweetener 29.220 29.220 29.220 – 
 Aspartame Artificial sweetener ND ND ND – 
 Cotinine Tobacco alkaloid 0.423 0.423 0.423 – 
 Galaxolide Synthetic musk 2.180 2.180 2.180 – 
 Musk ketone Nitro-musk 0.329 0.329 0.329 – 
 Neotame Artificial sweetener ND 0.010 0.010 – 
 Oxybenzone Sunscreen 0.017 0.061 0.029 0.021 
 Sucralose Artificial sweetener 124.000 459.433 274.478 170.354 
Steroids 
 4-androstene-3 Testosterone enhancer 0.010 0.026 0.018 0.011 
 Androstenedione Adrenal steroid 0.000 0.017 0.008 0.009 
 Epitestosterone Natural steroid ND ND ND 
 Progesterone Endogenous steroid ND 0.017 0.005 0.008 
Pesticides 
 Atrazine Herbicide 0.000 0.017 0.011 0.009 
 clofibric acid Herbicide 1.038 1.038 1.038 – 
 DEET Insect repellant 15 15 0.061 3.000 0.730 0.809 
 Diaminochlorotriazine Herbicide 0.035 0.035 0.035 – 
 Diuron Herbicide 0.072 0.545 0.281 0.161 
 Linuron Herbicide 0.027 0.063 0.045 0.026 
 Pentachlorophenol Pesticide ND ND ND – 
 Phenylphenol Preservative ND ND ND – 
 Quinoline Plant alkaloid 0.385 0.385 0.385 – 
 Simazine Herbicide 0.036 0.085 0.058 0.025 
Chemicals Chemical uses Number of tests Number of detects Number of unmeasurable detects Number of non-detects Minimum concentration (μg/L) Maximum concentration (μg/L) Average concentration (μg/L) Standard deviation 
Hormones 
 Diethylstilbestrol Estrogen ND ND ND – 
 Equilin Estrogen ND ND ND – 
 Estradiol Estrogen 11 10 ND 0.028 0.014 0.010 
 Estriol Estrogen ND 0.000 0.000 – 
 Estrone Estrogen 0.021 0.612 0.139 0.210 
 Ethynylestradiol Estrogen ND 0.017 0.006 0.010 
 Testosterone Androgen hormone ND 0.000 0.000 – 
Industrial chemicals 
 4-n-octylphenol Corrosion inhibitor ND ND ND – 
 4-tert-octylphenol Corrosion inhibitor ND 1.183 0.887 0.420 
 Bisphenol A Plasticizer ND 1.343 0.532 0.471 
 Nonylphenol Plasticizer ND 9.433 9.433 – 
 Para-chlorobenzene sulfonic acid Chemical producer ND ND ND – 
 Tris-2-chloroethyl phosphate Plasticizer 1.900 3.100 2.500 0.849 
Flame retardants 
 TCEP Hydrochloride salt 12 12 0.426 5.810 1.416 1.489 
 TCPP Flame retardant 3.033 3.033 3.033 – 
 TDCPP Flame retardant 1.233 1.233 1.233 – 
 Tetrabromobisphenol A Brominated flame retardant ND ND ND – 
Personal care products 
 Acesulfame potassium Artificial sweetener 29.220 29.220 29.220 – 
 Aspartame Artificial sweetener ND ND ND – 
 Cotinine Tobacco alkaloid 0.423 0.423 0.423 – 
 Galaxolide Synthetic musk 2.180 2.180 2.180 – 
 Musk ketone Nitro-musk 0.329 0.329 0.329 – 
 Neotame Artificial sweetener ND 0.010 0.010 – 
 Oxybenzone Sunscreen 0.017 0.061 0.029 0.021 
 Sucralose Artificial sweetener 124.000 459.433 274.478 170.354 
Steroids 
 4-androstene-3 Testosterone enhancer 0.010 0.026 0.018 0.011 
 Androstenedione Adrenal steroid 0.000 0.017 0.008 0.009 
 Epitestosterone Natural steroid ND ND ND 
 Progesterone Endogenous steroid ND 0.017 0.005 0.008 
Pesticides 
 Atrazine Herbicide 0.000 0.017 0.011 0.009 
 clofibric acid Herbicide 1.038 1.038 1.038 – 
 DEET Insect repellant 15 15 0.061 3.000 0.730 0.809 
 Diaminochlorotriazine Herbicide 0.035 0.035 0.035 – 
 Diuron Herbicide 0.072 0.545 0.281 0.161 
 Linuron Herbicide 0.027 0.063 0.045 0.026 
 Pentachlorophenol Pesticide ND ND ND – 
 Phenylphenol Preservative ND ND ND – 
 Quinoline Plant alkaloid 0.385 0.385 0.385 – 
 Simazine Herbicide 0.036 0.085 0.058 0.025 

A majority of the CECs found were pharmaceuticals. The rest belonged to other common chemical categories. The CEC category distribution is depicted in Figure 1.
Figure 1

CEC category distribution.

Figure 1

CEC category distribution.

CONCENTRATE STREAM TREATMENT METHODS

A wide range of treatment methods have been studied for removing concentrate stream CECs. The unique physical and chemical properties of the CECs were found to have a large impact on their removal by each treatment method. The findings of this report are summarized in the following sections.

Discharge reduction

The single most effective method to prevent CECs in the environment is reducing their presence in wastewater. Proper prescriptions by doctors and reduced source discharges by pharmaceutical companies are key steps being investigated. Phasing out the most dangerous medicines is also a move being researched (Ternes & Joss 2006).

Coagulation/flocculation

Coagulation/flocculation is a simple method of injecting chemicals that gather together charged particles into floc that can be settled or filtered out. Alum and ferric chloride (FeCl3) have been tested as viable coagulant chemicals. The process is popular as alum has been found to offer a 42% removal rate of dissolved organic carbon (DOC) while ferric chloride has DOC removal rates of 52% (Dialynas et al. 2008).

The effectiveness of CEC treatment has been found to be much lower. A jar test conducted on the removal of diclofenac, clofibric acid, bezafibrate, carbamazepine and primidone using iron(III) chloride found no impact on the pharmaceutical levels was made by the flocculant. These results were also found when the experiment was conducted on a full-scale system (Ternes et al. 2002).

Despite these poor results, further research is needed to determine the effectiveness of different coagulation/flocculation chemicals at removing CECs since their low cost and ease of operation are encouraging their widespread implementation to treat other contaminants.

Activated carbon adsorption

Activated carbon (AC) is a process that has long been used in water treatment. It involves the adsorption of contaminants into the pores on activated carbon pieces. It is often used in powder (PAC) or granular (GAC) forms. AC systems can be arranged in different configurations that include upflow-expanded bed, upflow-compacted bed, downflow-single stage and downflow-multistage. Efficient carbon utilization, costs, and operation procedures are some of the differences between these AC configuration systems (Rose n.d.).

Studies have found that GAC is capable of removing greater than 90% of most CECs, effluent organic matter (EfOM) and DOC (Snyder et al. 2007; Dialynas et al. 2008). Carbamazepine has shown high removal rates with GAC treatment while bezafibrate and diclofenac have shown slightly lower removal rates (Ternes et al. 2002). The high removal rates of most CECs are likely due to the high adsorption abilities of GAC. It has also been found to remove a wide range of the different fractions that compose DOC meaning it can be used to remove a broad range of contaminants (Dialynas et al. 2008). A mixture of sand and iron-coated GAC has also been found to be highly effective while allowing the reuse of previously waste iron in a beneficial manner (Joo 2014).

There are several issues with AC. Clofibric acid has shown resistance to AC treatment (Ternes et al. 2002). Contact time is lengthy to achieve high removal rates meaning GAC beds must be large and PAC doses must be high. The carbon must be disposed of or regenerated once it has been spent. Disposal requires hazardous waste-like handling and thermal regeneration requires large amounts of energy. Removal of CECs is also heavily impacted by the levels of natural organic matter present in the wastewater. These issues make implementation of AC difficult (Snyder et al. 2007).

Despite these issues, it appears that GAC may have a place in RO concentrate treatment. The decreased flow rates may limit the amount of GAC required while still offering high CEC removal rates. Some studies recommend only seasonal use of GAC in order to remove CECs during periods when their levels are known to be high (Snyder et al. 2007).

Biologically activated carbon

A variation of activated carbon is biologically activated carbon (BAC). This involves the creation of a microorganism biofilter within a bed of granular activated carbon. The water that passes through this filter is subjected to a generally aerobic environment of activated bacteria that provide high removals of most CECs (Sundaram & Emerick 2010a, b).

BAC is frequently implemented as a polishing step after ozonation. The constituent molecules and increased bioactivity that are created during the ozone process promote the growth of microbial communities. The microbial makeup of the biofilter has been found to develop and change for months after coming online due to variations in the system influent (Sundaram & Emerick 2010a, b).

BAC has the advantages of partial regeneration of the activated carbon by the microbial community, degradation of less biodegradable organics that are absorbed by the carbon and then exposed to the microbes, and increased levels of biological reaction rates due to a readily available food source adsorbed by the carbon. These factors are important as CECs are often less biodegradable organics (Ng et al. 2008).

Studies have also been conducted on pairing BAC with capacitive deionization (CDI). This links the high organic removal capabilities of BAC with the inorganic removal abilities of CDI. High removal rates for TOC have been found with this layout (Ng et al. 2008).

BAC systems have repeatedly shown consistent removal performance. The stability of the matured microorganism community allows for high removal rates of a wide range of organics, ozone byproducts, estrogenicity, CECs, and other associated toxicity. BAC treatment has also shown high removal levels of some problematic contaminants such as flame retardants, TCPP, TCEP and NDMA but does not display the same effectiveness against 1,4-dioxane and benzophenone. This has been attributed to the chemical makeup of these molecules (Sundaram & Emerick 2010a, b).

Concern has existed about the ability of BAC to handle the high salinity of RO concentrate. Fortunately, studies have lessened this concern by proving that sudden changes in organic load and salinity do not have drastic negative effects on BAC performance (Lu et al. 2013). The microorganism community will simply adjust to the different water quality parameters. Further research is needed, however, to verify these findings.

Full-scale implementation of BAC systems has been hindered by uncertainties. It is unknown how the units perform over the course of their lifetimes, how to handle the spent carbon when the media must be replaced, and what is the fate of CECs removed by the system. These areas must be further researched before use of BAC systems becomes more common (Sundaram & Emerick 2010a, b).

There is some concern of coliform regrowth in the BAC unit. The filter bed provides the opportunity for total and fecal coliform levels to increase which may also indicate pathogen levels will increase. BAC effluent routinely exceeded the requirement of 2.2 MPN/100 mL while treating municipal wastewater for reuse projects. This will likely also be the case when treating RO concentrate. It has been recommended to consider adding a downstream process such as UV disinfection to counteract this effect (Gerrity et al. 2011).

Ozonation

Ozone (O3) is a powerful oxidant that is gaining popularity in water and wastewater treatment as it has been found to be more effective than chlorination and chloramination. It can be turned into an advanced oxidation process (AOP) through the addition of either peroxide (H2O2) or ultraviolet (UV) light.

The organic compounds subjected to ozonation are oxidized by reacting with molecular ozone and with hydroxyl radicals generated by ozone decomposition. This allows the process to remove a wide range of contaminants (Justo et al. 2013).

Studies have found the process does well at removing estrogenic activity and most CECs that do not have a high resistance to oxidation. Sulfamethazine, acetaminophen, and naproxen have consistently displayed high removal rates under this process. Studies have found that most CECs are removed at rates of 80–100% (Snyder et al. 2006; Abdelmelek et al. 2011; Justo et al. 2013). Carbamazepine, sulfamethoxazole, and trimethoprim have even been suggested as indicator compounds to measure the performance of ozone/peroxide treatment (Snyder et al. 2006).

It should be noted that despite requiring higher ozone doses, the ozonation of brine concentrate requires less ozone than treating the full plant flow. This makes it a more economical means of implementing ozonation (Benner et al. 2008).

Not all CECs are subject to high removal rates during ozone treatment. Meprobamate was found to be among the most resistant to ozone treatment. Atenolol and carbamazepine have also displayed removal rates below 80% (Justo et al. 2013). Atrazine, iopromide, ibuprofen, and TCEP have also shown resistance (Snyder et al. 2006). Removal of clofibric acid may also be an issue (Ternes et al. 2002).

Ozone systems are often followed by BAC units. The ozone breaks down many compounds into their constituents which provide a more readily degradable food source for the microbial community living in the BAC unit. This treatment train yields lower effluent CEC levels than either system can obtain on its own (Sundaram & Emerick 2010a, b).

Despite its effectiveness, ozonation has some drawbacks. Some of the most concerning are the increase in bioactivity in the effluent, inadequate removal of engineered compounds such as flame retardants, and the formation of carcinogenic byproducts such as bromate and NDMA (Sundaram & Emerick 2010a, b). Residual DOC in the system effluent can further lead to byproduct formation when chlorine is introduced as a residual disinfectant (Snyder et al. 2006). There is also concern about the competition for the hydroxyl molecules between EfOM, the inorganic constituents, and the micropollutants of interest (Abdelmelek et al. 2011). It also does not leave a disinfecting residual in the effluent. These issues have limited the widespread implementation of ozonation.

The formation of bromate as a byproduct to the process causes concern. Studies have found that bromate formation can be limited by year-round peroxide addition, seasonal ammonia addition, and optimal ozone dosage. The dosage requirements and level of removals are water-specific. Peroxide addition interrupts bromate formation at the molecular level. A peroxide dose of 1:1 peroxide to ozone molar ratio has been found to help eliminate bromate formation. Ammonia converts bromide into bromamines which reduces the constituents available for the creation of bromate. An ammonia dose of greater than 1.0 mg N/L was found to reduce bromate formation. Ozone doses of 5 mg/L have been found to typically provide a good balance of CEC removal and bromate formation. A dose of 3 mg/L was found to yield lower CEC removal rates than desired while a dose of 7 mg/L yielded excessive bromate levels (Sundaram & Emerick 2010a, b).

It is important to note that the addition of peroxide changes the dynamics of ozone treatment. Peroxide addition results in faster creation of hydroxyl (—OH) molecules which yields shorter reaction times while involving a wider range of contaminants. The downsides to this augmentation, however, include the higher cost of using an additive, ensuring no residual peroxide is released to the environment, and creating a weaker disinfection environment than ozone alone. It is recommended to avoid peroxide dosing unless reaction time and bromate formation govern system design and operation (Gerrity et al. 2011).

Ultraviolet light (UV)/hydrogen peroxide (H2O2)

The AOP of UV/H2O2 is a proven method that is common in many advanced treatment systems. It employs UV light to break down chemicals while also breaking peroxide down into powerful hydroxyl radicals that further degrade contaminants.

It is important to note that the power output of UV systems vary greatly. Less powerful UV disinfection systems are not capable of achieving the energy output necessary to degrade CECs. More powerful units must also provide adequate exposure time to ensure proper removal rates. Systems capable of achieving CEC removal often apply over 20 times the UV dose used in a typical wastewater disinfection unit (Kim et al. 2008).

UV/H2O2 has been found to be superior to UV alone. The addition of peroxide allows for higher removal rates of a wider range of contaminants while using much lower UV doses. A study found that a UV dose of 2,768 mJ/cm2 resulted in far lower CEC removal rates than a UV dose of 923 mJ/cm2 combined with 7.8 mg/L of H2O2. The lower UV dose means less energy consumption helping the process become more cost-effective. The cost of the peroxide should also be factored into operating costs (Kim et al. 2008).

UV/H2O2 systems have been found to offer high removal rates of atenolol, diclofenac, and carbamazepine (Justo et al. 2013). This somewhat contrasts to ozone which displayed limited removal of these compounds. Tests have found it is capable of removing a broad range of CECs including ketoprofen, naproxen, metoprolol, clarithromycin and primidone at removal rates greater than 90% (Kim et al. 2008). This broad removal capability makes it a powerful technology.

Trimethoprim and paroxetine have been found to exhibit lower percentage removals (Justo et al. 2013). This has been attributed to their molecular structure. Norfloxacin and caffeine have also shown reduced removal rates (Kim et al. 2008). Research has also found that several chemicals including DEET and clarithromycin require higher UV doses even with the addition of peroxide in order to achieve removal rates above 90% (Kim et al. 2009).

A major drawback with UV/H2O2 systems is their operational costs. They have a high energy demand as the lamps require substantial amounts of energy to maintain proper operating parameters. Bulbs also require replacement when they burn out. UV/H2O2 systems have been found to be more costly to operate than ozone units (Pisarenko et al. 2012). Removal rates also appear to be affected by pH levels in the water receiving treatment (Canonica et al. 2008).

An important aspect of UV/H2O2 is its proven track record. Operating parameters have been researched heavily. The system also improves water quality using significantly less oxidant than ozone systems (Justo et al. 2013). These factors combined with the broad CEC removal abilities make UV/H2O2 the most potent system available for CEC removal in RO concentrate.

A promising form of UV treatment is high intensity UV from solar energy. This technology harnesses the power of the sun to treat waste flows with UV intensities much higher than typical systems. The addition of H2O2 or ammonia is also an option. It has been found to offer very high removal rates of all contaminants but is still in the development phase. The systems are currently being developed by Focal Technologies, Inc. and are just starting to come to market. (E. Steinmeyer, personal communication, April 10, 2015).

Alternative advanced oxidation processes

Applying the power of advanced oxidation using other methods is also being studied. An emerging concept is electrical treatment of RO concentrate. Electro-oxidation, electrochemical treatment, electrodialysis and electrochlorination have been studied as viable alternatives to the more common AOPs. They use various combinations of electricity, metals, and (in some methods) the addition of chemicals to treat the water. The use of light and catalysts in photocatalysis and ultrasound in sonolysis are also options under investigation.

Electro-oxidation in particular has shown great promise. The salinity and chloride concentrations in concentrate help improve these systems' performance. Using boron-doped diamond electrodes to apply a current density of 100 A/m2 has been found to yield removal rates greater than 97% for bisphenol A, hydrochlorothiazide, nicotine, atenolol, furosemide, and bezafibrate. Removal rates of 94% were also found for ofloxacin, fenofibric acid, 4-AAA, naproxen and gemfibrozil. Ibuprofen has been found to be the most resistant with removal rates of 70% (Pérez et al. 2010; Urtiaga et al. 2013).

Limited research has been conducted on the CEC removal capabilities of the other systems. Most studies focus on their DOC removal capabilities with promising results. Removal rates of DOC as high as 50% were recorded making them good candidates for combining with other processes that have opposing strengths (Dialynas et al. 2008).

Several drawbacks are known for these systems. Processes that employ electrolytic oxidation require very long oxidation times making the process energy-consuming and expensive (Dialynas et al. 2008). Electro generated chlorine has been found to lead to the formation of harmful by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) which must be removed by a polishing treatment (Bagastyo et al. 2011). These issues result in the need for much more research.

Less common and emerging technologies

There are many other treatment methods that are less mainstream but still worth noting. Many are aimed at the goal of making RO systems run at, or near, zero liquid discharge (ZLD) in order to avoid expensive discharge permits. These methods are separated into membrane-based and thermal-based technologies (Subramani & Jacangelo 2014).

Membrane-based technologies apply membrane treatment under different conditions. These include chemical softening of brine concentrate, chemical precipitation during RO cleaning activities, and disc tube membranes. Most of these methods are intended to reduce the concentrate volume to levels that can be more easily discharged or recycled to the beginning of the plant. It is not known if these methods offer any CEC removal but it is unlikely. The reduction of brine flow would actually result in more concentrated CEC levels in the concentrate (Subramani & Jacangelo 2014).

Thermal-based technologies use mechanical or natural methods to evaporate concentrate flows. Multi-effect distillation, brine concentrators and crystallizers, wind-aided intensified evaporation, and spray dryers all fall under this category. They are well-proven at reducing concentrate volume but are often expensive and complex to operate. The solid residue that is left behind is pure concentrations of the contaminants in brine including CECs meaning the handling of this waste is an environmental issue (Subramani & Jacangelo 2014).

Emerging technologies are also being developed which have great potential but need much more research. These systems include forward osmosis (FO), membrane distillation, thermoionic process, and eutectic freeze crystallization. They promise to solve some of the problems of existing methods but will likely be faced with the same CEC handling issues (Subramani & Jacangelo 2014).

FO is one of the most promising emerging technologies. FO is an emerging treatment process that involves driving water through a semi-permeable membrane using the pressure difference of the osmotic pressure between the feed and draw solution. The water diffuses from the lower osmotic pressure of the feed solution to the higher osmotic pressure of the draw solution (Martinetti et al. 2009). Since the travel of the fluid in FO is natural osmosis it in turn requires less energy to operate compared to other methods. The FO process is commonly operated by injecting an easily removed solution of high concentration (such as ammonia) into the feed in order to generate the hydrostatic osmotic pressure gradients. This causes the fluid (which can be brine) to cross the semi-permeable membrane where the membrane filters out the unwanted constituents. The saline solution is then removed with an additional process such as slight heating to remove ammonia.

The effectiveness of FO has been tested by injecting 1 M of concentrate into the feed along with the use of a cellulose acetate asymmetric semi-permeable membrane. This resulted in a water flux of a high initial value of 8.9 gallons/ft2/day which declined to 6.0 gallons/ft2/day after 18 hrs (1 gallon = 3.785 liters; 1 square foot = 0.0929 square meters). The resulting removal of organic contaminants in the concentrate was approximately 76%. Further research is needed to determine how much of this removed organic contamination was CECs (Tang & Ng 2008).

The FO process has less membrane fouling than other membrane treatment methods. This results in less contaminated cleaning water to handle. Despite the high potential, difficulties in manufacturing of the FO membranes and lack of knowledge on the ideal operating parameters have limited its implementation (Subramani & Jacangelo 2014).

FO has been under development for many years but has not yet been developed into commercially viable systems. Many of these problems appear to be due to the membrane manufacturing issue. It has been considered that the lower flows involved in RO concentrate streams could allow smaller membranes to be used which would likely help alleviate some of the manufacturing issues.

Although tests are being conducted, there has not been substantial research done on concentrate treatment using FO to date. Even less is known on CEC removal in FO. Despite this lack of current knowledge, FO's ability to remove constituents using less energy than other treatment options means that it has the potential to gain widespread use in the future.

CEC disposal options

Even the best RO concentrate treatment options produce hazardous products. Handling these products is site specific. Most RO concentrate is mixed with other treatment plant effluent and discharged to a receiving body of water. Deep injection wells are another less common environmental discharge option (Subramani & Jacangelo 2014). The preferred ZLD practice is to treat the concentrate to a level where it can be reintroduced at the beginning of the treatment plant but no plants were found during this study that currently have this ability.

Treatment systems that produce solid residue (such as brine crystallizers and evaporation ponds) pose a unique issue. This residue requires disposal at landfills which can be an unpopular and potentially expensive option (Subramani & Jacangelo 2014). No mention of burning this residue was found during the course of this study even though this is sometimes seen as a valid disposal option for other waste materials.

CONCLUSIONS

There are substantial concentrations of a wide range of CECs in concentrate even though RO systems usually follow several other treatment methods. The CECs that make it through these processes are removed from the main flow by the RO membranes but are concentrated into concerning levels in the brine flow.

Some of the most frequently found CECs with the highest average levels should be considered as potential indicator compounds. These include the following ten CECs: the beta blocker atenolol, the anti-epileptic carbamazepine, the cholesterol reducer gemfibrozil, the anti-inflamatory ibuprofen, the X-ray contrast media iohexol, the antibiotic sulfamethoxazole, the anti-bacterial triclosan, the flame retardant component TCEP, the insect repellant DEET, and the herbicide diuron. Each represents a different family of CECs with unique properties. All ten of these have been found to be common and offer a large enough range to be used for determining treatment process efficiency. The hormones estrone, estradiol, and ethynylestradiol and the androgen hormone testosterone should also be monitored due to their frequent presence and endocrine disrupting abilities.

The research repeatedly found UV/H2O2 and ozone-BAC to be the most viable treatment options. UV/H2O2 has the benefit of being a well-studied system that is already commonly employed for drinking water and wastewater treatment. It was found to be effective at removing a broad range of CECs but has the implementation issue of high power requirements for the UV bulbs. Typical operating parameters are also well-documented but more research needs to be done to determine if high CEC levels need higher contact-time values.

Ozone-BAC is a treatment train that has shown promise at removing CECs and seems to be effective on a different subset of CECs than UV/H2O2. The main concern is the limited knowledge on the BAC operating parameters and a concern of bromate and NDMA formation in the ozone process. It is an important system to research, however, as the lower operational cost could be a major benefit.

Some of the other treatment methods work well for removing particular CECs but they do not have the high removal rates for a wide range of CECs that are offered by UV/H2O2 and ozone-BAC. These options may be better choices if cost is the main concern or there is a particular CEC of interest.

Many of the emerging technologies are very promising. They have the potential to offer very high CEC removal rates while having low operational costs. Unfortunately, too little information is currently known about these systems for full-scale implementation. Much of this technology is at least 5–10 years from being considered as a viable alternative.

ACKNOWLEDGEMENTS

The authors would like to extend a special thank you to Jon Reynolds, PE, Krieger & Stewart, Incorporated; Dr Bruce Mansell, PE, California Polytechnic University, Pomona, Los Angeles County Sanitation District; Dr Monica Palomo, PE, California Polytechnic University, Pomona; and Steve Agor, PE, Skanska USA Civil. This work would not be possible without the support of the Bureau of Reclamation, California Polytechnic University, Pomona, and Skanska USA Civil. A very special thank you is also extended to Russ and Marilyn Romeyn for their support throughout the entire course of this research.

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