UC Berkeley

In response to water scarcity and an increased recognition of the risks associated with the presence of chemical contaminants, environmental engineers have developed advanced water treatment systems that are capable of converting municipal wastewater effluent into drinking water. This practice, which is referred to as potable water reuse, typically relies upon microfiltration and reverse osmosis (RO) treatment, followed by exposure of water to ultraviolet (UV) light and addition of hydrogen peroxide (H2O2). This process sometimes employs pretreatment with ozone, often followed by biological filtration. Depending on the reuse scenario, water produced by this advanced treatment process is sent to a reservoir or aquifer, or is blended with treated drinking water and sent to a drinking water distribution system. Potable water reuse systems face intense scrutiny from the public and regulators due to concerns about potential exposure to waterborne pathogens and chemical contaminants originally present in the wastewater.

To protect public health in communities where RO and UV/H2O2 are employed for potable water reuse, insight is needed into the ability of these processes to act as barriers against chemical contaminants. Previous research indicates that low molecular weight uncharged compounds, such as disinfection byproducts, odorous compounds, and industrial solvents, are not fully rejected during RO treatment. Transformation of these compounds during subsequent treatment (i.e., UV/H2O2) depends upon the susceptibility of the compounds to photochemical processes occurring upon exposure to UV light and their reactivity with hydroxyl radicals generated from photolysis of H2O2. Analysis of available data indicated that the low molecular weight, uncharged compounds that are routinely measured in water produced during advanced treatment only account for a small fraction of the dissolved organic carbon in recycled water.

On the basis of their potential health impacts and potential production during oxidative water treatment, low molecular weight, uncharged carbonyl compounds (e.g., aldehydes and ketones) were identified as compounds of concern in potable water reuse systems that could account for much of the unidentified dissolved organic carbon. Carbonyl compounds can be formed during oxidation processes commonly employed in water treatment (e.g., ozonation, chlorination), and might not be completely removed during subsequent advanced treatment processes (i.e., RO and UV/H2O2). Using a new analytical method, concentrations of carbonyl compounds were monitored throughout the treatment train in pilot- and full-scale potable water reuse facilities. In addition to monitoring saturated carbonyls (e.g., formaldehyde, acetaldehyde, acetone), the method was capable of detecting a,b-unsaturated aldehydes (e.g., acrolein, crotonaldehyde), which are more toxic than their saturated counterparts. Analysis of samples from the six water reuse facilities and one conventional drinking water treatment plant indicated that the highest concentrations of carbonyl compounds were produced when wastewater effluent was subject to ozonation. Biological filtration lowered concentrations of the carbonyl compounds by over 90%. Rejection of the carbonyl compounds during RO was a function of molecular weight, and ranged from 33 to 58%. Reactions of carbonyl compounds during UV/H2O2 resulted in decreases in concentration ranging from 10 to 90%, with trends in removal consistent with rate constants for reactions of the compounds with hydroxyl radical. Overall, carbonyl compounds accounted for 10 to 40% of the dissolved organic carbon in RO-treated water produced by advanced treatment plants employed for potable water reuse.

Carbonyl compounds are reactive and might not pose a health risk if they degrade before they are consumed. One possible mechanism through which carbonyl compounds might disappear is through the reaction with chlorine. Although saturated carbonyl compounds do not react with free or combined chlorine under conditions typically encountered in water systems, a,b-unsaturated carbonyl compounds reacted with free chlorine and free bromine over a wide range of pH values. Reactions of compounds with combined chlorine were too slow to be relevant on the timescale of a distribution system. For nearly all of the a,b-unsaturated carbonyl compounds, the rates of reaction with free chlorine were faster with increasing pH and followed a linear relationship with the fraction of free chlorine present in its deprotonated form. The reaction mechanism most likely involves nucleophilic addition of OCl- at the carbon—carbon double bond, which contains an electron-deficient b-carbon that results from the electron-withdrawing carbonyl group. Transformations of several of the compounds are expected to result in their disappearance under conditions typically encountered in a drinking water distribution system (i.e., 1 mg L-1 Cl2, residence times of 24-72 hours). To understand the health implications of these reactions, additional research is needed to identify the products of these reactions and their toxicity.