Biodegradation Kinetics and Adsorption of Aldehydes Formed from Ozonation of Natural Organic Matter
Creators: name: LeCourt, Thomas D.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1997

description: With the upcoming passage of the Disinfectants and Disinfection By-Products (D/DBP) Rule, stricter regulations will be placed on trihalomethanes (THMs) and haloacetic acids (HAAs), both of which are suspected human carcinogens resulting from the reaction of natural organic matter (NOM) with chlorine. Several options are available to utilities to achieve DBP control: 1) move the point of chlorine disinfection further down in the treatment train, 2) reduce the amount of NOM in water to lower the concentration of THM and HAA precursors before the point of chlorination, 3) remove the by-products once they are formed, and 4) use alternative disinfectants to chlorine. Halogenated DBP formation generally increases with chlorine contact time, so moving the point of disinfection further down in the treatment process will minimize the time for by-product formation and allow chlorine to be applied after NOM has been removed by earlier treatment processes. However, disinfection effectiveness maybe lowered because of the reduced chlorine contact time. Removal of THMs and HAAs after formation is difficult. Some common treatment practices for removing precursor material are coagulation and sedimentation, membrane separation, and adsorption onto granular activated carbon (GAC). Alternative disinfectants include the use of chlorine dioxide, ultraviolet (UV) irradiation, combined chlorine, and ozone. The use of ozone as an oxidant and disinfectant has long been practiced in Europe and is rapidly gaining popularity in the United States. Molecular ozone by itself is an excellent disinfectant and a very selective oxidant, but as ozone decomposes, hydroxyl radicals (0H°)are formed, which are much stronger and less selective oxidants. This susceptibility to decomposition indicates that ozone is very unstable in water and it is therefore difficult to maintain an ozone residual. Thus, ozone may be effective as a primary disinfectant, but must be combined with a secondary disinfectant to maintain an adequate residual in the distribution system. Although ozone has been shown to be effective at oxidizing THM and HAA precursors, ozone also reacts with NOM to form its own by-products; much less is known about ozonation DBPs than chlorination DBPs. In the past few years, our knowledge of ozonation DBPs has increased, but still lags behind compared to our knowledge of chlorination by products. It has been shown that ozone increases the biodegradability of NOM and that the by-products of ozonation are themselves biodegradable. Thus, ozonation followed by biological filtration can be an effective method of removing THM and HAA precursors before chlorination as well as removing the by-products of ozonation. Low molecular weight (MW) aldehydes (e.g., formaldehyde, acetaldehyde, glyoxal, and methylglyoxal) are a major class of by-products formed from ozonation. Formaldehyde concentrations immediately following ozonation have been reported at potentially carcinogenic levels. Acetaldehyde concentrations are generally lower than projected regulatory levels. Health effects data on glyoxal and methylglyoxal is not known; as such, these compounds are not currently regulated, but as toxicological studies are performed, they could become compounds of concern. This study seeks to examine the effect of ozone dose on the formation of aldehydes and to obtain information on aldehyde removal through biodegradation and adsorption. The kinetics of biodegradation are determined by passing solutions of ozonated NOM solutions containing aldehydes through a batch recycle reactor. The biokinetic data from the batch recycle experiments are fitted to kinetic models. The models for aldehyde removal are tested using a continuous flow system. Aldehyde removal by adsorption is examined by adding powdered activated carbon (PAC) to aldehyde-containing solutions. This study was part of an EPA-sponsored project that examined the biodegradation kinetics of ozonated NOM and the effect of biodegradation on the adsorbability of ozonated NOM (see Parameswar, 1997).