The water treatment industry is continually faced with water quality testing, and at times it is impractical to analyze for several species of interest. One method of reporting is to provide the collective concentration of one element in several species. One of the most common reporting terms to this industry is the total concentration of carbon of the organic compounds present. The method for testing organically bound carbon or NOM in water is via TOC analysis.
Natural Organic Matter (NOM), which occurs in all natural water sources, is produced by the breakdown of both, plant and animal material in the environment. Its chemical and physical characteristics vary according to environmental source. Typically, NOM comprises compounds ranging from proteins and amino acids to humic and fulvic acids. A range of functional groups can be associated with NOM, including carboxyl groups, carbonyl groups, phenolics, ethers, and esters. It is the carboxyl and phenolic groups that render the NOM negatively charged at typical water pH values. Neutralization of the negative charge surface is desired in order to get the particles to coagulate or attach to filter media during water treatment operations. With these vast characteristics of NOM it is easy to realize the importance of studying their relevance to activated carbon adsorption during water treatment.
Removal of NOM in water treatment plants has become extremely important in the last few decades as NOM serves as the precursor for many disinfection by-products (DBP) that are produced in the plants as well as in the distribution systems. Two sets of DBPs that are currently regulated by the EPA are Trihalomethanes (THMs) & Haloacetic Acids (HAAs) as they have been shown to be carcinogens. Removal of the DBP precursors prior to chemical disinfection can be enhanced by adsorption onto activated carbon. This can be achieved with PAC in mixing basins or pipes or with granular activated carbon (GAC) in filter beds or adsorbers. Each method had advantages and disadvantages; for example, PAC has a lower cost, strong kinetics and is adaptable to variance in NOM levels. However, since PAC is typically added prior to conventional treatment, it encounters the highest levels of NOM and high doses are necessary for superior removal. GAC is gaining popularity as a treatment method for lowering DBPs and DBP precursors. GAC is commonly used after sedimentation so NOM levels are lower and therefore the carbon can lower the TOC concentration of the effluent to below desired levels for many months before being replaced. Another advantage is that GAC can then be reactivated after it becomes spent and then reused. The disadvantages with GAC are higher capitol and material costs, as well as additional maintenance.