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 (Snoeyink et al., 1980). 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 (Benjamin, 2002).
With these vast characteristics of NOM, it is easy to realize the importance of studying their relevance to activated carbon adsorption during water treatment.
Detecting Disinfection Byproducts with TOC Testing
Removal of NOM in water treatment plants has become extremely important in the last few decades, because NOM serves as the precursor for many disinfection byproducts that are produced in the plants as well as in water distribution systems.
Two sets of disinfection byproducts that are currently regulated by the EPA are Trihalomethanes (THMs) & Haloacetic Acids (HAAs). THMs and HAAs have both been shown to be carcinogens. Removal of the disinfection byproduct 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 has advantages and disadvantages. 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 effective removal.
GAC is gaining popularity as a treatment method for lowering disinfection byproducts and disinfection byproduct precursors. GAC is commonly used after sedimentation so NOM levels are lower and therefore the carbon can lower the Total Organic Carbon 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 capital and material costs, as well as additional maintenance.