WaterSubject

The typical conventional water treatment train in Korea is composed of coagulation, sedimentation, sand filtration, and disinfection. Most of the water treatment plants (WTPs) (i.e., 97%) in Korea take their raw water from surface water, and turbidity of raw water can be increased up to 1000 NTU during the rainy season (June-August), but turbidity of tap water is less than 0.2 NTU in most Korean cities. Turbidity of surface water can be removed up to 99% in most developed countries’ WTPs, but removal of dissolved organic material such as dissolved organic carbon (DOC) is relatively low (e.g., 10-50%) in conventional WTPs. This may be an inherent defect of conventional WTPs as most of the conventional water treatment processes are designed for particle separation such as turbidity removal.

With the rapid industrialization and urbanization of Korea, the composition of contaminants has been shifting from inorganic to organic materials. The concentration of organic materials can be expressed as total organic carbon (TOC). A portion of the TOC can affect taste and odor or create disinfection by-products (DBPs) by reaction with disinfectants (e.g., chlorine). Certain DBPs have been reported to be associated with adverse health effects. Therefore, most countries regulate the concentration of DBPs, and many WTPs have been focusing on the control of DBPs through cost-effective methods.

Chlorinated DBPs can be created by the reaction of chlorine with bromide as well as natural organic matter (NOM), as shown in the following equation. NOM is the principal precursor of chlorinated DBPs in most water, and represents a significant portion of TOC content.

The most prevalent chlorinated DBPs in drinking water are trihalomethanes (THMs) and haloacetic acids (HAAs). There are four species in THMs and nine species in HAAs. The sum of five haloacetic acids (HAA₅), i.e., monochloroacetic acid (MCAA), monobromoacetic acid (MBAA), dichloroacetic acid (DCAA), dibromoacetic acid (DBAA), and trichloroacetic acid (TCAA), is regulated in the US, while only the sum of DCAA and TCAA, i.e., HAA₂, is regulated in Korea.

There are several methods to control the concentration of DBPs: (1) change of disinfection sites, (2) removal of DBPs after their formation, (3) removal of precursors prior to disinfection, and (4) use of alternative disinfectants which create fewer DBPs. One of the most effective and economical methods to control DBPs in conventional WTPs is to remove precursors before they react with disinfectants. Bromide ion is not removed in most separation processes used in conventional WTPs, while the organic materials can be removed. Therefore, many WTPs have focused on the removal of NOM to control. As free chlorine residual should be above 0.1 mg/L at the tap by Korean law, most of the WTPs in Korea have used chlorine disinfectants (e.g., chlorine gas, sodium hypochlorite) instead of non-chlorine disinfectants (e.g., ozone, UV).

One of the best available technologies (BATs) recommended by the US Environmental Protection Agency (EPA) for the control of DBPs is the adsorption of precursor such as NOM by granular activated carbon (GAC). In general, precursors (e.g., NOM) are more adsorbable than DBPs; therefore, adsorption by activated carbon is generally applied before chlorination. Many WTPs in Korea include some form of activated carbon adsorption, either powdered activated carbon (PAC) or GAC (mostly PAC), to remove taste, odor, or NOM. One of the main advantages of PAC is low capital cost, but its applicability is limited to the low concentration of organic materials due to the short contact time. In GAC adsorption, there are two options. One is to build a GAC adsorber generally after the sand filter. The other is to retrofit a sand filter to a GAC filter-adsorber (FA). Retrofitting the sand filter to the GAC FA may be considered when DBPs are hard to control in conventional water treatment as well as PAC, when more stringent standards for DBPs are imposed, or when the annual cost of PAC is too large.

In a GAC FA, removal of turbidity as well as organic materials can be achieved simultaneously found that GAC as a replacement for conventional rapid sand filter is as effective as conventional filtration media for the removal of turbidity, provided that appropriate medium size and empty-bed contact time (EBCT) have been selected.

Much research on DBPs removal has been focused on NOM removal only a few results have been recently reported on the removal of preformed DBPs in controlled experiments. Therefore, it was needed to investigate the removal of DBPs when the concentration of DBPs formed before postchlorination was high.

The objective of this research was to investigate TOC, DOC, DBPs, turbidity, and manganese removal in a GAC FA compared with a sand filter at a full-scale WTP, which had retrofitted a sand filter with a GAC FA. At the same time, specific removal mechanisms for THMs and HAA₅ were explored.

The following results were obtained based on 3 years of investigation of Buyeo WTP, which had retrofitted a sand filter to the GAC FA for the removal of preformed DBPs through pre-chlorination as well as DOC.

1. Removal of THMs and HAA₅ in the GAC FA by adsorption was high at the early stage of operation, but it was substantially decreased after 3 months of operation for THMs and 3.5 months for HAA₅. HAA₅ were more easily adsorbed to GAC than THMs, and earlier breakthrough was noticed in THMs than HAA₅.

2. The removal efficiency of HAA₅ was again increased (>99%) after 6 months of operation and it can be attributed to biodegradation on the surface of GAC FA. Biological activity was closely dependent on water temperature; the HAA₅ removal increased up to 99% during the warm season (April 2004-October 2004), but it was decreased to 34% during the cold season (January 2005-March 2005). Early in the GAC FA operation, removal was due to physical adsorption, but later on it was mainly caused by biodegradation.

3. The removal of DOC decreased to below 50% after 3.5 months of operation, but even after that a small portion (>10%) of DOC was continuously removed and it can be attributed to biodegradation.

4. The GAC FA was more efficient in terms of turbidity removal compared with the sand filter, but this difference was minimal considering differences of L/d in both filters.

5. Mn removal in the GAC FA was quite low compared with MOCS, therefore when there is a need to retrofit a sand filter to a GAC filter for water with high concentrations of Mn and DBPFP, partial retrofitting of filters is recommended. By doing so, operators can have flexibility in operating the filters depending on the raw water quality.