Pilot study of the removal of THMs, HAAs and DOC from drinking water by GAC adsorption

The objective of this pilot study was to evaluate the performance of a GAC postfilter-adsorber for the removal from the drinking water of Athens, Greece, of the two main groups of chlorination by-products, trihalomethanes (THMs) and haloacetic acids (HAAs), as well as of dissolved organic carbon (DOC). The analyses performed during the whole operation period (638 days) showed that the GAC breakthrough capacity for DOC was much higher than the capacity for total HAAs, which was higher than that for total THMs. The removal of THMs and the most part of the removal of HAAs and DOC should be attributed to adsorption by GAC, while that of a smaller part of HAAs and DOC may be attributed to biodegradation in the adsorber bed, where dechlorination, caused catalytically by the carbon surface, favoured microbial growth. Additionally, the GAC postfilter-adsorber showed a much higher adsorption efficiency than a GAC filter-adsorber, due to the smaller size of the carbon and the lower hydraulic loading rate. Also, observed desorption incidents of THMs (mainly) and HAAs, especially during the postfilteradsorber operation, were favoured by the same factors. Formation of THMs within the GAC bed was also indicated by the mass balance of total THMs during the whole cycle.

The pilot study of adsorption of THMs, HAAs and DOC by GAC postfilter-adsorber gave satisfactory results. The GAC breakthrough capacity for DOC was higher than the capacity for total-HAAs, which was higher than that for total-THMs (227.05, 4.37 and 2.83 g/kg GAC respectively). The same was observed concerning the GAC bed life, while the carbon usage rate (CUR) followed the opposite order, as expected (Table 3). The higher efficiency of GAC for the removal of the dissolved organic matter than of the smaller molecules of HAAs and THMs was according to the relevant literature. The removal of THMs and the most part of the removal of HAAs and DOC should be attributed to adsorption by GAC, while that of a smaller part of HAAs and DOC may be attributed to biodegradation in the adsorber bed, where dechlorination, caused by the catalytic action of the activated carbon, favoured microbial growth. The problem of continuous desorption of THMs after the saturation point was also encountered (Fig. 4). The reason for it could have been a reduction in TTHMs influent concentration or competitive adsorption. The mass balance of TTHMs during the whole cycle showed that the desorbed mass of TTHMs was more than the adsorbed mass. This fact indicates the probability of formation of new THMs molecules in the GAC bed, by either decomposition of adsorbed trihaloacetic acids or reaction of chlorine with the adsorbed organic matter. Two incidents of desorption of HAAs were attributed to a sudden reduction of THAAs influent concentration. No desorption of DOC was noticed. A comparison between the adsorption efficiency of the GAC postfilter-adsorber and that of the GAC filter-adsorber in the first part of this research showed that the postfilter-adsorber had a significantly higher breakthrough capacity for TTHMs, THAAs and DOC (3.9, 2.7 and 4.1 times higher respectively) (Table 4). The better adsorption efficiency of GAC postfilter-adsorber should be attributed to the smaller effective size of the carbon (larger surface area) and the longer EBCT (lower hydraulic loading rate). Phenomena of desorption of THMs and HAAs were observed more frequently during the operation of the GAC postfilter-adsorber. This fact can also be attributed to the above factors according to some researchers, since better adsorption efficiency of a system gererally produces a higher rate of desorption from a drop in influent concentration and a larger overshoot from competition. This work has demonstrated the significant efficiency of GAC postfilter-adsorber in the removal of THMs, HAAs and DOC from the drinking water of Athens, Greece. Further research is recommended on the importance of desorption from carbon beds and on methods to reduce it.