A new river-environment formed by the 4-River Restoration Project may require different efforts to improve river water quality from the ones that have been made before. Various methods were reviewed for this purpose, however, most of them had significant limits in applying to the new situation. Of those reviewed, filtration pond technology, a modified version of riverbed filtration which was increasingly popular in Korea, was regarded as to be the most appropriate for this purpose. In order to understand removal efficiency of various pollutants in the filtration pond, pilot and laboratory experiments were performed. Pilot experiment was to find the removal efficiency of various pollutants along the filtration depth according to the grain size and cross-flow velocity. Laboratory experiments were to find a way to remove phosphorus in order to prevent eutrophication in the major rivers using the filtration pond and also to understand the removal efficiency of toxic micro-pollutants in the filtration pond.
Pilot-scale test-bed was operated employing three soils with different grain sizes dredged from the Nakdong River to obtain the design and operation parameters of the multi-purpose filtration pond, such as the filtrate productivity of the filter sand, the appropriate removal period of the surface clogging and the contaminant removal efficiency. The cross-flow velocities were applied stepwise ranging from 0 to 40 cm/s in order to simulate the various velocities in the artificial stream of the pond. Results showed that a filtrate production rate of 5 ~ 3 m3/m2-day was maintained by removing the surface clogging every 7 to 13 days and that the filtrate quality was not affected by the factors of the filtrate production rate, the grain size of the filter sand and the cross-flow velocity. Results also showed that most of the removal occurred within 50 cm of the top soil and that the removal efficiencies with the filtration distance of 2.4 m were 80 ~ 95% for turbidity, 20 ~ 30% for COD, 75 ~ 90% for BOD, 5 ~ 20% for total nitrogen and 20 ~ 60% for total phosphorus, which suggested that particulate matters had a high removal efficiency.
In an effort to find a solution to the eutrophication of major Korean rivers, a method to utilize multi-purpose filtration pond was investigated. As literature showed that oyster shell is known to be the most adequate for the removal of dissolved phosphorus in Korean rivers, batch and column experiments were performed using oyster shell as an adsorbent in this study. The results of the batch experiment showed that the removal of dissolved phosphorus from river water through adsorption as a way of preventing algal growth was not practical. The results obtained from the column experiment, however, suggested that oyster shell may be utilized as an adsorbent under limited conditions. Based on the results of the experiments a methodology was proposed to remove algae from river water through the use of multi-purpose filtration pond. This method involves mechanically removing the accumulated algae cake from the surface of the artificial stream in the pond towards the condensing part located at the lower reach of the stream, where particles gather before the final removal. In addition, employment of oyster shell as an adsorbent in the condensing part allows prevention of phosphorus released from the dead algae re-entering the river water.
An experiment using a 5 m-long sand column was performed to evaluate the resisting capability against micro-pollutants of the infiltration gallery, multi-purpose filtration pond and riverbank/bed filtration, of which the filtration distance is becoming increasingly shorter in Korea. Results suggested that the Korean riverbed sand contained significant amount of organics, resulting in a relatively vigorous adsorption of chloride ion on the sand surface. Results also indicated that while phenol was not detected in the column filtrate, both 1,4-dioxane and diazinon were exposed to adsorption by the sand as they moved through the column, decreasing their peak concentrations during the movement. It can be expected that the peak concentrations will diminish significantly in the practical scale due to its longer travel distance.