No abstract

Biological filtration refers to the process of removing both particulate matter and biodegradable organic matter (BOM) from water [1]. Biological filters can remove organic compounds through the fixed biofilm that develops on various media such as sand, anthracite, granular activated carbon (GAC), or membranes [2]. The biologically active rapid filter and the biological activated carbon (BAC) filter have been widely used for decades in drinking water treatment plants [1-3]. The rapid filter is usually filled with sand or anthracite, and is considered the most economical way to remove particles and BOM with the same filter unit [1]. However, many previous studies have demonstrated that the BAC filter is a more appropriate process to remove BOM than the rapid filter [4, 5]. Generally, the combination of ozonation and GAC filter is referred to as the BAC process [3]. Ozonation converts high-molecular-weight refractory organic matter into low-molecular-weight BOM, and this increased BOM can be effectively removed by the bacteria attached to the BAC filter [1, 4, 6-8]. Biological filtration has many advantages for drinking water treatments. First of all, it is important to increase the biostability of drinking water by reducing the quantity of BOM which can cause bacterial regrowth in distribution systems [1, 3-5]. It can reduce the disinfection by-product

The feasibility of on-site generated chlorine (OSG chlorine) as an alternative disinfectant to chlorine gas was evaluated in terms of total organic carbon (TOC) removal, disinfection efficiency, biofilm control, disinfection by-products (DBPs) formation, chlorine decay rate, and volatility. Chlorine gas decreased the pH of the treated water by -0.1 per mg/L of free available chlorine (FAC) while OSG chlorine increased the pH by + 0.06 per mg/L. OSG chlorine with more hypochlorite ion (OCl-) at higher pH was less effective in the inactivation of suspended bacteria and TOC removal but remained in the distribution system longer and controlled biofilm formation more effectively than chlorine gas. The DBPs formation by OSG chlorine was not significantly different from that by chlorine gas except for the reduction of Haloacetonitriles. Hypochlorous acid (HOCl) was more volatile than OCl-, indicating the lower volatility of FAC in the OSG chlorine-treated water. Two types of OSG systems, “Mixed oxidants” and OSG hypochlorite, did not show any significant difference in disinfection, DBPs formation, and chlorine decay rate (paired t-test, p = 0.40, 0.11 ~ 0.70, > 0.42). A significant synergy effect by oxidants other than FAC cannot be expected in the use of “mixed oxidants” at a water treatment plant.

The seasonal effects on the biostability of drinking water were investigated by comparing the seasonal variation of assimilable organic carbon (AOC) in full-scale water treatment process and adsorption of AOC by three filling materials in lab-scale column test. In full-scale, pre-chlorination and ozonation significantly increase AOCP17 (Pseudomonas fluorescens P17) and AOCNOX (Aquaspirillum sp. NOX), respectively. AOC formation by oxidation could increase with temperature, but the increased AOC could affect the biostability of the following processes more significantly in winter than in warm seasons due to the low biodegradation in the pipes and the processes at low temperature. AOCP17 was mainly removed by coagulation-sedimentation process, especially in cold season. Rapid filtration could effectively remove AOC only during warm seasons by primarily biodegradation, but biological activated carbon filtration could remove AOC in all seasons by biodegradation during warm season and by adsorption and bio-regeneration during cold season. The adsorption by granular activated carbon and anthracite showed inverse relationship with water temperature. The advanced treatment can contribute to enhance the biostability in the distribution system by reducing AOC formation potential and helping to maintain stable residual chlorine after post-chlorination.