生物濾床對配水系統之生物穩定性之影響

Abstract

生物濾床較傳統過濾單元更容易去除有機物質及消毒副產物之潛質，近來常為自來水淨水廠加以利用。為了節省更改系統所需之成本及時間，本研究藉由降低前加氯之劑量，來提升現有濾床之生物降解功能。唯此措施對其配水系統之影響尚屬未知。故本研究架設一模擬傳統淨水程序之模廠，觀察以較低前加氯劑量產生之生物濾床將如何影響配水系統之生物穩定性。同時以鹵化乙腈(Haloacetonitriles, HAN) 作為氮系消毒副產物 (Nitrogenous disinfection byproducts, N- DBPs) 之測量指標，了解氮系消毒副產物在自來水處理系統的的生成與宿命。 模廠的前加氯劑量分別為8 mg/L、4 mg/L、2 mg/L。而模廠之過濾單元共有三組濾床，分別為GAC/石英砂、無煙煤/石英砂以及陶瓷珠。每一組濾床之出水經加氯消毒後，固定含有1 mg/L 之餘氯，停留24小時後之出水即為模擬之配水。 研究結果顯示，前加氯劑量對非揮發性之溶解有機碳 (NPDOC) 及生物可利用有機碳 (AOC) 之降解有顯著影響。生物濾床進流之餘氯降低至1 mg/L時，配水之NPDOC去除率自40%提升至60%。進流之餘氯降低至0.1 mg/L時，配水之平均AOC濃度自200 ug acetate-C/L降至50 ug acetate-C/L以下。同時，以即時定量聚合酶連鎖反應 (Quantitative Polymerase Chain Reaction, Q- PCR) 檢測顯示，所有配水均不含Escherichia coli 及Enterococcus sp.。 台灣有管制的三鹵甲烷中，僅有Chloroforms 測出，且其在清水生成之濃度低於80 μg/L之法規標準。所有HAN中，模廠僅測出Dichloroacetonitrile (DCAN)，在配水系統之濃度範圍為0.91 μg/L 到 2.49 μg/L，而DCAN生成潛質之濃度範圍為1.56 μg/L 到 9.84 μg/L。兩者之生成及去除皆受原水水質影響，而與濾床材質無關。 總結而言，降低前加氯劑量產生的生物濾床系統不僅可去除有機物質，亦具有良好的生物穩定性。但是本研究之系統無法降低DCAN生成潛質，故DCAN仍與消毒添加的餘氯反應，在配水系統中再度產生。

Biofiltration received much attention in recent years because it could remove organic matters and reduce formation potential of disinfection byprducts (DBPs) efficiently. By lowering prechlorination dosage in conventional drinking water treatment plant, the rapid sand filters could be converted to biofilters without renovation and supply better water quality. A pilot plant with three sets of rapid sand filters was established to study the impacts on the water quality resulted from biofiltration. The three filters received various prechlorination dosages and were packed with granular activated carbon (GAC), anthracite, and ceramic beads, respectively. Each filter effluent disinfected for maintaining the residual chlorine of 1 mg/L-Cl2 and entered to a simulated distribution system. The results showed that the prechlorination dosage directly affected the removal of non-purgeable dissolved organic carbon (NPDOC) and assimilable organic carbon (AOC) in distribution system. The removal of NPDOC rose from 40% to 60% as the chlorine residual of the filter influent was under 1 mg/L. The averages of AOC decreased from about 200 to 50 ug acetate-C/L as the chlorine residual of the filter influent was below 0.1 mg/L. By Quantitative Polymerase Chain Reaction analysis (Q-PCR), no Escherichia coli and Enterococcus sp. were found as 0.31 to 0.78 mg/L of residual chlorine were maintained in the distribution system. The detected trichloromethanes (THMs) species was chloroform, which was ranged from 8.5 to 24.1 μg/L in finished water. The dichloroacetonitrile (DCAN) was the only detectable haloacetonitriles chosen as the indicator of nitrogenous DBPs (N-DBPs) in this study. With 2 mg/L prechlorination dosage applied, the filtration process formed lower concentration of DCAN due to the low chlorine residual remained in the influent of filters. The DCAN formation potential ranged from 1.56 to 9.84 μg/L in the distribution system, which resulted the regeneration of DCAN from 0.91 to 2.49 μg/L with the chlorine residual. In conclusion, this study provides evidences that biofiltration not only reduced possibility for microbial regrowth but also released no pathogens. However, the biofiltration system in this study could not remove the precursors of N-DBPs. Therefore, DCAN could be regenerate by DCAN formation potential and chlorine residual remained in the distribution system.