逆滲透系統於水回收之最佳操作與維護

Abstract

本研究以迪化污水廠二沉池出流水為對象，利用逆滲透(reverse osmosis, RO)薄膜系統進行污水回收再利用，以達新加坡NEWater再生水水質標準，增加回收水的可利用性，並探討不同前處理設備、操作通量及CIP(clean in place)清洗薄膜時間對薄膜積垢行為的影響，進而以經濟效益分析找出最佳操作條件。 原水經RO系統處理後，其pH值為4.82-7.41，低於水質標準7-8.5，故需加藥調整，其他項目如氨氮、硝酸鹽氮、電導度、總溶解固體及總有機碳皆符合NEWater水質標準。 在無前處理的情況下，薄膜積垢速率為微濾(microfiltration, MF)及超濾(ultrafiltration, UF)膜作為前處理時的10-16倍，故前處理設備能大幅降低薄膜積垢速率。 以MF為前處理時，RO膜最佳操作條件為CIP清洗時間6小時，標準化通量回復率清洗後達96 %，操作通量為10 LMH，單位產水耗電量為15 及20 LMH的73 %及55 %。 以UF為前處理時，RO膜最佳操作條件為CIP清洗時間12小時，標準化通量回復率清洗後達96-100 %，操作通量為10 LMH，單位產水耗電量為20 LMH的63 %。 比較前處理MF及UF在最佳操作條件時之經濟效益，以UF作為前處理時，其最佳操作條件時的單位產水耗電量為MF的69 %。故在利用RO膜回收迪化污水廠二沉池出流水時，其最佳條件為UF作為前處理、CIP時間為12小時及操作通量為10 LMH。

The objective of the present study is to monitor the fouling of reverse osmosis (RO) membrane system in terms of economic analysis by optimization of pretreatment, operation flux rate and frequency of membrane cleaning, and also to achieve the RO permeate water quality to match the Singapore NEWater quality standards. Two types of membranes processes microfiltration (MF) and ultrafiltration (UF) were used for pretreatment before RO filtration at various operating fluxes. The performances of MF and UF processes were monitored in terms of flux recovery and the over all process pretreatment including RO filtration performance was monitored with energy consumption. The RO treatment with MF was carried out at initial operating fluxes of 10, 15 and 20 LMH with an intermittent chemical cleaning for every 6 hr. Results show that a good recovery of about 96 % normalized flux after chemical cleaning was achieved. The energy consumption calculations for only RO membrane filtration after the MF pretreatment at an initial flux of 10 LMH is 68.5 kWh/m3. In comparison the energy consumption for RO filtration after MF pretreatment at low operating flux (10 LMH) is 1.38 and 1.81 times lower than the energy consumptions at operating fluxes of 15 and 20 LMH respectively. In the case of RO system with UF, the pretreatment was carried out with initial operating fluxes of 10 and 20 LMH followed with an intermittent chemical cleaning for every 12 hr. Excellent recovery of 96-100 % was achieved. The energy consumption for only RO membrane filtration after the MF pretreatment at an initial flux of 10 LMH is calculated as 80.5 kWh/m3 and is 1.58 times lower than 20 LMH flux. The energy consumption comparison of whole process of RO filtration including MF and UF pretreatments by taking in to consideration of frequency of chemical cleaning shows that the process involving UF pretreatment results an energy consumption of 69 % with that of MF process. Further the RO permeate quality in terms of turbidity, NO3--N, NH4+-N, conductivity, total dissolved solids (TDS) and total organic carbon (TOC) matches with the Singapore NEWater quality standards, but the pH values of RO permeate was 4.82-7.41 that is lower than the standard values 7.0-8.5. In conclusion, from the energy consumption in achieving high quality standards of permeate, the RO system with a pretreatment of UF membrane is suggestible in the reclamation of water.