淨水場藻類分析及過濾除藻效能評估之研究- 以金門太湖淨水場為例

Abstract

缺水危機為外島地區居民帶來許多不便，儘管已建造多座水庫蓄水以供應民生所需，但受地形限制及氣候因素，降雨不豐沛加上蒸發量大，導致水資源嚴重匱乏。金門地區淨水場原水來源為水庫水，由於上游各種廢污水未經妥善處理直接排入水庫內，造成水庫優養化現象，不僅對水域生態環境造成衝擊，夏季甚至形成藻華。金門太湖淨水場原水存在藻類滋生嚴重之問題，藻類存在過濾單元濾床中，隨著過濾時間增加而於表面增生，縮短濾程造成濾池阻塞，亦增加淨水場前氧化劑與混凝劑的使用量。此外，藻體破裂或藻類死亡釋放藻體毒素到水體中，如未能妥善處理，對於飲用水安全亦造成危害。 由於含藻原水對於淨水程序產生極大負擔，為提升淨水場過濾單元除藻效能以改善金門自來水供水水質為水場重要課題之一。本研究針對淨水場各程序進行藻種調查及基本水質特性分析，藻類以電荷耦合元件搭配顯微鏡之鏡檢方式配合藻類圖鑑加以判定藻類種類及計數，瞭解含藻原水之藻種藻數現況以及現有淨水處理程序對藻類去除之效能。為同步瞭解不同濾料對於藻類物質之去除成效，藉由管柱過濾模組實驗，探討兩管柱間過濾後水質之差異變化，以獲得移除藻類成效最佳之濾料填充比。 淨水場各單元藻種藻數分析結果顯示，各單元優勢藻種皆含有微囊藻及柱孢藻，其餘藻種則隨季節而改變。各單元對於藻體的去除效率，浮除池藻類去除率僅31%，快濾池出水藻類去除率僅21%，快濾單元可有效欄截細針桿藻、微小色球藻及顫藻等，但針對微囊藻及小環藻等小尺寸藻類無法有效移除；部份單元甚至有藻類貫穿現象。管柱過濾模組評估結果顯示，管柱出流水中仍有部份微囊藻，但以錳砂搭配石英砂濾料，兩者填充量比例為1：1時，其過濾移除藻類效能略優於活性碳搭配錳砂，因錳砂及石英砂相較於活性碳其有效粒徑較小，因此錳砂搭配石英砂更可以有效攔截藻體，避免藻體貫穿濾池。 由研究結果可知，夏季高溫期淨水場原水含有大量藍綠藻(微囊藻及柱孢藻等)，然淨水場浮除及快濾單元除藻效能不佳，需重新檢視淨水處理設施整體效能及操作狀況，以提升淨水場除藻效能。對於過濾最適濾料之選擇，雙層濾料其藻類去除效果略優於單層濾料設計，且以錳砂搭配石英砂其過濾效能略優於活性碳搭配錳砂，但隨著濾程增加，濾料表面有藻類滋生現象，未來仍應進行較長時間模場試驗，以瞭解藻類繁生期間水質變化對過濾效能之影響。

The water crisis in the islands brings a lot of inconvenience for the residents. Even reservoirs required to supply water to meet people needs has been constructed, the topography characteristics and climate factors often limit the availability of water. The inadequate rainfall and high evaporation rate leads to severe water scarcity. Reservoir that provide raw water for Kinmen Water Treatment Plant (WTP) receives variety of waste water that has not been properly treated result in eutrophication that not only impacts the water environment but also causes algae bloom during summer. This phenomenon imposes serious problem to Kinmen WTP units. Besides, algae contained in raw water will grow in filter bed and causes clogging that result in the increase of filtration time and shorten the filter bed lifetime. It will also increase the requirement of oxidizing agents and coagulants. In addition, it will release toxins into the water that risks to unsafe drinking water when the algae die and the cell ruptured. Algae-laden raw water generated a great burden on the WTP for its removal processes. The filtration unit applied for algae removal to improve the quality of Kinmen tap water supply is one of the important topics. This study was aimed to investigate the effects of different filter media on the removal efficiency of algae removal by column filter module experiments. The media used includes quartz sand, mixed of manganese sand and quartz sand and mixed of manganese sand with activated carbon, in different ratios. Also, to evaluate the unit performance, algae species and its quantity in the influent and effluent were examined by a electron microscopy with charge-coupled device (CCD). The dominant algae species in every Kinmen WTP unit are Microcystis and Cylindrospermopsis, while other species also exist but change seasonally. Floatation unit had 31% algae removal efficiency while rapid filter unit algae removal rate was only 21%. Rapid filtration unit can effectively remove Synedra, Chroococcus and Oscillatoria, but failed to effectively retain the small size algae, such as Microcystis and Cyclotella. Some of the units even had the higher concentration of algae in its effluent compare to raw water. On the other hand, column filter module assessment results showed that there were still some of Microcystis remained in the water, but when manganese sand and quartz sand mixing ratio was 1:1, the algae removal slightly better than activated carbon mixed with manganese sand. Due to the smaller particle size of manganese sand and quartz sand compared to activated carbon, manganese sand with quartz sand can effectively intercept algae cell and avoid the algae passed through the filter. In conclusion, the high temperature during summer leads a large number of blue-green algae (Microcystis and Cylindrospermopsis, etc.) in WTP raw water. The existing flotation and rapid filtration unit cannot remove algae effectively. To further enhance the effectiveness of algae removal, the evaluation of overall performance and operating conditions of WTP are needed. To choose the optimal filter media, dual-media algae removal is slightly better than the single-layer filter design, especially when manganese sand was mixed with quartz sand in 1:1 ratio. Nevertheless, algae retained on the surface of the filter media will keep increasing, thus a long time mode field test should be performed in the future to learn more about the changes in water quality and filter performance during algae blooms.