以水質面向探討台灣自來水用戶水錶積垢成因

Effect of water quality on the scaling formation of water meters in Taiwan

吳宗昱(Tsung-Yu Wu)

指導教授 ： 黃志彬

國立交通大學/工學院/環境工程系所/碩士(2019年)

摘要

經濟部標檢局於民國105年抽驗台灣自來水用戶水錶積垢情形，於瑞芳地區發現了高達74%的不合格率，遠高於其他地區，猜測原因為用戶水錶內部的乳黃色積垢物所致，而此積垢物除了造成水錶內部阻塞以外，還可能因為積垢物累積而使入水口截面積縮小，使得水流流速加快，影響用水度數之準確性。
為了解決此一現象，本研究開始針對用戶水錶的積垢狀況做探討，以期解決台灣用戶水錶之積垢情形。除了瑞芳地區，本研究另外選定完全無積垢的板橋地區作為對照組，發現了不同地區由於水錶流經之自來水的不同使得水錶積垢狀況有所不同，加上過去文獻指出，黃銅合金鑄件在特定水質條件中很容易產生腐蝕而生成積垢，推測積垢之形成受水質影響之深，因此著手針對於水質面向探討積垢問題。
本研究可分為三個部分，第一部分首先對於有積垢與無積垢之地區進行各項水質分析，如pH、濁度、鹼度、水溫、溶氧、導電度、總溶解固體物、氧化還原電位、自由有效餘氯、結合有效餘氯、LSI值以及金屬含量，第二部分是對於積垢組成做有機物及無機物分析，結果顯示積垢物主要含有碳酸鋅、氫氧化鋅、氧化鋅，少量含有鋁矽氧化物以及部分脂肪酸，確立了積垢物為黃銅之腐蝕產物，最後是以統計分析找出具有顯著差異且相關性及代表性高的水質參數，結果顯示，當pH、鹼度及導電度越高，則積垢量生成越多並；反之，當溶氧、氧化還原電位、餘氯越高，則積垢量越少，推測瑞芳地區由於位於易腐蝕之pH區間，且餘氯量低，使得生物腐蝕可能性較高，加上高金屬含量使導電度提高，升高水中黃銅水錶之腐蝕電流，進而加劇腐蝕作用；而板橋地區無積垢生成之現象，推測為其自來水pH偏中性，避開了易腐蝕之區間值，且其餘氯量較高，使得氧化還原電位升高，致使黃銅水錶表面產生鈍化反應而抑制腐蝕反應，同時高餘氯量及偏中性的pH值使得水中不易產生生物腐蝕。
由本研究之結果得知主要促進因子為pH、鹼度、導電度，抑制因子為餘氯、氧化還原電位、溶氧，因此若在淨水廠出水水質中降低pH值至中性，且提高加氯量以升高氧化還原電位，促使黃銅水錶產生鈍化反應，也避免生物腐蝕，如此一來便可以使得黃銅水錶不受腐蝕侵蝕而生成積垢物。

關鍵字

水錶 ； 黃銅合金 ； 脫鋅腐蝕 ； 積垢 ； 自來水 ； 瑞芳

並列摘要

Brass dezincification corrosion of water meter is a significant problem in potable water systems due to corrosion product (scalants) build-up and pipe blockage, and pitting failure. In addition to cause internal blockage of the water meter, the scaling may also reduce the cross-sectional area of the water inlet. This might cause the increase of water flow of the water meter, even affecting the accuracy of the record of water consumption. It was pointed out that brass alloys are prone to corrosion in certain water quality conditions and to form scalants.
The formation of scaling is deeply affected by water quality. This study was thus to investigate the effect of the water quality on the scaling formation in the water meter in Taiwan. In addition to the Ruifang area, this study also selected the Banqiao area as the control group, which was found comprising insignificant scaling inside water meter. This study can be divided into three parts. The first part was to analyze various water quality in Ruifang area and Banqiao area, e.g., pH, turbidity, alkalinity, temperature. Then the second part was the analysis of organic matter and inorganic compositions of the scalants. The third part was to identify the correlation between water quality and scaling by statistical analysis.
It was found that different areas have different conditions of scalant due to the differences in the water quality. The results showed that the scalants mainly contained zinc carbonate, zinc hydroxide, zine oxide, a small amount of aluminum oxide and bismuth oxide and fatty acids. It is inferred to be the product of dezincification corrosion, as known as, meringue. The results also showed that the meringue dezincification was favored by higher pH (between 8.0 and 9.0), alkalinity and conductivity. Conversely, increased free chlorine and chloramine may inhibit meringue dezincification because of the high oxidation-reduction potential. the water meter in Ruifang area is in the corrosive pH range, and the residual chlorine content is low, which induced the possibility of microbiologically influenced corrosion(MIC). The high metal content of water increased the conductivity then rose the corrosion current of the surface of water meter, which aggravated the corrosion condition. However, there was negligible phenomenon of scaling formation inside the water meter in Banqiao area because the pH of water in this area was in neutral. Furthermore, the residual chlorine was higher (above 0.3 mg/L), which caused the oxidation-reduction potential to rise, resulting in a passivation reaction on the surface of the water meters. Therefore, the high residual chlorine and neutral pH keep the water meter away from MIC.
From the results of this study, the pH, alkalinity, and conductivity have positive correlation with scaling, and the inhibitors are residual chlorine, oxidation-reduction potential, and dissolved oxygen. Therefore, if the pH of the water is in neutral, and the chlorination amount is increased to increase the oxidation-reduction potential, the water meter might generate the passive layer, at the same time, avoid MIC. In this way, the water meter can be prevented from meringue.

並列關鍵字

water meter ； brass ； dezincification ； scaling ； potable water ； Ruifeng

參考文獻

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