Removal of natural organic matter from water sources by enhanced coagulation method

İçme suyu kaynaklarındaki doğal organik maddelerin zenginleştirilmiş koagülasyon yöntemi ile giderilmesi

Abstract

The disinfection of water with chlorine results in formation of disinfection by-products (DBP), such as trihalomethanes (THM) and haloacetic acids (HAA). It has been shown that natural organic matter (NOM) is a likely precursor material for THM and HAA formation. Among DBP found in chlorinated water, THM and HAA have been the focus of particular attention because they are considered potentially carcinogenic. Concerns about health risks associated with DBP have prompted several industrialized countries to establish maximum acceptable levels for THM and HAA concentrations in drinking water. In the United States (US), more stringent DBP regulations have been promulgated, placing limits on THM and HAA. The US Environmental Protection Agency (USEPA) drinking water limits for THM and HAA is 80 $mu g/L$, and 60 $mu g/L$, respectively. Recently most of the European countries regulated THM in their water at the level of 100 $mu g/L$. Further, there has been a 150 $mu g/L$ THM limit Turkish Drinking Water Regulation since 2005. Aluminum or iron coagulation may remove NOM by two general mechanisms. The first one is adsorption onto aluminum hydroxide [Al(OH)3] or iron hydroxide [Fe(OH)3] floc, and the second mechanism is formation of insoluble complexes in a manner analogous to charge neutralization. Generally, the former mechanism is more dominant at a higher coagulant dosages and higher pH values; whereas the later mechanism is relatively more dominant at lower dosages and lower pH conditions. Enhanced coagulation (EC) efficiently removes DBP precursors from source water. The term of EC refers to the modification of the coagulation process to achieve greater DBP precursors treatment. Reduction in the concentration of organic precursors prior to the application of disinfectant could be expected to result in a decrease in DBP formation. The objectives of this study were to evaluate the effect of alum and ferric chloride coagulation dosages on DBP precursors removal. Thus, DOC, ultraviolet absorbance at 254 nm $(UV_{254})$, and the associated reductions in THM formation potential (THMFP) removal in Büyükçekmece water were investigated The coagulant dosages were ranged from 20 to 160 mg/L, when treating Büyükçekmece water. It has been found that, at similar coagulant dosages, ferric chloride consistently outperformed alum in terms of DOC removal from raw water. As both coagulants dosages increased further, the reduction in DOC also increased gradually. It was concluded that ferric chloride has the maximum DOC removal capacity of 58% at the dosage of 120 mg/L while 120 mg/L alum treatment resulted in an overall DOC removal 48%. The overall fraction of NOM amenable to coagulation by ferric chloride is greater than for alum. Although other studies show a greater affinity of a fraction of NOM for ferric hydroxide than for aluminum hydroxide, in our study alum flcos adsorbed more DOC substances than ferric flocs. On the other hand, since ferric chloride produces more hydroxide flcos than alum, the overall DOC removal efficiency is found to be high for ferric than alum. $UV_{254}$ is instrumental in identifying the aromatic content of water. Ferric reduced the $UV_{254}$ level to 0.028 cm-1 with the dosage of 120 mg/L, while, 100 mg/L alum coagulation of $UV_{254}$ resulted in an absorbance level of 0.050 $cm^{-1}$. The percent removal of $UV_{254}$ with ferric chloride and alum was about 79, 62%, respectively. As observed in all the cases, at similar coagulant dosages, ferric chloride consistently outperformed alum for $UV_{254}$ removal. In this study, the $UV_{254}$ material was generally removed to a greater extent than DOC. These results are consistent with literature findings suggesting that aromatic materials are removed more effectively by chemical coagulation than other NOM fractions. The THMFP tests were used to evaluate one aspect of THM formation resulting from precursor removal by EC. Reduction in THMFP is attributed to removal of THM precursors. Thus, the trends in THMFP reduction as a function of ferric chloride and alum dosage are similar to those observed with DOC. It was found that ferric chloride was found to be more effective than alum in terms of THMFP removals. The maximum THMFP removal with ferric chloride coagulation was 71 percent for 120 mg/L ferric chloride dosage, and 50 percent for 120 mg/L alum dosages, respectively. Thus, EC appears to be an effective treatment technique for THMFP removal from Büyükçekmece water in this study. The results showed that at similar coagulant dosages, expresses as dry weight of coagulant, ferric chloride consistently outperformed alum in terms of DOC, $UV_{254}$ and THMFP removals.

Özet

İçme suyunda klorlama işlemi süresince, klor doğal organik maddelerle (DOM) reaksiyona girerek trihalometan (THM) ve haloasetik asit (HAA) gibi insan sağlığına zararlı olduğu belirtilen dezenfeksiyon yan ürünlerini (DYÜ) meydana getirir. Zenginleştirilmiş koagülasyon metodu, sudaki organik maddeyi gidererek içme suyundaki DYÜ oluşumunu azaltan bir arıtma tekniğidir. Bilimsel olarak, organik maddenin koagülasyonla giderilmesi, sudaki DOM’un konsantrasyonuna, DOM’un kimyasal yapısına ve bileşimine, koagülant türüne, koagülant dozuna ve koagülasyon pH’ına bağlıdır. Günümüzde, içme suyu arıtma tesislerinin en önemli problemlerinden biri sudaki DOM’u minimize etme konusunda güçlüklerle karşılaşmalarıdır. Alüminyum sülfat ve demir klorür DOM’u iki genel mekanizma ile giderebilmektedirler. Birinci mekanizmada DOM alüminyum ve demir hidroksit flokları üzerinde adsorbe edilir. İkinci mekanizmada ise çözünmez kompleksler oluşturularak, flokların su ortamından uzaklaştırılması sağlanır. Humik ve fulvik asitler alüminyum hidroksit veya demir hidroksit çökeltisinin oluşumundan önce, çoğu pH şartlarında çok güçlü kuvvetli ligant gibi davranan demir ve alüminyum katyonik türleri ile öncelikle kompleks oluşturmaktadırlar. Bu adsorbsiyon işlemi katılar ile birlikte çözünebilen kompleksleri de içerir. İkinci mekanizma, metal komplekslerinin oluşumu için ligant görevi yapan DOM’un asiditesinden etkilenmektedir. Bu çalışmanın amacı İstanbul’un en önemli yüzeysel su kaynaklarından biri olan Büyükçekmece Gölü suyunda zenginleştirilmiş koagülasyon ile jar testi deneyleri gerçekleştirerek, zenginleştirilmiş koagülasyon prosesinin ÇOK, $UV_{254}$, ve THM oluşum potansiyeli (THMOP) arıtma verimi belirlemeye çalışmaktır.