<![CDATA[Well water is the primary source of clean water widely used in Islamic boarding schools. However, its microbiological quality often fails to meet health standards due to contamination by fecal indicator bacteria such as Escherichia coli. Previous studies have demonstrated that sodium hypochlorite (NaOCl) is effective in reducing coliforms and E. coli; however, these studies generally used river water and relatively high NaOCl concentrations. Studies investigating the use of low-concentration NaOCl for well water disinfection while considering chlorine residual and pH as safety indicators remain limited. Preliminary testing of well water from Mamba’ul Hisan Combong Islamic Boarding School revealed the presence of E. coli contamination. This study aimed to analyze the effect of different NaOCl concentrations on Escherichia coli, total coliforms, chlorine residual, and pH, as well as to determine the most effective and safe NaOCl concentration for well water disinfection. An experimental study using a Completely Randomized Design (CRD) was conducted with five treatment groups: control (0 ppm), 0.5 ppm, 1.0 ppm, 1.5 ppm, and 2.0 ppm NaOCl, each with three replications. E.coli was analyzed using the Petrifilm method after a contact time of 10 minutes. The data were analyzed using the Shapiro–Wilk, Levene’s, and Kruskal–Wallis tests. The results showed that NaOCl concentration significantly affected E.coli (p = 0.008) and total coliform counts (p = 0.026), as well as chlorine residual (0.2–0.4 mg/L), pH (7.1–6.6), while the water temperature remained constant at 25°C. A concentration of 1.5 ppm was identified as the optimal concentration based on its effectiveness in reducing E.coli and total coliforms while maintaining acceptable chlorine residual and pH levels. These findings demonstrate that NaOCl is an effective and practical disinfection method for improving the microbiological quality of well water. Keywords: well water, disinfection, Escherichia coli, sodium hypochlorite (NaOCl), Islamic boarding school. ABSTRAK Air sumur merupakan sumber utama air bersih yang banyak digunakan di lingkungan pondok pesantren. Namun, kualitas mikrobiologisnya sering tidak memenuhi standar kesehatan akibat kontaminasi bakteri indikator pencemaran fekal seperti Escherichia coli. Penelitian terdahulu menunjukkan bahwa Natrium Hipoklorit (NaOCl) efektif menurunkan jumlah coliform dan Escherichia coli, tetapi umumnya menggunakan air sungai dan konsentrasi NaOCl yang relatif tinggi. Kajian mengenai penggunaan NaOCl berkonsentrasi rendah pada air sumur dengan mempertimbangkan residu klorin dan pH sebagai indikator keamanan masih terbatas. Hasil pengujian awal air sumur Pondok Pesantren Mamba’ul Hisan Combong menunjukkan adanya kontaminasi Escherichia coli. Penelitian ini bertujuan menganalisis pengaruh variasi konsentrasi NaOCl terhadap jumlah Escherichia coli, Total Coliform, residu klorin, dan pH serta menentukan konsentrasi NaOCl yang paling optimal dan aman untuk desinfeksi air sumur. Penelitian menggunakan metode eksperimen dengan Rancangan Acak Lengkap (RAL) yang terdiri atas lima perlakuan, yaitu kontrol (0 ppm), 0,5 ppm, 1 ppm, 1,5 ppm, dan 2 ppm NaOCl dengan tiga kali ulangan. Pengujian Escherichia coli dilakukan menggunakan metode Petrifilm setelah waktu kontak 10 menit. Data dianalisis menggunakan uji Shapiro–Wilk, Levene, dan Kruskal–Wallis. Hasil penelitian menunjukkan bahwa variasi konsentrasi NaOCl berpengaruh signifikan terhadap jumlah Escherichia coli (p=0,008) dan total coliform (p=0,026), serta nilai residu klorin (0,2-0,4 mg/L), pH (7,1-6,6), dan suhu 25oC. Konsentrasi 1,5 ppm merupakan konsentrasi paling optimal berdasarkan efektivitas penurunan bakteri Escherichia coli, total coliform, residu klorin, dan pH. Penelitian ini menunjukkan bahwa NaOCl efektif sebagai metode desinfeksi sederhana untuk meningkatkan kualitas mikrobiologis air sumur. Kata kunci: air sumur, desinfeksi, Escherichia coli, Natrium Hipoklorit (NaOCl), pondok pesantren]]>
