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Estimation of Short-term Chlorine Demand and Its Correlation with Available Iron in Drinking Water

Md. Rafiqul Islam
International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B), 68 Shahid Tajuddin Ahmed Sarani, Mohakhali, Dhaka 1212, Bangladesh.

Md. Mamun Hossain
Department of Chemistry, Jahangirnagar University, Savar, Dhaka-1342.

Abstract/ Executive Summary:

This study describes the effect of iron on chlorine demand and the efficiency of chlorination in drinking water. To find out the association between short-term chlorine demand and available iron, 15 raw water samples were analyzed for iron as well as other physicochemical parameters and then dosed with 7.8 mg/l & 3.9 mg/l of chlorine respectively. In chlorinated water samples, chlorine demands were estimated at 30 minutes and 24 hours of contact time respectively. It reveals that chlorination of water containing higher iron concentration lowers the amount of residual chlorine causing elevated chlorine demand and this may happen more rapidly within 30 minutes of addition of chlorine to water than that of 24 hours. Datapoint of all analyzed samples suggests that the concentration of available iron in drinking water is positively correlated (r >0.6; p<0.05) with chlorine demand.

Keywords: Chlorination; Chlorine demand; Correlation; Drinking water; Iron; Residual chlorine

Location: Kishoreganj district in Bangladesh

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Drinking water

Objectives:

To find out the extent of correlation particularly in between chlorine demand and available iron along with other physicochemical parameters in drinking water collected from a rural community in Bangladesh.

Methodology:

Raw water samples were collected from a rural community of Kishoreganj district in Bangladesh which is 130 kilometers away from the Dhaka Metropolitan City. Fifteen untreated drinking water samples were collected into high-density polyethylene containers maintaining 4 °C to 10 °C temperature in the cool box and transported to the laboratory within 24 hours of the collection with the logistic support of the International Centre for Diarrhoeal Diseases Research, Bangladesh (ICDDR, B). Then Samples were immediately processed for the measurement of pH using a pH meter (Orion 2star, benchtop pH meter, Thermo Scientific, USA), turbidity using a turbidity meter (Oakton, T-100 Turbidimeter, Singapore), temperature, and conductivity using a conductivity meter (Hach, sension5 conductivity meter, USA), residual free chlorine (method described below) and thereafter prepared for chlorine dosing. All experiments were carried out in the environmental laboratory of ICDDR, B. Sodium hypochlorite solution (Clotech, Global heavy chemical, bd.) containing 5.25% of free available chlorine was used as the chlorinating agent. All water samples (including chlorine demand free deionized water for the control run) were dosed with 7.8 mg/l and 3.9 mg/l of chlorine respectively for the particular contact time of 30 minutes and 24 hours respectively. The aim of the dosing was to produce a residual concentration of ≥0.5 mg/l as chlorine after the desired exposure time (at least 30 minutes contact time) at pH <8.0. PH of all analyzed water samples was maintained within the range of 6.0 to 8.0 throughout the entire experiment and was determined using a portable pH meter (Hach, sension1, USA). Chlorination was conducted in 300 ml chlorine demand-free glass stoppered bottles and after being dosed with chlorine, samples were stored headspace-free at 20 ^o C in the dark. Residual free chlorine of the water samples was measured at 30 minutes and 24 hours of exposure following the standard DPD (N, N-diethyl-p-phenylenediamine) ferrous titrimetric method. All reagents used in this experiment were ACS analytical grade. 2.4 Chlorine demand calculation Chlorine demand was calculated by subtracting the residual chlorine concentration from the concentration of chlorine dosed at a fixed contact time, after correction for blanks. The chlorine demand obtained in this way was quoted for a given chlorine dose, contact time, temperature, and sample pH. Following expression was used for the calculation of chlorine demand in water samples: Chlorine demand = Chlorine added concentration (mg/l) – Chlorine residual measured concentration (mg/l). In this study, the concentration of total iron was determined following the standard phenanthroline method. 50 ml (or a portion diluted to 50 ml volume if the sample contains more than 4.0 mg/L of iron) of sample water plus 1 ml of hydroxylamine hydrochloride (10 g dissolved in 100 ml deionized water) plus 2 ml of concentrated (37%) hydrochloric acid were taken in a 250 ml Erlenmeyer flask and were heated until the volume is reduced to 15 to 20 ml. Then after cooling to room temperature 10 ml of ammonium acetate buffer (125 g of ammonium acetate dissolved in 75 ml deionized water and 350 ml glacial acetic acid) and 4 ml of 1, 10-phenanthroline monohydrate (0.1 g dissolved in 100 ml deionized water) were added and finally diluted to 100 ml mark with deionized water. After 15 minutes, absorbance was taken at 510 nm using a UV-V is spectrophotometer (Varian, Cary 50, Australia) after correction for blank. The concentration of total iron was determined in milligrams per litre in the sample from the absorbance by reference to the calibration curve prepared by using a suitable range of iron standards containing the same amounts of phenanthroline, hydroxylamine, and ammonium acetate buffer as the sample [19]. Data analyses were performed using SPSS software for Windows (Version 16.0, Inc., Chicago, IL, USA). Arithmetic means, standard deviation, maximum, minimum was analyzed following basic descriptive statistics. Paired samples T-test was used to determine the significant differences between variables and within variables and significant criteria level was set at p≤0.05. Correlation among the studied parameters was analyzed using Pearson’s correlation matrix analysis which was applied to measure the strength of association between variables. Correlation coefficients(r) and p values were computed from the correlation matrix.

Source of Information: International Journal of Chemical Studies 2014; 2(4): 55-59

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Achievement/Findings:

Chlorine demand for water depends on various physicochemical factors present in the water body as well as its surrounding environment. It varies from sample to sample. Within all other organic and inorganic pollutants in water, iron is one of the most important parameters that are responsible for the consumption of chlorine. As a result, it interferes with the effectiveness of chlorination of water to a large extent. Iron plays an important role in decreasing the concentration of free residual chlorine within 30 minutes of the addition of chlorine in the water. So this study suggests that high iron-containing water requires an elevated amount of chlorine to maintain the efficiency of disinfection.

Knowledge Management: Journal