Abstract
Background/ Objectives: Chemical Oxygen Demand (COD) is a widely used aggregate organic pollutant index in national discharge standards and is one of the critical parameters for water quality assessment. The conventional COD determination by wet oxidative method encounters the problem of chloride interference in water samples with high chloride content. The dichromate reflux method oxidizes 85-95% of organic matter and also 100% of chloride ions in the samples, without an appropriate chloride masking strategy. The standard method cannot measure COD accurately in samples containing more than 2000 mg L-1 chloride. The present study demonstrates and validates a modified procedure for the determination of COD in industrial sample with high calcium chloride content and low organic load. Method: Petrochemical industries manufacturing Propylene Oxide and Propylene Glycol via chlorohydrin route generate wastewater with high calcium chloride content. Due to high chloride content, the standard analysis method could not produce reliable data for COD values with known precision and accuracy. The standard method based on dichromate was modified, with an additional mixing step for a specified time to enhance the chloride masking by mercuric sulphate. Undiluted sample (20 mL) was mixed with the required amount of mercuric sulpahte to maintain 10:1 ratio to chloride content and 10 mL of sulphuric acid at 150-175 RPM for 20 minutes at room temperature to increase the contact time/ chloride masking. Method validation attributes of the modified procedure were assessed and found to be complying with the acceptance criteria. Findings: We developed a modified procedure for the standard dichromate based chemical oxygen demand (COD) determination in industrial wastewater samples rich in calcium chloride. An additional mixing step is introduced in the standard procedure to enhance the chloride masking with mercuric sulphate in samples containing high chloride. The amended procedure resulted in accurate COD values in samples with low organic load, rich in calcium chloride and the modifiedmethod was successfully validated. Novelty: The modified procedure with mixing step to enhance chloride masking was found to be satisfactory for determining COD in calcium chloride-rich industrial effluent. Keywords: Chemical Oxygen Demand (COD); Chloride interference; Modification of standard method; Wastewater analysis; Calcium chloride; Validation
Highlights
Aggregate organic pollutant load in water can be assessed by measuring the water quality parameter chemical oxygen demand (COD), which is widely used as a national discharge standard to investigate the organic pollution in water(1)
In order to calculate method linearity, test samples (n=8) with varying COD concentration were prepared with chloride content of 35,000 mg L-1 and compared the measured value
The present study proposes a modified procedure of COD determination especially for industrial wastewaters containing high calcium chloride content and low organic load
Summary
Aggregate organic pollutant load in water can be assessed by measuring the water quality parameter chemical oxygen demand (COD), which is widely used as a national discharge standard to investigate the organic pollution in water(1). It is used widely as a measure of organic pollutant load in wastewaters (2,3). COD is considered as a critical parameter for water quality investigations(4). COD is defined as the number of oxygen equivalents required to oxidize organic components in a sample (1,5,6) and represents the measure of organic pollutant load in water(4). COD is considered as a more superior representative of organic matter in water than biochemical oxygen demand (BOD), total organic carbon (TOC) (7), and total oxygen demand (TOD) (8)
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