Measuring nitrate concentration in wastewaters with high chloride content

ABSTRACT

The presence of chloride ions in wastewaters in concentrations above 50 mg/L poses interference in several methods used for the measurement of nitrate-nitrogen. The aim of this study was to characterise the reliability and costs of some commonly available methods for the analysis of nitrate concentration in landfill leachate which contains high chloride levels. To investigate the effect of chloride interference, several widely used methods [ion chromatography (IC), continuous flow analysis (CFA), the German standard method (DIN), cuvette test (CUV), standard addition method (SAM) and reflectometric test (REF)] were used to measure the nitrate concentration in synthetic solutions containing varying concentrations of chloride and nitrate-nitrogen. Nitrate recoveries of the various methods were found to decrease in the following rank order: CUV (>95%) > IC (>90%) > CFA (89%) > DIN (88%) > REF (70%) > SAM (<80%). In the second part of the study, the same methods were used to measure nitrate concentrations in samples of biologically nitrified landfill leachate with and without chloride elimination. For leachate samples without chloride elimination, CUV results were well correlated (linear regression) with IC results (slope = 1.02/R² = 0.99) but to lesser extents with results obtained by CFA (0.91/0.86), DIN (0.89/0.97) and REF (0.86/0.77), and not correlated with SAM (0.74/–1.3). The incurred measurement costs per sample (in Euros) for the methods were as follows: CFA (<0.1) < DIN (0.6) < REF (0.7) < SAM (3) < CUV (3.8) < IC (15). Cuvette tests are recommended as the method of choice due to their accuracy and lower cost than IC.

KEYWORDS:

• chloride
• interference
• landfill leachate
• nitrate-nitrogen
• recovery

Acknowledgement

The authors acknowledge the two students of a school for educating chemical-technical assistants (Staatliche Gewerbeschule Chemie, Pharmazie, Agrarwirtschaft, Hamburg), Phillip Zink and Christopher Gordon, for executing the measurements using REF, DIN, SAM and CUV methods during their internships at TUHH. The authors also thank Susanne Eggers and the Central Laboratory of Analytical Chemistry, TUHH for executing measurements with a continuous flow analyser and ion chromatography, respectively. The authors thank RTS Rochem Technical Services GmbH and GFEU e.V. for funding this research work.

Disclosure statement

No potential conflict of interest was reported by the authors.