Evaluation of total phosphorus and total nitrogen methods in pulp mill effluents

Under the Clean Water Action Plan, the US Environmental Protection Agency is requiring states to establish numeric criteria for phosphorus and nitrogen. In preparation for the development of nutrient criteria NCASI undertook a research project to conduct a comparative study of methods for the determination of total phosphorus and total nitrogen in pulp and paper mill matrices. This paper presents results of a single laboratory method evaluation and comparative study of digestion techniques and analytical methods for the determination of total phosphorus (TP) and total nitrogen (TN) in pulp and paper mill secondary treated effluents. Analytical methods included EPA Methods 365.2 and 365.4 for TP. TN and total kjeldahl nitrogen (TKN) methods included EPA Methods 351.2, 351.4, and 353.2. Examinations of sample preservation and storage stability were conducted. Substitution of mercuric sulfate with copper sulfate during block digestion resulted in higher blank levels and method detection limits. TP measurements using EPA Method 365.4 (autoanalyzer) were found to be accurate with a positive bias as determined using matrix spike experiments. Sample digestion by acidic persulfate oxidation or mercuric sulfate block digestion in conjunction with EPA Method 365.4 yielded low blank levels (averages of 0.01 and 0.02 mg/L, respectively), precision of 2.1 and 2.4% relative standard deviations, respectively, and accuracy expressed as an average recovery (%R) of 117% for both. EPA Method 351.2 (autoanalyzer) was more precise than EPA Method 351.4 (ammonia probe). Accuracy (%R) for EPA Method 351.2 ranged from 81 to 95%, depending on the digestion technique applied, and was 55% when EPA Method 351.4 was utilized. Investigation of a method utilizing basic to acidic persulfate oxidation for the simultaneous determination of TN and TP using only two analytical techniques was found to be effective at concentrations above 1 mg/L in pulp mill effluents.