A sensitive and simplified analytical method for the concentration of insoluble nitrogen in activated sludge has been developed. Nitrogen concentration in activated sludge was measured by ultra-violet absorption spectrochemical analysis. The sensitivity of the nitrogen concentration was improved by H
2O
2 addition or O
3 injection. The H
2O
2 and O
3 content in 1mg activated sludge were 0.125ml to 1.0ml (30% H
2O
2), and 1.0ml (O
3), respectively. So far, the sensitivity difference between these methods and Kjeldal's method, which is the established quantitative method, was within 5%. The live slopes between these methods and Kjeldahl's method were about 1, and the correlation coefficients were 0.992 (H
2O
2 addition) and 0.964 (O
3 injection) . These methods were 33% faster than Kjeldahl' s method. These results indicate that the proposed simplified method using UV absorption spectrochemical analysis can be applied to nitrogen concentration measurements in activated sludge with high sensitivity.
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