Continuous Monitoring System for the Wastewaters Having Multiply, Randomly, and Small Effluent Characteristics -Approarch to Analysis of Chemical Oxygen Demand by Complete Flow Process-

A simple system was developed for the fully automatic and continuous measurement of chemical oxygen demand (COD) in wastewater samples based on colorimetry of dichromate. A sample and a solution of sulfuric acid (1+1) containing 2mM potassium dichromate are continuously pumped with a double-reciprocating micro-pump at each flow rate of 0.3 ml/min. The wastewater sample is filtered at first with a 100-mesh stainless filter and then mixed with the dichromate solution in the mixing joint. The mixture is introduced into a reaction coil made of poly(tetrafluoroethylene) tubing (1 mm i.d., 3 mm o.d., and 20 m length), being placed in an oil bath (120℃). After reaction, the mixture passes into a quartz tubular flow-through cell (10 mm path length, 18 μl volume) in a spectrophotometer, and the absorbance is measured at 445 nm. The COD value of the sample is automatically estimated from the amount of decreased absorbance. The system was successfully applied to COD measurement of some waters, and to continuous monitoring of COD in wastewater of university laboratories. The system was also evaluated by comparing with the flow injection analyzer system previously developed by the authors

Evaluation of Three Flow Injection Analysis Methods for the Determination of Chemical Oxygen Demand

Three methods for determining chemical oxygen demand (COD) by means of flow injection analysis (FIA) with potassium permanganate, potassium dichromate, or cerium(IV) sulfate as oxidant, developed in this laboratory, are described from the point of view of their operating properties. The permanganate method is the most sensitive and common, but forms manganese(IV) oxide precipitate which blocks the FIA lines and connectors. Addition of phosphoric acid in the reagent system is, however, effective to some extent in order to avoid blocking the flow system. The dichromate method is the most workable and stable, but produces toxic wastes containing chromium(VI). The cerium method is the most probable because cerium(IV) is the strongest oxidant of the three and less poisonous. The last method with cerium(IV) sulfate is therefore recommendable to apply to the continuous monitoring of COD in many situation