Continuous Heavy Metal Monitoring in Wastewater

Monitoring of trace metals as zinc, iron and lead has been carried out in effluents (outlet to the Trondheim fjord, Norway) from a Wastewater Treatment Plant (Høvringen Renseanlegg) using a automated voltammetric monitoring system constructed in our group. The assembled unit consists of an automatic sampling and measuring system, automatically supplied with waste water samples from an external pump system. Water samples are filtered through a coarse filter to avoid fouling in the cell, valves and tubes. In the voltammetric cell, supporting electrolyte (NH4Cl) is added by use of an automatic dosage pump. Voltammetric measurements have been carried out every half an hour over a seven month period. The electrodes used were a working electrode of solid dental amalgam electrode, an Ag/AgCl/KCl (3 M) reference electrode, and a platinum wire counter electrode. The voltammetric cell was automatically cleaned by use of citric acid (0.05 M) twice a day. The observed concentration ranges were as follows; iron up to 58.9 μg/l with an average concentration of 15.7 μg/l, zinc up to 46.5 μg/l with an average concentration of 0.3 μg/l, and lead up to 26.8 μg/l with an average concentration of 0.3 μg/l. Detection limit was estimated to 0.1 μg/l for the respective trace metals. Copper and cadmium was not observed, most probably because these metals totally are bound in non-electrolabile species. Variations in the different trace metal concentration in short and long time scale is discussed and assumed to relate to the water treatment cycle in the water plant and other factors as seasonal variation, local activities etc. Analyses by ICP-MS have been carried out for measurement of total amount of present trace metals in manual samples filtered through 0.45 μm filters to further study the speciation of the trace metals in the waste water samples. Preliminary results indicates that a the largest fractions of the trace metals is bound in suspended particles.