The effect of waste water treatment on river metal concentrations: removal or enrichment?

Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, PO Box 140, 4400 AC Yerseke, The Netherlands
Journal of Soils and Sediments (Impact Factor: 1.97). 11(2):364-372. DOI: 10.1007/s11368-010-0321-4

ABSTRACT PurposeDischarge of untreated domestic and industrial waste in many European rivers resulted in low oxygen concentrations and contamination
with trace metals, often concentrated in sediments. Under these anoxic conditions, the formation of insoluble metal sulfides
is known to reduce metal availability. Nowadays, implementation of waste water treatment plants results in increasing surface
water oxygen concentrations. Under these conditions, sediments can be turned from a trace metal sink into a trace metal source.

Materials and methodsIn an ex situ experiment with metal contaminated sediment, we investigated the effect of surface water aeration on sediment
metal sulfide (acid volatile sulfides (AVS)) concentrations and sediment metal release to the surface water. These results
were compared with long-term field data, where surface water oxygen and metal concentrations, before and after the implementation
of a waste water treatment plant, were compared.

Results and discussionAeration of surface water in the experimental setup resulted in a decrease of sediment AVS concentrations due to sulfide oxidation.
Metals, known to precipitate with these sulfides, became more mobile and increasing dissolved metal (arsenic (As), cadmium
(Cd), copper (Cu)) concentrations in the surface water were observed. Contrary to As, Cd, or Cu, manganese (Mn) surface water
concentrations decreased in the aerated treatment. Mn ions will precipitate and accumulate in the sediment as Mn oxides under
the oxic conditions. Field data, however, demonstrated a decrease of all total metal surface water concentrations with increasing
oxygen concentrations following the implementation of the waste water treatment plant.

ConclusionsThe gradual decrease in surface water metal concentrations in the river before the treatment started and the removal of metals
in the waste water treatment process could not be countered by an increase in metal flux from the sediment as observed in
the experiment.

KeywordsAcid volatile sulfides (AVS)–Metal availability–Oxidation–Redox chemistry–Sediments–Simultaneously extracted metals (SEM)

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