CO2, CH4, and N2O emissions from dredged material exposed to drying and zeolite addition under field and laboratory conditions

Abstract

Dredging, the removal of sediment from water courses, is generally conducted to maintain their navigability and to improve water quality. Recent studies indicate that dredging can significantly reduce aquatic greenhouse gas (GHG) emissions. These studies, however, do not consider the potential emission from the dredged material (sludge) in the depot. In addition, it is unknown if and how GHG emissions from sludge depots can be reduced. Here we present spatiotemporal variations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes, as well as environmental variables from a sludge depot located in the Netherlands. Measurements were conducted monthly from the time the depot was filled until the sludge was dry and the depot was abolished. We also experimentally assessed the GHG mitigation potential of 1) keeping the sludge permanently inundated, and 2) the addition of different amounts of zeolite to increase sludge nitrogen binding capacity to reduce N₂O emissions. In the depot and in the laboratory, a decrease in moisture content coincided with increased CO₂ and N₂O emissions while CH₄ emissions decreased. We observed that permanent inundation reduced emissions (∼4 times less CO₂-eq than in drying sludge). Adding zeolite lowered N₂O fluxes from permanently inundated sludge but did not reduce total GHG emissions. During the depot's operational period, average CO₂, CH₄, and N₂O fluxes were 5078, 27, and 5 mg m^-2 d^-1, respectively. GHG emissions from drying sludge occurred mainly in the form of CO₂ (73% of the total CO₂-eq emissions), with average GHG emission rates comparable to those reported for ditches and ponds. We estimate that approximately 14 tons of CO₂-eq were emitted from the 0.011 km² depot, which contained ∼20,000 m³ of sludge, during its entire operational period, and we argue that more studies are needed, considering different sludge origins, to expand our understanding of sludge depots.

Keywords: Depot; Dredging; Greenhouse gases; Mitigation; Sludge.