Remediation of polluted sediment assisted by tidal energy

Many estuaries and river sediments in the UK and worldwide are impacted by organic pollutants at levels which are harmful to the ecosystem. For instance, the Tyne Estuary in the North of England is heavily polluted with petroleum hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs), and recent studies demonstrated ecotoxicological effects in fish, e.g. flounder and eels. The EU water framework directive lists PAHs as priority substances in the field of water policy because of their carcinogenic potency.Natural processes such as the microbial break-down and the strong adsorption of pollutants like PAHs to certain sediment constituents (e.g. carbonaceous particles) can reduce the ecotoxicity of polluted sediment significantly. However, the limited availability of nutrients and oxygen, the formation of ecotoxic products from microbial break-down, a limited number of strong sorption sites in natural sediment, and slow mass transfer of pollutants from weak sorbents to microorganisms or strong sorbents may impede a natural recovery. It is thus proposed to devise a low-cost engineering system in which natural processes leading to a sediment recovery can be enhanced in a controlled setting.Tidal energy will be exploited to agitate the sediment and replenish it with nutrients and oxygen. This will accelerate the microbial break-down of the degradable portion of the pollution. The labile, but non-degradable portion of the pollution and eventual break-down products will be removed from the sediment by infiltrating the effluent from the system across a bed with an adsorbent. The residual pollution in the sediment will be sequestered by adding a dose of a strong, preferably natural sorbent (e.g. coal) to the sediment at the end of the treatment. Presumably this will result in a substantially reduced sediment ecotoxicity and longterm stabilization of the residual pollution.By exploiting tidal energy and natural processes the costs for this treatment can be kept low. The scheme could be implemented in situ to avoid the removal of sediment or ex situ, possibly within existing structures such as abandoned dry docks, to treat sediment from routine dredging operations. The in situ emplacement or ocean disposal of treated, stabilized sediment would implicate less risks than disposal of the untreated sediment, or the treated, stabilized sediment may become an acceptable material for use on shore, e.g. as a filling material.