Heavy Metals in Refinery Wastewater: Assessment and Treatment Using Diatomaceous Earth

Abstract

In refining crude oil, freshwater is applied in distillation, hydration, desalination, system cooling, firefighting and other cleaning operations. The Kenya Petroleum Refineries Limited consumes huge quantities of freshwater daily for various applications leading to generation of contaminated effluents. The effluents contain a cocktail of contaminants such as aliphatic hydrocarbons, polycyclic aromatic compounds, dissolved gases such as Hydrogen Sulphide like algae, fungi, and heavy metals. Except for heavy metals, the refinery wastewater treatment plant is able to remove most of the other contaminants from the wastewater through a combination of processes including filtration, coagulation, sedimentation, softening, de-aeration, chlorination, desulfurization, bioremediation, and ion-exchange. However, these techniques are expensive, generate huge amounts of sludge, and are also not effective for removal of trace levels of heavy metals. Using inductively coupled plasma coupled with optical emission spectroscopy (ICP-OES), the effluents were found to contain significant levels of Iron, Cadmium, Chromium, Manganese, Nickel, Lead, Vanadium and Zinc. Although the installed wastewater treatment could reduce heavy metals load to trace, effluents storage in the tank farm prior to disposal leads to accumulation of heavy metals over time. Diatomaceous earth (DE) was applied to treat the heavy metals from the tank farm to within allowable limits as recommended by the World Health Organisation. Hence DE should be applied to effluents exiting the wastewater treatment plant prior to storage in the tank farm.