Oil removal from industrial wastewater using flotation in a mechanically agitated flotation cell

D.A Deglon,M.L.S Welz,N Baloyi

doi:10.4314/wsa.v33i4.52939

Abstract

This paper investigates the flotation of oil from wastewater in a laboratory-scale mechanically agitated flotation cell. Mechanical flotation cells are used commercially for oil flotation but, to the authors’ best knowledge, there are no studies on their flotation performance in the literature. Some researchers have suggested that mechanically agitated systems are inappropriate for oil flotation as the high shear rates may break up the fragile organic flocs. The chemical pretreatment operating factors of pH and type of acid adjustor, coagulant type and dosage and flocculant dosage were firstly investigated. When using H3PO4 for pH adjustment, a preferential pH of 5.5 was identified. When using H2SO4, a pH of 5.5 was not optimal. The acid type was subsequently the determining factor. Al2(SO4)3 was a more effective coagulating agent than lime, based on coagulant and flocculant dosages required (0.5mg/ℓ when using Al2(SO4)3 and 1.5 mg/ℓ when using lime). The flotation cell operating factors of aeration rate and impeller speed were subsequently investigated. Oil removal rates were found to increase at higher levels of aeration and agitation. This suggests that oil flotation is both possible and beneficial in high-shear turbulent environments. However, residual oil concentrations could not be reduced to below 50 to 100 mg/ℓ which is higher than the desired target of 50 mg/ℓ.Keywords: oil flotation; wastewater, mechanical flotation cell

Highlights

The effluent treatment area at FFS Refiners (Pty) Ltd in South Africa, processes oil-contaminated water for release into municipal sewers
The aim of this paper is to investigate the flotation of oil from wastewater in a laboratory-scale mechanical flotation cell
The froth quantity and quality was superior when using H3PO4 compared to H2SO4

Summary

Introduction

The effluent treatment area at FFS Refiners (Pty) Ltd in South Africa, processes oil-contaminated water for release into municipal sewers. The company aims to reduce their residual oil content to below 50 mg/l for downstream removal of chloride ions and subsequent reduction in COD. The IAF attains residual oil contents in the vicinity of 200 mg/l which is higher than the desired specifications. Oil can be present in free, dispersed, emulsified or a soluble form up to 1 000 mg/l. Free oil is non-dispersed and floats on the surface or coats equipment. Non-emulsified dispersed oil can be removed by gravity separation, e.g. hydrocyclones, gravity filters or sedimentation (Da Rosa et al, 2005). Filters are generally avoided due to the high-pressure drop, recent studies on oil removal by microfiltration/ultrafiltration have proven successful (Schoeman and Novhe, 2007). If the oil droplets are small, the separation efficiency is accelerated through the addition of chemicals to the settling tanks to increase oil droplet size (Hamza et al, 1995)

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