Reducing Oil Content Of Sludge From Wastewater Treatment Plant

While industrialisation has been facilitated by the discovery, refinement, and use of petroleum, the production and use of petroleum also contributes to environmental pollution. Among all environmental pollutants, petroleum accounts for about 70% of the total environmental pollution. In particular, inappropriately managed oily sludge generated at modern petroleum refineries can pollute the environment (such as surface and groundwater and soils).

Disposal of this waste is costly, in part because the high oil content requires use of secure disposal methods akin to handling of hazardous wastes.

To reduce the oil content or break the structure of oily sludge, preliminary lab-scale experiments involving mechanical treatment, surfactant extraction, and oxidation are conducted. By applying surfactants, approximately 36% to 45% of oils are extracted from oily sludge.

Wastewater treatment plants at petroleum refineries often produce substantial quantities of sludge with relatively high concentrations of oil.

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By applying surfactants, approximately 36% to 45% of oils are extracted from oily sludge. Of this, about 33% of oils are rapidly oxidised via radiation by an electron beam within 10s of exposure.

In wastewater treatment plants, equalisation tanks are generally used for the efficient disposal of wastewater. The sludge that continuously flows into these tanks causes operational problems and large volumes of waste sludge are produced. To reduce the total volume, waste sludge is dehydrated by centrifugation. The resultant sludge cake is then incinerated. However, the oil content of the cake generally exceeds 5% and it is categorised as a specified waste by the waste management. The disposal cost of specified waste is about five times higher than that of non-specified waste.

If the oil content of the sludge is reduced below the legal standard of 5%, about 80% of the disposal cost can be saved. It is therefore necessary to develop suitable disposal methods for cost-effective disposal of oily sludge. The oily sludge includes 30% to 90% water, 4% to 7% sediments, and 5% to 60% waste oil; the waste oil in turn contains 40% to 60% saturates, 25% to 40% aromatics, 10% to 15% resins, and 10% to 15% asphaltenes. This complex structure of oily sludge inhibits the separation and degradation of waste oil.

The Ways of Traditional Oil-sludge Treatment

The traditional technique is to reduce the oil and water content below the level suitable for land disposal restriction criteria by thermal treatment. Heat treatment of 500~1000°C is currently applicable, but multi-step processes are required for efficient disposal, which is a notable drawback for large scale facilities.

Freezing/thawing recovers the oil from sludge through separation of oily sludge into oil, water, and a precipitate by the process of freezing and thawing the sludge. The equipment and operating costs of freezing/thawing, however, are high and the recovery rate of oil is low. To address this and enhance the recovery rate of oil, a combination of freezing/thawing and other methods (such as solvent extraction and ultrasonic treatment) have been examined.

Meanwhile, biological treatment using a specific microorganism is cost-effective and environmentally friendly compared with other recovery techniques. However, the high oil content of the sludge limits the activities of the microorganisms. Various other approaches (including electrokinetic methods, filtration, flotation, microwave irradiation, and centrifugation) to treat the oily sludge have been reported in the literature, but they showed low treatment efficiency or high costs.

Brief of Using Wastewater Plant to Treat Oil-sludge

As options for reducing the oil content of oily sludge below the standard for specified waste via the additional installation of simple facilities to the existing wastewater plants in petroleum refineries, mechanical treatment, oil extraction by surfactants, and oxidative degradation are adopted. The liquid (oil and water) and solid can be separated by mechanical treatment applying shear forces, and thus it becomes feasible to reduce the oil content in the solid component of the sludge.

The effectiveness of various methods to apply shear forces is examined in this work. The oil extraction from sludge by a surfactant is also evaluated. Surfactants have been widely used to recover or separate the hydrophobic contaminants from water, soil, and other wastes. Furthermore, separation using a surfactant is simple, fast, and efficient.

Lastly, the representative oxidative degradation methods, including the Fenton reaction, ozone, and electron beam radiation, are applied to reduce the oil content of sludge in a short time. Rapid oxidative degradation can be used for the direct disposal of sludge waste or pretreatment before the dehydration step.

Methods for reducing oil content of sludge

For mechanical treatment of the sludge, a sonicator, a ball mill, and a homogeniser were used to separate the phases of the sludge by applying shear force. The amount of sludge used in the mechanical treatment was 100 g. Sonication was conducted with a rod-type sonicator for 30min. Ball-milling was implemented for 48 h. To increase the shear force on the sludge structure, the homogeniser was used. The oily sludge was homogenised at a rate of 10,000 r/min-1 for 2 h. After applying the shear force, all samples were centrifuged and separated into bottom solids, middle liquids, and top solids. The surfactant has both hydrophobic and hydrophilic components and hence can be used for extraction of oil from sludge. In this study, anionic and non-ionic surfactants were selected to minimise the adsorption of the surfactants on solid surfaces that are charged positively.

Mechanical Treatment For the Separation of Oils

The oil content of each solid in the top and bottom phases was measured and the results are listed in Table 3. If the structure of the sludge is influenced by shear force, the oil in the solid precipitants will decrease. In all experiments applying shear force, oil content in the top and bottom phases changed slightly, but the application of shear force did not show significant performance in the separation of oils from the sludge. Therefore, the use of shear force did not cause a change in the phase structure of the oily sludge.