Ultraviolet Absorbance at 254 nm as an Indicator of Petroleum Hydrocarbon Removal by Activated Carbon: a case study

The California Department of Transportation (Caltrans) is redesigning the onsite wastewater treatment systems for several of its highway maintenance stations to achieve effective treatment, at lower cost and oversight. The onsite wastewater contains a mixture of petroleum hydrocarbons, natural organic material, soils, sands, road salt, surfactants and microorganisms. As part of the new treatment design, following slow sand filtration, activated carbon (AC) filtration is intended to remove petroleum hydrocarbons from the wastewater. In this study, ultraviolet absorption at 254 nm (UVA254) is analyzed as an indicator of the adsorption of total petroleum hydrocarbons (TPH) by the AC filter. Synthetic wastewater (washwater) blends were formulated to represent key constituents in the wash rack wastewater: petroleum products (diesel and motor oil at 1 ppm each), surfactants (degreasers), humic material and soils. With these washwater blends, five rapid small scale column tests (RSSCTs) were performed to model full-scale AC performance. The UVA254 of the influent and effluent was measured unfiltered and filtered (0.45 µm). Chemical oxygen demand (COD), TPH-diesel and TPH-motor oil were also measured. Results from the RSSCTs revealed significant preferential adsorption between UVA254, COD, and TPH related constituents, with UVA254 reaching 50% removal six times faster than COD, while TPH removal remained above 70% for the duration of the tests. Correlations between UVA254 and turbidity revealed average unfiltered UVA254 increase of 0.004 cm-1 per NTU. For humic material, maturation of fulvic acids to humic acids was also caused an increase in UVA254 . From the RSSCTs, it is predicted that, at full-scale treatment of 500 gallons per day, the AC filter will last 600 days before TPH removal decreases below 70%. To resolve issues regarding preferential adsorption and differences between synthetic washwater blends and maintenance station wastewater, it is suggested that a "trigger" UVA254 be set at 0.030 cm -1, indicating 10% removal of unfiltered UVA254 and 70%-80% removal of TPH. When the unfiltered UVA254 of the AC effluent has reached a 30-day average of 0.030 cm-1, it is recommended that the AC filter be visually tested with methylene blue, a common organic dye. If the dye is not being adsorbed, then the AC will be replaced; otherwise, the "trigger" UVA254 will be adjusted to better fit the wastewater at the maintenance station.