Bioremediation of Contaminated Soils with Hydrocarbons Using Industrial Wastewater Treatment Plant Effluents in a Circular Economy Model

Soil contamination by hydrocarbons poses a persistent threat to terrestrial and aquatic ecosystems, affecting soil quality, biodiversity, and human health. Given the limitations of traditional physical–chemical techniques, bioremediation has emerged as a sustainable alternative based on microbial activity to degrade toxic compounds. This study evaluated the suitability of secondary clarifier effluent and activated sludge from refinery and chemical industry wastewater treatment plants (WWTPs) for the bioremediation of petroleum-contaminated soils, with and without the addition of NPK (Nitrogen, Phosphorus, and Potassium) nutrients. This approach aligns with circular economy principles by promoting the reuse of industrial byproducts as biostimulants and supports the objectives of the Sustainable Development Goals. Microcosm tests were conducted over 20 weeks, assessing hydrocarbon degradation by chromatography and catalase activity as an enzymatic indicator. All treatments enhanced biodegradation compared to the control, achieving 65–79% total hydrocarbon removal. Activated sludge from La Rábida refinery with NPK (AS-RF + NPK) was the most effective, showing degradation from the first weeks. Light hydrocarbons (C6–C8) were almost completely removed, whereas long-chain compounds (C21–C35) were more resistant. Aromatic hydrocarbons, including benzene, toluene, ethylbenzene, and xylenes (BTEX), reached 100% removal in all treatments, demonstrating the strong biodegradation potential of microorganisms present in industrial effluents.