Evidence for PAH Removal Coupled to the First Steps of Anaerobic Digestion in Sewage Sludge

Anaerobic degradation of polycyclic aromatic hydrocarbons has been brought to the fore, but information on removal kinetics and anaerobic degrading bacteria is still lacking. In order to explore the organic micropollutants removal kinetics under anaerobic conditions in regard to the methane production kinetics, the removal rate of 12 polycyclic aromatic hydrocarbons was measured in two anaerobic batch reactors series fed with a highly loaded secondary sludge as growth substrate. The results underscore that organic micropollutants removal is coupled to the initial stages of anaerobic digestion (acidogenesis and acetogenesis). In addition, the organic micropollutants kinetics suggest that the main removal mechanisms of these hydrophobic compounds are biodegradation and/or sequestration depending on the compounds. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are nowadays considered as environmental pollutants by environmental and health agencies because of their toxic, mutagenic, and carcinogenic effects on living organisms [1]. PAHs are mainly formed during human activities such as crude oil spillage, fossil fuel combustion, and gasoline leakage. Through the air and runoff after rainy events, the PAHs can deposit to soil, water bodies, and sewage system [2, 3]. Due to their low water solubility and high octanol-water partition coefficients, these organic micropollutants (μP) are mainly associated with hydrophobic compartments such as the organic matter in sewage sludge and river sediments or the lipids in biota, with levels between 0.001 and 10？μg/gDM for PAHs [4–6]. μP can be also sorbed irreversibly in a short time scale to the organic matter. This phenomenon named sequestration has been reported by different authors as one of the abiotic mechanisms of μP removal in digested sewage sludge and soils [7–9]. PAHs are known to be biodegraded under aerobic conditions [10, 11]. However, most contaminated environments are anaerobic. In these environments, the anaerobic digestion can occur. Anaerobic digestion is a process whereby organic matter is broken down in the absence of oxygen into methane and carbon dioxide by naturally occurring microorganisms. The anaerobic digestion consists of four stages where different microbial populations participate: (1) hydrolysis, (2) acidogenesis, (3) acetogenesis, and (4) methanogenesis. The digestion process begins with the hydrolysis of insoluble organic polymers into monomers that are available for microorganisms. Acidogenesis is the step where these monomers are converted to carbon dioxide, hydrogen, and