Biodegradation of Persistent Organic Pollutant (POP) hexachlorocyclohexane in a hybrid reactor system

Abstract

Lindane or gamma- hexachlorocyclohexane (γ-HCH) isomer has been one of the most extensively used pesticides worldwide. However, four additional Hexachlorocyclohexane (HCH) isomers were produced during its manufacture, resulting in persistence in the environment due to their dissolvability and noxiousness. This study examines the sequential anaerobic-aerobic degradation of synthetically prepared solution of technical HCH (t-HCH) utilizing a hybrid reactor system with a UASB reactor coupled to the aerobic system. The ideal conditions for t-HCH degradation were initial substrate concentration (100 mg L−1), glucose as suitable electron donor, active biomass (12 g L−1), pH (neutral range), HRT (48 h) and organic loading rate (4.28 mg/g/day). Individual reactor findings demonstrated that the UASB reactor performed efficiently with these parameters, showing 90.03% degradation of 100 mg L−1 t-HCH in two days after a month of operation. In aerobic degradation, the selected isolate can break down 87% of 100 mg L−1 of t-HCH in eight days. Subsequent to individual reactor operations, the reactors were synchronized in the hybrid reactor and after 60 days of continuous operation, t-HCH was dechlorinated completely in the presence of glucose as a co-substrate. Therefore, compared to individual anaerobic and aerobic treatments, this sequential degradation showed an improvement in HCH removal efficiency of > 99%. This is the first study that demonstrates the mineralization of HCH intermediates in a hybrid reactor system. The metagenomic analysis of the anaerobic sludge and bacterial isolate’s whole genome sequencing further authenticates the genomic aspects of the degradation process by the presence of HCH degrading genera and functional HCH degrading genes.