Biodegradation of nonylphenol ethoxylates (NPEOs) in a membrane aerated biofilm reactor (MABR)

Abstract

The degradation intermediates of NPEOs surfactants (NP and short chain NPEOs) are of growing concern in environmental studies. These intermediates, recognised as endocrine disrupting chemicals (EDCs), are more toxic and refractory than their parent compounds. Their formation is assisted by anaerobic process, while their further breakdown to less harmful compounds is more easily achieved in aerobic environments. In this study, an hybrid MABR was exploited to completely degrade NPEOs, based on the concept of a multi-layered biofilm in the MABR that permits a simultaneous anaerobic-aerobic process to occur in a single reactor. This is the first study conducted on NPEOs biodegradation in an MABR. Batch microcosm experiments were conducted primarily to simulate NPEOs biodegradation behaviour in the MABR. The results showed that NPEOs removal was improved in a simultaneous anaerobic-aerobic system, as compared to a fully anaerobic system. A microporous polypropylene membrane with a non-woven polypropylene scrim heat-sealed to the surface was then used as an aeration device and biofilm support in a flat sheet MABR. Under steady state conditions (NPEOave9 surface loading of 0.49 g/m2.d; at 48 hr HRT), the reactor achieved an excellent removal of NPEOs (up to >99%) and organics in terms of COD (up to 93%). The disruption of MABR performance was less pronounced under hydraulic shock loads (reduced HRT) compared to organic shock loads (increased NPEOave9 concentration), and this was postulated to be due to improved NPEOave9 mass transfer into the biofilm. Despite the slow MABR recovery from shock loads, a stable NPEOs removal of more than 95% was achievable after the recovery periods. Based on HPLC-UV and GC-MS analyses, the EO units of NPEOave9 were sequentially shortened (commonly via a nonoxidative pathway) over 500 days of operation to the major intermediate of NPEO1. Nevertheless, complete removal of NPEO1 was unsatisfactory, and more work needs to be done to optimise and investigate the role of the aerobic layer in degrading the compound. Nevertheless, this study has shown that the MABR is very reliable for the removal of both COD and NPEOs under long term operation, and the presence of toxic intermediates did not appear to inhibit overall reactor performance.