Comparative study of dissolved organic matter generated from activated sludge during exposure to hypochlorite, hydrogen peroxide, acid and alkaline: Implications for on-line chemical cleaning of MBR

Although on-line chemical cleaning has been extensively employed for maintaining the MBR permeability, little attention has been given to the negative impacts of such prevalent membrane cleaning practice. This study thus comparatively investigated the potential release of dissolved organic matter (DOM) from activated sludge upon the exposure to different kinds of frequently-used cleaning reagents, i.e. NaClO, H₂O₂, HCl and NaOH. It was found that NaClO at 50 and 80 mg L⁻¹ triggered significant release of DOM, while NaOH strongly promoted soluble nitrogen release. However, the DOM generation induced by H₂O₂ in the range of 0–80 mg L⁻¹ was nearly negligible. The combined analysis by EEM-PARAFAC and LC-OCD-OND further revealed that NaClO-triggered DOM mainly originated from the breakdown of humic substances and other small humics with molecular weight (MW) less than 500 Da. In contrast, proteins and other biopolymers with higher MW highly contributed to DOM induced by NaOH. Most of DOM detected in this study belonged to low molecular weight (LMW) substances, which were not considered readily biodegradable or physically retainable by microfiltration membrane. It appears from this study that DOM generated from suspended activated sludge during membrane cleaning with different chemicals should be taken into serious consideration when water recycle and reuse are concerned.