Gravity selector-enabled kenaf recirculation and reuse as a renewable ballasting agent for sludge settleability enhancement in a municipal wastewater secondary process: A real wastewater pilot-scale study

Sludge densification can be achieved through granulation/densification, migrating carriers, ballasted flocculation, or a combination of these approaches. Hydrocyclone-enabled continuous flow densification has been applied for full-scale applications; however, it requires long startup times and, in some instances, results in poor stability of the densified floc. Migrating carriers hold promise to offset these disadvantages for continuous flow processes where reasonable feast-famine or plug flow conditions cannot be achieved while drawing on the strength of hydrocyclones to retain and return the ballasting agent. This idea was tested by dosing kenaf, a plant-based renewable migrating carrier in a pilot-scale plug flow reactor equipped with a gravity selector to mimic a hydrocyclone. Results showed that kenaf could gradually reduce the sludge volume index from 170 to 50 mL/g and increase zone settling velocity from 2.5 to 7 m/h over 110 days without compromising treatment performance. Because of the selective retention of the inert plant-based carrier represented as a volatile solid in the gravity selector underflow, a traditional SRT calculation based on mixed liquor volatile suspended solids tended to overestimate daily Waste Activated Sludge (WAS) requirements and resulted in active sludge inventory loss. An oxygen utilization rate-based method was developed and verified for easy SRT correction to avoid kenaf interference with the sludge wasting protocol. Denser kenaf incorporated into the sludge matrix during flocculant settling contributed to the settleability improvements. However, a lack of biofilm formation on the kenaf surface was observed even after 110 days of treatment is intriguing and different from other studies. Challenges with initial kenaf floating and interference with conventional SRT calculations were two significant lessons learned from this study, and countermeasures were provided accordingly. It was concluded that kenaf could work in synergy with gravimetric ballast separation, such as with a hydrocylone, for quick and sustainable sludge settleability improvements.