Free Ammonia Pretreatment to Enhance Biodegradation of Anaerobically Digested Sludge in Post Aerobic Digestion

In wastewater treatment plants (WWTPs), sludge reduction was implemented via sequential anaerobic-aerobic digestion. However, the performance of post aerobic digestion for anaerobically digested sludge (ADS) is limited. Free ammonia (FA)-based pretreatment technology is proposed in this study as an innovative method to enhance the degradation efficiency of post aerobic digestion for ADS. Pretreatment using FA at >440 mg NH₃-N/L for 24 h significantly increased ADS solubilization. The highest solubilization was reached at 1030 mg NH₃-N/L, which (0.12 mg COD/mg VS) is 6 times that (0.02 mg COD/mg VS) of no treatment. The batch experiments of post aerobic digestion demonstrated unpretreated ADS over the 8 days post aerobic digestion was degraded by 18.4%, whereas 31.3-33.6% of the pretreated ADS with FA at 440-1030 mg NH₃-N/L was degraded, representing a relative increase of 70-83%. Accordingly, inorganic nitrogen production increased in a similar way. Model analysis results revealed the enhanced ADS degradation was because of the increase in both hydrolysis rate and degradable percentage of ADS. Capillary suction time (CST) tests demonstrated FA-based pretreatment was able to generate ADS with greater dewaterability, as revealed by the decline of normalized CST from 77 s for ADS without pretreatment to 63-74 s for ADS with FA pretreatment at 65-1030 NH₃-N/L, with the best ADS dewaterability at 1030 mg NH₃-N/L of FA. Economic assessment showed that this FA pretreatment technology could be economically favorable.