Elucidating performance failure in the use of an Anaerobic-Oxic-Anoxic (AOA) plug-flow system for biological nutrient removal.

The Anaerobic-oxic-anoxic (AOA) process is a carbon-saving and high-efficiency way to treat municipal wastewater and gets more attention. Recent reports suggest that in the AOA process, well-performed endogenous denitrification (ED), conducted by glycogen accumulating organisms (GAOs), is crucial to advanced nutrient removal. However, the consensuses about starting up and optimizing AOA, and in-situ enriching GAOs, are still lacking. Hence, this study tried to verify whether AOA could be established in an ongoing anaerobic-oxic (AO) system. For this aim, a lab-scale plug-flow reactor (working volume of 40 L) previously operated under AO mode for 150 days, during that 97.87 % of ammonium was oxidized to nitrate and 44.4 % of orthophosphate was absorbed. Contrary to expectations, under AOA mode, little nitrate reduction (only 6.3 mg/L within 5.33 h) indicated the failure of ED. According to high-throughput sequencing analysis, GAOs (Candidatus_Competibacter and Defluviicoccus) were enriched within the AO period (14.27 % and 3 %) and then still dominated during the AOA period (13.9 % and 10.07 %) but contributed little to ED. Although apparent alternate orthophosphate variations existed in this reactor, no typical phosphorus accumulating organisms were abundant (< 2 %). More than that, within the long-term AOA operation (109 days), the nitrification weakened (merely 40.11 % of ammonium been oxidized) since the dual effects of low dissolved oxygen and long unaerated duration. This work reveals the necessity of developing practical strategies for starting and optimizing AOA, and then three aspects in future studying are pointed out.

Towards low carbon demand and highly efficient nutrient removal: Establishing denitrifying phosphorus removal in a biofilm-based system.

The combined denitrifying phosphorus removal (DPR) and Anammox process is expected to achieve advanced nutrient removal with low carbon consumption. However, exchanging ammonia/nitrate between them is one limitation. This study investigated the feasibility of conducting DPR in a biofilm reactor to solve that problem. After 46-day anaerobic/aerobic operation, high phosphorus removal efficiency (PRE, 83.15 %) was obtained in the activated sludge (AS) and biofilm co-existed system, in which the AS performed better. Phosphate-accumulating organisms might quickly adapt to the anoxic introduced nitrate, but the following aerobic stage ensured a low effluent orthophosphate (<1.03 mg/L). Because of waste sludge discharging and AS transforming to biofilm, the suspended solids dropped below 60 mg/L on Day 100, resulting in PRE decline (17.17 %) and effluent orthophosphate rise (4.23 mg/L). Metagenomes analysis revealed that Pseudomonas and Thiothrix had genes for denitrification and encoding Pit phosphate transporter, and Candidatus_Competibacter was necessary for biofilm formation.