Chemically enhanced primary treatment: Modelling and Resources Recovery

Wenjuan Mu, The University of Western Ontario

Abstract

The chemically enhanced primary treatment (CEPT) process is gaining momentum for carbon redirection, thereby lowering the downstream liquid train load and maximizing energy recovery from the primary sludge. CEPT technique enhances coagulation and flocculation, that enable faster settling of particulate and colloidal solids and dissolved ions such as phosphate to enhance removal efficiency. Comprehending the dynamic behaviour of CEPT clarifiers is critical to not only develop a reliable whole plant simulation but also fully understand the efficacy of the treatment plant performance. To achieve such purposes, a great amount of efforts has been contributed to the advancement of primary clarifier models. Modelling and controlling the primary clarifier have vital impacts to characterize the downstream biological and sludge treatment performance accurately, and subsequently, the full plant modelling (WWTP). This research was conducted to compares and contrasts the performance of three primary clarifier models, including volume-less point separator, three-compartments clarifier and layered flux clarifier concerning the accuracy in describing the CEPT performance. Most importantly, the characterization focused on the models' capability to accurately represent COD fractions, including colloidal COD (C_COD), soluble COD (S_COD), particulate COD(X_COD) and suspended solids (TSS) concentrations of the influent wastewater. For practical applications, our study has shown that among the three clarifier models, the three-compartments model accurately describes the effluent fractions, provides a better description of chemicals addition impacts, and comparatively a more straightforward calibration procedure. Furthermore, the impact of CEPT on the downstream solid train processes was also investigated. The experimental study was conducted on CEPT sludges with different pretreatments (ozonation and low-temperature thermal alkali pretreatments (LTTAP) in relation to the performance of anaerobic digestion for resource recovery. LTTAP process demonstrated the highest RP fraction of 53.33% in ferric based sludge (CEPT-I sludge), whereas ozonation process showed the highest RP fraction 76.38% in ferric alum- based sludge (CEPT-II sludge). Interestingly, after anaerobic digestion, not many differences were observed in the RP fraction from the pretreated samples for CEPT sludges and the control samples, implying pre-treatments may not be required due to the naturally occurred NRP conversion during digestion. Noteworthy, ferric-based sludge(control) produced 7% high methane yield than ferric alum-based sludge, indicated the inhibitory effect by the PACl coagulant on sludge digestion.