Densification Index/SVI Model: An Efficient Way to Predict Benefits of Biomass Densification in Full-scale MBR and MABR Wastewater Treatment Plants

Abstract Densification of biomass is an efficient technology to improve sludge settleability which benefits wastewater treatment plants (WWTPs) by increasing the capacity of the secondary clarifiers. In this study, a densification model was developed and validated to predict the level of densification performance to help capture expected benefits and new capacity limits for operations. The first version of the VWTS (Veolia Water Technologies & Solutions)-SUMO model was developed based on the growth kinetic for various microorganisms (e.g., nitrifiers and polyphosphate-accumulating organisms). Then the model was calibrated with the filed data collected from full-scale WWTPs (WWTPs with membrane-aerated biofilm reactors and WWTPs with membrane bioreactors). Although more filed data under steady-state operating conditions need to be collected to validate the model, the current simulation results demonstrated the settleability in the full-scale WWTP can be improved significantly by the densification processes. In addition, the proposed model can predict the settleability reliably. 1.INTRODUCTION Densification and granulation of biomass have been demonstrated to be efficient in improving sludge settleability which determines the design and performance of secondary clarifiers (i.e., the hydraulic capacity) in wastewater treatment plants (WWTPs). For instance, a full-scale granule-floc hybrid process in Dijon (France) proved that 20% of aerobic granular sludge (AGS) guarantees a sludge volume index (SVI) lower than 100 mL/g while 30% of AGS reduced the SVI to 50 mL/g [1]. The improved settleability (i.e., lower SVIs) further contributed to a better performance of clarifiers with two to three times higher surface loading rates (SLR) and surface overflow rates (SOR). However, the densification and granulation processes, configurations, and mechanisms still need to be understood more deeply. Thus, a densification model is needed to predict the level of densification performance (i.e., SVI) to help capture expected benefits and new capacity limits for operations. Densification Index (DI), which is the percentage of biomass larger than 200 µm in the mixed liquor, was proposed to quantify the level of biomass densification and partial granulation in continuous-flow biological treatment systems. In this study, a simple and reliable protocol was built to capture the DI in a full-scale WWTP with selective wasting (i.e., densification). These sites' DI and SVI data were used to implement and validate an SVI prediction model. With the predicted SVI from this model, the hydraulic capacity in WWTPs can be upgraded to reach new levels of process intensification. 2.METHODS 2.1 VWTS-SUMO Model The VWTS (Veolia Water Technologies & Solutions)-SUMO model [2][3] provides a simple method of calculating and predicting DI on the state variables from the process simulator. The model introduced separate growth kinetics for granular nitrifiers (i.e., ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB)), polyphosphate-accumulating organisms (PAOs), and ordinary heterotrophic organisms (OHOs) that contribute significantly to the formation of denser and granular biomass (> 200 µm). The half-saturation coefficient (Ks) of PAOs, AOBs, NOBs, and OHOs and their effects on granular growth were implemented based on the previous framework [4]. The contribution of influent inorganic suspended solids (ISS, > 200 µm) was also added to the model. 2.2 Field Data Collection and Calibration A simple and reliable protocol (i.e., sieve tests) has been implemented to measure the DI of a full-scale A2O (anaerobic-anoxic-aerobic)-MABR (membrane aerated biofilm reactor) WWTP and a full-scale A2O-MBR (membrane bioreactor) WWTP. The configuration of the treatment plant is shown in Figure 1. A SUMO model was developed and validated by the influent wastewater characteristics using VWTS blocks specially designed and developed for MABR, Secondary Settler, and External selector. RESULTS AND DISCUSSION The first version of the DI-SVI prediction model was proposed based on the VWTS-SUMO model (Table 1). The VWTS-SUMO model was re-calibrated by the collected data from the full-scale WWTPs. According to the site data, the model can reliably predict SVI before the densification process (Figure 1) for both MABR and MBR applications. However, modified equations should be used after the densification process since the SVI (i.e., settleability) in the full-scale WWTPs was improved significantly. For instance, the SVI can be reduced from 101 mL/g to 65 mL/g while the DI increases from 30% (typical DI in an aeration tank of CAS) to 50% in a densified MABR WWTP (Figure 2a). It should be noted that the densification process in this full-scale WWTP has not reached a steady state since the operating time for the densification process was around 7 months for the MABR plant and 5 months for the MBR plant. An updated model will be proposed by VWTS with more field data. In addition to the settleability improvement by the densification process, a modified VWTS-SUMO model will be developed to predict carbon, nitrogen, and phosphorus removal.