Defining Best Operating Points of Biomass Densification for MBR and Clarifiers: Lessons Learned from Two-year Operation at Full-scale Plants

Densified Activated Sludge (DAS) in Continuous Flow with MABR and MBR has been implemented at full-scale wastewater treatment plants for over 2 years. While new indicators such as SVI5, DI% and TTFOF have been revealed to be enhancing process control and helping operators better scrutinize and control process events (Shaw, Donnaz et al, 2024), steady-state operations also showed the existence of a climax for clarifier and ultrafiltration performance in terms of SVI30 and Densification Index. DI is defined as the sum of bio-aggregates > 200 µm in the Total Suspended Solids (gMLSS>200/gMLSS).

At Yorkville Wastewater Treatment Plant (WWTP), the long term operation of MABR DAS with clarifiers demonstrated the ability to operate with half of the hydraulic capacity of the plant (Regmi, Astrand et al, 2024; Roche et al, 2021; Donnaz et al, 2020).

From November 2023, SVI30 reached 72 mL/g at 95%ile and 58 mL/g on average. TSS in the outlet of the clarifier reached 13 mg/L at 95%ile and 5 mg/L on average (Figure 1).

However, when SVI30 reached a threshold below 40 mL/g, TSS in the secondary effluent increased from 5-7 to hit the 13 mgTSS/L mark. This was rapidly compensated by adjusting SVI30 back between 50 and 60 mL/g and by increasing the RAS rate from 45 to 50%.

Total SRT of Yorkville secondary biological system is 11-13 days, aerobic SRT 9-11 days, F/M 0.26 kgCOD/kgMLVSS/d (COD/BOD ratio 1.6).

At Detroit Lakes WWTP, the long term operation of MBR DAS with ultrafiltration provided benefits (Donnaz, Shaw et al, 2023) in terms of membrane resistance (-30%), colloidal fraction (-60% cCOD to -80% cTOC) and membrane cleaning (maintenance and recovery). Improvement of membrane filtration was correlated to rapid drops of SVI, time-to-filter and superfine colloids (sfCOD) in the membrane tank mixed liquor (Donnaz, Shaw et al, 2023). Time-to-filter is a direct test of filterability and membrane performance. Results of these tests show a very close correlation with settleability, meaning that improved settling and higher densification also improve filterability (Shaw, Donnaz et al, 2024). Total SRT of Detroit Lakes secondary biological system is 15-25 days, aerobic SRT 9-10 days, F/M 0.05 kgBOD/kgMLVSS/d.

This was confirmed in line with supporting research demonstrating that MBR mixed liquor quality can be enhanced by using hydrocyclones as gravimetric selectors, whereby MBR performance has been improved due to the removal in overflow of lighter flocs and colloidal fraction from the mixed liquor (Nogushi et al, 2018). External gravimetric selectors help regulate particles and granules size below 1 mm, which is believed to minimize pore blocking (Zhang & Jiang, 2018). Particle size distribution of densified biomass has shown to be 200 to 500 µm in majority, with 1,000 µm maximum (Roche et al, 2021). An external gravimetric selector such as memDENSE technology helps regulate particle and granule sizes above 100 µm and below 1 mm, which is believed to minimize pore blocking fouling observed with full granulation process by remaining below the 1 to 1.2 mm critical value range described by Zhang & Jiang (2018).

As DI% evolved from 30% to 80% in the biological system (Figure 2), the following was observed (Figure 3):
- optimum of TTF around SVI30 of 60 mL/g
- drop of permeability observed for SVI30 >70 mL/g and SVI30 < 40 mL/g
- optimum Permeability observed around DI 50%

DI-SVI correlation enables the prediction and the accurate assessment of DAS mixed liquor performance in secondary biological continuous flow systems (Guo, Donnaz et al 2024). Experimental data show that there is a linear correlation between DI and Particle Size Distribution (PSD) with DAS for all particle fractions.

Figure 4 shows how PSD evolves as DI increases over 50% in the system (aerobic and underflow):
- Fraction >550 µm increases when DI gets over 50% (blue)
- Fraction 200-550 µm is the one which increases the most when DI gets >50% (orange)
- Particles fraction < 100 µm (Σ<50;50-100 µm) decreases significantly as DI augment (yellow and red)
- Fraction 100-200 µm is the third that is decreasing the most after DI increases over 50% (green)
- Correlation between PSD and DI% shows that as DI increases from 40 to 80%, volume % of particles
> 200 µm increases while volume % of particles < 200 decreases

Evolution of DI above 50% in the biological system introduces the presence of big granules (>550 µm), while smaller granules are still abundant (relative vs absolute fractions). Therefore, there is a combined effect of the emergence of bigger granules (teal blue) close to the critical threshold of 1-1.2 mm and pore blocking by fine particles on the membrane permeability (Campo et al 2021). While Campo et al claim that one of the main factors for stable operation is to "verify that granules mean dimensions are far from critical values (1-1.2 mm)", these results imply a noticeable drop of permeability when d90 starts to flirt with 590 µm. It appears that the shear effect of the memDENSE selector's ability to self-regulate the size of the particles below 1 mm functions until DI reaches 60-80%.

Densified Activated Sludge in continuous flow has been coupled with membranes at full-scale with two different separators: secondary settling tank (MABR and clarifier) and ultrafiltration (MBR). In two projects, steady state densification has been implemented to find the best the best operating points.