MBR-DAS: Densification Improves MBR Performance at the City of Detroit Lakes

Membrane bioreactors are often selected as secondary treatment technologies where small footprint and water reclamation are desired. Sludge densification by means of an external selector such as memDENSE with adequate biological configuration helps maximize sludge densification and quality, thereby improving biological performance. We investigated the resulting oxygen transfer efficiency and membrane performance of an MBR before and after upgrade with densification and recorded an improvement in aeration performance and permeability. Densified biomass has been shown to be more resilient and efficient than CAS in terms of overall settleability performance (SOR, SLR, sludge blanket level, RAS rate, RAS concentration; Donnaz et al, 2020; Roche et al, 2021). 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). MLSS was found to be the main parameter controlling the α-factor in MBRs, corroborating several studies which have established the negative effects of solids concentration on oxygen transfer (inter alia, Krampe and Krauth, 2003; Germain et al., 2007). However, although several investigators focused on the concentration and viscosity of the sludge (e.g., Cornel et al, 2003; Racault et al, 2011; Amaral et al, 2018), none included MBRs with densified sludge. We investigated here the effects of densification on oxygen transfer in a full-scale MBR. Densified biomass is expected to have beneficial impacts on MBR membrane filtration performance. 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). A full-scale evaluation has been underway at the City of Detroit Lakes for over nine months to capture the benefits of biomass densification in a plant with challenging seasonal operation. Figure 1 shows the configuration of this MBR plant: anaerobic, anoxic and aerobic tanks in series (A2O configuration), with memDENSE incorporated as the external selection process on the RAS stream. Comprehensive characterization of mixed liquor properties and membrane fouling is being conducted before and during densification in this plant. The memDENSE system accumulates the large and dense biomass in the underflow while wasting the small and light biomass from the overflow (Fig. 1). In the first stage of intensification, the particle size and density increased in both underflow and overflow. Preliminary testing at full-scale at Detroit Lakes MBR facility showed early improvement of membrane filtration correlated to rapid drops of SVI, time-to-filter and superfine colloids (sfCOD) in the membrane tank mixed liquor (Donnaz 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. With results similar to those of SVI, Figure 2 shows that poorly filterable sludge is wasted while a much more stable biomass is achieved with densification. This led to a significant improvement in the MBR performance. Preliminary results shown in Figure 3 indicate that with selective wasting and continuous flow densification, permeability is improved and TMP stabilized. Allowing an acclimation period of 4 months of memDENSE operation to achieve a new early steady state, oxygen transfer efficiency (OTE, % and αSOTE, %) was compared at the two process conditions. The OTE was measured using the ASCE method (ASCE, 2018). Since the clean water performance of the disc diffusers was available, the alpha factors could be calculated and compared. An increase in air flux after densification corresponds to increased oxygen uptake (Table 1). The efficiency (OTE and αSOTE) is higher, despite the increase in aeration, indicating an improvement in the quality of the biomass-wastewater matrix. In fact, the α-factor improved and is now reaching the design value of 0.50, despite the diffuser aging. In the presentation, we will discuss in detail the field results and their comparison with the design criteria.