Investigation of Two-Stage Membrane Bioreactor Potential of Activated Sludge and Microalgae in Municipal Wastewater Treatment

The combination of activated sludge and microalgae in a membrane bioreactor (MBR) is a new promising solution in municipal wastewater treatment to remove nitrate (${\mathrm{NO}}_3^{-}$) and phosphate (${\mathrm{PO}}_4^{3-}$). Furthermore, municipal wastewater contains a considerable amount of ${\mathrm{NO}}_3^{-}$ and ${\mathrm{PO}}_4^{3-}$, which is suitable for microalgae growth. It can be concluded from the literature that there is a direct relationship between microalgae biomass production and nutrient removal. Thus, the light cycle and the initial microalgae concentration (as two main factors for microalgae growth) can be optimized. In this study, the biomass productivity in three light-dark (L-D) cycles (h) (24-0, 16-8, and 12-12 h) and initial concentrations (0.5, 1, and $1.5\mathrm{g}·{\mathrm{L}}^{-1}$) were measured. The L-D cycle of 24-0 and the initial concentration of $1\mathrm{g}·{\mathrm{L}}^{-1}$ was used as the chosen parameter in the membrane photobioreactor (MPBR) containing microalgae. The initial concentration of nitrate, phosphate, chemical oxygen demand (COD), and total dissolved solids (TDS) was investigated. The investigation was performed first on the MBR containing activated sludge and then on the treated wastewater via MBR to MPBR containing microalgae (MBR-MPBR). The duration of the experiment was 133 days. During this period, the removal amount of COD, nitrate, and phosphate was reported at 92.35%, 47.55%, and 36.46% in the MBR, respectively. In MBR-MPBR, the average of nitrate and phosphate removal increased to 93.88% and 88.44%, respectively. However, TDS did not show any notable alterations in both operating systems. According to the findings, the second stage of the treatment process, MBR-MPBR outperformed the MBR system in terms of ${\mathrm{NO}}_3^{-}$ and ${\mathrm{PO}}_4^{3-}$ removal.

A membrane bioreactor is a combination of the membrane with microorganisms used in the wastewater treatment process. Due to their simple structure, small size, and high efficiency in removing organic and inorganic substances, membrane bioreactors have been considered for industrial and urban wastewater treatment. Operational parameters in the design of these systems have a significant impact on nutrient removal efficiency. Furthermore, based on the high efficiency of microalgae in removing nutrients, its combination with activated sludge increases the efficiency of the bioreactor. In this work, some operating parameters such as the light-dark cycle and initial concentration of microalgae were optimized in the membrane bioreactor. Then, two various systems, namely a membrane bioreactor (containing activated sludge) and a two-stage membrane bioreactor (containing activated sludge and microalgae), were compared. In this regard, the total dissolved solids, chemical oxygen demand, nitrate, and phosphate removal were investigated. All these values were recorded under optimal conditions of the light-dark cycle and initial concentration of microalgae in two different systems.