Abstract
Photo-activated sludge (PAS) systems are an emerging wastewater treatment technology where microalgae provide oxygen to bacteria without the need for external aeration. There is limited knowledge on the optimal conditions for enhanced biological phosphorus removal (EBPR) in systems containing a mixture of polyphosphate accumulating organisms (PAOs) and microalgae. This research aimed to study the effects of substrate composition and light intensity on the performance of a laboratory-scale EBPR-PAS system. Initially, a model-based design was developed to study the effect of organic carbon (COD), inorganic carbon (HCO3) and ammonium-nitrogen (NH4-N) in nitrification deprived conditions on phosphorus (P) removal. Based on the mathematical model, two different synthetic wastewater compositions (COD:HCO3:NH4-N: 10:20:1 and 10:10:4) were examined at a light intensity of 350 µmol m−2 sec−1. Add to this, the performance of the system was also investigated at light intensities: 87.5, 175, and 262.5 µmol m−2 sec−1 for short terms. Results showed that wastewater having a high level of HCO3 and low level of NH4-N (ratio of 10:20:1) favored only microalgal growth, and had poor P removal due to a shortage of NH4-N for PAOs growth. However, lowering the HCO3 level and increasing the NH4-N level (ratio of 10:10:4) balanced PAOs and microalgae symbiosis, and had a positive influence on P removal. Under this mode of operation, the system was able to operate without external aeration and achieved a net P removal of 10.33 ±1.45 mg L−1 at an influent COD of 100 mg L−1. No significant variation was observed in the reactor performance for different light intensities, indicating the EBPR-PAS system can be operated at low light intensities with a positive influence on P removal.
Highlights
Removal of phosphorus (P) from wastewaters before discharge is essential to mitigate negative environmental impacts
VSS; Mara, 2004); f) adjusting the inorganic carbon concentration in the feed taking into account the CO2 generation of polyphosphate accumulating organisms (PAOs) so that microalgae should be the only source of oxygen for PAOs when implementing the illuminated stage; and g) light requirement was estimated taking into account light penetration/attenuation as a function of the TSS concentration using the Lambert-Beer equation (Swinehart, 1962) (Appendix B)
Phase 1 (P1) was the starting point of the enhanced biological phosphorus removal (EBPR)-Photo-activated sludge (PAS) cultivation system based on the steady-state model prediction (COD:HCO3:NH4-N: 10:20:1)
Summary
Removal of phosphorus (P) from wastewaters before discharge is essential to mitigate negative environmental impacts This can be achieved either chemically or biologically, biological removal, via enhanced biological phosphorus removal (EBPR) in activated sludge systems, is effective in achieving higher P removal efficiencies at lower operational costs than chemical alternatives (Bashar et al, 2018). In the conventional EBPR process, mixed liquor is recirculated through alternating anaerobic-aerobic stages to promote the growth of poly-phosphate accumulating organisms (PAOs). PAOs take up volatile fatty acids (VFAs) present in the influent and store them as poly-b-hydroxyalkanoates (PHA), using glycogen as a reducing agent, and obtain the required energy through the hydrolysis of intracellularly stored polyphosphates (poly-P) (Wentzel et al, 2008). PAOs oxidize the PHA, obtaining energy for replenishing glycogen for growth and PO4-P uptake (Wentzel et al., 2008). The oxygen (O2) required by the EBPR process is usually supplied by mechanical aeration, which may increase the operational costs of aeration, and contribute up to 45-75 % of the total energy costs of activated sludge systems (Rosso et al, 2008)
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