Static mixer application in enhancing biogranules treatment through hydrodynamic shear force in sequencing batch reactor

Biogranulation is considered a promising technology in biological wastewater treatment due to its high effluent treatment quality, strong ability to withstand organic loading, strong microbial structure and high capability to remove organics, nitrogen and phosphate. Hydrodynamic shear force in terms of superficial air velocity (SAV) is known as one of the significant factors in biogranulation process. High SAV would lead to the requirement of high aeration energy in biogranulation system while lowering the SAV will not be able to form biogranules of desired characteristics. The main objective of this study was to determine the effect of static mixer at low SAV on physical properties and removal performance of biogranules in treating synthetic textile wastewater. Experiments were divided into initial, development and post-development stages. Four static mixers namely SM1, SM2, SM3 and SM4 were designed and tested in the initial phase of the study. The best static mixer was chosen based on the mixing time study. The experiments were carried out in Sequencing Batch Reactor (SBR) made of acrylic with a height of 100 cm, diameter of 8 cm and a working volume of 1.5 L, at an exchange ratio of 50%. Three reactors with static mixer and another three operating without static mixer (R1-0.5N, R1-0.5Y, R2-1.4N, R2- 1.4Y, R3-21.N and R3-2.1Y) were used during development stage to investigate the properties and performance of developed granules under influence of static mixers and SAVs of 0.5, 1.4 and 2.1 cm/s. The influence of hydraulic retention time (HRT) and organic loading rate (OLR) on the properties and performance of aerobic granules were investigated during post development stage. Two reactors named as R4Y and R4N were used, where R4Y was a reactor with static mixer and R4N was without the static mixer. The mixing study revealed that SM1 was the best static mixer as compared to SM2, SM3 and SM4 due to the shortest mixing time of 8 sec. The results in development stage disclosed that the granules could be developed at low SAV using static mixer (R1-0.5Y) with integrity coefficient (IC) of 40.5%, sludge volume index (SVI) of 107.5 mL/g, settling velocity (Sv) of 70.3 m/h, chemical oxygen demand (COD) removal of 94.6% and color removal of 53.8%. The results also showed that the developed granules in others reactors with static mixer improved the physical properties and reactor performance. Reactor R2-1.4Y was the best with an IC of 27.5%, SVI of 29.6 mL/g, SV of 80.4 m/h, COD removal of 94.2% and color removal of 63.1%. The study also demonstrated that HRT and OLR affected the performance of biogranules. Again, the physical characteristics of biogranules and removal efficiencies of the reactor with static mixer seemed to be better as compared to the reactor without static mixer with increasing of HRT and OLR. The increase of HRT in the system resulted in decreasing of SRT (R4N-95.1 days, R4Y-96.8 days) with increasing of overall specific biomass growth rate (uoverall) (R4N-0.011 per day, R4Y-0.010 per day), endogenous decay rate (kd) (R4N-0.178 per day, R4Y-0.241 per day), observed biomass yield (Yobs) (R4N- 0.052 mg VSS/mg COD, R4Y-0.051 mg VSS/mg COD) and theoretical biomass yield (7) (R4N-0.938 mg VSS/mg COD, R4Y-1.230 mg VSS/mg COD). Meanwhile, the higher OLR of the system increased the SRT (R4N-96.2 days, R4Y-100 days) which led to the decreasing of Coverall (R4N-0.010 per day, R4Y-0.010 per day), k d (R4N-0.179 per day, R4Y-0.197 per day), Yobs (R4N-0.027 mg VSS/mg COD, R4Y-0.028 mg VSS/mg COD) and Y (R4N-0.497 mg VSS/mg COD, R4Y-0.587 mg VSS/mg COD). At lower SAV, the use of static mixer provided shorter SRT which resulted in higher ^ overau, k d, Yobs and Y compared to the reactor without static mixer. The findings proved that static mixer is capable to enhance hydrodynamic shear force at low SAV in biogranulation system and therefore, will lower the energy consumption and operational cost