SIMUTANEOUS NITIFICATION AND DENITRIFICATION (SND) AND ENHANCED BIOLOGICAL PHOSPHORUS REMOVAL (EBPR) IN SBR PLANTS IN INDIA

Abstract

Sequencing batch reactor (SBR) treatment plants are the most used wastewater treatment systems worldwide for biological nutrient removal from wastewater. Understanding the factors that impact the existence and behavior of microorganisms inhabiting selector-based SBRs is of the utmost value since it would direct for proper control of full-scale STPs for biological nutrient removal (specifically, Simultaneous Nitrification and Denitrification (SND) and Enhanced Biological Phosphorus Removal (EBPR)) and probably help in reducing operational expenditures. In developing countries like India, where the reactor temperature is relatively higher (25⁰C -30⁰C) compared to the developed countries (8⁰C-20⁰C), the low VSS/TSS ratio is ~0.50-0.65 for India, and >0.7 for developed countries, and low readily biodegradable COD at sewage treatment plant that is due to the silt ingress from largely unpaved areas, biological oxidation in drains which are further intercepted and diverted to sewage treatment plants, and intermittent small-scale industrial effluent discharges, it was essential to thoroughly study the wastewater composition, characterization, and the operating conditions responsible for the shifts in the microbial community structures which ultimately influences the biological nutrient removal (BNR) in the selector-based SBR wastewater treatment systems. Full-scale well-working SBR plants achieving COD < 50 mg/L, BOD<10 mg/L, TSS < 20 mg/L, TN< 10mg/L, and TP ≤1 mg/L have more functional species (i.e., Nitrifiers 15.7%, Denitrifiers 8.3%, and DPAOs 32.5%) compared to non-compliant or non-well-working SBRs, where the Nitrifiers are 7.4%, Denitrifiers are 7.2%, and DPAOs are 25.0%. High sewage and reactor temperatures in India (18-33⁰C) are effective in prevailing an abundance of Nitrifiers (Nitrospira and Nitrosomonas), Denitrifiers (Rhodococcus, Azoarcus, Zoogloea), and DPAOs (Burkholderia, Rhodocyclus, Rhizobiales, Pseudomonas, Paracoccus, and Flavobacterium) in several full-scale STPs. Nitrifiers, Denitrifiers, and DPAOs in domestic SBRs are 13%, 8%, and 32%, respectively, while in CETP, 3%, 13%, and 10%, respectively, though CETP has a higher abundance of cellulose degraders (41%) due to the cellulose fibers coming from textile industries. Average COD, TN, and TP removal efficiencies in domestic versus CETP were 85% and 69%, 73% and 61%, and 61% and 26%, respectively, due to the inclusion of high influent COD, TDS, Chlorides, Sulfates, and Heavy metals, in the wastewater of CETP. Detailed studies on a full-scale 3 MLD selector-based SBR showed > 95% COD and BOD5, > 96% NH4-N, >61% TP, and > 96% TSS removal and > 76% SND efficiency at 11.4 ±

2.6 days SRT with DO control of 0.5–2.5 mg/L in the SBR aeration tanks. A statistically significant result (R2> 0.8 and p < 0.001) was observed between rbCOD/ COD and SND, which showed a dependency of SND on rbCOD. The microbial community consists of Nitrifiers 37% (Nitrospiraand Nitrosomonas), Denitrifiers 10% (Rhodococcus, Azoarcus, Zoogloea), and Phosphorus removing organisms 17% (Burkholderia, Rhodocyclus, Rhizobiales, Pseudomonas, Paracoccus, and Flavobacterium) at lower SRTs. 100 KLD Pilot plant showed excellent removal efficiency with effluent COD 30 ± 6 mg/L, BOD was 9 ± 2 mg/L, TSS was 12 ± 3 mg/L, TN was 5.9 ± 1.6 mg/L, and TP was 1.1 ± 0.3 mg/L at low recirculation by total cycle time ratio R/T (0.017), and higher settling by aeration time ratios S/A (0.67 and 0.70). The key microbial community nitrifiers were Nitrospira(5.5%) and Nitrosomonas (5.4%) and the dominant denitrifiers were Flavobacterium(8.7%), Zoogloea (4.5%), Azoarcus(1.7%), Spirillum (1.1%), and Pseudomonas (1.2%). The putative PAOs or DPAOs belong to Burkholderia(17.0%), including Comamonas (8.9%), unclassified Burkholderiales (8.0%), and Burkholderiales bacterium (0.1%), Rhodocyclales(6.1%), Defluviicoccus(5.2%), Sphingomonas (2.6%), and Rhodospirillaceae (2.4%) contributing to SND-PR process. Moreover, Indian SBRs have higher numbers of nitrifiers, specifically Nitrospira in well working SBRs 11-27%, while STPs of developed countries have only 2.5-3% Nitrospira. The reason can be the favorable sewage temperature for Nitrospira’s growth and proper DO conditions (0.5-3.0 mg/L) in Indian selector-attached SBR plants. Indian SBRs have lower numbers of filaments than the STPs of developed countries. A2O plants in developed countries have M. Parvicella around 6%, and Indian STPs have no or much lesser <0.5%. Due to the low VSS/TSS ratio, low rbCOD, low P in the influent, and high SRT in the Indian STPs, the putative PAOs in the Indian STPs are lower than the developed countries (Candidatus Accumulibacter (2 8%) and Tetrasphaera (27%)). The study suggests that despite having the deficiency of CandidatusAccumulibacter and Tetrasphaera, other organisms like Paracoccus, Novosphingobium, Rhodococcus, Pseudomonas, Burkholderia, and Comamonas can conduct denitrification as well as biological phosphorus removal process, and Nitrospira can perform for high nitrification rates along with organic matter removal if proper rbCOD/ TP, SRT, and DO conditions are maintained in the SBRs in India.