Effect of organic loading rates on granulation characteristics and performance for rice mill wastewater treatment

Abstract

The impact of elevated OLR in conjunction with an anaerobic feeding strategy on the direct development of Aerobic granular sludge (AGS) for agro-based refractory rice mill wastewater is unexplored. This work utilized four identical sequencing batch reactors (SBRs) namely, R1, R2, R3, and R4 directly supplied with synthetically prepared lignin-rich rice mill wastewater, operating at varying OLR of 3, 6, 9, and 12 kg COD/m³·d, respectively to develop AGS. While granules in R1 and R2 achieved a stable average diameter of 1.23 ± 0.2 and 2.56 ± 0.1 mm, R3 and R4 could not meet the traditional AGS requirement due to excessive biomass development and loss of loose and fluffy filamentous granules. Higher EPS and PN/PS values led to compact and stable structures in OLR 3 and 6 kg COD/m³·d. Reactor performances were stable for R1 and R2. COD, lignin, TN, ${\mathrm{NH}}_4^{+}$N, and TP removal were 90 ± 2, 80 ± 3, 63 ± 2, 70 ± 2, and 52 ± 3 %, respectively for R1 and 86 ± 1, 73 ± 2, 68 ± 2, 81 ± 3, and 71 ± 2 %, respectively for R2 were observed. For OLR $\ge$ 9 kg COD/m³·d, stable granules did not form due to low DO and lignin inhibition led to overall unsatisfactory performance of R3 and R4. This study contributed to a better understanding of AGS formation and treatment performance in terms of organic and nutrient removal under different OLR conditions in lignin rich rice mill wastewater.