Optimum backwashing conditions and ammonium-nitrogen treatment efficiency of iron oxide modified sands coated in biofilm

Abstract

In this study, iron oxide modified sands with nano-pores (IOMSNP) were enveloped in biofilm (B-IOMSNP) in order to treat the ammonium-nitrogen (NH4+-N) in micro-polluted raw water. The biomass, optimum backwashing conditions, treatment efficiency, and surface morphological properties of the B-IOMSNP were investigated via bio-filtration experiments. The raw quartz sands (RQS) and IOMSNP exhibited biomass levels of 24.32 nmol-P/(g-sand) and 83.71 nmol-P/(g-sand), respectively. The B-IOMSNP filter exhibited a lower swelling ratio, smaller initial head loss, and longer filtration period when water and air were used alternately for backwashing rather than water alone. In addition, the B-IOMSNP was most effective when air alone (at a strength of qair,I = 7 L/(s·m2) for tair,I = 6 min), a combination of air and water (at strengths of qair,II = 7 L/(s·m2) and qwater,II = 5 L/(s·m2) for tair-water,II = 6 min, respectively), and water alone (at a strength of qwater,III = 7 L/(s·m2) for twater,III = 4 min) were used alternately for flushing. The results indicated that the proposed B-IOMSNP could efficiently resist the shock induced by high NH4+-N concentrations (4 mg/L). The ripening period of the B-IOMSNP column was equal to 30 minutes. Furthermore, the B-IOMSNP reduced the turbidity of the water to values of less than 0.2 NTU for 72 hours and exhibited an NH4+-N removal rate of up to 96% and a head loss of 132 cm. In contrast, the biofilm-coated RQS exhibited an NH4+-N removal rate varying from 60% to 82%, a filtration period of 16 hours, and a terminal head loss of 58 cm. Due to its nano-pores and rough surface, the IOMSNP exhibited a specific surface area 39 times greater than that of the RQS, resulting in a higher filtration performance and absorption capacity.