Study on the start-up operation and sludge characteristics of organic wastewater treated by the EGSB–two-stage anaerobic/aerobic biofilm process

Most wastewater treatment reactors have problems such as slow start-up and a poor treatment effect. This study investigates the rapid start-up and optimal conditions of the expanded granular sludge bed (EGSB)–two-stage anaerobic/aerobic (A/O) biofilm process. The combined process was successfully started by gradually increasing the organic load for approximately 50 d, during which the organic loading rate increased from 4.6 to 46 kg·COD/(m3·d). By investigating the operation effect of the EGSB reactor at hydraulic retention times (HRTs) of 4, 6, 8, and 10 h, reflux ratios of 6:1, 7:1, 8:1, and 9:1, and ammonia concentrations of 200–800 mg/L, the optimal operation conditions were determined. In the two-stage A/O process, hollow cylindrical fillers and hollow spherical fibrous fillers were used to observe the growth of biofilms on different fillers. The sludge structure and microorganisms were analyzed by scanning electron microscopy and high-throughput sequencing. The results show that when the combined process was successfully started, the chemical oxygen demand removal rate was 97%, and the ammonia nitrogen removal rate was 73%. The EGSB reactor had the best operation effect under the conditions of a low ammonia concentration, HRT = 8 h and reflux ratio of 8:1. In the two-stage A/O process, the biofilm growth condition of the hollow column filler with a high specific surface area was better than that of the hollow spherical filler containing flocculent fibers. The microstructure of the sludge was conducive to the metabolism of microorganisms. The primary microorganisms at the phylum level mainly included Proteobacteria, Firmicutes, Synergistetes, Actinobacteria, and Chloroflexi. These findings provide a new idea for the treatment of high-concentration organic wastewater and reactor start-up.