Aerobic granulation with industrial wastewater in sequencing batch reactors

Granular sludge formation was promoted in two laboratory scale sequencing batch reactors (SBRs), R1 and R2 fed with industrial wastewater produced in a laboratory for analysis of dairy products. Both reactors were operated under similar conditions during most of the experimental period. However, an anoxic phase between 10 and 30 min was included at the beginning of every cycle of operation of R1, but not in R2. Organic and nitrogen loading rates applied to both systems were high, up to 7 g COD/(L d) and 0.7 g N/(L d). Nitrogen removal efficiency was 70% in both units even considering that R2 was operated always under aerobic conditions. Granules with similar morphology were developed in both systems. Size distribution was comprehended between 0.25 and 4.0 mm for both systems. The presence of TSS in the effluent of the SBRs was strongly affected by either the length of the withdrawal period or by the particulated COD to biomass ratio (CODp/VSS) applied to the systems. The lower concentrations of TSS in the effluent were attained when the systems were operated with a CODp/VSS ratio lower than 0.12 g COD/g VSS. There was a strong reduction of the average TSS content in the effluent from 450 to 200 and 150 mg TSS/L when the length of the withdrawal period was diminished sequentially from 3 to 1 and 0.5 min, respectively. This was caused by a more intensive washout of small suspended biomass aggregates that took place when the length of this period was shortened.