BIOLOGICAL TREATMENT OF KRAFT MILL WASTE-WATER

Wastewater from an integrated bleached kraft pulp and paper mill was treated in seven parallel treatment trains. Each train consisted of three reactors in series, with the primary reactor (denoted L1) simulating an aerated equalization/stabilization lagoon with hydraulic retention time (HRT) equal to 1.5 days. The secondary (middle) reactor was different in each train. Three reactors (R1, R2, R3) were operated at HRT = 4.5 days and the solids retention time (SRT) equal to, respectively 10, 20, and 40 days. The next three secondary reactors (R4, R5, R6) were operated at HRT = 1.5 days and SRT equal to 10, 20, and 40 days, respectively. The secondary reactor in the seventh train (R7) was operated at an HRT equal to SRT set at 1.5 days, that is, effectively operating as an aerated lagoon. The last, third or tertiary reactor in each of the series was operated as a settling lagoon with all reactors (denoted E1 to E7) having identical HRT = 1.5 days. All seven treatment trains have removed 5-day biochemical oxygen demand (BOD5) down to 10 mg/L. The removals of adsorbable organic halides (AOX) and soluble organic carbon (SOC) were found to be directly proportional to the biomass concentration and the SRT of the process and were the highest in the R6 reactor with SRT = 40 days and highest mixed liquor suspended solids (MLSS) of 3320 mg/ L. In the activated sludge reactors R1 to R6 the removals of AOX and SOC were found to be inversely proportional to HRT, that is, the removals were larger for the reactors R4, R5, and R6 which operated at HRT = 1.5 days. Overall AOX removal was the highest (30%) in the train L1 --> R6 --> E6, and the lowest AOX removal (21%) was in the train of lagoons L1 --> R7 --> E7. The negative effect of increased HRT on the removal of AOX and SOC was explained by biosorption since at the same SRT, the lower HRT reactors contained more than twice as much mixed liquor suspended solids.