Pilot-scale comparison of thermophilic aerobic suspended carrier biofilm process and activated sludge process in pulp and paper mill effluent treatment

Thermophilic aerobic treatment of settled pulp and paper mill effluent was studied under mill premises with two comparative pilot processes; suspended carrier biofilm process (SCBP) and activated sludge process (ASP). Full-scale mesophilic activated sludge process was a reference treatment. During the runs (61 days) hydraulic retention times (HRTs) were kept 13 ± 5 h and 16 ± 6 h for SCBP and ASP, respectively. Corresponding volumetric loadings rates (VLR) were 2.7 ± 0.9 and 2.2 ± 1.0 kg COD_filt m^-3d^-1. Temperatures varied between 46 to 60°C in both processes. Mesophilic ASP was operated with HRT of 36 h, corresponding VLR of 0.7 kg COD_filt m^-3d^-1. Both SCBP and ASP achieved COD_filt (GF/A filtered) removals up to 85%, while the mesophilic ASP removal was 89 ± 2%. NTU values were markedly higher (100-300) in thermophilic effluents than in mesophilic effluent (30). Effluent turbidity was highly dependent on temperature; in batch experiment mesophilic effluent sample had NTU values of 30 and 60 at 35°C and 55°C, respectively. As a conclusion, both thermophilic treatments gave high COD_filt removals, which were close to mesophilic process removal and were achieved with less than half of HRT.