Thermophilic and mesophilic aerobic biological treament of wastewater

To achieve closed cycle operation and subsequent reuse of treated effluent, individual streams of pulp and paper mill effluents are being evaluated for their suitability fur in-mill treatment. This thesis includes two individual studies. The first part focused on the feasibility of thermomechanical pulping (TMP) condensate treatment using thermophilic and mesophilic sequencing batch reactors (SBR). The second part of this thesis, based on the results of the first part and literature, developed an improved fundamental understanding of how environmental (temperature) and operating (dissolved oxygen (DO" conditions affecting sludge properties and their role in bioflocculation and settling using a synthetic wastewater of glucose. The main results and conclusions are summarized below.;2.) Effects of temperature (mesophilic (35°C) vs. thermophilic (55°C)) and DO concentration (under thermophilic conditions) on sludge properties were studied using well-controlled sequencing batch reactors fed with a synthetic wastewater of glucose. Under similar dissolved oxygen level, the thermophilic sludge had a poorer flocculating and settleability than that of the analogous mesophilic sludge. Under thermophilic condition, an increase in DO level led to a slightly improved flocculating ability and a poorer settleability. A poorer settleability was related to a higher level of filaments. Analysis of extracellular polymeric substances (EPS) using chemical analysis indicates that thermophilic sludge had a higher level of total EPS than that of mesophilic sludge under similar DO level, and an increase in DO resulted in an increase in total EPS in thermophilic sludge. In the thermophilic system, an increase in sludge volume index correlated to an increase in total EPS, and a decrease in effluent suspended solids correlated to an increase in total EPS. But these correlations disappeared when the results of the mesophilic system are included, this is understandable due to the difference in microbial community between thermophilic and mesophilic systems. The results suggest that either the electrostatic interactions or the quantity of total EPS can not explain the differences in flocculating behavior between thermophilic and mesophilic sludge, pointing in the direction of the specific roles of individual EPS molecule governing flocculation in different biological systems. The strategy of increasing DO level can not solve the biomass separation problems associated with thermophilic sludge.;1.) The treatment of TMP condensate was feasible in terms of chemical oxygen demand (COD) removal for both thermophilic and mesophilic aerobic biological treatment. Thermophilic treatment showed COD removals of 74-84% with 13% being removed due to the aeration effects, while mesophilic demonstrated COD removals of 79-91% with 9% being removed due to aeration effects. The flocculation of the thermophilic sludge was found to be poorer than that of mesophilic sludge as evident by the higher amount of suspended particles in the treated effluent. Also, the settling of the thermophilic sludge was worse than that of mesophilic sludge as demonstrated by the amount of sludge bulking that occurred. The reason for the poorer sludge bulking is believed to be due to the higher level of filamentous microorganisms found in thermophilic sludge and the reason for poorer flocculation is unknown. Treatment of TMP condensate by thermophilic aerobic treatment is feasible in terms of COD removal; however sludge separation needs to be improved.