This study introduces a novel optical monitoring method to image and characterize activated sludge flocs and to study the dependency of sludge settling properties on the floc structure. The novel method can easily analyse thousands of particles in a short timeframe using the developed image analysis program. The main advantage of this method is its applicability for in situ use because the only required pre-treatment is sample dilution. This study tested real process samples from activated sludge plants treating wastewater from a pulp mill. The sludge samples were collected in bulking and non-bulking situations, and the image analysis results were compared to the settling speed of the samples. The structure of the activated sludge flocs was clearly different in bulking sludge situations as characterized by more fragile and elongated flocs. Additionally, excessive amounts of filamentous bacteria hold the flocs apart, hindering sludge settling. These results show that this method is suitable for studying and optimizing activated sludge processes.
A charge-coupled device camera was used for the optical monitoring of activated sludge flocs and filaments, and the image analysis results were compared with the effluent clarity at a full-scale activated sludge plant during a three-month period. The study included a maintenance stoppage at the wastewater treatment plant, which was followed by a settling problem. Thus, the study presents the development of floc morphology from poor flocculation to good flocculation. In this case, the evolution of flocs was a slow process, and the optimum floc morphology was achieved before the purification results improved. To diagnose the cause of the settling problems using optical monitoring, four major factors were found to be relevant: the mean area of the flocs, the amount of small particles, the amount of filament and the shape parameters of the flocs. In this case, the settling problem was caused by dispersed growth based on the image analysis results. In conclusion, the method used has the potential for usefulness in the development of monitoring applications to predict plant performance and also to diagnose the causes of the settling problems.
A mathematical model modified from the well established Activated Sludge Model no. 1 was used for modelling a full-scale wastewater treatment plant (WWTP) in a bleached kraft pulp mill. Effluents from the pulp and paper industry are typically nutrient deficient, which was considered in the model. The wastewater characterization and model calibration were based on respirometric batch experiments with sludge and wastewater sampled from the WWTP. The model performance was validated in a long-term simulation using routinely measured process data from the WWTP as the model inputs. The simulation results proved useful in evaluating nutrient dosage strategies at the WWTP and in troubleshooting poor treatment plant performance. However, in order to achieve a completely accurate description of nitrogen removal, more complex phenomena would have to be included in the model. Even though the simulated period was long compared to the brief measurement campaign used in the model calibration, the model was able to describe the treatment plant's behaviour. The calibrated model can be expected to stay valid for a long time, which allows the use of deterministic modelling in practical applications at pulp and paper WWTPs.
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