Particle size distribution as a major characteristic of domestic wastewater: implications for the modeling and design of membrane bioreactors

This study reviews the evolution of the concept and the methodology of particle size distribution (PSD) analysis for sewage. Evaluation of experimental data indicated that the analytical approach started with an empirical identification of soluble, colloidal and supra-colloidal organic matter. Then, it evolved into a sequential filtration and ultrafiltration methodology, yielding results useful enough to be an integral complement of respirometric chemical oxygen demand (COD) fractionation based on biodegradation characteristics. Default values could be defined for the distribution of COD fractions in terms of both size and biodegradation characteristics, which practically showed similar profiles, suggesting that PSD of COD could be directly used for model evaluation and design of activated sludge configurations. Size distribution of COD in sewage should be regarded as a prerequisite for activated sludge systems with a membrane module, mainly to assess the fate of COD fractions that would be entrapped and recycled back to the reactor by membrane filtration.