Monitoring and control of activated sludge mostly use non-microbial specific parameters (e.g. SS, COD, BOD etc.). This thesis suggests they are no longer precise enough to give adequate reliability of the removal of N and P to meet the new standards. Basic theory points to the importance of mass transfer, diffusion and biochemical reaction rates in determining microbial morphology and therefore settlement. These phenomena are influenced by particle characteristics both of the feed and the floc itself. The thesis researches particle and floc characteristics and correlates their variability with the changes in waste characteristics, biodegradability, settleability, plant design (nitrifying, non-nitrifying, biological phosphorus removal and MBR) and effluent quality.