v 3.3.3 Microbial analysis and the relationship between relative abundance and activities 39 3.4 Conclusions 44 CHAPTER 4 SYNERGY AND DYNAMICS OF CO-CULTURED AOB AND NOB AT DIFFERENT INITIAL AOB/NOB RATIOS 46 4.1 Introduction 46 4.2.1 Bacterial strains 47 4.2.2 Chemostat reactors 48 4.2.3 Chemical analysis 50 4.2.4 FISH and image analysis 50 4.2.5 Field emission scanning electron microscopy (FESEM) 51 4.3 Results and discussion 51 4.3.1 Concentration profiles of nitrogenous compounds 51 4.3.2 AOB and NOB abundances 53 4.3.3 Distribution of AOB and NOB in cell cluster and flocs 56 4.4 Conclusions 63 CHAPTER 5 EFFECT OF CHEMOTAXIS ON FLOC STRUCTURE AND METABOLIC ACTIVITY OF NITRIFYING BACTERIA