The competitive growth of Zoogloea ramigera and type 021N in activated sludge and pure culture ― a model for low F: M bulking

Abstract

Bulking is a common problem in activated sludge plants and is usually associated with excessive growth of filamentous bac teria. A variety of causes have been implicated including low dissolved oxygen (DO) concentration, nutrient deficiency, high sulfide concentration (septic wastewater), and low organic load ing, that is, a low food-to-microorganism ratio (F:M). Laboratory experiments and field studies have illustrated that creation of high initial wastewater substrate concentrations, or substrate concentration gradients, can prevent or cure low F:M bulking. High wastewater substrate concentrations can be created in practice by intermittent feeding of wastewater to the aeration basin, that is, a sequencing batch reactor (SBR),16 or modifi cation of the aeration basin configuration to incorporate a plug flow reactor or one or more small basins (selectors) upstream of the main aeration basin.4'718 These empirical laboratory and field studies all show that cre ation of a high spatial or temporal substrate concentration allows the system to select a flocculant rather than a filamentous ac tivated sludge. Several hypotheses exist to explain the preferential selection of floc-forming bacteria including the surface area to volume ratio theory,17 the kinetic theory,9 the accumulation regeneration theory,5 and the starvation theory.6 This paper presents experimental results that explain why fil amentous organisms such as Type 02IN are selected in com pletely-mixed low F:M systems and the mechanism by which selectors function to prevent filamentous organism growth and bulking at low F:M. Experiments included both activated sludge studies and pure culture work on a representative floc-forming bacterium (Zoogloea ramigera) and a filamentous bacterium (Type 02IN) that can cause bulking at low F:M. Growth and substrate uptake kinetics of these two organisms were incorpo rated into a mathematical model that can be used to design and predict aerobic selector performance in low F:M activated sludge systems.19