Evolution of the activated sludge process: the first 50 years

Abstract

The paper presents a critical overview of the first 50 years in the evolution of the activated sludge process. Recognition of the role of aeration and microbial activity in the purification of sewage in the early studies established the basis for the accidental discovery of the process, which was immediately adapted into practice. The problems encountered during operation started a period of empirical expansion with many process modifications. As scientific support lagged behind practice, efforts were then directed towards exploring and understanding the fundamentals of the system related to substrate removal mechanisms, microbiology, process kinetics and stoichiometry, nitrogen transformations and, then, translating accumulated information into design. Extensive research generated remarkable findings which should be regarded as major milestones for future progress in many areas such as fractionation of substrate and biomass, particle size distribution, and substrate storage. A rational foundation of the activated sludge process for the removal of organic carbon could then be established based on mass balance. This approach also defined essential parameters that could relate microbial reactions involved with plant design and operation. Scientific ideas and discoveries have also enabled conceptual development of many emerging technologies such as the sequencing batch reactor, enhanced biological phosphorus removal, the oxic-settling anaerobic process, and the super-fast membrane bioreactor.

The effect of iron dosing on reducing waste activated sludge in the oxic-settling-anoxic process

“…Historically, these ions were successfully traced and utilized for the understanding of microbial decay and substrate storage mechanisms (Gaudy and Engelbrecht, 1963;Orhon, 2014).…”

“…Separate handling of activated sludge has been historically tested and implemented for more effective stabilization; it was initially promoted as different re-aeration systems that basically involved aerating settled activated sludge in side-stream stabilization reactors prior to mixing with wastewater (Orhon, 2014). Later, more effective sludge stabilization was also reported to occur under anoxic/anaerobic conditions (Wanner, 1994;Eckenfelder, 1998).…”

“…Recently promulgated stringent regulations significantly increased the cost of sludge handling and disposal as they practically prohibited conventional and economically feasible approaches such as disposal to landfill (Wei et al, 2003); in fact, recent studies provided conclusive evidence that, technically, the organic carbon content of sludge could not be reduced to meet the new regulatory requirements (Orhon, 2014). Consequently, research is now focused on exploring different strategies that would potentially reduce the magnitude of sludge generation in the face of more expensive technologies such as drying, incineration, etc.…”

Assessment of anoxic volume ratio based on hydrolysis kinetics for effective nitrogen removal: model evaluation

“…System operation at the lower θ XD level of 3.5 days also achieved removals of 80% and close to 90% for total hydrolysable COD and total biodegradable COD respectively. These results are particularly important for activated sludge systems designed for carbon removal because, even at a θ X level of 3.5 days, they would be capable of removing approximately 90% of the biodegradable COD; any incremental increase of θ X would be purely to condition the biomass for appropriate gravity settling …”

Behavior of activated sludge systems with an active heterotrophic biomass inflow – a novel perspective for sludge minimization

“…Plant improvements were mainly focused on sustaining an activated sludge with good settling properties; this would ensure satisfactory system performance when coping with increasing sewage loads due to rapid population expansion and industrial development. Modifications mostly relied on experience and good judgment and were implemented on a trial and error basis …”