Reshaping the activated sludge process: has the time come or passed?

Abstract

Despite the amazing achievements over the years, the activated sludge (AS) process still maintains inherent drawbacks, namely gravity settling, high sludge ages, low excess sludge generation, etc., which hinder its successful operation and sometimes disrupt system performance. Substantial research on the subject has provided clear scientific evidence that time has come -or even passed -for reshaping the AS process. In the novel AS process, gravity settling will be replaced by membrane filtration, which will convert the system configuration into a membrane activated sludge (MAS) reactor; this configuration will be operated at an extremely low sludge age range of 2 to 4 days, which will maximize sludge generation and it will abandon the traditional anaerobic sludge digestion for optimizing energy recovery. It is suggested that super-fast membrane activated sludge (SFMAS) reactor be the new face of the AS process. SFMAS reactor will be the core unit of the AS process essentially limited to organic carbon (chemical oxygen demand) removal and energy recovery. The core SFMAS unit will also enable to benefit from all the assets of the AS process, such as nutrient removal, biopolymer recovery, etc., by means of hybrid systems in different configurations, i.e. by reactors with various functions that will be attached to the core unit.

Anatomy of super‐fast activated sludge process with gravity settling for biodegradation and energy recovery potential ‐ a review

“…In fact, initial studies with MBR systems operated at a sludge age range of 0.5–5.0 days reported stable system operation all while securing high COD removals, and obviously, with no particulate matter in the effluent 50,51 . A recent review on the performance of super‐fast MBRs (θ X lower than 2.0 days) designed for organic carbon removal and conducted with different substrates provided reliable experimental results that ‘… high sludge age values merely represent a design luxury for the MBR systems since complete removal of soluble biodegradable substrate could be achieved at θ X < 2.0 days .’ 52 . A new study proposed a novel model, MASM, for the membrane activated sludge process, which disclosed a significant but so far overlooked system attribute of much lower effluent COD levels; this was experimentally justified by means of entrapment and recycling of larger size particles compared to the effective filtration size of the membrane module 53 …”

Efficient energy recovery from wastewater with high rate activated sludge process: oversights and misguidances

“…Later, relevant microbial mechanisms of super‐fast MAS systems for organic carbon removal could be assessed by means of model analysis for domestic sewage and selected industrial wastewaters 48,49 . A review, after evaluating all existing information, argued that in the near future, the activated sludge process will be reshaped and ‘the novel AS configuration shall not include gravity settling and it will be operated at extremely low θ X levels of <2.0 d’ 50 …”

“…48,49 A review, after evaluating all existing information, argued that in the near future, the activated sludge process will be reshaped and 'the novel AS configuration shall not include gravity settling and it will be operated at extremely low ⊔ X levels of <2.0 d'. 50 The evaluation focused on the energy conservation aspect mainly because it constituted the rate-limiting step of the energy recovery mechanism. The paper under discussion adopted a biochemical technologyanaerobic digestion of conserved sludge for biogas generation.…”

Appraisal of Super-Fast Membrane Bioreactors by MASM—A New Activated Sludge Model for Membrane Filtration

“…Membrane filtration of biomass should be recognized as a cutting-edge landmark that would perhaps reshape the future of the activated sludge process. It took over the function of gravity settling, which accompanied the process since its discovery [1]. Gravity settling has always been the weak spot in the process, mainly because it could not cope with excessive biomass escape under conditions of filamentous bulking; it imposed limitations restricting biomass concentration in the reactor; and most importantly, it distorted system design, as the main concern had to be sustaining good settling conditions for biomass based on empirical experience, often leading to higher footprints [2].…”