Comment on “Bioflocculation management through high-rate contact stabilization: A promising technology to recover organic carbon from low-strength wastewater by Rahman, A., Meerburg, F.A., Ravadagundhi, S., Wett, B., Jimenez, J., Bott, C., Al-Omari, A., Riffat, R., Murthy, S. and De Clippeleir, H. [Water Research 104 (2016) 485–496]”

Which activated sludge configurations qualify for maximizing energy conservation – Why?

“…on HiAS, the average Y OBS characterizing 23 different experimental sets was 0.15 mg COD mg −1 COD; consideration of residual X SR could only raise the corresponding average Y OBS * to 0.18 mg COD mg −1 COD. These values are in the same Y OBS range as the one associated with extended aeration activated sludge, let alone sludge maximization . (ii) The HiAS experiments of Jimenez et al .…”

“…Only a small fraction of the sludge age can be associated with the contact tank. The contact sludge age clearly becomes insufficient to secure a sufficient level of substrate removal, especially in high‐rate systems operated at overall sludge ages of lower than 2.0 d. The kinetic basis of these arguments was explained in great detail in the recent literature . Furthermore, the vivid proof of the shortcomings in soluble COD removal is presented in Fig.…”

“…Unfortunately, the available data in the papers do not include the necessary kinetic information to assess X SR levels. Hence, an arbitrary X SR value was selected in the range 15–50 mg L −1 , and assigned to experimental runs as a function of decreasing θ X values from approximately 2.0 d ( X SR = 15 mg L −1 ) to θ X levels lower than 0.5 d ( X SR = 50 mg L −1 ), in compliance with the modelling results referred to in the preceding sections . The COD loss in the effluent, calculated on the basis of mass balance Eqn , is plotted in Fig.…”

“…Their systems were operated with the effluent of chemically settled sewage of low COD strength around 156–165 mg L −1 . The mass balance equation defining the fate of X S in the stabilization basin was solved for a k hx value of 1.0 d –1 , yielding residual (unutilized) X SR of 25–40 mgCOD L −1 for three systems operated at θ X values of 0.7, 1.1 and 1.7 d …”

“…In simpler terms, no matter how effective the energy recovery process may be, recovered energy may not be optimized without a compatible system design and operation that would maximize sludge generation. Hence, high‐rate AS configurations are becoming a prerequisite for this purpose . In short, the merit of an AS system to maximize energy conservation very much depends on the selected process configuration and operational conditions facilitating this priority.…”