Why have cascading vessels in the 'B' part of the 'AB' wastewater treatment process?

[MartinLe]

The AB process is wastewater treatment process that works like this (

https://en.wikipedia.org/wiki/Adsorption/Bio-oxidation_process):

'A Stage': Wastewater is (after mechanical treatment) subjected to a high load activated sludge process where also a lot of contaminats are adsorbed on the sludge.

'B Stage': Low load activated sludge process with further BOD destruction and nitrification and denitrification in anoxic zones. A part of the effluient of the B vessels (before settlement tanks) is recirculated to the beginning.

Each stage has it's own set of settlement tanks and return sludge.

Now my question: I've seen the claim (and actuall don't doubt it) that the B-stage is more effective (Per volume available) when the effluent of the A-Stage is added in a cascading way: Say (in a rectangular, long tank) 50% of effluent at the inflow end, 50% in the middle. Return sludge is all added at the beginning of the B stage. A carbon source can be added if required.

What is the process reason why cascading is more effective? In a cascading the low load reactor, is there still recirculation?

 

[ashtree]

Its a classic application of step feed. Typically step feed is about balancing oxygen demands and F:M ratio throughout the aeration tank. By spreading the feed over a number of different points it prevents a major oxygen sag at the inlet. Likewise with food distributed over a wider area then you do not get parts of the tank with very high F:M and parts with very low F:M. If this was in a large complete mix reactor the constant and relatively frequent movement of the biomass between high and low DO and high and low F:M creates a higher degree of stress and will favor certain bacteria groups, particularly filaments which may result in poorer settling sludge.Poor settling sludge will ultimately lead to larger wet sludge volumes. It does slightly favor de-nitrifiers as well because of a larger anoxic volume but potentially higher carbon low DO environment. But with most of the readily biodegradable COD gone in the A reactor this will be hardly any advantage.
However if you step feed the load is spread more evenly over the aeration tank. This is less likely to require a tapered aeration. The other thing is there is a lower chance of anoxic conditions occurring in the heavily loaded part of the aeration tank. This therefore makes better use of the aerobic volume,
If you were doing nutrient removal you would still need to recirculate between anaerobic , anoxic and aerobic but would depend upon your process aims , effluent and in-fluent requirements and actual plant configuration.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[MartinLe]

thanks, that's helpful.

 

[bimr]

Another aspect is that the activated sludge process treatment efficiency is limited in each stage.

Adding a second stage allows the treatment efficiency to be increased. One can achieve greater than 80+% treatment efficiency in each stage.

This scenario would be more appropriate when treating higher strength wastewater.