As a rough rule of thumb,  ...

As a rough rule of thumb,  the equalisation or balance tank should be big enough to do its job of balancing pH levels, loads and volumes, so there are no hard and fast rules.  It does depend on whether there are other materials other than domestic sewage from industrial or food sources.  I prefer to have a minimum of 24 hours when there is a food or dairy feeding the waste.  If the volumes vary throughout a day, then the volumes need to be balanced in the first tank.  I also prefer to mix this tank with air to prevent it from becoming septic.   As far as anoxic tanks is concerned, I like to have between 30-60 minutes contact time where returned sludge can be mixed with fresh liquor.  MBR plants do not require settlement tanks because the membranes carry out the separation process.  It also means that the plant can run at higher suspended solids levels and can  use many forms of aeration.  MBBR systems are a cross between fixed film and aerated systems.  In theory, they are supposed to have a smaller volume than an activated sludge plant.  Because my experience has been with industrial plants after the plant has been upset, I am not a fan of them.  I cannot see an real advantage of MBBRs.  As far as aeration is concerned, I avoid oxygen systems, because the carbon dioxide is not removed and causes the pH to run below ideal levels below 7.  With the advent of fine-bubble aeration systems, I prefer them to coarse air systems such as helixers.  If you have a plant, which uses a settlement tank, then I avoid jet mix or pumped venturi systems because it disrupts the bacterial floc and makes separation more difficult.  Good oxygen levels are needed to allow ammonia oxidising bacteria to produce nitrate, on which other bacteria may grow.  For design of plant I refer to Medcalf and Eddy.