Environmental Engineering

Accumulation = Input − Output + Generation − Consumption. At steady state, accumulation = 0.

It accounts for mass created or destroyed by chemical or biological reaction within the control volume; a sink makes it negative.

Contents are perfectly mixed, so concentration is uniform throughout and the effluent concentration equals the in-tank concentration.

Fluid moves as discrete plugs with no axial mixing; concentration varies along the reactor length like an ideal batch reactor in time.

θ = V / Q, the average time fluid spends in the reactor.