Optimal Operation of a Wastewater Treatment Unit Using Advanced Control Strategy Emad Ali Chemical Engineering Department King Saud University

Objectives Model Utilization: –optimum operating conditions determination – testing modern control strategies Improved plant operation via advanced control strategies Comparing PID with NLMPC

Assumptions: well mixed units well aerated reactor X i = 0 no reaction in settler no settler dynamics ideal settler: X out = 0 Process Model

Mathematical Model Without Recycle : With Recycle:

Optimal Operating Conditions Given: S i = 1.0 g/l K= 0.1 g/l  = 0.5 l/hr Y = 0.5 g/g k d = l/hr D c = 0.56 l/hr X rc = 0.23 g/l

S = substrate concentration X = biomass (cell) concentration D = dilution rate = Q/V K = saturation constant Y = yield coefficient  = maximum specific growth rate k d = endogenous decay constant D c =  (1+S i )/(S i -  (1+S i )),  = k d /  S i =S i /K X rc = S i /(1+  ) - 1/(  -1-  ),  =  /D for D>D c, conversion increase with U & X r for D X rc

Optimum operating point X S D X r U

Control Objectives Controlled Variables: Biomass Conc. (X) Substrate Conc. (S) Manipulated Variables: Dilution rate (D) Recycle ratio (U) Possible Disturbances: Inlet biomass conc. Inlet substrate conc. Inlet flow rate Recycle biomass Reaction activity Settler level Sludge flow rate Oxygen conc. Air flow rate

Controller design PI algorithm: Single loop scheme  u (k) = k c (e (k) - e (k-1) ) + k I e (k) NLMPC : Multivariable scheme

Closed-loop test for SISO case Rejection of +25% step change in Y S ss =0.04 X ss =0.38 D ss =0.2

Closed-loop test: MIMO case Setpoint change from arbitrary to optimal point S ss =0.04 X ss =0.38 S sp =0.01 X sp =3.5 D ss =0.2 U ss =2.0

Closed-loop test: MIMO case Rejection of + 10 % change in X r S sp =0.01 X sp =3.5 D ss =0.4 U ss =1.0

Closed-loop test: MIMO case Rejection of +0.2 step change in X i S sp =0.01 X sp =3.5 D ss =0.4 U ss =1.0

Closed-loop test: MIMO case Rejection of -20% step change in  S sp =0.01 X sp =3.5 D ss =0.4 U ss =1.0

Conclusions Optimum condition of maximum conversion and capacity is determined Optimal operation in the face of plant disturbances is obtained via good control Superiority of NLMPC over PI is observed