Dynamics of Wastewater Treatment Systems Gustaf Olsson Lund University, Sweden Gustaf.Olsson@iea.lth.se
Models Models for understanding mechanisms design control
Mass Balances Conservation of mass: (rate of change of vessel contents) = (rate of inflows) (rate of outflows) + (rate generated) (rate consumed)
Simple respirometer The DO (S O ) concentration: ds o = dt Respiration rate r : r = rmax K r o S o + S o
Dissolved Oxygen Dynamics
DO responses Air flow rate Oxygen concentration
Dissolved oxygen dynamics Oxygen transfer rate: rate = K a S S ) L ( O, sat O V DO mass balance: d( VS dt O ) = q S q S + K a ( S S ) V in O, in out O L O, sat O + r V
Controlled DO response Air flow rate Change in inlet DO (disturbance) DO conc InletDO conc Change in DO setpoint
Controlling the DO response Influencing the K l a K a const u L air u air = u airo + K ( S S O, ref O )
Organic carbon removal
Carbon removal activated sludge Influent Bio reactor Effluent Aeration Sludge recirculation Sludge outtake
Simple nutrient control
Simple biological kinetics Process Components Nutrient N Biomass B Kinetics Aerobic heterotrophic growth 1 YB 1 µˆ K N sn + s N X B
Simple bioreactor response Biomass Influent substrate decrease Substrate
Soluble carbon removal
Carbon removal kinetics Process Components Nutrient Oxygen Biomass Kinetics Aerobic heterotrophic growth 1 Y H YH 1 Y H 1 µˆ H K S ss + s S K O so + s O X H Heterotrophic decay 1 f P -1 b H X H
Carbon removal response Biomass Decrease in influent substrate Increase in air flow
Carbon removal
Nitrogen removal
Pre-denitrification plant Influent Anoxic reactor Aerobic reactor Effluent Internal recirculation Sludge recirculation Sludge outtake
Nitrogen Removal Process Ammonium Dissolved oxygen Nitrification Nitrate Easily degradable organic matter Denitrification Free gaseous nitrogen
N removal basic mechanisms
Nitrogen removal kinetics Processes Aerobic heterotrophic growth Anoxic heterotrophic growtn Aerobic autotrophic growth Heterotrophic decay Autotrophic Carbon Components Oxygen NH 4 NO 3 Biomass Biomass heterotrophic autotrophic Kinetics decay
Nitrification (batch)
Denitrification (batch)
Phosphorous removal
Plant design for bio-p removal Influent Bio-P reactor Anoxic reactor Aerobic reactor Effluent Internal recirculation Sludge recirculation Sludge outtake
P removal Processes Fermentation P release P uptake PAO growth PHA breakdown PP breakdown PAO breakdown Components S F fermentable COD S A volatile fatty acids Dissolved oxygen Phosphate PHA polyhydroxylalkanoates PP polyphosphate PAO
P removal basic mechanisms
P release basic mechanisms Fermentation of fermentable COD to VFA. VFA used by the organisms to store carbon as poly-hydroxylalkanoates (PHA) P release from poly-phosphate into solution while VFA is converted to PHA
P uptake basic mechanisms P uptake from solution to PP using the PHA and DO Growth of PAO biomass, utilizing PHA and DO
P removal mechanisms Anaerobic condition: no dissolved oxygen nor nitrate present Aerobic or anoxic conditions: Concnetration nitrate in reactor and/or dissolved oxygen presentcondition PO4-P, ppm Net P uptake Phosphate Accumulating Organism (PAO) PHA storage Accumulated poly phosphate storage PHA storage Accumulated poly phosphate storage Easy degradable organic matter, VFA Phosphate Carbon dioxide Phosphate
20 Typical nutrient variations NO3 NH4 PO4 15 NH 4 PO 4 ppm 10 5 NO 3 0 27-sep 28-sep 29-sep 30-sep 01-okt 02-okt 03-okt 04-okt 05-okt 06-okt
P release
P uptake
Hydraulic models
Simple tank hydraulics q out = const N b α h q in qout Volume V Area A dv dt = A dh dt = q in q out
Settling
Multilayer model Overflow Layer 1... Feed Layer m-1 Layer m Layer m+1......... Layer n Underflow
Multilayer model Underflow Solids flux dxi hi Ai qu dt i = m + 1,..., n ( x x ) + A ( f f ) = i 1 i i i 1 C2x f = x v = x C e i i i i i 1 i
Settler Feed Flowrate Increase
Settler Underflow Decrease
Settler Profile 5 layers
Settler Profile 10 layers
Settler Profile 20 layers
General dynamic models
State model dx = f ( x( t), u( t), d( t), p) dt y( t) = g ( x( t), u( t), d( t), p) x(t) = state variables u(t) = manipulated input variables d(t) = disturbances input variables y(t) = output variables
Input-output models ( ) p t d t u t y h dt dy ), ( ), ( ), ( = ) ( ) (... ) 2 ( ) ( ) (... ) 2 ( ) ( ) ( 2 1 2 1 t v t n t u b t t u b t t b u t n t y a t t y a t t y a t y n n + + + + + + + + + = Time discrete form:
Summary Mass balances of substrates, organisms and dissolved oxygen Processes for C, N and P removal Settler dynamics is crucial Many different time scales