HYDRAULIC MODELING OF PHYSICAL SYSTEMS
In the field of environmental engineering, a physical stream refers to a flowing body of water within a confined region in stream banks or in conduits. A hydraulic model is a mathematical model of a water or storm or sewer that is used to analyse the system's hydraulic behaviour. Hydraulic behaviour refers to the transportation of water through the tank and is of fundamental importance for the function of a reactor and the efficiency of the wastewater treatment process. Good hydraulic conditions are characterised by favourable conditions for high biochemical reaction rates and favourable growth rate for desirable microorganisms. Hydraulic models provide an understanding of the system. Hydraulic behaviour deals with how water is transported and moves inside the tank. Hydraulic models are mathematical models that involve algorithms and methodologies. Hydraulic modelling softwares have user friendly and customizable Graphical User Interfaces (GUIs). A knowledge of hydraulic modeling is essential to environmental engineers. Hydraulic modeling of wastewater streams help in- performance assessment and
- optimal planning and design.
- Plug flow reactor and
- Complete Mix Reactor (CMR) or Completely Stirred Tank Reactor (CSTR)
- The flow of wastewater follows the principle of first-in first-out.
- Particles pass through the tank in the same sequence in which they enter the tank
- Longitudinal mixing is assumed to be almost negligible
- Concentration of the reactant varies with time along the length of the reactor
- Mass balance of a reactant at steady-state conditions is given by
Change in concentration of reactant due to reaction of reactant in time, dt
=
Change in concentration of reactant due to change in position of fluid element in time, dt
Expressing the statement mathematically
-(dc/dt) = (dX/V)
Negative sign implies decrease in reactant concentration
V - Velocity of flow through the reactor
dX - Differential change in distance along length of reactor
Integrating both sides we have
(limits Co to Ce)integral(-dc/dt) = integral (dX/V)(limits 0 to l)
Ideal complete mix occurs when the fluid particles entering the tank are immediately dispersed throughout the tank. There are no concentration gradients in the tank and the composition is equal all over the tank. Hence, the effluent from the tank has the same composition as the fluid inside the tank. This type of flow is approximated in round or square tanks if the content of the tank is uniformly and continuously distributed. Complete mix reactors are also known as Continuously-Stirred-Tank-Reactors (CSTR)
The actual hydraulic behaviour of most tanks treating wastewater lies somewhere between PFR and CSTR. Hence it is necessary to characterise the hydraulic behaviour inside each tank in order to understand the effects on the treatment process.
Two common hydraulic phenomena that occur in reactors are
Dead zones are water volumes that are stagnant. They typically occur near the corner of a tank if the mixing is insufficient. In these zones there is no exchange with the bulk flow in the tank and the dead volume.
Improper design of tank can cause short circuiting of streams and
dead volume. Short circuiting leads to insufficient time for
completion of biodegrading reactions. Dead volume in the tank lowers
the capacity of the tank.
Mixing characteristics influence the sludge settleability. Reactors with hydraulic behaviour that imitates plug-flow conditions produce better settling sludge when compared to completely mixed reactors and hence are preferred.
Mixing also has an effect on concentration of substrate available to microorganisms. This inturn affects the population of microorganisms present.
Sludge bulking refers to an excess of filamentous organisms present in sludge. These microorganisms cause the biological flocs in tne reactor to become bulky and loosely packed. Bulky flocs do not settle well and are carried over in the effluent of the settling tank.
The actual hydraulic behaviour of most tanks treating wastewater lies somewhere between PFR and CSTR. Hence it is necessary to characterise the hydraulic behaviour inside each tank in order to understand the effects on the treatment process.
Two common hydraulic phenomena that occur in reactors are
- Short circuiting streams and
- Dead volume
Dead zones are water volumes that are stagnant. They typically occur near the corner of a tank if the mixing is insufficient. In these zones there is no exchange with the bulk flow in the tank and the dead volume.
The performance of a
reactor is influenced by its hydraulic behaviour. The hydraulic
behaviour is in-turn affected by the following factors
- The geometric design of the reactor
- Shape and position of the inlet and outlet
- External mixers
- Baffles
- Fluid viscosity
- Aeration and
- Water flow rate
Mixing characteristics influence the sludge settleability. Reactors with hydraulic behaviour that imitates plug-flow conditions produce better settling sludge when compared to completely mixed reactors and hence are preferred.
Mixing also has an effect on concentration of substrate available to microorganisms. This inturn affects the population of microorganisms present.
Sludge bulking refers to an excess of filamentous organisms present in sludge. These microorganisms cause the biological flocs in tne reactor to become bulky and loosely packed. Bulky flocs do not settle well and are carried over in the effluent of the settling tank.