Hydraulic models of physical streams

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

1. performance assessment and
2. optimal planning and design. 

 Modelling is also used for planning wastewater collection systems. In the context of wastewater treatment, hydraulic models are used to predict the dynamically changing concentrations of pollutants at various points in the physical stream. The models can also be used to demonstrate the effectiveness of proposed solutions. This is achieved by testing different alternatives and using models to arrive at the best possible alternative. The modeling capabilities are used to evaluate different alternatives to decide the best course of action. The two main types of theoretical hydraulic behaviour are

1. Plug flow reactor and
2. Complete Mix Reactor (CMR) or Completely Stirred Tank Reactor (CSTR)

The ideal plug flow reactor is charaterised by fluid particles passing through the tank and being discharged in the same sequence in which they entered. The particles remain in the tank for a time equal to the theoretical detention time. This type of flow is approximated in long tanks with high length to width ratio. These type of reactors are also known as tubular flow reactors.

•  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

1. Short circuiting streams and
2. Dead volume

In a 'short circuiting stream' the incoming flow or a part of the incoming flow, takes a "short cut" bypassing the reactor. With reference to wastewater treatment, this implies that a portion of the influent has a lower detention time in the reactor than the actual designed detention time resulting in lowering the efficiency of the treatment plant.
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

1. The geometric design of the reactor
2. Shape and position of the inlet and outlet
3. External mixers
4. Baffles
5. Fluid viscosity
6. Aeration and
7. Water flow rate

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.

Disposal of effluents into the ocean

OCEAN DISPOSAL OF WASTEWATER

• The ocean has been the ultimate sink for water-borne waste products coming from the land.
• Waste from urban and industrial communities has been disposed off in the ocean.
• The effluent which is a very dilute mixture of human and other waste is collected in a pipe system and carried to a central location. After treatment the effluent is discharged into the ocean.
• The disposal is carried out by constructing a pipeline on the bed of the ocean with a diffuser.
• The effluent that has a density close to that of fresh water, rises to the surface and entrains the surrounding salt water in the process. Hence surrounding salt water and becomes very dilute
• Of the total pollution of marine water, disposal by land or land based discharge amounts to 44%.
• Using oceans to dispose effluents is an example of dilute-and-disperse philosophy of waste disposal
• Ocean dumping is a small but potentially growing part of the overall ocean pollution picture
• Oceans will ultimately provide the most environmentally acceptable repository for limited types of man generated waste at specific sites and under specific conditions
• A marine outfall is a pipeline that discharges industrial or municipal wastewater, stormwater, combined sewer overflows, cooling water or brine effluents from water desalination plants into the ocean. Usually this is under the water surface (submarine outfall).
• In order to successfully dispose effluents into the ocean, the water quality should meet the objectives and requirements set by regulatory authorities
• The environmental (physical, chemical and biological) data of the proposed site should be collected for atleast one year. This describes the undisturbed pre-discharge condition and defines the environment in which plume mixing and dispersion occur.
• Plume behaviour is strongly affected by density stratification and currents
• Structural engineering design requires a detailed bathymetric map, information on the wave environment and geotechnical investigation of foundation conditions.
• Effluent quality is determined by degree of treatment. Buoyancy of the effluent relative to sea water is important to dynamics of the plume.
• Disposal of effluents will have some effects which depend on system design.
• The effects of ocean disposal of effluents will be muich less than the effects of other possible engineering solutions of effluent disposal to land or inland water bodies.