Design of Plants for Precipitation of Nitrogen Compounds

As mentioned in Section 11.1, the application of precipitation requires a three-step plant. Addition of chemicals is the first step. It requires some sort of automatic dosage equipment, where the amount of chemicals added to the waste water is determined by either pH, the flow or another parameter, that is feasible to measure and relates to the quality of the influent.

The design of the flocculation tank can be based on a first order process.

The number of particles/ volume, N, is transformed into the volume of particles per unit volume of suspension:

where R = the volume of colloidal particles per unit volume of suspension. Substitution of equation (11.37) into equation (11.19) gives:

dt n

- a first order reaction.

Integration of this equation for the boundary conditions N = No at t = 0



These considerations allow us to apply the equations for a complete mixed flow reactor in combination with an equation for a first order reaction.

A complete mixed flow (CMF) reactor is generally designed on the basis of the following equation; see Fig.11.13:


dt where r(Ci) = the reaction rate.

For steady state conditions, provided the reaction is a first order reaction, we have:

where k = the reaction coefficient.

Dividing this equation by Q * Ci, gives:

Ci where tm = V/Q, the mean residence time in the complete mixed flow reactor. The equation can also be written as:

Ci 1

However, there are advantages in applying a number of reactors in series. Let us consider m first order CMF-reactors each with volume, V. A mass balance identical to the one used for equation (11.43) gives for the second tank:

C2 1

Ci 1+ k * tm where C2 = the effluent concentration from tank 2.

The effluent concentration from reactor 2 can also be expressed in terms of inflow concentration of the first reactor by multiplying equations (11.43) and (11.45):

In a similar way, the effluent concentration, Cm, from the last reactor in a series of first order CMF-reactors may be expressed in terms of the concentration of the inflow to the very first reactor:

The total detention time required to achieve a given reaction will therefore mtm = QL ^ CO - (11.48)

k Cm

If this consideration is used for the flocculation unit the following equation can be set up:

4nGR Nm


Figure 11.13. Complete mixed flow reactor. Flow rate Q, volume of tank V, concentration in tank C1, and the input concentration is Co.

The third step is the separation of the suspended matter and the clear water phase. Several possibilities are available for this step, as mentioned in Section 11.1. Centrifugation and filtration are, however, rarely used due to their high costs for the great amount of waste water which must be treated in most cases. The design of these two operations is therefore not included, while settling and flotation will be covered in this section.

Suspended solid in waste water cannot usually be described as discrete particles. If any of the interacting particles have characteristics that might cause agglomeration, growth of individual particles to larger size is a natural consequence. Hence, the greater the tank depth, the greater is the opportunity for contact among particles and so sedimentation depends on the depth as well as on the properties of the fluid and the particles.

At present there is no satisfactory formulation for predicting the effect of flocculation on the settling rate. Thus flocculent settling requires extensive testing to define the characteristics of the waste water in this respect.

Evaluation of the sedimentation characteristics of flocculent settling can be accomplished by placing a quantity of the waste water in a column similar to the one shown in Fig. 11.14. The diameter of the column must be sufficient to ensure that the edge effect is almost eliminated. The suspension is settled and the concentration of the particles is determined from samples withdrawn at the different sampling points. The fraction of the particles removed at each step is used to construct lines showing equal fraction or equal percentage removal, as illustrated in Fig. 11.15. The lines are named iso-concentration lines; the per cent maximum settling path for the indicated per cent removal.

If the tank has an overflow of v1 = H4 /12, (see Fig. 11.15) all particles having a settling velocity 2 v1 will be removed from the tank and particles with a velocity v < v1 will be removed in proportion to v / v1. The figure shows that the remaining solid between Ra and Rb has settled with an average velocity of v = H' / t2, and the solid between Rc and Rd has settled with an average velocity of H" 1X2. An approximation for the total overall removal, R, by the chosen overflow is given by:

R = Rc+ H'*(Rb-Rc)/t2VI + H"* (Ra - Rb) /t2*v1 (11.50)

This approximation can be improved by adding more terms and decreasing the interval between the iso-concentration lines.

Figure 11.14.

Column with four sampling points for settling tests.


h' \

' h2

-—V- i \ \ \!




\d \


" h4


t2 Time Figure 11.15. The results of a settling test illustrated with iso-concentration lines.

t2 Time Figure 11.15. The results of a settling test illustrated with iso-concentration lines.

Zone settling of flocculated chemicals suspension occurs when the concentration of solids exceeds approximately 0.5 g/1 The particles form a mass, which settles as blanket with a distinct interface between the settling sludge and the clarified effluent. The interface can be observed in a batch settling test. Initially all the suspension is at a uniform concentration and the height of the interface as Zo; see Fig. 11.16, which shows the height of the interface plotted versus time. In the region A-B, settling is hindered, but proceeds at a constant rate. The region B-C shows a transition Into the compression zone, represented by C-D. The zones are further illustrated in Fig. 11.17.



Obtuse Angle


Figure 11.16. Height of interface in zone settling as function fo the time.


Figure 11.16. Height of interface in zone settling as function fo the time.

Figure 11.17. Illustration of the zones in zone settling.

Clarified zone

Discrete settling zone

Hindered settling zone

Transition zone

Compression zone

Figure 11.17. Illustration of the zones in zone settling.

It is possible to design a continuous clarifier based on the batch test. Two areas must be calculated; A1, the area required for clarification, and A2, the area required for thickening. A1 can be calculated from:

where vs is the velocity for hindered settling and Q is the rate of flow through the tank. To find A2 it is necessary to find the relationship between settling rate and the concentration of the sludge. The tangent is drawn at different points of the settling curve and the slope of the tangent indicates the settling rate, v; see Fig. 11.18. The corresponding concentration in the sludge is calculated from the following equation:

where Co is the slurry concentration at the start of the settling, Zo is the total height of the clarifier and Z is shown in Fig. 11.18. By this equation it is possible to calculate C, the concentration of suspended solid in the sludge layer, as a function of the settling rate. It is now possible to calculate Ws, defined as the weight of solid in sludge produced per minute per m2:

where Cs is the required concentration of suspended solid in the layer. Ws Is calculated for values of C, and the minimum value is used to determine the area necessary for thickening. The area per m3 /h, A, is found by dividing the sludge concentration Co by W8, where Co is defined above. It means that:

Settling Curves Secondary Settler
Figure 11.18. Sedimentation curve. Zo is total height. Slope of tangent (0 settling rate) is found asZ/t.

It is frequently possible to improve the performance in an existing settling tank by making modifications based on the results of a dispersion test. The addition of stream-deflecting baffles, inflow dividing mechanism and velocity dispersion feed wells may decrease short circuiting and increase efficiency.

Fig. 11.19 illustrates the principle of tube settlers. The design incorporates the use of very small diameter tubes in an attempt to apply the shallow depth principle as suggested by Camp (1946).

Flow through tubes with a diameter of 5-10 cm offers optimum hydraulic conditions and maximum hydraulic stability. Culp et al. (1968) have reported excellent results using tube settlers with a retention time of less than 10 minutes. The retention time can be calculated according to the following equation:


Q flow rate

A area of tube settler

L = length of tube

S = distance between the tubes (the diameter of the tubes)

13 = the angle of the tube to the horizontal (see Fig. 11.19)

vs = direct settling rate

As can be seen from this equation, Q/A will increase as f3 decreases. It should therefore be an advantage to place the tubes as near as possible to horizontal. However, the horizontal settler is not self-cleaning and must be back-washed. Therefore, the steeply inclined 60° tube settler is more commonly used. Continuous gravity draining of settled solid might be achieved from tubes inclined at angles between 45 and 60°.

The clarifier may be designed as a rectangular or circular tank, and may utilize either center or peripheral feed. The tank can be designed for center sludge withdrawal or for withdrawal over the entire tank bottom.

It is very difficult to design a full-scale sedimentation tank based on settling experiments, as presented above. Several important factors influencing particle behavior in a full-scale operation are neglected in such experiments. Tanks are subject to eddies, currents, wind action, resuspension of sludge, etc. A full-scale clarifier will therefore show a slightly reduced efficiency compared to settling experiments, but this can be considered by incorporating a safety factor. The choice of an acceptable safety factor requires experience. The practical factor might vary from 1.5 when the tank is very small, baffled and protected from wind, to 3.0 in the case of a large tank, unbaffled and unprotected from wind. Even with the use of the safety factor, however, perfect performance should not be expected.

Figure 11.19. Steeply inclined tube settler.

Flotation is used to remove suspended solid from waste water and to concentrate sludge. Thus flotation offers an alternative to sedimentation, especially when the waste water contains fat and oils.

Either a portion of the waste water or the clarified effluent is pressurized at 36 atm. When the pressurized water is returned to normal atmospheric pressure in a flotation unit, air bubbles are created. The air bubbles attach themselves to particles and the air-solute mixture rises to the surface, where it can be skimmed off, while the clarified liquid is removed from the bottom of the flotation tank.

Fig. 11.20 shows a flotation system with partial recirculation of the effluent. Generally it is necessary to estimate the flotation characteristics of the waste water by use of a laboratory flotation cell:

Tube Settler Figure

27 outlet

Figure 11.19. Steeply inclined tube settler.

27 outlet

1.The rise of the sludge interface must be measured as a function of time.

2.The retention time must be varied and the corresponding saturation of pressurized water determined.

3.The effluent quality must be determined as a function of the air/solids ratio, Based on such results it is possible to scale up appropriately.

Figure 11.20. Flotation unit.

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