Tapered Aeration Figures

Sludge return

Secondary clarifier

Effluent

Sludge waBte

CompreaBed air

Primary effluent

Secondary clarifier

Effluent

Sludge return

Sludge waste

Tapered Aeration

Compressed air

Compressed air

Pure oxygen

Primary effluent

Oxygen return

Waste gas

Secondary clarifier

Effluent

Reactor Sludge return

Sludge waste

Brush-type aerator

Brush-type aerator

(e) clarifier
Activated Sludge Process PointsStep Aeration

Figure 6.7. Overview of common applications of the activated-sludge process, (a) step aeration; influent addition: influent addition at intermidate points provides more uniform removal throughout the tank, (b) Tapered aeration: air added in proportion to nutrient exerted, (c) Contact stabilization: biomass adsorbs organlcs in contact basin and settles out in secondary clarifier; the thickened sludge is aerated before being returned to the contact basin, (d) Pure-oxygen activated sludge: oxygen added under presurre keeps dissolved oxygen level high, (e) Oxidation ditch, plan view, (f) High rate: short detention time and high food/mass ratio in aerator to maintain culture In log-growth phase, (g) Extended aeration: long detention time and low food/mass ratio to maintain culture in endogeneos phase.

This limited efficiency makes contact stabilization less attractive as a design alternative for nitrification.

6.4 The Kinetics of the Activated Sludge Process

The kinetics of the nitrification process are well-defined for the suspended-growth systems. From experience, it has been found that the following factors have a significant effect on the kinetics of the nitrification process.

1) Ammonia and nitrite concentration, 2) COD/total N ratio, 3) Dissolved-oxygen concentration, 4) Temperature and 5) pH.

The impact of these variables on the nitrification and denitrification processes and the approach developed to account for them are reported in Chapters 3 and 4. Table 6.2 shows typical kinetic coefficients for the activated-sludge nitrification process. The kinetic expression used for analysis of suspended-growth nitrification and denitrification are summarized in Table 6.3.

6.5 Modification of Activated Sludge Plants for Biological Nitrogen Removal

Today's high standards for nitrogen removal from waste water often demand modification of existing plants. The approache necessary to convert an existing waste water treatment plant to a biological nitrogen removal plant is dependent on the site conditions and on the level of treatment required.

For existing systems that accomplish only removal of organic material, a higher solid retention time will have to be provided for nitrification to occur. This can be done by increasing the size of the aeration tank and/or the sludge concentration. This will need a greater quantity of oxygen.

If the system is already designed for nitrification, additional volume may be required to provide anoxic zones for denitrification. The anoxic volume in an activated sludge nitrification-denitrification system may account for 20 to 40% of the total tank volume. If denitrification is required the oxygen supply must be reduced.

A number of activated-sludge designs have been developed for the combined removal of nitrogen and phosphorus. Some of these processes were developed originally for phosphorus removal and later developed into combined phosphorus and nitrogen removal systems.

Table 6.2 Typical coefficient for the different parameters in the nitrifying activated sludge process.

Coefficient

Unit

Value

Range Typical

Nitrosomonas Um K.

d"1

Nitrobacter

Overall max

After: Schroeder (1976); EPA (1975) and Eddy and Metcalf (1991).

The most commonly used processes for combined nitrogen and phosphorus removal are: 1) the A2/0 process (Hong et al. 1984), 2) the five-stage Bardenpho process, 3) the UCT process and 4) the VIP process. They are all described in Metcalf and Eddy (1991). Stensel et al. showed in Table 6.4 the nitrification rate obtained, based on both total MLVSS and on calculated Nitrosomonas biomass for the biological nutrient removal (BNR) and the conventional activated sludge process.

Table 6.3 Summary of kinetic expressions used for the analysis of activated-sludge nitrification and denitrification. See also Chapters 3 and 4.

Equation Definition of terms s

Ks+S

H = specific growth rate, time"1

ds/dt = substrate utilzation rate, mass/unit

S = concentration of growth limiting substrate in solution, mass/unit volume.

Y = maximum yield coefficient, mass of cell formed per mass of substrate consumed.

Ks = maximum rate of substrate utilization.

k = k = maximum rate of substrate utilizaion.

<|>c = design mean cell residence time, time.

U = substrate utilization rate, time"1.

— = Yk-kd (|)cm = minimum mean cell-residence time.

SF = safety factor u = s°~ s S0 = influent substrate concentration mass/unit

X = conc. of microorganisms.

Table 6.4 Summary of specific Nitrification Rates and Ammonia Oxidation Rates in tbe biological nutrient removal process (BNR) and the conventional activated sludge process.

System SRT Aerobic SRT T Total NH3-N Aerobic Specific Nitrosomonas Ammonia d d °C Oxidized MLVSS Nitrification VSS CMcfefcn ate mg/l mg/l Rate mg/l mgN/mg mgN/gMLVSS/h Ntasaronas, d

System SRT Aerobic SRT T Total NH3-N Aerobic Specific Nitrosomonas Ammonia d d °C Oxidized MLVSS Nitrification VSS CMcfefcn ate mg/l mg/l Rate mg/l mgN/mg mgN/gMLVSS/h Ntasaronas, d

BNR

15

8.3

20

18.8

2636

1.783

122

0.834

5

2.7

20

23.2

1014

5.720

74

1.729

2.7

1.5

20

21.6

749

7.210

42

2.695

1.5

0.9

20

12.3

446

6.895

14

4.382

Conventional

15

15

20

21.2

1348

1.986

101

0.631

15

15

15

26.6

2177

1.527

143

0.560

5

5

20

26.5

1284

2.580

72

1.107

2.7

2.7

20

27.1

658

5.148

47

1.716

Source: Stensel etal. (1992)

Source: Stensel etal. (1992)

6.6 Modelling the Activated Sludge Process

A mathematical model, Activated Sludge Model No. 1, for the removal of carbonaceous biodegradable material, nitrification and denitrification was developed by the IAWPRC Task Group (Henze etal. 1987) and modified by Wentzel etal. (1991) and Dold (1991).

A total of ten dissolved and seven particulate components are used to characterize the wastewater and the activated sludge. These include:

1) Dissolved oxygen, bicarbonate alkalinity, and soluble phosphorus.

2) Three forms of biomass (Heterotrophs and two types of autotrophs, all represented in terms of COD)

3) Five forms of nitrogen (particulate and soluble biodegradable organic nitrogen, ammonia, nitrite and nitrate).

4) Six forms of COD (inert soluble and particulate in feed, two forms of biodegradable soluble, enmeshed slowly degradable particulate, and inert particulate COD from endogenous decay).

For a detailed overwiev of the formula matrix the authors recommend consulting the Activated Sludge Model No 1. (Henze et al. 1987), because the most recent attempts at modelling the activated sludge have been made with this model.

6.7 Advantages and Disadvantages of the Separate and Combined

Activated Sludge Process

The following gives an overview of some of the advantages and disadvantages of the activated sludge process, both as A) a separate stage process and B) as a combined stage process.

A) Separate stage activated sludge process for nitrification. Advantages:

1) Good protection against most toxicants.

2) Stable operation.

3) Low effluent ammonia concentration possible.

Disadvantages:

1) Sludge inventory requires careful control when BOD5/TKN ratio is low.

2) Stability of operation linked to operation of secondary clarifier for biomass return.

3) Greater number of unit processes required than for the combined oxidation and nitrification unit.

B) Combined carbon oxidation and nitrification activated sludge process for nitrification.

Advantages:

1) Combined treatment of carbon and ammonia in a single stage.

2) Low effluent ammonia is possible.

3) Inventory control of mixed-liquor sample due to high BOD5/TKN ratio. Disadvantages:

1) No protection against toxicants.

2) Only moderate stability of operation.

3) Stability linked to operation of secondary clarifier for biomass return.

4) Large reactors required in cold weather.

Part C

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  • ALGISA
    When tapered aeration?
    5 months ago

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