In this article we will discuss about:- 1. Introduction to Activated Sludge Process 2. Oxygen Requirements and Transfer 3. Secondary Settling 4. Advantages and Disadvantages of Activated Sludge Process.
Introduction to Activated Sludge Process:
The activated sludge process was developed in England in 1914 by Ardern and Lockett and was so named because it involved the production of an activated mass of micro-organisms capable of aerobically stabilizing a waste. Since then various modifications of the original process have been made but fundamentally they are all the same.
The activated sludge process of sewage treatment is based on providing intimate contact between the sewage and activated sludge. The activated sludge is the sludge which is obtained by settling sewage in presence of abundant oxygen so as to be enriched with aerobic micro-organisms. Thus activated sludge is biologically active and it contains a large number of aerobic bacteria and other micro-organisms which have an unusual property to oxidize the organic matter.
In the activated sludge process either raw sewage or the effluent from the primary settling tank is mixed with 20 to 50 percent of its own volume of returned activated sludge. The mixture enters an aeration tank where the organisms and sewage are mixed together with a large quantity of air. Under these conditions three more-or-less distinct activities occur.
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First, the organisms oxidize a portion of the organic matter present in the sewage to carbon dioxide and water, and other end products to obtain energy for cell maintenance and the synthesis of new cell tissue. Second, synthesize the other portion of the organic matter and convert it into new microbial cell tissue using part of the energy released during oxidation.
Finally, when the organic matter is used, the new microbial cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endogenous respiration.
These three activities are defined by the following generalized chemical reactions:
As a result of intense mixing of activated sludge with sewage in the presence of ample quantity of oxygen the following effects take place:
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(i) Organic matter present in sewage is oxidized; and
(ii) Suspended and colloidal matter coagulate and form flocculent masses which are readily settleable.
The mixture of returned activated sludge and sewage in the aeration tank is referred to as mixed liquor. The mixed liquor then enters a settling tank where the flocculent masses enriched with micro-organisms settle and are removed from the sewage. Thus new activated sludge is continuously being produced in this process.
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A portion of the settled sludge enriched with micro-organisms or activated sludge is then recycled to the head end of the aeration tank to be mixed again with sewage. Once the required concentration of micro-organisms in the mixed liquor has been reached so as to maintain proper food/micro-organisms (F/M) ratio for optimum operation, its further increase is prevented by regulating the quantity of sludge recycled.
The excess sludge produced each day (termed as waste activated sludge) is disposed of together with the sludge from the primary settling tanks. Fig. 13.1 shows the generalized biological process reactions in the activated sludge process.
Aerobic and facultative bacteria are the predominant micro-organisms which carry out the above reactions of organic matter i.e., oxidation and synthesis. Their cellular mass contains about 12% Nitrogen and 2% Phosphorous. These nutrients should be present in sufficient quantity in the sewage or they may be added as required for the reactions to proceed satisfactorily. A generally recommended ratio of BOD5 : N : P is 100 : 5 : 1. Domestic sewage is generally balanced with respect to these nutrients.
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The effluent obtained from a properly operated activated sludge plant is clear and sparkling and it is of high quality, usually having a lower BOD than that from a trickling filter. Typical BOD5 and suspended solids concentrations in the effluent obtained from an activated sludge plant vary from 10 to 20 mg/l for both constituents. In general the percentage of removal of suspended solids and BOD in an activated sludge plant may be as high as 90 to 95 percent.
The main advantage of activated sludge process is that it offers secondary treatment and an effluent of high quality with a minimum land area requirement.
However, in this process a rather close degree of control is necessary in its operation to ensure that:
(i) An ample supply of oxygen is present;
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(ii) There is intimate and continuous mixing of the sewage and the activated sludge; and
(iii) The ratio of the volume of activated sludge added to the volume of sewage being treated is kept practically constant.
Moreover, there is a problem of obtaining activated sludge at the start of a new plant. Hence, when a new plant is put in operation, a period of about 4 weeks is required to form a suitable activated sludge, and during this period almost all the sludge from the secondary settling tank will be returned through the aeration tank. However, a new plant may also sometimes be seeded with activated sludge from another plant, so as to quickly start the process in the new plant.
Index of the Mass of Active Micro-Organisms:
The suspended solids concentration in the aeration tank liquor, also called Mixed Liquor Suspended Solids (MLSS), is generally taken as an index of the mass of active micro-organisms in the aeration tank. However, the MLSS will contain not only active microorganisms but also dead cells as well as inert organic and inorganic matter derived from the influent sewage.
Thus composition of MLSS in the aeration tank is given by the following equation:
The suspended-solids concentration maintained in the mixed liquor (MLSS) of a conventional activated sludge process ranges from 1000 to 4000 mg/l. The concentration of volatile suspended solids (VSS) in the mixed liquor is usually 60 to 85 percent of the total suspended solids.
Oxygen Requirements and Transfer:
Oxygen is required in the activated sludge process for the oxidation of a part of the influent organic matter and also for the endogenous respiration of the micro-organisms in the system. The former is a function of BOD removal, while the latter is a function of MLVSS in the aeration tank. However, a part of BOD is removed from the system, without being oxidized, in the form of wastage of excess sludge (synthesized biomass) from the system.
Hence the oxygen requirement will be equal to the amount that would be required to remove all the BOD by oxidation less a credit for the fraction of BOD removed by sludge wasting. The theoretical oxygen requirements can thus be determined by knowing the BOD5 of the sewage and the amount of organisms wasted from the system per day.
If all the BOD5 is converted to end products, the total oxygen demand may be computed by converting BOD5 to BODL, using an appropriate conversion factor. A portion of the organic matter is converted to new cells that are subsequently wasted from the system: therefore, if the BODL of the wasted cells is subtracted from the total, the remaining amount represents the amount of oxygen that must be supplied to the system.
The BODL of a mole of cells can be estimated as follows:
Therefore, the theoretical oxygen requirements for the removal of the carbonaceous organic matter in sewage for an activated sludge system can be computed as-
The formula does not allow for nitrification but allows only for carbonaceous BOD removal. The extra theoretical oxygen requirement for nitrification is 4.56 kg O2 per kg of ammonia nitrogen (NH3 – N) oxidized to nitrate nitrogen (NO3 – N).
The total oxygen requirements per kg of BOD5 removed for different activated sludge processes are given in Table 13.3. The amount of oxygen required for a particular process will increase within the range shown in the table as the F/M value decreases.
After having determined the total oxygen requirement, the actual quantity of air to be supplied is found by considering the fraction of oxygen in air and the oxygen transfer efficiency of the aerators. The specific weight of air, at mean sea level is 1.2 kg/m3 at 20°C (and 1.16 kg/m3 at 30°C) and the fraction of oxygen in air is 23.2%.
The air supply must be adequate to:
(1) Satisfy the BOD of the sewage,
(2) Satisfy the endogenous respiration by the sludge organisms,
(3) Provide adequate mixing, and
(4) Maintain a minimum dissolved oxygen concentration of 1 to 2 mg/1 throughout the aeration tank.
The following information about air supply is noteworthy:
(1) For F/M ratios greater than 0.3, the air requirements for conventional process amount to 30 to 55 m3 per kg of BOD5 removed.
(2) At F/M ratios less than 0.3, endogenous respiration, nitrification, and prolonged aeration periods increase air use to 75 to 115 m3 per kg of BOD removed.
(3) For diffused air aeration, the amount of air used has commonly ranged from 3.75 to 15.0 m3/m3 at different plants with 7.5 m3/m3 an early rule-of-thumb design factor.
(4) To meet the sustained peak organic loadings it is recommended that the aeration equipment be designed with a safety factor of at least 2.
(5) The air diffusion system should be capable of delivering 150 percent of normal requirements which are assumed to be 62 m3 per kg of BOD in the sewage applied to the aeration tanks.
Secondary Settling:
Secondary settling assumes considerable importance in the activated sludge process as the efficient separation of the biological sludge is necessary not only for ensuring final effluent quality but also for return of adequate sludge to maintain the MLSS level in the aeration tank.
The secondary settling tank of the activated sludge process is particularly sensitive to fluctuations in flow rate and on this account it is recommended that the units be designed not only for average overflow rate but also for peak overflow rates.
The high concentration of suspended solids in the effluent require that the solids loading rate should also be considered. The recommended overflow rates and solids loading rates for secondary settling tanks of activated sludge process are given Table 13.4.
Operationally, secondary settling tank must perform two functions:
(i) Separation of the mixed liquor suspended solids from the treated sewage, which results in a clarified effluent; and
(ii) Thickening of the sludge to obtain the return sludge of desired concentration xr.
Both these functions must be taken into consideration for proper design of the secondary settling tank. Furthermore because both the functions will be affected by the depth, adequate consideration must be given to the selection of a depth that will provide the necessary volume for both the functions. For example – ample volume must be provided for storage of the solids during periods in which sustained peak plant loadings are experienced. Also, peak daily flow rate variations must be considered because they affect the sludge removal requirements.
Settling Characteristics of Concentrated Suspension:
The settling characteristics of concentrated suspensions (that contain high concentrations of suspended solids), like the mixed liquor of the activated sludge process, are very much different from those of dilute suspensions. In the case of concentrated suspensions, both hindered or zone settling (Type 3) and compression settling (Type 4) usually occur in addition to discrete (free) and flocculent settling.
The behaviour of a particular concentrated suspension in both clarification and thickening is determined by conducting settling tests in a graduated cylinder.
This test is usually known as column settling test, which is described below:
A column of height Ho is filled with a suspension of solids of uniform concentration Co. As the suspension settles the time taken by the solid-liquid interface to cross each mark of known height is recorded and plotted as a height v/s time graph as shown in Fig. 13.20.
At the beginning of the test, because of high concentration of particles, the liquid tends to move up through the interstices of the contacting particles. Due to this, the contacting particles tend to settle as a zone or ‘blanket’, maintaining the same relative position with respect to each other, giving rise to what is known as hindered settling represented by the portion AB of the curve (Fig. 13.20).
In the hindered settling (or zone settling) the solid-liquid interface visibly subsides in the column at a uniform rate till the concentration at the interface approaches a critical concentration Cc.
At this stage of critical concentration the rate of subsidence of interface decreases due to increased density and viscosity of the suspension, and a transition region occurs, as indicated by portion BD of the curve. After that as the time passes, compression settling occurs, when the solid particles come into physical contact with each other, and get compacted due to the weight of the sludge in the upper layers. Compression settling is represented by portion DE of the curve.
Area Requirement Based on Single-Batch Test Results:
For purposes of design, the final overflow rate selected should be based on a consideration of the following factors:
(i) Area required for clarification (or settling);
(ii) Area required for thickening; and
(iii) Rate of sludge withdrawal.
The area required for thickening is usually greater than the area required for clarification (or settling) and hence the area required for thickening is the controlling factor in the design of secondary settling tank.
The area required for thickening is determined according to a method developed by Talmadge and Fitch. A column of height Ho is filled with a suspension of solids of uniform concentration Co.
The position of the interface as time elapses and the suspension settles is determined and plotted against time as shown in Fig. 13.20. The rate at which the interface subsides is then equal to the slope of the curve at that point of time. Accordingly the area required for thickening is given by the expression
In which
A = area of tank required for sludge thickening (in m2);
Q = flowrate into tank (in m3/s);
Ho = initial height of interface in column (in m); and
tu = time to reach desired underflow concentration (in s)
The critical concentration controlling the sludge handling capability of the tank occurs at a height Hc where the concentration is Cc. This point is determined by extending the tangents from the hindered settling and compression settling regions of the subsidence curve to their point of intersection and bisecting the angle thus formed as shown in Fig. 13.20. The bisector will then intersect the subsidence curve at Cc, the point of critical concentration.
The time tu can be determined as follows:
1. Draw a tangent to the subsidence curve at point Cc.
2. Determine the height Hu that corresponds to the height at which the solids are at the desired underflow concentration Cu, using the following expression-
3. Draw a horizontal line at height Hu, and at the intersection of this line with the tangent at Cc draw a vertical line to the time axis to determine the value of tu.
Knowing time tu required to reach the desired underflow concentration Cu, the area required for sludge thickening is found using equation 13.65. The area required for clarification is then determined as indicated below, and the larger of the two areas is the controlling value.
For determining the area required for clarification the slope of the subsidence curve in the hindered settling region (i.e., slope of line AB) is determined which gives the settling velocity in the hindered settling region. The area required for the clarification is obtained by dividing the overflow rate of the settling tank by the settling velocity.
The overflow rate of the settling tank is proportional to the liquid volume above the critical sludge zone in the settling tank, and it may be determined.
Surface Overflow Rate (SOR) and Solids Loading Rate:
The design of secondary settling tanks for the activated sludge process is based on both Surface Overflow Rate (SOR) as well as solids loading rate.
Surface Overflow Rate (SOR):
The surface overflow rate (SOR) represents the hydraulic loading per unit surface area of tank in unit time expressed as m3/day per m2, or simply as m/day. The surface overflow rates must be checked both at the average flow and peak flow. The values of surface overflow rates to be adopted for the design of secondary settling tanks as recommended in the Manual on Sewerage and Sewage Treatment of the Ministry of Urban Development, New Delhi are given in Table 13.4.
The smaller values in the ranges given are applicable to small plants of capacities less than 5 Mld. These values of surface overflow rates should be applied to the sewage flow and not to the mixed liquor flow to the settling tank.
Solids Loading Rate:
The solids loading rate represents the total solids applied per unit surface area of tank per unit time and is expressed as kg SS/m2 per day. The solids loading rates must also be checked both at average flow and peak flow. The values of solids loading rates to be adopted for the design of secondary settling tanks as recommended in the Manual of Sewerage and Sewage Treatment of the Ministry of Urban Development, New Delhi are also given in Table 13.4.
The solids loading rate is related to the MLSS and the surface overflow rate (SOR) by the following expression-
In effect, the solids loading rate represents a characteristic value for the suspension under consideration. It has been observed that in a settling tank of fixed surface area the effluent quality will deteriorate if the solids loading is increased beyond the characteristic value for the suspension. Typical solids loading rate values for activated sludge mixed liquor vary from 72 to 144 kg SS/m2 per day.
The surface area for secondary settling tank is to be designed for both surface overflow rate and solids loading rate and the larger value should be adopted.
Types of Secondary Settling Tanks:
The secondary settling tanks for activated sludge process may be either circular or rectangular. The diameter of circular tanks may range from 3 to 60 m, with the more common range being from 10 to 30 m. The tank radius should preferably not exceed five times the side water depth.
Circular tanks may be either of centre-feed type or rim-feed type. Both types use a revolving mechanism to transport and remove the sludge from the bottom of the tank. In rectangular tanks, the maximum length, where possible, should not exceed ten times the depth, though lengths upto 90 m have been used successfully in larger plants. Width of tank is generally limited to 6 m, but where width of tank exceeds 6 m, multiple sludge collection mechanisms may be used to permit tank widths upto 24 m.
Flow through Velocity:
In order to avoid troubles due to density currents and scouring of the deposited sludge, the horizontal flow through velocities based on maximum mixed liquor flow should be limited to 30.5 m/h in rectangular tanks. In circular tanks of centre-feed type, the inlet baffle should have a diameter of 15 to 20 percent of the tank diameter and should not extend more than 1 m below the surface to avoid scouring of deposited sludge from the sludge draw off sump.
Placement and Loading of Effluent Weir:
Circular settling tanks are manufactured with overflow weirs located near both the centre and the perimeter of the tank. The minimum water depth below effluent weirs so located should be 3.1 m to prevent overflow of density currents. When weirs are located at the tank perimeter or at end walls in rectangular tank, the minimum depth should be 3.7 m. Optimum position of the circular weir troughs to intercept well clarified effluent is at a distance of 2/3 to 3/4 of the radial distance from the centre.
Weir loading rates in large tanks at maximum flow should be kept below 375 m3/m per day for weirs located away from the zone where density currents might turn upward, and below 250 m3/m per day when located at the zone where density currents might turn upward.
In small tanks, the weir loading rate should be kept below 125 m3/m per day at average flow and below 250 m3/m per day at maximum flow. The upward velocity in the immediate vicinity of the weir should be limited to about 3.7 to 7.3 m/h. This can be used to determine the spacing of multiple weirs in rectangular tanks.
Advantages and Disadvantages of Activated Sludge Process:
Advantages of activated sludge process:
1. Clear sparkling effluent of high quality is obtained.
2. Process requires small area of land and hence, the design may be made compact.
3. There is freedom from fly and odour nuisance due to high degree of treatment given to the sewage in this process.
4. Process is highly efficient. Removal of SS, BOD and bacteria are around 90% each.
5. Low cost of installation as compared to that of trickling filters.
6. There is comparatively very small loss of head through the treatment plant.
7. The excess sludge has higher fertilizing values as compared to sludge obtained from other treatment methods.
8. Degree of stabilization or nitrification is controllable within limits, so as to match with the quantity and character of receiving sewage. The treatment may be partial or full as desired or required.
Disadvantages of Activated Sludge Process:
1. If there is sudden increase in the quantity of sewage, or if there is sudden change in the character of sewage, there are adverse effects on the working of process, and consequently, the effluent of poor quality is obtained.
2. Operating cost of the process is high.
3. Large quantity of wet sludge obtained at the end of the process requires suitable method for its disposal.
4. Process is sensitive to certain types of industrial sewages, particularly in respect of those which may cause sludge bulking.
5. Process requires skilled supervision for its efficient working. It becomes necessary to ascertain that the sludge actually remain active during the process.