The activated sludge process of sewage treatment is based on providing intimate contract between the sewage and biologically active sludge. The sludge is developed initially by prolonged aeration under conditions which favor the growth of organisms with special ability to oxidize organic matter.
When the sludge containing these organisms is brought into contact with the sewage, organic materials are oxidized and suspended and colloidal matters tend to coagulate and form a precipitate matter, which settles quite readily.
A rather close degree of control is necessary in operation to ensure that:
- An ample supply of oxygen is present;
- There is intimate and continuous mixing of the sewage and the sludge liquor; and
- The ration of the volume of activated sludge added to the volume of sewage being treated is kept practically constant.
As a first step in the activated sludge process, the sewage is given preliminary treatment to remove interfering substances, such as grit, coarse solids, and oils and grease by utilizing sedimentation and other devices described elsewhere. From the settling tanks, the sewage flows to mixing and aeration tanks where activated sludge, returned from the final settling basin, is added, usually in an amount between 20 and 30 percent of the volume of the incoming sewage.
In the aeration tank, the mixture of sewage and sludge is aerated and agitated by applying compressed air or by stirring with specially designed mechanical devices, or by a combination of the two methods. After aeration for 4 to 8 hours, the period depending on the strength and character of the sewage and the degree of treatment desired, the sewage flows to final settling tanks where the activated sludge is separated by sedimentation.
A portion of this settled sludge is returned to the inlet end of the aeration tanks to inoculate the incoming sewage, and the remainder is pumped to digesters or otherwise disposed of by other means.
PRIMARY AND FINAL SETTLING TANKS
Removal of the grit and the larger solids in the raw sewage is generally considered necessary before aeration. However, some large plants utilizing the activated sludge process do not employ sedimentation as a phase of preliminary treatment. Removal of materials which may settle rather rapidly is helpful in preventing deposits on, and reduction of the efficiency or, aeration devices.
In addition, such materials may settle on the tanks bottom and, by decomposition, interfere with the treatment process. Accordingly, grit removal, screening or comminution, and sedimentation are normal phases of preliminary treatment.
Since it is desirable to keep the sewage as fresh as possible, a somewhat shorter detention period is provided in the primary settling tanks than is required for most other treatment processes. The period of primary may vary with the size of the plant and the characteristics of the sewage, but tank sizes will generally provide an overflow rate of about 100 gpd per sq. ft. For a depth of 8 ft, the detention period will be about 1.4 hr.
From the aeration tanks, the sewage flows to a final settling tank or clarifier. This will normally be of a general type described in other section, but there will be some modifications. Since there are no floating solids, provisions for the removal of scum or flotage are not needed. The suspended particles in the aeration-tank effluent are light in weight and are thus markedly influenced by currents.
Therefore, a considerable length of overflow weir is desirable to reduce the velocity of approach. Good design should provided a weir overflow rate of 10,000 gpd per lineal foot for plant with a capacity of 1 mgd or less, and not more than 15,000 gpd per foot for larger plants.
The tank overflow rate in a large plant should not exceed 1,000 gpd per sq ft of surface. However, for plants having a capacity of less than about 1 mgd, the overflow rate should be held to 800 gpd per sq ft. Some consideration has been given to establishing a ratio of depth to length of flow. It may be about 1 to 5 for circular tanks and about 1 to 7 for rectangular tanks. Since final settling is always required in an activated sludge plant to provide the return sludge, duplicate settling tanks are generally considered necessary.
CHARACTERISTIC OF PROCESS
A high degree of treatment can be obtained with a conventional activated sludge process. The effluent will be clear and low in BOD. However, a number of modifications of the conventional have been developed, and they will be almost any lesser degree of treatment that is desired. In normal effluent from complete treatment plants of conventional design, the BOD ranges from 15 to 30 mg/l.
Most of the modifications are design to remove 50 to 70 percent of the applied BOD, as compared to 90 percent or better for the conventional plant. The extreme flexibility provided by the various modifications constitutes one of the important advantages of this method of treatment. Another advantage, especially applicable in the case of many large cities, is the relatively small area required for the plant.
The first cost of an activated sludge plant is comparable to that of any other process providing the same degree of treatment. Operating costs are often higher, and a considerable degree of skill is required in operation, especially where organic loads and volume of flow vary materially over short periods of time.
The necessity for operational skill and, in most cases for 24â€“hr operating attention may limit the economical application of this method to communities of large population. Utilization of digester gas for power production may offset some of the operating costs, but this is feasible only where 24-hr operation is provided and the population exceeds 75,000 to 100,000.
Following preliminary treatment, a conventional design will normally provide for an aeration period of 4 to 8 hours, settling of the aerated mixture, and return of the settled activated sludge to the aeration tank inlet. When compressed-air aeration is used, the air required is in the range of 500 to 700 cu ft per lb of BOD removed. The average BOD load on the aeration thank is not in excess of 50 lb per 100 lb of suspended solids in the aeration system. When these criteria are followed, the effluent will be clear and highly nitrified, with a BOD content less than 30 and often as little as 10 to 15 mg/l.
MODIFICATIONS IN THE PROCESS
The effluent from a conventionally designed activated sludge plant is well-nitrified and low in 5-day BOD. However, a considerable reduction in costs of treatment can be achieved if the process is not carried to the nitrification stage, though the same degree of removal of 5-ay BOD and suspended solids is obtained. In general, the modified methods employ a shortened period of aeration with a reduced quantity of suspended solids in the mixed liquor.
Methods employed or being tested range from treating only a portion of the sewage along conventional lines and blending this with effluent from a primary treatment plant to the utilization of oxygen gas.
Step aeration provides for adding return sludge or settled sewage at various points as the sewage passes through the aeration tank. This process permits control of the rates and points of application of the sewage or return sludge to provide maximum efficiency of treatment. Less aerator volume is required, but BOD loadings and removal are quite high.
Tapered aeration is based on the fact that, as the mixed liquor progresses through the aeration tanks, less air is required. Therefore, air is applied at a higher rate near the influent end of the tank. This procedure is relatively normal in most well-designed plant and is not strictly a modification of the basic process.
Activated aeration utilizes a pair of conventional activated sludge plants operating in parallel. Excess activated sludge form the final settling tanks on one units supplies both aeration tanks, and the sludge from the second unit is pumped to final disposal. This arrangement reduces construction costs, and probably operating costs, but does not modify operating results.
The Biosorption process, represented in the diagram below, involves preaeration of the sewage with activated sludge in a special mixing chamber for about 30 minutes, following which the mixture of sewage and sludge is passed to the aeration tank where it remains for about 2.5 hours. The total aeration time is considerably reduced, and plant capacity is thereby increased.
The bioprecipitation process utilizes oxygen gas. The wastes are pretreated in a countercurrent oxygenation unit, and are then passed upward through a blanket of biological active floc. This process is shown diagramatically in the figure below.
The high-rate modified processes operate at considerably higher loading in the aeration tank than do the conventional plants. The BOD is usually in excess of 100 lb per day per 100 lb of sludge solids, and is often much higher.
THE SLUDGE VOLUME INDEX
Since the weight of the mixed solids in the aeration tank at activated sludge plant is an important factor in its design and equally important in its operation, a simple and usable test is desirable. The test for the sludge volume index is quite universally used. This test indicates the percentage of suspended matter in the mixed liquor, by volume, within acceptable limits of accuracy.
It is obtained by allowing 1 liter of the mixed liquor to stand quiescent for 30 minutes and measuring the volume of settle sludge. The dry weight concentration of the suspended solids in the mixed liquor is then determined.
The ratio of the second result to the first is the sludge volume index. For instance if the percentage of sludge resulting from the settling test is 20 and the dry weight concentration of suspended solids is 0.25 percent, the sludge volume index is:
20 / 0.25 = 80
In a well-operating plant, usual values are 50 to 150 for a diffused-air installation and between 200 and 300 for mechanical aeration. The sludge volume index is useful in operation as an indication of the volume of sludge to be returned in order to maintain the desired percentage of suspended solids in the mixed liquor. The amount of solids carried in the aeration tank is 1500 to 3000 mg/l in most plants of the diffused air type, and normally ranges from 300 to 900 mg/l in mechanical type plants.
The period of aeration, the volume of sewage flow, and the probable volume of return sludge are of importance in the design of an activated sludge plant, since they determine the size of the aeration tank. However, there are many variables in operation which have to do with the volume of flow, the strength of the sewage, and the most desirable mixed-liquor concentration in the aeration tank.
To maintain optimum operating conditions, it may be necessary to vary the solids proportion in the aeration tank, and hence to vary the volume of return sludge. Provision is therefore made in design for return-sludge flows of 150 percent of the design requirements.
The return sludge is drawn from the final settling tank. Since prolonged storage permits the sludge to become stale, it should be drawn promptly. It may be either returned to the aeration-tank inlet, with a step of aeration if necessary, or pumped to disposal.
EMPIRICAL PRACTICES IN DESIGN
Activated sludge plant serving more than 5000 to 10,000 population usually utilize compressed air for aeration and agitation, though larger plants utilize combinations of air and mechanical equipment. Design of plants may be based on empirical practices, such as are laid down by the various regulatory agencies, or on rational calculations, which consider the BOD loading and the weight of suspended solids in the mixed liquor.
General requirements for design have been prepared before. A general summary of these requirements indicates the sued of the following empirical practices as bases for design:
â€¢ Removal of 90 percent of the applied BOD;
â€¢ Provision of two or more aeration units equipped for independent operation;
â€¢ Depth not less than 10 ft and usually not greater than 15 ft;
â€¢ Provision to maintain at least 2 mg/l of air in all parts of the tanks except at inlets;
â€¢ Delivery., for diffused air tanks, of 1000 cu ft of air per pound of 5-day BOD;
â€¢ A capacity of 150 percent overload; and
â€¢ Retention in aeration tanks as follows: 7 hrs. for capacities between 0.2 and 0.8 mgd. 6 to 7.5 hours for capacities between 0.8 to 1 mgd and 6 hours for capacities beyond 1 mgd.
Another basis of design sometimes used is BOD loading per unit volume of aeration tank. Usual practice is to provide for 30 to 50 lb of 5-day BOD per 1000 cu ft of aeration-tank volume.
The efficiency of the activated sludge process as measured by the removal of BOD is directly related to the weight of the activated sludge solids, either total or volatile, carried in the aeration tank and is inversely related to the BOD load applied to the tanks.
With an average daily BOD load (from municipal sewage) not exceeding 50 percent by weight of the suspended solids being aerated, a 90 percent reduction of the applied BOD is likely in a plant of conventional design. With the heavier loading characteristic of high-rate modifications, ranging from 1 to 4 lbs of BOD per pound of solids in the aeration tanks, BOD reduction is less.
Plant operating experience has demonstrated a definite relationship between the sludge volume index, the minimum percentage of return sludge, and the solids concentration in the mixed liquor. This relationship is as follows:
Percentage of return sludge = 100 / [100/ip) - 1]
Where i = sludge volume index;
p = percentage of solid in the mixed liquor.
For plants employing compressed air, a sludge volume index of 80 and a solids concentration in the mixed liquor of 2500 mg/l indicates a return-sludge volume of about 25 percent; but with a sludge volume index of 100, the return-sludge volume will be 33 percent.
Since in most plants the sludge volume index will change quite considerably from time to time varying from a probable low of 50 to a high of perhaps 250, the volume of return sludge must be modified in order to maintain a desirable solids concentration in the mixed liquor. With a sludge volume index of 150 and a solids concentration of 2000 mg/l, the volume of return sludge must be increased to more than 40 percent.
The solids content of the mixed liquor is essentially that due to the returned solids, since the solids contribution of the settled sewage is normally too small to have any effect.
Aeration tank volume can be readily approximated for design purposes if the mixed-liquor solids concentration, the BOD loading, and the ration of BOD loading to solids are established or assumed. For instance, with mixed-liquor solids assumed at 2000 mg/l, a loading of 35 lb of BOD per day per 100 lb of aeration-tank solids, and an over-all BOD loading of 2380 lb (from a population of 20,000 with primary treatment removing 30 percent), the required volume of the aeration tanks, in millions of gallons, will be:
100 x 2380 / (35 x 2000 x 8.3) = 0.41
This is equivalent to 54,700 cubic feet.
DESIGN OF AERATION TANKS
As indicated, there three quantities on which the design of an aeration tank can be based. These are detention period, BOD loading, and solid under aeration. For purposes of illustration, it will be assumed that the sewage flow is 1 mgd for a population of 10,000, the removal of BOD in the primary settling tank will be 30 percent, and the flow of return sludge will be 25 percent.
For a design based on the detention period, a 6-hr detention wil be provided for a flow of 1 mgd plus 25 percent for return sludge, or a total flow of 1.25 mgd. The capacity required is:
6 x 1.25 / 24 = 0.3125 million gallons or 41,700 cu. Ft.
Assuming a tank 15 ft deep and 20 ft wide, the length required is 139 ft.
For a design based on the BOD loading per unit of volume, provision will be made for 30 lb of BOD per 1000 cu ft of tank capacity. Since the BOD load to the aerators is:
10,000 x 0.17 x 0.70 = 1190 lb per day,
the capacity required is :
1190 x 1000 / 30 = 39,700 cu ft.
if the 15 by 20 ft cross section is used, the length will be 133 ft.
When the design is based on solids under aeration, provision will be made for 35 lb of BOD PER 100 LB of solids in the mixed liquor and for a solids concentration of 2000 mg/l in the liquor. The BOD load to the aerators will be 1190 lb per day, as before. If and additional 25 percent is allowed for the return sludge, the total loading will be:
1190 x 1.25 = 1488 lb per day.
The corresponding amount of solids will be 1488 x 100 / 35 = 4230 lb, and the amount of solids in 1 million gal of the mixed liquor will be:
2000 x 8.34 = 16,680 lb.
For 4,230 lb of solids, the volume of the aeration tank must be:
4230 / 16,680 = 0.254 million gal, or 33,900 cu ft.
If the 15 20 ft cross section is used, the required length is 113 ft.
The computation using the solids under aeration is preferable if all factors are known accurately. If the assumed values represent actual values, the results do not depend on sewage or return-sludge flows.
DIMENSIONS AND NUMBER OF TANKS
Local condition may influence the design of aeration tanks, such as the depth. However, shallower tanks, while reducing the air pressure required, do not provide as much contact time between the air bubbles and the sewage as do deeper tanks. In practice, depths range from 10 to 15 ft; and the bottoms are flat or nearly so. Widths are 1.5 to 2 times the depth, this ratio being based on construction economy and on experience which indicates that velocities adequate to prevent settling of solids can be maintained in channels of such dimensions.
At least two units should be provided, and more are desirable. Except in plants of less than about 5-mgd capacity, or unless local conditions clearly indicate otherwise, four aeration tanks should be provided.
Spiral flow tanks, are less costly to build that the ridge-and-furrow type formerly used. Deposition of solids on the bottom is prevented by maintaining a transverse velocity across the bottom of about 1.5 fps. By rising the diffusers somewhat above the bottom of the tanks, deposits of suspended matter on the diffuser during tank drawdown or dewatering is reduced.
Two methods for aerating and agitating the mixture of sewage and activated sludge are in general use. One involves diffused air, usually in connection with mechanical devices for agitation and dispersion of the air. The other method uses induced or aspirated atmospheric air and surface aeration. In the diffused-air process, most of the oxygen required comes from the small bubbles of air which are diffused under pressure at the bottom of the aeration tank.
In mechanical aeration, atmospheric air is carried into sewage by draft tubes, propellers, or other devices. The object of aeration is to supply the needed oxygen and the more efficient the oxygen transfer, the greater the treatment capacity. In some cases both aeration methods are used.
When compressed air is employed, probably not more than 10 percent of the oxygen in it is used. In mechanical aeration, the transfer from air to droplets is efficient, but the proportion of droplets to the entire tank area is small and the oxygen transfer per unit of time is les than in diffused-air aeration.
With a combination of mechanical dispersion and diffused air, the transfer is accelerated and treatment efficiency is increased. Since cost also increases, however, a limit is placed on the efficiency that can be obtained.
There are numerous criteria for the amount of diffused air required for treatment. Two that are commonly used are air per gallon of sewage and air per pound of BOD removed. A long-established basis, which does not take into account the strength of the sewage, is the provision of 1 to 1.5 cubic feet of free air per gallon of sewage treated. For a flow of 1 mgd, the amount of air required will be 700 to 1,050 cfm.
According to another basis, 500 to 700 cubic feet of free air will be required per pound of BOD removed. In practice, 150 percent to 200 percent of the capacity required on this basis is usually installed, exclusive of the air required for air lifts or for reaerating the sludge.
Freshness of the sewage affects the air requirements, since stale sewage utilizes the oxygen in the air more slowly or less efficiently than does fresh sewage. Also, stale sludge has high oxygen demand, and unduly long detention in the secondary settling tanks is therefore undesirable.
Diffused air is normally applied at or near the bottom of the tank and is delivered under a pressure of 8 to 10 psi. In smaller plants, rotary blowers of the positive-displacement dry type are mainly used. In large plants, centrifugal or turbo blowers are preferred. The air supplied to a blower should be filtered to limit dust and oils, which tend to clog the diffusers. The dust content of air, as supplied, should not exceed 0.5 mg per 1,000 cubic feet of free air and less is desirable. Some manufacturers recommend no more than 0.1 mg per 1,000 cubic feet.
The rotary positive blowers have efficiencies in the range of 70 to 80 percent of the power input and capacities to about 50,000 cubic feet per minute. Centrifugal blowers range up to 125,000 cfm. About 20 to 25 hp will be required per million gallon of sewage having an average BOD content.
Many kinds of materials are used for diffusers making, but those from stainless steel have been used in the majority of the plants recently. Flexible bags and tubes which are self-cleaning when the air pressure is release are more convenient.
Standard plates are 12 inches square and 1 inch thick. The average effective area of such a plate is 0.84 square feet. Standard tubes are 24 inches long, have an internal diameter of 3 inches, and a wall thickness of five eights of an inch.
Plates and tubes clog on the air side from suspended matter in the air, and clog on the water side from sediment and aquatic growth.
As an example, assume that a plant is to treat 2 mgd of sewage having a BOD content of 200 mg/l, which is reduced to 130 mg/l by primary treatment, and the applied BOD is 2,166 pounds per day. The air requirements based on 500 to 700 cubic feet of air per pound of BOD will range from 1,80,000 to 1,500,000 cubic feet per day. If we have the higher figure and apply a factor of 150 percent for peak loadings and similar demands, the total required air supply will be 2,250,000 cubic feet per day. This assumes a BOD loading of not less than about 30 pounds of BOD per 100 pounds of solids in the aeration tank., since at lower ratios of BOD lodadings to solids air requirements increase quite rapidly.
To the basic requirement of 2.25 million cubic feet of air per day, which is:
2,250,000 / (24 * 60) = 1,560 cfm
should be added the amount necessary for such other uses as air lifts and sludge reaeration.
An average rate of applying diffused air is 2.5 cfm per square foot of diffuser area. Thus, the diffuser area required to apply 1,560 cfm of air will be:
1,560 / 2.5 = 624 square feet.
If plates are used and the average effective area of each plate is assumed to be 0.84 square feet, the number required will be:
624/0.84 = 743 square feet.
Diffusers are classified by their ability to pass air under specified standard conditions. For purposes of establishing a standard, permeability is defined as the volume of air in cubic feet at 70 oF and 10 to 25 percent relative humidity, which will pass through an area of 1 square foot of dry porous plat in 1 minute under an equivalent pressure of 2 inches of water.
Plates having a permeability rating of 40 to 60 are generally used. The coarser plats clog less rapidly and require somewhat less power for air compression, but deliver less efficient bubbles.
When mechanical aeration is employed, round, square, rectangular, and exagonal tanks are used. Tanks are 8 to 18 feet deep and the diameters or other horizontal dimensions range from 14 to 30 feet. Tanks may have hopper-shaped or flat bottoms, depending on the design of the aerator equipment.
Because oxygenation capacity in a mechanical aerator is generally lower than that obtained by diffused air, the solids concentration in the mixed liquor in the aeration tank is also lower, a longer aeration period and a larger aeration tank are required.
Design usually provides for a retention period of 8 hours for the raw sewage plus the returned sludge. However, various devices and methods have been developed to shorten this period. Most health department require retention periods of 8 hours, but will consider plants utilizing high-rate methods.
Based on an 8-hr retention period and 30 percent of return sludge, the tank capacity for an average flow of 1 mgd is:
[1,000,000 + (0.30 * 1,000,000)] * 8 / (7.48 * 24) = 57,900 cubic feet.
Types of Mechanical Aerators
The mechanical aerator is intended to produce surface aeration of the sewage by agitation and, at the same time, produce a circular or helical motion, for prevention of solids settlement and to attain maximum aeration. This motion has been provided in the past by rotating paddles or by a type of propeller. More recent developments have combined the use of air and mechanical equipment.
A high-speed turbine mixer has been combined with an air sparger to produce the fine bubbles needed for efficient oxygen transfer. In another method, air is drawn down through a rotating hollow shaft. These and other developments are aimed to higher oxygenation rates, which will permit higher solid concentration and a higher floc volume in a given tank volume. In addition, these developments can facilitate design for any needed degree of treatment, partial or complete.
Sludge age is defined as the number of days found by dividing the weight of the dry suspended solid in the aeration tanks by the weight of the dry suspended solids in the primary-tank effluent added daily. It is also suggested that this age be determined by the formula:
Sludge age, in days = VA / QC
Where. V = aerator volume, in million gallons;
A = average concentration of suspended solids in the aerator, in
milligrams per liter;
Q = sewage flow, in million gallons per day;
C = suspended solids in primary tank effluent; in milligrams per liter.