In this essay we will discuss about:- 1. Meaning of Activated Sludge 2. The Activated Sludge Process 3. Variables 4. Capacity of Aeration Tank and Detention Period 5. Return Activated Sludge 6. Advantages and Disadvantages 7. Operation of Activated Sludge Plant.
Essay # 1. Meaning of Activated Sludge:
When sewage is passed or trickles through a grit bed, the sewage is purified. This purification is due to the actions of the aerobic bacteria which remain in the organisms film. The main disadvantage of such filters is that they cannot treat the concentrated sewage continuously and get clogged. Due to this difficulty long ago experiments were carried out for many years by blowing air in the sewage and it was discovered that the floe having living organism is formed.
When the so-formed floe was examined by a microscope, it showed the presence of variety of freely moving bacteria, protozoa, moulds and yeasts which were similar to the organism film formed in sewage filters. When the circulation of air was stopped, the floe settled. The floe if added in another fresh sewage caused its digestion. This floe is called the activated sludge. It was named so, because it is very active and can treat the fresh sewage.
The term activities sludge is used to indicate the sludge which is obtained by settling sewage in presence of abundant oxygen. The activated sludge is biologically active and it contains a great no. of aerobic bacteria and other m.o. which have got an unusual property to oxidise the organic matter.
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The activated sludge process is an aerobic, biological sewage treatment system. The essential units of the process are an aeration tank, a secondary settling tank, a sludge return line from the secondary settling tank to the aeration tank and an excess sludge waste line.
Essay # 2. The Activated Sludge Process:
In the early stages, the activated sludge process was carried out on ‘Fill and Draw, basis, but later on when it became popular, it became continuous process.
The following process is adopted in the current practice:
(i) The raw sewage is given the primary treatment in the primary settling tank. The detention period is kept short 1- 1.5 hours. The primary settling tank removes less percentage of settleable solids. Due to less removal of solids (suspended), the filter media does not clog.
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(ii) After primary treatment, the raw sewage is mixed up with the required quantity of activated sludge which is called ‘Return sludge’ and sent in the aeration tank. The mixer is known as ‘mixed liquor’.
(iii) In the aeration units the mixed liquor is aerated and simultaneously agitated for 4- 10 hours depending on the degree of purification desired and the strength of the sewage.
(iv) The aerated mixed liquor is sent in final settling tank, where the sludge settles.
(v) The effluent from the final settling tank which is cleans disposed off. A portion of the effluent is mixed in the raw sewage before sending it in the primary settling tank.
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(vi) A part of settled sludge is sent back in the aeration tank for seeding the raw sewage and the excessive quantity is treated and disposed off in a suitable manner.
Fig. 17.1 illustrates the general outlines of the activated sludge process diagrammatically.
Primary Treatment:
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The main object of primary treatment is to remove suspended matters, oils, sand, grit, floating matter etc. from the sewage. In the primary treatment, the sewage is passed through screens, grit chamber, detritus tank and sedimentation tanks.
Returned Sludge:
It is the activated sludge obtained from the final settling tank for mixing with the effluence of the primary treatment before going in the aeration units, to prepare a mixed liquor. Generally for preparing the mixed liquor, the activated sludge is injected in the influent of aeration tank by a volumetric ratio.
The quantity of returned sludge varies from 20% to 35% of the presettled sewage by volume. The activeness of the returned sludge can be judged by its golden brown colour and denseness. At some treatment plants the return sludge is reaerated or reactivated before mixing with influent.
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It has been seen that the efficiency of the activated sludge process is increased if a part of the activated sludge is mixed up in the sewage before its primary sedimentation.
Sludge Index. Previously the quantity of return sludge was determined on volumetric basis. But it is better to do it by ‘Sludge Index’. The sludge-index is the ratio of the volume of the activated sludge. In milli-litres for one gm of dry weight of the sludge. The value of this index varies from 150 to 300.
Oxidation Ditch:
It is one form of an extended aeration system having certain special features like an endless ditch for the aeration tank and a rotor for the aeration mechanism. The oxidation ditch consists of a long continuous channel usually oval in plan as given in Fig. 17.15. The channel may be earthen with lined sloping sides and lined floor or it may be built in concrete or brick with vertical walls.
The aeration of the sewage is done by a surface rotor placed across the channel. In addition to the aeration, the rotor also imparts a velocity of 0.3-0.4 m.p.s. to the mixed liquor preventing the biological sludge from settling out. Cage type aeration rotors are usually provided, but angle iron type rotors are also used. The dia. of cage type rotor is about 70 cm and speed is 75 rpm.
They have standard oxygen transfer capacity of 2.8 kg. O2/m length per hour at 16 cm depth of immersion. The rotor impart adequate circulation for 120-150 m3 of ditch volume per metre length of rotor. The ditch volume is designed based on the criteria given for extended aeration system in Table 17.1.
The design of the width is done to accommodate the length of aerator required to meet the oxygen demand either as a single rotor or as multiple rotors. The oxidation ditch has 1.0 to 1.2 m depth and the length is provided to give the required aeration tank volume.
The return sludge and raw sewage are discharged into the ditch upstream of the rotors. The outlet of the ditch is located geometrically opposite to the inlet. Baffles are provided in the outlet weir so that the ditch water level does not rise excessively and overload the rotors at peak flow period.
The operation of the oxidation ditches may be intermittent or continuous. In intermittent operation, the oxidation ditch functions also as a settling tank.
Intermittent operation cycles are:
(a) Closing inlet and aerating the sewage,
(b) Stopping the rotor and letting the contents settle, and
(c) Letting in fresh sewage which displaces an equal quantity of clarified effluent.
Removal of the sludge is done from the mixed liquor or a sludge sump at the bottom of the ditch. Only in small plants, intermittent operation is adopted. In case of continuous operation the oxidation ditch is followed by a secondary settling unit as in case of any other extended aeration system.
Nitrification:
Activated sludge process plants are usually designed for the removal of only carbonaceous B.O.D. However, there may be incidental nitrification in the process, which will consume part of the oxygen supplied to the system and reduce the level in the aeration tank.
Nitrification also leads to subsequent denitrification in the secondary settling tank causing a rising sludge problem known as blanket rising. Nitrification is aided by low F/M and long aeration time. In hot weather, it may be pronounced in extended aeration. In the case of modified aeration unit, here is little or no nitrification.
Nitrification though generally not desired may be required in spacial cases, e.g. when ammonia has to be eliminated from the effluent in the interest of pisciculture or when nitrification-cum-denitrification is proposed for elimination of nitrogeneous matter from the effluent for the control of the entrophication.
Under such cases, the trend is towards the design of two stage plants with only carbonaceous B.O.D. removal in the first stage and nitrification in the second stage. The design of the first unit is done with high F/M to prevent nitrification. The two stage process eliminates the problem of denitrification and rising sludge in the first stage settling tank.
Essay # 3. Variables of Activated Sludge Process:
The main variables of the activated sludge process are the loading rate, the mixing regime and the flow scheme.
(i) Loading Rate:
This is expressed as the rate at which sewage is applied in the aeration tank. A loading parameter which has been developed empirically over the years, is the hydraulic retention time (HRT) and is expressed as
Where, V = Volume of aeration tank in n?
Q = Sewage inflow excluding sludge recycle in mLd
Another common equation is,
Volumetric loading
Where, La = influent BODs to aeration tank, mg/l.
The organic loading rate is defined as the ratio of kg BOD$ applied per day (representing microbial feed) to kg. MLSS in aeration tank (representing micro-organism) or
Where Xt = MLSS mg/l. The F/M ratio is the main factor controlling BOD removal. Lower the F/M value, higher will be the BOD removal in the plant. The F/M can be varied by varying MLSS concentration in the aeration tank.
For checking the design of activated sludge system, third parameter known as Solid Retention Time (SRT) also known as Mean Cell Residence Time (MCRT) or the sludge age is used, which is given by
(ii) Mixing Regime:
The mixing regime employed in the aeration tank may be plug flow or completely mixed flow. Plug flow method implies that the sewage moves down progressively along the aeration tank, essentially unmixed with the rest of the tank contents. In the complete mix flow method, it involves the rapid dispersal of the incoming sewage throughout the tank.
In the plug flow method, the F/M and the oxygen demand will be highest at the inlet end of the aeration tank and will then progressively decrease. In the complete mix system, the F/M and the oxygen demand will be uniform throughout the tank.
(iii) Flow Scheme:
The flow scheme of the activated sludge process involves the pattern of sewage addition and sludge return to the aeration tank and also the pattern of aeration. Sewage addition may be at a single point at the inlet end of the tank or it may be at several points along the aeration tank.
The sludge return may be directly from the settling tank to the aeration tank or through a sludge reaeration tank. The compressed air may be applied uniformly along the whole length of the tank or it may be tapered from the head of the aeration tank to its end.
Essay # 4. Capacity of Aeration Tank and Detention Period:
The size or capacity of the aeration unit to be provided is generally determined by the detention period and the quantity of the sewage to be treated. The detention period depends on the aeration effect or B.O.D. to be removed.
The design of detention period is determined by the following two empirical formulae:
(a) American Public Health Association Formula:
Where B.O.D. of the aeration tank sewage influent in mg/litre to be removed
(b) M/s Ames Crosta Mills and Co. Ltd. (England) have recommended the following formula:
= Aeration period in hours.
Practically it has been noted that 50% of the B.O.D. is removed in the first one hour, 25% in the next two hours and the remaining 25% in the last three hours in the 6 hours detention period tank. Therefore, it has been observed that longer detention period increases the cost of aeration units and operational cost too high, whereas the efficiency is not so high. The design of aeration units is therefore done keeping in view all the facts.
The usual practice is to keep the detention period between 6 to 8 hours. The volume of the aeration tank is also decided by considering the return sludge which is about 10% to 25% of the sewage volume. The capacity of aeration tank is usually kept 18.5 m3/kg of B.O.D. load.
Essay # 5. Return Activated Sludge:
The activated sludge is in the shape of a floe, which is similar to the gelatinous film formed on the trickling filter media. The aerobic bacteria are responsible for the formation of this floe. The activated sludge mainly consists of a Zoological matrix having protozoa and other living organism which eat the organic matters of the sewage. These assimilate the colloidal and dissolved organic material and convert them into cell substance and matrix, which easily settle down in the settling tanks.
The quantity of activated sludge required for seeding the sewage is very small-about 5% of the sewage quantity. The efficiency of the process is slow in the beginning. It requires 1 to 3 weeks for building up the necessary floe. After this time the plant attains the required efficiency.
Quantity of Returns Sludge:
The quantity of return sludge to be returned to the aeration units mainly depends on the quality and quantity of the raw sewage coming in the aeration unit, as well as the degree of treatment to the primary effluent.
The quantity of the return sludge is so adjusted for the better efficiency that certain concentration of suspended solids remain in the aeration units. Usually solid concentration of about 1000 mg/litre is kept in the diffused air aeration units and forms 1000 to 2500 mg/litre in the mechanical aeration units.
Mathematically, if the solid contents in the primary effluent be SP and the solid contents in the return sludge be SR the ratio of the volume of return sludge to the volume of the primary effluent (sewage) for obtaining a solid contents of Sa.
The values of SP SR and Sa are in mg/lilre. The above formula (17.1) is of great practical use in design for determining the volume of the return sludge. The value of this ratio is usually kept between 0.2 to 0.5 i.e., 20% to 50%. The design of the aeration units, return sludge pumps, channels, secondary settling tanks etc. can be easily determined after knowing the quantity of the sewage and the quantity of the return sewage.
Design Loading Rates:
The working of the aeration tank is more or less similar to the trickling filters. The sludge solids remaining in suspension act like the contact media of the trickling filters and remove the B.O.D. of the sewage. The rate of the aeration tanks or units loadings are expressed in terms of B. O. D. applied per unit weight (or surface area) of the suspended solids.
Practically, it has been noted that good results are obtained if the weight of the suspended solids is kept between 2 to 3 times the daily B.O.D. load of the incoming sewage. When the load is increased the efficiency of B.O.D. removal drops down.
Fig. 17.8 shows practical results plotted on the graph, how the B.O.D. removal drops with the increase in B.O.D. load.
Quality of Return Sludge:
The quality of the return sewage mainly depends on the presence of biological life in it. The quality of the sewage will be better or healthy when it will contain ciliated protozoa and gelatinous masses of bacteria in large numbers.
On the other hand if the return sludge contain preponderance of other protozoa and filamentous organisms, it will show poor quality of sludge. The purifying power of fresh sludge is much more than old or stale sludge. For better results, it is most necessary to go on changing the old sludge.
The above ratio or the sludge age should not be more than 10 to 12 (i.e., 3 to 3.5 days old).
In certain units the return sludge is aerated before mixing it with the sewage on its way to the aeration units in the channels. Sometimes holding tanks are used for aeration of the return sludge.
Sludge Volume Index:
It is the ratio of the volume of the activated sludge in cu.cm for one gm of dry weight of the sludge. It is also defined as the volume in millilitres occupied by 1 gm of sludge (dry wt.) after 30 minutes of settling. The sludge volume index of good sludge is between 50 to 100 ml/ gm.
The sludge volume index of bulked sludge is about 200 ml/gm or more. The higher value of sludge index indicates a light and fluffy sludge which takes more time in settling, which also indicates the low efficiency of the process.
Essay # 6. Advantages and Disadvantages of Activated Sludge Process:
Following are the main advantages of the activated sludge process of treatment:
(i) The initial cost of this process is lower than the trickling filters units.
(ii) Smaller area of land is required and design can be made compact.
(iii) The effluent is clear, sparkling liquid free from putrescibling and odour.
(iv) The total process is free from offensive objectional smells, and therefore there is no fly nuisance.
(v) The efficiency of the process is very high, it can remove suspended solids B.O.D. and bacteria more than 90%.
(vi) The quality or nature of the effluent can be varied depending upon the quality and the quantity of the natural water courses available for final disposal.
(vii) The excessive sludge has very high fertility value than the sludge obtained from other sewage treatment units.
(viii) The water head required for the operation is smaller than all other treatment processes, such as trickling filters.
Following are the disadvantages of the activated sludge process of treatment:
(i) The operational cost is higher than that of trickling filter.
(ii) It requires skilled workmen for its constant operation and maintenance.
(iii) It has sensitivity to change in the quality of the influent.
(iv) Large quantity of sludge is produced, which causes difficulty in disposal.
(v) If the process goes out of order it gives much trouble and takes time in setting it in right operation.
(vi) There is uncertainty of results under all conditions.
Essay # 7. Operation of Activated Sludge Plant:
During the operation of an activated sludge plant, F/M should be maintained properly, which is achieved by increasing or decreasing the MLSS levels in the aeration tank to suit the influent BOD5 loads. The MLSS in the aeration tank can be regulated by controlling the rate of return sludge based on SVI determined experimentally. Wastage of excessive sludge is controlled on the basis of experience.
In small plants, sludge is often wasted only intermittently when the sludge fills up the settling tank and starts showing up in the effluent. The quick settle ability of sludge is an important factor for the efficient performance of the plant. The SVI serves also as an index of sludge settle ability. With MLSS of 800-3500 mg/l for satisfactory operation, the SVI value should be kept between 80-150. Poor settling characteristics sludge is known as bulking sludge.
Sludge bulking is usually due to inadequate air supply, low pH-value or septicity and also due to growth of filamentous organisms consequent to the presence of industrial wastes containing high concentration of carbohydrates in sewage. Sludge bulking is controlled by eliminating the causes and application of chlorine to the sewage or to return sludge. The dose of the chlorine is kept 0.2 to 1.0% of dry solids weight in return sludge.
Sometimes the secondary settling tank may function poorly even when the sludge volume index is satisfactory and sludge may rise up in the tank and escape with the effluent. Rising sludge may be due to denitrification in the settling tank releasing nitrogen bubbles which buoys up the sludge. This problem can be overcome by increasing the return sludge rate increasing the speed of the sludge scraper mechanism and increasing the sludge wasting rate.
Example 1:
Following data are given:
(i) Flow of raw sewage = 5450 m3/day (I mg.d)
(ii) Suspended solids in the raw sewage = 550 ppm.
(iii) B. O. D. of the raw sewage = 300 ppm.
(iv) The primary treatment remove 55% of the suspended solids and 33/% of the B.O.D.
Design the aeration unit for the activated sludge process treatment for removing 90% of the B. O. D. and the suspended solids each for the proper disposal of effluent of treated sewage.
Solution:
Example 2:
Calculate the sludge volume index for a mixed liquor with 2700 mg/litre, suspended solids having settled volume of 200 ml with 1 litre sample. Indicate whether this volume index is good or poor?
Solution:
Sludge volume index
... 2700 mg settling gives 200ml volume gm
... 1 gm settling give = 200/2.7 ml = 74ml
... S.V.L = 74
This is good sludge volume index. Ans.
Example 3:
Design the activated sludge unit treatment with the following data for a town of population of 65,000.
(i) Average sewage flow = 210 litres/C/day
(ii) B.O.D. of the raw sewage = 210 mg/litre
(iii) Suspended solids in raw sewage = 300 mg/litre
(iv) B.O.D. removal in primary treatment = 40%
(v) Overall B.O.D. removal desired = 90%.
Solution:
