Conventional Activated sludge process for a beginner

 

Conventional Activated sludge process

·         Conventional biological treatment of wastewater under aerobic conditions includes activated sludge process (ASP) and Trickling Filter. The ASP was developed in England in 1914. The activated sludge process consists of an aeration tank, where organic matter is stabilized by the action of bacteria under aeration and a secondary sedimentation tank (SST), where the biological cell mass is separated from the effluent of aeration tank and the settle sludge is recycled partly to the aeration tank and remaining is wasted (Figure 19.1). Recycling is necessary for activated sludge process.

·         The aeration conditions are achieved by the use of diffused or mechanical  aeration.

 

Loading Rate: The organic matter loading rate applied to the reactor is quantified as kg of BOD

applied per unit volume of the reactor per day, called as volumetric loading rate, or kg of BOD

applied per day per unit mass of microorganisms present in the reactor (i.e. in the aeration tank),

called as organic loading rate or F/M. This can be calculated as stated below:

 

                                          Vol. = Volume of aeration Tank, m3

 

 

 

 Quantity of Return Sludge: Usually solid concentration of about 1500 to 3000 mg/L (MLVSS

80% of MLSS) is maintained for conventional ASP and 3000 to 6000 mg/L for completely

mixed ASP. Accordingly the quantity of return sludge is determined to maintain this

concentration. The sludge return ratio is usually 20 to 50%. The F/M ratio is kept as 0.2 to 0.4

for conventional ASP and 0.2 to 0.6 for completely mixed ASP.

 

Aeration units can be classified as:

1) Diffused Air Units

2) Mechanical Aeration Units

       3) Combined Mechanical and diffused air units.

 

1. Diffused air aeration

 

·        In diffused air aeration, compressed air is blown through diffusers.

·        The tanks of these units are generally in the form of narrow rectangular channels. The air diffusers are provided at the bottom of tank.

·        The air before passing through diffusers must be passed through air filter to remove dirt. The required pressure is maintained by means of air compressors.

Air Supply:

·         Normally air is supplied under pressure of 0.55 to 0.7 kg/cm2.

·         The quantity of air supplied varies from 1.25 to 9.50 m3/m3 of sewage depending on the strength of the sewage to be treated and degree of treatment desired. The oxygen transfer capacity of the aerators depends on the size of air bubbles, for fine bubble oxygen transfer capabilities of aeration device is 0.7 to 1.4 kg O2/KW.h. For medium bubble it is 0.6 to 1.0 kg O2/KW.h, and for coarse bubble it is 0.3 to 0.9 kg O2/KW.h.

 

Mechanical Aeration Unit

 

·         The main objective of mechanical aeration is to bring every time new surface of wastewater in contact with air. In diffuse aeration only 5 to 12% of the total quantity of the air compressed is utilized for oxidation and rest of the air is provided for mixing. Hence, mechanical aeration was developed.

·         For this surface aerators either fixed or floating type can be used (Figure 19.3).

·         The rectangular aeration tanks are divided into square tank and each square section is provided with one mixer. The impeller are so adjusted that when electric motors starts, they suck the sewage from the centre, with or without tube support, and throw it in the form of a thin spray over the surface of the wastewater.

 

Types of Activated Sludge Process

1. Conventional aeration

Ø  In conventional ASP the flow model in aeration tank is plug flow type. Both the influent wastewater and recycled sludge enter at the head of the tank and are aerated for about 5 to 6hours for sewage treatment (Figure 19.4).

Ø The influent and recycled sludge are mixed by the action of the diffusers or mechanical aerators. Rate of aeration is constant throughout the length of the tank.

Ø During the aeration period the adsorption, flocculation and oxidation of organic matter takes place. The F/M ratio of 0.2 to 0.4 kg BOD/kg VSS.d and volumetric loading rate of 0.3 to 0.6 kg BOD/m3.d is used for designing this type of ASP.

Ø  Lower mixed liquor suspended solids (MLSS) concentration is maintained in the aeration tank of the order of 1500 to 3000 mg/L and mean cell residence time of 5 to 15 days is maintained. The hydraulic retention time (HRT) of 4 to 8 h is required for sewage treatment. Higher HRT may be required for treatment of industrial wastewater having higher BOD concentration. The sludge recirculation ratio is generally in the range of 0.25 to 0.5.

2. Tapered Aeration

Ø  In plug flow type aeration tank BOD load is maximum at the inlet and it reduces as wastewatermoves towards the effluent end. Hence, accordingly in tapered aeration maximum air is appliedat the beginning and it is reduced in steps towards end, hence it is called as tapered aeration (Figure 19.5).

Ø  By tapered aeration the efficiency of the aeration unit will be increased and it will

           also result in overall economy. The F/M ratio and volumetric loading rate of 0.2 to 0.4              kg BOD/kg VSS.d and 0.3 to 0.6 kg BOD/m3.d, respectively, are adopted in design.

Ø  Other design recommendation are mean cell residence time of 5 to 15 days, MLSS of 1500 to 3000 mg/L, HRT of 4 to 8 h and sludge recirculation ratio of 0.25 to 0.5.

Ø  Although, the design loading rates are similar to conventional ASP, tapered aeration gives better performance.

19.3Tapered aeration activated sludge process

 

 

 

3. Step aeration

 

Ø  If the sewage is added at more than one point along the aeration channel, the process is called as step aeration (Figure 19.6). This will reduce the load on returned sludge. The aeration is uniform throughout the tank.

Ø   The F/M ratio and volumetric loading rate of 0.2 to 0.4 kg BOD/kg VSS.d and 0.6 to 1.0 kg BOD/m3.d, respectively, are adopted in design. Other design recommendation are mean cell residence time of 5 to 15 days, MLSS of 2000 to 3500 mg/L, HRT of 3 to 5 h and sludge recirculation ratio of 0.25 to 0.75. In step aeration the design loading rates are slightly higher than conventional ASP. Because of reduction of organic load on the return sludge it gives better performance.

                4. Completely mixed

Ø  In this type of aeration tank completely mixed flow regime is used. The wastewater is distributed along with return sludge uniformly from one side of the tank and effluent is collected at other end of the tank (Figure 19.7). The F/M ratio of 0.2 to 0.6 kg BOD/kg VSS.d and volumetric loading of 0.8 to 2.0 kg BOD/m3.d is used for designing this type of ASP. Higher mixed liquor suspended solids (MLSS) is maintained in the aeration tank of the order of 3000 to 6000 mg/L and mean cell residence time of 5 to 15 days is maintained.

 

                               

 

 

 

 

 

 

 

 

 

 

 

 

                                              

 

 

 

 

Reaction rate kinetics, Conventional Activated sludge process

 

 

Y = yield coefficient i.e., fraction of substrate converted to biomass, (g/m3 of biomass) / (g/m3 of substrate). The value of Y typically varies from 0.4 to 0.8 mg VSS/mg BOD (0.25 to 0.4 mg VSS/mg COD) in aerobic systems.

Sequencing batch reactor (SBR)

Ø A sequencing batch reactor (SBR) is used in small package plants and also for centralized treatment of sewage. The SBR system consists of a single completely mixed reactor in which all the steps of the activated sludge process occurs (Figure 19.10). The reactor basin is filled within a short duration and then aerated for a certain period of time. After the aeration cycle is complete, the cells are allowed to settle for a duration of 0.5 h and effluent is decanted from the top of the unit which takes about 0.5 h. Decanting of supernatant is carried out by either fixed or floating decanter mechanism. When the decanting cycle is complete, the reactor is again filled with raw sewage and the process is repeated. An idle step occurs between the decant and the fill phases. The time of idle step varies based on the influent flow rate and the operating strategy.

Ø  During this phase, a small amount of activated sludge is wasted from the bottom of the SBR basin. A large equalization basin is required in this process, since the influent flow must be contained while the reactor is in the aerating cycle.

 

 

Ø This process is popular because entire process uses one reactor basin. In areas where there is a limited amount of space, treatment takes place in a single basin instead of multiple basins, allowing for a smaller footprint. In the effluent low total-suspended-solid values of less than 10 mg/L can be achieved consistently through the use of effective decanters that eliminate the need for a separate clarifier.

Ø  The treatment cycle can be adjusted to undergo aerobic, anaerobic and anoxic conditions in order to achieve biological nutrient removal, including nitrification, denitrification and some phosphorus removal.