Secondary Treatment Biological

Physical treatment of raw municipal wastewater by sedimentation removes most of those pollutants that will either float or settle out by gravity, which accounts for only approximately 35% of the BOD. The major purpose of secondary treatment is to remove nonsettleable (colloidal and dissolved) solids in the wastewater. "Secondary treatment" is generally considered to mean at least 85% efficiency in reducing BOD and now represents the minimum degree of treatment required by law in most cases. Some of the plant nutrients are also removed. The removal of organics and nutrients helps to protect the receiving watercourse. Secondary treatment processes are almost always biological systems.

Biological treatment systems are living systems that rely on mixed biological cultures to break down waste organics and remove organic matter from the solution. A biological waste treatment system provides an artificial and controlled environment suitable for the growth of microorganisms that can stabilize the organic pollutants in the wastewater before it is discharged into the surface waters. These living microorganisms, including bacteria and protozoa, consume the organic pollutants as food. They metabolize the biodegradable organics, converting them into carbon dioxide, water, and energy. The primary use of energy is for synthesis. The maximum rate of synthesis occurs simultaneously with the maximum rate of energy yield (maximum rate of metabolism). Aerobic metabolism requires oxygen for the processes of metabolism and synthesis. In anaerobic metabolism, the metabolism and synthesis take place in the absence of oxygen. A low energy yield per unit of organic matter results from an incomplete reaction. When the supply of biologically available energy is exhausted, the processes of metabolism and synthesis cease.

To keep the microorganisms productive in their task of wastewater treatment, they require an ample supply of oxygen, suitable temperatures and ph, a nontoxic environment, and other favorable conditions. The design and operation of the secondary treatment is based upon these factors.

Several types of biological treatment systems are stabilization pond, oxidation ditch, biotowers, trickling filter, and activated sludge. The latter two are the most common.

1. Trickling Filters

A trickling filter consists of a bed of coarse material, such as stones, slats, or plastic media, over which wastewater is applied. A widely used design is a bed of stones approximately 5-7 ft (1.5-2.1 m) deep; it is usually circular and may be as large as 200 ft (60 m) in diameter. Figure 5 shows a trickling filter, and Figure 6 is a flow diagram of a trickling filter plant.

Biotowers Wastewater Treatment
Figure 5 A trickling filter.

Trickling filters are usually preceded by primary treatment to remove large and settleable solids. The wastewater is typically distributed over the surface of the rocks by a rotating arm that has nozzles along its length and sprays wastewater evenly over the surface of the trickling filter. The distributor arm is mounted on a center column in the trickling filter. It is driven around by the reaction force of the wastewater. The underdrain system serves to collect and carry the wastewater from the bottom of the bed and also permits air to circulate through the bed.

The primary effluent is sprayed on a bed of crushed rock or other media coated with biological films. The biological slime layer consists of bacteria, protozoans, and fungi. Sludge

Trickling Filter Column
Figure 6 Flow diagram of a trickling filter plant.

worms, rotifers, filter-fly larvae, and other higher animals also grow in this environment. As the wastewater flows over the microbial film, the soluble organics are metabolized and the colloidal organics are absorbed onto the surface, thus removing organic substances from the wastewater. Air circulating through the void spaces in the bed of stones provides the oxygen for stabilization of the organics by the microorganisms.

The rocks in the trickling filter are usually approximately 3 in. (75 mm) in size to provide a large surface area for the biological growths, and the large voids allow air circulation. Sometimes materials such as modules of corrugated plastic or redwood are used as the medium instead of rocks.

As the microorganisms grow and multiply, the slime layer thickens. Due to its weight and the flushing action of flowing wastewater, the slime is washed off the rock surfaces. The trickling filter effluent is collected in the underdrain system, from where it flows to a sedimentation tank called a secondary or final clarifier.

To maintain a uniform flow rate through the trickling filter and to keep the distributor arm rotating during periods of low wastewater flow, a portion of the effluent is pumped back to the trickling filter inlet. Recirculation also serves to improve the treatment efficiency of the trickling filter; it allows a certain portion of the wastewater to make a second pass through the film of microbes on the rocks. There are many recirculation patterns and configurations of trickling filter plants, and these may be direct or indirect.

Recirculation. The amount of recirculation can vary. It is characterized by a recirculation ratio, which is the ratio of recycled flow to raw wastewater flow,

where R is the recirculation ratio (dimensionless), QR is the recirculated flow rate (ft3/sec or m3/sec), and Q is the wastewater flow rate (ft3/sec or m3/sec). The recirculation ratio, R, is generally in the range of 0-3.0.

Hydraulic Load and BOD Load. The rate at which the wastewater flow is applied to the trickling filter surface is the hydraulic load. The rate at which organic material is applied to the trickling filter is called the organic or BOD load.

The hydraulic load depends on the recirculated flow QR\ the total flow through the trickling filter is equal to Q + QR.

where As is the trickling filter surface area in square meters or square feet. Hydraulic load can be expressed in cubic feet per square foot per day, cubic meters per square meter per day, or millions of gallons per acre per day.

The rate at which organic material is applied to the trickling filter is expressed as BOD load. It does not include the BOD of the material in the recirculated flow. The formula for BOD load can be expressed as



Feet Sewer Sludge
Figure 7 An activated sludge unit.

where BOD is the biological oxygen demand of the primary effluent in milligrams per liter or parts per million and V is the volume of the trickling filter bed in cubic meters or cubic feet.

Efficiency. The BOD reduction efficiency of a trickling filter unit depends on organic load, recirculation ratio, and temperature. Generally, the efficiency increases with increasing recirculation and temperature and decreasing organic load.

2. Activated Sludge Treatment

The activated sludge process is a biological wastewater treatment technique in which a mixture of wastewater and microorganisms is agitated and aerated. Wastewater is fed continuously into an aerated tank, where the microorganisms metabolize and biologically flocculate the organics using oxygen provided in the compressed air. Figure 7 shows an activated sludge unit.

Aeration and mixing are achieved by continuously injecting compressed air into the mixture through porous diffusers located at the bottom of the tank (Figure 8a). Sometimes mechanical devices such as propeller-type mixers located at the liquid surface are used (Figure 8b). The propeller blades mix air with the wastewater and keep the contents of the tank in suspension.

The aerobic microorganisms in the tank grow and multiply, forming an active suspension of biological solids called activated sludge. The mixture of the activated sludge and the waste-

Types Aeration Tank

Figure 8 An activated sludge unit achieves aeration and mixing (a) with diffused air or (b) with a mechanical aerator.

Figure 8 An activated sludge unit achieves aeration and mixing (a) with diffused air or (b) with a mechanical aerator.

water in the aeration tank is called mixed liquor. In most cases the aeration period is 6-9 hr. The biological solids are subsequently separated by gravity from the mixed liquor under quiescent conditions in the final clarifier. The clarified water near the surface (the supernatant) is discharged from the final clarifier over a weir. The settled sludge is pumped out from a sludge hopper at the tank bottom. A portion of the sludge is returned to the aeration tank so that active and acclimatized microorganisms can absorb and metabolize organics more efficiently. Since the organisms grow and multiply greatly, it is not possible to pump all the sludge to the aeration tank. Therefore, excess sludge is diverted to the sludge handling unit for treatment and disposal.

General loading and operational parameters for the activated sludge processes used in the treatment of municipal wastewater are listed in Table 2. The wide range of aeration periods and BOD loadings used in activated sludge processes differentiate the processes from each other. Also, the aeration tank's size and shape influence the process.

An important indicator used in the design and operation of activated sludge systems is the food/microorganism (F/M) ratio. The "food" or the BOD in the influent wastewater (without regard to return sludge) is expressed in pounds or kilograms per day of liquid volume in the aeration tank. The concentration of the suspended solids, which mainly consist of active microorganisms, is called the mixed liquor suspended solids (MLSS). F/M can be computed from the formula

Table 2 Loadings and Efficiencies of Activated Sludge Systems

Loading (lb BOD/day


Efficiency of


per lb MLSS)

period (hr)

BOD removal (%)

High rate








Extended aeration




Waste sludge
Multi Chamber Septic Tank
Figure 9 Activated sludge plant; (a) conventional (b) step aeration.

where BOD is the applied 5-day BOD, in mg/L or ppm; Q is the wastewater flow rate, in mL/day or mg/day; MLSS is mixed liquor suspended solids, mg/L; and V is the volume of the aeration tank, mL.

Types of Activated Sludge Processes. Several types of systems have been developed. Many of these serve to increase plant capacity or reduce the tank volume requirement.

1. In the conventional activated-sludge process, the aeration basin is a long rectangular tank with air diffusers along one side of the tank bottom to provide aeration and mixing. Settled raw wastewater and returning activated sludge enter the head of the tank and flow down in a spiral flow pattern. The air supply is tapered to provide a greater amount of diffused air near the head where the rate of biological metabolism and consequently oxygen demand are the greatest (Figure 9).

Screen & Grit Chamber

Screen & Grit Chamber

Grease Interceptor

Figure 10 Activated sludge systems; (a) contact stabilization plant, (b) completely mixed activated sludge plant, and (c) extended aeration plant.

Figure 10 Activated sludge systems; (a) contact stabilization plant, (b) completely mixed activated sludge plant, and (c) extended aeration plant.

2. The step-aeration activated-sludge process is a modification of the conventional process. It provides multiple feed points of the primary effluent into the aeration tank, unlike in the conventional process where flow enters only at the head end. Distributing the influent load along the tank produces a more uniform oxygen demand (see Figure 10).

3. Extended aeration systems are generally small packaged plants used for treating low wastewater flow rates from hotels, schools, suburban residential development, and other iso-

lated sources. In this process the screened wastewater flows directly into the aeration tank without any primary settling, and the detention time is 24 hr or greater. The system operates on a low food/microorganism ratio (F/M). (See Table 2.) The low FIM ratio and the "extended" period allow for the stabilization of most of the organics. Excess sludge is generally not wasted continuously, only periodically. Figure 10c is a flow diagram of an extended aeration system.

4. Table 2 indicates that the high-rate (completely mixed activated sludge) process operates with a high BOD load per unit volume of aeration tank and a short aeration period. Many such units use a combination of compressed-air aeration and mechanical mixing. Also, in large, completely mixed aeration basins a mechanical impeller is placed above an uptake tube for deep mixing along with surface aeration. (See Figure 10b.)

5. The contact stabilization activated-sludge process provides for reaeration of the returned activated sludge from the final clarifier, allowing the use of a smaller aeration tank and therefore relatively shorter aeration periods. After a short contact time, the mixed liquor enters a clarifier and the activated sludge settles out; the clarified liquid flows over effluent weirs. The settled sludge is pumped into another aerated tank for reaeration. This type of process is shown in Figure 10a.

6. Air is approximately 21 % oxygen. The major components of a pure oxygen aeration system are an oxygen generator, a covered aeration tank, a final clarifier, and recirculation pumps. Primary effluent, return activated sludge, and oxygen are introduced into the first compartment of a multistage covered tank. Mechanical agitators mix the oxygen with the wastewater in the tank. High-purity-oxygen activated sludge has several advantages. High efficiency is possible at increased BOD loads and reduced aeration periods. Waste sludge production is also less.

Operation and Control of Activated Sludge Processes. Operation of an activated-sludge treatment plant is regulated by the quantity of air supplied, the rate of activated sludge recirculation, and the amount of excess sludge withdrawn. The settleability of the mixed liquor in the final clarifier governs the rate of activated sludge recirculation. Poorly flocculated particles and filamentous growths that do not separate by gravity in the final clarifier contribute to the BOD and suspended solids in the effluent. Excessive carryover of floe is called sludge bulking. This condition can be controlled by appropriate adjustments in the mixed liquor suspended solids concentration and food/microorganism ratio. Also, sludge bulking may be caused by excessive agitation or insufficient aeration.

The settleability of mixed liquor is defined by the sludge volume index (SVI), which is equal to the volume occupied by 1 g of settled sludge and is expressed in milliliters per gram (mL/g). To determine SVI, a sample of mixed liquor from the aeration tank is allowed to settle for 30 min in a 1-L graduated glass cylinder. The volume of settled sludge is read from the scale on the cylinder, and the MLSS is also measured. The SVI is then computed as

where V is the volume of settled sludge (ml/L), and MLSS is the mixed liquor suspended solids (mg/L).

3. Rotating Biological Contractors

A rotating biological contractor (RBC) is a secondary treatment device that consists of a series of large plastic disks mounted on a horizontal shaft. The lightweight disks are approximately 10 ft (3 m) in diameter and are spaced approximately 1.5 in. (40 mm) apart on the shaft. The disks are usually made of corrugated plastic sheets bonded together.

Table 3

High rate


BOD loading

[lb/(1000 ft3-day)]



Hydraulic loading




The disks are approximately 40% submerged in primary effluent. As the shaft rotates, the disks are alternately in contact with air and with the wastewater. The result is similar to that of the trickling filter system. When disks are rotated out of the tank, air enters the spaces while the liquid trickles out over the biological film on the disks. The microbes that form the film absorb the organic material in the wastewater. These microbial solids grow on the medium. Excess microbial solids (slime) break away from the medium due to their weight and hydraulic forces and are carried out in the process effluent for gravity separation in the final clarifier.

4. Stabilization or Oxidation Ponds

For suburban or rural areas with seasonal industries such as fruit and vegetable canning facilities and where land is relatively cheap, wastewater lagoons may be used for secondary treatment. These lagoons are also called stabilization or oxidation ponds. A stabilization pond is a flat-bottomed pond enclosed by an earth dike. It can be round, square, or rectangular of length approximately three times the width. The operating liquid depths are 2-6 ft (0.6-1.83 m) and 3 ft (0.91 m) dike freeboard. A minimum of 2 ft (0.6 m) liquid depth is needed to prevent the growth of weeds. Liquid depths of 6 ft (1.83 m) or greater may give out odors because of anaerobic decomposition near the pond bottom. Where the required lagoon area is greater than 6 acres, it is good engineering to have multiple cells that can be operated individually, in series, or in parallel. If the soil is pervious, the pond bottom and dikes should be sealed with clay or other sealant to prevent groundwater pollution. Dikes and surroundings are seeded with grass, graded to prevent runoff water from entering the pond, and fenced.

A majority of lagoons are facultative ponds; i.e., both aerobic and anaerobic biochemical reactions take place. Raw wastewater enters the pond without primary treatment. Organic solids that settle in the bottom decompose anaerobically, producing organic acids, methane, and hydrogen sulfide. Near the mid-depth of the pond, most of the organic matter is decomposed by facultative bacteria, bacteria that can grow in either an aerobic or anaerobic environment.

In the presence of oxygen, aerobic decomposition occurs mostly in the top half of the liquid. A major portion of the oxygen added to the wastewater in the pond is due to the mixing of air at the pond surface by wind action. A minor portion of the oxygen is from photosynthesis as algae present in the pond use energy from sunlight. The algae grow and multiply by consuming carbon dioxide and other inorganic compounds released by bacteria. Bacteria and protozoa in turn use oxygen released by algae and from surface aeration to decompose the organic matter in the wastewater. The overall process in a stabilization pond is the sum of the reactions of the bacteria, protozoans, and algae.

BOD loadings on a stabilization pond are generally in the range of 20-40 lb of BOD per acre per day or 0.46-0.92 lb of BOD per 1000 ft2 [2.2-4.4 g/(m2-day)]. Higher loadings are possible in areas with warmer climates. Although the algae play a role in the purification process in a lagoon, they can cause a problem when they are carried out of the pond in the effluent flow by increasing the levels of total suspended solids.

Despite many drawbacks, such as difficulty with total suspended solids removal efficiency and potential groundwater pollution, wastewater lagoons are being used where land is relatively cheap. The low construction costs and ease of operation and maintenance are distinct advantages.

5. Secondary Effluent Chlorination

The final step in the secondary wastewater treatment process is usually disinfection by chlorination. The main purpose is to destroy pathogens in the effluent before discharging it into a body of water used for swimming or water supply downstream. The chlorine demand of secondary effluent is usually high. Chlorine dosage should be adjusted so that a combined chlorine residual is approximately 0.5 mg/L in the secondary effluent.

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  • gilda trevisani
    What is secondary treatment device?
    2 years ago

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