Actibated Sludge

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Activated sludge is a common method used to biologically treat municipal wastewater. The method was first developed in England in the early 100's. In 11, H. W. Clarke studied the use of microorganisms in the purification of sewage through aeration. Edward Arden and William Lockett followed by carrying out similar experiments. They determined that high levels of purification could be reached through this biological process. They also integrated the use of recycle to the aeration stage. Arden and Lockett reported their findings to the Society of Chemical Industry in 114. Since then, the theory of activated sludge as a wastewater treatment process has continued to develop in small steps to the present day.

The activated sludge method typically consists of a well-agitated and aerated reactor and a settling tank. Depending on the design, the reactor can either model a plug-flow reactor, or a CFSTR. If the reactor is designed to be long and narrow, it will model a PFR, while circular tanks approximate a CFSTR.

It was discovered that wastewater could not be purified solely by aeration, but that it needed to be activated by the living organisms that are contained in it. Thus, the sludge consists of undecomposed materials as well as living organisms. The sludge is recycled to the reactor, but overgrown sludge is known as excess sludge, which needs to be disposed of. A schematic of a typical activated sludge process is shown in Figure 1.

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Figure 1-A schematic representation of an activated sludge process

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The activated sludge process has been extremely successful in treating wastewater, in spite of all the variable process conditions that can make purifying water complex. The process has the potential to metabolize a wide variety of organic compounds and can also remove compounds containing nitrogen, sulfur, phosphorus, etc. There are many variations of the process that have been developed to increase purification, such as membrane-coupled systems, and high purity oxygen systems. Some argue that activated sludge is the best method compared to all other biological processes, from both a technical and economical standpoint.

Overview

As mentioned before, wastewater treatment is a very complex process. Wastewater is made up of a vast amount of different compounds and solids. The purification process often includes a primary treatment before the wastewater even enters the sludge tank. The primary treatment may involve some type of screening or filtration to remove large solids. It also may involve conditioning treatments to adjust the pH, or add specific nutrients, which will be vital to the growth of cells. This treatment can remove 45-60 percent of the suspended solids, and even higher removals can be reached with chemical additions. After the primary treatment, the wastewater is then fed to the sludge tank. The activated sludge process generally occurs in two steps. The first step is the transformation of the contents of the wastewater into the activated sludge, and the second step involves the separation of the sludge from the cleaned water, typically through sedimentation.

The contents of the wastewater can be transformed into activated sludge by hydrolysis, adsorption, enmeshment, accumulation or storage. Larger solids that are found in wastewater generally are transformed through adsorption or enmeshment. Colloids, which have a diameter of approximately 10- microns, mostly adsorb on sludge flocs, while larger particles tend to become enmeshed in the void spaces between the sludge flocs. Accumulation, like enmeshment and adsorption, is a fairly fast process. It involves the uptake of small organic molecules. Experiments have shown that mono- and disaccharides, lower fatty acids, alcohols, amino acids, and other similar substances can be accumulated under rich nutritional conditions1.

Hydrolysis and storage, on the other hand, are rather slow processes. Hydrolysis is a process in which surface bound substrate is utilized by the cells. This process transforms some particles into a solved matter, typically a degradable organic matter. Solved matters can then be further transformed either into new cell substances by the bacteria or can be oxidized.

Once the wastewater components have been transformed into the sludge, it then moves on to the final clarifier. The sedimentation process heavily relies on the formation of flake-shaped sludge flocs in order to be efficient. The flocs then settle to the bottom of the clarifier where they thicken and are scraped into the withdrawal cone. The sludge is then recycled back to the sludge tank, while the excess is removed. The cleaned water effluent is removed from the process.

Terminology

Several parameters are of great importance in the understanding and designing of an activated sludge process. One of these parameters is the degree of biological oxygen demand, BOD. The BOD is typically reported for a 5-day time period. To find the BOD5 for a sample of wastewater, the sample is seeded with bacteria and is incubated at 0°C for 5 days along with an unseeded blank. The BOD5 is then calculated according to Equation 7.

()

Another important parameter that needs to be considered when designing an activated sludge process is the MLSS, or mixed-liquor suspended solid concentration, which represents the concentration of activated sludge in the sludge tank. The MLSS is often determined through filtration, with a pore size of 4.5 microns, followed by drying at 105°C, although this does not represent the actual solid concentration, it is a good approximation4.

Design

Contracted engineers do the design of most activated sludge process plants, however, they are restricted by state standards. A guide often used is The Water Pollution Control Federation (WPCF) Manual of Practice 8 (MOP 8)1. Most standards are used to control the amount of certain compounds in the effluent of the treatment process. These standards are set up by local and state agencies and are designed to meet current U.S. EPA Standards.

The first step in designing a purification process is to determine the type of wastewater that needs to be treated. For existing plants, this can be accomplished by studying past plant data. For new plants, however, much of the criteria need to be estimated based on the population. Table 1 shows typical characteristics of wastewater in the United States. It also is necessary to consider the recycle stream of the sludge. The recycle stream can increase toxins in the effluent by as much as 0%.

Concentration

ItemWeakMediumStrong

Biochemical oxygen demand(BOD)5, mg/l1100400

Chemical oxygen demand (COD), mg/l505001000

Suspended solids, mg/l

Total100050

Volatile8016575

Settleable solids, mg/l5100

Nitrogen (as N), mg/l

Organic8155

Ammonia1550

Total04085

Phosphorus (as P), mg/l

Organic15

Inorganic510

Total4815

Table 1-United States Wastewater Characteristics 1

In designing an activated sludge plant, the main tasks are to design the sludge tank and the clarifier, which will be discussed here. However, it is also important to note that in the final design of purification plants, the engineer must also design certain accessories, such as aeration diffusers and return sludge pumps.

When designing a sedimentation tank as a clarifier, both depth and shape need to be determined. Typically, the shape of the clarifier is either circular or rectangular. Most often rectangular clarifiers are used in large plants, or in plants that do not have a lot of space. Table compares the most documented circular clarifier, known as a "deep circular clarifier with a center flocculation well", to the most documented rectangular clarifier, a "Gould clarifier".

Deep, Center Floc CircularItemGould Rectangular

15-0 ftSidewater depth1 ft

Difficult, erraticScum/floating solids removalEasy, complete

GoodEase of covering odor/VOC controlExcellent

HighHead loss for flow distribution among clarifiersLow

HighSite requirementLow

NoCommon wall constructionYes

Table -Comparison of a Rectangular and Circular clarifier 1

Typically, state regulations set a maximum surface overflow rate, SOR. In most instances, the smallest clarifier that meets the state regulations for SOR is an efficient way to design the clarifier.

The main unit that needs to be designed in a purification plant is the activated sludge tank. Designing an activated sludge tank utilizes the rate expressions for cell growth and substrate use, and material balances for both the biomass and the substrate. The Monod equation can be used to approximate cell growth in the system.

(1)

where

m=Specific growth rate

mmax=Maximum specific growth rate

Ks=Saturation constant

S=Substrate concentration

Figure shows the flow rates and concentrations of both the cell mass and the substrate

in an activated sludge system.

Figure -Flow rates and concentrations used in material balances 7

By writing mass balances for the biomass and the rate-limiting substrates around the sludge tank

and the clarifier, an equation to determine the volume of the sludge tank can be derived 7.

()

where

V=Volume of sludge tank

F=Feed flow rate to sludge tank

=Substrate yield coefficient

=Solids' residence time

kd=Tate of cell death

X=Concentration of cells

S0-S=BOD5 removal rate

Thus, for a specified BOD removal rate, it is possible to determine the volume of reactor required.

As of 11, the depth of most typical sludge tanks is approximately 15 ft, or 4.6 m. However, more recent studies have shown that deeper reactor tanks may be equally efficient while consuming less space. Gnirss and Frolich conducted studies on both 10 and 15 m deep reactor tanks from 10 to 15. It was determined that sedimentation as the clarifying process would not work with deeper reactor tanks, so experiments were conducted with 10 and 15 m tanks with dissolved air flotation unit as an alternative clarifier. The 15 m tanks were found to be too costly, however, the 10 m tanks seem to be a good alternative to conventional purification processes. Some advantages of the deeper tanks with flotation are that they take up less than half the area of the typical sludge tank, the return sludge volume is very small, and there are not any problems with scum on the secondary clarifier. The only disadvantage is the small amount of experience with the deeper tanks. At the time of publication of the experiments, no full-scale plant of this type exists, although the authors reasoned that scale-up of their pilot plant should work at least as well .

Variations

Improvements are constantly being developed to advance the quality of wastewater treatment plants. For example, high-purity oxygen is often used in place of air to improve the quality of the purification system. Another improvement that has been found is the membrane-coupled activated sludge process, MCASP. This process uses the typical sludge tank, but varies from the conventional process in that it uses membrane filtration as a clarifying process rather than sedimentation. The MCASP can either have the cross-flow microfiltration unit located outside of the sludge tank or directly within. Figure gives a schematic of both configurations.

Figure -Schematic representation of MCASP with external (a) and internal (b) membrane filtration 4

The MCASP require cross-flow to ensure that the membranes are not clogged too quickly. The main advantage of using a MCASP is that higher quality effluents can be achieved. This is because there is a complete separation of solids due to the membrane depending on the pore size. The main disadvantage of this system is that it requires almost double the energy as a sedimentation system. Thus, each plant must be considered independently to determine if it is cost-efficient or not to install a MCASP4.

The addition ozone has also proved useful in the advancement of wastewater treatment. When ozone is added to the sludge tank, it increases its biological degradability. Therefore, the amount of excess sludge can be greatly reduced through ozonation. The process involves sludge digestion and wastewater treatment that are conducted simultaneously in the same sludge tank. Figure 4 shows a schematic representation of the ozonation process. Studies have shown that in most wastewater can actually be treated without any excess sludge. More recent studies have found that intermittent ozonation can be used in place of continuous ozonation in order to cut expenses, while still greatly reducing the amount of excess sludge6. Another advantage to ozonation is that it improves the settleability of the sludge.

Figure 4-Schematic Representation of an ozonation process 8

Separation Problems

Several problems can occur in the sedimentation tank. If the sludge to does not exhibit certain qualities, such as fast settling velocities ( 1m/h), leaving a clear supernatant after settling, and not rising within a - hour period after settling, the efficiency of the system is greatly damaged. Dispersed growth is one problem that might occur in the sedimentation tank. This occurs when the bacteria do not form flocs and instead grow in small clusters. The small clusters then are able to rise out of the settling tank with the effluent. This problem, however, rarely occurs in industry1.

Pinpoint flocs are similar to dispersed growth in that they are small flocs that escape with the effluent. Pinpoint flocs are generally flocs with a diameter that is less than 100 microns. They can occur from a low production of glycocalyx, which helps to form a polymetrix matrix of firm sludge flocs that aids in keeping the small flocs from escaping.

Rising sludge also creates a problem since the sludge is not settling to the bottom. This can occur due to either a high adsorption of oils and fats or from the occlusion of gasses. Non-filamentous bulking is a condition where the sludge loses its ability to flow. Yet another possible problem that can occur in sedimentation is filamentous bulking which leads to foaming.

Environmental Concerns

As mentioned before, the EPA regulates the amount of certain pollutants, such as nitrogen, phosphorus, and sulfur that can be contained in the effluent and released into the environment. These pollutants are originally contained in proteins, which are found in the influent to the sludge tank. When the bacteria degrade the proteins in the wastewater, certain inorganic products are released. Table summarizes these inorganic products. A high concentration of some of these inorganic substances can have an adverse affect on the surrounding environment.

Element Contained In ProteinsInorganic Product Released

CarbonCarbon Dioxide (CO)

HydrogenWater (HO)

NitrogenAmmonium Ion (NH4+)

OxygenWater (HO)

PhosphorusPhosphate ion (PO4-)

SulfurSulfate ion (SO4-)

Table -Inorganic Products released from the oxidation of Proteins

Nitrogen can lead to an assortment of environmental problems. Released nitrogen leads to a depletion of dissolved oxygen. Nitrogen follows the path shown below where each ammonium ion consumes two molecules of oxygen gas before it is converted to a nitrate ion.

NH4 + O NO-

NO- +O NO-

Nitrogen can also lead to the toxication of fish and other aquatic life. Ammonium, nitrite, and nitrate ions are among the most toxic. Nitrogen wastes, as well as phosphate ions, greatly contribute to eutrophication, which is the premature aging of lakes and ponds. Since both of these wastes are nutrients to algae and other plant life, the disposal of nitrogen and phosphorus ions lead to the overgrowth of these plants.

While phosphate can be easily removed from sludge plants by precipitation with alum or possibly another flocculant, nitrogen requires an extended aeration along with periods of anoxia. This leads to denitrification in the plant. However, this and other known methods are extremely costly. One interesting new finding in the area of nitrogen removal is the possibility of using wetlands to enhance the natural process of bacterial denitrification5. There exist bacteria that can naturally convert nitrite and nitrate ions into the harmless nitrogen gas. The nitrogen gas can then be released into the atmosphere. While wetlands may provide a cost-effective way to remove nitrogen from the effluent of wastewater treatment plants, they do require a lot of space and only are appropriate for specific locations.

Conclusion

Activated sludge as a municipal wastewater treatment has proven to be a very efficient way of purifying wastewater. Although, some problems can occur with this process, many of them have been overcome due to the lengthy history of this type of purification. With added technologies of ozonation and deeper sludge tanks, activated sludge can only improve as a cost-effective way to treat municipal wastewater.

References

1Eckenfelder, Wesley W., and Grau, Petr. 1. Activated Sludge Process Design and Control

Theory and Practice. Technomic Publishing, Lancaster.

Gerardi, Michael H. 00. Nitrification and Denitrification in the Activated Sludge Process.

Wiley-Interscience, New York, NY.

Gnirss, R. and Frolich, A. Peter. 16. Biological Treatment of Municipal Wastewater with

Deep Tanks and Flotation for Secondary Clarification. Wat. Sci. Tech. (4).

4Gunder, Berhold. 001. The Membrane-Coupled Activated Sludge Process in Municipal

Wastewater Treatment. Technomic Publishing, Lancaster

5Horne, Alexander. 15. Nitrogen Removal from Waste Treatment Pond or Activated Sludge

Plant Effluents with Free-Surface Wetlands. Wat. Sci. Tech. (1).

6Kamiya, T. and Hirotsuji, J. 18. New Combined System of Biological Process and

Intermittent Ozonation for Advanced Wastewater Treatment. Wat. Sci. Tech. (8).

7Shuler, Michael L. and Kargi, Fikret. (00). Bioprocess Engineering, Basic Concepts.

Prentice Hall, Upper Saddle River, NJ.

8Yasui, H, Nakamura, K. Sakuma, S. Iwasaki, M, and Sakai, Y. 16. A Full-Scale Operation

of a Novel Activated Sludge Process Without Excess Sludge Production. Wat. Sci. Tech

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