The Activated

Figure 5.1 The activated sludge process. In the activated sludge process, a tank (aeration tank) or series of tanks is used to degrade or oxidize wastes. The oxidation of the wastes is achieved by mixing the wastes, with air and a high concentration of bacteria, for sufficient time. When the wastes are oxidized more bacteria are produced. The bacteria in the form of floc particles leave the aeration tank and enter a clarifier. Here the floc particles settle to the bottom of the clarifier and are returned to the aeration tank to degrade more incoming wastes or are removed from the process. If a clarifier is placed upstream of the aeration tank, the clarifier is called a ''primary clarifier.'' The purpose of this clarifier is to remove heavy solids and fats, oils, and grease. The clarifier following the aeration tank usually is called a "secondary clarifier.'' Wastes oxidized in the aeration tank are converted to carbon dioxide (CO2), water (H2O), ammonium ions (NH£), nitrite ions (NO;,), nitrate ions (NO^), sulfate ions (SO2phosphate ions (PO^~), and more bacterial cells (MLVSS).

Figure 5.1 The activated sludge process. In the activated sludge process, a tank (aeration tank) or series of tanks is used to degrade or oxidize wastes. The oxidation of the wastes is achieved by mixing the wastes, with air and a high concentration of bacteria, for sufficient time. When the wastes are oxidized more bacteria are produced. The bacteria in the form of floc particles leave the aeration tank and enter a clarifier. Here the floc particles settle to the bottom of the clarifier and are returned to the aeration tank to degrade more incoming wastes or are removed from the process. If a clarifier is placed upstream of the aeration tank, the clarifier is called a ''primary clarifier.'' The purpose of this clarifier is to remove heavy solids and fats, oils, and grease. The clarifier following the aeration tank usually is called a "secondary clarifier.'' Wastes oxidized in the aeration tank are converted to carbon dioxide (CO2), water (H2O), ammonium ions (NH£), nitrite ions (NO;,), nitrate ions (NO^), sulfate ions (SO2phosphate ions (PO^~), and more bacterial cells (MLVSS).

TABLE 5.1 Inorganic Products Released in the Aeration Tank from the Oxidation of Proteins

Element Contained in Proteins

Inorganic Product Released

Carbon

Hydrogen

Nitrogen

Oxygen

Phosphorus

Sulfur

Carbon dioxide (CO2) Water (H2O) Ammonium ion (NHJ) Water (H2O) 4 Phosphate ion (PO|_) Sulfate ion (SOf")

Ultimately, under appropriate operational conditions and adequate aeration time, the bacteria convert the substrate to simplistic products through biochemical reactions. Some organic-nitrogen compounds such as proteins contain carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The sulfur is found in thiol groups (-SH) bonded to the proteins. When proteins are degraded in the aeration tank by bacteria, the bacteria obtain carbon for growth, energy for cellular activity, and release inorganic products in the aeration tank (Table 5.1).

Ammonium ions that are produced in the sewer system and the aeration tank through hydrolysis and deamination are the substrate for the bacteria that oxidize nitrogen in the form of ammonium ions. The oxidation of the ammonium ions by bacteria is nitrification. When ammonium ions are oxidized, the bacteria obtain energy and release nitrite ions in the aeration tank.

The nitrite ions that are produced in the aeration tank are the substrate for the bacteria that oxidize nitrogen in the form of nitrite ions. The oxidation of nitrite ions by bacteria is nitrification. When nitrite ions are oxidized, the bacteria obtain energy and release nitrate ions in the aeration tank.

When bacterial cells oxidize substrate in the aeration tank, reproduction occurs or an increase in the bacterial population results. Bacteria represent a portion of the solids in the aeration tank. Therefore, as the bacterial population increases through reproduction, the solids inventory in the aeration tank also increases.

Solids in the aeration tank are referred to as sludge. Because the sludge is aerated, and the bacteria become very active during aeration, the term ''activated sludge'' is used to describe the process where bacterial solids are active in the purification of the wastes within the aeration tank.

As the bacteria in the aeration tank age, many bacteria stick together to form floc particles or large solids. These particles contain a large number and diversity of bacteria that degrade the wastes in the aeration tank. As the solids flow into the secondary clarifier, the solids settle to the floor of the secondary clarifier and a clear supernatant develops above the settled solids. After additional treatment, the supernatant is discharged to the receiving water.

The settled solids may be returned to the aeration tank or may be removed from the activated sludge process. The removal of solids from the activated sludge system is referred to as wasting. Solids are ''wasted'' to another treatment unit for additional treatment and disposal.

There are several operational factors that can be used to monitor and regulate the activated sludge process. These factors include F/M and MCRT (Appendix II). These factors are critical for monitoring and regulating nitrification and denitrification in the activated sludge process.

F/M is the food-to-microorganism ratio. This factor measures the quantity of BOD or food (the ''F'' in F/M) available per day per quantity of bacteria or microorganisms (the ''M'' in F/M). The F/M increases when additional food enters the activated sludge process or more bacteria are wasted from an activated sludge process. The F/M decreases when less food enters an activated sludge process or fewer bacteria are wasted from an activated sludge process.

The MCRT is the mean cell residence time as measured in days. The MCRT is the average time the solids or bacteria are retained in an activated sludge process. The higher the MCRT is the older the bacteria are.

The MCRT is increased in an activated sludge process by decreasing the quantity of solids wasted. The MCRT is decreased in an activated sludge process by increasing the quantity of solids wasted.

Part II

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