Variables That Impact Plant Operation | Activated Sludge Process | Wastewater Treatment Plant

 Variables That Impact Plant Operation 

The operator of an activated sludge treatment plant is responsible for creating an environment conducive to the efficient conversion of colloidal and dissolved solids into settleable biological floc. However, the quality of the influent wastewater as well as the plant processes can affect this process and, as a result, the plant effluent quality. Some of the variables to think about are as follows:

1. Influent BOD and changing waste characteristics: 

• Organic overload can occur when BOD levels are high.
• A change in influent temperature or pH can alter biological reaction rates as well as the normal equilibrium of wastewater constituents, potentially affecting the availability of food for organisms or the rate at which they can process it. When temperatures fall below 5 degrees Celsius, biological activity decreases significantly.
• Toxic substances introduced into wastewater influent can kill or inhibit organisms. 

2. Waste Activated Sludge Rate 

• As the rate at which sludge is wasted changes, so does the nature of the organisms that predominate in the activated sludge plant.
• The rate of activated sludge waste must be regulated to account for daily or seasonal fluctuations in BOD loading (in order to maintain an appropriate F/M ratio), as well as to respond to abrupt or shock BOD loads to the system.

3. Dissolved Oxygen Level 

• Low DO levels in the aeration tank can inhibit desired organism activity while allowing undesirable (i.e., facultative and filamentous) organisms to thrive. If there is too much air, the solids will floc (clump together) and not settle properly. For conventional secondary treatment, the desired goal is 2.0 mg/l measured at the aeration tank's end (nitrification).
• The oxygen requirements of the activated sludge system are affected by changes in the organic strength (amount of food) of the influent. To maintain an adequate dissolved oxygen concentration to support the organisms without over aerating the wastewater, the operator or system must be able to respond to these changes.
• The temperature of the liquid in the aeration tank, as well as the size of the air bubbles, influence the rate of oxygen transfer from air to water. As the temperature of the liquid and the size of the air bubbles decrease, the rate of oxygen transfer increases, and vice versa. This means that finer bubbles and colder water result in optimal dissolved oxygen saturation.
• The solubility of oxygen in water varies with temperature, with 0 °C displacing about twice as much (14.6 mg/L) as 20 °C. Consider the formation of bubbles in a pot of water just before it begins to boil; these bubbles are oxygen that was dissolved at room temperature but is being ejected as the temperature rises. Without causing the water molecules to separate, oxygen can enter the crevasses or "holes" in the loose hydrogen-bonded network. When the water is colder, the water molecules move less and the oxygen remains trapped in the aqueous solution, according to a very physical perspective on oxygen solubility in water.
• The surface area of the bubbles can be increased by fine bubble diffused aeration, allowing more oxygen to be supplied to the water per bubble. Furthermore, smaller bubbles take longer to reach the surface, maximizing not only the surface area but also the number of seconds each bubble spends in the water, giving it more time to transfer oxygen to the water. In general, smaller bubbles and a deeper release point result in a higher oxygen transfer rate.