Mixed-Liquor Settling Characteristics Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index(s) and the zone settling rate The SVI is determined by placing a mixed-liquor sample in a 1-to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample mlss concentration For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a 1-L cylinder would have an SvI of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth
Mixed-Liquor Settling Characteristics Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids. Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index (SVI) and the zone settling rate . The SVI is determined by placing a mixed-liquor sample in a 1- to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample MLSS concentration. For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a 1-L cylinder would have an SVI of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth
Use of selectors Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector (a smal contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well, An appropriate selector design can be added before the activated- sludge aeration basin
Use of Selectors Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector (a small contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well. An appropriate selector design can be added before the activatedsludge aeration basin
Process Control To maintain high levels of treatment performance The principal approaches to process control are( maintaining dissolved oxygen levels in the aeration tanks,(2) regulating the amount of return activated sludge (RAS) and (3)controlling the waste-activated sludge(WAS) The parameter used most commonly for controlling the activated-sludge process iS SRT
Process Control To maintain high levels of treatment performance The principal approaches to process control are (1) maintaining dissolved oxygen levels in the aeration tanks, (2) regulating the amount of return activated sludge (RAS), and (3) controlling the waste-activated sludge (WAS). The parameter used most commonly for controlling the activated-sludge process is SRT
Dissolved Oxygen Control. When oxygen limits the growth of microorganisms filamentous organisms may predominate and the settleability and quality of the activated sludge may be poor In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1. 5 to 2 mg/L in all areas of the aeration tank
Dissolved Oxygen Control. When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the settleability and quality of the activated sludge may be poor. In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1.5 to 2 mg/L in all areas of the aeration tank
Return Activated-Sludge control Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L Settleability If the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL /725 mL)* 100]. If the plant flow were 2 m/s, the return sludge rate should be 0.38X 2 0.76m3/s
Return Activated-Sludge Control Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L . Settleability If the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL / 725 mL) * 100]. If the plant flow were 2 m3 /s, the return sludge rate should be 0.38 × 2 m3 /s = 0.76 m3 /s