Editing Activated sludge (section)

===Process control===

The general process control method is to monitor sludge blanket level, SVI (Sludge Volume Index), MCRT (Mean Cell Residence Time), F/M (Food to Microorganism), as well as the biota of the activated sludge and the major nutrients DO ([[Dissolved oxygen]]), [[nitrogen]], [[phosphate]], BOD ([[Biochemical oxygen demand]]), and COD ([[Chemical oxygen demand]]). In the reactor/aerator and clarifier system, the sludge blanket is measured from the bottom of the clarifier to the level of settled solids in the clarifier's water column; this, in large plants, can be done up to three times a day.

The SVI is the volume of settled sludge occupied by a given mass of dry sludge solids. It is calculated by dividing the volume of settled sludge in a mixed liquor sample, measured in milliliters per liter of sample (after 30 minutes of settling), by the MLSS (Mixed Liquor Suspended Solids), measured in grams per liter.<ref name="MECC">{{cite web|url=https://water.mecc.edu/courses/ENV149/lesson7.htm|title=Lesson 7: Activated Sludge|website=Water/Wastewater Distance Learning|publisher=[[Mountain Empire Community College]]|date=2013-03-19 <!--Based on browser-derived page modification date as of access date-->|access-date=2022-02-19}}</ref><ref name="WastewaterMath">{{cite web|url=http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%2017%20-%20Mathematics.pdf|title=Mathematics For Wastewater Operators|archive-url=https://web.archive.org/web/20120907112903/http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%2017%20-%20Mathematics.pdf|archive-date=2012-09-07|url-status=usurped}}</ref> The MCRT is the total mass (in kilograms or pounds) of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate (in kilograms/pounds per day) of mixed liquor suspended solids leaving as WAS and final effluent.<ref name="MECC" /><ref name="WastewaterMath" /> The F/M is the ratio of food fed to the microorganisms each day to the mass of microorganisms held under aeration. Specifically, it is the amount of BOD fed to the aerator (in kilograms/pounds per day) divided by the amount (in kilograms or pounds) of [[Mixed Liquor Volatile Suspended Solids]] (MLVSS) under aeration. Note:  Some references use MLSS (Mixed Liquor Suspended Solids) for expedience, but MLVSS is considered more accurate for the measure of microorganisms.<ref name="MECC" /><ref name="WastewaterMath" /> Again, due to expedience, COD is generally used, in lieu of BOD, as BOD takes five days for results.

To ensure good bacterial settlement and to avoid sedimentation problems caused by filamentous bacteria, plants using atmospheric air as an oxygen source should maintain a dissolved oxygen (DO) level of about 2 mg/L in the aeration tank. In pure oxygen systems, DO levels are usually in the range of 4 to 10 mg/L. Operators should monitor the tank for low DO bacteria, such as S. natans, type 1701 and H. hydrossis, which indicate low DO conditions by elevated effluent turbidity and dark activated sludge with foul odours. Many plants have on-line monitoring equipment that continuously measures and records DO levels at specific points within the aeration tank. These on-line analysers send data to the SCADA system and allow automatic control of the aeration system to maintain a predetermined DO level. Whether generated automatically or taken manually, regular monitoring is necessary to favour organisms that settle well rather than filaments. However, operating the aeration system involves finding a balance between sufficient oxygen for proper treatment and the energy cost, which represents approximately 90% of the total treatment cost.<ref>{{cite web|title=Lesson 8: The Activated Sludge Process|website=water.mecc.edu|url=https://water.mecc.edu/courses/Env149/lesson8.htm|access-date=19 August 2024}}</ref>

Based on these control methods, the amount of settled solids in the mixed liquor can be varied by wasting activated sludge (WAS) or returning activated sludge (RAS).{{citation needed|date=December 2019}}
The returning activated sludge is designed to recycle a portion of the activated sludge from the secondary clarifier back to the aeration tank. It usually includes a pump that draws the portion back. 
The RAS line is designed considering the potential for clogging, settling, and other relatable issues that manage to impact the flow of the activated sludge back to the aeration tank. This line must handle the required flow of the plant and has to be designed to minimize the risk of solids settling or accumulating.