SRTmaster: real-time sludge age control
SRTmaster is a real-time control system for sludge age, also called solids retention time (SRT). It calculates the waste activated sludge flow continuously from on-line suspended solids and flow measurements, and sends the waste flow setpoint to the plant's SCADA system. The first real-time SRT control system was implemented in 1996 at a 170 MGD biological nutrient removal plant.
Why a constant sludge age matters
A constant sludge age keeps the food-to-microorganism ratio stable. Too little wasting can overload clarifiers, cause bulking and foaming, and raise air demand. Too much wasting can cause poor removal of soluble pollutants and, where the plant nitrifies, higher chlorine demand. Day-to-day variation in wasting also raises polymer use in sludge thickening. Wasting calculated once or twice a day from laboratory samples is adjusted late and on uncertain data.
Published results
- Sludge age held within 5% of setpoint, and within 2.5% most of the time (WEFTEC 2016).
- At the Pomona Water Reclamation Plant, an average sludge age control error of 0.13 days, less than 2% (U.S. Department of Energy final report).
- Daily variation in the mass of sludge wasted reduced from 23% to 3% of the daily average.
- Secondary treatment capacity increased by up to 30% through better sludge settling (Victor Valley Wastewater Reclamation Authority, 30%).
- Foaming caused by Nocardia, a filamentous bacterium, significantly reduced at Sacramento Regional and at other plants.
- Polymer for sludge thickening reduced by 25% at Oxnard.
- Laboratory suspended solids tests for process control reduced by as much as 90%.
- In the twenty years to 2016, no recorded incident of the controller acting on an erroneous instrument signal.
How it works with the plant's equipment
A basic installation uses two suspended solids analyzers (in the mixed liquor and in the return or waste sludge), a waste flow meter and a flow control element, such as a control valve or a variable-speed pump. SRTmaster communicates with the SCADA system using the standard OPC protocol. The instruments can be bought from any supplier; Ekster & Associates sells none of them, but provides recommended performance specifications.
Operators keep authority
Operators set the sludge age target and the limits within which SRTmaster operates, and can override it at any time. Every reading is checked before it is used; faulty readings are detected and not acted on, and an activated sludge model detects suspended solids meter drift and alerts operators when recalibration is needed. Before any waste flow is sent, a safeguard checks that it will not overload the clarifiers or sludge processing.
Where it is used
Plants described in the published record include Bellingham (Washington), Victor Valley Wastewater Reclamation Authority, Sacramento Regional, East Bay Municipal Utility District, Oxnard, San Jose/Santa Clara, San Francisco's Oceanside plant, Chico and Topeka, as well as the Pomona Water Reclamation Plant. SRTmaster has been in service at the City of Chico since 2014.