Aysix Technologies

- Model SRT - Sludge Retention Time Controller



The Aysix controller is a premier automation system that maintains constant solids retention time (SRT) by changing waste flow rate of an activated sludge system. The control software uses information provided by on-line total suspended solids meters and a waste flow meter. SRT controller is a combination of a control and expert systems that utilizes the latest advances in modeling of activated sludge processes, fuzzy logic and on-line instrumentation.

The most critical parameter to control in a plant is the Food to Mass ratio or F/M. F/M balance is very difficult to accomplish since the F/M test takes several days. A better way of control is to control SRT based on the following:

(Y x F/M) - K = 1/SRT

Y = yield coefficient  (constant)
K = decay coefficient (constant)
If we can control SRT, we can control F/M.
How do we determine SRT:

SRT = Mass of Solids under aeration
Mass of Solids Wasted

Mass of Solids under Aeration = MLSS x Volume
Mass of Solids wasted = WAS (RAS) SS x WAS flow

Basic Architecture of the System

The control system is designed to accept inputs from TSS meters and WAS flow. The unit then sends these valves to a calculation module for SRT calculation. This occurs every 15 minutes.

This block diagram shows the simplicity of the Automatic Waste Control system. The brains of the system is the controller containing custom designed algorithms. The heart of the system is the Aysix Model 1500 Analyzers with 15/17 Sensors.

The Aysix SRT controller is an expert system that examines all the input data and calculated data for correctness and effect on plant operation. The blow table will give an idea as to the data flow.

Values from input instruments for MLSS, WASSS (RASSS) and WAS flow. Note tank volume and number of tanks are used here.

All input values are assumed to be wrong until verified by the system. The processing block will look at historical and future (calculated) value trends to verify the validity of any input values. It will also complete any averaging if more than one input for any one parameter is used.  Very sophisticated statistical analysis is performed on all input valves in the processing block. Once verified the numbers are passed on.

Calculation Block:
This block will do the actual SRT calculation and for the new waste setpoint top the filtration block. This block uses several different SRT calculations to determine the new setpoint.

Filtration Block / Safeguard block
These blocks look at the new setpoint and determine if it complies with all the safety parameters set into the controller are met. It will not pass on a setpoint that would cause problems. These parameters include things like MAX and MINs for MLSS, waste Flow, Waste Load and others. Please see the parameter list.

Actuation Block:
This block supplies either a linear output to be used by an outside control system as a new setpoint or it will control the waste system directly but the use of a built in PID control algorithm.

The combination of these blocks makes the Aysix SRT controller a true expert control system for SRT.

The entire system is a family of Aysix instruments designed to work together to ensure reliable process values and proper control output to make your Activated Sludge Process (ASP) run at its best. The Aysix SRT system uses proven algorithms from Dr. Alex Ekster*, one of the worlds leading authorities on the automatic control of the ASP. In conjunction with Aysix Technologies a simple user friendly control system has been proven effective in reducing sludge wasting volumes, reducing foaming problems, and optimizing nutrient removal, only to mention a few of the key benefits.


  • Settling
  • Nutrient Removal
  • Thickener Performance
  • Clarification
  • Foam Control

  • WEF Award winning SRT control algorithm and sensor fault detection system
  • Computer modeling of each activated sludge system
  • Providing SRT and process optimization
  • Quick SRT change
  • Unique support program
  • Providing guarantees
  • Sable reliability
  • PC based or field mount controllers
  • Optional dual channel systems
  • Expert, field proven control algorithms
  • Aysix proven suspended solids analyzers
  • Custom tuned to your facility
  • Unique sensor fault detection software
  • Output data filtration to ensure no plant upsets
  • Automatic operator notification of potential problem with suggested corrective actions

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