How Shepherd Works

Based on BACTEST’s patented CYTOMAIA technology platform, Shepherd uses manometric respirometer technology which measures the activity of biomass in Activated Sludge and translates this into an accurate measure of the loading on the plant (through BOD – Biochemical Oxygen Demand), and the response of the biomass to the incoming food source.  Using this measure, the correct aeration can be applied to optimise the process and its associated costs, carbon footprint, and other parameters such as biological phosphorous removal.

By measuring the activity of the biomass Shepherd’s software can determine toxic events, equipment failure and other process issues that affect respiration and the health of the Floc. Similarly, nutrient availability associated with chemical dosing (e.g. phosphate removal) or nutrient additions, can be monitored and controlled, based on the response of the biomass.

Manometric respirometers have the advantage of measuring pressure changes associated with microbial activity, and utilising barrier-protected sensors, require very little maintenance, and no calibration once aligned with the plants’ operation.

Shepherd comprises a combination of a robust test chamber, sensors and a data logger as well as algorithms and curve stabilizing software which monitors the chamber, processes the information, and converts it into a visual output. Proprietary software converts the BOD5 result into a recommended aeration requirement taking into account the oxygen delivery characteristics of the plants aeration system.


The test chamber is a reaction vessel of 2 litre working volume and 1 litre headspace which is supported in a buoyancy collar and tethering device, and floats in the activated sludge lane. The sample is drawn directly from the activated sludge, analysed and returned on a programmable cycle of approximately one hour. The absence of pumping for sample loading and unloading prevents fouling of the system.

The floating sensor head allows consistent sample volume due to simple hydraulics and mediates temperature to the operation of the process at that time.


When a sample is captured, the chamber is sealed, and the test begins.

The test chamber houses an air recirculation system that draws air from the headspace and aerates the sample using a fine bubble diffuser, and converts the biomass’ respiratory gas exchange, and hence its activity, into a headspace pressure variable. Barrier-protected sensors monitor sensitive pressure changes within the test chamber and data loggers convert them into distinct respirograms. Proprietary software interrogates the respirograms, performs proprietary calculations, and converts them into a near real-time BOD5 proxy and an overall estimate of status, suitable for managing and optimizing plant processes.

Further calculations are performed which determine the recommended aeration based on the mass transfer and oxygen delivery capability of the AS aeration system. Results are communicated to a local display and to a cloud-based dashboard, the latter including trending and historical data.

A calculation of F:M ratio is also performed, based on a manual MLSS value.

STEP 1. Sample Gathering:

Top and bottom valves open allowing the sample to enter the test vessel to a pre-determined volume as the sample reaches the same height as the outside sludge.

STEP 2Monitoring:

  • Top and bottom valves close making the vessel completely air tight.
  • The circulation pump drives head space air through a fine bubble diffuser for mixing and rapid gas exchange.
  • Oxygen used by respiring microorganisms creates a pressure variable in the vessel, which is measured by barrier protected sensors.
  • Data is transferred to software that converts the pressure change into information and operational advice that is communicated through the local control panel display, cloud-based dashboard, and E mail.

STEP 3. Sample expulsion:

  • Top and bottom valves open and air is forced into the vessel under pressure to drive out the sample and clean components.
  • Pressurised air-flow stops, and another sample enters again in Step 1.

STEP 4. Data processing:

On completion of the test cycle, the captured data is analysed to generate a BOD5 proxy and a recommended airflow, which is then displayed directly on the control panel.
This process information is sent to the cloud-based dashboard which in turn generates an email that is sent to all subscribers.

Process information can be communicated from the local control cabinet to SCADA or other monitoring and control systems via 4-20mA analogue or digital Modbus signal for process control purposes.