This portable volatile fatty acids (VFA) sensor is an optical sensor developed for in-situ monitoring of VFA in anaerobic processes to support maximising the digestion performance and optimizing the biogas production. VFA are essential intermediate compounds produced during the initial phases of anaerobic digestion and their monitoring is important for the following reasons: their concentration in the reactor is a sensitive indicator for the state of the anaerobic digestion process. VFA monitoring can provide real-time information of the status of the process and thereby support operational decision-making to increase the waste treatment performance. Also, VFA monitoring allows to control the sub-products generated and to foresee and avoid inhibitions of the treatment process.
The schematic (Fig. 1) below shows the light absorbance spectroscopy measurement principle used in the VFA sensor. In this optical setup, ultraviolet (UV) light is collimated through a liquid or gas sample and then measured to calculate the sample light absorbance and correlate it to the VFA molecule concentration.
Figure 1: Light absorbance spectroscopy measurement principle. Source: Own illustration (2023a)
The liquid phase design is based on spectroscopy absorption measurements in a glass cuvette. Ultraviolet/visible (UV/VIS) light from a Deuterium/Halogen lamp is guided through fibre to the glass cuvette, where absorption takes place, then collimated back into a fibre and guided back to a UV/VIS spectrometer. The value peak of the absorption is directly correlated with the VFA concentration (this has been demonstrated for laboratory synthetic samples).
The VFA sensor is put in a portable suitcase and measurements can be carried out by a trained operator, who collects the sample and deposits on the device.
Figure 2 Design for the final integrated portable VFA measurement unit. Source: Own illustration (2023b)
Figure 2 above shows the design for the final integrated prototype automatized and portable for VFA measurement and data analysis and transfer.
The VFA sensor is employed to determine single-observation measurements on-site. The samples are drawn manually and prepared for analysis by being diluted and then filtered with a syringe filter of 0,45 µm (micrometre).
For user-friendly operation and maintenance, the sensor is equipped with a touchscreen with an intuitive interface that allows to work in two different modes:
- Maintenance mode which enables to adapt calibration and operation parameters.
- Operator’s mode in which the system only allows to run the measurements program and start the measurement.
The obtained results are an average of 20 consecutive measurements carried out by the sensor and displayed on the screen in less than 60 seconds.
This sensor is prepared to connect to a communication module which sends data to a network gateway and allows upload to a cloud via GPRS (General Packet Radio Service). Also, there is an in-built storage module in the sensor that stores the last measurements and allows to export data to an USB (Universal Serial Bus) pen drive for off-line analysis of the data.
The sensor was designed to for in-situ monitoring of anaerobic digestors. The system contains a battery which allow to work several hours off-grid, which is ideal for measuring at remote places or as a portable device. As the measurements require only a few seconds, the autonomy of the sensors will allow measurements to be taken for weeks without recharging the battery.
Operation and Maintenance
Being integrated in a portable suitcase, the sensor is designed as straight-forward device, which can be used by any operator after having received basic training. Every component was selected to work with a minimum number of consumables.
The measurement operation is automated, except for the sampling. The operator needs to collect and place the sample on a designated cuvette inside the suitcase sensor and launch the measurement. The measurement process, the sampling preparation, the data treatment, the results display and the data upload to the cloud will run automatically without further instructions.
The maintenance requirements are minimal, with the main activity to be prepared for is the cleaning of the measurement cavity after each measurement. The UV lamp has a limited lifetime, so it will have to be replaced at some point. Recalibration is also contemplated, and operators will be trained for it in case any malfunction is detected.
Experiences in India
This device has been designed and tested under PAVITR and has prior to that not been deployed anywhere else in India.
This device has been designed and tested under PAVITR and has prior to that not been deployed anywhere else.