Endress Hauser have launched an inline analyser for municipal and industrial wastewater treatment plants. The new STIP-scan device overcomes the costly sampling and maintenance procedures required by historical cabinet analysers since it operates directly in the process.
On-line analysis has been used for monitoring and process control in wastewater treatment for approximately 20 years. The on-line analysis market started with cabinet analysers which require sample preparation systems containing components such as pumps and filtration units. These components increase the cost of ownership and require maintenance and prompted the development of in-situ analysers in the mid 1990s.
The in-situ analyser probes are immersed directly in the channels or basins of the wastewater treatment plant and the results indicate real conditions directly at the sampling point. By doing so, additional costs for pumps, pipe work and their maintenance are avoided.
Endress+Hauser’s STIP-scan inline analyser has the added benefit of allowing the measurement of multiple parameters simultaneously. Using a single probe, nitrate, chemical oxygen demand (COD) or total organic carbon (TOC), total solids (TS), sludge volume (SV), sludge index (SI) and spectral absorption coefficient (254 low) can be measured.
STIP-scan is a UV/ Vis-spectroscopic sensor which operates on the principle of light absorption. The core of STIP-scan technology is a miniaturised spectrometer. A xenon lamp sends light flashes through a measurement cell containing the sample. After being transmitted through the sample, the light is collected and recorded by a photo diode array spectrometer in the wavelength range between 190nm and 720nm. The water sample does not require any pre-treatment and is drawn directly at the measuring chamber. The quartz settling chamber also serves as the spectrometer cell. Immediately after sampling, the particles and sludge flocs start to settle. The settling conditions in the measuring cell are not influenced by turbulence outside the measuring system.
During the settling process, continuous, rapid measurements are taken of the total solids (TS in g/l) using the absorption of visible light. From the results of the settling kinetics, a settling curve is recorded and the sludge volume (SV in ml/l) is calculated. Particles in the light-path normally influence the accuracy of the measurements. The settling procedure compensates for this interference, making the resulting data very accurate.
After approximately 30 seconds of settling, the sample is sufficiently clear for analysis. Nitrate, spectral absorption coefficient (254 low) and chemical oxygen demand (COD) or total organic carbon (TOC) are measured. The system determines the end of the measuring cycle when the measured values become stable. The sample is expelled out of the measuring cell and a new measurement cycle is initiated. Depending on the settling characteristics of the wastewater, the complete measurement cycle takes between one and five minutes.
The fluctuation of the lamp intensity, precipitation and discoloration of the cell are usually limitations of spectroscopic measurements. These effects have been taken into account when developing STIP-scan. Each time a sample is drawn or expelled, the inner part of the quartz cell is mechanically cleaned by special seals on the piston. This routine prevents the formation of precipitates; STIP-scan typically, does not need cleaning or calibration.
Discoloration is compensated for by means of a reference measurement before each measurement cycle. The sample piston stops, and the light-beam of the spectrometer is directed through a hole in the piston. The drift of the light source and any potential discoloration of the cell are then automatically compensated for.
Endress+Hauser’s STIP-scan inline analyser does not need any chemicals for calibration or operation and essentially operates with no maintenance. The system can be installed in the inlet, in the aeration basin and in the effluent.