From a watcher’s point of view
Mike Pearce of Envitech examines new innovations in wastewater plant monitoring and control.
On-line monitoring in wastewater facilities, although widely used, has often been viewed with at best scepticism and at worst hostility, due to excessive cost, regular skilled maintenance requirements, pretreatment requirements and high overall cost of ownership.
In response to this, the development of ‘in-situ’ analyzers commenced in the mid 1990s. The sensors for these devices were immersed directly in the channels or basins of the wastewater treatment plant, resulting in real time measurements directly at the sampling point, without the need for pumps, pipe work and pretreatments.
They were however single parameter devices, which still required regular calibration and maintenance.
Now a new generation of instrumentation is about to be introduced to the waste industry, amongst which is the STIP-scan analyzer from Isco-Stip. This analyzer is a multi-parameter in-situ instrument based on UV/Vis-spectroscopy. It is now possible to measure, with one simple probe:
- Spectral absorption coefficient (254 low)
- Chemical oxygen demand (COD)
- Total organic carbon (TOC)
- Total solids (TS)
- Sludge volume (SV)
- Sludge index (SI)
No chemicals are needed for calibration or operation and it is an essentially no maintenance device.
STIP-scan is a UV/Vis-spectroscopic sensor, which operates on the principle of light absorption. The core of which is a miniaturized 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 190 and 720nm. The water sample does not require any pretreatment and is drawn directly into the cell from its immersion position in the process. The quartz settling chamber also serves as the spectrometer cell.
An electrically actuated plunger with wiping seal draws the sample into the settling/measuring chamber. Immediately continuous, rapid measurements are taken over the complete spectrum. The initial values in the visible part of the spectrum are used to indicate the total solids level (TS in g/l). With time 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, an absorption profile is obtained. Mathematical processing of this curve yields the sludge volume (SV in ml/l), and the sludge index (SI).
After approximately 30 seconds of settling, the sample is sufficiently clear for other 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 1 and 5 minutes.
One prerequisite of UV/Vis spectroscopy is the presence of chromophoric (colour giving) groups in the sample water. They show absorption in the UV/Vis range by transferring light energy into an electronic excitation (vibration and rotation) of molecules. Chromophoric groups usually are double bond systems like C=C, C=O, C=N or N=O or triple bond systems in the molecules. Aromatic compounds also absorb in the UV range.
Many chromophoric groups are hydrophilic, increasing the solubility of organic molecules into water. This is the reason why UV light can be used for dissolved organic carbon (DOC) detection. Characteristic spectral absorption can be calibrated against COD or TOC standard laboratory tests and the UV-based parameter with good correlation. Nitrate and some organic molecules exhibit absorption in a narrow spectral range.
With earlier UV-analysers it was impossible to differentiate between nitrate and other organic absorption bonds, however the Stip-scan utilises a mathematical algorithm which compensates for the interfering influences, resulting in reliable measurements of nitrate and COD or TOC in the water sample. In addition to nitrate and COD or TOC, STIP- scan also provides the UV254 absorption. This is a well-established parameter often used to indicate organic loads in natural waters. Due to the use of one absorption wavelength (at 254nm), the parameter gives limited information. COD and TOC better represent organic loads and are environmentally more relevant.
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 in or expelled, special seals on the piston mechanically clean the inner part of the quartz cell. This cleaning routine prevents the formation of precipitates. 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. STIP-scan does not need routine manual cleaning or calibration and is essentially maintenance free.
The analyzer can be used in municipal and industrial wastewater treatment plants for the monitoring and control of:
- MLSS control (sludge wastage rate)
- Organic load
- control and balancing
- De-nitrification control
- Effluent quality monitoring
- Treatment efficiency monitoring
- Unexpected Discharge monitoring and control
- Effluent fingerprinting and tracing
In industrial plants specific organic compounds often exhibit a characteristic absorption spectrum. A defined absorption value within the spectrum can be used as an alarm level. Malfunctions in the plant and potential danger for the environment can be detected quickly and reliably.
The capability to run up to three sensors on one controller also permits the progress of treatment to be followed through the unit processes. It should therefore now open the way for easy application of neural net or fuzzy logic systems to predict effluent outcomes from measured inlet parameters. This same ability may be applied for sensor performance verification by comparing the predicted values from the model with the value obtained by the sensor, alarming when differences exceed a preset tolerance level. The ‘fingerprinting’ of the effluent at different treatment stages also allows identification of unexpected discharges, their possible source and the progress through the treatment plant.
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