Reagent free testing: COD, Nitrates, Nitrites
Spectral measuring technology in wastewater treatment plants and laboratories
The cumulative parameter COD, is a measure for the organic load of wastewater. Nitrate, nitrite and ammonia are important parameters for the nitrogen elimination in wastewater plants. Along with phosphate, nitrate is an essential plant nutrient that contributes to the eutrophication of open waters. High concentrations of nitrite are toxic to fish. This is why it is important to keep the concentrations of these parameters as low as possible and within the required limits in the effluent of wastewater treatment plants.
At a wastewater treatment plant it is part of the daily routine to use cell tests for determining the chemical oxygen demand (COD) in the influents and effluents, as well as the nitrogen parameters such as nitrate and nitrite in the effluent. Years ago the continuous monitoring of these parameters in the influents and effluents of wastewater treatment plants together with the stabilisation and optimisation of those plants required expensive, high maintenance and technically complex wet-chemical analysers.
Optical Online Spectral Sensors
The first online spectral sensors came into use in wastewater treatment plants at the beginning of the new millennium. They were able to measure the COD, nitrate and nitrite parameters in the influents and effluents as well as the aeration tanks continuously. Using a spectral measurement and its evaluation it was possible to calculate and then immediately show the concentration of these parameters. In the second generation of spectral sensors it became possible to reduce the manual maintenance work through the introduction of ultrasonic cleaning. The optimised measuring procedure also ensured higher measuring accuracy.
The operating principle is based on a direct, spectral absorbance measurement covering a wavelength range of 200–390 nm (UV sensors for COD, nitrate and nitrite), or of 200–702 nm (UV-VIS sensors for COD, nitrate and TSS). The measured spectrum is analyzed over the whole wavelength range and the concentration is calculated using a complex algorithm or evaluation model which is stored in the sensor. There are different evaluation models for the various parameters of the influent, the biology and the effluent in wastewater treatment plants depending on the composition of the water.
The evaluation models used can be employed at municipal wastewater treatment plants and can be individually adjusted for maximum measuring accuracy at each plant. This adjustment is carried out using a 1 or 2 point calibration with reference values from conventional laboratory procedures – usually with commercial photometric test kits.
Meanwhile the spectral measuring technology in wastewater treatment plants has established itself as a reliable method for monitoring and as a procedure for regulating and control of biological processes. It provides very good results, simple application, low maintenance costs and does not require the use of chemicals. Its fields of use range from the detection of COD influent load and load peaks to the monitoring of COD, nitrate and nitrite in the effluent. The COD measurements taken in the influent and effluent make it also possible to carry out carbon balances. This measuring technology also plays an essential role in the optimised and cost efficient regulation and control of processes such as nitrification and de-nitrification.
Transfer to the Laboratory
The determination of COD, Nitrate and Nitrite for documentation for self-monitoring purposes usually takes place in the laboratory: nowadays commercial photometric tests are used almost exclusively – the advantage over DIN methods being the very low volume of reagents needed and their simple handling. Good laboratory practice (GLP) requires at least to duplicate if not triplicate the determination withcell tests as well as a control standard and therefore represents a certain cost factor. COD cell tests in their use of potassium dichromate and mercury sulphate are hazardous to the health and the environment. Not least a reason why REACH has put potassium dichromate on the list of substances causing high concern. These environmental and health aspects are other reasons why spectral measuring procedures have been introduced in laboratory analytics. The optical reagent free (OptRF) methods for COD, nitrate and nitrite are now available in the laboratory.
Unlike spectral sensors, the methods with the laboratory photometer are limited to measurements in the effluent of wastewater treatment plants. Contrary to the influent and the aeration, under normal conditions, there are hardly any particles in the effluent which can have a disturbing influence on the measurement because of their sedimentation behaviour. The algorithm could not be transferred 1:1 due to the technical differences between the online measuring process and the laboratory photometer measurement. Instead there was an optimisation regarding the evaluation and calculation in the photometer.
The measurement is menu-guided and it is quite simple. A representative sample is taken from the effluent, pipetted into a 10mm quartz cuvette and the desired measurement for COD, nitrate or nitrite started. The result can be read immediately. During the development of the test method a lot of work was put into making it easy to handle for the user. The user can also read extra information and tips on the screen. If the concentration of the measuring parameter is too high, the user is asked to dilute the sample. If the measured spectrum deviates substantially from the underlying spectra it is an indication that the composition of the sample is different from the “standard” effluent wastewater. A recommendation to carry out a user calibration with the aid of a 1 or 2 point calibration using reference values will then pop up. If the measured spectrum deviates from the underlying spectra too much, it is impossible to carry out a determination of the sample. An appropriate message will appear stating that there is an unsuitable sample matrix.
As all wastewater treatment plants are slightly different to each other in regards to the effluent matrix, it is generally advisable to carry out a user calibration for each parameter. This will ensure the best results.
How does the procedure contribute to the future?
The composition of the sample is an essential criteria for the application of the measurement. The method is already being used in municipal outlet channels, even in some surface waters. However, before being put into use it should be tested and then regularly verified. Even if tests for reference and self-monitoring continue to be required, the OptRF measurements in the laboratory make it possible to carry out daily controls, retention samples and preliminary checks of the expected measuring range quickly and free of charge.