Sewage treatment process generally consists of mechanical treatment, biochemical treatment and chemical treatment, involving liquid, solid and gas components. Instruments for monitoring these stages can be simply divided into two categories: general-purpose and special-purpose ones.
2. General instrument for sewage treatment process
General measuring instruments include temperature, pressure, liquid level, flow rate, pH value, conductivity, suspended matter and other sensors.
① Temperature sensor is more important, because temperature control is often implemented during anaerobic digestion. A typical temperature measuring element is a thermal resistor.
② Pressure measurement is often used as an alarm parameter for aeration and anaerobic digestion.
(3) Liquid level measurement is used for water level monitoring. Buoy, differential pressure transmitter, capacity measurement, ultrasonic water level detection and other methods are usually adopted.
④ Flow monitoring instruments mainly include diaphragm, rotor flowmeter, turbine flowmeter, target metering tank, electromagnetic flowmeter and ultrasonic flowmeter.
⑤pH value is an important variable in biochemical process, and also a key value in anaerobic digestion and nitrification. Usually, pH electrodes are installed in sewage treatment plants to soak human sludge, and long-term maintenance-free can be achieved through different cleaning strategies. For wastewater with high buffering capacity, pH measurement may be insensitive to process changes, so it is not suitable for process supervision and control. In this case, a carbonate measuring system can be used.
⑥ Conductivity sensor is used to monitor the change of influent composition, and it is also the basis of chemical phosphorus removal control strategy.
⑦ The traditional biomass measurement is based on the scattering and absorption of incident light by suspended particles. With the emergence of sensitive light detectors, sensors that can automatically measure light effects have come out. Most commercial sensors use light sources that emit low visible or infrared light, and most media show low absorption rate in this region. Biomass concentration can also be determined according to the speed difference between suspended solids and microorganisms in free solution by ultrasonic wave.
3. Sensors in anaerobic digestion process
The measurement of biogas flow is widely used in anaerobic digestion, which can indicate the overall activity of the reactor. In recent years, some special technologies have been used to monitor gas composition. The typical laboratory method is bottle washing and separation, and the gas composition can be determined according to the flow ratio before and after entering the bottle. For example, alkali washing bottles will be able to collect all CO2 and H2S and allow CH4 to pass through. A more professional gas analyzer can directly monitor the content of gas components. For example, infrared absorption instrument is used to determine the contents of CO2 and CH4, and a special hydrogen analyzer is also developed based on chemical power supply. The gas-phase H2S measuring instrument can determine the content of H2S by monitoring the reaction of sulfide to lead dissolution.
The main problem of monitoring system based on gas analysis is that it can not directly predict the concentration of corresponding gas in liquid phase. An immersion sensor which can directly measure dissolved hydrogen is developed. Fuel cell is the core of the sensor. Direct measuring instruments for H2S and CH4 have not been reported so far.
It is not easy to detect the unbalanced anaerobic digester by pH measurement, especially when the alkalinity of the mixed solution is high. In this case, CO2 and carbonate in the mixed liquid can be measured. Alkalinity mainly depends on carbonate buffer, so it is often used in the control strategy of anaerobic digestion. The carbonate monitor has been developed and applied to the actual anaerobic digestion process.
There are two basic principles for estimating carbonate alkalinity. One is titration. Advanced on-line titration sensors can simultaneously monitor different components, such as ammonia and carbonate. Another method for on-line determination of alkalinity is based on the quantification of gaseous CO2 obtained by sample acidification. A gas flowmeter can be used to measure the volume of generated gas.
All biological activities can be characterized by heat production. Measuring heat with calorimeter can directly insight into the changes of biological processes. Flow calorimeter is the first choice for sewage treatment.
Volatile fatty acids (VFA) are the most important intermediates in anaerobic digestion. Their aggregation will lead to the decrease of pH value and the failure of anaerobic digestion. VFA concentration monitoring is usually used as an index of process performance, but online sensors are rarely used. The most advanced measuring instruments include gas chromatograph or high pressure liquid chromatograph. As an on-line multi-parameter sensor, Fourier transform infrared spectrometer can simultaneously measure COD, TOC, VFA and other parameters. FT-IR does not need to add any chemicals and rarely needs maintenance, but its calibration is difficult. A more reliable measurement method is to provide the VFA content in the sample by two-step titration or back titration with a titrator.
In recent years, biosensors have been developed and applied in sewage treatment industry. VFA analyzer can determine the concentration of VFA in digestive juice; MAIA biosensor can measure metabolic activity; RANTOX biosensor is used to detect the upcoming organic overload and toxic load.
4. Sensors in activated sludge process
Oxygen plays a very important role in activated sludge process, and the related aeration cost accounts for about 40% of the total operating cost, so oxygen sensor has become the most widely used measuring and monitoring instrument in sewage treatment plants. Oxygen measurement is based on the electrochemical reaction of diffused oxygen in liquid. Dissolved oxygen (DO) sensor is a reliable and accurate measuring instrument, but the appropriate measuring position must be carefully selected to prevent scaling. At present, automatic cleaning system is quite common, and some dissolved oxygen sensors with cleaning system and self-calibration have been applied. DO sensor is widely used in the control of aeration process, which saves a lot of investment, and the information obtained can also be used to monitor any activated sludge treatment process.
Respiration is the measurement and explanation of activated sludge respiration rate, which is defined as the oxygen consumed by microorganisms in unit volume of activated sludge in unit time. It is a common tool to characterize the kinetics of wastewater and sludge. Breather is essentially a reactor, and the measurement results are easily influenced by the changes of experimental conditions.
Biodegradable components of wastewater were obtained by off-line measurement of BOD5. BOD5 is the amount of dissolved oxygen required for biological oxidation of organic solute within 5 days. BOD5 experiment is not suitable for automatic monitoring and control, because it takes a long time to complete the experiment and it is difficult to achieve consistent and accurate measurement. On-line measurement of wastewater load is realized according to short-term BOD estimation. At present, there are two online BODst methods: breath measuring instrument and microbial sensor. The respiratory sensor RODTOX proposed by Vanrolleghem et al. can monitor the potential toxicity of BODst and wastewater. The sensor consists of a batch reactor with constant aeration and complete mixing, which contains 10 liter of sludge, and can obtain BODs with large dynamic range. Microbial sensor is composed of solidified battery, membrane and a dissolved oxygen detector, which is most suitable for activated sludge system containing many microorganisms. In order to maintain its efficacy, microbial BOD sensors need to be carefully maintained and stored. Most microbial BOD sensors have a short life span, from a few days to several months.
The most widely monitored variable in sewage treatment plants is chemical oxygen demand. COD automatic monitor can be automatically monitored once every 1~2 hours, and can be divided into acid monitor and alkaline monitor according to the oxidative decomposition. The main limitation of COD experiment is that it is impossible to distinguish biodegradable and inert organic matter.
TOC represents the content of total organic carbon in sewage, and it is also an index to characterize the degree of organic pollution in water. The main principle of TOC measurement is to convert organic carbon into CO2, and then measure this product in the gas phase to calculate the concentration of organic carbon in the water phase. The typical measuring instrument is infrared extraction analyzer. TOC is considered as a good monitoring parameter, especially for monitoring drainage quality.
Many waste water components absorb ultraviolet rays. The absorption of ultraviolet rays is closely related to the organic matter in wastewater. Automatic ultraviolet absorption monitor is introduced into wastewater treatment system to detect the degree of water pollution or evaluate the discharge quality. In recent 10 years, remarkable progress has been made in optical technology, which makes it possible to measure at long distance and multi-points, and greatly facilitates the implementation of sewage treatment process monitoring. Infrared spectrum measurement has great potential in the estimation and on-line monitoring of TOC, COD, BOD and other special parameters. The main disadvantage of infrared spectrometer is that the scaling of photovoltaic module will reduce the sensitivity and need frequent recalibration.