Research status of sewage treatment process instrumentation

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　1. Introduction The monitoring and control system of the sewage treatment process consists of four parts: model, sensor, local regulator and upper-level monitoring strategy. Among them, the sensor is the weakest, most important and basic link in the sewage treatment plant monitoring system. Increasingly stringent sewage discharge standards have led to the complexity of sewage treatment processes and equipment, and have also put forward higher requirements for the performance of sensors used for monitoring and controlling sewage treatment processes. This has promoted the development of sensor technology in the field of sewage treatment. Some new sensors suitable for sewage treatment processes have come out one after another. The sewage treatment process is a complex biochemical reaction process involving a wide variety of instruments and meters. Most sensors are unique to the sewage treatment process and are used in different situations to reflect changes in status information of one or more specific variables.             The sewage treatment process generally consists of mechanical treatment, biochemical treatment and chemical treatment, which involves three material components: liquid phase, solid phase and gas phase. Instruments that monitor these phase states can be simply divided into two categories: general purpose and specialty.            2. General instruments for sewage treatment process General measuring instruments include temperature, pressure, liquid level, flow, pH, conductivity, suspended solids and other sensors.             ①Since temperature control is often implemented in the anaerobic digestion process, temperature sensors are even more important. A typical temperature measuring element is a thermal resistor.             ②Pressure measurements are often used as alarm parameters for aeration and anaerobic digestion processes.             ③Liquid level measurement is used for water level monitoring, usually using methods such as buoys, differential pressure transmitters, capacity measurements, and ultrasonic water level detection.             ④Flow monitoring instruments mainly include Kanban, rotor flowmeter, turbine flowmeter, target metering tank, electromagnetic flowmeter, ultrasonic flowmeter, etc.             ⑤pH value is an important variable in the biochemical process, and it is also a key value in the anaerobic digestion and nitrification process. pH electrodes are usually installed in sewage treatment plants and immersed in the sludge. Long-term maintenance-free can be achieved through different cleaning strategies. For wastewaters with a high buffering capacity, pH measurement may be insensitive to process changes and therefore not suitable for process monitoring and control. In this case, a carbonate measurement system can be used instead.             ⑥Conductivity sensors are used to monitor changes in incoming water composition and are the basis of chemical phosphorus removal control strategies.             ⑦Traditional biomass measurements are based on estimates of the scattering and absorbance of incident light by suspended particles. With the advent of sensitive light detectors, sensors capable of automatically measuring light effects were developed. Most commercial sensors use a light source that emits low visible or infrared light, a region in which most media exhibit low absorbance. Biomass concentration can also be determined based on the speed difference of ultrasonic waves in the free solution between suspended solids and microorganisms.            3. Sensors in the Anaerobic Digestion Process The measurement of biogas flow is widely used in the anaerobic digestion process, and it can indicate the overall activity of the reactor. In recent years some specialized techniques have been used to monitor gas composition. A typical laboratory method is the bottle washing separation method. The gas composition can be determined based on the flow ratio before entering the bottle and after exiting the bottle. For example, a caustic wash bottle will be able to collect all CO2 and H2S while allowing CH4 to pass through. More professional gas analyzers can directly monitor the content of gas components. For example, infrared absorption measuring instruments are used to determine the content of CO2 and CH4. Special hydrogen analyzers have also been developed based on chemical power sources. Gas phase H2S meters can determine H2S levels by monitoring the reaction of sulfides to lead stripping.             The main problem of monitoring systems based on gas analysis is that the concentration of the corresponding gas in the liquid phase cannot be directly predicted. Immersion sensors that can directly measure dissolved hydrogen have been developed. The fuel cell is at the heart of this sensor. Direct measuring instruments for H2S and CH4 have not been reported so far.             pH measurement is not easy to detect in unbalanced anaerobic digesters, especially when the alkalinity of the mixed liquor is high. In this case, CO2 and carbonate in the mixed liquid can be measured. Alkalinity depends primarily on carbonate buffers and is therefore often used in control strategies for anaerobic digestion. Carbonate monitors have been developed for use in practical anaerobic digestion processes. There are two basic principles for estimating carbonate alkalinity. One is the titration method. Advanced online titration sensors can monitor different components such as ammonia and carbonate at the same time. Another method for online determination of alkalinity is based on the quantification of gaseous CO2 obtained by acidifying the sample. A gas flow meter can be used to measure the volume of gas produced.             All biological activities can be characterized by the production of heat. Measurement of heat by calorimeters provides direct insight into changes in biological processes. The first choice for wastewater treatment processes is the flow calorimeter.             Volatile fatty acids (VFAs) are the most important intermediate products in the anaerobic digestion process. Their aggregation can cause a decrease in pH and lead to failure of the anaerobic digestion process. VFA concentration monitoring is commonly used as an indication of process performance, but online sensors are rarely implemented. State-of-the-art measuring instruments include gas chromatographs or high-pressure liquid chromatographs. Fourier transform infrared spectrometer (FT-IR), as an online multi-parameter sensor, can simultaneously provide measurements of COD, TOC, VFA and other parameters. FT-IR does not require the addition of any chemicals and requires very little maintenance, but it is difficult to calibrate. A more reliable measurement is to use a titrator to provide the VFA content in the sample via a two-step titration or titration backtitration.             Biosensors have been developed and applied in the sewage treatment industry in recent years. VFA analyzer can determine VFA concentration in digestive fluids ； MAIA biosensor measures metabolic activity ； RANTOX biosensors are used to detect impending organic overload and toxic loads.            4. Sensors in the activated sludge process Oxygen plays a very important role in the activated sludge process, and the related aeration costs account for about 40% of the total operating costs. Therefore, oxygen sensors have become the most widely used measurement and monitoring instruments in wastewater treatment plants. Oxygen measurement is based on the electrochemical reaction of diffusing oxygen in a liquid. Dissolved oxygen (DO) sensors are reliable and accurate measuring instruments, but the appropriate measurement location must be carefully selected and fouling must be prevented. At present, automatic cleaning systems are quite common, and some dissolved oxygen sensors equipped with cleaning systems and capable of self-calibration are already in use. DO sensors are widely used for the control of aeration processes, saving a lot of investment, and the information obtained can also be used to monitor any activated sludge treatment process.             Respiratory capacity is the measurement and interpretation of the respiration rate of activated sludge, which is defined as the oxygen consumed by microorganisms in unit volume of activated sludge in unit time. It is a common tool for characterizing wastewater and sludge dynamics. The respirometer is essentially a reactor, and the measurement results are easily affected by changes in experimental conditions.             The biodegradable composition of wastewater was obtained by a standard method of offline measurement of biological oxygen demand (BOD5). BOD5 is the amount of dissolved oxygen required for biological oxidation of organic solutes within 5 days. The BOD5 experiment is not suitable for automated monitoring and control because it takes a long time to complete the experiment and it is difficult to achieve consistent accurate measurements. Online measurement of wastewater loads is based on short-term BOD estimates. There are two online BODst methods currently used: Breathalyzers and microbial sensors. The respiration measurement sensor RODTOX proposed by Vanrolleghem et al. can monitor BODst and potential toxicity of wastewater. The sensor consists of a constantly aerated, fully mixed batch reactor containing 10 liters of sludge and can obtain BODs within a wide dynamic range. The microbial sensor consists of a cured cell, membrane, and a dissolved oxygen detector and is best suited for activated sludge systems containing a wide variety of microorganisms. To maintain their efficacy, microbial BOD sensors require careful maintenance and storage. Most microbial BOD sensors have a short life span, ranging from days to months.             The most widely monitored variable in wastewater treatment plants is chemical oxygen demand (COD). The COD automatic monitor can perform automatic monitoring every 1 to 2 hours. According to the conditions of oxidation and decomposition, it is divided into acid method monitor and alkaline method monitor. The main limitation of the COD experiment is the inability to distinguish between biodegradable and inert organic matter.             TOC represents the total organic carbon content in sewage and is also an indicator of the degree of organic matter pollution in water bodies. The main principle of TOC measurement is to convert organic carbon into CO2, and then measure this product in the gas phase to determine the organic carbon concentration in the water phase. A typical measuring instrument is an infrared extraction analyzer. TOC is considered a good monitoring parameter, especially for monitoring drainage quality.            Many wastewater components absorb UV light. The absorption of ultraviolet rays is closely related to the organic matter in wastewater. Automatic ultraviolet absorbance monitors are introduced into wastewater treatment systems to detect water pollution levels or evaluate discharge quality. In the past 10 years, optical technology has made significant progress, making remote and multi-point measurements possible. * * It facilitates the implementation of sewage treatment process monitoring. Infrared spectrum measurement has great potential for the estimation and online monitoring of special parameters such as TOC, COD, and BOD. The main disadvantage of infrared spectrometers is that fouling of photovoltaic components can cause a reduction in sensitivity and require frequent recalibration.            5. Sensors for nutrient removal processes The purpose of nutrient removal systems is to remove nitrogen and phosphorus from wastewater through biological, chemical or combined treatment methods. The current mainstream method is biological nitrogen and phosphorus removal. Under oxygen-rich conditions, ammonia in wastewater is oxidized to nitrate (nitrification process), and phosphorus-accumulating bacteria absorb phosphorus in wastewater and store it in the body in the form of polyphosphorus (phosphorus absorption) ； Under anoxic conditions, nitrate in wastewater is converted into nitrogen and eliminated (denitrification) ； Under anaerobic conditions, polyphosphorus decomposes and releases inorganic phosphorus into the sludge (phosphorus release). To ensure satisfactory operation of the instrument, most commercial measurement systems still require the use of pretreated samples. Ultrafiltration units (UF) are often used to implement sampling preprocessing. The principle of semi-micro continuous flow analysis system established based on diaphragm technology is widely used in ammonia, nitrate, phosphorus and other nutrient sensors. These sensors are based on colorimetry and can be automatically calibrated. The disadvantage of this type of sensor is that multiple measuring points cannot be connected to one measuring device, whereas UF units allow multiple parallel UF units connected to different sampling points using one meter. Since reliable sampling preparation units have become available, considerable efforts have been devoted to the automated online application of typical experimental methods in wastewater treatment plants. There are currently three implementation options: batch sample chemical analysis, continuous flow-through system based on the principle of flow injection analysis (HA), and sequential injection analysis (SIA). FIA is the most commonly chosen online measurement method. Its main feature is that the analysis reaction does not need to reach equilibrium, because the reaction time of sample dilution and injection and detection can be regenerated at a constant carrier flow rate, but the pump must be selected carefully. SIA is an improvement of HA. Its main feature is to replace the multi-pipeline of FIA with a multi-position valve. SIA increases measurement flexibility. SIA and FIA systems offer the advantages of small samples, low reagent utilization, and high sampling throughput compared to batch systems. The colorimetric NH4+ analyzer consumes a large amount of reagents and is sensitive to changes in sampling temperature. Colorimetric automated orthophosphate analyzers have proven accuracy but are expensive to operate.             ORP (Oxidation Reduction Potential) electrodes can be commonly used to indicate the oxidation status of the system being monitored. Compared with DO electrodes, ORP electrodes can also provide information on biochemical processes that occur under anoxic and anaerobic conditions. From a technical perspective, ORP measurements can be considered accurate and unproblematic, but processes should not be controlled based on absolute ORP values. ORP measurements can be interpreted in terms of breaks or inflection points on the ORP curve. The inflection point can characterize the appearance or disappearance of a redox buffer system and can be compared to the pH buffer system in acid titrations. The most famous ORP breakpoints are DO breakpoint and NO3- breakpoint. The DO breakpoint means the disappearance of NH4+ in the oxygen-rich phase (the end point of nitrification), while the NO3- breakpoint means the disappearance of NO3- in the anoxic process (the end point of denitrification).             A large number of ion selective electrodes (ISE) use electrochemical reactions to monitor specific chemical components such as NH4+, NO3-, S2- and so on. Nitrate ISE has the advantages of low chemical consumption, no or only a small amount of pretreatment, and short response time. However, the system is sensitive to electrode pollution, electrode drift, ion interference, etc. However, the phenomenon of electrode drift in nitrate detectors can be overcome by implementing an automated field calibration method. NH4+ISE is the preferred method for measuring NH4+, with limited operational issues related to clogging, electrode drift, hydroxide poisoning of the electrode, and bubble retention at the end of the electrode.             Nitrate content can be determined using the absorption of ultraviolet (UV) light by nitrate at 210 nm. The advantage of the UV absorption nitrate analyzer is that it does not require much maintenance and has a short response time (only 10s). UV technology is more suitable for wastewater with low organic content. However, large amounts of organic matter are also present in wastewater in the W absorption region, and despite many efforts to compensate for this, UV absorption measurements are still subject to such interferences. To prevent baseline drift, frequent zero calibration is required. Automated cleaning and autocalibration are already integrated into commercial products.             The titration sensor obtains relevant information about the nitrification process based on the stoichiometric relationship of NH4+ converted to 2H+. There is a clear relationship between the ammonium added to the sludge and the ammonium measured by a titration sensor, which can be obtained by applying a stoichiometric conversion factor to measure the amount of protons produced during ammonium nitrification. This measurement principle is used for online measurement of nitrification reaction rates in activated sludge, online ammonium concentration measurement, wastewater toxicity measurement, and measurement of nitrifiable nitrogen. Compared with existing online NH4+ analyzers, the titration sensor does not require sampling pre-treatment. In addition, the titration process does not require expensive and environmentally friendly chemicals. The disadvantage of the titration sensor is that its response time changes with the concentration of NH4+ in the sludge sample and the nitrification rate of the sludge.             The distinguishing feature of nitrification processes is the consumption of large amounts of oxygen, so respirometers can be used to monitor these processes. The application of respirometers in nitrogen removal processes is not limited to the estimation of nitrification rates, but can also be used to determine the concentration of nitrifiable nitrogen in wastewater treatment plant influents. A combined respiration-titrator was used to monitor the degradation process during activated sludge batch experiments. This respirometer consists of an open aeration tube and a closed non-aeration breathing chamber, and collects two-channel oxygen absorption rate information at high frequency through two oxygen probes. The respirometer is combined with a titration unit that maintains the pH, the added acid and the amount of matrix as complementary sources of information on the degradation process. An integrated sensor has recently emerged that can monitor nitrification, denitrification and oxygen-enriched carbon source degradation processes in one device. This sensor obtains rich informative data from breath titrators and nitrate ISE measurements at high frequencies.             Oxygen uptake rates under oxygen-enriched conditions are a good indicator of sludge activity, but evaluation of the metabolic status of cells under anoxic and anaerobic conditions in nutrient removal wastewater treatment plants cannot use this reliable measurement method. In this case, monitoring NADH fluorescence can be used instead. The measurement of intracellular redox status by the NADH fluorescence signal is valuable in determining the metabolic status of microorganisms. The end point of denitrification in the alternating activated sludge process can be detected using an NADH fluorometer.            6. Measurement of the sedimentation process. As the last process in the sewage treatment plant, any mistakes in the secondary sedimentation tank will directly affect the quality of the effluent. However, in current scientific research, relatively little attention has been paid to this process monitoring and measurement issue.             There are currently three sludge interface positioning measurement principles put into practical application.: Ultrasonic absorption and turbidity equipment detects suspended solids interfaces, and ultrasonic scanning devices provide concentration profiles. The third method is considered the best measurement method. Turbidity sensors with rotating drums are the most widely used. The accuracy of the turbidity detector can be reduced until it reaches the sludge layer, and the distance it extends is the depth of the sludge layer. As long as proper maintenance and cleaning are carried out, such measuring systems give reliable results. There is a different detection instrument consisting of three turbidimeters, which are fixedly installed in different positions of the sedimentation tank and can detect whether the sludge layer appears in these positions. This is a more reliable instrument because it avoids mechanical problems caused by the presence of a rotating drum. The sludge layer can be adjusted through a control strategy based on the conditions detected by the intermediate position probe. The signals from the other two turbidimeters can be used for alarm triggering.             Sludge settling characteristics are usually expressed by the sludge volume index (SVI). This parameter is calculated by dividing the 30-minute sludge settling volume by the suspended solids concentration. SVI is severely affected by sludge concentration. Advances in science and technology have promoted the development of sensors for measuring sludge settling characteristics. The main feature of this type of sensor is that the central glass cylinder brings the mixed liquid sample into a batch sedimentation experiment close to the conditions of the secondary sedimentation tank. It uses light transmission to track the decline of the sludge layer interface in the batch experiment and measures it through a fixed row of light-emitting diodes (LEDs) on one side and photodiodes on the other side or moving LED photodiode pairs. The sedimentation meter introduced by Vamolleghem uses a moving light detection system to record changes in the height of the sludge layer, and the maximum sedimentation rate and sludge volume index can be obtained from the corresponding sludge sedimentation curve.             With the increasing performance and price ratio of image analysis systems, the application of microscopic image processing technology in the sewage treatment industry has been promoted. For example, timely monitoring of changes in the sedimentation characteristics of activated sludge in the secondary sedimentation tank based on image collection and analysis is of great significance to preventing excessive expansion of sludge (filamentous bacteria). Grijspeerdt et al. used low-magnification microscopy combined with image analysis to develop an online instrument for estimating activated sludge settling characteristics, which can measure activated sludge flake morphology and provide a rapid and reliable estimate of suspended solids concentration.             The measurement of flocculation size and its particle size distribution can detect changes in flocculation characteristics at different treatment stages and provide valuable information on the treatment process. There are different ways to measure floc. Laser scattering technology has recently been used to obtain online flocculation size and particle size distribution information. The flocculation size measuring instrument is made based on Fraunhofer diffraction theory. ； There is also a detector based on the focused beam reflectivity method that can measure the sludge particle size distribution in the secondary sedimentation tank.             7. Conclusion The increasingly stringent sewage discharge standards have put forward higher requirements for process measurement instruments and promoted the development of increasingly complex sensor technology. However, due to reasons such as reliability and cost, there is still a big gap in the sensors actually used in sewage treatment systems. In terms of future academic research and industrial applications, further improvements need to be made in the reliability of the developed instruments and the availability of information provided by sensors in automatic monitoring and control systems for sewage treatment processes.