The sewage treatment process is generally composed of mechanical treatment, biochemical treatment and chemical treatment, which involves the liquid phase, solid phase, gas phase of the three material components. The instrumentation for monitoring these phases can be simply divided into two main categories: general and specific.
2, general-purpose instrumentation for wastewater treatment process
General-purpose measuring instruments include temperature, pressure, level, flow, pH, conductivity, suspended solids and other sensors.
①Anaerobic digestion process is often implemented due to temperature control, temperature sensors become more important. Typical temperature measurement elements are RTDs
②Pressure measurement is often used as an alarm parameter for aeration and anaerobic digestion processes.
③Level measurement is used for water level monitoring, and is usually measured by buoys, differential pressure transmitters, volume measurement, ultrasonic level detection and other methods.
④Flow monitoring instruments are mainly Kan plate, rotor flowmeter, turbine-type flowmeter, target-type metering tank, electromagnetic flowmeter, ultrasonic flowmeter and so on.
5 pH is an important variable in the biochemical process, but also the key value of anaerobic digestion and nitrification process, usually in the wastewater treatment plant are installed with pH electrodes immersed in the sludge, through different cleaning strategies can be realized in the long term maintenance-free. For wastewater with a high buffering capacity, pH measurements may not be sensitive to process changes and are therefore not suitable for process monitoring and control, in which case a carbonate measurement system can be used instead.
6 Conductivity sensors are used to monitor changes in the composition of the influent water and are also the basis of the chemical phosphorus removal control strategy.
⑦Traditional biomass measurements are estimated based on the scattering of incident light by suspended particles and absorbance. With the advent of sensitive light detectors, sensors capable of automating light effect measurements were introduced. Most commercial sensors use a light source that emits low visible or infrared light in a region where most media exhibit low absorbance. The biomass concentration can also be determined from the difference in velocity of the ultrasound in the free solution between the suspended material and the microorganisms.
3. Sensors in anaerobic digestion
Measurement of biogas flow is widely used in anaerobic digestion processes to indicate the overall activity of the reactor. In recent years a number of specialized techniques have been used to monitor gas composition. Typical laboratory methods are wash bottle separation methods, where gas composition can be determined based on the ratio of the flow rate before entering the bottle and after exiting the bottle. For example, an alkali flask will be able to collect all the C02, H2S and allow CH4 to pass through. More specialized gas analyzers can directly monitor gas composition levels, such as infrared absorptiometry to determine C02 and CH4 levels, and specialized hydrogen analyzers have been developed based on chemical power sources. Gas-phase H2S meters can determine H2S content by monitoring the reaction of sulfide to lead stripping.
The main problem with monitoring systems based on gas analysis is that they cannot directly predict the concentration of the corresponding gas in the liquid phase. Immersion sensors that can directly measure dissolved hydrogen have been developed. Fuel cells are at the heart of such sensors. direct measurement instruments for H2S and CH4 have not yet been reported.
PH measurements are not easy to detect in unbalanced anaerobic digesters, especially when the alkalinity of the mixture is high. Measurements of C02 and carbonate in the mixed liquor can be made in this case. Alkalinity is largely dependent on the carbonate buffer and is therefore often used in control strategies for anaerobic digestion. Carbonate monitors have been developed for use in actual anaerobic digestion processes.
There are two basic principles for estimating carbonate alkalinity. One is titration, where advanced online titration sensors can simultaneously monitor different components such as ammonia and carbonate. The other method for on-line determination of alkalinity is based on the quantification of gaseous C02 obtained by acidification of the sample. A gas flow meter can be used to measure the volume of the resulting gas.
All biological activities can be characterized by the production of heat. Measurement of heat by a calorimeter provides direct insight into biological process changes. Flow calorimeters are preferred for wastewater treatment processes.
Volatile fatty acids (VFA) are the most important intermediate products of anaerobic digestion processes. Their aggregation can cause a decrease in pH and lead to failure of the process anaerobic digestion process. VFA concentrations are usually monitored as an indication of process performance, but on-line sensors are rarely implemented. State-of-the-art measurement instruments include gas chromatographs or high-pressure liquid chromatographs. Fourier Transform Infrared Spectroscopy (FT-IR) can be used as an on-line multi-parameter sensor to provide simultaneous measurements of COD, TOC, VFA, etc. 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 titrometer to provide the VFA content of the sample through a two-step titration or titration back-titration.
Biosensors have gained traction in the wastewater industry in recent years, with VFA analyzers determining the concentration of VFA in digested liquids, MAIA biosensors measuring metabolic activity, and RANTOX biosensors detecting impending organics overload and toxic loads.
4. Sensors in the activated sludge process
Oxygen plays a very important role in the activated sludge process, and the associated aeration costs account for about 40% of the total operating costs, so 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 diffused oxygen in a liquid. Dissolved oxygen (DO) sensors are reliable and accurate measuring instruments, but care must be taken to choose the right measurement location and to prevent fouling. Automatic cleaning systems are now quite common, and a number of DO sensors equipped with cleaning systems and capable of self-calibration are already in use.DO sensors are widely used in the control of aeration processes, saving considerable investment, and the information obtained can also be used to monitor any activated sludge treatment process.
Respiration is the measurement and interpretation of the rate of respiration of activated sludge, defined as the oxygen consumed by microorganisms per unit volume of activated sludge per unit time. It is a common tool for characterizing wastewater and sludge kinetics. The respirometer is essentially a reactor and the measurements are susceptible to changes in experimental conditions.
The biodegradable content of wastewater is obtained by the standard method of offline measurement of biological oxygen demand (BOD5), which is the amount of dissolved oxygen required for the biological oxidation of organic solutes over a period of 5 days.BOD5 experiments are not amenable to automated monitoring and control because of the length of time it takes to complete the experiments and the difficulty in achieving consistently accurate measurements. Online measurements of wastewater loading are realized based on short-term BOD estimates. There are two on-line BODst methods currently in use: respirometry and microbial sensors.The respirometry sensor RODTOX proposed by Vanrolleghem et al. is capable of monitoring BODst and potential toxicity of wastewater. The sensor has consists of a constant aeration, fully mixed batch reactor containing 10 liters of sludge to obtain a large dynamic range of BODs.The microbial sensor consists of a curing cell, a membrane, and a dissolved oxygen detector, and is best suited for activated sludge systems containing a wide range of microorganisms. In order to maintain their effectiveness, microbial BOD sensors require careful maintenance and storage. Most microbial BOD sensors have a short life span, ranging from a few days to a few months.
The most widely monitored variable in wastewater treatment plants is chemical oxygen demand (COD).COD automated monitors can be automated at 1 to 2 hour intervals, and are categorized as acidic or alkaline based on the conditions of oxidative decomposition.The main limitation of the COD assay is the inability to differentiate between biodegradable and inert organics.
TOC indicates the content of total organic carbon (TOC) in wastewater, and is also an indicator of the degree of organic pollution of a water body.The main principle of TOC measurement is to convert organic carbon to C02, and then measure this product in the gas phase, from which the concentration of organic carbon in the aqueous phase is derived. Typical measurement instruments are infrared pumping analyzers.TOC is considered to be a good monitoring parameter, especially for monitoring the quality of drainage water.
Many wastewater constituents absorb ultraviolet light. There is a close relationship between UV absorption and organic matter in wastewater. UV absorbance automatic monitor introduced into the wastewater treatment system for detecting the degree of water pollution or evaluation of the quality of discharge. In the last 10 years, significant advances in optical technology have made remote and multipoint measurements possible, greatly facilitating the implementation of wastewater treatment process monitoring. Infrared spectroscopic measurements have a great potential for the estimation and on-line monitoring of specific parameters such as TOC, COD and BOD. The main disadvantage of infrared spectroscopy is that fouling of the photocell components causes a reduction in sensitivity and requires frequent recalibration.