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Pressure-detecting coulometric BOD analyzer
The principle of the pressure-detecting coulometric BOD analyzer is shown in Figure 2-38. The water sample in the culture flask is stirred using an electromagnetic stirrer. When dissolved oxygen in the water sample is consumed by microbial degradation of organic matter, oxygen in the culture flask dissolves into the water sample. The generated carbon dioxide is selected from the water and absorbed by the adsorbent placed inside the flask, causing a decrease in the oxygen partial pressure and total pressure inside the flask. The decrease is detected by an electrode-type pressure gauge and converted into an electrical signal. This signal is amplified and sent to a relay circuit to connect the constant current power supply and synchronous motor. The electrolysis bottle (containing neutral copper sulfate solution and electrolysis electrodes) automatically electrolyzes to produce oxygen to supply the culture flask. When the pressure inside the flask returns to its original level, the relay disconnects, and the electrolysis electrodes and synchronous motor stop working. This process is repeated to maintain a constant pressure in the culture flask.
According to Faraday's law, the oxygen consumption can be calculated from the amount of electricity consumed by constant current electrolysis. The instrument can automatically display the measurement results and record the biochemical oxygen demand curve.
Pressure Measurement Method
In a closed culture flask, dissolved oxygen in the water sample is consumed by microorganisms degrading organic matter. The resulting CO2, equivalent to the oxygen consumption, is absorbed, causing a pressure drop in the closed system. This pressure drop is measured with a pressure gauge, allowing the determination of the water sample's BOD value. In actual measurements, a relationship curve is first plotted between the BOD value of a standard glucose-glutamic acid solution and the corresponding pressure difference. This curve is then used to calibrate the instrument scale, allowing direct reading of the water sample's BOD value.
Microbial Electrode Method
A microbial electrode is a sensor combining microbial technology with electrochemical detection technology. Its structure is shown in Figure 2-39. It mainly consists of a dissolved oxygen electrode and an immobilized microbial membrane tightly adhering to its permeable membrane surface. The principle of BOD response is as follows: when a substance is inserted into a constant-temperature, BOD-free substrate with a fixed dissolved oxygen concentration, the rate at which dissolved oxygen molecules diffuse through the microbial membrane into the oxygen electrode is constant due to the constant respiratory activity of the microorganisms. This results in a steady-state current output by the microbial electrode. If BOD is added to the substrate, its molecules diffuse into the microbial membrane along with oxygen molecules. Because the microorganisms in the membrane assimilate the BOD, consuming oxygen, the number of oxygen molecules entering the oxygen electrode decreases, thus reducing the diffusion rate and decreasing the electrode output current, which drops to a new steady-state value within minutes. Within a suitable BOD concentration range, the decrease in electrode output current is linearly related to the BOD concentration, and the BOD concentration has a quantitative relationship with the BON value.
The working principle of the microbial membrane electrode BOD analyzer is shown in Figure 2-40. The analyzer consists of a measuring tank (containing a microbial membrane electrode, an aeration tube, and the water sample to be tested), a constant-temperature water bath, a constant-voltage source, a temperature controller, an aeration pump, and a signal conversion and measurement system. The constant-voltage source outputs an o... A 72V voltage is applied to the Ag-A8C1 electrode (positive) and the gold electrode (negative). The polarization current change generated by the gold electrode due to the varying BOD concentration in the test solution is sent to an impedance conversion and micro-current amplification circuit. The amplified micro-current is then sent to an A-D conversion circuit, followed by an A-V conversion circuit. The converted signal is then displayed digitally or recorded by a recorder. After calibration with a standard BOD solution, the instrument can directly display the BOD value of the test solution and complete the measurement of one water sample within 20 minutes. This instrument is suitable for monitoring the BOD of various easily degradable wastewaters. In addition to the above method, there are also activated sludge methods and correlation estimation methods.
