With the rapid economic development in our country in recent years, there has been a significant increase in urban industrial and domestic waste, leading to frequent outbreaks of eutrophication and algal blooms in water bodies. The primary method for waste disposal is currently sanitary landfilling, which involves open-air operations. As waste is compacted, the decomposition of biological matter within the waste, coupled with rain and snow during inclement weather, allows for leachate to form as rain and snowwater infiltrate the landfill site. Leachate is a high-concentration organic wastewater with a wide range of concentration values, containing hydrocarbons, nitrates, sulfates, and trace amounts of heavy metals like copper, cadmium, and lead, as well as high bacterial levels. If untreated leachate is discharged into water bodies, it can severely pollute the local water environment. To protect water resources, it is essential to strengthen the monitoring of wastewater discharge. The design of monitoring points and the quality of detection instruments (mainly water quality analyzers) play a crucial role in water environmental monitoring.
I. Principle of Operation for Water Quality Analyzers
The wastewater treatment plant utilizes two analytical instruments: a pH meter and a dissolved oxygen analyzer.
1. The Working Principle of a pH Meter
The pH level of water varies depending on the amount of dissolved substances, thus, the pH level can sensitively indicate changes in water quality. Changes in pH have a significant impact on the reproduction and survival of organisms, and also greatly affect the biochemical activity of activated sludge, influencing treatment efficiency. The pH level of wastewater is generally maintained between 6.5 and 7.
Water is chemically neutral, and some water molecules spontaneously decompose according to the following equation: H2O = H+ + OH-, meaning they break down into hydrogen ions (H+) and hydroxide ions (OH-). In a neutral solution, the concentration of hydrogen ions (H+) and hydroxide ions (OH-) is both 10^-7 mol/L, resulting in a pH value. The pH is the negative logarithm of the hydrogen ion concentration to the base 10: pH = -log. Therefore, the pH value of a neutral solution is 7. If there is an excess of hydrogen ions, the pH is less than 7, indicating an acidic solution; conversely, if there is an excess of hydroxide ions, the solution is basic.
pH levels are typically measured using the potentiometric method, which involves forming a primary cell with a reference electrode of constant potential and a measuring electrode. The electromotive force (EMF) of the primary cell depends on the concentration of hydrogen ions and the acidity or alkalinity of the solution. Our factory utilizes the CPS11 pH sensor and the CPM151 pH transmitter. The specific structure is shown in Figure 1. The measuring electrode features a special glass probe that is highly sensitive to pH changes. Made from a special glass that conducts electricity and permeates hydrogen ions, this probe boasts high measurement accuracy and excellent anti-interference capabilities. When the glass probe comes into contact with hydrogen ions, a potential is generated. This potential is measured by a silver wire suspended in an silver chloride solution against the reference electrode. Different pH levels correspond to different generated potentials, which are then converted into a standard 4-20mA output by the transmitter.
Section II: Working Principle of the Dissolved Oxygen Analyzer
Oxygen content in water adequately reflects the degree of water purification. For biological treatment plants using activated sludge, understanding the oxygen levels in aeration tanks and oxidation ditches is crucial. Increased dissolved oxygen in wastewater promotes biological activity excluding anaerobic microorganisms, thereby removing volatile substances and ions that are easily oxidized naturally, leading to the purification of wastewater.
There are primarily three methods for measuring oxygen content: automatic colorimetric analysis and chemical analysis, paramagnetic measurement, and electrochemical measurement. The electrochemical method is generally used to measure dissolved oxygen in water.
Oxygen is soluble in water, with its solubility depending on temperature, total pressure at the water surface, partial pressure, and the salt content in the water. The higher the atmospheric pressure, the greater the water's ability to dissolve oxygen, a relationship determined by Henry's Law and Dalton's Law. Henry's Law posits that the solubility of a gas is directly proportional to its partial pressure.
