The flowing current method is a new technology of automatic control of coagulant dosing that appeared in the 1980s. Using the microscopic characteristics of coagulation-the change of colloidal charge, we can realize the continuous control of dosing only by controlling the flowing current, which is an important breakthrough in the field of coagulation dosing technology. In China, since the first paper was published by 1989, the research and application of flowing current coagulation control technology has a history of 10 years. In recent 10 years, China has made remarkable progress in the basic theory, technical research, special equipment development, application and popularization of flowing current. This paper briefly introduces this series of progress.

1. Detection mechanism of flowing current detector

The detection mechanism of the flowing current detector (SCD) can be summarized as follows: the charged charges on the cylinder and the piston surface in the SCD probe are separated, and the flowing current produces electrode response and signal output.

Generally speaking, the electrification of solid surface is caused by ionization, ion adsorption, ion dissolution, lattice replacement, friction and so on. For the cylinder and piston in SCD probe, due to their own properties, their surfaces can absorb different amounts of positive and negative ions in water to obtain charges, which makes their surfaces charged. Because of the different hydration abilities of ions, the ions with strong hydration ability tend to stay in the solution and the weak ions are easy to be adsorbed on the solid surface, which makes the solid surface more negatively charged, and the ions with the same composition as the solid surface are the easiest to be adsorbed. Therefore, the cylinder and piston surface of SCD probe in natural water are generally negatively charged. Due to the negative charge on the surface of cylinder and piston, positive ions in water will be selectively adsorbed on the interface between cylinder and liquid. The negative charge layer attached to the solid surface and the charge layer of positive ions diffused in water with a small distance from it constitute the so-called electric double layer, which makes the charge distribution on the interface uneven. Due to the air suction and compression caused by the movement of the piston, the liquid flows in the narrow annular space between the cylinder and the piston, so that the charge of the diffusion layer in the solution is separated from the charge of the fixed layer and moves directionally with the water, thus generating a flowing current. The flowing current generated in the probe is detected by the electrodes at both ends and output after amplification and synchronous rectification. The output current signal is converted into recognizable digital signal for the reference of operators, so as to control the dosage of coagulant.

2. Relationship between flowing current and coagulation mechanism

A large number of suspended solids in natural water bodies are the main factors that constitute the turbidity of water bodies, and the purpose of coagulation and sedimentation is to effectively remove them. Suspended substances in water basically exist in the form of colloidal particles, and the electric double layer formed by negatively charged colloidal particles on the surface makes them repel each other and exist stably in water. According to the theory of electric double layer model, the thickness of colloidal electric double layer is the main factor affecting its stability and the decisive factor of flowing current. Therefore, due to the negative charge on the surface of colloidal particles such as clay in natural water, when inorganic coagulants such as iron and aluminum are added, a large number of positive ions will diffuse into the colloidal diffusion layer or even the adsorption layer, resulting in the thinning of the electric double layer and the decrease of the flowing current value. When a large number of positive ions flood into the adsorption layer to make the diffusion layer disappear completely, the thickness of the electric double layer is zero, and the flowing current value is also zero, then the colloid is completely unstable and aggregated. If the dosage of coagulant continues to increase, more positive ions will flood into the adsorption layer, which will make the surface electrical properties of colloidal particles positive, but the thickness of electric double layer will increase and the flowing current will increase. Therefore, only for colloidal particles in water, when the isoelectric point is zero, it will be the best dosage of coagulant in theory. However, in the actual process operation, an optimal flowing current range is often determined by experiments, and the optimal operation of water plant is achieved by controlling the automatic dosage of coagulant within this range.

3. Design principle of automatic control system for flowing current.

Coagulation dosing automatic control system generally consists of SCD sensor (SCS), controller (SCC) and variable frequency speed control system (FCS). The control system mode diagram is as follows: