Basic knowledge of water purification

Natural water sources, including surface water and groundwater, contain various impurities. The impurities contained in the water source can be divided into suspended substances, colloidal substances and dissolved substances according to their particle size and existing form. The impurities in the water can also be divided into inorganic substances, organic substances and microorganisms.

The main feature of suspended solids is that they are suspended in moving water, while in still water they are light ones floating up and heavy ones sinking. The inorganic suspended solids in the surface water are mainly sediment, large particle clay or mineral waste residue, which have a large proportion and are easy to sink. In the organic suspended solids, the large ones such as water plants, the small ones such as the reproduction and death debris of some plankton (such as algae, bacteria or protozoa), and the organic substances from sewage are also included. Suspended solids such as aquatic plants with large particles are easy to remove, while those with small particles are difficult to remove.

Colloidal impurities in natural water can be divided into two categories: inorganic colloid (silicic acid colloid, clay colloid) and organic colloid (various proteins and humus, etc.). Colloidal impurities are characterized by being relatively stable in water and will not naturally precipitate when they are left standing for a long time.

Dissolved substances in natural water include: oxygen (O2) and carbon dioxide (CO2) mainly exist in water in molecular state. The ionic state in water is basically the result of inorganic salts dissolved in water, such as calcium, magnesium, iron, carbonate, sulfate, chloride, etc. Dissolved impurities cannot be removed by any mechanical method or coagulation method. They are stably and uniformly dispersed in water.

Due to the contradiction and gap between the water quality of natural water source and the user's requirements for water quality, we must use advanced water purification technology and feasible scientific water treatment methods to change the original natural water containing various impurities into water that can meet the requirements of life or production.

Some common water supply treatment methods are briefly introduced as follows:

1. Clarification:

The objects of water clarification treatment are mainly suspended solids and colloidal substances in raw water to reduce the turbidity of these substances in raw water. The specific treatment process can be divided into coagulation, sedimentation and filtration.

1.1 Coagulation

The agent (water purifying agent) is put into the raw water to make the agent and raw water fully mixed and reacted (i.e. the coagulation process is carried out in the reaction tank), so that the suspended solids and colloidal impurities in the water can form large particle flocs that are easy to precipitate, commonly known as "alum flowers".

1.2 Sedimentation

The raw water through the coagulation process carries large particle flocs and flows into the sedimentation tank at a certain water flow rate. Gravity separation is carried out through the sedimentation tank, and the heavy impurity particles in the water are sunk to the bottom of the sedimentation tank for discharge.

The above purification process can also be completed through the clarifier, which is a treatment structure integrating reaction and sedimentation.

1.3 Filtering

After the raw water passes the coagulation and sedimentation process, the turbidity of the water is greatly reduced, but there are still some small impurities left in the sedimentation water flowing into the pool through the collecting tank. The small impurities in the water are intercepted by the granular filter materials (such as quartz sand, anthracite, etc.) in the filter tank, so that the turbidity of the water is further reduced.

When the turbidity of raw water is low, the raw water after the reagent is put can also be directly filtered without coagulation, sedimentation and other treatment processes.

The above clarification processes (coagulation, sedimentation and filtration) can not only reduce the turbidity of raw water, but also remove the chromaticity, bacteria and viruses effectively.

For raw water with high turbidity, sedimentation tank or pre sedimentation tank is usually used to remove sediment particles with large particle size.

2. Disinfection

After coagulation, sedimentation and filtration, the raw water flows into the clean water tank through pipes and must be disinfected. The method of disinfection is to put chlorine, bleach or other disinfectants into the water to kill pathogenic microorganisms in the water. Others use ozone or ultraviolet radiation to disinfect water.

In addition to the above Class II water supply treatment methods, other commonly used treatment methods include deodorization, deodorization and iron removal; Softening, desalination, desalination, etc.

According to different raw water quality and water quality requirements after treatment, the above treatment methods can be used alone or combined to form different treatment systems. In water purification, several treatment methods are usually used together.