Basic concepts of sewage treatment

Basic concept

The processing of raw water (raw water) to meet the quality requirements of finished water (domestic or production water and waste water for final disposal).

1, processing raw water for domestic or industrial water, called water treatment;

2. When processing wastewater, it is called wastewater treatment. The purpose of wastewater treatment is for the discharge of wastewater (into water bodies or land) or reuse.

In the recycling water system and the recycling treatment of water, the raw water is the wastewater, the finished water is the water, and the processing process has the nature of both water supply treatment and wastewater treatment. Water treatment also includes the treatment and final disposal of wastewater and sludge generated during the treatment process, and sometimes the treatment and discharge of exhaust gases.

Water treatment can be summarized in three ways:

① The most commonly used is to obtain the required water quality by removing part or all of the impurities in the raw water;

② By adding new ingredients in the raw water to obtain the required water quality;

The processing of raw water does not involve the removal of impurities or the addition of new ingredients.

Impurities in water and treatment methods

Impurities in water include coarse substances, suspended solids, colloids and dissolved substances. Coarse materials such as water plants floating in rivers, garbage, large aquatic organisms, grit in wastewater, and large pieces of dirt. In the water supply project, the coarse impurities are removed by the facilities of the water intake structure and are not included in the scope of water treatment.

In wastewater treatment, the removal of coarse impurities generally belongs to the pretreatment part of water. Suspended solids and colloids include sediment, algae, bacteria, viruses, and insoluble substances that are present in water and produced during water treatment. Dissolved substances include inorganic salts, organic compounds and gases. There are many treatment methods for removing impurities in water, and the scope of application of the main methods can be roughly divided according to the particle size of the impurities. Because the impurities contained in the raw water and the impurities allowed in the finished water differ greatly in type and concentration, the water treatment process also varies greatly.

As far as domestic water (or urban public water supply) is concerned, raw water from high-quality water sources (well water or well-protected water supply reservoirs) only needs to be disinfected to become finished water; Raw water from general rivers or lakes should first be removed from muddy impurities such as sediment, and then disinfected; For raw water with serious pollution, organic matter and other pollutants need to be removed. Raw water containing iron and manganese (such as some well water) needs to be removed. Domestic water can meet the water quality requirements of general industrial water, but industrial water sometimes needs further processing, such as softening, desalt and so on.

When the water quality requirements of wastewater discharge or reuse are low, only coarse impurities and suspended matter need to be removed by screening and precipitation (often called primary treatment); When organic matter is required to be removed, biological treatment (often called secondary treatment) and disinfection are generally adopted after primary treatment; For the wastewater after biological treatment, the treatment process is collectively referred to as tertiary treatment or advanced treatment. For example, when the water body into which the wastewater is discharged needs to prevent eutrophication, the nitrogen and phosphorus removal process belongs to the tertiary treatment. When wastewater is used as a source of water, the quality requirements of the finished water and the corresponding processing process depend on its use.

In theory, modern water treatment technology can make any high-quality finished water from any inferior water.

Method classification

The process of water treatment is actually designed according to the gap between the quality of raw water and the quality of finished water, so the processing procedure must be taken for raw water.

Each process produces a physical, chemical, or microbiological effect on raw water. A process is generally completed in a processing equipment, but there are also several processes completed in a processing equipment. A process or a combination of several processes can form a unit method of water treatment, such as coagulation, precipitation, filtration and disinfection are the most commonly used unit methods of water treatment.

According to the applied theoretical basis, the unit methods can be divided into two categories: physicochemical method and biological method. All treatment methods based on the principles of physics or chemistry (including physicochemistry) or both are incorporated into physicochemistry. All treatment methods based on the theoretical basis of microbial life activities are incorporated into biological methods, also known as biochemical methods.