Briefly describe the main aspects of wastewater grading treatment.

1. Primary treatment, also known as pretreatment, refers to the process of removing floating and suspended solids from wastewater, while also adjusting the pH value of the wastewater in order to reduce its corrosiveness and the load on subsequent treatment processes. 2. Secondary treatment refers to the further processing of wastewater through biochemical processes, building on the results of primary treatment. The purification process that utilizes biological treatment methods to further remove colloids and soluble organic matter from wastewater primarily targets BOD and COD substances, with removal rates of over 90%, thereby bringing the organic pollutants to within the allowable emission standards. 3. Tertiary treatment, also known as advanced wastewater treatment or deep treatment, is the final stage among the three levels of wastewater treatment. After secondary treatment, wastewater still contains extremely fine suspended particles, phosphorus, nitrogen, biodegradable organic substances, minerals, pathogens, and other substances that require further purification.

The degree of wastewater treatment can be divided into primary treatment, secondary treatment, and tertiary treatment. Primary treatment mainly involves removing suspended solids, colloids, floating oil, or heavy oil from water, and methods such as screen filtration, natural sedimentation, flotation, and oil separation can be employed. Secondary treatment primarily aims to remove biodegradable organic solutes and some colloidal pollutants, in order to reduce the BOD and COD of wastewater. It is usually carried out using biochemical methods, which constitute the main part of the treatment process for wastewater containing organic matter. Tertiary treatment is primarily aimed at removing biologically refractory organic pollutants as well as dissolved inorganic pollutants in wastewater. Common methods include activated carbon adsorption and chemical oxidation; ion exchange or membrane separation techniques can also be employed.