Pretreatment system
brief introduction
Sewage pretreatment is a pretreatment facility set up according to the water quality requirements of the follow-up treatment process before the sewage enters the traditional sedimentation and biological treatment, which is the throat of the sewage treatment plant. For urban centralized wastewater treatment plants and decentralized sewage treatment plants within pollution sources, the pretreatment mainly includes grid, screen, grit chamber, sand water separator and other treatment facilities.
Principle of sewage pretreatment
（1） Coagulation
It is generally believed that coagulation is composed of coagulation and flocculation. Coagulation mainly refers to the process of colloid destabilization after being compressed by electric double layer; flocculation mainly refers to the process of colloid coagulation into large flocs after colloidal destabilization.
The main coagulation mechanisms are as follows:
(1) The compression mechanism of electric double layer: when the electrolyte is added to the solution, the ion concentration in the solution increases, and the thickness of the diffusion layer will decrease. When the two colloidal particles are close to each other, due to the decrease of the thickness of diffusion layer and the decrease of zeta potential, the repulsive force between them decreases and the colloidal particles can condense rapidly.
(2) Mechanism of adsorption electric neutralization: adsorption electric neutralization refers to the strong adsorption of the surface of colloidal particles to the parts with different charge. Because this adsorption neutralizes part of its charge and reduces the electrostatic repulsion force, it is easy to be close to other particles and adsorb each other.
(3) Mechanism of adsorption bridging: adsorption bridging mainly refers to the mutual adsorption of macromolecular substances and colloidal particles, but the colloidal particles do not directly contact with the colloidal particles themselves, so that the colloidal particles agglomerate into large flocs.
(4) The mechanism of sediment netting: when metal salts or metal oxides and hydroxides are used as coagulants, and the dosage is large enough to form metal hydroxide or metal carbonate precipitates rapidly, colloidal particles in water can be caught by these precipitates during the formation.
（2） Precipitation
(1) In the process of precipitation, the solid particles are separated and the settling velocity is constant.
(2) Coagulation sedimentation: solid particles collide with each other in the process of sedimentation, and the particle size and settling velocity gradually increase.
(3) Layered sedimentation: the relative position of solid particles remains unchanged during the sedimentation process and sinks as a whole.
(4) Compression sedimentation: the solid particles compress the lower particles by gravity during the sedimentation process, and the water in the gap between the lower particles is squeezed out.
Types and characteristics of sedimentation tank
| category | Applicable conditions | advantage | shortcoming |
| Horizontal flow sedimentation tank | 1. It is suitable for areas with high groundwater level and poor resistance 2. It is suitable for large, medium and small sewage treatment plants | 1. The structure is simple and the cost is low 2. It has strong adaptability to the change of water quality and quantity | 1. It covers a large area and is difficult to discharge mud 2. When using mechanical sludge discharge, the equipment is immersed in water and easy to rust |
| Sedimentation tank with sheet loss | 1. It is suitable for areas with high groundwater level 2. It is suitable for large and medium sewage treatment plants | 1. It has large capacity and strong adaptability to the change of water quantity 2. There are fixed sludge discharge equipment, and the management is simple | 1. The flow velocity of the pool water is unstable 2. The cost of sludge discharging machinery is high, and it is more complex, which requires higher construction |
| Vertical loss sedimentation tank | 1. Suitable for small sewage treatment plant | 1. Small floor area 2. Sludge discharge is convenient and easy to manage | 1. The depth of the pool, the diameter of the pool should not be too large, and the bottom of the pool is conical, so the construction is difficult 2. It has poor adaptability to shock load and temperature change 3. High cost |
| Inclined tube sedimentation tank | 1. Water treatment 2. Sewage oil separator | 1. The land occupation is small, the sedimentation area is larger, the water depth is reduced, and the water yield is increased 2. High separation efficiency | 1. Short residence time, poor buffering capacity and high requirements for coagulation 2. Consumables, high cost, difficult mud discharge. |
Air flotation
(1) Dissolved air flotation (DAF)
Dissolved air flotation (DAF) is a kind of air flotation. It uses the characteristics of water solubility under different pressures to pressurize and aerate all or part of the water to be treated (or treated), so as to increase the air dissolved amount of water. After passing into the water added with super coagulant, it is released under normal pressure. The air precipitates to form small bubbles and adhere to the impurity flocs, resulting in the overall density of flocs less than that of water Rise, so that the solid-liquid separation.
(2) Cavitation air flotation (CAF)
Cavitating air flotation (CAF) system is a water treatment equipment, and it is also a recommended export technology of the U.S. Department of Commerce and environmental protection agency. Caf is a system specially designed to remove grease, colloid and suspended solids (SS) from industrial and municipal wastewater.
Pretreatment of chemical fiber wastewater
In the chemical fiber wastewater pretreatment system, because the chemical fiber wastewater is acidic or alkaline, it must be neutralized before treatment, and the pH value should be adjusted to neutral. Generally, the acid wastewater is neutralized with alkali, and the alkaline wastewater is neutralized with acid. If conditions permit, the acid-base wastewater can be neutralized by mixing the acid-base wastewater. After pH adjustment, the wastewater needs pretreatment to remove SS and oil substances, such as removing oil by air flotation and removing suspended solids by coagulation sedimentation. The pretreatment process can improve the biodegradability of wastewater. The pretreated wastewater enters the biological treatment unit, and most of the impurities and other pollutants are removed.
Pretreatment of papermaking wastewater
The wastewater produced by the paper industry has the characteristics of various types, large amount of water and high content of pollutants. It is one of the industrial wastewater that is difficult to treat. The wastewater comes from various processes of pulp and paper making, and its physical properties and pollutant concentration are different. According to the characteristics of the wastewater, the relevant treatment process is determined. Currently, the main methods used for the pretreatment of paper industry wastewater are precipitation, sedimentation, and so on Air floatation, adsorption, membrane separation, aerobic and anaerobic biological treatment methods, as well as a combination of several treatment methods. No matter what method is used, the wastewater needs to be pretreated. The pretreatment is mainly to improve the wastewater quality, so as to meet the requirements of each process.
To improve the overall effect of wastewater treatment and ensure the stability of the whole treatment system, pretreatment plays a very important role in wastewater treatment of paper industry. Pretreatment in papermaking industry wastewater treatment can be divided into in-house pretreatment and off-site pretreatment. In plant pretreatment is mainly used to recover pulp from white water, and filtration and air flotation are often used for recycling, which can avoid a large number of pulp entering the wastewater treatment system, which not only improves the pulp yield but also saves the cost of wastewater treatment; the off-site pretreatment is mainly to ensure the quality of pulp The wastewater discharged into the physicochemical and biochemical treatment systems can meet the process requirements and make the system run stably.
The pretreatment process mainly includes: grid, screen, fiber recovery system, regulating water and water quality, etc. According to the different paper industry wastewater quality, different pretreatment methods can be adopted to remove some pollutants and modify the wastewater quality, so as to improve the treatment effect of the whole wastewater treatment system.
Pretreatment technology of hospital wastewater
The composition of hospital sewage is very complex and harmful. The main source is the sewage discharged from the clinic, laboratory, ward, laundry room and operating room of the hospital. There are a lot of bacteria and chemicals in the sewage, which has the characteristics of space pollution, acute infection and latent infection. If the hospital sewage containing pathogenic microorganisms is discharged into urban sewer or environmental water without treatment, it will often cause water pollution, cause various cases and infectious diseases, and seriously endanger people's health. The grid should be set in front of the water pump in the sewage pretreatment system of the hospital. The grid well and the regulating pool can be built together. Automatic mechanical grille is used in infectious disease hospital and automatic mechanical grille is used in general hospital. The grid well shall be sealed and equipped with ventilation hood to collect waste gas for centralized treatment; the grid slag and sludge generated from sewage treatment shall be treated together and transported to the outside for incineration. Babbitt steam or lime can be used. A regulating tank should be set up in hospital sewage treatment, and its volume should be calculated according to 30 ~ 40% of the daily treated water volume during continuous operation. In intermittent operation, its volume is calculated according to the process operation cycle. The regulating tank should be divided into two groups, each group is calculated according to 50% water volume. The regulating tank should be of closed structure with exhaust outlet, and the underwater mixing method should be adopted to prevent sedimentation. The sludge produced in the regulating tank is cleaned regularly and treated together with the sludge generated from sewage treatment.
How to evaluate the operation effect of pretreatment system in sewage treatment plant
To evaluate the operation effect of the pretreatment system mainly depends on whether the water quality meets the design requirements and standards after the pretreatment section. For example, if the pretreatment section has grid, grit chamber and other process facilities to remove suspended solids, then the suspended solids removal rate between the inlet and the effluent of the pretreatment section is an assessment parameter. Similarly, the parameters mentioned in the design of pretreatment section can be used as evaluation criteria.
In addition, a multi-faceted operation effect evaluation should also include the operation status of the process equipment in the process section, reagent consumption, management methods, personnel operation, etc.
Scope of application
1. Water treatment: domestic water, industrial water;
2. Urban sewage treatment;
3. The treatment of industrial wastewater, sewage and sludge, and the recovery of some residues in the sewage, etc;
4. For some industrial wastewater which is difficult to be treated, the composite PAC is prepared by mixing PAC with other chemicals, which can achieve remarkable results.