How to treat catering wastewater

Due to the particularity of its water quality, the treatment of catering wastewater is generally divided into two stages: pretreatment and treatment. In practical applications, multiple technologies are often used in combination, and the main goal of treatment is to remove COD, suspended solids and animal and plant oils in catering wastewater. Pretreatment is the basis of the treatment process, and treatment is the deepening of the pretreatment process. Both are indispensable.

1. Pretreatment technology of catering wastewater

Catering wastewater contains a large amount of suspended matter and animal and plant oils, which will block the dissolved oxygen in the atmosphere from entering the water body. During the treatment process, the oil will also wrap around the microorganisms, causing them to die due to lack of oxygen, affecting the treatment effect. A large amount of suspended matter is mostly food debris, which is large in size and difficult to be used by microorganisms. In addition, it is easy to cause blockage of treatment facilities during the treatment process, which brings difficulties to the treatment. Therefore, pretreatment of catering wastewater has become a very important link and means in the treatment process.

The main pretreatment technologies used are coarse granulation method, adsorption method, air flotation method and electrochemical method.

(1) Coarse granulation method

The coarse granulation method is also called coalescence filtration method. Using oleophilic and hydrophobic materials, when the oily wastewater passes through, tiny oil droplets agglomerate on its surface to form an oil film. When it reaches a certain thickness, it will separate from the surface of the filter material under the action of buoyancy and water shear force, forming large oil droplets that float to the water surface for oil-water separation.

The treatment effects of two coarse-grained materials, W-type and H-type modified polypropylene fiber, on emulsified edible oil wastewater were compared. The results showed that the H-type had better oil removal performance than the W-type. The coarse-grained technology can effectively reduce the oil content in catering wastewater and significantly reduce the COD concentration, which is beneficial to the subsequent biochemical treatment.

Ultrasonic wave was used to compare the traditional static flotation method to treat emulsified oil in catering wastewater. The results showed that the order of influence on oil removal rate was time, power, oil volume fraction, temperature, and emulsifier volume fraction. An oil-water separator was designed by the coarse-grained method to study several factors affecting oil removal rate. The test results showed that the oil removal rate could reach 82% when the oleophilic coarse-grained polypropylene plate was placed at an angle of 15°; the temperature was increased (the oil removal rate could be increased); and the water volume flow rate was about 150L/h. The oil removal process was effective and feasible, and had broad application prospects.

(2) Adsorption method

The adsorption method is a method of removing one or more substances in wastewater by adsorbing them on the solid surface using porous materials. According to the different adsorption forces of solid surfaces, it can be divided into physical adsorption and chemical adsorption. But they are not isolated, and often occur together. Due to the influence of adsorbents, adsorbents and other factors, the adsorption effect is also different. Therefore, the research and development of high-efficiency adsorbents and the recycling of adsorbents in the future are the research focuses of adsorption separation technology.

Using magnetic materials as materials, the magnetization effect of oil droplets is used to mix magnetic powder with oily wastewater, so that the oil adheres to the magnetic powder, and then the magnetic powder and the adsorbed oil are left in the magnetic field through a magnetic separation device, so as to achieve the purpose of separation from water. The results show that the oil content of the effluent is greatly reduced after adding magnetic powder, and it is verified that the emulsified oil is negatively charged, the magnetic powder particles are positively charged, and the combination of magnetic powder and oil is not only directly adsorbed, but also finer magnetic powder can be aggregated with oil droplets by magnetic flocculation.

After sand filtration, the catering wastewater is aerated in the intermediate pool, and then the effluent is measured through the biological activated carbon fiber column with fixed microorganisms. The results show that the sand filtration-immobilized biological activated carbon fiber treatment of catering wastewater has good removal effects on turbidity, COD, and UV254, and the effluent water quality is stable.

The grid-precipitation-adsorption process is used to treat catering wastewater. After filtering with special non-woven fabrics and modified fiber balls, walnut shell particles are used for adsorption. It is found that: without changing the pH, 30g/L of walnut shell particles are directly added, adsorbed for 20min, oscillated at 120r/min, and at 30℃, the removal rates of COD and oil content are both above 69%.

(3) Flotation method

The flotation method is to generate a large number of microbubbles through a certain method, so that they adhere to solid or liquid pollutant particles with a density close to that of water in the wastewater, forming a floating body with a density less than that of water, floating to the water surface to form scum, and then separated. It is mainly used to remove suspended matter, oil and fat with a specific gravity less than 1. Combined with the use of flocculants, the removal effect can be further improved.

When pre-treating oily catering wastewater, it was found that under the same or similar conditions, the new micro-bubble flotation process and the ordinary pressure dissolved air flotation process were comparable.

A chemical dissolved air flotation system integrating coagulation, flocculation and flotation processes was used to treat catering wastewater. The results showed that the removal rates of turbidity, grease, TSS, BOD5 and COD in catering wastewater were very high, indicating that the chemical dissolved air flotation system is an effective pre-treatment method for removing pollutants in catering wastewater.

The flotation method has a good effect and occupies a small area, but it consumes a lot of power, has a complex structure and is difficult to maintain in the later stage. The research and development and improvement of the bubble generation and dissolved air system in the new flotation device have become the difficulties in breaking through the existing flotation device.