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Reasons for the Ineffective Performance of the Dissolved Air Flotation(II)

  1. Impact of Flocculants and pH on Dissolved Air Flotation (DAF) Effect

Generally, the higher the COD concentration in raw water, the more chemicals need to be dosed; the higher the concentration of suspended solids in raw water, the greater the amount of chemicals required. There is an optimal dosage for flocculation, exceeding this amount can decrease the flocculation effect, and exceeding it too much can have the opposite protective effect. In addition, attention should be paid to the proportion and sequence of chemical dosing.

Dissolved Air Flotation

Coagulants (such as PAC) can break hydrophobic groups during the hydrolysis process, causing suspended solids and fats to coagulate with each other, while some dissolved proteins precipitate in the form of small particles.

Flocculant PAM has a large molecular weight, with adsorption, bridging, linking, and sweeping effects. It allows small particles formed under the action of coagulants to adsorb and connect with each other, forming large flocs that quickly separate from water. In the joint action of PAC and PAM, larger flocs are formed, and wastewater needs to be dosed with PAC first, followed by PAM.

The flocculants PAM commonly used now are acidic flocculants, and they have an optimal pH value. When the pH value of sewage exceeds the optimal pH value, it can cause the dissolution or fragmentation of flocculants, adversely affecting the separation of dissolved air flotation machines. Therefore, the pH value of the influent should be monitored and controlled during operation.

  1. Sludge Scraper Frequency

The frequency of the sludge scraper is related to the thickness of the floating sludge on the top of the dissolved air flotation (DAF). The higher the thickness, the lower the water content, which may lead to poor flowability of the sludge, blockage of the sludge discharge pipeline, and an increase in sludge concentration and viscosity in the subsequent sludge tank, making it difficult to lift. Excessive floating sludge can increase the overall density, making sludge-water separation difficult and affecting the effluent water quality. Therefore, the sludge scraper frequency should be adjusted according to the amount of generated floating sludge.

  1. Cleaning of DAF System

Due to the presence of a considerable proportion of Ca2+ and SO42- in industrial wastewater and sewage, and the addition of some flotation agents during the DAF process, after the dissolved air flotation (DAF) system operates for a certain period, scale may adhere to the DAF machine wheel and bearings, reducing the efficiency of the DAF system. Therefore, regular cleaning of the DAF plates and diffusers is necessary. At the same time, check the integrity of the diffusers, whether they are corroded, whether the pipelines are leaking, and whether they are blocked.

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