Most SS, most insoluble COD and some soluble COD and BOD5 can be removed by air flotation or sedimentation and adding coagulant. The typical treatment process is as follows:
Waste water → screen → water collection tank → air flotation or sedimentation → discharge.
Air flotation and sedimentation are both physical and chemical treatment methods, and the treatment effect is related to the selected equipment, process parameters and coagulant. The removal rate of COD is generally higher than that of the middle stage water in pulping, usually reaching 70% ~ 85%. For small and medium-sized waste paper papermaking enterprises with a ton of paper wastewater discharge of > > 150m3 and low concentration, the effluent quality index can reach or approach the national discharge standard through air flotation or sedimentation treatment.
For large and medium-sized waste paper and papermaking enterprises with low discharge per ton of paper wastewater and high COD content in wastewater, it is difficult to meet the national first-class discharge standard by using single-stage air flotation or precipitation, because soluble COD and BOD5 can be effectively removed mainly by biochemical methods. Generally, when the discharge standard of COD≤ 100mg/L is implemented, the COD concentration of raw water should not exceed 600 ~ 800 mg/L; When the discharge standard of COD≤ 150mg/L is implemented, the original COD concentration should not exceed 800 ~1000mg/L. Therefore, when the SS and COD concentrations in raw water are high, biochemical treatment should be carried out after primary physical and chemical treatment, so that the treated effluent can finally meet the requirements of national discharge standards.
The typical process flow of physicochemical and biochemical treatment methods is as follows:
Waste water → screening → regulation → sedimentation or air flotation →A/O or contact oxidation → secondary sedimentation tank → discharge.
The requirements for COD are strict, and the substitutability of the process makes the requirements higher. MBFB process has been adopted by the United States, Europe and other countries. MBFB can effectively remove toxic organic substances such as ammonia nitrogen, COD and other small molecules that are difficult to degrade. .
Membrane biological fluidized bed process (MBFB) is based on biological fluidized bed, with powdered activated carbon (PAC) as the carrier, combined with the solid-liquid separation technology of membrane bioreactor (MBR), so that the reactor integrates physical adsorption of activated carbon, microbial degradation and high-efficiency membrane separation. The refractory low molecular weight organic matter in water is fully mixed with fluidized activated carbon powder under aeration condition, and adsorbed and enriched on the surface of activated carbon, forming a local pollutant concentration area on the surface of activated carbon; Powdered activated carbon also provides a special surface for microbial reproduction, and its porous surface absorbs a large number of microbial flora, especially those with target pollutants as metabolic substrates. At the same time, powdered activated carbon has a strong adsorption capacity for dissolved oxygen in water. Under the condition of high dissolved oxygen, microorganisms oxidize and decompose small molecular organic substances enriched on the surface of activated carbon, and then use ceramic membrane separation system to separate water from suspended particles such as powdered activated carbon adsorbed with organic substances, and further purify sewage through cross-flow filtration to make it reach the standard of reclaimed water reuse.
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