Views: 890 Author: Site Editor Publish Time: 2024-12-04 Origin: Site
Nantong X Paper Co., Ltd. is located in Nantong City, Jiangsu Province. It is a papermaking company that uses recycled paper as raw material. During the production process, the daily discharge of wastewater is 8,000 to 10,000 m3. The recycled paper of the company is mainly made of waste newspapers, waste board paper and waste book and magazine paper.
The characteristics of recycled paper wastewater are not only large fluctuations in treatment load; but also due to the diversity of raw materials and the use of a wide variety of chemical additives, the discharged wastewater has complex components and many difficult-to-degrade substances; this type of wastewater contains almost no ammonia nitrogen and phosphorus.
At the same time, due to the difficult-to-biodegrade lignin and cellulose in the raw materials, their dissolution enters the wastewater, resulting in the BOD5/COD in the wastewater being about 0.3, and the biodegradability is poor.
The designed effluent quality of the plant area meets the "Water Pollutant Discharge Standard for Pulp and Paper Industry" (GB3544-2008). The system inlet water quality and discharge standards are shown in Table 1.
| Project | COD/mg/L | BOD5/mg/L | pH | Chroma/times | SS/mg/L |
| Influent water quality | 1 500 ~ 3 500 | 450 ~ 1 100 | 6.5 ~ 8 | 400 ~ 900 | 11 000 ~ 13 500 |
| Emission standards | ≤ 100 | ≤ 60 | 6 ~ 9 | ≤ 80 | ≤ 50 |
Table 1 System inlet water quality and discharge standards
Figure 1 Process flow
In view of the complex composition, high concentration of suspended solids and large changes in water volume in wastewater, pulp fiber recovery and water volume adjustment are first carried out through screens and regulating tanks to achieve the purpose of removing solid suspended solids and adjusting water volume.
The effluent from the regulating tank enters the shallow flotation tank, and the suspended solids with smaller particles in the wastewater are further removed by adding PAC and PAM in the pipeline to reduce the working pressure of the subsequent treatment units. The generated scum enters the sludge thickening tank. The sludge returned from the secondary sedimentation tank is fully mixed with the effluent from the shallow flotation tank after activation in the adsorption regeneration tank.
The mixed wastewater enters the jet aeration oxidation ditch after flocculation and adsorption. The effluent from the oxidation ditch is separated from the mud and water in the secondary sedimentation tank and then enters the rapid filter tank for filtration and discharge.
The sludge and scum enter the sludge thickening tank for concentration, and then are dehydrated by the belt filter press and the mud cake is transported out. The specific process flow is shown in Figure 1.
Shallow flotation tank. The shallow flotation tank of this process adopts a high-efficiency shallow flotation tank with a diameter of 1600 mm, a height of 4530 mm, a dissolved gas pressure of 0.5 MPa, a reflux ratio of 70%, a hydraulic retention time of 60 min, and integrates mixing, flocculation, flotation, slag skimming, sedimentation, and mud scraping.
Using high-efficiency dissolved gas technology, the reflux water is under a certain pressure condition to maximize the dissolution of gas into the water, so that it is in a saturated state. Then the formed pressure dissolved gas water and raw water are mixed and released through decompression. The generated fine bubbles come into contact with the suspended flocs in the water, so that the suspended flocs in the water adhere to the fine bubbles and float to the water surface with the fine bubbles, forming scum and scraping off the scum, thereby removing SS and suspended COD in the raw water.
At the same time, this system changes the way of water inlet and outlet and sludge separation in traditional plug-flow flotation tanks, and adopts the form of dynamic water inlet and static water outlet.
Shallow flotation section
In order to achieve the removal of high SS and part of COD in wastewater, flocculants need to be added to reduce the load of subsequent treatment units. The simultaneous addition of PAM (polyacrylamide) and PAC (polyaluminum chloride) can bridge the colloidal particles formed by flocculation to form large and solid floccules, which greatly accelerates the particle settling speed and achieves the effect of enhancing flocculation.
The treatment efficiency of the shallow flotation tank is closely related to the dosage of PAC and PAM. In order to determine the optimal dosage of the shallow flotation tank, relevant experiments were carried out, and the results are shown in Table 2.
| PAC/mg/L | PAM/mg/L | COD removal rate/% | SS removal rate/% |
| 260 | 10 | 46.78 | 93.38 |
| 260 | 8 | 44.17 | 89.54 |
| 250 | 10 | 45.32 | 94.56 |
| 250 | 8 | 43.53 | 87.12 |
Table 2. Shallow flotation tank dosage commissioning data
When the optimal dosage of PAC and PAM in the shallow flotation section is controlled at 250 mg/L and 10 mg/L respectively, the SS removal rate can reach 94.56%, and the COD removal rate can reach 45.32%.
