At present, domestic and foreign enterprises producing reconstituted tobacco leaf have adopted the papermaking forming technology similar to the traditional paper industry. Each ton of reconstituted tobacco leaf product produces about 30 ~ 60t wastewater. The wastewater has large discharge, complex composition, high chromaticity, high total chemical oxygen demand (TCOD), many fiber suspended solids and large concentration fluctuation. It is a high concentration organic wastewater that is difficult to treat. For the waste water produced in the production process, recycling tobacco enterprises at home and abroad mainly use the means of end treatment (adding agents such as polyaluminum or polyferric sulfate) to control the pollution, which has some problems, such as the discharge standard of waste water is not ideal. Domestic researchers have done more research on the wastewater treatment technology of reconstituted tobacco leaves by papermaking method, but most of them are limited to the laboratory stage and have not really realized industrialization. There are still some problems in the production process of reconstituted tobacco leaves, such as insufficient wastewater treatment capacity and high treatment cost.
In particular, it is difficult to degrade the organic matter in the secondary sedimentation effluent in the production of reconstituted tobacco enterprises, which is specifically reflected in the high content of COD and pigment in the water quality. In the sewage treatment, enterprises mostly use polyaluminium chloride, and the flocculation effect is general, which makes it difficult for the external drainage to meet the standard. In order to make the sewage discharge index meet the standard, the secondary effluent from the production of reconstituted tobacco leaves was treated with polyferric sulfate (PFS). The test results were compared with the results of air flotation effluent of the existing process, so as to provide technical support for the application of polyferric sulfate in the sewage treatment of reconstituted tobacco leaves. Polyferric sulfate is a high-efficiency inorganic polymer coagulant developed in the late 1970s. Compared with aluminum sulfate and polyaluminium chloride coagulants, it has the characteristics of large floc, fast settling speed, wide pH adaptation range and no secondary pollution. It is widely used in the coagulation treatment of municipal sewage, industrial water and drinking water purification.
Test material
The wastewater produced online by a paper-making tobacco reconstituting enterprise is discharged from the secondary sedimentation tank after biochemical treatment.
Polyaluminium chloride, sodium hypochlorite, sulfuric acid, polyferric sulfate, sodium hydroxide, cationic polyacrylamide. All chemical reagents are industrial products.
test method
The pH adopts glass electrode method, and the TCOD adopts potassium dichromate method.
Test method for coagulation and sedimentation aid effect of polyferric sulfate
According to the drug instructions, the optimal configuration concentration of liquid PFS is 10% ~ 50%. The beaker experiment is divided into three stages, and the optimal conditions of each stage are as follows:
(1) Coagulation stage (micro alum formation stage): 250 ~ 300r • min-1, stirring for 10 ~ 20s, generally not more than 2min;
(2) Flocculation stage (alum aggregation stage): 150r • min-1. After stirring for 6min, adjust the speed to 60R • min-1 and stir for 4min;
(3) Sedimentation stage (flocculation sedimentation process): 20 ~ 30 R • min-1, stirring for 5 min and standing for 10 min.
Results and discussion
Analysis of secondary sedimentation effluent quality
TCOD and pH are the two main factors affecting the flocculation effect. The secondary sedimentation effluent is monitored for 10 days. The determination results and change trend of TCOD and pH are shown in Table 1. It can be seen from table 1 that the mean value of TCOD is 389.17 mg • L-1 and the mean value of pH is 7.72. TCOD increased significantly on May 29 and May 31, increased by 19.6% and 25.5% respectively compared with the mean value; After June 1, it returned to below 400 mg • L-1, showing a decreasing trend and tending to be stable; During the ten days of monitoring, except May 29, other data showed an increasing trend. Among them, there were three days when the pH exceeded 8.00, which were May 31 (pH = 8.01), (pH = 8.01), June 3 (pH = 8.08) and June 4 (pH = 8.16).
Analysis on the relationship between TCOD and the dosage of polyferric sulfate
Using the amount of polymeric ferric sulfate and TCOD removal rate as Figure 1, it can be seen that the amount of polymeric ferric sulfate and TCOD removal rate are not a simple linear relationship, but the TCOD removal rate tends to the theoretical optimal value when the amount of polymeric ferric sulfate tends to the middle section; Ignoring the difference of TCOD in raw water, when the dosage of polyferric sulfate is less than 78 drops (3.12 ml), the compliance rate of TCOD is 44.44%; When the dosage of polyferric sulfate is greater than 78 drops (3.12ml), the compliance rate of TCOD is 75.00%.
The secondary effluent from reconstituted tobacco production was treated with polyferric sulfate. PFS instead of PAC was used to treat the secondary effluent. Under the laboratory conditions, the TCOD and chromaticity of the effluent could be effectively reduced, and the effluent could meet the secondary discharge standard; The content of PFS can be effectively reduced by reducing the pH value of secondary sedimentation effluent; The pH value of the liquid treated with polyferric sulfate must be controlled between 4.5 ~ 5.5 in order to effectively reduce TCOD and chromaticity; The pH value of the liquid treated by PFS should be adjusted after floatation
