Printing and dyeing wastewater is a kind of industrial wastewater produced by the dyeing of wool textiles and synthetic fiber cloth and its printing and dyeing industry. At present, there are many methods for treating printing and dyeing wastewater, such as coagulation/sedimentation, coagulation sedimentation/ozone oxidation, flocculation / Fenton oxidation, microorganisms, nanofiltration and ultrafiltration / reverse osmosis, pretreatment/ultrafiltration, biological Treatment/reverse osmosis, coagulation pretreatment/nanofiltration and other combined processes. However, with the rapid development of the printing and dyeing industry, there are more and more synthetic dyes and auxiliaries that are difficult to biochemically degrade in the printing and dyeing wastewater, resulting in long traditional biological and biochemical treatment cycles, large footprint, poor treatment effect, and even inability to biochemically, However, the Fenton-type oxidation method also has the problems of large consumption of chemicals, a large amount of slag, and secondary pollution of slag. The direct membrane separation process can easily lead to blockage of the membrane system and shorten the service life of the membrane. In order to reduce the contamination of the membrane surface by pollutants such as suspended particles, colloids, and soluble molecules in the influent, the membrane pores become smaller and blocked, etc., flocculation pretreatment is often used, such as micro flocculation direct filtration/ultrafiltration, micro flocculation/microfiltration, Microflocculation/variable pore direct filtration, micro flocculation/ultrafiltration/membrane system, micro flocculation/direct filtration, micro flocculation/reverse osmosis and other combined processes. The reported “micro-flocculation” pretreatment processes mostly use traditional poly ferric chloride (PFC), polyaluminum chloride (PAC), FeCl3, Al2(SO4)3, and their combination agents and polymer flocculant polyacrylamide (polyacrylamide). PAM). In the process of use, Fe3+ will lead to contamination of the membrane surface, while Al3+, PAM, and its decomposition monomer acrylamide have certain risks of neurotoxicity to humans and animals.
In recent years, microbial flocculant (MBF), as an emerging green and environment-friendly flocculant, has the characteristics of low price, high efficiency, non-toxicity, wide adaptability, etc., and has become one of the research hotspots at home and abroad. The use of MBF for micro flocculation pretreatment, combined with membrane separation technology, forms a new micro flocculation/ultrafiltration combined process, and its application in the field of practical printing and dyeing treatment is rarely reported. In this study, MBF was selected as the flocculant, and the influencing factors of the micro-flocculation process, the orthogonal experiment of micro-flocculation, and the operation parameters of the ultrafiltration treatment process were investigated, which can be helpful for the engineering application of MBF/membrane separation combined process in the field of wastewater treatment in the future. Provide reference.
1.1 Wastewater source and water quality
The printing and dyeing wastewater is taken from the wastewater outlet of the dyeing and finishing process of a dyeing and finishing factory in Quanzhou City, Fujian Province. It mainly contains a small number of dyes, auxiliaries, flocculants, and a small number of inorganic salts.
CaCl2 is industrial grade; H2SO4 is analytical grade; MBF (code: XN188). Pleated filter element filter (50μm); external pressure ultrafiltration membrane separation equipment (with backwash function, membrane material is polyvinylidene fluoride, membrane separation area 0.9m2, average molecular weight cut off 10000u); UV-2102C UV-Vis Spectrophotometer; HCA-100 COD digester; XZ-0101S turbidimeter; ALC-110.4 electronic analytical balance; PHS-3C pH meter.
1.3 Experimental steps and methods
Take printing and dyeing wastewater, add the appropriate amount of H2SO4 to adjust pH, and filter to remove impurities with larger particle size (≥50μm). At room temperature, add MBF (mass fraction 0.1%) and coagulant aid CaCl2 to 200 mL of the filtered water sample, stir at 450 r/min for 2 min, let it stand for flocculation, take a small amount of supernatant, and measure its decolorization rate and COD. The effluent from the flocculation pretreatment is introduced into the security filter (the filtration precision is 5μm), and then enters the ultrafiltration membrane unit for advanced treatment.
1.4 Analysis method
Calculate the decolorization rate according to the OD550 of the incoming and outgoing water. Membrane permeation flux (JV, L/(m2·h)) refers to the permeation amount of substances per unit membrane area per unit time during the membrane separation process, which is calculated by the formula (1).
Where: V is the volume of permeate, L; S is the effective area of the membrane, m2; t is the sampling time, min.
Experimental results and analysis
2.1 Optimization experiment of a single micro flocculation factor
2.1.1 Influence of CaCl2 dosage
Under the conditions of MBF dosage of 25mg/L, pH of about 12, and flocculation of 30min, the effect of CaCl2 dosage on the flocculation effect of printing and dyeing wastewater was investigated, as shown in Table 2. MBF is 25mg/L, the decolorization rate in the effluent increases first and then decreases with the increase of the dosage of CaCl2. When the CaCl2 is 800mg/L, the decolorization rate reaches 86.72%, and the COD in the effluent decreases to 515.88mg/L. The COD The removal rate was 81.46%. Therefore, the best mass ratio of MBF to CaCl2 (MBF: CaCl2 for short) is 1:32.
2.1.2 Influence of MBF dosage
Under the conditions of MBF: CaCl2=1:32, pH of about 12, and flocculation of 30min, the influence of MBF dosage on the flocculation effect of printing and dyeing wastewater was investigated, as shown in Table 3. When the MBF is 30mg/L, the decolorization rate reaches 87.50%, the effluent COD is reduced to 477.20mg/L, and the COD removal rate reaches 82.85%; when the MBF is 40mg/L, the decolorization rate reaches 88.28%, and the effluent COD is reduced to 461.34mg /L, the COD removal rate reached 83.42%. Considering the cost of the agent and the amount of residue, the appropriate dosage of MBF is 30mg/L.
2.1.3 The effect of pH
Under the conditions of MBF dosage of 30mg/L, MBF: CaCl2=1:32 and flocculation for 30min, the effect of pH on the flocculation effect of printing and dyeing wastewater was investigated, as shown in Table 4. The decolorization rate first increased and then decreased with the increase of pH. When the pH was 7.5, the flocculation effect was the best.
2.1.4 Influence of flocculation time
Under the conditions of MBF dosage of 30 mg/L, MBF: CaCl2=1:32 and pH of 7.5, the influence of flocculation time on the flocculation effect of printing and dyeing wastewater was investigated, as shown in Table 5. The decolorization rate first increased and then decreased with the extension of flocculation time. The decolorization rate reached 88.28% at 20 minutes, the COD of the effluent decreased to 452.99mg/L, and the removal rate was 83.72%.
To sum up, using MBF as microflocculant, the optimal conditions of the pretreatment process are MBF: CaCl2=1:32, MBF30mg/L, pH=7.5, and flocculation time 20min.
2.2 Orthogonal experiment of the micro flocculation process
Based on the experimental results of pretreatment, the L9(43) orthogonal experiment was carried out by selecting the dosage of MBF (A), the dosage of CaCl2 (B), pH (C), and flocculation time (D) as the influencing factors, as shown in Table 6. Show. It can be seen from Table 7 that the influence order of the four factors is MBF dosage>CaCl2 dosage>pH>flocculation time.
2.3.1 Influence of process on membrane permeate flux
Select the operating pressure of 0.20 MPa and the recovery rate of 90%, adopt the method of “cross-flow filtration – no backwashing”, run continuously for 90 minutes, and record the membrane permeation flux at 10-minute intervals, as shown in Figure 1. The decreasing rate of membrane permeation flux in the treatment of printing and dyeing wastewater by micro flocculation/ultrafiltration combined process is better than that of a single ultrafiltration process; MBF has little effect on the membrane permeation flux of ultrafiltration membrane. Therefore, adding MBF for micro flocculation pretreatment can significantly improve the membrane permeation flux in the subsequent ultrafiltration process, and has little effect on the life of the ultrafiltration membrane.
2.3.2 Effect of operating pressure on membrane permeate flux
The operation mode of “cross-flow filtration – no backwashing” was adopted, the recovery rate was 90%, the membrane permeates flux was recorded every 10 minutes, and the influence of the operating pressure on the membrane permeate flux was investigated, as shown in Figure 2. The membrane permeation flux increases with the increase of operating pressure; after fitting, the decay rates are 14.6%, 7.3%, 13.2%, 22.7%, 19.2% when the operating pressure is 0.10, 0.12, 0.14, 0.16, and 0.18MPa, respectively. %, indicating that the membrane permeation flux is the most stable when the operating pressure is 0.12MPa.
2.3.3 Influence of operating period on membrane permeate flux
Under the conditions of the operating pressure of 0.12 MPa, the recovery rate of 90%, and alternating “run-backwash” mode, the effect of the operating cycle on membrane permeation flux was investigated, as shown in Table 8. The membrane permeation flux first increased and then decreased with the extension of the operating period. When the operating period was 18min, the membrane permeation flux was the largest, reaching 338.33L/(m2·h).
2.3.4 Effect of recovery rate on membrane permeate flux and COD removal
The influence indicators of ultrafiltration membrane mainly include ultrafiltration membrane material, molecular weight cut-off, and raw water matrix concentration. Under the conditions of the operating pressure of 0.12 MPa, the recovery rate of 90%, operating period of 18 min, and alternating “run-backwash” mode, the effect of recovery rate on membrane permeation flux and COD removal was investigated, as shown in Table 9. The membrane permeates flux decreased gradually with the increase of recovery rate, while the COD removal rate showed a trend of first increase and then decrease. When the recovery rate is 83%, the effluent COD is reduced to 109.39mg/L, the COD removal rate reaches the maximum value (75.85%), and the comprehensive COD removal rate reaches 96.07%, and the membrane permeation flux is also large, reaching 351.21L/(m2 h).
2.4 Analysis of Chemical Cost and Sludge Yield
According to the treatment capacity of 15m3/h, the costs of H2SO4, CaCl2 (industrial grade), and MBF are 0.12, 0.92, and 1.28 yuan/m3 respectively; the direct operating cost of ultrafiltration is 0.49 yuan/m3, including power consumption, cleaning, and membrane core costs. They are 0.09, 0.03 and 0.37 yuan/m3 respectively. Therefore, the cumulative operating cost of the micro flocculation/ultrafiltration combined process is 2.81 yuan/m3, and the direct treatment cost is low, which has certain promotion prospects.
The sludge moisture content is calculated as 97.5%, the sludge output of traditional biochemical treatment of printing and dyeing wastewater is about 30-50kg/m3, and the sludge output of iron salt or aluminum salt as pretreatment is about 8.00-50kg/m3. 26.00kg/m3。In the micro-flocculation/ultrafiltration combined process, the sludge output is about 19.20kg/m3 when the pH of the wastewater is adjusted to about 7 by adding H2SO4 solution, the sludge volume produced by the MBF treatment unit process is about 14.70kg/m3, and the total sludge output is about 33.90kg/m3. m3, the sludge yield is between the traditional biological method and the iron salt or aluminum salt flocculation treatment process, but its sludge is biodegradable, non-toxic, and free of secondary pollution problems, and can be directly disposed of in sanitary landfills, while the iron salt, The sludge produced by aluminum salt has poor stability, easy dissolution, and non-biodegradation. As a flocculant, PAM may also produce a biotoxic acrylamide monomer, which poses a certain ecological risk to the storage site of the slag.
(1) The optimal operating conditions of micro flocculation were MBF: CaCl21:32, MBF dosage 30mg/L, pH=7.5, and flocculation time 20min. Orthogonal experiments show that the order of influence of four factors on the micro flocculation process is MBF dosage>CaCl2 dosage>pH>flocculation time. The addition of MBF for micro flocculation pretreatment can significantly improve the membrane permeation flux in the subsequent ultrafiltration process and has little effect on the life of the ultrafiltration membrane.
(2) The optimal operating conditions of ultrafiltration are the operating pressure of 0.12MPa, the operating period of 18min, the recovery rate of 83%, and the alternate “running-backwash” mode.
(3) Under the optimal operating conditions of the micro flocculation/ultrafiltration combined process, the COD in the actual printing and dyeing wastewater was reduced from 2782.50mg/L to 109.39mg/L, and the comprehensive COD removal rate reached 96.07%.