Keywords: environmental protection, leather-based cloth wastewater, AB method, example analysis
With the development of economy and the progress of science and technology, synthetic leather has been more and more widely used in the use of leather products, due to the increase of leather products and the lack of leather, promoting the continuous updating of synthetic leather technology, the development of synthetic leather technology has also led to the development of the leather-based cloth industry. Through the introduction of advanced equipment at home and abroad, the development of marketable high-grade synthetic leather base cloth products to improve the economic efficiency of enterprises has an important role.
Polyurethane and other polymer (PU) leather base cloth production process desizing, bleaching, dyeing and cleaning sections will produce a certain amount of wastewater, in addition to a certain amount of rinse water on the ground of the workshop. At present in the Chinese literature there is no leather-based fabric wastewater treatment methods introduced, we learned in practice, leather-based fabric wastewater and printing and dyeing wastewater have similarities, but different. According to the relevant literature [1-4], at present, the treatment methods of printing and dyeing wastewater are mainly chemical (chemical coagulation, chemical redox, photocatalytic oxidation, electrochemical method), physicochemical (adsorption (air flotation), membrane separation technology, ultrasonic gas vibration technology), and biological methods. We believe that the combination of chemical coagulation and biological treatment is effective, technically and economically feasible for the dyeing and finishing wastewater generated by the leather-based fabric production process.
I. Water treatment process plan
The composition of wastewater discharged by printing and dyeing enterprises is complicated, and the wastewater contains substances that are difficult to be biochemically degraded, such as various dyes, chemical pastes and chemical auxiliaries of large molecular weight, and substances that are easy to be biochemically degraded, such as starch. The chromaticity and pH value of the wastewater are high, which makes it difficult in wastewater treatment technology. Wastewater discharged during the dyeing and finishing process of leather-based fabric is different from general printing and dyeing wastewater. Due to the production process of leather-based fabric, as well as the use of dyeing agents, auxiliaries and other dosage and variety. Therefore, the concentration of pollutants in leather-based fabric dyeing and finishing wastewater is higher than that of general printing and dyeing wastewater; secondly, leather-based fabric in the finishing and dyeing process, a lot of fine fluffy fibers will be dropped, and the wastewater has high suspended solids, which must be precipitated through multiple grills and many times during the wastewater treatment process in order to achieve the desired treatment effect; in addition, as most of the leather-based fabric blanks are treated with chemical pulp or starch pulp, after boiling and desizing, most of the pulp has to be transferred to the wastewater treatment plant. In addition, since most of the leather-based fabrics are treated with chemical pulp or starch pulp, after desizing by cooking, most of the pulp has to be transferred to the wastewater, which makes the sludge produced by the leather-based fabric wastewater treatment is large and sticky, and the sludge dewatering and drying has become a big problem. We use chemical coagulation combined with two-stage biochemical method, i.e., biological adsorption - aerobic hydrolysis - aerobic biochemical as the main body of the improved AB biochemical method, a better solution to the leather-based fabric production process produced by the dyeing and finishing wastewater treatment problems, and achieved the desired results.
The main features of the process:
a. Multi-stage biochemical, bacterial species, complete degradation of pollutants. The process is set up in two sections of aerobic hydrolysis, giving full play to the function of aerobic hydrolysis, difficult to biochemical macromolecules and macromolecular compounds degraded into easy to biochemical low molecular compounds, for the subsequent aerobic biochemical treatment to create favorable conditions, aerobic biochemical function can be given full play to. At the same time, because the partially oxygenated section operates under low dissolved oxygen and high pollution load, the energy consumption for removing unit COD load is low.
b, the biochemical section of the isolation, to prevent different strains of bacteria compete with each other to improve the removal rate of pollutants. The process is set up in the inclined plate isolation pool, so that the parthenogenetic microorganisms in the parthenogenetic section and the aerobic microorganisms in the aerobic biological section are isolated, avoiding the disadvantages of the two different microorganisms mixing and competing with each other to inhibit the aerobic biochemical function. It improves the aerobic biochemical function.
c, make full use of biological coagulation, reduce the amount of coagulant and sludge production. Process flow in the partially aerobic and aerobic section of the sludge reflux, and set up a biological adsorption reaction section, so that the reflux sludge and sewage pollutants in the adsorption, roll band. Compared with the sludge non-return process, the coagulant dosage can be reduced by about 30%, and the amount of sludge produced is also reduced accordingly.
d, art layout is reasonable and compact, covers an area of small, easy to operate and manage. Conditioning pool is arranged underground, the rest of the treatment pool are arranged on the ground, the same horizontal surface is the same large pool separated into different functional pools, the whole system continuous flow operation, continuous water.
e, partially aerobic aerobic joint treatment, nitrogen and phosphorus removal effect is good.
Two examples of application analysis
(a), Overview
A leather-based fabrics Co., Ltd. main products are leather-based fabrics, the project scale for the annual output of leather-based fabrics 25 million meters of leather-based fabrics, the main raw materials: blank fabrics, sulfur dyes, disperse dyes, additives and so on. The main sources of wastewater: desizing, bleaching, dyeing, cleaning section of the wastewater generated, in addition to the workshop ground washing water and sewage. The company's wastewater treatment facilities design capacity of 800 tons / day, three-shift system, the average hourly treatment of 34 tons of water. Water quality before design treatment: CODcr 1450mg/L, BOD5 500mg/L, SS 800mg/L, chromaticity 1000 times.
The company's wastewater treatment scale is 800 tons/day, process
Flow diagram is as follows:
(2), the main unit process parameters
a, grating ditch: 4m3 brick, built-in three coarse and fine grating to remove coarse debris, fibers, etc..
b, regulating tank: 533m3, effective volume 426m3, HRT13h.
c, inclined plate primary sedimentation tank 1: 191m3 concrete, effective volume 153m3, HRT4.5h.
d, inclined plate primary sedimentation tank 2: 191m3 concrete, effective volume 153m3, HRT4.5h.
e, partially oxygenated Hydrolysis biological adsorption tank: 191m3 concrete, effective volume 153m3, HRT4.5h.
f. Inclined plate isolation tank: 191m3 concrete, effective volume 153m3, HRT4.5h.
g. Aerobic biological contact oxidizing tank: design volume 573m3, effective volume 458m3, HRT13.5h.
h, Inclined plate secondary sedimentation tank: design volume 191m3, effective volume 153m3, HRT4.5h.
i. Sludge thickening tank: design volume 173m3, sludge thickening time 36h.
(C), the operating effect
In order to understand the wastewater treatment facility's treatment effect, we carried out an actual test on the wastewater treatment facilities of the company.
Closer working conditions during the monitoring period
The production load during the monitoring period was 90%, in line with the requirements of the technical specifications for monitoring the completion of the acceptance of environmental protection facilities.
Peak monitoring points and analytical items
There is a monitoring point in the wastewater treatment, primary sedimentation tank effluent, biochemical tank effluent, treatment facilities discharge. The analysis items are pH, CODcr, BOD5, SS, sulfide, color.
3 Monitoring results and comments
The average results of wastewater monitoring are shown in Table 1, and the effect of wastewater treatment in each section is shown in Table 2.
Table 1 Summary of monitoring results Unit: mg/L (except for pH and color)
Monitoring points
CODcr
BOD5
SS
Color
Sulfide
pH
Treatment Facility Inlet
1190
424
745.5
729
31.18
7.45
Primary Sedimentation Tank Effluent
512
205
36
25
0.17
8.08
Biochemical tank effluent
67.5
24.4
13
25
0.02
7.80
Treatment facilities Outfall
43.4
17.2
6
20
<0.02
7.58
Table 2 Table of treatment rate for each section
Item
Coagulation treatment system (%)
Biochemical treatment system (%)
Total Removal rate (%)
CODcr
57.0
91.5
96.4
BOD5
51.7
91.6
95.9
Color
96.6
20
97.3
SS
95.2
83.3
99.2
Sulfide
99.5
94.1
99.9
The enterprise discharges an average of 663.5 tons of wastewater per day, and 199,000 tons of wastewater annually. The amount of pollutants generated, reduced and discharged are shown in Table 3.
Table 3 Discharge of each pollutant Unit: tons/year
Pollutant name
SS
CODcr
BOD5
Generation
148.4
236.9
84.4
Reduction
147.2
228.3
81.0
Emission
1.2
8.6
3.4
From Tables 1 and 2, the discharge of the wastewater treatment facility complies with the GB4287-92 "Textile Dyeing and Finishing Industry Water Pollution Emission Standard" of the Ⅰ level discharge standard. It shows that the wastewater treatment facility has a good removal effect on CODcr, BOD5, chroma, SS, sulfide, and its pollutant removal rate: 96.4% for CODcr, 95.9% for BOD5, 97.3% for chroma, 99.2% for SS, and 99.9% for sulfide.
Three, discussion
(A), coagulant selection
The selection of coagulant is a key to this process, leather-based fabric dyeing and finishing process using a larger proportion of sulfur dyes, so leather-based fabric wastewater has a high chromaticity, organic pollution is serious, if coagulant selection is not appropriate, it will often produce a large number of hydrogen sulfide gas, resulting in secondary pollution. Selection of ferrous sulfate as coagulant, ferrous sulfate in the divalent iron and divalent sulfur to generate a very small solubility product of ferrous sulfide precipitation, cohesion and precipitation under certain pH conditions is more ideal, almost no hydrogen sulfide gas, the treated wastewater chromaticity and sulfide content is greatly reduced, and the actual operation of the desulfurization rate of up to 95% or more.
(2), primary sedimentation tank design
Georgian cloth wastewater has a deep color, high content of suspended solids, so precipitation decolorization coagulation treatment process is the key, coagulation treatment effect is good, the subsequent biochemical treatment effect will be better. Therefore, the primary sedimentation tank adopts two-stage serial design, and the actual operation shows that after coagulation of wastewater precipitation by two primary sedimentation tanks, the removal rate of color and suspended solids can reach more than 95%.
(C), precipitation sludge dewatering and disposal
Flocculation and sedimentation is an important part of the sewage treatment process, but good flocculation and sedimentation is not equal to good water. Leather cloth wastewater with high content of suspended solids, wastewater treatment of sludge produced by a large amount, to obtain stable and good water requirements, it is necessary to precipitate sludge timely discharge after dewatering and timely transportation, safe disposal.
(D), regulating pool odor suppression measures
Because of the leather-based fabric production process using sulfur dyes and sulfide alkali, sulfur-containing wastewater into the regulating pool after a long residence time, anaerobic sludge at the bottom of the pool, and in addition to the regulating pool due to the acidic wastewater into the (water film dust spray water), so that the regulating pool in the pH value is reduced when the upper part of the aeration will be released in part of the hydrogen sulfide, so that the surrounding environment produces an unpleasant odor. The surrounding environment produces an unpleasant odor. Sulfide in the wastewater only to form free hydrogen sulfide, can be released into the air to produce a bad smell, we know from theoretical analysis, sulfide in the free hydrogen sulfide content and pH value has a direct relationship, if the pH value of wastewater in the regulating pool to 9 ~ 10, the wastewater in the free hydrogen sulfide percentage will be close to zero. So into the regulating pool of waste water droplets of liquid alkali, control pH can eliminate the bad smell.
(E), operating cost analysis
Operating cost consists of electricity, chemicals and labor costs. Each treatment of a ton of wastewater electricity costs about 0.32 yuan; each treatment of a ton of wastewater ferrous sulfate costs about 0.45 yuan, alkali agent costs about 0.04 yuan, the total cost of chemicals about 0.49 yuan; each treatment of a ton of wastewater labor costs about 0.10 yuan. The running cost is about 0.91 yuan per ton of wastewater treated.
The modified AB biochemical method is used to treat the leather-based fabric wastewater, the removal rate of CODcr, BOD5, chromaticity, SS, sulfide can reach more than 95%, and the treated effluent complies with the GB4287-92 "Textile Dyeing and Finishing Industry Water Pollution Discharge Standards" of the Ⅰ discharge standard. This method has the advantages of high efficiency, stable operation and low operation cost.
References
[1] Shi Xiangyu, Fu Dexue, Chang Zhaorong. Research Progress of Dyeing and Printing Wastewater Treatment Methods[J]. Journal of Jiaozuo University, 2004, 4:32-35
[2] FANG Xu, FU Zhenqiang, HAN Hongda. Complex aerobic biological treatment of printing and dyeing wastewater[J]. Environmental Protection Science, 2004, 30(6): 20-23
[3] BI Dongsu, LI Yongmei, GU Gu Guowei. Discussion on the potential of printing and dyeing wastewater treatment process and case analysis[J]. Water Supply and Drainage, 2004, 30(5): 48-51
[4] Chen Qunyan, Zhong Weiguo. Application of AB biochemical method in printing and dyeing wastewater treatment[J], Water Treatment Technology, 2003, 29(4):236-238
.