A STUDY ON REUSE OF WATER IN A WOOLEN MILL

Transcription

1 21 1 A STUDY ON REUSE OF WATER IN A WOOLEN MILL Hu Jhjua, Professor Cni Bute, Professor Gno Tiagyno, Associate Professor Gu Youghin, Lecturer Wnng Huizheag, Lecturer Department of Environmental Engineering Tongji University Shanghai China INTRODUCTION In the Shanghai Second Woolen Mill, there are two workshops discharging colored wastewater (lop dyeing workshop and dyeing-finishing workshop). Main raw materials are wool and synthetic fiber. Products are woolen and blend fabrics. Dyestuffs used are acidic mordant dyes, disperse dyes, sulfur dyes, neutral dyes, reactive dyes and direct dyes. Also, different dyeing auxiliaries are used. The Shanghai Environmental Protection Bureau demands treatment of colored wastewater. From September 1974 through June 1976, field experiments were carried out under the conditions in this mill, resulting in making use of biotower-flotation process and working out design parameters and flowsheet. Consequent design and construction followed. At the end of 1979, construction of the treatment facilities was finished. The facilities were put into operation and since then the effluent has been meeting the criteria for discharge but has a distinct tint to exclude it from reuse. In order to make use of the effluent and further improve the environmental quality, it was sought to upgrade the treatment process in use so as to further reduce color of the effluent for recycle. So research was carried,)ut in 1982, resulting in the development of a coagulation-adsorption process for decolorization by which the effluent could be recycled with only insignificant modification of the existing facilities. Biotower EXPERIMENTS ON BIOLOGICAL TREATMENT The area available for constructing the treatment works was only 250 m2. In making plans for experimentation, a biotower was chosen. The filter media of the experimental biotower was honey-comb plastic blocks. The effect of hydraulic loading rate was evaluated after the biotower had run as smoothly as expected. After a two-month operation period, it was found that when the hydraulic loading rate was lower than 80 m3/m2/day, the openings of the 19 mm honey-comb media were clogged. Thereafter 25 mm-blocks were used, and the hydraulic loading rate was raised. When the loading rate was up to m3/m2/day, the effluent met the national criteria for discharge. BOD, and COD removal *as ~O-WVO and , respectively.

2 Fluctuation of Waste Quality and Flowrate and Ita Effect on Treatment The fluctuation of both flowrate and quality of waste was significant with the latter having a great effect on the treatment. Table I summarizes the data of one year and shows the quality of colored wastewater. Fluctuation of wastewater quality was caused by the change of dyestuffs used. When ph reached 4, or toxic auxiliaries were used, upset of the biotower would occur. However, there was the case when a large number of microorganisms died while ph value was normal. This was caused by methyl salicylate in the dyeing auxiliaries. The variation of water consumption rate during a working shift was investigated twice. The coefficient of hourly variation ranged from 0.6 to 1.3. Average water consumption in these two workshops was 1460 "/day, and the maximum was 2000 m3 /day; the minimum was 1280 m3/day. The hourly average was approximately 60 m3 with hourly maximum 110 m3 and hourly minimum 30 m3. An equalization tank was needed. EXPERIMENTS ON FLOTATION Experiments on biotower showed that effluent settled poorly. So dissolved air flotation with pressurized recycle was chosen. The pressure in the recycle system was 3-4 kg/cm2. Recycle ratio was Retention time in the flotation tank was 15 minutes. Overall COD removal was 65-75%. If 100 mg/l aluminum sulfate was added to the biotower effluent, COD removal would increase by approximately loolo. EXISTING FACILITIES AND THEIR EFFICIENCY I Wastewater treatment facilities were designed and built based on the results obtained. Construction of the facilities was finished at the end of The biowater has been running well while the flotation system not so well. Table I1 shows the data obtained in April, The volume of the equalization tank is equal to that of wastewater discharged during one working shift, which makes influent varying less in quality and flowrate. Data show that the color of the effluent is still relatively deep and sometimes COD is higher than 100 mg/l which is the limit of discharge criteria. The flotation tank did not work well as far as BOD, removal was concerned. In order to see if it could be reused in the case of closed circuit, further research was conducted in The Method of Color Determination EXPERIMENTS ON DECOLORIZATION In colbr determination of colored industrial waste, we usually use method of dilution. The sample is diluted time after time with colorless water until it is tintless, and the ratio of the final volume to the sample volume is called "times of dilution." Apparently, this method takes time and is cumbersome and not sensitive. It is not suitable for this study. A spectrophotometric method was used. According to this method, color of wastewater is defined in terms of optical density which is the log of the ratio of incident light intensity to transmission light intensity and is measured with a spectrophotometer. The procedure is simple, and the result is definite and therefore reproducible. However, the spectrophotometric method cannot substitute for the dilution method. The color of two samples was determined with both methods. Results are shown in Table 111. According to the dilution method, the color of sample B is greater than that of sample A (times of dilution of A and B are 24 and 64, respectively). However, according to spectrophotometric method, the color of sample Table I. Quality of Influent T ("C) PH BOD (mg/l) COD (mg/l) NH3-N (mg/l) Cr+6 (m/l) O-8.9 about

3 ~~~ ~ Table 11. Data Obtained from Operation (April, 1982) Term Range Average Temperature ("C) Air Wastewater Flowrate (m3/day) PH 6 6 SS (mg/l) Influent < Effluent from biotower Effluent < BOD, (mg/l) Influent Effluent from biotower Effluent COD (mg/l) Influent Effluent from biotower Effluent Cr (mg/l) Influent Effluent from biotower Effluent Influent Color, times of dilution Effluent from biotower Effluent Table 111. Color of Two Samples Times of Dilution Sample A B Optical Density A is greater than that of sample B (optical densities of A and B are 0.35 and 0.25, respectively). Since in the decolorization study, color determinations were used to compare the determinations of samples taken from the wastewater before and after treatment with the process studied; therefore, the spectrophotometric method was adopted. Selection of Decolorization Method Coagulation and adsorption were chosen for study since both would require less modification of the existing facilities. The adsorbent selected was waste powdered activated carbon from a nearby pharmaceutical factory. The colored waste under study was effluent from the biotower. Experiments on Coagulation Operation of existing facilities showed that the abnormal work of the flotation tank might be caused by poor coagulation. Therefore, in this study of coagulation, clarification was noticed as well as decolorization. A jar test was used in the experiments. The equipment consisted of 8 sets of one-liter beakers and a stirring device. Since treated wastewater was going to be used for dyeing, aluminum salts, rather than ferric or ferrous salts, were preferred as the coagulant. Commercially available goods were aluminum sulfate with A1203 content greater than 16% and polyaluminum chloride (PAC) with A 1203 content greater than 8%. The prices were 350 Yuan/ton and 130 Yuan/ton, respectively. 213

4 While decolorization efficiency increased with increasing coagulant dose, there was an economic limit of removal which was 55-60%. After that efficiency, increase of efficiency would be insignificant (about 5% for even double the dose). Dose of aluminum sulfate ranged from 100 to 150 mg/l with a typical value of mg/l, while PAC dose (calculated as AI,O,) was mg/l. Relative costs of the former and the latter were 1 and 1.5, aluminum sulfate being cheapcr. Besides, in using aluminum sulfate, no new feeding mechanism would be needed because it was already in use in the existing facilities. Clarification efficiency also increased with increase of coagulant dose. It was found that a dose of more than 100 mg/l of aluminum sulfate would be necessary to provide an effluent with a turbidity of 10 units (a requirement for reuse). Therefore aluminum sulfate dose for both decolorization and clarification was almost the same, about 130 mg/l. Experiments on Adsorption Problems of study were decolorization characteristic of the adsorbent, contact time, point of dosage and dose as far as the use of waste powdered activated carbon was concerned. All studies were carried out in the same set of equipments as used in coagulation study. The decolorization characteristic of the waste activated carbon was compared with that of some cheap commercial grades, namely, medical activated carbon (4.95 Yuan/kg), sugar refining activated carbon (3.5 Yuanlkg) and industrial activated carbon (3.16 Yuanlkg). The procedure of testing each carbon was the same, namely: (1) dose 100 mg/l; (2) contact time 10 minutes; and (3) determination of color of treated sample after clarifying with aluminum sulfate. The results showed that the decolorization characteristic of the waste powdered activated carbon was as good as those kinds of commercial powdered activated carbon, while it was cheaper. It cost only 0.02 Yuan/kg, with 50% moisture. Therefore, waste activated carbon was an appropriate adsorbent in this case. TO study dose and contact time, five samples from biotower effluent were taken. Doses used were 50, 100, 150, 200, 300 mg/l; contact times were 5, IO, 60 minutes. For each sample, different combinations of dose and contact time were used. Results showed that no matter what dose and what contact time were used, the decolorization effect was the same. It was evident that decolorization finished within five minutes and removal was too low to satisfy requirements for reuse as well as that by coagulation. Next, the effect of coagulation and adsorption in combination was tested. Experiments were carried out to see whether aluminum sulfate or activated carbon should be fed first. As it is shown in Table IV, activated carbon should be fed 5 minutes before feeding aluminum sulfate. In the determination of the dosage of activated carbon and aluminum sulfate, 90, 110, 130, 150 mg/l of aluminum sulfate, and 100, 150, 200, 250, 300, 350 mg/l of activated carbon in various combinations were tested. Conclusions are as follows: (1) Aluminum sulfate should not be less than 100 mg/l; (2) Dose of activated carbon is dependent on color of wastewater to be treated, the greater the color, the higher the dose; (3) There is a relationship between dose of aluminum sulfate and that of activated carbon, increase in one will decrease the other; (4) There is a narrow range of aluminum sulfate dose (higher dose only increases the efficiency a little and lower dose fails to decolor. It is reasonable to set the dose at 130 mg/l); and (5) It is impossible to obtain colorless effluent no matter how high the dose is. The color of the best effluent is with two times dilution. Effect of Waste Quality It was found :ha: :he wasie quaiity (especially its color) effected removal greatly. Dyestuff can composition of dyeing liquors were changed from time to time, so the conclusions stated above were only applicable to samples taken during the course of the experimentation, and it was necessary to investigate the influence of waste quality on decolorization with coagulation and adsorption. The effect of temperature, ph value and dyestuffs used were considered. Generally speaking, waste temperature ranged from 25 C to 40 C, ph value 6-7, and dyestuffs frequently used were more than ten species. Study showed decolorization was independent of waste temperature and ph value in the ranges mentioned above. With the effect of dyestuffs, dyeing liquors prepared with 14 species of frequently used dyestuffs were used in experiments. Dyeing liquors were diluted prior to experiments to serve as waste. Each sample was then treated as biotower effluent with coagulation and adsorption according to procedure mentioned above. 214 I

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6 Results might be outlined as follows: For acidic mordant dyes and direct dyes, both coagulation and adsorption are effective in decolorization, but coagulation is more effective than adsorption. For neutral dyes, only coagulation functions well and adsorption fails to decolor For reactive dyes, coagulation is useless in decolorization, and adsorption has some effect but not very great. For sulfur dyes, coagulation functions well and adsorption fails to function. It seemed that, for dyes forming color lake, coagulation would function well and for soluble dyes adsorption would function better. Since adsorption is selective and flocs formed by aluminum sulfate have some adsorption ability, sometimes coagulation will contribute some results to removal of color developed by soluble dyes. Note, however, that all waste samples were not true waste but prepared by dilution of dyeing liquors to imitate wastewater to be decolored. FEASIBILITY OF WATER REUSE After biological treatment and decolorization, could the wastewater be reused for preparing dyeing liquors and wool potting and washing processes? If the water would be recycled, dissolved solids in the water would be accumulated. However, in dyeing and finishing operations, some water is evaporated and fresh water must be added in water recycling. Therefore, the dissolved solids in recycling water would have an upper limit and would not be rising unlimitedly. It could be estimated by using the following formula: S= S + A AS l + r where S = soluble solids in fresh water, mg/l; AS = soluble solids increment in each cycle, being mostly sodium sulfate, 120 mg/l; r = recycle ratio, namely, the ratio of waste (about 1200 m3 /day) to consumption of water (1460 /day), 0.8. Substituting values of S, AS and r into the formula, we got the value of S to be about IO00 mg/l. Effluent from biotower was taken and decolored with coagulation-adsorption process and then diluted with 20% tap water. This make-up water and tap water were used to prepare dyeing liquors, and a neutral dye and a mordant dye were selected. With these 4 liquors, 4 dyeing samples were made. The pair dyed with the neutral dye had no difference in tint, while another pair dyed with the mordant dye did have some difference. The sample dyed with liquor prepared with the make-up water had a deeper color. It was due to the toxic 6-valence chromium-ion content in the wastewater (about 0.1 mg/l). However, it could be overcome by modifying the dyeing liquor formula. For wool potting and washing tests, dissolved solids of the make-up water were raised to IO00 mg/l by adding Glauber s salt. Dyed samples were potted and washed with either the make-up water or tap water. Although both waters were acceptable, tap water was preferred. The conclusion was that water recycling was practicable and dyeing liquors and final washing could use complimentary water. In reinstalling water pipes of the two workshops, existing water pipelines should be retained and a second system would be for water recycling. Endneering Fcssih!!!!y The experimentation of decolorization demonstrated that if the biotower effluent was further treated with coagulation and adsorption, it would settle well. Therefore, flotation seemed to be unnecessary. It could be substituted by sedimentation and rapid filtration. The operation would be easier and power cost would be lower. The dimensions of the flotation tank are 8.7 m x 4.2 m x 4.0 m (Figure 1) with free-board being 30 cm. The effective volume is 135 m3. When flowrate is 50 m3/hr, its retention time will be 2.7 hr. Therefore, the volume of flotation tank is large enough for reconstructing it into flocculation and sedimentation tanks. The clean water reservoir has an area of 3.8 m x 9.0 m and one third of it will be 216

7 0 Dl E Fac i I ity Pumping Station NO I D&en s I on - _-_. 30x45 a50 Equalization TanK 15 5 r87x 4 0 Pumping Stotion NO 2 a o * Flowmeter Figure 1. Flowsheet of existing wastewater treatment facilities. sufficient for a rapid sand filter. Besides, feeding mechanism for activated carbon is necessary. Layout of the treatment facilities is shown in Figure 2. COST ANALYSIS Comparison of costs for the existing system and water recycling system was made vable V). EXpense on reconstruction was estimated lower than 10,OOO Yuan, therefore its depreciation charge was negligible. Analysis shows that reuse of water will save 90 Yuan each day. 217

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9 Figure 2. Layout of treatment facilities. CONCLUSIONS Research has been done twice for treatment of wastewater from the Shanghai Second Woolen Mill. The first time, in-plant wastewater treatment facilities were constructed. The second time, it was demonstrated by experiments and analysis that water from top dyeing and dyeing-finishing workshops could be recycled. This strategy makes it possible to refrain from discharge of colored wastewater and is economically feasible. The cost for reuse is 0.2 Yuan/m3. Although it is higher than the price of tap water (0.12 Yuan/m3), yet it includes the cost for wastewater treatment which is 0.16 Yuan/m3. They key here is the availability of waste activated carbon from a nearby pharmaceutical factory.