Treatment of chemical fiber wastewater by coagulation sedimentation and integrated oxidation ditch process

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1 Water quality and quantity of wastewater: The Yi Jing Shui Wang project: A foreign-funded company that operates as a textile printing and dyeing enterprise producing simulated silk polyester fabrics; its production wastewater includes several types: textile wastewater, alkali reduction wastewater, refining wastewater, dyeing and finishing wastewater, and miscellaneous wastewater. In addition, there is domestic wastewater from dormitory buildings, canteens, bathrooms, office buildings, and other places. The quality and quantity of each wastewater stream are shown in Table 1. Table 1: Water volume and quality of wastewater
Wastewater type | CODcr (mg/L) | pH | Water volume (m3/d)
----------------|--------------|----|---------------------
Alkali-reduction wastewater | 10,000–40,000 | >14 | 100
Refining wastewater | 1,000–1,600 | 9–11 | 144
Dyeing and finishing wastewater | 700–1,300 | 5–6 | 853
Textile wastewater | 60–95 | 6–8 | 657
Miscellaneous wastewater | 600–1,000 | 6–9 | 308
Domestic sewage | 300–400 | 6–8 | 1,050

2. Wastewater treatment processes
The CODCr level in alkali-reduction wastewater accounts for a large proportion of the total wastewater discharged from production processes. It is reasonable, from a design perspective, to carry out acid addition and filtration as pre-treatment steps; however, this process has disadvantages such as high acid consumption, complex operation, and high operating costs. Due to its low volume, it is considered to be mixed with the other wastewater for combined treatment. The combined use of physical and biochemical methods is a conventional and typical process for treating printing and dyeing wastewater [1]. Coagulation sedimentation has the advantage of strong decolorizing and suspended solids removal capabilities; meanwhile, the coagulant added can also play a role in regulating pH. The biochemical method has a strong ability to remove soluble organic matter. In this project, since the main pollutants in the wastewater are insoluble terephthalic acid and various disperse dyes at a pH of 6–9, the combined wastewater is first subjected to coagulation and sedimentation. This process effectively removes organic pollutants and color, adjusts the pH value, and improves the biochemical properties of the wastewater, thereby creating favorable conditions for subsequent biochemical treatment. The biochemical treatment method uses the oxidation ditch process, which is suitable for the characteristics of this printing and dyeing wastewater as well as the high requirements regarding the quality of the treated water. ? The wastewater treatment process is shown in Figure 1. 3 Main Treatment Structures and Design Parameters 3.1 Collection Tank The collection tank has dimensions of 5.6m×2.5m, with an HRT of 20 minutes; it is equipped with two FLYGT submersible sewage pumps at the bottom. The submersible sewage pump is equipped with an automatic level control device to protect the pump and adapt to changes in the inflow flow rate. 3.2 Homogenization Tank The homogenization tank has dimensions of 14.7m × 5.2m, with an effective water depth of 4.1m. The mixing time is 2.5 hours, and two floating jet agitators are installed on the water surface to facilitate mixing. Due to the high impact load resistance of the oxidation ditch, the HRT in the homogenization tank is shorter. 3.3 Neutralization tank The neutralization tank has dimensions of 2.5m×2.1m, with an effective water depth of 2.5m and a reaction time of 6 minutes. Polyferric sulfate and a small amount of concentrated sulfuric acid are added to the neutralization tank to regulate the pH value and facilitate rapid coagulation. The neutralization tank is equipped with a mechanical mixer, as well as a coagulant dosing system and an automatic and manual acid addition system for pH control. Keep the pH value between 7 and 9. There are also acid storage tanks and coagulant storage tanks, both of which are lined with UPVC panels for corrosion protection; their effective capacities are 2.5 m3 and 8 m3 respectively. 3.4 Coagulation and sedimentation tank The coagulation reaction tank is of the swirl chamber type, consisting of six square-shaped chambers with chamfered edges; it has holes at the top and bottom, and is divided into two series. The total reaction time is 20 min. 　　Each series of reaction tanks is equipped with a sedimentation tank, which is of the multi-bin horizontal flow type. The HRT is 2.5 hours, the surface load is 1.4 m3/(m2·h), and the dimensions are 14.7 m × 3.2 m. The sludge from the reaction tanks and sedimentation tanks is sent to a drying site for treatment. 3.5 Primary and secondary integrated oxidation ditches The integrated oxidation ditch is the most important part of this process, as well as its main feature. The integrated oxidation ditch combines aeration, sludge-water separation, and sludge return in one unit, eliminating the need to build a separate secondary sedimentation tank. Its main features are: ① Simple process, automatic sludge return, and few pieces of equipment required ； ②Resistant to impact loads, operates stably and reliably, with simple maintenance and management ； ③Sludge can be stabilized, with very little excess sludge remaining ； ④Low infrastructure and operating costs ； ⑤The solid-liquid separation efficiency is higher than that of ordinary secondary sedimentation tanks. ? The integrated oxidation ditch used in this project is the Carrousel oxidation ditch from the American company EIMCO, along with its in-channel modular solid-liquid separator. The bottom of this solid-liquid separator features a series of evenly arranged inclined isosceles triangular crossbeams, which ensure uniform mixing of the liquid and rapid return of the settled sludge. ? The HRT for the primary and secondary integrated oxidation ditches is 15.5 hours and 14.7 hours respectively; the SRT is greater than 30 days in both cases. The organic sludge load is 0.15 kgBOD/(kgMLSS·day) and 0.05 kgBOD/(kgMLSS·day) respectively. The depth of the ditches is 3.6 m and 3.4 m respectively, the width is 6.0 m in both cases, the length of the straight sections is 43 m each, and the radius of the semi-circular bends at both ends is 6.0 m. ? The HRT for the solid-liquid separator sections is 1.9 hours and 1.7 hours respectively, with a surface load of 1.4 m3/(m2·h) in both cases. Their dimensions are: length of 15 m, width of 6 m, and depths of 2.6 m and 2.4 m respectively. At the outlet of the solid-liquid separator in the primary integrated oxidation ditch, four openings measuring 300 mm × 200 mm each are installed along the edge of the ditch at intervals of 3 meters apart; the water flows directly into the secondary integrated oxidation ditch. The outlet water from the solid-liquid separator in the secondary integrated oxidation ditch is handled by four collection channels spaced 3 meters apart, with an outlet flow rate of 0.8 L/(m·s). ? The aeration equipment consists of inverted umbrella-type slow surface aerators produced by the American company EIMCO; one such aerator is installed at each stage, located at one end of the channel. The motor power for each aerator is 73.5 kW, while the oxygenation capacity is 2.1 kgO2/(kW·h). 　　The excess sludge is removed from the oxidation ditches; a submersible pump is installed in each oxidation ditch to pump the sludge to the drying facility. During actual operation, since the oxidation ditch is in the delayed aeration phase, its sludge has approached stabilization, with very little excess sludge remaining; sludge discharge occurs only once every year or so. 3.6 Drying Area The drying area consists of 10 cells that are used in turn; each cell has dimensions of 10m×5m, and it is equipped with sand and gravel graded filter media. Dried sludge is transported for disposal, while leachate is returned to the collection tank. 3.7 Control Room and Laboratory The control room has dimensions of 3.3 m×3.3 m. The laboratory at the wastewater treatment plant is combined with the laboratory used for production. 4 Actual Operation Results Table 2 presents some of the data on the operation measurements taken during debugging and at the completion inspection; the water treatment volume was generally between 2000 and 2500 m3/d. Table 2: Water quality of influent and effluent in the wastewater treatment system – Sampling dates, CODcr (mg/L), colority (times), pH. August 26, 1997: 138, 980, 94.23; 012591.71, 1.37, 7.6. August 27, 1997: 236, 291, 96.13; 762094.71, 2.57, 7.3. August 28, 1997: 120, 397, 91.92; 861594.81, 1.67, 7.7. September 17, 1998: 206, 868, 96.74; 831596.91, 2.57, 7.5. September 18, 1998: 193, 984, 95.74; 031596.31, 1.87, 7.6. September 19, 1998: 216, 978, 96.43; 701296.81, 2.07, 7.3. As can be seen from Table 2, the CODCr concentration in the influent water varies considerably, but the effluent water quality remains within the specified standards. On September 17, 1998, this treatment facility passed the completion inspection and monitoring by the local environmental protection authorities, with all discharge parameters meeting the first-class discharge standards specified in the \"Comprehensive Wastewater Discharge Standards\" (GB8978—88). 5 Main Technical and Economic Indicators The comparison of the main technical and economic indicators of this treatment process project with those of similar wastewater treatment projects carried out by domestic manufacturers is shown in Table 3. Table 3 Comparison of Key Technical and Economic Indicators: Wastewater treatment projects for textile factories – Jiaxing Tongxiang Qunhe Gas-Liquid Textile Factory, Shantou Xinchang Textile Dyeing and Printing Factory. Water volume (t/d): 3000, 2000, 800. Project investment (yuan/t): 432.6, 313.4, 440. Investment related to water consumption (yuan/t): 1442, 1567, 55000. Electricity cost (yuan/t): 0.53, 0.62, 0.44. Chemical cost (yuan/t): 0.51, 0.92, 0.9. Land area required (m2/t): 0.85, 0.61. Effluent quality standards: GB8978-SS Class I standard, GB8978-SS Class I standard, GB8978-SS Class II standard. As can be seen from Table 3, the investment per ton of water as well as the operating costs (electricity and chemical costs) for this treatment process are relatively low. Transferred from Yijingshui Network