Treatment of Cotton Weaving and Textile Wastewater
Several printing and dyeing factories in Shenzhen used to be the focus of the Shenzhen Environmental Protection Bureau. They were all foreign-funded projects introduced by the local government. When the factories were built, they were equipped with sewage treatment facilities. However, due to backward technology, aging facilities, poor management, and expansion of production, the sewage discharge exceeded the standard, seriously polluting the waters of Mipeng Bay. The Shenzhen Municipal Environmental Protection Bureau selected one of the textile factories as an early recommendation for treatment. The treatment project lasted more than a year, from 1996 to the end of 1997. Since continuous operation in 1998, the quality of the effluent water has been stable and up to standard. The Shenzhen Municipal Environmental Protection Bureau recommended it as a model project.
1. Introduction of the mill
The textile factory is a large-scale, wholly-owned enterprise in Hong Kong. It was relocated from Hong Kong in 1991, with a total investment of 200 million Hong Kong dollars. It integrates spinning, dyeing, and weaving. The product is denim. The wastewater mainly comes from the dyeing factory, and its main components are indigo powder (including lime, dyestuff, and cassava flour), as well as NaOH and low sodium sulfate. The wastewatervolume is about 1000m3/d. The wastewater quality under normal conditions is shown in Table 1.
Table 1 Wastewater quality
|Project||pH||Chroma (times)||CODcr (mg/L)||Water temperature (°C)|
The design treatment capacity of this process is 1000m3/d, and the water volume was 300m3/d when it was first put into production, and then gradually increased to 1000m3/d.Still, t the effluent water quality gradually deteriorated, and the chroma and CODCr seriously exceeded the standard. The main reasons are as follows:
1) The raw water quality was underestimated during the design. The CODCr of the raw water was originally designed to be 700-1 300 mg/L, but now the CODCr of the raw water is 1000-2000 mg/L.
2) Two circular biological filters, both 13m in diameter and 2.5m high, are equipped with a central water distribution system. These filters are packed with 10cm-sized stones and have ventilation holes at their base for natural oxygenation. However, due to the anaerobic hydrolysis tank’s low oxygen levels and high effluent concentration, the biofilter’s interior becomes anaerobic. This results in anaerobic sludge clogging the gaps between the stones, leading to water flow disruption. Consequently, the efficiency of the aerobic biofilter is compromised, with the secondary biofilter only degrading 30% to 40% of organic matter.
There is a need for more skilled management personnel, and the quality of operating workers could be better.
2. Process transformation
The Jiede Textile Factory adopts the method of production while reforming. Since the degradation rate of organic matter in the secondary biofilter is only 30% to 40%, and it breeds mosquitoes and flies, it is decided to use the existing anaerobic hydrolysis pool to remove the secondary biofilter and replace it with a biological contact oxidation pool. However, the COD removal rate of the anaerobic hydrolysis tank is only 10% to 20%, and a large load will be brought to the subsequent treatment structures. In order to reduce the pollution during the biochemical transformation, a set of physical and chemical treatment equipment (i.e., chemical coagulation and sedimentation) is added in front of the anaerobic hydrolysis tank to reduce the load.
Due to the strong practicability of this transformation project, it is actually a transformation process that involves the production, research, and formulation of plans. When determining the physical and chemical treatment process, a large number of chemical coagulation and decolorization tests were done first. Using the traditional method for ferrous sulfate, water purifying agent, poly aluminum, and ferric chloride, the main goal is to remove chroma and COD. Through small tests, it is determined that ferrous sulfate is suitable for the decolorization effect and COD removal of our factory. The dosage is 800-1000mg/L, and the pH range is between 9.5-11.
Various parameters of the physical and chemical treatment process: the design flow rate Q is 1000m3/d; two lift pumps, each with a Q′ of 80m3/h; two static mixers, each with an external dimension L of 800mm and D of 250mm; two mechanical reaction tanks, 1.5m×1.5m×4m, with a design residence time of 20min; one inclined tube sedimentation tank, with an area of 5m×5m—galvanized pipe.
After physical and chemical treatment, the average removal rate of chroma reached 60%, and the average removal rate of COD was more than 40%, which achieved the goal. After ferrous sulfate was added, mud production increased greatly. The existing mud storage tank can only accommodate the scraping and mud discharge of the air flotation tank, resulting in the mud accumulated in the inclined tube sedimentation tank cannot be discharged in time. According to this actual situation, the factory purchased another plate and frame filter press.
The effluent after physical and chemical treatment is stable. Under the conditions of controlling the dosage and pH, the effluent COD is between 600 and 900 mg/L. During the biochemical transformation, it is an operation exceeding the standard discharge.
Main structures and equipment to be transformed:
1) Regulating pool. The effective residence time of the raw water regulating pool is designed to be 8 hours, and the size of the whole regulating pool is 20m×10m×2.5m, with six grids inside. Before reaching the adjustment tank, raw water flows through a narrow channel where ferrous sulfate is added to maintain a pH level of 7-9. The wastewater undergoes preliminary settling in the regulating tank. Annually, during the Spring Festival holiday, the textile factory removes and transports the sludge accumulated at the tank’s bottom.
2) Anaerobic hydrolysis tank. It is rectangular, with a size of 10m×7.5m×5m and an effective residence time of 10h. Split into two sets, each has a pulse water distributor responsible for intermittent water distribution in the hydrolysis tank. This system also serves to agitate and rinse the anaerobic tank. Within the anaerobic tank, the wastewater is segregated into three layers: sludge, adhesive, and clear water. The agitation process ensures water doesn’t stagnate and flow isn’t cut short while rinsing helps avoid blockage in the adhesive layer. The anaerobic tank features a tilted plate adhesive layer, and the anaerobic hydrolysis tank uses a perforated tank for water distribution.
3) contact with the oxidation pool. It is built on the site of the original biological filter, with external dimensions of 20m×15m×4.5m, using three corridors, with semi-soft filler (thickness 3m) suspended inside. The filler contact time is 20h, the bottom is equipped with a microporous aeration head, the air-water ratio is 15:1, and the dissolved oxygen concentration in the tank is 3.8mg/L after testing.
4) Sedimentation tank. It is a vertical flow type, square, with a size of 6 m × 5 m × 5 m. Water is introduced from the pool’s central top and exits via the zigzag collection tank. The primary purpose of the vertical flow sedimentation tank is to settle and interact with the biofilm shed from the oxidation tank. The treated water then proceeds to the coagulation air flotation tank. All the settled sludge is redirected to the anaerobic hydrolysis tank to replenish nutrients, while the originally designed sludge discharge pipe leading to the sludge storage tank remains intact.
5) Coagulation air flotation tank. Further, remove the fine particles that cannot be removed by sinking in the sedimentation tank, and add a water purifying agent. The water purifying agent is a light yellow liquid provided by Shizhong Chemical Factory in Nanhai City, Guangdong Province. It is made of scrap aluminum, scrap iron, and concentrated sulfuric acid. Its main component is a trivalent salt of iron and aluminum, with a pH of 2.3 to 3. Following the dosing process, the wastewater moves into the air flotation zone via the partitioned reaction area. The floating debris is channeled into the sludge storage tank, where it’s processed through a plate and frame filter press, resulting in a compacted sludge cake ready for export. The treated water is then measured and released directly into the sea.
3. Operation effect and analysis
After the film in the contact oxidation pool matures, the physical and chemical treatment equipment will be stopped. See Table 2 for the day and night monitoring results of the Shenzhen Environmental Protection Bureau at the discharge outlet.
Table 2 Monitoring data
|Date||Sampling point||Analysis Project|
|September 9th||total outlet||7.14||105||18.3||64||0.03|
|September 12th||total outlet||6.26||147||12||32||0.02|
|September 13th||total outlet||6.78||41.3||5.3||32||0.01|
|September 19th||total outlet||7.52||87.7||9.5||32||0.01|
|September 20th||total outlet||7.03||93.3||23||32||0.01|
After two years of continuous operation, the monitoring data show that it is up to standard operation, indicating that the treatment project is successful. The following experiences and conclusions were also drawn from it:
1) Biological filters aren’t ideal for treating highly concentrated printing and dyeing wastewater aerobically. While the biofilter offers benefits like energy conservation and simplicity in operation, its efficiency is compromised, degrading only 15% to 40% of organic matter. Additionally, it becomes a breeding ground for mosquitoes and flies. When overburdened, the biofilter turns anaerobic, leading to sludge clogging its pores and hampering ventilation. Cleaning a blocked filter material proves to be a challenging task.
2) In biochemical treatment, the hydrolysis tank plays a very important role. Its removal rate for chroma is usually 40% to 70%, and the removal rate for COD is only 10% to 20%, but it can improve the biodegradability of wastewater. Prior to setting the specifications for the contact oxidation tank, a laboratory oxygen consumption test was conducted using the filter material from the biofilter and the effluent from the anaerobic tank. The findings revealed that with extended residence time, the COD removal rate can exceed 90%. This suggests that the discharge from the anaerobic hydrolysis tank possesses high biodegradability, paving the way for optimal conditions in the following biochemical processes.
3) For such printing and dyeing wastewater with high concentration and strong alkalinity, it needs to be neutralized before biochemical treatment. Ferrous sulfate uniquely impacts this kind of printing and dyeing wastewater. Pretreatment adopts the method of adding ferrous sulfate, which can remove part of the chroma and reduce COD to below 900mg/L.
4) After contacting the oxidation tank with film formation, the food chain composed of microorganisms on the filler can maintain the balance of the biofilm, and all the precipitated sludge is returned to the anaerobic hydrolysis tank. The coagulation air flotation tank produces only about 1m3/d of sludge, compared to the 7-8m3/d before the modification. This is an approximately 87% reduction in sludge volume, significantly lightening the burden on the sludge treatment system.
5) The contact oxidation tank is divided into three corridors. During the later operation, it was found that if the first corridor stopped aeration to make it an anaerobic section, the CODCr of the effluent would only rise slightly to about 140mg/L, and the chroma would be about 50 times. In this way, the actual aerobic residence time is reduced to 14h, which saves energy consumption on the one hand and creates a very favorable condition for expanding the processing capacity in the future on the other hand.
4. Project investment and operating costs
The sewage treatment investment in supporting construction is 900,000 yuan, the renovation project investment is 1.2 million yuan, and the total project investment is 2.1 million yuan. For 1000m3/d water volume, the project investment cost is 2,100 yuan/m3. See Table 3 for running costs.
Table 3 Operating costs
|Serial number||Cost item||Treated water volume (m3)||Total cost (yuan)||Remark|
|1||salary||348956||66000||This table does not take depreciation into account. The factory produces its own olefins to generate electricity. There are 5 operators in total, working in three shifts, with 1 person in each shift, 1 person in the laboratory, and 1 person as the monitor.|
|Of which: electricity fee||135000|
|mud cake shipping||28000|
|adjusting pool dredging||10000|