Design plan for reclaimed water reuse project in a chemical industry group

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Design Proposal for Reuse Project of Wastewater in a Chemical Industry Group – Preface 　　 Wastewater (reclaimed water) is one of the sources of water supply in cities. It has a large and stable water supply, is available locally, and results in low water usage costs. Therefore, from both practical and long-term perspectives, effectively treating urban wastewater and making good use of reclaimed water is one of the strategic measures to alleviate water shortages in cities, address urban water scarcity issues, and promote sustainable development. 　　 Dahua Group is a major water consumer in Dalian. The group’s normal water consumption quota is 38,160 tons per day in winter, and 34,660 tons per day in other seasons. In recent years, the group has placed great emphasis on water conservation, resulting in a continuous reduction in actual water consumption. The current water usage quota is 22,714 t/d, representing a 34.5% reduction in actual use. To further reduce water usage targets, the issue of reclaimed water reuse must be considered. 1 Reuse of treated water 1.1 Water volume and quality 　　 The source water for the treated water reuse treatment plant comes from the secondary effluent of the Chunliu River Wastewater Treatment Plant in Dalian, with a treatment capacity of 6000 t/d. Secondary treatment is employed; the majority of the treated reclaimed water is used as make-up water for the circulating cooling water in ammonia synthesis plants (5,000 t/d), while the remaining amount is used for chemical production and landscaping (1,000 t/d). The water quality parameters of the secondary effluent from wastewater treatment plants, that is, the inlet water quality for water reclamation stations, are shown in Table 1. 1.2 Process Flow Given the aforementioned characteristics of the water quality, the main parameters to be treated are total alkalinity, total solids, chloride ions, and COD, etc. The process flow for treatment is shown in Figure 1. 　　The primary treatment process employs the CASS process: in the pre-reaction zone, there is a high-load biological adsorption process, followed by a low-load matrix degradation process in the main reaction zone. The CASS art collection integrates reaction, precipitation, and drainage; the microorganisms are subjected to periodic changes between aerobic, anoxic, and anaerobic conditions, enabling effective nitrogen and phosphorus removal. Given the low organic content in the wastewater, elastic packing is added to the tank to provide a habitat for microorganisms. Table 1 Water quality indicators – Analysis items: Secondary effluent from wastewater treatment plants, Primary effluent, Secondary effluent
pH: 6.5–8.5, 6.5–8.5, 6.5–8
Turbidity/NTU: 50, 5, 5
Total solids/(mg.L-1): 1200, 1000, 150
Total hardness (as CaCO3)/(mg.L-1): 500
Total alkalinity (as CaCO3)/(mg.L-1): 500
Chloride ions/(mg.L-1): 300
CODcr/(mg.L-1): 120
BOD5/(mg.L-1): 40
Ammonia nitrogen/(mg.L-1): 30
Total phosphorus/(mg.L-1): 3
Petroleum content/(mg.L-1): 10
Total iron/(mg.L-1): 2
Suspended solids/(mg.L-1): 50
Total heterotrophic bacteria count/(mg.L-1): 5×105
Note: Primary effluent is used for chemical production and landscaping, while secondary effluent is used as makeup water for circulating cooling systems. The process equipment in the ammonia synthesis plant is a complete set of equipment imported from German company Linde. The water quality requirements for the circulating cooling water system are quite high; after primary treatment, the quality of the water still differs significantly from those required for water used to replenish the circulating cooling water, especially in terms of the Cl- content. In industrial applications, chemical precipitation or gas conversion methods cannot be used to remove Cl-, and membrane treatment is the only viable option. To this end, an RO reverse osmosis system was chosen for the secondary treatment process; this process utilizes membrane technology for desalination. We opted for imported composite membranes, which possess the advantages of low pressure, high throughput, and high desalination efficiency associated with composite membranes, while also overcoming the issue of the negative charge on the surface of traditional composite membranes. Therefore, this membrane also possesses the special advantage of resistance to contamination. 2 Main treatment structures and process equipment file:///E:/DOCUME~1/zzjjaa/LOCALS~1/Temp/ksohtml/wps_clip_image4.png 2.1 Main treatment structures 　　① CASS biochemical reaction tank 　　Its external dimensions are 21m×10m×6m; it has a steel-concrete structure, an effective volume of 1200m3, an effective water depth of about 5.5m, and a retention time of 4.8 hours. 　　②Composite coagulation tank: The external dimensions are 6m×2.5m×4m; it has a steel-concrete structure with anti-corrosion treatment applied inside. Its effective volume is 104 m3, and it is divided into 3 compartments. Retention time: 10 min. 　　③Tranquilization tank: The external dimensions are 24m×7m×4m; it has a reinforced concrete structure, an effective volume of 500m3, and a retention time of 2 hours. 　　④Salt dissolution tank, steel structure, 12m×5.5m×2.5m, total effective volume of 160m3. 　　⑤Sludge tank, steel pole structure, 4m×4m×3.5m, effective volume 48m3. This tank is located in the sludge dewatering room. 　　⑥Sludge dewatering room, brick-concrete structure, with an area of 200 m2. 　　⑦Clear water lift pump room, brick-concrete structure, with an area of 260 m2. 　　⑧Integrated factory building, brick-concrete structure. Including chlorine dioxide generators. Chemical dosing system, blower, sewage lift pump, regeneration system for adsorption tanks, and control room. 　　⑨The adsorption filtration plant is of brick-concrete structure, with an area of 280 m2 and external dimensions of 20m×14m×8m. It is equipped with 5 Φ3600 adsorption filters inside. 　　⑩The reverse osmosis plant is built with brick-concrete structure, measuring 24m×5m×3.5m. It is equipped with the reverse osmosis unit itself, safety filters, a water storage tank with a capacity of 10m3, high-pressure pumps, acid tanks, as well as acid washing pumps and other equipment. 2.2 Main process equipment and facilities 3.3 Economic benefits 　After the reuse of reclaimed water, 220×104 m3 of fresh water can be saved each year. At an industrial water usage cost of 2.5 yuan per ton in Dalian, the direct economic benefit amounts to 1.5 million yuan per year. 3.4 Environmental Benefits After treatment, the amount of pollution discharged into the environment can be reduced each year as follows: CODcr: 　　　　 {（120－30）×6000 + (30－20)×5000×10-6 ×365 = 215.35 tons} Suspended solids: 　　　　 (50－5)×6000×10-6×365 = 98.55 tons 4 Conclusion In June, the Municipal Economic Commission organized relevant experts to conduct a technical evaluation of this plan, which was well received by all the experts present. They agreed that the process design of this plan is reasonable and that it features a high level of technical sophistication. Once completed, this treatment station will help to address the shortage of water resources and bring economic benefits to enterprises ； On the other hand, it brings significant social and environmental benefits, reduces environmental pollution, and serves as a model for the development, utilization, and protection of water resources in the industry.