Wastewater treatment in chemical analysis

Collect centrally, process centrally, and release only after it meets the standards

Process centrally, and discharge only after it meets the standards

Phosphides——addition of alkaline earth metal salt solutions (Mg2+/Ca2+, precipitation); potassium compounds (unknown); heavy metals (S2- ions, or replacement with iron powder) ---------long explanation--------------- Wastewater treatment process: Modern wastewater treatment technologies can be classified into primary, secondary, and tertiary treatment, based on the level of purification achieved. Primary treatment primarily removes solid pollutants in wastewater that are in a suspended state; most physical treatment methods can only meet the requirements of primary treatment. For wastewater that has undergone primary treatment, approximately 30% of the BOD can usually be removed, but this level does not meet the discharge standards. Primary treatment is a pre-treatment for secondary treatment. Secondary treatment primarily removes organic pollutants in the form of colloids and dissolved substances from wastewater (such as BOD and COD), with a removal rate of over 90%, thereby ensuring that these organic pollutants meet the discharge standards. Tertiary treatment involves further processing of refractory organic substances, as well as soluble inorganic substances such as nitrogen and phosphorus that can cause eutrophication in water bodies. The main methods include biological nitrogen and phosphorus removal, coagulation sedimentation, sand ratio method, activated carbon adsorption, ion exchange, and electroosmotic analysis. The entire process involves raw wastewater, which has been pre-treated by coarse screening, being pumped up using a sewage lift pump. It then passes through further screening devices or sieves before entering a grit chamber. The wastewater after sand-water separation proceeds to a primary sedimentation tank; this constitutes the first stage of treatment, namely physical treatment. The effluent from the primary sedimentation tank is sent to biological treatment units, which can use either the activated sludge method or the biofilm method. (The activated sludge method includes reactors such as aeration tanks and oxidation ditches, while the biofilm method encompasses biological filters, biological rotors, biological contact oxidation processes, and biological fluidized beds.) The effluent from these biological treatment units goes to a secondary sedimentation tank. The water from the secondary sedimentation tank is either discharged after disinfection or sent on to tertiary treatment. Once the first stage of treatment is complete, we reach the second stage of treatment. Tertiary treatment methods include biological nitrogen and phosphorus removal, coagulation sedimentation, sand filtration, activated carbon adsorption, ion exchange, and electrodialysis. Part of the sludge from the secondary sedimentation tank is returned to the primary sedimentation tank or biological treatment equipment, while another part goes to the sludge thickening tank and then to the sludge digestion tank. After passing through dewatering and drying equipment, the sludge is finally utilized. The above is the basic process flow of a sewage treatment plant; the flowchart is shown in Figure 1 on the next page. II. Energy consumption analysis of various treatment structures 1. Sewage lift pump room: The sewage entering the wastewater treatment plant passes through a coarse screen before reaching the sewage lift pump room, where it is then lifted by pumps to the pre-chamber of the grit chamber. The operation of water pumps consumes a large amount of energy, accounting for a significant portion of the total energy consumption in wastewater treatment plants; this is related to the flow rate of wastewater and the head that needs to be lifted. 2. Sand settling tank: The function of the sand settling tank is to remove inorganic particles with a higher specific gravity. Sedimentation tanks are generally installed in front of pump stations and inverted siphons to reduce the wear caused by inorganic particles on pumps and pipes ； It can also be placed in front of the primary sedimentation tank to reduce the load on the sedimentation tank and improve the operating conditions of the sludge treatment facilities. Commonly used grit chambers include horizontal flow grit chambers, aeration grit chambers, Dor grit chambers, and bell-type grit chambers. The components in the grit chamber that require energy supply are mainly the sand-water separator and sand suction pump, as well as the aeration system of the aerated grit chamber, as well as the power systems of the Dor and bell-type grit chambers. 3. Primary sedimentation tank: The primary sedimentation tank is the main treatment structure in primary wastewater treatment plants, or it is used as a pretreatment structure in secondary wastewater treatment plants, located in front of the biological treatment units. The targets for treatment are SS and part of the BOD5, which can improve the operating conditions of biological treatment facilities and reduce their BOD5 load. Primary sedimentation tanks include horizontal flow sedimentation tanks, radial flow sedimentation tanks, and vertical flow sedimentation tanks. The main energy-consuming equipment in the primary sedimentation tank is the sludge removal system, such as chain-type sludge scrapers, sludge and scum skimmers, and sludge pumps; however, due to the impact of the sludge removal cycle, the energy consumption of the primary sedimentation tank is relatively low. Figure 1: Typical process flow for municipal sewage treatment 4. Biological treatment facilities: The energy consumption associated with the biological treatment of sewage accounts for a significant proportion of the total direct energy consumption in a sewage treatment plant; together with the energy consumption related to sludge treatment, these two aspects account for over 60% of the plant’s total direct energy consumption. Aeration in the aeration system of the activated sludge process consumes a large amount of electrical energy; it operates continuously and with high power, otherwise satisfactory aeration results cannot be achieved, and the treatment efficiency will also be poor. The aerators installed in the oxidation ditch treatment process are also equipment that consume a large amount of energy. Compared to the activated sludge process, biofilm treatment equipment requires less energy, but it is currently used less frequently; it is a treatment technology that needs to be widely promoted in the future. 5. Secondary sedimentation tank: The main energy consumption in a secondary sedimentation tank is related to the pumping of sludge and the removal of floating substances from the surface water; its energy consumption is relatively low. 6. Sludge treatment: The thickening tanks, sludge dewatering, and drying processes in sludge treatment require a large amount of electrical energy. The energy consumption of sludge treatment units is quite high, as these devices all have high power demands. III. Energy-saving approaches for various treatment facilities 1. Sewage lift pump rooms To reduce energy consumption in sewage lift pump rooms, it is important to consider how to save electricity used by these pumps. Choosing the right type of pump and ensuring that it operates within its most efficient range are effective strategies. Making use of the terrain to reduce the height to which sewage needs to be lifted is also an effective way to lower the shaft power required by the pumps. Regular maintenance of the pumps to minimize friction can likewise help reduce electricity usage. 2. Sand settling tank: Horizontal flow sand settling is used, avoiding the use of sand settling tanks that require power-driven equipment, such as horizontal flow sand settling tanks. Using gravity sand discharge instead of mechanical sand discharge can **reduce energy consumption**. 3. Primary sedimentation tank: The energy consumption of the primary sedimentation tank is relatively low; the main energy use occurs in the sludge discharge equipment. Employing the hydrostatic pressure method will undoubtedly reduce energy consumption significantly. 4. Biological treatment facilities: Scholars abroad have compared various biological treatment processes through energy consumption and cost-benefit analyses. They believe that most of the energy consumption in such treatment facilities occurs in devices such as motors; therefore, energy savings should be achieved by improving the power factor across the entire facility, selecting high-efficiency mechanical and electrical equipment, and reducing peak electricity demand. The energy-saving measures they proposed include improving the electrical performance of motors, addressing operational process issues, as well as energy recovery from the outputs of wastewater treatment plants. The energy consumption of aeration systems is quite high, and research on improving the energy efficiency of these systems always involves the modification and innovation of the aeration equipment. Although new types of aeration equipment keep emerging, they can still be divided into two categories: the first category uses submerged porous diffuser heads or air nozzles to create air bubbles that deliver oxygen into the aqueous solution; the second category employs mechanical methods to stir the wastewater in order to dissolve oxygen from the atmosphere into the water. Micro-porous aeration, the layout of the aeration diffusers, and the adjustment of the aeration system are all effective measures for saving energy. An energy-saving, biological phosphorus removal scheme that creates an anaerobic zone at the front end of the aeration tank in traditional activated sludge treatment plants, using submersible mixers for mixing. This simple modification can save nearly 20% in aeration energy consumption; when the energy used for mixing is taken into account, the energy savings amount to 12%. Automatic control systems are used to save energy in wastewater treatment; the aeration system operates in stages, creating a concentration gradient of dissolved oxygen. This not only reduces energy consumption but also improves the treatment efficiency and decreases the amount of sludge generated. The use of anaerobic treatment in the biofilm process can significantly reduce energy consumption. 5. Secondary sedimentation tank: Research on sludge discharge equipment and improvements to sludge discharge methods in the secondary sedimentation tank are effective ways to reduce energy consumption. 6. Sludge treatment: Research on energy conservation in sludge treatment systems focuses mainly on energy recovery from sludge treatment. The recovery of energy from organic pollutants in sewage sludge for use in treatment processes was put into practice as early as the beginning of the last century, but it was not given much attention until before the energy crisis. Currently, there are two recycling methods: one is the utilization of biogas generated from sludge anaerobic digestion, and the other is the utilization of the heat produced by sludge incineration. Digestate has stable properties and is easy to store; it can be converted into mechanical or electrical energy through internal combustion engines or fuel cells, and the waste heat can also be recovered to heat the digestate sludge. Therefore, utilizing biogas can address the issue of varying degrees of energy self-sufficiency in wastewater treatment plants. Lin Rongchen and others compared the two utilization methods of biogas generators and fuel cells, concluding that fuel cells have a high energy efficiency and hold great potential for development. Maximizing the utilization of digestate gas is the main way to improve energy efficiency. Research and application of biogas generator sets for grid-connected power generation already have practical examples in China, representing a viable approach for the comprehensive utilization of biogas in large-scale sewage treatment plants. Another method of energy recovery is to locate urban solid waste incineration plants next to wastewater treatment plants, where the solid waste is burned along with wastewater sludge, and the electricity generated is used to power the operation of the treatment plants. Research on energy consumption analysis in municipal wastewater treatment often does not keep pace with the development of energy-saving technologies and methods. Due to the lack of research on energy balance analysis methods for wastewater treatment, the formulation and implementation of energy-saving measures often proceed ahead of schedule. Most energy-saving approaches and methods are often proposed by the operators of treatment plants, taking into account the specific conditions of each treatment facility; they are based on experience and are specific to particular situations, so they may not be applicable to other wastewater treatment plants, even those with similar processes. On the other hand, in a broader sense, technological innovations within the field of wastewater treatment, as well as the use of new materials and equipment, all hold potential for saving energy and improving efficiency; therefore, the avenues for energy savings are generally quite diverse. IV. Conclusion Water treatment is an energy-intensive comprehensive technology. For some time now, high energy consumption and high operating costs have, to a certain extent, hindered the construction of urban sewage treatment plants in China. Some of the plants that have been built are also shut down or operating at reduced capacity due to energy-related issues. For a considerable period to come, energy consumption will remain a bottleneck in urban wastewater treatment. Whether it is possible to address the energy consumption issues in wastewater treatment plants and to allocate energy efficiently has become a key factor determining the operational efficiency of such plants. Low energy consumption is also a decisive factor in the feasibility analysis of new wastewater treatment plants in the future. Developing wastewater treatment technologies with higher energy efficiency, as well as designing and operating wastewater treatment plants in a rational manner, will undoubtedly be the way forward for their design and operation.

Citing the laboratory rules and regulations compiled by the moderator of \"A Grain of Sand in the Vast Ocean\", 3: Treatment of chromium-containing wastewater. 3.1 Principle of the treatment method: Adjust the pH of the chromium-containing wastewater to below 3, add sodium bisulfite to reduce Cr(VI) to Cr(III), and then adjust the pH of the wastewater to between 7.5 and 8.5 so that Cr(III) forms Cr(OH)3 precipitates. (If the wastewater also contains metal ions such as mercury and silver, use Ca(OH)2 to prepare lime slurry, adjust the pH of the wastewater to between 8 and 9 to cause Cr(III) to form Cr(OH)3 precipitates, and then add NaHS to cause mercury and silver to form sulfide precipitates.) 3.2 Operating Steps 3.2.1 Add concentrated sulfuric acid to the waste liquid bucket, stir thoroughly, and adjust the pH of the solution to below 3 (measure using pH paper or a pH meter). If the solution is already acidic, there is no need to adjust the pH value. 3.2.2 Add solid sodium sulfite in small amounts over time, while stirring, until the solution changes from yellow to green. 3.2.3 If the solution contains only chromium ions, add a 50 g/L sodium hydroxide solution to adjust the pH of the solution to 7.5–8.5, thereby causing Cr(III) to form a precipitate (note: if the pH is too high, the precipitate will dissolve again). The waste liquid is left to stand for one night; the precipitate is filtered out, dried, and stored properly. The filtrate is tested for total chromium and hexavalent chromium in accordance with the \"Methods for Monitoring and Analyzing Water and Wastewater\" (compiled by the Environmental Protection Bureau); if it meets the requirements of GB8978 \"Comprehensive Wastewater Discharge Standards\", it can be discharged directly into the sewer system. 3.2.4 If the solution also contains metal ions such as mercury and silver (e.g., waste liquid used for COD measurement), after reducing hexavalent chromium with sodium sulfite, calcium hydroxide prepared as lime milk is added. The solution is stirred thoroughly to bring its pH to 8–9. Once the solution becomes clear, an appropriate amount of sodium hydrosulfide is added (the amount added in moles is equal to the number of moles of precipitable metal ions present), and the mixture is stirred again to maintain a pH of 8–9. The waste liquid is left to stand overnight, after which the precipitate is filtered out, dried, and stored properly. The filtrate is tested for total chromium, hexavalent chromium, total silver, and total mercury in accordance with the \"Methods for Monitoring and Analyzing Water and Wastewater\" (compiled by the Environmental Protection Bureau). After it meets the requirements of GB8978 \"Comprehensive Wastewater Discharge Standards\", it is checked to determine whether sulfur ions are present in the filtrate (a small amount of the filtrate is mixed with a few drops of 1 mol/L zinc acetate solution; if no precipitate forms, then no sulfur ions are present; otherwise, sulfur ions are present). If sulfur ions are present, they can be oxidized using hydrogen peroxide, and after neutralization, the mixture can be discharged directly into the sewer system. 4 Treatment of arsenic-containing wastewater 4.1 Principle of the treatment method First, calcium hydroxide solution is added to the wastewater to precipitate most of the arsenic; then ferric chloride is added to cause arsenic to precipitate together with iron, thereby separating the arsenic. 4.2 Procedure: When the waste liquid contains large amounts of arsenic, add a saturated calcium hydroxide solution to adjust the pH of the waste liquid to around 9.5. Stir thoroughly, allow it to settle and become clear, and then filter it. Solid ferric trichloride is added to the filtrate to achieve an arsenic-to-iron ratio of 50 (on a mass basis). The pH of the filtrate is adjusted to 7–10 using sodium hydroxide, and it is left to stand for one night. After that, filtration is carried out, and the residues from both filtrations are dried and stored properly. The final filtrate is tested for total arsenic in accordance with the \"Methods for Monitoring and Analyzing Water and Wastewater\" (compiled by the Environmental Protection Bureau); once it meets the standards specified in GB8978 \"Comprehensive Wastewater Discharge Standards\", it is neutralized to a neutral pH before being discharged directly into the sewer system. 5 Treatment of zinc-containing wastewater 5.1 Principle of the treatment method When the pH of the wastewater is between 8 and 9, sodium hydrosulfide solution is added, causing the zinc ions in the wastewater to react with sulfide ions to form zinc sulfide precipitate, thereby removing the zinc ions. 5.2 Procedure: Dilute the concentration of zinc ions in the waste liquid to below 1% using water. Adjust the pH of the waste liquid to 9.0–9.5, add an appropriate amount of sodium hydrosulfide and stir thoroughly; then add a small amount of ferric trichloride and stir again. Adjust the pH of the waste liquid to above 8.0, and leave it to stand for one night. Filter the precipitate using the decantation method. Dry and store the precipitate properly. The filtrate is tested for zinc ions in accordance with the \"Methods for Monitoring and Analyzing Water and Wastewater\" (compiled by the Environmental Protection Bureau). After meeting the requirements of GB8978 \"Comprehensive Discharge Standards for Wastewater\", it is then tested for sulfur ions; if a small amount of the filtrate is mixed with a few drops of 1 mol/L zinc acetate solution and no precipitate forms, then there are no sulfur ions present, otherwise sulfur ions are present. If sulfide ions are present, they can be oxidized using hydrogen peroxide; after neutralization, the solution can be discharged directly into the sewer after dilution. 6 Treatment of cadmium-containing wastewater 6.1 Principle of the treatment method Calcium hydroxide is used to convert divalent cadmium ions into cadmium hydroxide, which is insoluble in water, thereby separating cadmium. 6.2 Procedure Add calcium hydroxide, prepared as lime milk, to the waste liquid to adjust its pH value to 10.6–11.2, stir thoroughly, and let it stand for one night. Filter the precipitate using the decantation method. Dry and store the precipitate properly. The filtrate is tested for cadmium ions in accordance with the \"Methods for Monitoring and Analyzing Water and Wastewater\" (compiled by the Environmental Protection Bureau); once it meets the requirements of GB8978 \"Comprehensive Wastewater Discharge Standards\", the filtrate is neutralized and discharged directly into the sewer system.