This article analyzes the hazardous factors and hazardous point sources of sewage treatment plants based on the A/A/O process for sewage treatment and the sludge digestion and dewatering process for sludge treatment. It also introduces the management and control measures for hazardous point sources.
Process flow and structures of sewage treatment plants
With the promulgation of the "Pollutant Discharge Standards for Urban Sewage Treatment Plants" (GB18918-2002) by the state in December 2002, sewage treatment technology has evolved from focusing only on removing organic matter to having phosphorus and nitrogen removal functions. Many new technologies, processes, and equipment have been introduced from abroad. AB method, oxidation ditch, A/O process, A/A/O process, SBR, CASS and other processes have been widely used in urban sewage treatment plants in China. Imported grilles, water pumps, blowers, dehydrators, mixers and other equipment are commonly used. The main structures include metering wells, inlet coarse grids, inlet pump rooms, fine grids, sedimentation tanks, primary sedimentation tanks, biological reaction tanks, distribution wells, secondary sedimentation tanks, outlet pump rooms, sludge concentration tanks, sludge storage tanks, sludge pump rooms, sludge digestion tanks, dewatering machine rooms, sludge silos, biogas compressor rooms, and biogas generator rooms. The main buildings include office buildings, substations, blower rooms, chlorination rooms, dosing rooms, dehydration rooms, etc.
Identification and distribution of hazardous point sources
According to the design and operation of the sewage treatment plant, the main hazards of major hazardous point sources are.
Distribution of occupational poisoning hazard sources
The source water of the sewage treatment plant comes from urban domestic sewage and industrial wastewater. When transported through the municipal pipeline network, it is already in a state of hypoxia. During the treatment process, toxic and harmful gases such as hydrogen sulfide and biogas in the sewage will be produced, dissolved, deposited or overflowed. Therefore, poisoning incidents may occur when workers enter the following areas: inlet grille, submersible pump room, sedimentation tank, distribution well, process gate well and culvert, sludge tank, digestion tank, biogas tank, dewatering room, rainwater and sewage pipeline and inspection well. The liquid chlorine, sulfuric acid, chemical flocculants used in the production process, and analytical reagents used in the laboratory that are released or inhaled by the human body can also cause poisoning incidents.
2 Distribution of Electric Shock Hazard Point Sources
The sewage treatment plant is a major electricity consumer, designed with high and low voltage transformation and distribution systems, and about 300 sets of equipment control boxes. During maintenance and operation, operators are prone to electric shock accidents due to improper operation, equipment failure, and the grounding and lightning protection system no longer being in a safe state. The main parts are: high and low voltage substation, inlet pump room distribution room, dosing room distribution room, blower room distribution room, ultraviolet disinfection channel distribution room, sludge control room distribution room, dewatering room distribution room, central control room, equipment control box
3 Distribution of fire hazard sources
In addition to the process structures, the sewage treatment plant also needs to construct auxiliary structures. The structures not only store flammable materials, but also have combustible materials. When the power supply of the structures ages, lightning strikes, improper use of electrical appliances, use of open flames, and other unsafe behaviors occur, fire hazards may occur: warehouses, comprehensive office buildings, high and low voltage substations, training buildings, access pump rooms, mechanical repair workshops, blower rooms, dosing rooms, sludge control rooms, and dewatering rooms.
4 Distribution of explosion hazard sources
The biogas generated during the sludge digestion process is not only toxic and harmful gas, but also flammable and explosive gas. Therefore, when workers enter the digestion tank, biogas tank, and sludge control room area, they must take effective measures and obtain a hot work order from the security and safety department before they can work. Equipment with high-pressure vessels or pipelines used in the production process (such as air compressors, blowers, and high-density pumps in the dehydration room) may experience explosion accidents due to the failure of safety devices.
Distribution of 5 drowning hazard sources
The process of sewage treatment requires a certain amount of residence time, and the effective water depth of the treatment structure is generally 3-6 meters. If a person falls into the water, it may cause drowning accidents due to the presence of toxic and harmful gases and sludge. The main structures are: inlet grille channel, sedimentation tank, primary sedimentation tank, reactor tank, secondary sedimentation tank, disinfection tank (channel), inlet and outlet water pump room collection tank, and mud storage tank.
6 Distribution of falling hazard sources
The structures for sewage and sludge treatment have the characteristic of large volume. In order to ensure gravity flow during the treatment process, the top of the structure is generally 2-3 meters above the ground in elevation design, and some structures will reach more than 10 meters. The pool depth of the structure is generally 3-7 meters. If the operator accidentally falls into the pool or on the ground, it may cause a fall accident. The main structures include: inlet grille channel, sedimentation tank, primary sedimentation tank, reactor, secondary sedimentation tank, disinfection tank (channel), pump room collection tank, sludge storage tank, digestion tank, biogas tank, and sludge silo.
7 Distribution of Hazardous Points for Mechanical Injuries
Wastewater treatment is a mechanized and automated production process. Each wastewater treatment plant has thousands of sets of mechanical equipment (coarse grid and press machine, fine grid and press machine, primary sedimentation tank scraper, blower, secondary sedimentation tank scraper, dosing pump, digestion tank sludge pump, dewatering machine sludge inlet pump, high-density pump, crane, electric gate). Its rotating parts can cause mechanical injuries to personnel, and lifting objects or steel wire rope breakage can cause lifting injuries.
Management and control of hazardous sources in sewage treatment plants
Safety management is a systematic work that requires not only establishing safety management institutions, implementing safety production responsibility systems, conducting effective safety inspections, education, and training, developing and implementing various safety operating procedures and technical operating procedures, and formulating various safety emergency plans to ensure that everyone understands and knows safety. And it is necessary to manage and control the hazardous sources of sewage treatment plants, take necessary technical measures, and control them to ensure that people, things, and the environment are in a safe state.
Control measures for occupational poisoning hazard sources
1. Install toxic and harmful gas detectors, automatic alarm devices, safety belts, safety ropes, air respirators, and personal protective equipment.
2. Protective masks and gloves must be worn during homework and rescue operations.
3. Install necessary ventilation equipment.
4. Set up safety warning signs at hazardous locations.
2 Control measures for electric shock hazard point sources
1. Regularly inspect and check electrical equipment.
2. Strengthen the insulation protection of voltage testers, insulated boots, insulated gloves, insulated rubber, etc., and ensure proper grounding protection.
3. Regularly inspect the lightning protection and grounding system.
4. Install a leakage protector.
5. Use electrical facilities that comply with regulations.
6. Develop safety operating procedures, adhere to job training for electrical operators, and ensure that they are certified to work.
7. Set up safety warning signs at hazardous locations.
3 Control measures for fire hazard sources
1. Regularly inspect, check, repair, and replace fire-fighting equipment and devices.
2. Regularly inspect the fire protection system and structural facilities.
3. Regularly replace fire extinguishers.
4. Use fire protection facilities that comply with regulations.
5. Implement the fire command system.
6. Strengthen personnel training.
7. Set up safety warning signs at hazardous locations.
4 Control measures for explosion hazard sources
1. Regularly inspect pressure vessels, pressure gauges, safety devices, and pressure relief devices.
2. Strengthen the operation and maintenance of electrical equipment.
3. Regularly inspect and detect hazards.
4. Develop safety operating procedures and strengthen personnel training.
5. Set up safety warning signs at hazardous locations.
5 Control measures for drowning hazard sources
1. Regularly inspect the protective railing.
2. Equipped with safety belts, safety ropes, lifebuoys, life jackets and other life-saving equipment.
3. Timely remove accumulated water, snow, and debris from the pool.
4. Develop safety operating procedures and strengthen personnel training.
5. Set up safety warning signs at hazardous locations.
6 Control measures for falling hazard sources
1. Regularly inspect ladders, protective railings, and baseboards.
2. Equipped with safety belts, safety ropes, lifebuoys, life jackets and other life-saving equipment.
3. Timely remove accumulated water, snow, and debris from the pool.
4. Strengthen personnel training and improve attention.
5. Set up safety warning signs at hazardous locations.
7 Control measures for hazardous sources of mechanical injury
1. Regularly inspect and check the transmission parts of the equipment.
2. Strengthen the safety protection devices for exposed moving parts.
3. Regularly inspect special equipment such as overhead cranes and forklifts.
4. Strengthen the training of special operators of field trucks and forklifts, and ensure that they hold relevant certificates before taking up their posts.
5. Develop safety operating procedures.
6. Set up safety warning signs at hazardous locations.
The safety control of sewage treatment process should be effectively indicated based on the status of safety risks. Necessary technical and engineering measures should be taken to strengthen the maintenance and upkeep of equipment and facilities, ensuring that they are in good working condition. Strengthen team safety management, implement job safety responsibility system, strictly implement safety and technical operation procedures to ensure normal production operation.
