Common problems and solutions for initial rainwater collection tanks and accident tanks

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Common Problems and Solutions for Initial Rainwater Collection Ponds and Contingency Ponds I. Initial Rainwater Collection Ponds 1. Common Problems with Initial Rainwater Collection Ponds The initial rainwater from chemical plants usually contains high concentrations of chemicals. In accordance with the principle of separating clean water from polluted water, such polluted initial rainwater needs to be collected separately and treated properly. Rainwater with a lower level of pollution, after undergoing simple pre-treatment to remove suspended solids and particulate impurities, can then be discharged directly into natural water bodies through the rainwater system. Common problems are as follows: 1) The calculation and collection of initial rainfall are not scientific; the design of the initial rainfall collection tanks is either too large or too small ； 2) No initial rainwater collection tank has been installed, or the initial rainwater tank is used interchangeably with the emergency accident tank ； 3) The layout of the pollution convergence areas is unreasonable; areas that may experience leaks and contamination as well as those affected by rainfall erosion have not been taken into account ； 4) Treat wastewater generated in indoor or underground areas as well as from manual cleaning operations as polluted initial rainwater ； 5) The rainwater and wastewater switching device relies on manual control, which results in a reaction delay; as a result, some of the rainwater that exceeds the specified standards overflows and is discharged outside ； 6) The collected initial rainwater was not treated after collection and was discharged directly. 2. Countermeasure: All rainwater within the factory premises is fed into the wastewater treatment system ； Or separate systems for rainwater and wastewater, with the rainwater drainage system having all of the following measures: ① A collection tank or rainwater monitoring tank to collect initial rainwater ； A shut-off valve is installed on the outlet pipe of the pool; under normal conditions, the valve remains closed to prevent contaminated water from being discharged ； The pool is equipped with lifting facilities that can transport the collected material to the wastewater treatment facilities within the plant for processing ； When it is not possible to use the device cofferdams or the fire dikes around the tank arrays to contain the accidental liquid, the outlet valves of the rainwater system and the shutters on the silt traps should be closed to cut off the connection between the flood control facilities and the outside world, thereby directing the accidental liquid into the intermediate accident buffer facility ； If no intermediate accident buffer facility is installed, discharge directly into the end accident buffer facility ； ②It is equipped with monitoring and closing facilities for the main discharge outlets of the rainwater system (including flood discharge channels), with dedicated personnel responsible for closing these outlets in emergency situations (including when the same drainage system is used for both rainwater and wastewater), in order to prevent rainwater, fire water, and leaks from entering the external environment ； When the regional drainage ditch passes through the plant area: ③ If there is a drainage ditch, it does not pass through the production area and tank area, and there are measures in place to prevent leaks and contaminated fire-fighting water from entering the regional drainage ditch. When the regional drainage ditch passes through the plant area: 1) It is not advisable to pass through the production area ； 2) Measures should be taken to prevent leaking flammable liquids and contaminated fire-fighting water from entering the area’s drainage ditches. 3) Liquefied hydrocarbon tank farms or flammable liquid tank farms should not be located adjacent to drainage ditches. II. Accident Tank 1. Common Issues with Accident Tanks An accident tank is a pool used to temporarily collect and store accident wastewater when the existing accident wastewater collection systems are not capable of meeting the required storage capacity. Common issues are as follows: 1) Lack of accident handling pools, resulting in poor capacity to respond to emergencies ； 2) The accident tank is used for other purposes; for example, the initial rainwater tank and the accident emergency tank are used interchangeably, with initial rainwater entering the accident tank ； Use the accident pool as a regulating tank ； 3) The accident collection system is not properly configured; during the handling of accidents, untreated wastewater ends up in the accident sewage collection system and subsequently in the accident emergency tank ； 4) The accident tank is improperly designed; its small capacity leads to overflow of accident wastewater. The accident tank is located at a higher elevation, so in the event of an accident, the wastewater from the accident cannot flow there by gravity ； 5) It does not take into account the situation where the volume of the accident tank is insufficient to meet the requirements for storing accident wastewater, in which case pressurization facilities are used to discharge the wastewater to other storage facilities ； The overflow pipe of the accident emergency pool is connected to the rainwater system ； 6) There is no wastewater treatment plan for accidents; the contaminated water is discharged directly. 2. Countermeasures: 1) Install accident drainage collection facilities such as emergency accident ponds, accident liquid storage ponds, or buffers for clean wastewater discharge in accordance with relevant design specifications; determine the capacity of these facilities based on the sensitivity of downstream environmental receptors to pollution and the likelihood of extreme weather conditions ； 2) The accident liquid storage tanks, emergency accident water tanks, and buffer tanks for the discharge of clean wastewater – such as facilities used for collecting accident-related wastewater – are located in appropriate positions; they enable the passive collection of leaks and fire-fighting water under accident conditions, while also ensuring an adequate capacity for storing accident-related wastewater on a daily basis ； 3) Install pumping facilities connected to the sewage pipelines, so as to transport the collected waste to the wastewater treatment facilities within the plant for processing ； 4) The installation of the accident liquid storage tank shall comply with the following provisions ； 5) Tank groups equipped with accident liquid storage tanks should be provided with drainage pipes (ditches) to allow any spilled liquid to flow out of the tank group smoothly. The distance between the accident liquid storage tank and the fire dike should be no less than 7 m, while the distance between the accident liquid storage tank and open flames should be no less than 30 m. The accident liquid storage tank must be equipped with drainage facilities ； 6) Treatment of accident wastewater: The materials contained in accident wastewater should be recovered ； Accidental wastewater should be sent to a sewage treatment plant for treatment; if it cannot be sent there, it must be disposed of properly ； Accidental wastewater that can be treated biologically should be restricted in flow as it enters the biological wastewater treatment system ； The monitoring parameters for accident discharge should be determined based on the type of material ； When handling accident wastewater, safety precautions should be taken based on factors such as the volatility and toxicity of the substances involved. According to the \"Code for Fire Protection Design of Petrochemical Enterprises\" (GB50160-2008), wastewater containing flammable liquids and heavily polluted rainwater should be discharged into the production wastewater pipelines. However, condensates of flammable gases, as well as certain types of water, must not be discharged directly into these pipelines. Water with a temperature exceeding 40°C, as well as wastewater that can undergo chemical reactions upon mixing that may lead to fires or explosions, must also not be discharged directly; instead, it should be mixed with the wastewater already present in the pipelines. 1) According to the \"Design Code for Sewage Treatment in Petrochemical Industries\" (GB50747-2012), 3.1.1 states that the designed sewage volume should include the amount of production wastewater, domestic wastewater, polluted rainwater, and unforeseen wastewater. The amounts of various types of wastewater shall be determined in accordance with the following provisions: 1. The amount of production wastewater shall be determined by combining the wastewater discharge volume during continuous operating hours and that during intermittent operating hours for each unit or device ； 2 The volume of domestic wastewater shall be determined in accordance with the relevant provisions of the current **standard, the ‘Code for Design of Outdoor Drainage Systems’ GB500014 ； 3. The capacity of the storage facility for polluted rainwater should be calculated as the product of the area of the polluted area and the rainfall depth. It can be calculated using the following formula: V = Fh/1000. Where: V – capacity for storing polluted rainwater (m3) ； h – rainfall depth; it is advisable to use a value between 15 mm and 30 mm. Based on an analysis of the intensity of heavy rains in more than a dozen cities across the country, it was found that after 5 minutes of initial rainfall, the polluted areas were essentially cleaned up. The 5-minute rainfall depth is mostly between 15 mm and 30 mm ； F — Area of the polluted zone (m2). 4. The amount of polluted rainwater should be determined by calculating the storage capacity of water that can be contaminated by a single rainfall event, as well as the time required for that polluted water to be converted into a continuous flow. It can be calculated using the following formula: Qr = V/t. Where: Qr – amount of polluted rainwater (m3/h) ； t —— the time (in hours) during which polluted rainwater is converted into a continuous flow, and can be selected between 48 h and 96 h. 5 The unforeseen sewage volume should be set at 10%~20% of the continuous hourly wastewater discharge volume of each process unit (cell), including water leakage due to accidents and seepage. Code for Environmental Protection Design of Chemical Engineering Construction Projects (GB50483-2009) 6.1.8/6.6.1 Chemical engineering construction projects shall be equipped with emergency accident ponds. 6.6.2 The wastewater discharged into the emergency accident tank shall be subject to necessary monitoring, and the following treatment measures shall be taken: 1 It shall be reused if possible ； 2 Wastewater that does not meet the requirements for reuse but complies with emission standards can be discharged directly ； 3 For wastewater that does not meet the emission standards but satisfies the requirements for inlet water at the sewage treatment plant, its flow rate should be restricted to allow it to enter the plant for treatment ； 4 For wastewater that does not meet the requirements for inlet water at the sewage treatment plant, treatment measures should be taken or the wastewater should be sent elsewhere for treatment. 6.6.3 The capacity of the emergency accident tank should be determined comprehensively based on factors such as the capacity of the equipment where an accident occurs, the amount of water required for firefighting during an accident, and the amount of precipitation that may enter the emergency accident tank (Capacity of emergency accident tank = Maximum calculated volume of emergency accident wastewater – Net capacity within the enclosure of the plant or tank area – Capacity of the pipes for carrying accident wastewater). The capacity of the emergency accident pool should be determined comprehensively based on factors such as the capacity of the equipment where an accident occurs, the amount of water required for firefighting during an accident, and the amount of precipitation that may enter the emergency accident pool. The maximum amount of wastewater from emergency incidents is calculated as: 1, the amount of material in the largest-capacity equipment or tank ； 2 The amount of water required for firefighting in the plant area or tank storage area in the event of a fire or explosion, including the water needed to extinguish the fire and the water volume for sprinkler systems to protect adjacent equipment or tanks (a minimum of 3) ； 3 The maximum rainfall in the area. When calculating the wastewater volume from emergency accidents, accidents in the plant area or storage tank area are not considered to occur simultaneously; the maximum value among them is taken).