May I ask about the capacity of the accident emergency tank and the initial rainwater tank? Do they need to be entirely underground? What are the criteria for determining whether they can be built at the sewage treatment plant?

This issue mainly depends on applicability. The purpose of the so-called accident emergency tank is to handle water when the treatment equipment stops operating, yet sewage continues to flow into the treatment station. A reservoir established to hold wastewater that cannot be treated during a certain period of time. The basic requirement for this pool is that it must be impermeable. The specific size is determined by the amount of water flowing in from upstream; of course, the larger, the better. You can choose the depth underground as you wish, but one thing is for sure: leakage must be prevented. This is an environmental inspection item. As for where to build it, as long as it’s in your area, does not cause any odor pollution to the outside environment, and no one reports you, you can build it wherever you want

Is that accident tank different from the emergency tank? How is the volume or something like that calculated?

The accident tank is also known as an emergency tank; its capacity can be determined based on the normal water treatment volume and the downtime of the upstream facilities. For example, in the event of a power outage, especially when the time of restoration is unknown. This requires notifying the upstream companies to stop production as quickly as possible, because it is not possible to create infinitely large sewage containment ponds. It is by integrating these relative factors that an appropriate volume is determined. (For reference).

GB50483-2009 \"Code for Environmental Protection Design of Chemical Engineering Construction Projects\" specifies the method for calculating the capacity of accident emergency ponds. The capacity of such ponds should be determined based on factors such as the capacity of the equipment involved in the accident, the amount of water required for firefighting during an accident, and the amount of precipitation that may flow into the emergency pond. For general new construction, expansion, renovation, and technical upgrading projects, the capacity of the emergency accident tank shall be calculated using the following formula. V Accident Tank = (V1 + V2 + V_rain) – V3. Where: (V1 + V2 + V_rain)max represents the maximum calculated volume of emergency accident wastewater (in m3) ； V1 is the material storage volume (in m3) of the device (apparatus) or tank with the maximum capacity ； V2 represents the maximum amount of water required for firefighting in the equipment area or storage tank area in the event of a fire, explosion, or leakage. It includes the water needed to extinguish the fire as well as the water volume required for sprinkler systems to protect adjacent equipment or storage tanks (a minimum of 3), and can be determined in accordance with relevant standards such as GB50016, GB50160, and GB50074 ； Vrain refers to the maximum local rainfall that may enter this wastewater collection system in the event of an accident, and should be determined in accordance with the relevant provisions of GB50014 ； V3 is the sum of the net capacity within the equipment of the accident wastewater collection system or the tank area dike and fire dike (in m3), and the capacity of the pipes for conveying accident wastewater (in m3).