## 3 Results and Discussion  
### Causes of Poisoning and Suffocation Accidents in Pre-Treatment Pools  
The Bow-Tie model for biogas poisoning and suffocation accidents in pre-treatment pools is presented in Figure 2. The meaning of each symbol can be seen in Table 1.  
The Bow-Tie model was used to analyze Biogas poisoning and suffocation in the pre-treatment pool. In the Bow-Tie model, the top event represents the biogas poisoning and suffocation accidents in the pre-treatment pool. The fault tree on the left analyzed the root cause of the top event through logical combination, including aspects of the ventilation system, gas detection alarm, protective equipment, and so on. The event tree on the right analyzed consequences after the occurrence of the top event. It can be known from the analysis results that as long as the harmful gases can be controlled within the standard, the top incident will not occur. Effective ventilation equipment, prevention of personnel entering illegally and wearing protective equipment are all effective mitigation barriers.  
In accordance with the provisions of 'Technical Specifications for Biogas Engineering Part 1: Engineering Design', the pre-treatment pool workshop should be equipped with combustible gas alarm devices and toxic gas alarm devices, namely, a methane gas alarm and a hydrogen sulfide gas alarm. When natural ventilation cannot meet the requirements  
Figure 2: The Bow-Tie model for biogas poisoning and suffocation accidents in pre-treatment pools.  
of the factory, forced exhaust can be used. At this time, the gas alarm devices in the workshop should be interlocked with the forced ventilation facilities.  
The pre-treatment pool workshop meets the definition of confined space in the 'Safety Regulations for Confined Space Operations in Industrial and Trade Enterprises', that is, confined space refers to a closed or partially closed space not designed as a fixed workplace and can be entered by personnel for work, which is prone to cause hazardous materials accumulation (toxic, harmful, flammable and explosive substances) and insufficient oxygen content. If operators need to enter the pre-treatment pool workshop to carry out operations, the operations are called confined space operations. Before entering the pre-treatment pool workshop, operators must strictly abide by the confined space operation requirements. Before entering, operators must conduct a gas test to check whether the oxygen concentration in the workshop meets the standard. If it does not meet the standard, forced ventilation is required. After the concentration test meets the standard, operators can enter the workshop. In addition, as the pre-treatment pool workshop is prone to biogas poisoning and suffocation accidents, protections such as respiratory equipment and safety belts should be provided.  
The Bow-Tie model elucidated the logical relationship for biogas poisoning and as-phyxiation accidents. It is evident that the prevention of such incidents can be addressed through the following measures: establishing an efficient forced ventilation system, prohibiting unauthorized entry into the pre-treatment pool workshop by workers, and ensuring strict adherence to protective requirements.