Study of outdoor rainwater drainage pollution in a hospital

The sanitary safety of the outdoor rainwater drainage system in a hospital has always been an essential aspect of controlling the spread of viruses (Hong et al. 2023). When a hospital's rainwater drainage system is imperfect, viruses, pathogenic bacteria, and other pathogens will be transmitted through water bodies, causing severe public health safety hazards (Zhang & Wang 2020; You et al. 2023). While focusing on the sewage drainage system in a hospital, it is crucial to ensure the sanitary safety of the rainwater drainage system (Fontanella et al. 2021; Niu et al. 2023).

Relevant standards include Outdoor Drainage Design Standards (GB 50014-2006), Building Design Code for Infectious Disease Hospitals (GB 50849-2014), Environmental Quality Standards for Surface Water (GB 3838-2002), Emission Standards for Water Pollutants in Medical Institutions (GB 18466-2005), Comprehensive Hospital Building Design Code (GB 51039-2014), Building Water Supply and Drainage Design Standard (GB 50015-2019), and Infectious Disease Hospital Building Construction and Acceptance Code (GB 50686-2011).

The hospital is a comprehensive hospital with an infectious disease area, which is located in the south of Zhejiang Province, covering a total area of 350,000 m², 3,380 approved beds, 5,162,300 outpatient and emergency visits per year, 196,500 discharges, and an annual water consumption of nearly 1 million tons, with a treatment capacity of 2,850 m³/d at the sewage treatment station. The hospital has also built new functional buildings such as negative pressure isolation wards, square cabin wards, and nucleic acid testing sites, and many changes have been made to the structure of the pipe network of the drainage system.

The hospital is in the centralized living drinking water surface water source secondary protection zone. The hospital rainwater collection through the outdoor rainwater pipe discharges into the surrounding river. The rainwater quality is required to meet the water quality criterion of Class III indicated in the Environmental Quality Standard for Surface Water (GB 3838-2002). Rainwater quality testing (Suleman et al. 2023) is a vital judgment basis for analyzing and judging whether the operation of the rainwater drainage system is standard. Due to the enormous rainwater pipe network, this project requires more times of water quality testing. After careful consideration of efficiency and economy, the ammonia nitrogen and chemical oxygen demand is an essential comprehensive index to evaluate the pollution degree of rainwater drainage (Wei et al. 2019; Wang et al. 2022). In this paper, the determination of ammonia nitrogen was performed by the nascent reagent spectrophotometric method (HJ 535-2009), and the resolution of chemical oxygen demand was performed by the dichromate method (HJ 828-2017). The hospital rainwater pipe network drains on the non-rainy day. The test results of the outlet water quality on the non-rainy day are shown in Table 1. The locations of test sections Q1, Q2, and Q3 are shown in Figure 1. Testing data show that the hospital discharge of rainwater quality in ammonia nitrogen and chemical oxygen demand is seriously exceeded. The rainwater pipe has been polluted. Medical sewage is discharged into the river through the rainwater pipe. There is the risk of environmental pollution, chemical substances, and pathogens (Ma & Xiong 2022). Analysis of the causes of pollution of rainwater discharge and exploring the corresponding countermeasures have important research significance.

The data for water quality analysis were obtained from field sampling and testing. Statistical analysis was carried out using SPSS25 software to derive the data from the samples (Suleman et al. 2023). The test of normality distribution was carried out to obtain the level of significance under Kolmogorov–Smirnov and Shapiro–Wilk tests. The test of normality was carried out by the statistical test method. When the result of the Kolmogorov–Smirnov test and the Shapiro–Wilk test had a p-value less than 0.05, the data were considered not to satisfy normality. On the contrary, the data are considered to satisfy normality. However, the above tests have some limitations. Therefore, the normality of the data is determined by histogram. In the histogram, the data show a bell-shaped distribution and high in the center, and gradually decrease at both ends. The left and right sides show symmetry or near symmetry.

Skewness characterizes the degree of asymmetry of the density function curve of a probability distribution with respect to the mean. If the skewness of a normal distribution is 0, then both tail lengths are symmetrical (Raimondi et al. 2023). If the skewness is negative, then there is less data to the left of the mean than to the right. This is visualized by the fact that the tails on the left are longer relative to the tails on the right. The left tail of the curve is dragged out because of the small values of a few variables. The left tail is longer than the right. The majority of values, including the median, lie to the right of the mean. Values to the left of the mean are few and small. A positive skewness means that fewer data lie to the right of the mean than to the left. This is visualized by the fact that the tails on the right are longer relative to the tails on the left. The right tail of the curve is dragged out because of the large values of a few variables. The right tail is longer than the left, and the majority of values lie to the left of the mean.

It is known from the settlement mechanism that buried pipelines are prone to fracture due to uneven settlement at their indoor–outdoor junctions. For the study of pipe mechanics analysis (Winter et al. 2023), Teodoru (2009) calculated and analyzed the force deformation of buried pipes by analyzing elastic beams placed in a semi-infinite space elastic foundation. Limura (2004) proposed an equation for calculating the deflection of buried pipes based on the Winkler elastic foundation beam model under the effect of settlement to analyze the pipe forces. However, in practical engineering, the relative settlement is a dynamically changing quantity (Auddy et al. 2022). The buried pipelines often fail due to progressive damage during settlement occurrence.

where M is the bending moment acting on the cross-section, N·m; W_Z is the bending section coefficient of the pipe, m³; E is the modulus of elasticity of the pipe, GPa; y is the deflection of the pipe, m; and l is two times the horizontal distance between two adjacent extreme points, m.

where σ_r is the pipe equivalent stress, MPa; σ_a is the pipe axial stress, MPa; σ_b is the pipe circumferential stress, MPa; σ_c is the pipe radial stress, MPa; and σ is the allowable stress of the pipe, MPa.

In this paper, the repair of geologically significant settlement areas is achieved using reinforcement treatment with the replacement of geological foundations. According to the current national standard ‘Design Code for Building Foundation’ (GB 50007-2011), a composite foundation of medium-coarse sand, sand and gravel, and gravel soil is used for the pipe network area with plastic material pipes. A blended foundation of C20 concrete, sand and gravel, and gravel soil is used for the pipe network area with metal pipes. In addition, for the area where no obvious geological settlement occurs but does not meet the requirements of the characteristic value of foundation bearing capacity (Ye & Zhong 2019), its geological foundation is also reinforced accordingly to avoid pipe breakage and pipe well settlement caused by the aggravation of geological settlement at a later stage. At the same time, the degree of geological settlement is evaluated through regular elevation measurements of the buried pipeline laying area to prevent damage to the pipeline network caused by uneven geological settlement in many aspects.

The pipeline interface should be determined according to the pipeline material and geological conditions and should conform to the relevant provisions of the current national standard ‘Seismic Design Code for Outdoor Water Supply and Drainage and Gas Heat Engineering’ (GB 50032-2003). The drainage pipe is made of polyethylene (PE) solid wall pipe with high resistance to deformation, and the drainage pipe is prevented from breaking and leaking by using hot fusion and electric fusion connection. For flexible connections to avoid the leakage of plastic pipes, such as the connection of socket-type elastic seal, the interface sealing needs to be focused on. The current national code is still mainly based on the traditional gravity drainage system, the vacuum drainage system can reduce outdoor drainage lines and has advantages in reducing outdoor pipe breakage and leakage (Hong et al. 2023).

The connection of inspection wells and plastic pipes should comply with the relevant provisions of the current national standard ‘Seismic Design Code for Outdoor Water Supply and Drainage and Gas Heat Engineering’ (GB 50032-2003). The connection between the drainage pipe and the inspection well is prone to rupture, which is repaired by strengthening the sealing to prevent leakage at the rupture. Plastic-finished inspection wells have the advantage of good sealing and are used in the inspection wells of outdoor drainage pipes, which can effectively prevent the risk of leakage caused by broken pipe wells. The elevation of the similar rainwater pipe network should be higher than the sewage pipe network and set the isolation layer at essential locations to prevent pollution of the rainwater pipe when the sewage pipe leaks.

In addition to solving the damage of pipes and tube wells caused by uneven geological settlement, the project also rehabilitated and repaired the problems of mixed rainwater and sewage pipes and polluted water sources in the pipe network. Through the water test of water collection well pumps and sewage pipes, the mixed rainwater and sewage pipes were identified, and the mixed pipes were repaired according to the requirements of rainwater and sewage diversion. The water quality in the drainage pipes of condensate, equipment water, fire water, and rainwater catchment wells was tested, and the pipeline was repaired for polluted water sources to prevent polluted water sources from entering the rainwater pipes (Wilkinson et al. 2016; Winter et al. 2023).

For the hospital's new negative pressure isolation wards, square cabin wards, nucleic acid testing sites, and other functional buildings, the prevention of cross-infection needs to be considered. Its outdoor rainwater will increase the risk of virus transmission once it is affected by sewage. Therefore, the health and safety requirements of rainwater drainage are higher than those of other conventional buildings in the hospital. Its drainage pipes need to be set up separately. They are required to prevent leaking sewage and polluted rainwater from being discharged directly into the river or seeping down into the groundwater system. High density polyethylene (HDPE) impermeable membranes are used to pave the ground in areas such as negative pressure isolation wards (Qin et al. 2020; Peng et al. 2021), and rainwater runoff is collected uniformly with a rainwater runoff coefficient of ψ = 1.0. When negative pressure isolation wards are not in operation, the initial rainwater is discharged to the sewage pipeline, and other rainwater is released to the rainwater pipeline. When negative pressure isolation wards operate, the collected rainwater runoff is disinfected with 10% sodium hypochlorite to meet the standard and then discharged into the sewage pipe.

The results of the water quality test data after the implementation of the measures are shown in Table 2. The concentration of ammonia nitrogen far exceeded the water quality range of Class III water in the Environmental Quality Standard for Surface Water (GB 3838-2002) (Fan et al. 2014). It indicates that the water quality is poor. After implementing the measures, the NH₃-N concentration in the rainwater pipe decreased significantly. The average removal rate was 97.4%, and the water quality was stable within the range of Class III surface water.

Before the implementation of the countermeasures, the COD value greatly exceeded the water quality range of surface water category III. After implementing the measures, the COD concentration also decreased significantly and remained stable. The average removal rate was 91.8%, which was within the water quality range of Class III surface water.

The results show that implementing countermeasures can effectively deal with the problem of outdoor rainwater drainage pollution in the hospital.

This paper takes a large general hospital as the research object and analyzes the causes of its outdoor rainwater drainage pollution. The systematic analysis method using segment-by-segment screening found that the broken pipe network caused by uneven geological settlement is the main reason for the pollution of rainwater pipes. By studying the mechanism of geological settlement and analyzing its influence, it is known that the stress of the pipe increases with the increase in the relative settlement of the pipe, and finally, the pipe failure occurs at the junction of indoor and outdoor of the building. Countermeasures were carried out in terms of settlement repair and geological foundation reinforcement, rainwater and sewage pipes transformation, the repair of mixed connection of rainwater and sewage pipes and polluted water sources, drainage health safety in isolated areas and emergency storage tank, to achieve regular operation and safe discharge of the hospital rainwater pipeline network.

After implementing the measures, the NH₃-N measured value was within 0.82 mg/L, and the average removal rate was 97.4%. The COD concentration also decreased significantly, with an average value of 12 mg/L, and the average removal rate was 91.8%. The results show that the reconstruction can effectively deal with the problem of outdoor rainwater drainage pollution in the hospital. Through this study, there is no problem of water quality exceeding the standard in the river on neighbored areas. The problem of outdoor rainwater drainage pollution of the surrounding environment is solved.

Notably, the limitation of this study is that the broken pipeline caused by settlement is a process that slowly intensifies with time, and the risk of broken rainwater and sewage pipelines still exists after repair. This will require more time to observe the relationship between the settlement and the repaired pipeline. On the other hand, more research data are needed to see if the research methods in this paper can help other hospitals. We will continue to study the relationship between the settlement and the repaired pipeline for the next three years. We will apply the research methodology to other hospitals with the same problem. The research methodology will be refined by comparing the data. This paper's ideas and technical methods provide some reference for the study of rainwater drainage sanitary safety in medical buildings.

This work was supported by the Fundamental Research Project for the Wenzhou Science & Technology Bureau under Y20220803 and the Fundamental Research Project for the Wenzhou Science & Technology Bureau under Y20211155.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.