International Journal of Environmental Research and Public Health Article Turbidity in Combined Sewer Sewage: An Identification of Stormwater Detention Tanks Yang Liu 1 , Liangang Hou 1, Wei Bian 2, Banglei Zhou 1, Dongbo Liang 1 and Jun Li 1,* 1 The College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China; [email protected] (Y.L.); [email protected] (L.H.); [email protected] (B.Z.); [email protected] (D.L.) 2 China Energy Investment Corporation, Beijing 100011, China; [email protected] * Correspondence: [email protected] Received: 10 March 2020; Accepted: 21 April 2020; Published: 28 April 2020 Abstract: Combined sewer overflow remains a major threat to surface water quality. A stormwater detention tank is an effective facility to control combined sewer overflow. In this study, a new method for the selective collection of combined sewer sewage during wet weather based on real-time turbidity control is established to reduce the load of pollutants entering a river using a stormwater detention tank with a limited volume. There was a good correlation found between turbidity and the concentrations of total suspended solids (TSS) (R2 = 0.864, p < 0.05), total phosphorus (TP) (R2 = 0.661, p < 0.01), and chemical oxygen demand (COD) (R2 = 0.619, p < 0.01). This study shows that turbidity can be used to indicate the concentration of TSS, TP, and COD in the sewage of the combined sewer systems in wet weather. Based on the adopted first flush detection approach, total nitrogen (TN) and TP showed the first flush effect, whereas the first flush effect of TSS and COD was not obvious. The results show that it is impossible to effectively control combined sewer overflow by only treating the initial rainwater. Keywords: combined sewer system; correlation; stormwater detention tank; turbidity 1. Introduction In many large cities all over the world, the combined sewer system is mostly used in old urban areas to receive rainwater runoff, domestic sewage, and industrial wastewater and is a common method of wastewater transportation [1]. Under sufficient rainfall, combined sewer overflow (CSO) occurs when sewage in the combined sewer exceeds the capacity of the wastewater treatment plants (WWTPs) [2–4]. In CSO events, the discharge of untreated domestic sewage and rainwater into surface water means that numerous nutrients, suspended solids, heavy metals, and pathogenic microorganisms are imported. This phenomenon has become one of the main causes of pollution for many receiving water bodies around the world [5–7]. It is not easy to reconstruct a combined sewer system in large cities. The construction of stormwater detention tanks is considered to be an effective measure to reduce the negative impact of CSO [8,9]. However, due to factors such as floor space and construction investments, some WWTPs in China are not equipped with stormwater detention tanks of sufficient capacity. Reducing CSO as much as possible using limited storage volume has become a serious problem in the treatment of non-point pollution in China. At present, most detention tanks are designed and constructed based on information such as catchment area, rainfall depth, rainfall time, and pipe network system [10,11]. Nevertheless, Suarez [12] carried out the monitoring and analysis of the confluence areas of five different cities in Spain. All the catchment areas showed no significant pollution first flush effect, and the main pollution load of a rainfall event was discretely distributed. Similarly, the results of Rauch [13] showed that reducing the Int. J. Environ. Res. Public Health 2020, 17, 3053; doi:10.3390/ijerph17093053 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 3053 2 of 10 discharge of wastewater in wet weather may not necessarily improve the water quality of the receiving water body, which raised doubts about the effect of using only the volumetric method to control the pollution of combined sewer overflow. Rathnayake [14] found that relying solely on rainfall depth control can increase the design volume of the stormwater detention tank and cannot reasonably reduce overflow pollution from the combined system. The operating state of a detention tank is regulated based on rainfall data, which are relatively one-sided, especially when the concentration of influent pollutants is low. Studies have shown that the initial rainfall depth is large, but the first flush effect of pollutants is not obvious [15–17]. In this case, if the concentration of pollutants is very high in the later period, due to insufficient volume, the storage facilities will have little effect on the removal of pollutants, which will cause great harm to the receiving water. Therefore, Stormwater detention tanks based on water quality judgment should be developed, rather than rainfall variables. However, there are relatively few studies on the selective collection of combined sewer sewage based on the high frequency turbidity measurements used to estimate the concentration of pollutants. In this study, water samples from 21 typical rainfall events from 2017 to 2019 were collected in the southeast area of Beijing, and the water quality characteristics of the combined wastewater were measured and analyzed. The correlations between turbidity and the primary pollutant concentrations in the combined sewer sewage, including total phosphorus (TP), total suspended solids (TSS), chemical oxygen demand (COD), and total nitrogen (TN), were investigated. Using the information on the concentration of primary pollutants provided by turbidity measurements, heavily polluted sewage can be retained selectively in stormwater detention tanks, while lightly polluted sewage is preferably supposed to be discharged into the receiving water. In this way, the volume of the storage tank was used effectively, and the pollution load of the receiving water during wet weather was reduced. 2. Materials and Methods 2.1. Study Site The examined areas located in Tongzhou District and Chaoyang District covered 37.09 and 46.42 ha, respectively, and the impervious area fractions were 32.5% and 67.8%, respectively. This is an important diplomatic and commercial area of Beijing and a sub-central area of the city under vigorous construction. The average annual rainfall in Beijing is 585 mm, and most of the rainfall occurs in April to October, accounting for more than 85% of the total annual rainfall. Four monitoring points were selected in this study: BS1, BS2, BGD, and XDW. The specific Int. J. Environ. Res. Public Health 2020, 17, x 3 of 11 locations are shown in Figure1, and all are located in the combined sewers before entering the wastewater treatment facilities or WWTPs. Figure 1. Locations of the four monitoring points in the southeast area of Beijing. Figure 1. Locations of the four monitoring points in the southeast area of Beijing. 2.2. Sampling and Sensors To establish the correlation functions between turbidity and the primary pollutants, 25 sampling sessions were performed between 2017 and 2019 at the study site. Four were conducted during dry weather, and the remaining 21 were performed during different rainfall events. The four monitoring points were equipped with turbidimeters and flow meters. The turbidity sensor is a Viso Turb 700 IQ electrode (WTW, Munich, Germany), which is used to measure turbidity in the range of 0–4000 NTU (nephelometric turbidity unit). Every 5 min, the average turbidity value is calculated by the integration of 15 s of data. On-site calibration is checked using kieselguhr [18], and no sensor drift was observed during the experiment. At the monitoring point, a flow meter (Hydreka, Lyon, France) based on the acoustic Doppler principle recorded the flow, and the rainfall information was monitored by a tipping bucket rain gauge located in the Tongzhou study area, as shown in Figure 1. In order to measure the other water quality parameters (except for turbidity), a sample was collected in the combined sewer every 5 min at the beginning of the rainfall, and the interval of the later sampling was 10–15 min. The specific operation depended on the intensity of the rainfall. Samples were transported to the laboratory in polyethylene bottles and stored in a refrigerator at 4 ℃ before testing. 2.3. Analytical Methods The NH4+-N, TSS, COD, and TP analyses of wastewater were performed in accordance with the American Public Health Association (APHA) Standard [19]. TN was measured by a vario TOC-TN instrument. In particular, TN included the total kjeldahl nitrogen, NO3–-N, and NO2–-N. Dissolved oxygen (DO), pH, and temperature were monitored by a WTW Multi 3420i meter (WTW, Munich, Germany). All water quality data were analyzed using Microsoft Excel, Origin, and SPSS. A one-way analysis of variance was used to analyze the treatment effect. The SPSS 24.0 software (IBM, Armonk, NY, USA) was used for a linear regression analysis and curve fitting. The p value was less than 0.05, indicating that the two pairs had good statistical significance. 2.4. Data Treatment Concentrations of pollutants often change greatly during the same rainfall process. The use of the event mean concentration (EMC) can better characterize the pollution characteristics of rainfall events [12]. The EMC represents the flow-weighted average concentration computed as the total Int. J. Environ. Res. Public Health 2020, 17, 3053 3 of 10 2.2. Sampling and Sensors To establish the correlation functions between turbidity and the primary pollutants, 25 sampling sessions were performed between 2017 and 2019 at the study site. Four were conducted during dry weather, and the remaining 21 were performed during different rainfall events. The four monitoring points were equipped with turbidimeters and flow meters. The turbidity sensor is a Viso Turb 700 IQ electrode (WTW, Munich, Germany), which is used to measure turbidity in the range of 0–4000 NTU (nephelometric turbidity unit). Every 5 min, the average turbidity value is calculated by the integration of 15 s of data.