Search

Industrial wastewater and sewage are both important sources of heavy metals and metalloids in urban wastewater treatment plants (WWTPs). China has made great efforts to control heavy metal and metalloid pollution by setting discharge limits for WWTPs. There is, however, limited discharge data and no systematic methodology for the derivation of discharge limits. In this study, 14 heavy metals and metalloids (Hg, alkyl mercury, As, Cd, Cr, Cr6+, Pb, Ni, Be, Ag, Cu, Zn, Mn, Se) that are listed in the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002) were selected for the analysis of discharge characteristics while using the supervised monitoring data from more than 800 WWTPs located in nine provinces in China. Of the 14 heavy metals and metalloids, all but alkyl mercury were detected in the discharge water. There was a high rate of detection of As, Cu, Mn, Se, and there were some samples that exceeded the standard concentrations of Cr, Cr6+, Pb, and Ni. Removal rates of Hg, As, Cd, Cr, Cr6+, Pb, Ni, Cu, Zn, Mn, and Se were higher than 40%, comparable to values from other countries. Hg and As were selected to analyze the influencing factors of effluent and derive discharge limits of WWTPs using a statistical method, because these two metals had more detected data than other metals. The study used supervised monitoring data from Zhejiang WWTPs with 99 for Hg and 112 for As. Based on the delta-lognormal distribution, the results showed that geographic location was significantly closely correlated with Hg (P = 0.027 &lt, 0.05) and As (P &asymp, 0 &lt, 0.05) discharge concentrations, while size (for Hg P = 0.695 &gt, 0.05, for As P = 0.088 &gt, 0.05) and influent concentration (R2 &lt, 0.5) were not. Derived Hg and As discharge limits suggest that it is necessary to establish stricter discharge limits for WWTPs, which is more consistent with the real-world situation in China. The study here comprehensively researches the discharge characteristics of heavy metals and metalloids in effluent of WWTPs in China, and developed for the first time in China heavy metals and metalloids discharge limits based on statistical methods. The results may inform special discharge limit settings for WWTPs in China.

With the rapid economic development, water pollution has become a major concern in China. Understanding the spatial variation of urban wastewater discharge and measuring the efficiency of wastewater treatment plants are prerequisites for rationally designing schemes and infrastructures to control water pollution. Based on the input and output urban wastewater treatment data of the 31 provinces of mainland China for the period 2011&ndash, 2015, the spatial variation of urban water pollution and the efficiency of wastewater treatment plants were measured and mapped. The exploratory spatial data analysis (ESDA) model and super-efficiency data envelopment analysis (DEA) combined Malmquist index were used to achieve this goal. The following insight was obtained from the results. (1) The intensity of urban wastewater discharge increased, and the urban wastewater discharge showed a spatial agglomeration trend for the period 2011 to 2015. (2) The average inefficiency of wastewater treatment plants (WWTPs) for the study period was 39.2%. The plants&rsquo, efficiencies worsened from the eastern to western parts of the country. (3) The main reasons for the low efficiency were the lack of technological upgrade and scale-up. The technological upgrade rate was &minus, 4.8%, while the scale efficiency increases as a result of scaling up was &minus, 0.2%. Therefore, to improve the wastewater treatment efficiency of the country, the provinces should work together to increase capital investment and technological advancement.

Most guidelines for urban wastewater treatment plants (WWTPs) are established on a time basis determination to determine performance and compliance with discharge standards and limits (i.e., maximum daily-average). Nevertheless, there is a lack of a systematic analysis of the relationship between instant sampling and daily average discharge concentration values. The present study used the chemical oxygen demand (COD) automated monitoring data that were collected from 1738 WWTPs in China to discuss the relationship between instant sampling and daily average values. A ratio model (K value) was developed to study the relationship between the reliability of the instant sampling value and the daily average limit for the COD measurements. The K value revealed that the ratio of COD instant and daily average measured concentrations for WWTPs in China collectively ranged from 1.00 to 1.45. The results of this study suggest setting the K value of COD to 1.3 for WWTPs in China to estimate the corresponding instant sampling limit of COD, as well as for most WWTPs in China, in order to ensure the stability of compliance to the daily average limits. The instant sampling value of COD in 24 h should be controlled no more than five times out of the instant sampling limit, which is 1.3 times of the daily average limit.