COD<sub>Cr</sub> degradation coefficient of urban river recharged with reclaimed water and its impacting factors

Abstract

化学需氧量（COD）是影响地表水体水质的主要污染指标.明确地表水中COD的降解系数，可为把握有机物污染过程变化以及有效控制污染提供科学依据.北运河是城市再生水河流，COD污染严重.以北运河从上游至下游的5个典型断面（马坊桥、火沙路、东关大桥、榆林庄桥、杨洼闸）为例，通过室内模拟实验，估算了不同断面COD_Cr降解系数，并对COD_Cr降解系数进行校正；测定了颗粒物沉降速率、复氧系数，评估了水体自净能力；分析了不同环境条件（温度、溶解氧、流速）对COD_Cr的降解影响.结论如下：北运河水体自净能力较差，颗粒物沉降速率范围为1.09~3.22 m/d，杨洼闸断面颗粒物沉降速率最低为1.09 m/d，其复氧系数偏低，为0.016 d^-1.北运河水体COD_Cr降解系数符合一级动力学方程；经水力、温度经验公式校正后，COD_Cr降解系数范围为0.0184~0.0883 d^-1，低于我国其他平原地区河流.北运河COD_Cr降解系数存在明显的空间差异性，上游断面的降解系数高于下游断面，不过下游北京界最后出水断面杨洼闸表现出闸坝特殊性，降解系数最高（0.0416~0.0883 d^-1）.水质参数中温度、溶解氧、氧化还原电位、营养盐以及总有机碳是COD_Cr降解系数的主要影响因子.COD_Cr降解系数随着温度、溶解氧、外源动力增加而增大；COD_Cr降解系数与营养盐、TOC呈显著正相关，与氧化还原电位呈显著负相关.研究结果可为我国相关城市再生水体水质改善和城市黑臭水体治理提供科学依据.;Chemical oxygen demand (COD) is one of the major indicators of surface water pollution in China. Degradation coefficient of pollutant may provide scientific basis for understanding the process of organic polluter change and effective pollution control. As a typical urban river recharged with reclaimed water, Beiyun River is at very serious state of COD pollution. In this research, five typical sections from upstream to downstream (Mafang Bridge, Huosha Road, Dongguan Bridge, Yulinzhuang Bridge and Yangwa Sluice) on Beiyun River were selected for estimating COD_Cr degradation coefficients and calibration by laboratory simulation experiments. Taking the section of Yangwa sluice as a typical example, the sedimentation rate and reoxygenation coefficient were measured on site to evaluate water self-purification ability. Then the impacts of different environmental factors (temperature, dissolved oxygen and flow rate) on COD_Cr degradation were discussed based on laboratory simulation experiments. The conclusions were summarized as follows:The water purifying ability of Beiyun River was very low. The sedimentation rates ranged from 1.59 to 3.22 m/d, among which, the lowest rate was in Yangwa Sluice with a reoxygenation coefficient of 0.016 d^-1. The variation of COD_Cr degradation coefficients conformed to the first-order kinetic equation, ranging from 0.0184 d^-1 to 0.0883 d^-1 corrected by the hydraulic and temperature condition. The COD_Cr degradation coefficients showed significant spatial variation. The degradation coefficients of upstream sections was higher than that of downstream sections, except Yangwa Sluice (last section of Beijing at downstream) as the highest (0.0416-0.0883 d^-1), showing the characteristics of the sluice. Water quality parameters such as temperature, dissolved oxygen, redox potential (ORP), nutrients, and total organic carbon (TOC) were the main influence factors. The COD_Cr degradation coefficients increased with the temperature, dissolved oxygen and flow rate. In addition, COD_Cr degradation coefficient was positively related to nutrients and TOC, but negatively to ORP. The results of this study can provide a scientific reference for urban river improvement and pollution control.