Effects of rainwater intrusion on an activated sludge sewer treatment system

Abstract Effluent collection and stormwater management make use of different drainage systems, which should remain unconnected with one another. If stormwater drains into sewage collection systems, it often causes changes in effluent quality indicators at sewage treatment plants. This study sought to quantify the changes caused by rain in a Wastewater Treatment Plant (WWTP) located in Paulista, PE, which uses the activated sludge system. Accordingly, changes in pH, temperature, BOD, and treatment efficiency at the inlet and outlet, as well as inlet flow were analyzed on dry and rainy days at the plant, which has a sewage capacity of 400 L s-1. The input volume was found to be greater than the design flow of the station, about 25%. With this, some relevant operational controls were identified, such as temporarily shutting down sewage pumping stations in order to maintain full operation of the system and avoid overload. The pH increases slightly in alkalinity, about 0.3, but this does not interfere with the treatment process. The temperature of the influent on rainy days is about 3 to 5 degrees cooler, depending on the intensity of the event. Using the T and Z tests, it was possible to identify that BOD concentrations in the influent and the effluent were not significantly different (uneq. var. t: 1.18 > 0.26 and uneq var. z: 0.71 > 0.48, respectively), and the organic load removal efficiency was maintained, contrary to what is expected with other, more simple types of treatment.

摘要：污水收集与雨水管理需采用各自独立的排水系统，二者不得相互连通。若雨水汇入污水收集系统，往往会导致污水处理厂的出水水质指标发生变化。本研究旨在量化巴西伯南布哥州保利斯塔市某采用活性污泥法工艺（activated sludge system）的污水处理厂（Wastewater Treatment Plant, WWTP）因降雨引发的水质变化。该污水处理厂的设计处理规模为400 L·s⁻¹，本研究对比分析了其在旱天与雨天条件下，进水端、出水端的pH值、温度、生化需氧量（Biochemical Oxygen Demand, BOD）、处理效能以及进水流量的变化情况。
研究发现，实际进水量超出该厂设计流量约25%。据此明确了若干关键运行调控措施，例如临时关停污水泵站，以保障系统满负荷稳定运行并避免过载。雨天时进水pH值小幅升高约0.3个单位，但该变化并未对处理工艺造成干扰；同时雨天的进水水温较旱天低3至5摄氏度，具体降幅取决于降雨强度。
通过T检验与Z检验分析可知，进水与出水中的生化需氧量浓度无显著差异（方差不齐T检验统计量为1.18＞0.26，方差不齐Z检验统计量为0.71＞0.48），有机负荷去除效率得以维持，这与其他更为简易的污水处理工艺的预期表现相悖。