下水道体系(sewage system)
由回收管、总管和排放管网组成的综合体系，用於处理社区废水。早期文明社会常在城区修建排水系统，以解决暴雨的排放问题，罗马人精心建造的系统还可排放公共浴池的废水。中世纪时这些系统年久失修，随着城市人口的增加，由於没有把污水和饮用水有效地分开，导致霍乱、伤寒等时疫流行。19世纪中期人们开始采取措施处理废水。工业革命时期，人口的集中以及制造业废水的增加，提高了有效污水处理的必要性。下水管道沿街道的路线铺设，并附有加金属盖的出入孔，供定期检查和清理用。街角的集水池和沿街的沟渠收集地表的雨水而引入雨水下水道。工程师们确定可能的污水量、系统的路线以及有时有必要的管道倾斜度，保证雨水在重力作用下不留固体残物均匀流走。在平坦地区有时须增设泵站。现代的下水道体系包括日常生活污水和工业污水以及雨水。污水处理厂通过一系列的步骤清除掉废水中的有机物。污水进厂後，先通过巨大的网栅，以清除如木块和砾石等大的杂物；接着通过沈积或较细的滤网清除灰砾和泥沙。然後流入初级沈淀池，使许多悬浮物（污泥）沈淀下来。接下来在污水中加入化学气体或微生物，使其中的有机物分解。再经过二级沈淀池，让所有残余的固体物质都沈淀下来，然後将余下的净水排入水体。从各沈淀池中收集的污泥可填埋洼地、倾入海中、用作肥料或置於加热池（煮解池）中进一步分解产生沼气，为处理厂提供能源。
English version：
sewage system
Collection of pipes and mains, treatment works, and discharge lines (sewers) for the wastewater of a community. Early civilizations often built drainage systems in urban areas to handle storm runoff. The Romans constructed elaborate systems that also drained wastewater from the public baths. In the Middle Ages these systems fell into disrepair. As the populations of cities grew, disastrous epidemics of cholera and typhoid fever broke out, the result of ineffective separation of sewage and drinking water. In the mid-19th century the first steps were taken to treat wastewater. The concentration of population and the addition to sewage of manufacturing waste that occurred during the Industrial Revolution increased the need for effective sewage treatment. Sewer pipe is laid following street patterns, and access holes with metal covers allow periodic inspection and cleaning. Catch basins at street corners and along street gutters collect surface runoff of storm water and direct it to the storm sewers. Civil engineers determine the volume of sewage likely, the route of the system, and the slope of the pipe to ensure an even flow by gravity that will not leave solids behind. In flat regions, pumping stations are sometimes needed. Modern sewage systems include domestic and industrial sewers and storm sewers. Sewage treatment plants remove organic matter from waste water through a series of steps. As sewage enters the plant, large objects (such as wood and gravel) are screened out; grit and sand are then removed by settling or screening with finer mesh. The remaining sewage passes into primary sedimentation tanks where suspended solids (sludge) settle out. The remaining sewage is aerated and mixed with microorganisms to decompose organic matter. A secondary sedimentation tank allows any remaining solids to settle out; the remaining liquid effluent is discharged into a body of water. Sludge from the sedimentation tanks may be disposed of in landfills, dumped at sea, used as fertilizer, or decomposed further in heated tanks (digestion tanks) to produce methane gas to power the treatment plant.