Blackwater is wastewater which is loaded with biological material such as feces or grease. Many people colloquially refer to blackwater as “sewage.” Because blackwater contains dangerous materials, it must be treated before being released or reused, otherwise it could be a source of disease. There are a variety of ways for dealing with blackwater, ranging from setting up home composting toilets to render such waste inert to flushing blackwater through a municipal sewer system, where it eventually winds up at a sewage treatment plant so that it can be treated and safely disposed of.
One very obvious source of blackwater is the toilet. Human waste is generally recognized as hazardous, especially when people are sick, because it can contain a multitude of bacteria which could potentially be passed on to others. Kitchen sinks are also viewed as a source of blackwater by many people, because they contain grease, oils, and chunks of food which could go rancid or carry pathogens. In addition, laundry water from a house with a sick person or a baby could be considered to be blackwater, since it would presumably be loaded with pathogens.
Historically, humans have come up with a variety of ways for handling blackwater, since it is an ever-present problem. It is important to isolate blackwater from sources of potable water, including the water table and surrounding rivers and streams. In rural areas, this is often accomplished with the use of a septic tank, which allows the pathogens in the blackwater to break down before releasing leachwater; most septic tanks need to be pumped periodically to remove solids.
People have also used outhouses, essentially composting their blackwater in pits in the ground, as well as composting toilets and buckets in the house. In fact, at some periods, human waste was actually a valuable product, and it would be collected by “night soil men” who would sometimes pay a small fee for the privilege; the urine could be used in a variety of industries, from munitions to laundry, while the feces would be used as fertilizer.
The use of sewer systems for handling blackwater is also quite old, with some examples of such systems in Asia and the Middle East dating back to around 4000 BCE. Historically, many such systems simply dumped the blackwater somewhere else, or into holding tanks and sump pits which were imperfectly designed for containing and decontaminating waste. Modern sewer systems are quite advanced, with complex treatment plants which are capable of rendering sewage inert, and sometimes safe enough to use on agricultural crops.