Sewage

Sewage, often called domestic wastewater, is the used water produced by a community of people. It is a complex mixture of water and various waste products. This wastewater comes from many different sources within a locality. These sources include private residences and commercial buildings. They also include institutional facilities like schools, hospitals, and prisons.

Managing this liquid is vital for public health and environmental safety. It must be collected and moved through a system of pipes.

Scientists divide sewage into two main sub-types based on their source. The first type is greywater. This is the wastewater from sinks, bathtubs, showers, dishwashers, and clothes washers.

The second type is blackwater. Blackwater is the water used to flush toilets. It carries human excreta, which includes feces and urine. Sewage also contains soaps, detergents, and food waste from dishwashing. In some areas, toilet paper is also added to the mix.

Most sewage is actually composed of water. It is approximately 99.9% pure water. The remaining 0.1% consists of solid matter. These solids can be dissolved or suspended in the liquid. About one-third of these solids are kept suspended by turbulence. The rest are dissolved or exist as tiny colloidal particles. Organic matter makes up a large part of these solids. This includes proteins, carbohydrates, oils, and grease. Scientists use two indirect methods to measure organic matter. These are called Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD).

Sewage also contains nutrients like nitrogen and phosphorus. Nitrogen often comes from ammonia, which stems from urea in urine. Phosphorus is often found as phosphates from detergents. If untreated sewage enters lakes or reservoirs, it can cause eutrophication. This is a process where high nutrient levels lead to pollution. Sewage also carries pathogens, which are organisms that can cause disease. There are four main types of pathogens found in sewage. These include pathogenic bacteria, viruses, protozoa, and helminths, which are worms.

To check for fecal contamination, experts look for specific bacteria. They often track Escherichia coli, commonly known as E. coli. These are intestinal bacteria found in all warm-blooded animals. E. coli is easy to track because it exists in very high concentrations. You might find 10 to 100 million E. coli per 100 mL of sewage. Beyond germs, sewage can contain micro-pollutants. These include microplastics from synthetic clothing and pharmaceuticals excreted by humans. Some people even accidentally or intentionally pour oils and solvents into drains.

The amount of sewage produced depends on water consumption. Many factors influence how much water a person uses. Climate plays a role, as warmer climates may increase consumption. Economic levels and the size of a community also matter. The way water is priced or metered affects flowrates. In some regions, water is scarce, making the sewage more concentrated. In the United States, higher water use leads to more diluted sewage. This means the concentration of pollutants might be lower even if the total volume is higher.

Managing sewage requires careful collection and transport. Sewage travels from plumbing into either a sewer or an onsite facility. There are sanitary sewers, which exclude stormwater. There are also combined sewers, which take in both sewage and stormwater.

Once collected, it must be treated before release. Treatment makes the water compatible with local requirements. Disposal options include land disposal or using sewage farms. Some areas use marine outfalls to release treated water.

However, all disposal methods carry a risk of water pollution.