Sewer treatment systems collect wastewater from a community or a larger urban area via a sewer system consisting of a network of sewer pipes to a treatment plant. The sewers from individual homes and business, are typically a privately owned lateral network of small sewer lines that feed into larger community sewer lines and finally into very large main sewer lines (mains) that deliver the wastewater to the treatment plant. These shared public sewer lines typically rely on gravity to deliver the wastewater to the treatment plant, so the lines are constructed at progressively deeper levels as the wastewater drains to the treatment plant. Consequently, the treatment plant’s collection structures may be deep in the ground or conveyance through the sewer may include pump mechanisms to raise the wastewater level to flow into the plant at a higher level. The effluent is processed through a variety of plant dependent mechanisms and finally, when cleaned of pathogens and most of the nutrients, is released to the environment. The sludge and solid waste are removed and disposed of offsite in landfills or by incineration, or in the case of biosolids, recycled to the soil by use in agriculture, mine reclamation, landscaping, or horticulture. Sludge can potentially undergo additional processing at another plant for recovery and be recycled or used as fuel.

In most of the urban developed world, where incomes are high enough for significant taxes that can fund infrastructure investments, centralized sewer systems are the norm.

There are three types of sewers:

• Sanitary sewers take kitchen and bathroom waste to either a discharge point, such as a river or estuary, or in more regulated environments, to a sewage treatment plant, where it is treated and discharged to the environment.
• Surface water sewers take rainwater from roofs, roads and other surfaces to a discharge point in a river, estuary or other waterway.
• Combined sewers collect both sanitary and surface water and take it to a sewage treatment works, where it is treated and discharged to the environment (Blackburn et al., 2017). During high flow events, such as storms, combined sewage and surface water may be discharged into groundwater, compromising local water quality.

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