SEWERAGE: The Invisible Health Network
SEWERAGE is one of the most fundamental and at the same time invisible systems of modern urban life. It is a complex mechanical and hydraulic infrastructure, designed to efficiently and safely remove domestic and industrial wastewater, stormwater runoff and other liquid waste from residential, work and public spaces, transporting them to treatment facilities. Its proper functioning is a prerequisite for public health, environmental protection and the sustainability of cities.
Basic principles and operation of sewage
The sewage system is based on two main physical principles:
- Gravity . This is the main driving force. The pipes are laid with a slight slope (usually 1-2% or 1-2 cm per meter of length) in the direction of flow. This ensures that the waste and water flow naturally downwards and forwards without the need for constant pumping, significantly reducing energy costs.
- Water Flow: Solid waste cannot flow on its own. Water (from toilets, sinks, showers, faucets, even rain) acts as the transport medium. It flushes and carries the waste through the pipes. This is an important reason why water conservation should be done judiciously – too little water flow may not be enough to effectively transport the solids, leading to blockages.
The journey of wastewater from home to treatment facilities
The internal sewage network
It starts with every plumbing fixture within a building: toilet, sink, bathtub, kitchen sink, bathroom sink, laundry, basement sinks, etc.
Each device directs the flow to a vertical pipe (central vertical duct) through horizontal branches. The pipes run through the building from the top to the basement/ground floor.
At the top of each vertical pipe is the vent that protrudes above the roof. This is very important for the proper functioning of the sewer because:
- It introduces atmospheric air into the system, preventing the creation of a vacuum that would impede the flow of water and “suck” water from the siphons (causing unpleasant odors).
- It vents gases (such as methane and hydrogen sulfide) produced by the decomposition of organic substances in wastewater, preventing their accumulation inside the building.
At the base of the vertical pipes, in the basement or ground floor, the wastewater enters the main horizontal outlet pipe that exits the building and is connected to the public network.
The external network (Public sewer network)
The horizontal outlet pipe of the building is connected to a lateral pipe of the public network, usually located under the street or sidewalk.
Many lateral pipes are concentrated into larger diameter sewer headers .
These in turn empty into even larger mains . These huge pipes (sometimes large enough to walk through) collect wastewater from very large areas and direct it to pumping stations or treatment plants.
Pumping Stations: When geography does not allow gravity flow over the entire length (e.g., in very flat areas or when the wastewater must rise to a higher level to reach the treatment plant), pumping stations are used. These contain powerful pumps that “push” the wastewater upward or over long distances, where gravity can take over the flow again. The wastewater first passes through filters (screens) to remove large solids that could damage the pumps.
Wastewater treatment
This is the final destination of sewage. Here, raw wastewater undergoes a series of physical, mechanical and biological processes to remove pollutants before the water is returned to the environment (usually a river, lake or sea).
Primary treatment .
Involves physical processes. The wastewater passes through filters for large solids (e.g., garbage, plastics, cloth). It then flows into sedimentation tanks , where heavy inorganic solids (sand, gravel) settle. Finally, it enters primary settlers , where the flow is slowed and biodegradable solids (organic matter) sink to the bottom forming primary sludge (mud) , while fats and oils float to the surface and are removed.
Secondary Treatment .
Based on biology. The water (that has remained) is mixed with communities of natural bacteria and microorganisms in special activated sludge tanks , biological filters , flowing membrane (used in reverse osmosis or water filtration systems. These membranes are made of materials that allow water to flow while preventing the passage of dissolved solids, ions and other pollutants). These microorganisms consume the dissolved organic matter and nutrients (nitrogen, phosphorus) that remained. The water then goes to secondary sedimentation , where the microorganisms (which have now formed biological flakes) sink, forming secondary sludge . The purified water (bottom) usually proceeds to tertiary treatment or is distilled.
Tertiary Treatment (optional, but increasingly common) .
Provides additional purification for specific contaminants. May include chemical treatment (e.g., addition of chemicals to remove phosphorus), filtration through sand or carbon, disinfection (usually with chlorine, ultraviolet light, or ozone) to destroy pathogenic bacteria, viruses, and parasites, and advanced oxidation .
Sludge Treatment: The sludge collected in primary and secondary sedimentation is subjected to further treatment. This may include thickeners , anaerobic digestion (where bacteria decompose organic matter in the absence of oxygen, producing biogas – mainly methane – which is often used to generate energy on site), aerobic digestion, and dewatering (usually by centrifugation or pressure filters). The final product, known as bio-sludge (biosolids), can be used as fertilizer in agriculture (if it meets strict quality criteria), incinerated, or landfilled.
Sewerage system design and components
- Pipe Material: Historically, clay, wood and lead were used. Today , concrete , asbestos cement (in the process of being replaced due to risks), PVC (Polyvinyl Chloride), HDPE (High Density Polyethylene) and in some cases cast iron or stainless steel (especially in pumping stations) dominate. The choice depends on cost, overall strength, corrosion resistance and ease of installation.
- Pipe Diameter: Gradually increases as more wastewater is collected from more households and businesses. It starts at a diameter of 10-15 cm for lateral pipes and can reach 3 meters or more for main pipes.
- Ventilation: As mentioned above, it is vital for the smooth flow and prevention of the accumulation of hazardous gases. The ventilation system is a network that connects to the vertical pipes and exits above the roofs, allowing the free circulation of air.
- Controls and Connections:
- Manholes. Cylindrical structures made of brick or concrete that connect sections of pipe and allow personnel access for inspection, cleaning and unblocking . They are located at curves, intersections and at regular intervals along straight sections.
- Sewer overflow diversions: In combined sewer systems (which collect both wastewater and stormwater), when they overflow from heavy rainfall, there are special structures that allow the excess volume (mainly stormwater) to flow directly into natural water sources (e.g., rivers). This prevents flooding of streets and basements, although it can cause pollution.
- Drain Traps: A curved section of pipe under any drain fixture (e.g., the toilet bowl, the traps under sinks and toilets). It creates a permanent pool of water that acts as a hydraulic barrier, preventing odorous gases from the system from entering the building.
Types of sewage systems
- Separate sewer system: The most modern and preferred system. It uses two completely separate pipe networks :
- Sewage network: Transports only domestic and industrial wastewater from toilets, sinks, showers, etc. to the treatment plant.
- Stormwater drainage network: Collects only rainwater from roofs, roads and squares and sends it directly (without treatment) to natural water resources (rivers, lakes, sea).
- Combined sewerage system (Pandorroikos): This is an older system, still present in many historic cities. It uses a single network of pipes that collects both domestic/industrial wastewater and rainwater. During dry periods, the entire volume is sent for treatment. However, during heavy rainfall, the total volume can exceed the capacity of the network or treatment plant, leading to combined sewer outfalls . These discharge raw or partially treated wastewater together with rainwater directly into the environment, causing significant pollution. Conversion to separate systems is a high priority.
Problems and maintenance
The effective operation of the sewerage system requires constant attention, development and maintenance. The sewerage system is a common network for everyone and it is very important that we all be responsible, both private and public. Various problems can occur either due to misuse or due to incomplete construction or due to abnormal climatic conditions.
Blockages caused mainly by the disposal of unsuitable materials in the network (oils/greases that freeze, hygiene products, liquid soap, hair, baby wipes, plastics, garbage). They cause flow reversal and flooding. They are removed with hydrodynamic machines, special steels or chemicals (less common now due to environmental problems).
Cracks and collapses in pipes due to age of the network, erosion, ground movement, pressure from traffic or inadequate construction. This leads to leaks contaminating the subsoil and groundwater. Inflows can also occur as groundwater or rainwater enters the system through cracks, overloading the network and treatment plants. This dramatically increases operating costs and can cause flooding.
Sediment deposits (sand, mud) can accumulate at the bottom of pipes even if they have the correct slope, reducing the flow capacity of the wastewater. They are removed by periodic cleaning (often with high-pressure hydrodynamic machines).
Corrosion: from wastewater which produces hydrogen sulfide (H₂S), which is converted to sulfuric acid by bacteria on the pipe walls (above the water line), causing severe corrosion, especially in concrete pipes. This reduces the life of the pipes.
Environment and health
The proper functioning of sewage and the smooth treatment of wastewater are essential for healthy living, especially in cities that have large populations and heavy and diverse use.
- Prevents the spread of serious diseases (cholera, typhoid fever, hepatitis A, gastroenteritis) transmitted through contaminated water or food. Removes pathogenic organisms and parasites.
- It prevents the contamination of rivers, lakes, groundwater and seas by organic matter (which consumes oxygen, killing aquatic organisms), nutrients (which cause eutrophication and “dead zones”), toxic chemicals and pathogens.
- It protects ecosystems as it maintains the healthy balance of aquatic ecosystems.
- Modern treatment plants can produce reusable water (for irrigation, industrial uses) and bio-sludge for fertilizer, contributing to the circular economy.
- It combats unpleasant odors , reduces insects and improves the aesthetics and value of cities.
Sewage is an extremely complex mechanical and biological system that operates silently beneath our feet. From the moment we flush the toilet or empty the sink, waste begins a journey under the force of gravity, crossing a vast network of pipes, sometimes with the help of pumps, until it reaches specialized facilities where it undergoes multiple physical and technological processes to be cleaned. The effectiveness of this system relies on proper design, regular maintenance, investment in upgrades and, above all, on the responsible behavior of citizens (avoiding the disposal of inappropriate materials). The benefit is immeasurable: protecting public health, preserving clean water resources and creating healthy cities and ecosystems for current and future generations. It is the invisible shield that protects us every day.