Sewage treatment

Dirty water from homes and shops needs cleaning.

This water is called sewage. It travels through long pipes to a plant.

At the plant, the water is cleaned. This stops the water from hurting the earth. Some plants use tiny living things to clean it. These tiny things eat the bad stuff in the water.

Some places use big ponds to clean water.

These ponds need a lot of land. Other plants use machines to work fast.

After cleaning, the water is safe to use. It can go back into the rivers. This keeps our world healthy and clean.

Dirty water from homes and shops is called sewage.

This water can hurt our rivers and land. We must clean it first. This is called sewage treatment.

Some systems are small. They work right where the waste is made. These are called decentralized systems. Large cities use centralized systems. These use a big network of pipes. The pipes carry waste to a large plant.

Cleaning often happens in steps. The first step is primary treatment. This removes large parts from the water. The next step is secondary treatment. This uses tiny living things to clean the water. These living things eat organic matter. This is a type of biological process.

Some plants use high-tech machines. These work fast but cost more money. Other plants use nature. They use big ponds or wetlands.

These need lots of land. They use very little power. After cleaning, the water is called effluent.

This water is safe to put back into nature.

Sewage treatment is a vital way we clean dirty water. This water, called sewage, comes from our homes and businesses. It can also include water from factories or rain that washes through city streets. If we let this dirty water flow straight into rivers, it causes water pollution. The main goal of treatment is to remove these contaminants. This makes the water safe to release back into nature.

Sometimes, the clean water is even reused for other jobs.

Cleaning the water usually happens in a few main steps. First, primary treatment removes large bits of waste. Next, secondary treatment uses biological processes to clean the water. This means tiny living things eat organic matter in the water. Some systems use aerobic processes, which need air, or anaerobic processes, which do not.

For even cleaner water, plants might use tertiary treatment. This step removes extra nutrients like nitrogen and phosphorus. Some advanced plants even use a fourth step to remove tiny traces of medicine.

There are different ways to collect and move this waste. In many big cities, a centralized system is used. This involves a huge network of pipes and pump stations called sewerage. These pipes carry the waste to one large treatment plant.

In rural areas, people often use decentralized systems. These work right where the waste is made. Examples include septic tanks or vermifilter systems.

These smaller systems are often used when there are not many people living nearby.

Engineers must make smart choices when building these systems. They look at how much land is available and how much energy is needed. They also think about the cost to build and run the plant.

Some systems are high-tech and use machines to work quickly. These can be expensive and use a lot of power. Other systems are nature-based and use big ponds or wetlands.

These use very little energy but need a lot of space to work.

Cleaning water is a huge job for the whole world. Right now, about 52% of the world's sewage is treated. However, this is not the same everywhere. High-income countries treat about 74% of their sewage. In contrast, developing countries treat an average of only 4.2%.

This work is part of a bigger field called sanitation. Sanitation also helps manage human waste and even stormwater. Keeping our water clean helps keep our whole planet healthy.

Sewage treatment is a vital form of waste management designed to protect our environment. It involves removing contaminants from sewage to produce an effluent, which is the treated water released after processing. This process prevents water pollution caused by raw sewage discharges. Sewage includes wastewater from households and businesses, and sometimes pre-treated industrial wastewater. The ultimate goal is to make the water safe for discharge into the surrounding environment or for reuse in other applications.

To understand how this works, we must look at the different ways sewage is collected. In a centralized system, a network of pipes and pump stations known as sewerage carries waste to a large municipal treatment plant. Some cities use combined sewers, which also carry urban runoff, or stormwater, to the plant. In contrast, decentralized systems treat waste closer to where it is created. These are often used in rural areas with low population densities. Examples include on-site sewage systems (OSS), septic tanks connected to drain fields, or vermifilter systems.

Most sewage treatment follows a specific sequence of stages. Primary treatment is the first step, often using settling tanks to remove solids. Secondary treatment follows this stage to reduce organic matter. This is measured by biological oxygen demand (BOD), which tracks how much oxygen is needed to break down organic material. Secondary treatment uses biological processes that can be aerobic, meaning they require air, or anaerobic, meaning they function without air.

Advanced facilities may incorporate even more complex steps. Tertiary treatment is an additional stage used for polishing the water and removing nutrients like nitrogen and phosphorus. Some very advanced plants include a quaternary treatment step. This fourth stage is designed to remove organic micropollutants, such as traces of pharmaceuticals. This specific type of advanced treatment has been implemented at a full scale in Sweden.

Engineers categorize treatment technologies into different groups based on their complexity and cost. High-tech or intensive systems are often mechanized and compact, but they can be expensive and energy-intensive. An example is the activated sludge process, which achieves high effluent quality. Low-tech or extensive systems are often nature-based and use natural processes. These systems, such as waste stabilization ponds or constructed wetlands, require large areas of land but use very little energy.

Selecting the right technology is a complex task for decision makers. They must consider many technical and economic criteria. These include the desired quality of the effluent and the expected costs for construction and operation. Engineers also look at land availability, energy requirements, and sustainability. In industrialized countries, the focus is often on efficiency, reliability, and space. In developing countries, the focus may shift toward simplicity and lower costs.

Globally, the scale of sewage treatment is highly unequal. It is estimated that only 52% of the world's sewage is treated. High-income countries treat approximately 74% of their sewage. However, in developing countries, the average treatment rate is only 4.2%. This work is part of the broader field of sanitation, which also includes managing human waste, solid waste, and stormwater drainage.

Even after treatment, there are leftovers to manage. Some processes produce sewage sludge, which is the solid material remaining after treatment. This sludge can be treated before disposal or reuse. Under certain conditions, treated sludge is called biosolids and can be used as a fertilizer. Managing these residuals is a key part of the entire wastewater treatment process.