The 4 stages of wastewater treatment

Before anything else, wastewater passes through screens that remove large debris—things like plastic, paper, and food scraps. Without this step, these materials could clog pipes or damage equipment. Next, the water moves to a grit chamber, where heavier particles such as sand, gravel, and small stones settle at the bottom. These particles could wear down machinery over time, so removing them early helps keep the system running smoothly. By the end of this stage, the wastewater still looks dirty, but at least the biggest physical pollutants are gone. Now, it’s ready for the next step.

At this stage, gravity does most of the work. Wastewater enters a large settling tank, where heavier solids called sludge sink to the bottom, while oils and grease float to the top. Special equipment skims off these floating substances, while the sludge is collected separately.

The ASTRASEPARATOR® is a compact, low-maintenance sedimentation tank and is typically used in this stage of wastewater treatment. This step removes about 50-60% of suspended solids, but the water still contains dissolved pollutants and harmful microorganisms. That’s why primary treatment alone isn’t enough — secondary and tertiary treatment is necessary to further break down contaminants. The subsequent use of anaerobic and aerobic treatments depends on the specific design and requirements of the wastewater treatment system.

Once large solids are gone, it’s time to break down organic waste and nutrients like nitrogen or phosphorus. This stage uses biological processes to clean the water.

One common method is the activated sludge system. The water to be treated is left in a tank under varying oxygen conditions (aerobic and anoxic) depending on the required removal requirements. The different types of bacteria that live in the tank or reactor feed on the organic matter and the nutrients, removing them from the water (aerobic COD removal) or converting them into biogas (anaerobic treatment). Some industrial wastewater plants in the food industry, breweries, distilleries, and pulp and paper mills use anaerobic digestion already in the primary treatment stage to break down high-strength organic waste.

After this, the water enters another settling tank, where any remaining solids sink to the bottom and the sludge can be separated from the water. At this point, most contaminants are gone, but tiny particles and bacteria may still be present. That’s why wastewater treatment doesn’t stop here, it moves on to the final stage. It is common in wastewater treatment plants for water treatment to end at this point, when the treated water meets the defined discharge requirements and there are no additional water quality requirements for reuse or further use.

Tertiary treatment is the final cleanup step. It removes any remaining bacteria, nutrients, or chemicals that could harm the environment.

Some of the most common methods include:

• Filtration: Carbon or sand filters remove fine particles that weren’t caught earlier.
• Chemical treatment: Chlorine or other disinfectants kill any remaining bacteria.
• Advanced disinfection: UV light or ozone gas neutralizes harmful microorganisms without adding chemicals.

After stage 4, the water is clean enough to be released into rivers, lakes, or the ocean. Efficient treatment systems can also purify the water in such a way that it can be reused for irrigation, replenishing groundwater supplies, industrial cooling and other industrial processes. This makes wastewater treatment a critical part of water conservation efforts. In the paper industry, reuse of wastewater is already common practice. The water is mostly reused for making paper pulp. Paques have helped many paper and pulp facilities along. Paques also has a growing number of references for industrial water reuse in the food and beverage industry, where water is frequently reused as boiler feed water and for cleaning.

Paques offers various world-renowned wastewater plant treatment systems that are economically ánd ecologically sustainable. For wastewater recycling and resource recovery, as well as for biogas production and purification.

Since 1981, the wastewater treatment systems of biotech company Paques has offered high returns to 3300+ customers in 61+ countries. The world-renowned biological treatment systems help commercial industries and water service providers on the way to a circular economy by imitating nature. With our systems, companies can reduce fresh water usage, recover precious resources, decrease carbon footprints and meet strict effluent requirements.

For example, 25.8 million kg of organic pollution is treated with over 1500 anaerobic BIOPAQ® installations of Paques worldwide. These biological treatments systems produce an amount of biogas equal to the natural gas requirements of 2.8 million households in the Netherlands – and additionally reduce 11.2 Mt CO₂ per year. And since Paques pioneered the first biological Anammox system in 2002, ANAMMOX® has become the world’s most used nitrogen removal system.

Our experts are ready to help you select and design the best combination of our 18 world-renowned wastewater treatment systems to meet the specific wishes and demands of your plant.
Our experts can help you turn:

• wastewater into clean water
• biogas into grid-ready energy
• regulation challenges into sustainable leadership
• an inefficient, low-rate treatment system into an efficient, high-rate recycling system