The Role of the Secondary Sedimentation Process

What is the Sedimentation Process?

Sedimentation is a very important process used in wastewater and water treatment. It is used to remove suspended solids from water. The solids, or sediments, can be anything from sand and silt to larger pieces of debris.

Sediments are classified into three types, based on their shape and size:

Fines - Fines (as in fine solids) are the most problematic of the three. They cause the most damage to wastewater systems by clogging up the air intake and creating excessive pressure.
Pea - tend to collect in the spinner of the tank which requires regular cleaning.
Silt - Silt sludge can be easily removed with coarser particles.

Usually, the process of sedimentation is divided into two processes - primary and secondary sedimentation. For more information on primary treatment sedimentation, read here . To learn about secondary sedimentation, read on!

The secondary sedimentation process removes the smaller particles not captured by primary sedimentation. This is done by using a trickling filter, activated sludge or a chemically enhanced treatment. Secondary sedimentation is used when pathogens and fecal matter are high in concentration. At this time, viruses, bacteria, and protozoa are removed at a high rate to produce drinkable water. Some of the residue is recycled into feedstock.

When is Secondary Sedimentation Used?

Secondary sedimentation is utilized after primary sedimentation where larger waste such as fats, oils, and uneaten food is removed. Once that’s done, the resulting water is called secondary effluent. At this stage, secondary sedimentation is used to further cleanse the water.

Major constituents in secondary wastewater are silt (solids), sand, organic matter, and chemicals. To remove these, chemical treatment processes are used to aid biological sedimentation.

A Biological Treatment

Many wastewater treatment plants use an activated sludge process (ASP) in secondary sedimentation. ASP is a biological process that speeds up the decomposition of wastes in wastewater by using an aeration process. It blows air or oxygen into raw, unsettled sewage in aeration basins to break-up the solids. This creates a biological 'soup' which digests the organic content and pollutants in the water.

Secondary sedimentation tanks use devices like scum collectors that capture waste, which is rotated in the wastewater using blowers. These tanks also use spray systems for moving scum, and pumps for draining tanks.

Secondary sedimentation tanks are often used to separate sewage sediment into wet and dry components for disposal. This is different from primary sedimentation tanks which separate the sewage solid particles from water. These are predominantly used during the treatment of rivers polluted by sewage as well as wastewater in a treatment plant. Sedimentation systems are also used to transport suspended solids from a wastewater treatment plant to a settling basin or sludge pond. The blowers used in wastewater treatment applications are responsible for removing these solids from the wastewater.

Like its primary cousin, secondary sedimentation also uses gravity to separate solid waste from wastewater. It also uses specialized filters to aid in the process. The solids that settle out and are collected at the bottom of the tank are pushed through filters using blowers. The clear water is then discharged from the tank.

A Source of Feedstock

In a typical secondary sedimentation process, wastewater solids settle out, combine into a sludge at the bottom. The sludge can be used in a specialized phase to organically regenerate the cleansed effluent. This can create good feedstock for land application.

Once suspended matter is settled in the tanks, the liquid lying above is recycled back to another tank for reuse. It has an optimum density for pumping out of the plant without clogging air pipes while achieving higher quality effluent.

Blowers Used in Secondary Sedimentation Process

There are a few factors to consider when selecting a blower for secondary sedimentation. These include:

the size of the particles to be removed;
the flow rate of the fluid stream or body of water;
energy consumption;
·noise level of the blower.

Specially designed pumps and blowers use compressed air to mechanically rotate the wastewater in a tank and loosen sediments. Blowers are also used to clean devices involved in the process of sedimentation. They apply pressure to loosen and unclog pipes, machinery and intakes valves.

In some cases, the high sludge concentration at the bottom of the secondary sedimentation tank can inhibit the flow of effluent. This hinders the effluent from reaching an acceptable concentration as required by municipal wastewater treatment methods. Aeration provided by blowers is used to help this process. Blowers force air through the liquid in order to help this process.

Lobe Blowers for Secondary Sedimentation Tanks

Learn About the Robuschi Blower Portfolio for Secondary Sedimentation Tanks in Wastewater Treatment

Lobe blowers are the most common technology used in the secondary sedimentation stage of wastewater treatment. The process requires small concentrated amounts of air generally in the form of coarse bubbles that enter the aeration tank. Lobe Blowers are the ideal technology for this secondary sedimentation process. They offer the optimal flexibility of a wide flow range, and the ability to work non-stop or cyclically.

Rotary Lobe Blowers

Pressure up to 1.000 mbar (g)
Capacity up to 25.000 m³ / h