Sedimentation

A.    Sedimentation Basin

 

Method diagram

Method and installation description

The aim of sedimentation is to remove undissolved particles from wastewater.  If the density of the particles is greater than that of the water, they will sink to the bottom under the force of gravity (sedimentation). Sedimentation takes place in sedimentation basins in which wastewater flows slowly. The upward flow of the water must be less than the downward speed of the sinking particles. In larger basins, a sludge scraped is normally fitted to the bottom of the basin in order to dispose of the settled sludge. Sedimentation basins are available in various guises – round or rectangular, with or without sludge scraper and floating layer clearer. To improve separation, a coagulant or flocculant can be added to sludge that is difficult to sink.

Specific advantages and disadvantages

A sedimentation basin is a very simple and relatively cheap installation. On the other hand, a sedimentation basin does take quite a lot of space. The design of a sedimentation basin is always based on a maximum volume. The operator has few opportunities to modify the set-up once the installation has been designed.

Application

A preliminary sedimentation basin is used to remove sinkable substances from wastewater. Together with the sinkable substances, part of the COD, BOD, nitrogen (N), phosphorous (P) and metals is also reduced. This allows the aerobic water purification system to be smaller in size. Sludge from preliminary sedimentation can be further processed in an anaerobic digester.

Most biological water purification systems also feature a conventional post-sedimentation basin. This involves using gravity to separate the biological sludge from the purified water. The sludge is then pumped back to the aeration basin.

Sedimentation basins can also be used for the separation of inorganic sludge.

Boundary conditions

The maximum load (effluent discharge per meter of overspill edge per hour) of sedimentation tanks is approximately 10 m3/m; that of post-sedimentation tanks is 3-5 m3/m.hour.

The surface load may not exceed 1.5 to 2.5 m3/m2 hour and 0.5 to 1 m3/m2.h respectively for preliminary and post-sedimentation tanks. The limit for wastewater retention time in preliminary and post-sedimentation tanks is 1 to 1.5 hours and 1.5 to 2 hours respectively.

The size of the sedimentation tank is limited by the technical construction of the squeegee mechanism.

Effectiveness

A very high removal yield can be achieved if the installation is designed correctly. The suspended matter concentration after a sedimentation basin is typically <10 mg/l.

Support aids

Flocculants are used in some applications if sedimentation does not offer optimum results.

Environmental issues

A watery sludge flow is released as by-product. In biological purification, this flow is partly re-circulated to the water purification system. In other cases, the sludge must be further diluted or treated.

There will be no odour problems if the installation is correctly dimensioned. Odour problems may be encountered in some cases if sludge is kept in the basin for too long.

Costs

Most sedimentation basins are made from concrete. The cost price for standard concrete work is estimated at 200-300 €/m³.  Because the bottom must be slanted, concrete work for a sedimentation basin costs more than standard concrete work.

A squeegee system is fitted in order to separate the sludge centrally. The squeegee must be built by a specialised company. The cost is determined by the diameter of the squeegee. For smaller diameters (up to 5 meters) the cost comes to approximately € 25.000. The cost will be considerably higher for larger diameters, because the construction weight will also be much higher.

The operational costs are restricted to energy consumption for the squeegee system.

Comments

none

Complexity

A sedimentation basin is not very complicated.

Level of automation

Not relevant because this process seldom involves active management.

References

  • Baeyens J., Hosten L. and Van Vaerenbergh E., Wastewater purification, Environment Foundation - Kluwer Editorial, 1995
  • EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
  • VITO-SCT, revision of technical notes WASS, 2009

 

 B.     Lamella Separator

Method diagram

 

Method and installation description

The aim of sedimentation is to remove undissolved particles from wastewater.  If the density of the particles is greater than that of the water, they will sink to the bottom under the force of gravity (sedimentation). A lamella separator is a special, space-saving separator. The actual separation mechanism consists of a separator with a series of plates inside it and a collection system for sunken sludge at the bottom. Particles that land on the plates under the influence of gravity, slide down the plates to the bottom.  The system can be a lot more compact than a conventional separator because the sinkable matter only needs to travel a limited distance (a few centimetres) between two plates.  A lamella separator is also referred to as a plate separator.

Specific advantages and disadvantages

A lamella separator requires little supervision or maintenance. Separation via gravitational force is often the most cost-effective method for separating sinkable matter from a liquid.

Application

A lamella separator is used to keep the surface area of a sedimentation basin or tank to a minimum or to increase the hydraulic capacity of an existing basin.

Boundary conditions

The capacity per unit is circa 1-300 m3/hour. The type of pollution and its concentration determine the retention time and thus also the size of the installation.  Large particles, or the presence of fatty matter, could block the series of plates.

Effectiveness

A very high removal yield can be achieved if the installation is designed correctly.  The suspended matter concentration after a sedimentation basin is typically <10 mg/l.

Support aids

Flocculants may be used as a support aid.

Environmental issues

A watery sludge flow is released as by-product.  In biological purification, this flow is partly re-circulated to the water purification system.  In other cases, the sludge must be further diluted or treated.

There will be no odour problems if the installation is correctly dimensioned.  Odour problems may be encountered in some cases if sludge is kept in the basin for too long. 

Costs

A lamella separator (RVS 304) for 240 m3/h costs approximately 150.000 €. One for 20 m3/h costs approximately 20.000 €.

A lamella separator requires very limited operational supervision.

Comments

none

Complexity

limited

Level of automation

A lamella separator is very easy to automate. It is a continuous system, possibly featuring an operable valve for sludge discharge.

References

  • Baeyens J., Hosten L. and Van Vaerenbergh E., Wastewater purification, Environment Foundation - Kluwer Editorial, 1995
  • EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
  • VITO-SCT, revision of technical notes WASS, 2009

Version February 2010