Proposed features/Process tanks

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Process tanks
Status: Proposed (under way)
Proposed by:
Tagging: equipment=*
Applies to: area Areas
Definition: Structures used for the processing of chemicals, feeds, wastewater, or other components

Drafted on: 2021-03-07
RFC start: 2021-03-20


A process tank is a type of  storage tank that is used for the processing of chemicals, feeds, wastewater, or other components, leading to the production of a desired substance or product.

In water treatment facilities,  aeration tanks,  clarifiers and  oxidation ditches are specific types of process tanks used to separate sludge (which settles to the bottom) and scum (which floats to the top) from untreated or waste water. These structures are often found adjacent to each other in water treatment facilities, although not all water treatment facilities have all types of structures.

Proposed are that:

While this proposal proposes specific tagging for features associated with water treatment plant, equipment=* and equipment:stage=* are generic tags that can be used and/or extended for any industry.

Four clarifiers and two oxidation ditches at a water treatment facility in The Netherlands.

Summary of tagging changes

Summary of Tagging Changes
Tag Action Description
man_made=storage_tank Expand Definition Update wiki definition to include storage tanks that also contain processing equipment.
equipment=aerator Approve Used to tag an  aeration tank.
equipment=clarifier Approve Used to tag a  clarifier.
TBD Used to indicate whether a clarifier (tagged man_made=storage_tank + equipment=clarifier) is a primary or secondary clarifier.
equipment=oxidation_ditch Approve Used to tag an  oxidation ditch.
equipment=baffles Approve Used to tag process tanks which contains  baffles.
Deprecate These tags have little usage and are wholly described by other tagging.


  1. Clarifiers, oxidation ditches, and aeration tanks are large constructed features, usually with steel and/or concrete housing. Therefore, it is not appropriate to categorize these features as basins, which are of earthen construction. Similarly, it is not appropriate to categorize these features as a general water feature (natural=water), as these are industrial structures which may contain sewage, waste water, or untreated reservoir water.
  2. Discussion on the proposal for tailings ponds indicated that people felt that tailings should not be categorized as water, even though tailings are mostly water. Similarly, sewage, waste water, and sludge, should not be classified as water. The tagging natural=water + water=wastewater causes sewage and waste water to be grouped in with traditional waterbodies such as lakes, ponds, and reservoirs. By providing specific tagging for wastewater processing tanks, we reduce the usage of natural=water on non-water industrial processing features.
  3. The tags water=wastewater and reservoir_type=sewage are used on water treatment features other than process tanks, such as settling ponds. Therefore, it is not proposed to deprecate these tags. However, this question may be addressed in a future proposal.
  4. Due to the lack of approved, dedicated tagging for these features, they are often tagged as natural=water, landuse=basin, or building=yes in order to achieve rendering. By creating dedicated tagging scheme for these features, we create a clear path to uniformity in both tagging and rendering for these features. Since man_made=storage_tank currently renders in many styles, mappers can adopt this tagging and achieve render support without change to renderers.
  5. The tag water=wastewater conflates clarifiers and oxidation ditches with settling basins. However, these are entirely different features.
  6. The proposed key equipment=* is unused for other purposes and provides a clear path to defining additional values which represent various types of processing equipment suitable for many industries.
  7. Primary and secondary clarifiers are tagged in the map today, by mappers with specific domain knowledge of water treatment facilities, using a variety of invented tags. The proposed key equipment:stage=* provides a uniform scheme for this tagging.
  8. The tags man_made=wastewater_basin and man_made=sewage are barely used and have unclear and redundant meaning, therefore they are proposed for deprecation.

How to map

Draw as an area area along the outline of the tank.

Tag it with man_made=storage_tank. Add the tag equipment=* if this is known.


Clarifiers, oxidation ditches, aeration tanks, and baffled tanks are visually distinct and easily identified on imagery, as show in in the examples section. Draw a way on the outline of the structure, and tag as follows from the table below. For conjoined adjacent tanks, it is acceptable to tag the entire combined tank with a single polygon. If the mapper cannot identify the specific type of process tank, it is acceptable to tag it man_made=storage_tank without additional tagging.

The specific types of process tanks described in this proposal are easily identified on overhead imagery, and are usually open on top. In cases where a feature identified on imagery is ambiguous between a tank or a building, mappers may only make an educated guess based on the surrounding context or other external information, or else leave these objects to be mapped by a future mapper with more information.

Tagging Examples
Tag Description
Used as to tag an aeration tank


Used as to tag an aeration pond or basin[1]
Used to tag a primary clarifier. The information on whether a clarifier is primary or secondary likely requires addition information about the facility that cannot be determined from overhead imagery alone.
Used to tag an oxidation ditch
Used to tag a baffled tank. In water treatment plants, baffled process tanks are sometimes used when disinfecting chemicals are added to treated water. In open tanks, the baffles are typically visible as either a labyrinth pattern or as axial protrusions from the wall of a circular chamber. Photos of baffled tanks are provided in the examples section.


The following image from the way Rioolwaterzuiveringsinstallatie Nieuwegein in the Netherlands shows examples of both clarifiers and oxidation ditches.

Clarifiers and oxidation ditch.png

This image from the way Klärwerk Ruhleben treatment plant near Berlin, Germany, shows examples of aeration tanks, both circular and rectangular clarifiers, and oxidation ditches.

Water treatment 3 way.png

Aeration Tanks

Aeration tanks are used to treat wastewater by introducing air into wastewater. These tanks are identifiable by the presence of turbulent water in the tank. However, unlike oxidation ditches, the water does not circulate in a loop.



The purpose of a clarifier is to mechanically separate both scum and sludge from wastewater. Clarifiers are recognizable by their mechanical skimming arm. Below are examples of clarifiers from water treatment plants. There are two basic designs:

Circular clarifiers use a radial skimming arm which turns around a central axis. A trough around the edge of the clarifier catches the scum trapped by the arm.


Rectangular clarifiers use a series of parallel skimming arms which move from one end of the clarifier to the other, with a scum trough on one end. Note in the example photos that the skimming arms of adjacent clarifiers are slightly offset from each other -- this is a visual indication that these are clarifiers since the sets of skimming arms in each tank typically are driven independently and will not be in sync.


Oxidation ditches

Oxidation ditches are found in round or oval configurations which allow the liquid to flow in a loop. A portion of the loop is turbid, as this is where the aerator machinery is introducing aeration to the water. The key identifying characteristics of an oxidation ditch are the round or oval shape, and the visible trails from horizontal aerators as the liquid flows around the loop. In some cases, the oxidation ditches will be rectangular in shape, but with semi-circular guides at each end to direct the liquid in a looping flow.

Oxy ditch annotated.png

Below are examples of oxidation ditches:

Baffled tanks

Baffled tanks are found in water treatment facilities and can be identified by their geometric baffle walls. These tanks are sometimes known as contact chambers, and are used to slow the movement of liquid through the tank. They can be found in various shapes.

Below are examples of baffled tanks:

Features/Pages affected

Create these pages:

Update the following pages consistent with this proposal:

External discussions


Please comment on the discussion page.

  1. Note that aeration basins, which are earthen structures which have aeration equipment installed, should not be tagged as a tanks, but rather as a basin.