Passive area sources consist of extended solid or liquid surfaces without an outward airflow, which are exposed to the atmosphere and therefore may emit volatile odorous compounds.

Examples of passive area sources that are typically associated with odour emissions are for instance wastewater treatment tanks (liquid) or composting piles (solid).

One way to reduce emissions from these types of sources is to minimize the evaporation of odorous substances by reducing the portion of the odorous surface that is directly exposed to the atmosphere. This can be achieved by enclosing the emitting surface with specific covers.

There are many cover styles available on the market: fixed covers, floating covers, inflatable covers, retractable covers, covers made from flexible geomembranes, and covers made from rigid materials such as aluminum, fibreglass, or steel.

Covers can – in principle – be applied both to liquid and to solid surfaces. When evaluating the possibility to cover an odour source, some important factors should be taken into consideration such as:

• Possibility of normal operation/ maintenance of the source after covering
• Oxygen demand for sources that require aerobic conditions (e.g., composting piles)
• Possibility that flammable conditions are reached under the cover

Some examples of covers are shown in Figure 1 to Figure 4.