On pig farms, an exhaust air purification system (ARA) is mandatory when constructing new enclosed, mechanically ventilated housing units if the farm requires a permit under air pollution control legislation. This is the case, for example, if the number of animal places exceeds 2,000 for fattening pigs or 750 for sows. This also applies to several smaller individual sheds which together form a complex and exceed the aforementioned thresholds. Since the 2021 revision of the Technical Instructions on Air Quality Control (TA Luft), exhaust air purification systems (the term ‘exhaust air purification facility’ (ARE) is also used in the TA Luft) are considered state of the art and are generally mandatory. Only systems that have been tested for suitability (certified) and meet certain minimum requirements for purification performance and monitoring may be used. Exceptions apply only to so-called “quality-assured husbandry methods that demonstrably serve animal welfare” (TA Luft, 2021) and where, due to natural ventilation and outdoor access, an exhaust air purification system cannot be used. Existing livestock buildings must be retrofitted by December 2026, provided this is technically feasible and, in particular, economically proportionate. If this is not possible or economically viable, measures set out in Annex 11 or equivalent mitigation measures must be implemented, enabling a 40% reduction in ammonia emissions for conventional livestock housing or a 33% reduction for animal-friendly outdoor climate stables. Furthermore, exhaust air purification may also be required for smaller farms if, when constructing a new barn, the distance to protected areas such as sensitive biotopes or residential areas is insufficient. Keeping track of this is often not that straightforward. What types of exhaust air purification systems are available? Which is suitable for which farm? What are the specific features of the systems? This short article provides you with basic information. For further details, please refer to specialist literature.

Exhaust air purification systems operate in various ways. These include different basic types as well as combinations thereof (Hahne et al., 2023a; KTBL, 2023), enabling the use of systems tailored to specific operational requirements.

Examples of different systems

Not every system or combination of systems can treat ammonia, odour and dust. Table 1 shows the suitability of various systems for reducing the different pollutants: ammonia, odour and dust. Which system is necessary and suitable for a particular operation must be decided on a case-by-case basis.

The choice of which air purification system to use must be decided on a case-by-case basis for each barn. It is essential that the air purification system is certified for its intended use. Other criteria include, for example, the system’s performance or the barn’s technical infrastructure, such as feeding systems or ventilation systems. If an ARA is mandatory on a farm due to its approved number of animal places, it may only be dispensed with if, for example, the air cannot be cleaned via the ARA due to an open-air barn and the above-mentioned requirements for a quality-assured husbandry method that serves animal welfare are met.  

In the case of underfloor extraction, care must be taken to ensure that it is correctly dimensioned and operated (maximum slurry channel level at least 50 cm below the concrete grates, level no higher than 30 cm below the lower edge of the ventilation opening, low flow velocity (VDI 3894/1; TA Luft, 2021)). For example, trickle-bed filters may fail to demonstrate a consistently adequate purification performance if incorrectly designed, as – depending on the fill level and depth of the slurry pit – air is constantly drawn over the slurry and thus high loads of harmful gases are drawn into the filter, and the biological system is unable to filter out these substances. When using food waste (whey, distiller’s grains, baked goods, etc.), there is a risk that, in particular, the requirement regarding odour concentration in the purified gas cannot be consistently met. In this case, a multi-stage system with a biofilter is recommended.