**Acknowledgements**

We would like to thank Lukas Lesmeister (DVGW, Karlsruhe), Melanie Mechler and Jörn Breuer (Center of Agricultural Technology Augustenberg, Karlsruhe) and Prof. Karl-Heinrich Engesser (ISWA, Stuttgart) for their continuous support. This work was supported by the German Federal Environment Agency at grant number FKZ 3718 74 210 0; AZ: 91 007 – 2/82.
