Since autumn 2008 a new L-band 7-Feed-Array receiver is used for an \ion{H}{i} 21-cm line survey performed with 100-m Effelsberg telescope. The survey will cover the whole northern hemisphere comprising both, the galactic and extragalactic sky in parallel. Using state-of-the-art FPGA based digital Fast Fourier Transform spectrometers - superior in dynamic range and providing high temporal resolution of spectra - offers the possibility to apply sophisticated radio frequency interferences (RFI) mitigation schemes.

We will discuss the severe degredation of RFI-signals on radio astronomical \ion{H}{i} data and the enormous gain in scientific yield when applying modern RFI-mitigation schemes. For this aim we performed simulations of the galaxy distribution within the local volume ($z\leq 0.07$) with and without RFI degradation. We will highlight in particular the bias introduced by RFI-signals on the derived results. We briefly discuss the data reduction software including RFI mitigation, gain-curve correction, intensity calibration, stray-radiation correction, gridding, and the source detection.