Let me make your life easier. In this case using DMA has merely educational benefits. Both 18-bit ADC and UART are slow (<1kHz), so there are no benefits utilizing DMA.
What is your suggested solution then, if I have to filter the signal? My input signal is in low frequency area (lower then 1kHz)
You best choose interrupt driven filter output. In the interrupt handler you just read the filter value and send to UART.