Process for preparing an agricultural fertilizer from sewage

Contaminated water is contacted with an agent which contains salts or cations of mono-, di-, or trivalent metals in a maximum amount of 50 masspercent and rock granules containing at least 25 masspercent zeolite, preferably clinoptilolite and/or mordenite. The amount of metal salts respectively cations present in the parent zeolitic rock can be increased according to necessity by ion-exchange, adsorption and/or impregnation, i.e. by combination of these processes following eventual dehydration. The agent obtained is homogenized. The exhausted bed can be regenerated biologically or with aqueous solution of a potassium salt.