Complex sentence: Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or else the septic tank fills up and wastewater containing undecomposed material discharges directly to the drainage field.
Split into simpler sentences: Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank. Otherwise the septic tank fills up and wastewater containing undecomposed material discharges directly to the drainage field.