The effects of fouling on the cross-flow ultrafiltration of a natural groundwater are modeled in both the constant pressure and constant flux operational modes. Semi-empirical equations are derived which accurately predict the various flows and pressures in an ultrafiltration system when used to treat both a raw groundwater and a groundwater pretreated with powdered activated carbon. The effects of both reversible and irreversible fouling are modeled, and the deviation between the predicted flows and pressures and the experimental results generally remained below 10%. The overall efficiency of water production is compared for the constant flux and constant pressure modes, and it is found that neither mode provides a higher efficiency. A method is also presented for the optimization of backflushing frequency in constant pressure ultrafiltration.