Most odor problems in water result from low oxygen levels. To avoid the corrosive effects of chlorine, add some hydrogen peroxide to the water as needed to eliminate the smell. The sump pump should not be connected to a sewage system, but collects groundwater. Oxygen demand by minerals in the water, and lack of agitation causes the oxygen depletion. Hydrogen peroxide (H2O2) adds what you need. Oxyclean is similar if you have it.