Water Infrastructure Improvement Act (Sec. 3) This bill amends the Federal Water Pollution Control Act (commonly known as the Clean Water Act) to allow municipalities to develop a plan that integrates wastewater and stormwater management. A permit for a municipal discharge under the national pollutant discharge elimination system that incorporates an integrated plan may integrate all requirements under the Act addressed in the plan, such as requirements relating to combined sewer overflows, sanitary sewer collection systems, and total maximum daily loads. A plan that is incorporated into a permit may include the implementation of green infrastructure and projects to reclaim, recycle, or reuse water. Green infrastructure includes measures that mimic natural processes to store, reuse, or reduce stormwater. Those permits may include a schedule of compliance that allows actions for meeting water quality-based effluent limitations to be implemented over more than one permit term if the compliance schedules are authorized by state water quality standards. A municipality under an administrative order or settlement agreement may request a modification of the order or settlement based on the municipality's integrated plan. The EPA must report on each integrated plan developed and implemented through a permit, order, or judicial consent decree since June 5, 2012, including a description of the control measures, levels of control, estimated costs, and compliance schedules for the requirements implemented through such a plan. (Sec. 4) The bill establishes an Office of the Municipal Ombudsman in the Environmental Protection Agency (EPA) to provide: (1) technical assistance to municipalities seeking to comply with the Clean Water Act, and (2) information to the EPA to ensure that agency policies are implemented by all EPA offices. (Sec. 5) The EPA must promote the use of green infrastructure.