Rural Water Infrastructure Improvement Act of 2018 This bill amends the Consolidated Farm and Rural Development Act to reauthorize through FY2023 and modify the Department of Agriculture Water and Waste Disposal Loan and Grant Program. (The program provides funding for clean and reliable drinking water systems, sanitary sewage disposal, sanitary solid waste disposal, and storm water drainage to households and businesses in rural areas.) The bill modifies the program to: increase from $100,000 to $200,000 the maximum amount of financing from a revolving loan fund that may be made available to an entity for water and wastewater projects, authorize loans for local utilities with experience delivering water or wastewater programs to provide service to unserved and underserved rural communities, increase the population ceiling that applies to certain loans, and provide mandatory funding for applications that are pending when this bill is enacted.