The County of Maui Department of Water Supply (DWS) provides water to the nearly 150,000 residents of the island. DWS operates five water districts on the island and treats a variety of ground and surface water sources. The Mahinahina Water Treatment Facility (WTF) in the Lahaina district is one of six surface water treatment plants operated by DWS. The Mahinahina WTF is a 2.5 million gallon per day (mgd) surface water treatment plant constructed in the early 1990s. The existing treatment process consists of rapid mix, tapered flocculation, and deep bed dual-media (anthracite/sand) filtration. Due to poor source water quality and a need for additional supply, DWS wishes to make improvements to the treatment process and expand the capacity of the WTF to 8 mgd. This paper presents the results of the treatment process and capacity improvement analyses conducted for the Mahinahina WTF. It will discuss development of treatment and capacity goals, identification of improvement alternatives, and prioritization and ranking of selected improvements. The results from bench-scale testing will be presented and include the ability of the existing pre-treatment (rapid mix and flocculation) to be high-rated and still achieve finished water quality objectives. The hydraulic and capacity analysis of the existing facility will also be discussed. This paper will also compare the use of pressurized versus submerged membranes to replace the existing filters, and discuss how the existing filter box configuration influenced the selection of the preferred improvements, including cost, construction and operational impacts.