Effective conventional slow sand filter use is typically limited to good quality raw water sources that are low in color, turbidity, and algal content. Conventional slow sand filter plants are also typically custom-engineered and constructed with cast-in-place concrete that can make this treatment option relatively high in capital cost for small systems (<0.5 MGD capacity). This paper presents recent pretreatment enhancements that have been proven effective in the field for extending the slow sand filtration process for marginal water supply sources, along with a case study of a pre-fabricated ozone/roughing filter/slow sand filter installation in North Haven, Maine. Includes 7 references, tables, figure.