a rainwater collection system is described which includes generally a rainwater routing system and a collection unit . the rainwater routing system may comprise a first flush diversion unit designed to discharge to a run - off path , an initial amount of rainwater such that initial particulate matter from rooftops , etc ., are not collected . the rainwater routing system may comprise a pre - treatment unit , the pre - treatment unit designed to remove particulate matter from the rainwater . the collection unit of the rainwater collection system comprises a storage tank having an internal day tank compartment . the described rainwater collection system allows for the collection of rainwater with a substantially reduced amount of particulate matter .