Filtration devices with filters for filtering and purifying tap water have conventionally been widely used. However, relatively long-term use of such a filter causes a decrease in filtering performance due to trapped impurities (e.g., inorganic particles, etc.) therein. Thus, there are known some filtration devices that are capable of removing the impurities by causing purified water, which is obtained by filtration of tap water by the filters, to flow backward to filters according to backwashing methods.
As a backwashing method, there is a method for pooling some purified water in a tank beforehand and then causing the purified water to flow backward to a filter when backwashing needs to be performed. This method, however, requires such a device as a tank, leading to an increase in size of the entire filtration system. For this reason, there is proposed a method that can avoid the increase in size of the system by providing two filters in parallel and switching a water current using a switching valve so that some of purified water in one of the filters can flow backward to the other filter (Patent Documents 1 and 2 , for example).