A system for treating water includes an ozone injector combined with a monitoring apparatus 400 that includes a flow meter 402 and a pressure sensor 404. The monitoring apparatus includes a flow path 424 having inlets 420, 422 communicating with the inlet 304 and outlet 306 of the ozone injector main water pipe 302. The flow path inlets are spaced respectively upstream and downstream of the ozone injection point. In operation, water filtered in filter 108 is ozonized in venturi 316 and then sterilized by UV lamp 210. The lamp is also used to generate ozone that is delivered to the venturi via check valve 308. The pressure sensor positioned within the flow path produces signals to control the switch of the UV lamp. The flow meter is used to determine when the filter should be replaced. The pressure sensor may be in the form of a silicon chip.