A water treatment system (10) includes an ozone generator (3) combined with an electrolytic chlorine generator (4010) in a compact, efficient and serviceable assembly. The system (10) may include a modular and replaceable ozone generator (3), which allows a damaged or non-functional ozone generator (3) to be quickly and efficiently replaced. In order to protect the ozone generator (3) from damage, a fail-safe drain valve assembly may also be provided which will expel backflowing pool water before it is allowed to backflow into the ozone generator (3). The water treatment system (10) may further include an insulated electrolytic chlorine generator (4010) that mitigates or eliminates leakage current for efficient operation.