Water treatment of small water reservoirs with limited infrastructure and no addition of chemicals

Water treatment systems and methods are provided, for treating a reservoir without addition of chemicals. Such requirements arise mainly for relatively small reservoirs with limited filtering infrastructure, such as private pools. Ultraviolet (UV) treatment is employed, in addition or possibly in place of filtering, to overcome infrastructural limitations on water throughput, such as filtering and/or pumping capacity. UV may be used over longer periods and/or with respect to higher throughputs, to reduce bacterial growth rates more significantly than filtering, and thereby prevent bacterial growth in the reservoir. UV treatment may be applied according to a pre-defined temporal pattern selected to prevent biomass growth in the reservoir and optionally be updated with respect to water quality measurements.