Abstract: A mobile water purification system is configured as modules to be air-lifted to a site requiring potable water. Raw water is pumped to a pre-treatment module that separates suspended solids and preferably oxidizes organic contaminants. At least one, and preferably two or more filtration modules coupled in parallel, then remove contaminants using ultrafiltration and/or reverse osmosis, and pass treated water to a storage tank. A control system operates valves to stagger backwashing of filtration modules, allowing one such module to be backwashed while allowing others to continue filtering water. The control system can selectively bypass a reverse osmosis unit, and disable related booster pumps, depending upon raw water quality. During backwash cycles, sodium bisulfite is added to fill water directed into ultrafiltration units to neutralize free chlorine present in backwash fluid.