To understand reverse osmosis (RO), one must first understand osmosis. Osmosis uses a semi-permeable membrane that allows ions to pass from a more concentrated solution to a less concentrated solution without allowing the reverse to occur. Reverse osmosis overcomes the osmotic pressure with a higher artificial pressure to reverse the process and concentrate the dissolved solids on one side of the membrane. Normal operating pressures are 300 to 900 psi. Reverse osmosis will reduce the dissolved solids of the raw water, making the final effluent ready for further pretreatment. Although sometimes expensive, this process can be used on any type water.