Proposed Advanced Treatment Process

The Regional Groundwater Sustainability Project will build on the existing treatment processes at the Pismo Beach and SSLOCSD WWTPs to provide advanced treatment that will include: MF/UF, RO, and UV/AOP. These processes are described below.

MF/UF = Microfiltration/Ultrafiltration

Microfiltration/Ultrafiltration is exactly what it sounds like. The water is filtered through a physical membrane barrier with very small pores. These pores come in all sizes, ultrafilter pores range from 0.2 micrometers (μm) to 0.005 μm. For comparison, 0.2 μm is 1/300th the diameter of a human hair. MF/UF removes very small particles and preps the water for the next step, reverse osmosis.

RO = Reverse Osmosis

Reverse osmosis removes dissolved solids from the flow. It also uses a physical membrane barrier with pore sizes that range from 0.001 μm to 0.0001 μm depending on the membranes used. For comparison, the width of a strand of spider web silk is 3-8 μm. Unlike microfiltration/ultrafiltration, reverse osmosis produces a clean water stream and a waste stream of salty water, or brine. This means that not all the water is recovered from this process as purified water. A percentage of the water becomes brine (about 10-20%), which contains a higher concentration of the dissolved particles that were in the flow. This brine will ultimately be discharged to the ocean through the existing ocean outfall that currently receives all the flow from the Pismo Beach WWTP. While the brine stream is more concentrated than typical drinking water it is still much less salty than ocean water or brine from ocean desalination facilities. After the dissolved solids have been removed, the water that passed through the RO membranes is of very high quality and can ready for the Advanced Oxidation treatment process.

UV/AOP = Ultraviolet disinfection with Advanced Oxidation

The last treatment step in this advanced treatment train is UV disinfection and advanced oxidation. Advanced oxidation uses ultraviolet light and oxidation chemicals, to initiate a series of chemical reactions to break down compounds in the water that cannot be broken down by biological treatment or removed using the membranes.


Advanced treatment results in purified water ready for injection into the groundwater aquifer. The months the water spends in the aquifer before being pumped back out is known as the environmental buffer, after which the water can be re-pumped to the surface and used as drinking water, pure and clean.