Filtration

Next, the water is filtered to remove microscopic particles. Rinconada has six filters each capable of filtering 16.7 million gallons of water per day. Each filtration tank is 13 feet deep and 77 feet long, about half the length of an Olympic-sized swimming pool.

At the bottom of each filter are layers of coarse sand (6 inches), fine sand (18 inches), and anthracite goal (1 foot). As the water seeps down the layers of sand and coal, tiny particles as small as one micron are left behind.

Each of our plants will use granular activated carbon, or GAC, in the future instead of anthracite coal. GAC, the same type of material used in many home filtration systems, removes tiny particles and also chemical compounds that affect the water’s taste and odor.

Just like any home water filtration system, these filters get dirty and must be maintained. To keep them functional, they have to be washed periodically. The process, called "backwashing," involves several steps. First, the filter is taken off line and the water is drained down to the filter bed. Then, the air wash cycle is started which pushes air up through the filter bed causing the filter bed to appear to boil. This breaks up the compacted filter bed and forces the accumulated particles into suspension. The air wash cycle lasts for about three minutes.

After the air wash cycle stops, the backwash cycle starts with water flowing up through the filter bed. Most of the accumulated particles are flushed out. This cycle continues until the backwash water looks clean. The filter is then refilled with water and put back on-line. The backwash water flows into the recovery ponds where the solids in the washwater settle out and the water is pumped back to the beginning of the process to be treated again.

At the Santa Teresa plant (starting in the spring 2006), the waste backwash water enters a mini-treatment plant to pre-clean it before it is pumped back to the starting point again. This will further reduce the possibility of parasites like giardia and cryptosporidium cycling back to the main treatment process. The other two plants may have this capability in the future.