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1.4: Water Recycling

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    Learning Outcomes

    • Understand the requirements of Title 22 for recycled water
    • Know what level of treatment is needed for different recycled water uses


    By now the wastewater has been completely treated. By undergoing preliminary, primary, biological, secondary, tertiary, and disinfection treatment the water should now meet the rigorous requirements of a National Pollutant Discharge Elimination System (NPDES) permit and be able to discharge in a nearby waterbody. Often, the addition of this treated water provides a riparian habitat to many aquatic species. However, a lot of time, money, and energy were put into treating this water. If advanced treatment methods like BNR and membrane filtration were utilized, then the water can be used for other beneficial reuses.

    Non-Potable Reuse & Reclamation

    To reuse treated wastewater for non-potable reuse, meaning it cannot be consumed, it must comply with Title 22 of the California Code of Regulations. Title 22 outlines what level of treatment is required for different types of reuse applications. There are four different types of classifications of water under Title 22, undisinfected secondary recycled water, disinfected secondary-23 recycled water, disinfected secondary-2.2 recycled water, and disinfected tertiary recycled water. Undisinfected secondary recycled water is the lowest level of treatment and therefore has limited application uses. It can be used for orchards or vineyards where the recycled water does not come into contact with the edible portion of the crop. It can also be used for ornamental nursery stock and non-food-bearing trees as longs as no water is applied to the plants for 14 days prior to harvesting, retail sale, or allowing access by the general public.

    Disinfected secondary 23 and 2.2 differ in the level of disinfection. The 23 and 2.2 are referring to results from the Most Probable Number (MPN) test used to determine the effectiveness of the disinfectant. An MPN of 2.2 is the lowest level of detection for the test and therefore a lower chance of pathogenic organisms existing in the sample. As the wastewater goes through higher levels of treatment it can be used for more non-restrictive uses. Disinfected secondary-23 recycled water can be used for irrigation purposes but only in areas where there is a limited chance of the water coming in contact with the general public. For example, it can be used to water freeway landscaping, cemeteries, and golf courses with restricted access. Disinfected secondary 2.2 recycled water can be used for irrigation of food crops where the edible portion is produced above ground and not contacted by the recycled water.

    Disinfected tertiary recycled water is the highest level of treatment defined by Title 22 and can be used for pretty much any type of irrigation. It can be used on food crops where the recycled water does come into contact with the edible portion of the crop. It can be used to irrigate parks, playgrounds, schoolyards, and residential landscaping. Disinfected tertiary recycled water can also be used for other purposes such as flushing toilets, industrial processes, fire fighting, decorative fountains, and car washes.

    Indirect Potable Reuse

    Title 22 also regulates using recycled water for indirect potable reuse. It outlines two ways to do this, either by surface application or subsurface application of groundwater replenishment. Surface application is when the recycled water is sent to a percolation basin. The water is forced by gravity through the voids in the soil which adds an extra layer of filtration as the water enters a groundwater aquifer. The water is then pumped out of the aquifer and sent to a water treatment facility where it is further treated and ultimately is used for potable water.

    Subsurface application is when the water is pumped directly from the treatment plant discharge to the groundwater aquifer. Since this method does have the added treatment benefit of the water percolating through the soil, additional advanced treatment methods are needed at the wastewater treatment facility. The tertiary treated wastewater will typically go through reverse osmosis membrane filtration prior to being injected into the groundwater aquifer.

    This page titled 1.4: Water Recycling is shared under a CC BY license and was authored, remixed, and/or curated by Nick Steffen (ZTC Textbooks) .

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