Water Recycling and Treatment Requirements

What is Wastewater Reuse?

When the word drought is present, the words recycled water will follow. The most obvious solution to water scarcity is the capture, treatment, and reuse of wastewater. However, wastewater reuse, also called water reclamation, is not without its complexities and technicalities. While all water on Earth is cycled and reused eventually, wastewater reuse specifically regards water that has been collected into sewers and has been treated sufficiently for various reuse applications. There are three different types of water recycling: Non-Potable Reuse, Indirect Potable Reuse, and Direct Potable Reuse. The table below lists definitions and examples for each case.

Definition Examples
Non-Potable Reuse (NPR) Recycled water is used for purposes other than drinking, including golf course irrigation, agriculture, public parks, and artificial snow production. Most water recycling plants focus on non-potable reuse.
Indirect Potable Reuse (IPR) Treated wastewater is sent to a natural water body, either on the surface or underground, before it is extracted again for drinking water treatment. California has the most IPR plants of any state, including the Groundwater Replenishment System in Orange County, which has been in operation since 1975, though recently upgraded in 2007.
Direct Potable Reuse (DPR) Highly treated wastewater becomes the drinking water source. The first fully operational DPR project in the United States is the Big Spring Raw Water Production Facility in Big Spring, Texas, which came online in 2013. A year later, a second DPR plant opened in Texas, only 230 miles from the original plant in Big Spring.


Treatment Requirements for Conventional Wastewater Discharge

Wastewater treatment plants typically treat to a secondary level—that is, water is treated beyond simple removal of floating and suspended solids. Secondary water treatment usually employs biological methods to reduce chemical and biological loadings to the environment. California defines secondary oxidized wastewater as having no more than 10 mg/L of total organic carbon (TOC), 30 mg/L of suspended solids, or 30 mg/L of biochemical oxygen demand (BOD).

Treatment Requirements for Water Recycling

Water recycling requires that wastewater be treated to a tertiary level with targets of reducing nutrients and achieving a higher level of disinfection than is required for conventional wastewater discharge.

Legislation Regulating Reclaimed Wastewater

Currently, only a few U.S. states have passed legislation regarding regulations for reclaimed wastewater. Unsurprisingly, these states are those with the heaviest water worries, such as California, Texas, Arizona, and Florida.

California Title 22 Regulations for Reclaimed Wastewater

In California, the regulations for recycled water are stipulated by Title 22. Details and definitions pertaining to NPR and IPR via groundwater recharge are described. However, the regulations do not encompass DPR nor surface reservoir recharge, perhaps owing to the fact that groundwater sources are more common than surface water sources in California. California is currently in the last stages of a feasibility study for creating DPR regulations.

Tertiary Recycled Water

Under Title 22, disinfected tertiary recycled water is the most stringent water definition for water reuse. It describes water that has been both filtered and disinfected.

Filtration Requirements for Tertiary Recycled Water

The filtration process must produce an effluent that meets the following requirements:
• Average daily turbidity less than 2 NTU (nephelometric turbidity unit)
• Effluent cannot exceed 5 NTU more than 5% of the time
• Effluent cannot exceed 10 NTU at any time
• If the influent to the filter exceeds 5 NTU for more than 15 minutes, or 10 NTU at any time, coagulant must be added

Disinfection Requirements for Tertiary Recycled Water

The disinfection process must meet either of the following requirements:
• A chlorine residual that provides 450 milligram-minutes per liter under a minimal contact time of 90 minutes, which corresponds to a residual of 5 mg/L or less
• Any process that achieves 5-log virus removal
Additionally, the median total coliform count in the disinfected water cannot exceed an average most probable number (MPN) of 2.2 per 100 mL, and no more than one sample per month can read an MPN of over 23 per 100 mL.

Which disinfection processes are permitted for recycled wastewater under CA Title 22?

Title 22 leaves the method of disinfection open to any process that can achieve 5-log virus removal, but suggests a chlorine disinfection method as one alternative. Another alternative is UV disinfection.

Chlorine Disinfection for Wastewater Reclamation

The chlorine definition is not rigid, as it can be met with either free or combined chlorine. Free chlorine is chlorine present as hypochlorous acid (HOCl) and hypochlorite ions (OCl-) while combined chlorine is present in the form of monochloramines, dichloramines, and trichloramines, which form in the presence of nitrogen and ammonia. Chloramines are significantly less effective at reducing virus levels than free chlorine.

UV Disinfection for Wastewater Reclamation

While not directly addressed by Title 22, ultraviolet disinfection can achieve 5-log virus removal using a UV dose of 100 mJ/cm2. This is the recommended dose according to the National Water Research Institute (NWRI) in partnership with the Water Research Foundation (WRF) for disinfection of water filtered by non-membrane processes for reuse applications.

Chlorine Disinfection vs. UV Disinfection

UV Transmittance for Recycled Water Treatment

Unlike chlorine disinfection, UV disinfection does not depend on water quality parameters such as pH and nitrogen content. UV disinfection only depends on the UV transmittance of the filtered wastewater. A minimum UV transmittance of 55% is sufficient for 5-log virus removal using the recommended UV dose of 100 mJ/cm2.

Chlorine Residual

Lastly, unlike drinking water systems, which require a chlorine residual for distribution, water treated for reuse only needs to be disinfected to remove enteric viruses present in the wastewater. Thus, UV disinfection can be a standalone disinfection process for recycled wastewater without any addition of chlorine.

Sensors for Recycled Water Treatment

Sensorex offers sensors, transmitters, and online process meters to help control treatment processes in water recycling plants. Chlorine disinfection processes may need to monitor pH or free chlorine, while UV disinfection processes will benefit from UV Transmittance monitoring.

Online Process pH Sensors

inline pH probe

UV Transmittance Sensors

submersible sensor for UVT testing

Free Chlorine Sensors

Accurate Free Chlorine Sensor