2009 E3 Research Grand Prize

An Innovative Approach to Disinfection for Recycled Water

Los Angeles County, California
ENTRANT:Sanitation Districts of Los Angeles County
ENGINEER IN CHARGE: Stephen R. Maguin, P.E., BCEE

The Sanitation Districts of Los Angeles County (Districts) provide wastewater treatment and solid waste management services to over 5 million people in the Los Angeles County. For wastewater treatment, the Districts operate 11 treatment facilities with a combined treatment capacity of 510 MGD. These treatment plants are indicated on this map. Other than the Joint Water Pollution Control Plant, or JWPCP, which is an ocean discharge plant, the other 10 facilities are water reclamation plants, seven of these are tertiary WRPs. These tertiary WRPs produce approximately 150 MGD of reclaimed effluent. Approximately one-third of the disinfected effluent is reused for a variety of applications including indirect potable water recharge.	Historically, chlorination is the most commonly practiced wastewater disinfection technology. Depending on the ammonia levels in the water, chlorine may be present as either free chlorine or chloramines. Free chlorine is an effective disinfectant, but it generates elevated levels of trihalomethanes (THMs) and haloacetic acids (HAAs), both of health concerns. It also generates cyanide and cyanogen chloride. Chloramines generate lower levels of THMs, HAAs, cyanide, and cyanogen chloride, but they are less effective disinfectant than free chlorine against viruses. Recently, it was found that chloramines generate nitrosamines, including n-nitrosodimethylamine (NDMA), which are chemicals with high carcinogenic potency.	The Districts staff developed a disinfection process that applies free chlorine and chloramines sequentially to fully nitrified secondary effluents produced at their tertiary water reclamation plants. This process, named sequential chlorination, is shown on the diagram. Not only is sequential chlorination an effective disinfection process, it also minimizes the formation of various byproducts typically formed from chlorination including THMs, HAAs, cyanide, and cyanogen chloride.
The Districts conducted sequential chlorination research in four phases including laboratory, pilot, and plant scale testing. Objectives and scope of each phase are summarized on this table.	Bench-scale studies were conducted in water quality and microbiology laboratories using samples collected from reclamation plants. Viruses were seeded into the water samples which were then disinfected by chlorine, chloramines, or a combination of both.	Virus inactivation was evaluated using a pilot-scale chlorine contact channel shown in the picture. Male-specific coliphage (MS2) was seeded to fully nitrified secondary effluent which was disinfected by free chlorine in the contact channel.

Entrant Profile

The Sanitation Districts of Los Angeles County (Districts) function on a regional scale consisting of 24 independent special districts. The Districts protect public health and the environment through innovative and cost-effective wastewater and solid waste management, and in doing so convert waste into resources such as reclaimed water, energy, and recycled water. The Districts' service area includes over five million people in 78 cities and unincorporated County territories.

The Districts' 1,400 miles of main truck sewers and 11 wastewater treatment plants manage approximately 480 millions gallons per day (mgd) of which 170 mgd are available for reuse in arid Southern California. Tertiary effluent from the Districts' water reclamation plants is reused at more than 530 sites throughout the county including groundwater recharge, industrial uses and irrigation of parks, schools, golf courses, nurseries and greenbelts.

Three active sanitary landfills handle approximately 18,000 tons per day (tpd) of which 15,000 tpd are disposed (approximately forty percent of the County-wide disposal capacity) and 3,000 tpd are recycled. The Districts also operate four landfill energy recovery facilities, two recycle centers, and three materials recovery/transfer facilities, and participate in the operation of two refuse-to-energy facilities. Recycling efforts at Districts' facilities recover and recycle items such as green waste, tires, dirt, asphalt, large appliances and ash from refuse-to-energy facilities.

Project Description

The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs) in Los Angeles County, California. Treatment processes employed at these plants include primary sedimentation, activated sludge with biological nitrogen removal, secondary sedimentation, media filtration, chlorine disinfection, and dechlorination. During fiscal-year 2007-2008, the seven plants produced 153 million gallons per day (MGD) of recycled water, of which 52 MGD was reused. Groundwater replenishment represented two-thirds of the water reused.

Challenge

Recycled water must be properly disinfected before reuse. In California, disinfection requirements are specified in California Water Recycling Criteria. For groundwater replenishment, the recycled water must meet drinking water standards. The disinfection method used for recycled water must be effective for pathogen inactivation, and should minimize the potential generation of harmful disinfection byproducts (DBPs).

Historically, chlorination is the most commonly practiced wastewater disinfection technology. Depending on the ammonia nitrogen levels in water, chlorine may be present as either free chlorine or combined chlorine (chloramines). Free chlorine is an effective disinfectant, but it generates elevated levels of DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs). Chloramines generate lower levels of THMs and HAAs, but they are less effective than free chlorine with respect to virus inactivation. Recently, it was found that chloramines react with dimethylamine and form N-nitrosodimethylamine (NDMA), a chemical with high cancer risk. Dimethylamine is present in the cationic polymer used at the Districts' tertiary WRPs to enhance settling of mixed liquor and for foam control.

Although California has not established a drinking water standard for NDMA, the Districts have proactively decided to minimize the generation of NDMA at its tertiary WRPs. Because these WRPs produce fully nitrified effluent, it is possible to use either free chlorine or chloramines for disinfection. Relying solely on free chlorine (breakpoint chlorination) would not generate NDMA, but would result in total THM (TTHM) levels exceeding the drinking water standard (80 μg/L). Using chloramines with alternative polymers not containing dimethylamine was investigated, but these polymers were not as effective as a settling aid. The Districts also investigated ultraviolet irradiation, but rapid conversion to this technology at multiple WRPs would not be practical. A new disinfection method was needed that could be rapidly implemented using existing infrastructure in order to continue protecting public health while minimizing the formation of DBPs of concern. <

Innovation: The Sequential Chlorination Process

In 2006, the Districts' research staff developed a unique, two-step chlorination process to meet the above-mentioned objectives. This process, named "sequential chlorination", applies chlorine to fully nitrified secondary effluent, and then chloramines (formed in the field by adding ammonia nitrogen and additional chlorine) to the media filtered effluent. Free chlorine provides three important functions: inactivating bacteria and viruses in the fully nitrified secondary effluent; reacting with NDMA precursors, thus reducing NDMA formation; and controlling biofouling on the filter media. Chloramines added to the filtered effluent then stop the formation of additional THMs and HAAs from free chlorine residuals, and provide further disinfection of the effluent.

Process Performance

The Districts tested the sequential chlorination process in four phases. Plant-scale testing was conducted at the Long Beach WRP (20 MGD average daily flow), San Jose Creek East WRP (55 MGD), San Jose Creek West WRP (30 MGD), and Whittier Narrows WRP (8 MGD). Typically, 5 mg Cl2/L was added to the secondary effluent, and 1 mg NH3-N/L and 5 mg Cl2/L to the filtered effluent to form chloramines. Only minor modifications to the existing chlorination system were needed to implement the process.

The results indicated that sequential chlorination

provided effective disinfection against total coliform bacteria;
reduced NDMA formation by 50 to 85% in comparison to chloramination;
produced effluent consistently meeting the TTHM limit;
generated insignificant amounts of cyanide and cyanogen chloride (both DBPs);
caused no aquatic toxicity.

Virus inactivation by the sequential chlorination process was studied initially at the laboratory scale using seeded male-specific bacteriophage (MS2) and poliovirus. Laboratory experiments were conducted with fully nitrified secondary effluent samples collected from the Districts' tertiary WRPs. Results from the laboratory experiments showed at least 5- log inactivation of both MS2 and poliovirus under simulated sequential chlorination conditions.

To verify the results from the laboratory study, the Districts conducted pilot-scale testing on virus inactivation at one of the WRPs in 2008. A rectangular cross-section open channel was constructed and fed with secondary effluent. Free chlorine was added to inactivate seeded MS2. Results indicated that free chlorine alone, at 3.7 to 5.8 mg Cl2/L, generally inactivated 5 logs of MS2 at contact times less than two minutes. These results were consistent with those from the laboratory experiments.

Originality

The sequential chlorination process has never been used for wastewater disinfection other than at the Districts WRPs mentioned above. The Districts have filed a patent application for the process (US Patent and Trademark Office Publication No. US-2008- 0257832-A1).

Economic Advancement

Recycled water disinfected by the sequential chlorination process contains lower NDMA levels than effluent disinfected by chloramination making the water more amenable to reuse. As an example, the Districts' Long Beach WRP provides effluent to the Leo Vander Lans Advanced Water Treatment Facility. This facility (2.7 MGD) uses microfiltration and reverse osmosis followed by ultraviolet oxidation to remove NDMA. Effluent is blended with imported water and pumped into groundwater aquifer to form a seawater intrusion barrier. Maintaining low NDMA levels in the Long Beach WRP effluent is critical for this reuse application because ultraviolet oxidation is highly energy intensive. Energy use depends on influent NDMA concentrations.

The sequential chlorination process also has the potential to significantly reduce the size of chlorine contact tanks currently required by California regulations. Current disinfection requirements are based on using chloramines. Because free chlorine is the primary disinfectant in the sequential chlorination process and free chlorine is a more powerful disinfectant than chloramines, shorter chlorine contact time, which translates to smaller chlorine contact tanks, would be needed to achieve the same level of public health protection.

A close-up view of the pilot-scale chlorine contactor.	Full-scale testing of the sequential chlorination process was conducted at four water reclamation plants operated by the Districts. Testing included extensive sampling of disinfected effluent for analyses of various disinfection byproducts and total coliform.	This table compares NDMA results in disinfected effluents at the four water reclamation plants tested. Median NDMA concentrations are 50 to 85% lower in effluents treated with sequential chlorination compared to historical values when chloramination was practiced.
This table summarizes the total THM concentrations in chlorinated final effluent under chloramination and sequential chlorination. Total THM concentrations are higher under sequential chlorination, but the levels are well within the drinking water standard, 80 μg/L.	This table summarizes the cyanide and cyanogen chloride concentrations in effluents disinfected by sequential chlorination at the four water reclamation plants. Cyanide is usually below the laboratory reporting limit of 5 μg/L. Cyanogen chloride concentrations are occasionally above the reporting limit of 5 μg/L. Currently, there is no regulatory limit for this compound.	This chart shows log inactivation of two surrogate viruses, MS2 and poliovirus, by the laboratory simulated sequential chlorination process. For both viruses, >5-log inactivation was achieved at very low CT values.
Inactivation of total coliform by the laboratory simulated sequential chlorination process is shown on this chart. The results show that the sequential chlorination process can effectively reduce total coliform to levels below that required by California Title 22 (<2.2 per 100 mL) at CT values much less than Title 22 required 450 mg Cl2-minute/L.	Free chlorine inactivation of seeded virus was investigated using the pilot-scale chlorine contactor shown in Pictures 6 and 7. The pilot-scale testing results, shown on this picture, confirm that free chlorine alone, the first step of the sequential chlorination process, can inactivate >5-log of seeded virus at relatively short CT values.