July 2013, Vol. 25, No.7

Small Communities

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Achieving water reuse and 3 mg/L total nitrogen standards using decentralized techniques

Pio Lombardo

The city of Malibu, Calif., was established in 1991, largely as a method to stop Los Angeles County’s proposed large-scale sewering, which had the capacity to serve 400,000 people. With a 2010 population of 12,645, Malibu covers 34 km (21 mi) of prime Pacific coastlines and world-famous beaches. The city also has focused on properties relying on decentralized systems for wastewater management. 

The city has developed many progressive practices in this regard, including an operating permit program for onsite wastewater treatment systems. Many commercial properties within Malibu must comply with both Los Angeles Regional Water Quality Control Board (LARWQCB) orders on waste discharge requirements in addition to City of Malibu wastewater system requirements. The Malibu Village Plaza shopping plaza is one of these properties. At the time of its wastewater system design, the plaza consisted of restaurants with 375 seats and 2415 m2 (26,000 ft2) of retail businesses.  


Project background
 

The Malibu Village Plaza sits adjacent to the environmentally sensitive Malibu Lagoon and near the world-famous Surfrider Beach. Based on water quality studies, LARWQCB required the plaza to produce tertiary quality effluent prior to subsurface disposal. The table (see p. 60) presents the wastewater system effluent requirements. LARWQCB also encouraged the owner to consider upgrades that would enable the treatment system to meet water reclamation standards and thereby provide greater flexibility for disposal and/or reuse of recycled water. 

In addition to the effluent limits listed in the table, monthly analysis for 89 volatile and semivolatile organic compounds and an annual priority pollutant scan of 19 contaminants is required. Quarterly groundwater sampling is required for the constituents listed in the table, as well as 89 volatile and semivolatile compounds and 19 priority pollutants. Samples must be collected from wells both upgradient and downgradient of drain fields, which are located under the parking lot. 


Wastewater treatment system
 

Meeting the stringent standards required a wastewater system having: 

  • a septic-tank effluent-collection system, 
  • a recirculating-media filter for biochemical oxygen demand removal and nitrification, 
  • a denitrification filter, 
  • rapid-mixed-media-cartridge filters, 
  • ultraviolet and ozone disinfection, and 
  • discharge to new drain fields. 

The treatment system was designed to handle 60,560 L/d (16,000 gal/d) of high-strength wastewater — equivalent to about 573,000 L/d (151,400 gal/d) of residential-strength wastewater — and has experienced flows from 22,700 to 60,560 L/d (6000 to 16,000 gal/d) with no effect on effluent quality. The treatment system is predominantly passive, with electricity only needed for the various lift pumps and disinfection system. Operation and maintenance requirements comprise 2 to 4 hours per week, with remote monitoring and alarm notifications by pager and Internet/telephone connections. Odor management relies on positive ventilation and use of carbon canisters. Despite being adjacent to restaurants with outdoor seating and within 9 m (30 ft) of retail establishments, there have been no odor complaints. 


System performance
 

The treatment system commenced operation in July 2007 and has been continuously achieving 95.4% nitrogen removal. It has been achieving permit compliance 99% of the time, with minor pH and bacterial excursions caused by equipment malfunctions. Of particular note is the achievement of total nitrogen concentrations near 2 mg/L, which is better than the U.S. Environmental Protection Agency (EPA) limit of technology and expectation of the most sophisticated centralized facility of 3 mg/L. The Malibu system also has average achieved turbidity readings of about 0.4 NTU and never exceeded the water reuse standard of 2 NTU. Water reuse disinfection standards generally have been met — some higher values of enterococcus and fecal coliform indicate minor upsets, which would be addressed with required redundancy in a water reuse system. 

Based on the Malibu experience, it seems likely that total nitrogen concentrations less than 1 mg/L are readily achievable. Biochemical oxygen demand and total suspended solids levels are nondetectable, as the detection limit is 5 mg/L. The priority pollutants and semivolatile and volatile organic compounds have had 5785 analyses, with only eight samples (0.014%) found to exceed detection limits. These samples were metals in low microgram-per-liter concentrations. The ultraviolet–ozone disinfection system is recognized as effective for removing emerging contaminants of concern. 

The central component of the plaza wastewater treatment system is the denitrification filter, which has been independently evaluated by the EPA, Montana, Florida, and the Massachusetts Alternative Septic System Test Centers. The filter is permitted to achieve total nitrogen concentrations less than 10 mg/L in Suffolk County, N.Y., as well as in Rhode Island, Florida, New Mexico, Arizona, and Oregon. Many other states, including California, North Carolina, and Massachusetts, have approved the use of the passive denitrification system to achieve total nitrogen concentrations less than 10 mg/L on a project-by-project basis. 

The average total nitrogen removal is greater than 93% — achieving typical effluent concentrations of less than 3 and 5 mg/L in warm and cold climates, respectively. This performance compares well to enhanced nitrogen removal technologies used at sophisticated centralized water resource recovery facilities.  

Pio Lombardo is owner of Lombardo Associates Inc. (Newton, Mass.).