CH2M HILL and the City of Henderson recently completed a study to evaluate pathogen removal efficiencies through a full-scale membrane bioreactor system (MBR) that produces recycled water for the City of Henderson, Nevada. The results of the study show that MBR systems are very efficient for reducing viruses and pathogenic organisms—greatly reducing the need for downstream disinfection treatment.
In order to save on pumping costs and alleviate constraints in the collection system, the City of Henderson decided to build a strategically located reclamation facility. The Southwest Water Reclamation Facility (SWRF), which opened in 2011, was built to treat wastewater produced from south and west Henderson for reuse. The facility is a membrane bioreactor (MBR) facility with a design capacity of 8.4 million gallons a day (mgd) (maximum month) and handles a peak hourly flow of up to 13.6 mgd. The plant currently utilizes both MBR treatment and medium‐pressure UV disinfection to provide disinfected tertiary reclaimed water for non-potable uses and irrigation purposes.
Membranes are an effective treatment process for removing coliform bacteria, and sampling of MBR permeate at operating reuse facilities frequently demonstrates that no coliform bacteria are present because of excellent rejection of bacteria through the membranes. Additionally, virus studies conducted at reuse facilities have also indicated substantial virus removal (0.5-log to as high as 6-log removal) depending upon virus types used, pore sizes of membranes (MF vs. UF) and characteristics of membranes utilized in MBR systems, as well as MBR design and operating conditions. Given these results, many utilities question why stringent disinfection requirements are needed, in addition to MBR, since reclaimed water pathogen limits are mainly met through MBR alone.
Energy use and the associated cost for UV disinfection may be a strong incentive for some facilities to consider decreasing the operating settings of UV reactors to save energy and operating costs without compromising regulatory compliance.
CH2M HILL helped conduct a study at the SWRF to determine virus removal capabilities of the MBR system and UV disinfection, and offered operating recommendations for MBR and UV disinfection to address the disconnect between the bacterial discharge limits for disinfection and the implicit regulatory goals for reuse disinfection at the SWRF.
The objective of the study was to evaluate pathogen removal capabilities of the MBR and UV disinfection systems under various operating conditions by:
- Understanding the synergy of MBRs and UV disinfection for pathogen reduction
- Developing operational goals to protect public health while optimizing energy and chemical use to reduce operational cost of the treatment facility
The study results showed:
- Without disinfection, the MBR was very effective for reducing virus and bacteria concentrations to very low levels(in most cases to ND levels). The UF membranes used in the SWRF MBR had a nominal pore size of 0.04 micron.
- The lowest virus rejection by the MBR was observed for MS-2 Coliphage and under clean membrane conditions. Under these conditions, the MBR permeate contained viruses and additional removal was required by the disinfection system (either UV disinfection or chlorine disinfection).
- NWRI and Title 22 recycled water regulations do not set numeric concentration limit for viruses in recycled water. Conservatively assuming that recycled water contains non–detectable virus, the additional virus removal required by disinfection was about 2-log removal. The 2-log additional virus inactivation can be satisfied with a UV dose of 40 mJ/cm2 or with a free chlorine CT of 3 mg/L*min.
Following completion of this study, the City of Henderson presented the results to the State of Nevada, with the recommendation to reduce or eliminate UV disinfection at the SWRF. The State of Nevada agreed that UV disinfection following membrane treatment was not necessary to properly treat the effluent water to reclaimed standards. By eliminating UV disinfection, the City of Henderson will achieve significant cost savings in power (an annual saving of over 1.2 million kWh is expected), equipment, and staff time.
By: Ufuk G. Erdal, PhD, PE, CH2M HILL West (US) Region Regional Technology Leader in Reuse
Larry Schimmoller, PE, CH2M HILL Global Technology Leader in Reuse
Paul Swaim, PE, Principal Technologist, CH2M HLL Deputy Water Infrastructure Leader
Julian Hoyle, PE, CH2M HILL Project Manager
This article has been republished with permission by CH2M Hill. To view the original article, visit CH2M Hill's blog here.