Maximizing Site Production at New Membrane Plant
The growing scarcity and cost of high-quality water supplies in California have put pressure on water purveyors ...
by Vincent Roquebert, Dan Hugaboom Robert Avera and John Dotinga
The growing scarcity and cost of high-quality water supplies in California have put pressure on water purveyors, engineers and treatment process equipment suppliers to design highly efficient water treatment systems. The Eastern Municipal Water District (EMWD), Carollo Engineers, and GE-Zenon responded to this pressure in the design of the 12 mgd Hemet Water Filtration Plant (HWFP).
The plant treatment process includes coagulation, one-stage flocculation, ultrafiltration (UF), chlorine disinfection, and chloramine residual. The plant is the first 98% recovery membrane filtration plant to receive the maximum 4-log disinfection credit for cryptosporidium from the California Department of Public Health (CDPH). The facility was designed to achieve these high numbers without the use of a backwash wastewater recovery system in order to minimize the building footprint on the five-acre project site and control the initial project cost.
Figure 1. Small increase in recovery sharply raises VCF and risk of pathogen breakthrough associated with a membrane breach
The three main features that were developed and implemented to optimize process efficiency, maximize site capacity, and enhance system availability include minimizing waste wash water, re-rating the plant to 12 mgd, and minimizing reliance on space intensive equalization tanks.
Minimizing Waste Wash Water
Previously approved California membrane treatment systems had recoveries of 95% or less. For the initial Hemet plant capacity of 10 mgd, the increase in recovery from 95% to 98% reduced wastewater volume by 360 acre-foot (117 million gallons) annually.
However, the innovative process raised a new regulatory challenge. Increasing the recovery of the membrane system increased the maximum solids concentration or Volumetric Concentration Factor (VCF) and therefore the maximum pathogen concentration from 20 to 50 in each of the six membrane tanks. Consequently, the higher recovery makes it more challenging to get the full 4-log cryptosporidium removal credit that is usually awarded to a membrane system in the state of California.
To minimize the solids concentration in the membrane feed water while maintaining 98% recovery, a specific membrane tank design was developed based upon guidelines from the USEPA Membrane Filtration Guidance Manual (MFGM). The high recovery is achieved by using a two-zone membrane tank and a periodic partial concentrate drain.
The end part of the membrane tank exhibits maximum solids concentration that is 10 times the solids concentration in the front part of the tank. In order to enhance the efficiency of the deconcentration phase, the end part (i.e., zone 2) of each tank can be isolated and drained separately while the low solids concentration feed water in the front part (i.e., zone 1) is saved for further filtration.
A six-month pilot test and a full-scale sampling campaign were performed respectively during construction and start-up to obtain CDPH approval and confirmed the solids concentration in the membrane tank over time. Pilot testing protocol and sampling protocol were based on the MFGM. The two and a half years of operation confirmed the sustainability of the approach by allowing EMWD to extend the chemical cleaning interval from one month up to three months.
Re-rating to 12 mgd
Because the facility was designed for an ultimate capacity of 40 mgd on a small site, some of the infrastructure and equipment built into the initial phase of the project have capacity in excess of the rated 10 mgd. Once the membrane system had been demonstrated to reliably produce its design capacity, efforts began to re-rate the plant to 12 mgd.
Figure 2. VCF sampling shows that design and actual tank maximum VCF agree well.
The capacity increase evaluation study included a systemic and detailed evaluation of critical conveyance, pretreatment, membrane filtration, disinfection systems and distribution systems for their ability to support plant production of 12 mgd. Figure 3 highlights the approach developed for each treatment process unit.
The 10 mgd capacity was achieved with five of the six membrane tanks in service and hot standby capacity for the other treatment process units. The HWFP is now allowed to produce 12 mgd with the six membrane tanks in service.
Figure 3. CT basin and FWPS capacity evaluation.
Regulatory requirements for redundancy are provided by alternate water supply sources outside of the plant, including existing wells, another existing surface water treatment plant, and connections to imported finished water pipelines. Because the re-rating was achieved without increasing the membrane flux, the re-rating did not compromise the membrane warranty. Also, the re-rating did not create substantial downtime or require additional capital investment.
Minimizing Reliance On Equalization Tanks
Membrane processes are characterized by periods of production at flows higher than the rated capacity followed by short periods of downtime for backwashing and cleaning. In many cases, flow equalization either upstream as pretreatment or downstream as on-site storage capacity is used to minimize variability in flows from finished water pumps. Site constraints did not allow this approach at Hemet. Instead, a combination of the use of a raw water sleeve valve and careful tuning of production and cleaning cycle sequencing was used to minimize flow variations from the six membrane tanks.
Figure 4 depicts the disinfection basin and high service pump arrangement. Figures 5 and 6 illustrate the difference in performance prior to and following tuning of the membrane system sequencing.
The 280,000-gallon CT basin includes an inlet and outlet control structure insuring adequate water volume to meet disinfection goals at 40 mgd build out capacity. The overflow from the disinfection zone of the tank feeds a forebay from which the high service (distribution system) pumps are fed. The limited volume of water above the inlet weir and in the forebay provides the equalization volume available to accommodate periods of downtime for all the membrane trains.
Initial testing at 12 mgd demonstrated that when multiple trains were offline for cleaning operations, water level in the forebay dropped below critical levels, initiating an automated turndown sequence provided to protect the pumps from running dry. This prevented the plant from meeting the goal of 12 mgd of production.
Figure 5 shows the variation in filtered water total flow from membrane tank cleaning operations, and corresponding decreases in high service pump flows. As shown in Figure 6, modified sequencing designed to stagger non-production cleaning intervals of the membrane trains results in more stable flow allowing the pumps to operate at a stable rate of 12 mgd.
The Hemet Water Filtration Plant project demonstrates that a partnering approach between owner, engineer and supplier allows the owner to take full advantage of a membrane filtration process without negatively impacting the interests of the supplier. This is a win-win situation that could certainly apply to many future membrane projects. —m
About the Authors:
Vincent Roquebert is a project manager with Carollo Engineers. His 22-year experience in water and wastewater is highly focused on membrane filtration. Dan Hugaboom is a senior project engineer with Carollo Engineers. He has been working as a membrane technologist for the last 8 years. Robert Avera is a senior civil engineer with Eastern Municipal Water District. He has been involved in the design and the engineering support for the HWFP. John Dotinga is the water production manager with Eastern Municipal Water District. He is familiar with the operation of membrane filtration plants and membrane desalination plants.