Membrane Treatment of Complex Well Water: A Case Study
By Gary Battenberg, Nancy Prosser and Stephen Wiman, PhD
For much of the Southwest, water is in very short supply and conservation is a top priority for state governments, municipalities and commercial and residential users. Of equal concern are the quality and aesthetics of the water. Common contaminants include extreme hardness, silica, total dissolved solids, iron, manganese, arsenic and uranium. For private well customers in northern New Mexico, water treatment specialists are faced with the daunting task of converting poor-quality water of limited quantity to high-quality water with minimal waste. One local company has been successfully meeting these challenge since 1988 and the case study presented here details the most difficult project in its 25-year history.
A private well near Abiquiú, NM
The Abiquiú area attracts many new residents because of its stunning physical beauty and the fact that it is commonly associated with a world-famous artist and described as Georgia O’Keefe Country. It is also home to some very challenging groundwater chemistry and, as a result, Abiquiú constitutes a large customer base for water treatment. Travel times to this general area range from 1.5 to 2.5 hours, as many residences require significant backroad travel (see Figure 1).
This project began when a local water treatment company received a call from a preeminent custom home builder in late November of 2009. The project superintendent relayed complaints from the stone masons that their work on the project had come to a halt because something strange in the water was preventing the mortar from binding to the stone work. A water sample was collected and a comprehensive water analysis was ordered as the first requisite step in ascertaining the root cause of the problem. It turned out that the mortar problem was only a small part of a very big problem that would quickly reveal the severity of the water quality and cause collateral damage that would impact service plumbing, faucets, fixtures, appliances and water heaters without chemistry-based water treatment. And then there was the issue of drinking water to be addressed.
The water treatment company has a policy of recommending that treatment issues should be addressed in the earliest possible stages of a building project; but, this one was at a rather advanced stage with limited time to install a (less than ideal) solution before the move-in date for this part-time occupancy, single-family residence. Water usage requirements were actually rather modest (150-600 gpd [567.8-2,271.2 L/d], depending on guest-house occupancy), but the water chemistry exceeded anything the water treatment specialists had seen, even in this area known for its complex water. A review of the raw water chemistry would give pause to most seasoned veterans of the water treatment industry (see Table 1). For those who deal with this kind of water as a matter of course, this case study offers insight on how the company handled some issues others may be facing.
Table 1 illustrates in red the constituents that were of major concern in the well. Other constituents were below US EPA’s maximum contaminant level (MCL) or were of minor concern.
The well and cistern
After the well had been drilled, the cistern installed and the home reached an advanced stage of construction, the water treatment company was contacted. The first question was whether or not the well had been production-tested to determine the pump rate and to detect any sediment issues. The driller remarked that the well would pump at the rate of 8-10 gpm (30.28-37.8 L/m), but producing the well at that rate had some serious consequences with regard to sediment incursion. No pump testing had been conducted (this is not uncommon) and that was one more reason why water treatment issues should have been considered at the earliest possible stage of property development.
After reviewing the comprehensive lab tests, suspicion of the need for a complex, multi-stage treatment system was confirmed. The next step was the site visit. The company’s senior water treatment specialist visited the construction site and met with the project superintendent for the purpose of assessing the installation site and the space available for the water treatment system that would likely be recommended. The company also needed to know all the infrastructure details so that it could properly estimate installation costs. The specialist collected the physical dimensions of the proposed footprint, then sketched a layout of the existing garage and affected walls, electrical conduits and water service locations. The area between the garage and the private shop area presented a footprint of 66 x 81 x 100 inches high (1.67 x 2.05 x 2.54 meters), which was designated for the equipment gallery. Space engineering, which the company commonly practices, would be required in this compact space, with respect to what needed to be installed. The intent was to both confine the entire treatment array within the limits of this space and ensure serviceability as well, with sufficient ingress and egress access as stipulated by the National Electric Code (NEC) for the main electrical service panel, also located in this space.
In addition to the equipment array, a suitable drain system to carry wastewater away had to be considered in the scope of this project. Given the topography and in keeping with the indigenous plant life, an infiltration system was the logical choice. An infiltrator (commonly used in this area because it is installed below grade) would not interfere with the natural charm of the environment. The total volume of wastewater to the infiltrator for the initial 550-gallon-tank fill (2,081.87 liters) was calculated to be approximately 357 gallons (1,351.39 liters). Thereafter, the average waste volume was calculated to be 124 gallons (469.39 liters) per harvest of 206 gallons (779.79 liters). The proposed location of the infiltrator and trunk line from the equipment gallery was discussed with the superintendent to identify the best location for the infiltrator relative to the well, the building pads on the property and the landscaping plan.
A very detailed proposal and cover letter were submitted to the owner outlining installation of the pretreatment, the RO system, posttreatment, storage, repressurization and posttreatment layout as follows.
Installation consisted of interfacing a weather-rated ozone generator next to the raw-water cistern with a diffuser mounted internally in order to maintain bacteriostasis in the cistern and to oxidize iron and manganese prior to entering the sediment/ conversion backwash filters. The inlet plumbing at the point of entry transitioned to corrosion-resistant, rigid Schedule 80 PVC piping into parallel triplex granular activated carbon filters, each prepared with a cap of Filter-Ag to intercept particulates that could migrate from the raw-water cistern; carbon converted any residual ozone to oxygen prior to entering the RO safety filter. This triplex filter array was fitted with a clear-water backwash manifold that provided treated water from the on-line twins to flush off the intercepted sediment filter cake and prevent cross contamination of the media beds (see Figure 2). The three filters were designed to backwash sequentially to maintain equal flow through the filters at all times.
The low turbidity water was then injected with an NSF-certified, food-grade antiscalant to control membrane scaling from silica, calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, metal silicates and colloids. Water then passed through a five-micron safety filter on the RO unit to protect the membranes. The RO system was controlled by a twin-float level tree that initiated the RO to make up the storage tank when the water level dropped to a predetermined level and the float tree shut down the RO when the storage tank was full. A low tank level switch/alarm disabled the repressurization pump should the storage tank water level become too low for the repressurization pump to safely pump water. The permeate water from the RO passed through dual parallel upflow neutralizer vessels prior to entering the storage tank (see Figure 3). The purpose of the neutralizers was to reduce free carbon dioxide (CO2) to a non-aggressive level and eliminate the corrosive attack currently experienced with the untreated well water. The neutralized water was then repressurized using a transfer pump, passed through a UV sterilizer, then finally through activated carbon filters to remove any taste and odor from the storage tank and piping prior to delivery to the residence and guest houses.
A return line from the UV outlet provided continuous recirculation of water in the storage tank at 2.5 gallons (9.46 liters) per hour (0.04 gpm) or 64 gallons (242.26 liters) per day (see Figure 4). An exclusive feature from the company, it ensured no heat build-up inside the UV system and bacteriologically safe water in the storage tank at all times. This did not increase the overall pumping cycles during normal daily activities. Additionally, because the system was in stand-by mode during prolonged absence from the property, the system remained in a working configuration, ready for immediate use, as well as keeping the water moving at all times to prevent stagnation.
A self-contained, fully automatic RO system specially designed for this challenge water, with eight 4” x 40” brackish water membranes mounted on a powder-coated frame, was installed. The projected recovery rate for the system on the raw well water chemistry, as originally tested, was 65 percent. The RO system featured a microprocessor controller, a backlit LED screen that displayed product water quality (TDS), system status text for system on, tank full, run-time hours, feed-water temperature, low-feed pressure and high-pump pressure transducers for realtime monitoring and a time delay on system start-up to prevent pump damage. A high-pressure, stainless steel, multi-stage pump was specified to maintain optimum applied system pressure and crossflow for the membranes. The system included four glycerinfilled inlet and system operating pressure gauges. Stainless steel pressure regulating valves maintained consistent, reliable operating pressure. Automatic membrane flush at system shutdown ensured long membrane life. The RO system was designed to produce five gallons (18.92 liters) per minute (300 gallons/ 1,135.62 liters per hour) with a waste stream of 3.69 gallons (13.96 liters) per minute (234 gallons/885.78 liters per hour).
Listed below is a description and purpose of the various components; Figure 5 denotes the system schematic of the initial system installation:
- Ozone treatment diffuses gas-phase ozone into the water of the cistern to oxidize iron and manganese and to maintain bacteriostasis of the well water.
- Triplex sediment/conversion filters collect sediment and convert residual ozone to oxygen to protect the RO membranes.
- Antiscalant injection controls membrane scaling from silica, calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, metal silicates and colloids.
- RO system separates water at the molecular level into two streams, flushing high-solids water to drain and diverting low-solids water for domestic household use.
- Dual parallel remineralizing tanks with pH control. Specially prepared pH correction system to neutralize low pH water to a non-aggressive state of equilibrium.
- 550-gallon RO water storage tank contains level controls to start and stop RO based on preset water levels.
- Repressurization pump/pressure tank repressurizes RO water for delivery to residences.
- UV sterilizer maintains continuous disinfection of RO water both in the storage tank and in the service plumbing of residences.
- Postfiltration removes any taste and odor from the storage tank and piping system. Installation commenced on May 24 and was completed on May 27, 2011. The system was started up and commissioned, and finished water samples were collected for post-analysis testing and reporting.
Test results—treated water
This system was very effective in removing contaminants (see Table 2); but it was limited by the cistern configuration and the fact that water was being pumped for construction purposes. The unmonitored use of the well resulted in heavy sediment accumulation in the cistern and in suspension when water was being conveyed to the treatment array.
Two months after the initial installation, the water treatment company received a call that the system was not running due to a lowpressure event and a water treatment specialist was dispatched to assess the cause of the problem. Inspection of the pretreatment array indicated heavy accumulation of clay in the triplex filter array. The safety prefilter was removed and inspected for loading and was found to be coated with a thick layer of heavy clay. Further inspection of the well cistern revealed that the well was pumping extremely turbid water and this water had completely migrated through the pretreatment to the RO membranes. This was evidenced by the presence of clay and sediment in the flowmeters (see Figure 6). The bottle on the left is the (settled) raw water feeding the treatment system from the cistern at that time.
A report was generated for the property owner and a recommendation made to reconfigure the pretreatment. It was also recommended that the well driller service the well to prevent heavy clay and sediment from being pumped. The homeowner was presented with the reconfiguration indicated in Figure 7.
Because the sediment had been such a limiting issue and the cistern had not been reconfigured, the company added additional components to the treatment array. The function of these added components is described below:
- Triplex micro-sand filters collect sediment as fine as five microns.
- Carbon conversion filter removes residual ozone and converts it to oxygen.
- Dual-gradient particulate filter. The larger diameter of the prefilter reduces the particle load to the postfilter, allowing higher flowrates and particulate reduction.
Decision point: drill another well or reconfigure the existing well?
After it became apparent that maintenance requirements of the original system (as configured) would be very high due to silt and sand incursion, the property owner solicited the water treatment company’s advice as to whether or not he should drill a new well. The existing well had been drilled to a total depth of 800 feet (243.84 meters) below ground surface and completed in the Upper Triassic Chinle Formation. An earlier well had collapsed after a brief period of production and it was replaced by the current well. Perforations in the six-inch (15.24- cm) steel casing were located from 680-780 feet (207.26-237.74 meters) and the 1.5 hp pump was set at 520 feet (158.49 meters) on a 1.25-inch (3.175-cm) PVC drop pipe. Even beyond the company’s ‘every well is a wildcat’ philosophy, to drill a new well was a particularly problematic and expensive decision due to the great depth to water, the hard formations to be penetrated and the lack of surrounding well control. The original driller had to call in a bigger rig as he had reached the limits of his drilling capabilities. The homeowner was confident of the presence of water because it was known locally that a deeper oil and gas exploratory well on the property had to be subsequently re-cemented after abandonment because of water leaking on the surface.
Generally the company would advise the client to thoroughly evaluate an existing well before considering a replacement well. The owner agreed with that recommendation to conduct an extended pump test of the well. The water treatment specialists contacted a local hydrologist who had done some water-rights work for the owner and was very accomplished in drawdown testing.
Constant rate discharge (pump test) of the well
In April 2012, nearly a year after the original system installation, the hydrologist conducted a 500-minute pumping test of the well, witnessed by staff of the water treatment company. The white and foaming water produced during the test resembled soapsuds (see Figure 8). There was no suitable explanation as to why the well yielded CO2, but was probably due to faulting associated with the Rio Grande rift and connections with even deeper strata. The well was at a much greater depth than most area wells and it was completed in much older and less well-known, waterbearing strata.
After the pump test was concluded, the hydrologist recommended that the well only be pumped at a constant rate of 2.5 gallons (9.46 liters) per minute, not to exceed five hours daily to alleviate silt and sand production and also to avoid drawing the water level down near to the pump, thereby causing stress on the pump and aquifer. The well was then tested at that rate and duration and at the end of the pump test interval no sediment production was observed.
Proposal to reconfigure the cistern and supplement the treatment array
The water treatment specialists met again with the hydrologist to review his conclusions about the capabilities of the well. Internally, they had discussed how to install a redundant system incorporating much of the same equipment, yet making additional provisions for preventing or minimizing damage due to sediment incursion. Now they had test-based verification of the production limitations of the well. They knew the cistern had been installed below ground by digging into a sandstone outcrop and completely abandoning and removing the cistern did not seem to be a desirable or practical option. It was determined that the cistern and pump assembly could be rededicated for storage of lesser-quality water, either trucked in or pumped to the cistern from a nearby river (which proved to be cost prohibitive) to irrigate landscaping on the property. This solution would help satisfy the goal of maximizing sunk costs in the cistern and also reduce the cost of irrigation. The treated water was much too precious for landscaping. (Figure 9 is a schematic of the current water treatment and sediment handling system.) The water treatment company recommended the current cistern be abandoned as the primary water source to the water treatment gallery and that the raw water line from the well be reconfigured to feed directly to the equipment gallery, where the first phase of the 14-stage treatment process began. It was recommended this rework be done by the plumbing or contracting company that set the tank and/or subsurface piping in the first place, since they would be familiar with the materials and location of the piping. After this work had been completed, the company would then commence reconfiguration of the water treatment gallery (as noted in Figure 9).
The components added to the existing system are listed and described below. Certainly the biggest modification involved taking over the garage space to accommodate the settling and transfer system designed to handle silt and sand in the feed water. See Figure 10 for the operational details of this system of cone-bottom tanks.
- 500-gallon cone-bottom settling tank provides presedimentation and clarification of raw water storage with ozone oxidation.
- 1,050-gallon cone-bottom settling and transfer tank provides additional sedimentation to further clarify rawwater feedstock to RO system.
- Ozone feed. An ozone generator pumps gas-phase ozone into both cone-bottom tanks to maintain bacteriostasis of raw-water storage. Ozonation was moved from the cistern to the main treatment array.
- Transfer pump repressurizes water to pretreatment and provides motive flow and pressure to RO system.
- Dual-parallel nano prefilters intercept and trap iron, manganese and sulfur precipitates from the raw water and also convert ozone to oxygen to protect RO membrane assemblies.
- Turbidity monitor constantly samples raw-water feed between filtration arrays and provides alarm condition in the event of turbidity migration to the RO system safety filter.
Because of the uniqueness of the circumstances and the amount of money the owner spent on this project, the proposed design was submitted to an outside consulting group for evaluation to critique the system reconfiguration and to make any recommendations based on the company’s functional and technical synopsis. The information provided to this consulting group included the field report, water analysis, system reconfiguration flow chart and cone-bottom tank array to provide a full description of the project. After a thorough evaluation and clarification of questions from the consulting group, it was concluded that the design was fundamentally sound and very comprehensive relative to the potential of the well to produce sand and heavy sediment. The company also thought that sediment issues could be controlled by the pump rate and duration of production, as per the hydrologist’s report. Additionally, the volumetric water storage and supplemental capacity calculations were confirmed as being sufficient to ensure that 750-gallon (2,839.05-liter) maximum pumping every third day would not be exceeded, even with full residence and guesthouse occupancy.
The owner accepted the recommendation after reviewing the pump test data and considering the options. The company provided the general contractor with a list of the roles and responsibilities of his subcontractors, in order for the water treatment specialists to install the additional equipment required. These tasks included work by the drilling contractor (2.5 gpm/9.46 L/m flow regulator and tank level electrical float), plumber, mechanical contractor (roof penetrations for venting) control, etc. The system reconfiguration began after the infrastructure work had been completed.
Maintenance requirements and water quality
The key to longevity of the system was regular and diligent maintenance. The water treatment company’s maintenance contract stipulated that it would provide monthly visits to the property to maintain oversight of the maintenance requirements; the owner’s representative would be on-site once each week. The company provided a system profile data sheet for recording weekly inspections and all service. All inspection and maintenance service were logged into the company’s service database for automatic scheduling and notification to ensure timely compliance for both parties.
As currently configured, the system is operating without problems and is producing water of 191 ppm TDS, which is very impressive considering the rawwater TDS of 11,000 ppm. The system also removes uranium and provides drinking water to the residence and guest houses.
One of the big breakthroughs in the treatment system was the use of cartridge-tank nanofilter systems ahead of the prefilter of the RO membranes (see Figure 11). The electro-positive filtration characteristics of these filter cartridges made it possible for them to intercept colloidal sediment, which is much smaller than the 20-micron rating the product label indicated. This duplex parallel array treated 26,000 gallons (98,420.7 liters) of water over five weeks and collected a total of 54 pounds of sediment (27 pounds each). A new filter weighed approximately three pounds and the spent filters weighed 30 pounds each. This filter was selected because the challenge water is ozonated and the powdered activated carbon (PAC) was instrumental in scrubbing residual ozone from the feedstock. After the water passed through these filters, it then passed through a 4.5” x 20” carbon block to ensure that any free available ozone was converted to oxygen to protect the membrane array. The carbon block filter was virtually pristine, with only minor iron oxide from the deposits in the preexisting piping. (The nanofilters referred to in Figure 12 are rated for 20 microns but are capable of much better filtration due to the nature of how they function. The filters are actually capable of intercepting colloidal particulates because the filtration media is technically electro-positive filtration. Since most sediment is negatively charged, the extremely fine particulates are prevented from migrating through to the effluent stream because of the attraction of the inherent filter design and composition.)
Turbidity meter readings indicated turbidity of 0.080 NTU even with the loading on the cartridges. The cartridge filters continued to perform despite the loading (see Figure 13). The extreme amount of water treated by these filters was due to an irrigation line that had been damaged during the final phases of the landscaping. Instead of the 90 gallons (340.68 liters) per day that was the approved allotment for irrigation, the system was using 600 gallons (2,271.24 liters) per day. The total loading in five weeks with irrigation leakage was equivalent to three months of normal household usage with planned irrigation water consumption.
The initial installation of a water treatment solution has been modified twice. The single reason this was necessary was the sedimentprone well being connected to an improperly plumbed cistern. The cistern pump was installed too low in the tank, creating very high turbidity when the pump was activated. The problem was compounded by high output from the well at pump rates required for ongoing construction. If the well had been pump-tested at the time it was drilled, it would have been known that the well would produce silt and fine sand at rates over 2.5 gpm. The pump testing established the limits of the pump rate and the maximum duration (five hours of pumping).
This case study is a textbook example of why water treatment should be considered from the time a well is drilled. Acquiring comprehensive water chemistry and the hydraulic characteristics of a well are of paramount importance, but it is difficult to predict sediment production rates (especially in the absence of pump testing) and to anticipate all the downstream issues of equipment fouling. In this particular case, the water treatment specialists were called in to rescue a construction project nearing completion. There is no question that the company would have done things differently if involved from the onset. And the property owner would have saved a considerable amount of money by configuring the well as it is today.
About the authors
Gary Battenberg, Managing Director of Good Water Company, Inc., has 31 years of combined experience in the fields of domestic, commercial and high-purity water treatment processes. His areas of expertise include sales, service, design and manufacturing utilizing filtration, ion exchange, UV sterilization, RO/nano membrane separation, UF and ozone technologies. Battenberg can be reached via email, gary@ goodwatercompany.com.
Nancy Prosser, Business Manager, has an extensive background in economics and biology. She has been involved with water treatment since 2004. Prosser handles water testing requests, test interpretation, proposal generation and customer service. She can be reached via email, email@example.com. S Stephen Wiman, PhD, Owner of Good Water Company, has a background in earth science (geology). He is a member of the City of Santa Fe’s Water Conservation Committee and writes a monthly column, Our Water Quality, for the Santa Fe New Mexican HOME Real Guide. Wiman can be reached via email, firstname.lastname@example.org.
About the company
Good Water Company has been the leader in designing, installing and servicing residential, commercial and industrial water treatment systems in Santa Fe and throughout northern New Mexico since 1988. The locally owned company offers superior products and unparalleled expertise as it strives to base its efforts on integrity, scientific principles and service after the sale. A local real estate broker commented that “Good Water Company makes it possible to live where we want to live.” Its philosophy is to ‘evaluate the water quality before you buy the property.’ The company takes the approach that every well is a wildcat and each well will have unique water chemistry as a function of its depth and the geology of the aquifer(s) encountered. It thrives on solving challenging well water problems. The company’s prerequisite in specifying equipment to treat well water is that the well owner commission a comprehensive water test from an independent, third-party lab which uses US EPA-approved testing methods and is not older than 12 months. “We resolve difficult water problems immediately, but the impossible takes just a little longer.”