Detail on Concentrate Handling and Disposal Options A number of options are available for disposing of concentrate including direct disposal as well as additional handling and/or treatment designed to reduce concentrate volume prior to disposal. The following sections address what are identified as the more traditional methods for handling and disposal which include: 1) surface water discharge, 2) publicly owned treatment works (POTW) treatment, 3) deep well injection, 4) evaporation, and 5) groundwater discharge. Emerging methods of handling that involve concentrate reduction and recovery (i.e., zero liquid discharge) are discussed in the main body of the report. Each option is presented in terms of the technical aspects involved along with the regulations governing the permitting process (all regulations referenced herein are from 5 CCR 1002). Information is also offered on the applicability of each method for specific types, sizes, and/or geographic locations of treatment facilities in Colorado. For additional discussion of the relative merits of alternative concentrate disposal options, please refer to Appendix E for a reprint of the article, Current Perspectives on Residuals Management for Desalting Membranes published in the December, 2004 issue of the Journal of the American Water Works Association. Surface Water Discharge Technical Description. This option consists of direct disposal to surface water. This can include disposal to lakes, reservoirs, or rivers. Regulatory Considerations. For a surface water discharge, a permit is issued in accordance with the requirements defined in Regulations 31 and 61. This includes a calculation of the available assimilative capacity (Regulations 31.14 and 61.8) at low flow conditions (Regulation 31.9) to determine the water quality based effluent limitations (WQBELs). The WQBELs that are determined based upon the site-specific water quality standards and available assimilative capacity may become more restrictive based upon the application of the Mixing Zone Regulations (Regulation 31.10 and Mixing Zone Guidance document). The Mixing Zone Regulations may limit the amount of dilution that can be used in the assimilative capacity calculations. This is based upon site-specific physical characteristics of the receiving stream. Also, the presence of threatened and endangered species may further limit the amount of dilution available by prescribing end of pipe limitations or a change to how the discharge. If the discharge is to a stream that is classified as undesignated or outstanding waters, the antidegradation regulations will apply (Regulation 31.8 and the Antidegradation Guidance document). This regulation limits the discharge to a fifteen percent increase of the available assimilative capacity between the baseline water quality as of September 30, 2000 and the water quality standard, or to a non-impact limit, which is based upon an existing limitation for that particular discharge. H-1
A surface water discharge may also be subject to the other regulations and policy documents listed above such as any existing control regulations (Regulations 71-75), salinity regulations (Regulations 39 and 61.8((2)(l)), as well as existing total maximum daily loads (TMDLs) and whole effluent toxicity (WET) testing. The salinity regulation applies to discharges to the Colorado River basin and limits the amount of total dissolved solids (TDS) that can be discharged to 500 mg/l or 1 ton per day. Limitations for radionuclides, organic chemicals, and TDS may also be included in accordance with narrative standards for water quality (Regulation 31.11), in order to protect the designated uses of a receiving stream. WET testing may also be a limiting factor on the discharge of a concentrate. TDS can be the cause of failed WET tests at concentrations as low as 700 mg/l, depending on the specific chemistry of the discharge. Other pollutants that have been concentrated into the discharge may also individually cause failure of WET testing. A surface water discharge may also include a discharge to an irrigation ditch. If the ditch does not return to another water of the state (river, creek, lake or reservoir), then the only applicable regulatory requirements are the water quality standards pertaining to agricultural use in Regulation 31, and the technology based standards in Regulation 62. If the ditch flows to another water of the state, then limitations would be considered as described in the paragraphs above, for the protection of the regulated receiving water. Site-Specific Applicability. Typically, the driving force behind the use of membrane technology is that a pollutant(s) is present in high concentrations in the source water, often times at or above a water quality standard, or in the case of TDS, above the secondary drinking water standard (500 mg/l). After treatment, the concentration of this pollutant(s) can be two to ten times greater, depending upon the specific treatment system. For the discharge of this concentrate, having available assimilative capacity is often necessary. In more remote areas where there are less dischargers, this assimilative capacity may be available. However, in systems with numerous dischargers and/or receiving streams with small low flows, the assimilative capacity may already be fully allocated. At this time it has been identified that there is no further capacity for the South Platte within and downstream of the Denver metro area to assimilate additional loads of nitrate and specific metals from reverse osmosis (RO) brine. Issues that affect this include low flow and current loading from point sources. Wastewater POTW Technical Description. This option consists of disposal to the sewer. There are two advantages to this approach. First, the concentrate can be blended with the sewer flow, reducing the concentration of TDS and other contaminants. Second, a National Pollutant Discharge Elimination System (NPDES) permit is not required for the membrane system, since the ultimate discharge is permitted by the POTW. Regulatory Considerations. This option for discharge is subject to pretreatment regulations (Regulation 63) and any additional requirements dictated by the facility accepting the waste. This also requires a POTW to have a pretreatment program in place, and be willing to take on the concentrated discharge with considerations of its own permit, the hydraulic capacities of the facility, and the potential effect of the concentrate on its treatment system. H-2
Any additional permitting requirements from the State regulations for a POTW would be incorporated in the POTW s discharge permit. The facility discharging to the POTW would have limitations and monitoring requirements set by the POTW. Site-Specific Applicability. Unless the wastewater flow is large relative to the concentrate flow, little dilution of the concentrate will occur and discharge standards for the receiving stream may not be met. In addition, high TDS levels in the concentrate may be detrimental to the performance of the wastewater treatment plant. Close coordination between the membrane plant and the wastewater treatment plant would be required. Issues that need to be evaluated are distance to the waste water treatment plant (WWTP) from the RO treatment facility, does the WWTP accept commercial RO discharge, does the WWTP have capacity for dilution, and will the discharge still meet the NPDES permit limits. There are several WWTPs that discharge to segment 14 and 15 of the South Platte with the Metro facility being the largest. At present, Metro does not accept RO concentrate. Deep Well Injection Technical Description. This option consists of disposal by injecting the concentrate into porous subsurface rock formations. The applicability of deep well injection is limited by geologic and hydrogeologic conditions, and may not be available at a location convenient to the discharger. To be an effective option, the receiving aquifer must have sufficient capacity to accept the volume of concentrate produced over the life of the plant and must be hydraulic isolated from other aquifers to prevent contamination of potential drinking water sources. Regulatory. Deep well injection is subject to the Underground Injection Control (UIC) program which was established under the Safe Drinking Water Act to protect current and future underground sources of drinking water. The Environmental Protection Agency (EPA) Region VIII is the permitting authority for such a discharge. Underground injection is grouped into five classes. A Class I permit, for Industrial and Municipal Wells That Inject Beneath the Lowermost Underground Source of Drinking Water (USDW), is the most strictly regulated and is defined as the placement of hazardous and nonhazardous fluids (industrial and municipal) beneath the lowermost underground drinking water source. This class is also subject to the Resource, Conservation and Recovery Act. Class II is for the discharge of brine and other fluids associated with oil and gas facilities. Class III is for the discharge of solution mining wastes. Class IV addresses injection of hazardous or radioactive wastes into or above a USDW and is banned unless authorized under other Statutes for ground water remediation. Class V includes all underground injection not included in Classes I-IV. A Class 1 permit is required for concentrate disposal. Site-Specific Applicability. There are approximately 500 Class 1 permits nationwide, with the bulk of the permits in Florida and Texas where the subsurface geology is favorable for this type of disposal. In 1999 there were six Class 1 well permits in Colorado. One Class 1 well is operating in Northeast Colorado in Weld County. At this time there are no wells within Colorado accepting RO concentrate. H-3
Conversations with the Region VIII UIC office indicated the volume of concentrate that would be injected from a large scale RO plant would be unprecedented in the region and that substantial geological investigations would be required. It was pointed out that in the 1980 s a deep well injection program operating at the Rocky Mountain Arsenal was halted due to concern over fracturing of the aquifer and the triggering of small earthquakes. Additional investigation regarding this option is required, but it appears unlikely this option would be permitted for the disposal of large volumes of concentrate, at least for Segments 14 and 15 of the South Platte. Issues to be evaluated include distance to the well, capacity of the well, and permitting issues with the EPA. Evaporation Technical Description. This option consists of pumping concentrate into man made shallow ponds where it evaporates, leaving the salt to accumulate in the pond. The pond can be designed for periodic removal of the salt or such that salt continues to accumulate over the life of the pond. The pond must have an impervious liner which reliability isolates the contents of the pond from surface or groundwater. Regulatory Considerations. State regulations require that evaporation ponds be lined or otherwise constructed to meet the 1 x 10-6 centimeter per second infiltration rate to not be considered a discharge to groundwater. Site-Specific Applications. For this option to be feasible, a large area of low cost flat land must be available. The climate must be warm and dry with a high rate of evaporation. Spraying the brine in the air within the pond can increase evaporation rate. However, even in desert areas, this option is only feasible for small volumes of concentrate. For larger systems, the required area of the pond becomes excessive, on the order of several square miles. An evaporation rate on the order of 50 inches per year is typical for northeast Colorado. At this rate, approximately 1 or more acres of evaporation pond will be required for every 2 to 3 gallon per minute (gpm) of concentrate to be evaporated. For a large scale RO plant the concentrate flow would be several million gallons per day (mgd) and evaporation ponds on the scale of square miles would be required. Issues that should be evaluated include the cost of the land and public perception. Groundwater Discharge Technical Description. Groundwater discharges may be accomplished by allowing the infiltration of wastewater through a single or multiple pond systems, or by the application of wastewater to the land. The overall size of the system would depend upon site-specific hydrogeology, infiltration rates and soil types. Regulatory. Groundwater discharges may be permitted in accordance with Regulations 41, 42 and 61. A permit to discharge to groundwater would either have to meet limitations based on a site-specific groundwater designation in Regulation 42, or the more stringent of the limitations found in Tables 1-4 of Regulation 41. Compliance with the applicable groundwater standards can be determined either at the end of pipe, or via monitoring wells. TDS limitations for groundwater discharges are typically based upon ambient water quality as shown in Table 4 of Regulation 41. However, discharge to groundwater that is hydrologically connected to a surface water stream may be subject to H-4
the surface water regulations, including the Colorado Salinity Regulation (Regulation 39) if the discharge is located on the West slope. Site-Specific Applicability. Depending upon the size of the treatment system, and the amount of concentrate to be discharged, this option may be limited to smaller systems with a lesser amount of concentrate to discharge. A larger system would need a large land area for infiltration ponds or for land application type discharges. Zero Liquid Discharge (ZLD) Technical Description. This option refers a class of developing technologies that process the concentrate stream to the point where there is no liquid discharge. Because mechanical evaporation produces distilled water, most or the entire concentrate stream recovered from the ZLD process is pure enough to be blended with the effluent of the membrane treatment plant for consumption. This substantially reduces water loss from the overall system. Approaches for zero liquid discharge include the use of evaporators, brine concentrators and crystallizers to completely separate dissolved salts from the water. All of the approaches are relatively complex and energy intensive. ZLD approaches have been successfully implemented for industrial water treatment. However, the technology is immature when applied to drinking water treatment and it has not been implemented at a large scale. Hence, it is a costly and technically risky disposal approach for a large scale RO system. Regulatory Considerations. This option would not require a discharge permit as there would be no discharge of pollutants to state waters. However, solids would need to be disposed of in accordance with solid waste regulations. Evaporation Ponds Evaporation ponds can further concentrating brine from 3 grams per liter (g/l) to approximately 150 g/l (Figure AH.1). Residuals from the evaporation pond can either be taken to a monofill or further dewatered via a mechanical vaporization. Figure AH.1. Typical Evaporation Pond. H-5
The merits of evaporation ponds are discussed earlier. In general, this method of brine reduction may be feasible for small volumes of concentrate, but restricted by land availability and cost for larger scale operations. Evaporators The basis of operation for evaporators involves a heat transport phenomena where heat is transferred from condensing steam to brine. As the brine is heated it vaporizes and becomes more concentrated. The vapor formed by the process is collected and cooled, producing liquid water that is very low in TDS. The remaining heat in the steam is removed by a condenser and dissipated to the environment via a cooling tower. This is called a single effect evaporator. Alternatively, if sufficient energy is contained in the vapor it can be used to re-heat the steam and repeat the evaporation process. This is called a multiple effect evaporator. While more complex and capital intensive, a multiple effect evaporator operates at greater thermal efficiency than a single effect evaporator. Brine Concentrators Brine concentrators operate in a similar fashion to evaporators except that the vapor released from the brine is compressed. In turn, the vapor is recycled and heated with the steam and used to evaporate even more brine. Brine concentrators can reduce the amount energy required to evaporate a pound of liquid water by a factor of 10 over ambient atmospheric conditions. Both evaporators and brine concentrators are capable of treating several hundred gallons per minute of brine. Figure AH.2 illustrates a brine concentrator designed by Ionics. Figure AH.2. Brine Concentrator. Crystallizers Crystallizers can effectively treat small flows in the 1 to 50 gpm range. Similar to brine concentrators, the vapor from the brine is compressed and used to provide energy to evaporate brine, forming crystals that are extracted from the stream. In a RO ZLD process, H-6
crystallizers are most likely to be used as the last step in the brine concentration process. Figure AH.3 is a diagram of a crystallizer designed by Ionics. Figure AH.3. Brine Crystallizer. Sustainable Brine Management Geo-Processors have patented a unique process that takes RO concentrate and produces beneficial byproducts. A conceptual study was performed assuming South Platte River water treated by conventional RO with 90% recovery. Study results indicated that such a project was technically and financially feasible. The financial feasibility was determined using a benefit to cost ratio analyses. The benefits were the revenues generated from byproducts and costs were the amortized capital costs plus the annual operating costs. A ratio of 1.0 meant the project would be considered break-even, a ratio greater than 1.0 meant the project would generate money, and a ratio less than 1.0 meant the project would loose money. The beneficial by-products obtained from treating South Plate River water would be precipitated calcium carbonate (PCC) and magnesium hydroxide. PCC is used for brightening paper and magnesium hydroxide is used for light weight concrete. A chemical manufacturing company presently producing PCC in Denver was contacted and interest was expressed in purchasing the chemical grade product. H-7
Figure AH.4. SAL TECH Process Flow Diagram. H-8