Chloride capture process for stormwater runoff

Abstract

A chloride capture process is described for the injection of chemicals to mix, react and precipitate Friedel's salt following Primary Treatment for 80% removal of sediment and suspended solids from stormwater and meltwater runoff. The process directs stormwater with known chloride levels >250 mg / L through a process of chemical dosing with sodium aluminate and calcium hydroxide, introduced as a molar ratio of the mass of chloride present. Friedel's salt production and precipitation begins at dosing; sufficient reaction time is considered >10 minutes, while time sufficient for settlement of Friedel's salt is an additional >10 minutes. Reaction and precipitation both occur in plug or continuous flow fashion. Process steps can occur in chambers, vessels or lined basins, sized for expected flows and settling time. Settled Friedel's salt shall be kept separate or removed from the treated supernatant. Treated supernatant following precipitation is sparged with carbon dioxide (or other acid) to reduce pH below 9 S.U. before discharging to surface water.

Description

BACKGROUND OF THE INVENTION

[0001] The problem - road salts in stormwater runoff add chloride to groundwater and surface water. Chloride in surface water is a pervasive issue in northern regions of the United States, where snowfall and cold temperatures require deicing treatments on roadways.1These deicing treatments often use different types of salts, which lower the freezing point of water and keep roads clear. When temperatures rise following deicing treatments, snowmelt and precipitation can wash road-applied salts into stormwater conveyances that ultimately discharge to groundwater, streams and lakes. Once dissolved in surface water, dissociated chloride ions (Cl') from these salts can cause several issues with aquatic organisms, primarily through interference of osmoregulation (Evans et al., 2003; Hunt et al., 2012).23

[0002] Once dissolved in water, chloride ions are difficult to remove.Flows and chloride concentrations are variable, intermittent and hard to remove. Desalination has posed serious challenges to human infrastructure for many years due to the costly nature of typical treatment processes. Modern desalination methods typically involve either evaporation, Ion exchange (IX) or reverse osmosis filtration (ROF), all of which are power-intensive, site limited, and impractical for treating stormwater.4Passive conservation practices, such as vegetated filters and settling basins designed to remove sediment and nutrients from stormwater, have little impact on lowering chlorideconcentrations in receiving waters.51Corsi, S. R., Graczyk, D. J., Geis, S. W, Booth, N. L., & Richards, K. D. (2010). A Fresh Look at Road Salt: Aquatic Toxicity and Water-Quality Impacts on Local, Regional, and National Scales. Environmental Science & Technology, 44(19), 7376-7382.2Evans, M., & Frick, C. (2001). The effects of road salts on aquatic ecosystems.3Hunt, M., Green, L., (2012). Chlorides in Fresh Water.4Ghaffour, N., Missimer, T. M., & Amy, G. L. (2013). Technical review and evaluation of the economics of water desalination: Current and future challenges for better water supply sustainability. Desalination, 309, 197-207.5Stagge, J. H., Davis, A. P., Jamil, E., & Kim, H. (2012). Performance of grass swales for improving water quality from highway runoff. Special Issue on Stormwater in Urban Areas, 46(20), 6731-6742. >1 KIESER 1 PCTOBJECTS OF THE INVENTION

[0003] With the instant invention, we have adapted Friedel’s salt precipitation. In industrial settings, chloride is typically removed from water or waste streams via ROF or IX, but recent research has shown that chloride can be precipitated from solution through Friedel’s salt formation.6Friedel’s salt has a chemical formula of Ca4Al2Cl2(OH)i2, meaning every mole of chloride theoretically requires 2 moles of calcium and 1 mole of aluminum to bind it. In practice, the minimum theoretical additive ratios do not achieve 100% chloride removal, whereas higher additive molar ratios achieve better chloride removal rates.7When used in a stormwater setting, the reaction must contend with other competing ions and low temperatures, regulatory limits mandating stormwater discharges to surface waters at a pH of 9 S.U. or less, as well as regulatory permitting of additives to discharges. Recent published research and art8suggests this process applies to the stormwater setting.BRIEF DESCRIPTION OF THE FIGURESFigure 1 - CHLORIDE CAPTURE Stormwater Treatment Schematic (black arrows show process flow direction).SUMMARY OF THE INVENTION

[0004] The present invention can be described as a stormwater treatment process, comprising:- a Primary T reatment of stormwater where eighty percent(80%) removal of sediment and total suspended solids is achieved via conventional stormwater treatment practices,6Wang, L. P., Lee, W. H., Tseng, S. M., & Cheng, T. W (2018). Removal of chloride ions from an aqueous solution containing a high chloride concentration through the chemical precipitation of Friedel's salt. Materials Transactions, 59(2), 297-302.7Fang, P., Tang, Z. J., Chen, X. B., Huang, J. H., Tang, Z. X., & Cen, C. P. (2018). Chloride ion removal from the wet flue gas desulfurization and denitrification wastewater using Friedel’s salt precipitation method. Journal of Chemistry, 2018.8Alhinaai, Marwan A., Daniel P. Cassidy, Donald M. Reeves, Stephen E. Kaczmarek, Michael Foster, John Jacobson, and Mark S. Kieser. (2025). Chloride Removal from Stormwater and Lakes Impacted by Road Salt Application: Experimental Assessment of Friedel’s Salt Reactions. Environmental Science & Technology Letters, 2025 >2 KIESER 1 PCT- a Secondary Treatment process comprising removal of Cl'in urban stormwater or meltwater associated with roadsalting activities following Primary Treatment of sediment and TSS, achieved through the production of Friedel’s salt using chemicals8mixed with runoff flowing through aReaction Chamber to a Precipitation Chamber, precipitant removal, then buffering treated supernatant to reduce pH prior to discharge to groundwater or surface water.

[0005] The present invention also embraces such a process wherein eighty percent (80%) removal of sediment and total suspended solids is achieved via conventional stormwater treatment practices.

[0006] The present invention also embraces such a process wherein chloride is present in stormwater from deicing activities on roadways, parking lots and other impervious surfaces at concentrations >250 mg / L of chloride.

[0007] The present invention also embraces such a process wherein stormwater flows from the Primary Treatment to a Reaction Chamber wherein sodium aluminate and calcium hydroxide, at a molar ratio of up to 4:1 (to chloride) for sodium aluminate and up to 10:1 (to chloride) for calcium hydroxide, are added to the stormwater flows for the expressed formulation of Friedel’s salt in the presence of Ch

[0008] The present invention also embraces such a process wherein Friedel’s salt production and settling occur immediately after chemical additions and mixing and wherein the time needed for satisfactory reaction in the Reaction Chamber is at least 10 minutes to facilitate the formulation of Friedel’s salt.

[0009] The present invention also embraces such a processwherein the flow from the Reaction Chamber is directed to and through a Precipitation Chamber for settlement of Friedel’s salt for a flow-throughtime of at least 10 minutes using one or more vessels.3 KIESER 1 PCT

[0010] The present invention also embraces such a stormwater treatment process, wherein the precipitate is settled out and captured inthe precipitation chamber, wherein such precipitation can occur naturallyor be enhanced by addition of a polyacrylamide or other enhancedsettling technology such as slant plate clarification, filter fabric baffles, or discharge through a filter fabric system.

[0011] The present invention also embraces such a stormwater treatment process, wherein the precipitate is separated from or removedprior to any pH modification or dilution actions.

[0012] The present invention also embraces such a stormwater treatment process, wherein the flow from the Precipitation Chamber issparged with carbon dioxide to create a carbonic acid, reducing treated supernatant to a pH below 9 prior to discharge.

[0013] The present invention also embraces such a stormwater treatment process, wherein the pH of the flow from the PrecipitationChamber is reduced by utilization of a stronger acid or by dilution.

[0014] The present invention also embraces such a stormwater treatment process, wherein pH-buffered flow is discharged to surfacewater or groundwater at reduced chloride levels.DETAILED DESCRIPTION OF THE INVENTION

[0015] The inventors have surprisingly discovered that utilization of Friedel’s salt precipitation for stormwater treatment, i.e. , CHLORIDE CAPTURE, is a six-step treatment-train process of Primary Treatment and Secondary Treatment, including:1. Sediment and Total Suspended Solids (TSS) removal treatment (Primary Treatment) from stormwater4 KIESER 1 PCT2. Dosing of chemicals (Secondary Treatment)3. Friedel’s salt formation (Secondary Treatment)4. Addition of polyacrylamide (PAM) to accelerate precipitation of Friedel’s salt (Secondary Treatment)5. Friedel’s salt precipitation and consolidation (Secondary Treatment) (flow to be recirculated to 2 above if additional treatment is needed); Friedel’s precipitate to be separated or removed from treatment prior to any buffering or dilution6. pH buffering or dilution of the supernatant discharge (Secondary Treatment)

[0016] This CHLORIDE CAPTURE process is shown in Figure 1.

[0017] Primary Treatment - Sediment and TSS removal: The primary treatment process step is necessary to remove sediment particles in stormwater, allowing greater Friedel’s salt precipitation of chloride. Urban stormwater often carries elevated levels of sediment and TSS, which interfere with the Friedel’s salt reaction process as well as prematurely fill treatment basins. Sediment, particularly silt and clay, contains reactive surface areas that could intercept chemicals before they are able to react with chloride in stormwater. The bulk of sediment and TSS (80% or more) can be removed from stormwater via traditional sedimentation ponds or mechanical treatment devices (MTDs). MTDs are well-established stormwater treatment practices and available in a variety of sizes. For the CHLORIDE CAPTURE process, ponds, MTDs or other sediment and TSS removal elements need to be installed ahead of the Secondary Treatment train. This Primary Treatment of stormwater is typically incorporated into most stormwater systems mandated by regulatory agencies.

[0018] Secondary Treatment - Dosing of Chemicals: Following the primary treatment of sediment and TSS removal, two chemicals are introduced to the treatment train; sodium aluminate (38% solution) and calcium hydroxide.5 KIESER 1 PCTSodium aluminate, NaAIO2, is highly soluble and can be stored as an aqueous solution without issue. Calcium hydroxide, Ca(OH)2, exhibits limited solubility, and thus its dosing storage container must be stirred intermittently to prevent it from settling out of solution before dosing. Chemicals should be introduced to the treatment train via a storm sewer manhole or other suitable impermeable mixing vessel for addition to stormwater.

[0019] Chemicals are introduced to stormwater at a molar ratio of the mass of chloride present. It is important that ratios be calculated on a molar basis rather than a mass basis. Maximum dosing rates are considered a molar ratio of up to 4:1 for sodium aluminate to chloride and up to 10:1 for calcium hydroxide to chloride, as ratios any higher than this produce limiting returns in removal efficiency. These ratios may be constrained by the mass of carbon dioxide required to reduce effluent pH to acceptable discharge levels. Higher additive ratios result in a higher mass of hydroxide ions that are not consumed by the Friedel’s salt reaction and must be neutralized prior to discharge. Additive ratios should be guided by the mass of chloride required to be removed in order to meet relevant water quality goals. Using the minimum ratios required to meet these goals will maximize treatment cost-effectiveness.

[0020] If chloride concentrations in the stormwater fall into a consistent range, adding at fixed ratios can be accomplished by monitoring flow. If chloride concentrations are highly variable, conductivity can be measured as a surrogate and incorporated into the logic or triggers that operate(s) dosing pumps and / or valves.

[0021] Secondary Treatment - Reaction Chamber: Turbulent flow must be present following the additive dosing stage such that stormwater becomes fully mixed. This can be done with flow baffles within the system or as a separate mixing chamber (mechanical or forced air mixing) before the precipitation chamber. The stormwater then flows into a basin or chamber that is sized to allow for at least 10 minutes of reaction time, given the expected system flow.Stormwater moves through this chamber as plug or continuous flow while the6 KIESER 1 PCTreaction takes place. Under-sizing this stage will hamper chloride removal efficiency, while over-sizing will result in a larger installation footprint (and thus installation expenses) than what is required. The reaction chamber should be lined or made otherwise impermeable to prevent interactions between the stormwater being treated and external ground surfaces or local groundwater.

[0022] Secondary Treatment - Addition of polyacrylamide (PAM): This is an optional step to accelerate the settling of the Friedel’s salt precipitate following the Reaction Chamber. PAM is a commonly used flocculant to remove suspended particles in industrial and municipal waste streams. The addition of this or an alternative compound increases Friedel’s salt particle diameter, accelerating the settling or retrieval process.

[0023] Secondary Treatment - Precipitation Chamber: The Precipitation Chamber follows the reaction chamber and is where the newly formed Friedel’s salt drops out of solution for later retrieval. This chamber should be sized for 10 minutes of settling time or greater given expected system flows and settlement technologies.

[0024] Secondary Treatment - The Reaction and Precipitation Chambers can be either separate or combined into a single volume, separated by a vertical baffle. Precipitated Friedel’s salt can be removed from the Precipitation Chamber via suction pumping, loaders, or excavators with suitable access and design to maintain the chamber’s structural integrity.

[0025] Secondary Treatment - pH Buffering and Discharge: This process follows the Precipitation Chamber, whereby the treated stormwater supernatant must be buffered down to a pH of 9 S.U. or less prior to discharge. This is accomplished via carbon dioxide (CO2) sparging of treatment flow, which forms carbonic acid, buffering out excess hydroxide ions. Sufficient time, space, and CO2 mass must be supplied such that the hydroxide ions in the supernatant stormwater that were not consumed in the Friedel’s salt reaction can be neutralized. Sparging can be introduced either in-pipe or in a dedicated mixing7 KIESER 1 PCTstructure such as a manhole or other suitable impermeable mixing vessel. Other methods of pH reduction such as the use of other acids or dilution are acceptable for buffering prior to discharge.EXAMPLES

[0026] The following examples illustrate the invention without limiting its scope.Example 1 - CHLORIDE CAPTURE Components

[0027] The minimum process elements and conditions required for the CHLORIDE CAPTURE process to generate Friedel’s salt precipitate include:• Stormwater runoff with chloride concentrations >250 mg / L• Primary T reatment of stormwater runoff to eighty percent (80%) removal of sediment and TSS• Sodium aluminate storage tank• Calcium hydroxide mixing tank• Dosing pumps for sodium aluminate and calcium hydroxide injection• Piping system to accommodate dosing injections in the treatment process• Reaction / Precipitation Chambers (vessels or lined basins) sized for the appropriate treatment flow times; 10-minute reaction time and 10-minute precipitation time• PAM dosing pump between the Reaction / Precipitation Chambers• Precipitation Chambers and removal (or standard sludge removal system)• Carbon dioxide (CO2) tanks for sparging to buffer pH prior to supernatant discharge• Discharge piping system for supernatant >8 KIESER 1 PCTExample 2 - Operational CHLORIDE CAPTURE Process

[0028] The CHLORIDE CAPTURE process targets removal of chloride associated with deicing applications in areas where freezing conditions require deicing. The process targets winter and spring stormwater runoff where chloride concentrations in meltwater from deiced surfaces of roadways, parking lots and impervious cover may exceed 250 mg / L. Sampling or monitoring is necessary to confirm the presence of chloride at levels in stormwater or meltwater that may exceed groundwater or surface water quality standards. The process first requires Primary Treatment of stormwater runoff prior to Secondary Treatment to remove chloride. Secondary Treatment adds chemicals to the stormwater to precipitate chloride ions (Cl ). The process of adding chemicals to stormwater that has received Primary Treatment is permitted by states according to the National Pollution Discharge Elimination System (NPDES). Individual treatment configurations for CHLORIDE CAPTURE must be permitted for each discharge accordingly by the appropriate permitting agency.

[0029] The first step in the stormwater treatment process is Primary Treatment of runoff or meltwater to eighty percent (80%) removal of sediment and TSS. Primary treatment may be accomplished by traditional stormwater Best Management Practices (BMPs) such as settling ponds or MTDs. Primary Treatment is not described here as numerous stormwater BMPs and standard practices typically address sediment control. The CHLORIDE CAPTURE process assumes that a Primary Treatment process is in place, maintained and operable. If this is not the case, a MTD of sufficient size should be designed into the process such that 80% of the sediments and TSS are removed prior to the Secondary Treatment system.

[0030] A pump or a piping system (either by pumping or gravity) directing the flow of primary treated stormwater through and / or to Secondary Treatment is the next step of the CHLORIDE CAPTURE process. The pump and piping system are sized specifically to control the flow of stormwater through Secondary Treatment to meet treatment time requirements. Stormwater is pumped or directed by gravity into the headworks of Secondary Treatment9 KIESER 1 PCTsystem (manhole / catch basin, flocculation or mixing tank) where it receives chemical additions.

[0031] A sodium aluminate storage tank of sufficient size for the normal operation of the Secondary Treatment system is required to provide enough sodium aluminate for anticipated flows. An onsite storage tank should have spill control elements in place per standard operation.

[0032] A mixing tank for preparing Ca(OH)2 solution is required. Calcium hydroxide normally is delivered in a powdered form and is mixed on-site.Calcium hydroxide exhibits limited solubility and thus its storage container must be stirred intermittently to prevent it from settling out of solution.

[0033] Dosing Pumps for sodium aluminate and calcium hydroxide must be capable of providing dosing rates using a molar ratio of up to 4:1 for sodium aluminate to chloride, and up to 10:1 for calcium hydroxide to chloride.

[0034] Sodium aluminate and calcium hydroxide are delivered to a mixing tank for dosing of the chemicals ahead of the Reaction / Precipitation Chambers. The mixing tank discharges completely mixed chemicals into the Reaction Chamber to initiate the Friedel’s salt precipitation process. Reaction Chamber flow-through time should be >10 minutes. A separate mixing chamber, using mechanical or forced air mixing, ahead of the reaction / precipitation may be included.

[0035] Reaction Chamber flows move to the Precipitation Chamber (a tank or lined pond) for >10 minutes of flow-through time for Friedel’s salt capture with passive settling and / or standard sludge handling methods. Stormwater movement through the Reaction / Precipitation Chambers is plug or continuous flow to allow the reaction of the mixed chemicals and the precipitation of the Friedel’s salt to occur in a laminar flow environment. The Reaction Chamber and the Precipitation Chamber may be the same vessel with a separation baffle, or they may be separate chambers where each is sized for necessary treatment process timing.

[0036] Polyacrylamide can be added after the Reaction Chamber to10 KIESER 1 PCTincrease Friedel’s salt particle size and settling in the Precipitation Chamber.The increase in particle size diameter enhances the physical recovery of Friedel’s salt precipitate from the Precipitation Chamber. Other settling enhancement devices such as slant plate clarifiers or filter baffles may be used for settling of the precipitate. The precipitate shall be separated and or removed prior to any pH modification or dilution.

[0037] Carbon dioxide (CO2) supplied by compressed gas tanks for sparging must be in sufficient on-site supply to reduce supernatant pH to <9 S.U. This supernatant buffering may occur in a tank or piping prior to groundwater or surface water discharge. The dosing of a stronger acid in the buffering vessel or dilution of the flow may also be used in the process as an alternative to CO2 sparging.*****

[0038] The present invention is not to be limited in scope by thespecific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description. Such modifications areintended to fall within the scope of the appended claims.

[0039] All patents, applications, publications, test methods, literature, and other materials cited herein are hereby incorporated by reference.11 KIESER 1 PCT

Claims

CLAIMS1. A stormwater treatment process, comprising:- a Primary T reatment of stormwater where eighty percent(80%) of sediment and total suspended solids (TSS) isremoved via conventional stormwater treatment practices, - a Secondary Treatment process comprising removal of Cl'in stormwater or meltwater associated with road saltingactivities, or other chloride-laden discharges to surface or groundwater, following Primary Treatment of sediment and TSS, achieved through the production and removal ofFriedel’s salt using chemicals mixed with runoff flowingthrough a Reaction Chamber to a Precipitation Chamber, then buffering treated supernatant to reduce pH prior todischarge to groundwater or surface water.

2. The stormwater treatment process of claim 1 , wherein eightypercent (80%) removal of sediment and total suspended solids is achieved via conventional stormwater treatment practices.

3. The stormwater treatment process of claim 1 , wherein chloride is present in stormwater from de-icing activities on roadways,parking lots and other impervious surfaces at concentrations>250 mg / L.

4. The stormwater treatment process of claim 1 , wherein stormwaterflows from the Primary Treatment to a Reaction Chamberwherein sodium aluminate and calcium hydroxide, at a molar ratioof up to 4:1 of sodium aluminate to chloride and up to 10:1 forcalcium hydroxide to chloride, are added to the stormwater flowsfor the expressed formulation of Friedel’s salt in the presence ofCh.12 KIESER 1 PCT5. The stormwater treatment process of claim 4, wherein Friedel’ssalt production and settling occur immediately after chemicaladditions and mixing and wherein the time needed for satisfactory reaction in the Reaction Chamber is at least 10 minutes tofacilitate the formulation of Friedel’s salt.

6. The stormwater treatment process of claim 5, wherein the flowfrom the Reaction Chamber is directed to and through aPrecipitation Chamber for settling the Friedel’s salt for a flow- through time of at least 10 minutes using one or more vessels.

7. The stormwater treatment process of claim 6, wherein theprecipitate is settled out and captured in the precipitationchamber, wherein such precipitation can occur naturally or beenhanced by addition of a polyacrylamide or other enhancedsettling technology such as slant plate clarification, filter fabricbaffles, or discharge through a filter fabric system.

8. The stormwater treatment process of claim 7, wherein theprecipitate is separated from or removed prior to any pHmodification or dilution actions.

9. The stormwater treatment process of claim 7, wherein the flowfrom the Precipitation Chamber is sparged with carbon dioxide tocreate a carbonic acid, reducing treated supernatant to a pHbelow 9 prior to discharge.

10. The stormwater treatment process of claim 7, wherein the pH ofthe flow from the Precipitation Chamber is reduced by utilizationof a stronger acid or by dilution.

11. The stormwater treatment process of claim 1 , wherein pH- buffered flow is discharged to surface water or groundwater atreduced chloride levels.13 KIESER 1 PCT

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