ADVANCES IN NITRATE REMOVAL BY ELECTRODIALYSIS REVERSAL

Transcription

1 ADVANES IN NITRATE REMOVAL BY ELETRODIALYSIS REVERSAL David Elyanow Process Manager Ionics Incorporated Janet Persechino Product Manager Ionics Incorporated ABSTRAT Electrodialysis has been used for treating saline waters for fifty years. Advancements in membrane and system technology have made Electrodialysis Reversal (EDR) an especially attractive technology recently for treating waters contaminated with high Nitrate levels. Technological improvements are discussed including the following areas: 1. The Reversal Process effects on scaling and fouling 2. Advances in membrane technology leading to more chemical and fouling resistance 3. Advances in spacer design and hydraulics Electrodialysis Reversal is a competitive technology to Reverse Osmosis for many Nitrate removal applications. A comparison is made between the applications of these technologies to illustrate where the high recovery advantages of EDR may be a greater advantage as compared to the typically higher rejection capabilities of RO. Several examples of Nitrate removal by Electrodialysis Reversal are presented including recent experience in Israel on a system recovering drinking water at over 94% recovery from wells containing 135 mg/l of Nitrate. THE PROBLEM OF NITRATES Nitrate contamination of groundwater is a common problem throughout the world. The problem is prevalent in many parts of Europe, including Great Britain, France, the Netherlands, Germany, and Switzerland; several parts of the United States, and even here in Israel. Most nitrate contamination is a result of intensive agriculture and the use of nitrogen containing fertilizers, although occasionally it may be a result of natural nitrates or contamination from wastewater sources. The primary health risks associated with elevated nitrate levels are methemoglobinemia, which causes the blue baby syndrome in infants (1), and the potential formation of

2 carcinogenic nitrosamines. In infants, the higher ph of their upper respiratory tract accelerates the conversion of nitrate to nitrite. The nitrite in turn oxidizes the infant s hemoglobin to methemoglobin, which is unable to carry oxygen in the bloodstream. In severe cases the result may be fatal. The cancer causing effects of nitrates are not well documented, but nitrate reaction products, nitrosamines and nitrosamides are strongly considered as potential human carcinogens. As a result of the health consideration the Maximum ontaminant Limit for nitrate in drinking water has been set at 10 mg/l nitrate as nitrogen (44.3 as NO 3 ) in the United States and anada. A similar guideline of 50 ppm as NO 3 has been set by the WHO (2), while the EE standards allow a maximum admissible concentration of 50 mg/l as NO 3 and a guide level of 25 mg/l as NO 3. TREATMENT TEHNOLOGY There are a number of treatment technologies available for the reduction of nitrates in drinking water. These include the following: 1. Ion Exchange using strong base anion resins regenerated with Nal. 2. Biological Denitrification using Methanol or Ethanol addition 3. Electrodialysis or Electrodialysis Reversal 4. Reverse Osmosis Ion exchange generally substitutes chloride anions for the nitrate anion without substantially changing the salinity or the cation makeup of the feedwater. Sulfate is preferentially removed and replaced by chloride unless special nitrate selective resin is used. Because of the batch nature of the process the anion composition and ph of the product changes over the service cycle of the ion exchanger. Since salt is used for the regeneration of the ion exchanger, the waste contains substantial added sodium chloride. This can pose a problem for waste disposal since high salt loads can effect the performance of waste treatment plants or may be otherwise regulated. Since chloride is exchanged in this process, the chloride level of the product is elevated and there is no benefit of TDS reduction as with the membrane processes. In summary, ion exchange can be a cost efficient method for nitrate removal as long as waste disposal is not a major issue and as long as hloride or TDS reduction is not desired. Biological treatment for nitrates is not common for drinking water applications but widely practiced in wastewater treatment. The process generally involves the reduction of nitrates to nitrogen by bacteria in an anoxic bioreactor. A carbon source is generally required by the bacteria, which for waste treatment can usually be supplied by the natural BOD. The relatively low organic content of drinking water sources generally requires the addition of methanol or ethanol to drive the reaction. oncerns about potential discharge of bacteria or methanol into drinking water has generally limited the acceptance for this approach. As with ion exchange, 168

3 the salinity of the water from biological is not significantly changed and this can be a disadvantage for areas that have elevated salinity or hardness. The membrane processes Electrodialysis Reversal (EDR) and Reverse Osmosis (RO) are both seeing increasingly widespread use for nitrate reduction. Since both processes remove other ions along with nitrate, they also result in lower product salinity along with lower levels of sodium, chloride, hardness, etc. For areas where the salinity as well as the nitrate is high, this results in a substantial water quality improvement as compared to the non-membrane processes above. For waters with moderate levels of nitrate the product of either RO or EDR may be blended with feedwater to achieve the desired nitrate level at higher production level and recovery. These processes and their advantages will be discussed in more detail below. THE ELETRODIALYSIS REVERSAL PROESS Electrodialysis is an electrically driven process that uses a voltage potential to drive charged ions through a semi-permeable membrane, reducing the TDS in the source water (3). The process uses alternating, semi-permeable cation (positively charged ion) and anion (negatively charged ion) transfer membranes in a direct-current (D) voltage potential field. The source water flows between the cation and anion membranes via flow spacers that are placed between the membranes. The spacers are used to provide a flow path for the water, support the membranes, and create turbulent flow. The D voltage potential induces the cations to migrate toward the anode through the cation membrane, and the anions to migrate toward the cathode through the anion membrane. The cations and anions accumulate in the reject water side of the membranes and low TDS product water is produced on the dilute side of the membranes. The electrodialysis reversal system periodically reverses the polarity of the electric field, and consequently the dilute and concentrate compartments, to help flush scale forming ions off the membrane surface and minimize membrane cleaning. " X " IN L E T A A A A N O D E (+ ) _ l " X " O U T L E T P R O D U T A T H O D E (-) " Y " IN L E T " Y " O U T L E T W A S T E N e g a tiv e P o la rity A = ANION TRANSFER MEMBRANE " X " IN L E T A A A A T H O D E (-) " X " O U T L E T W A S T E A N O D E (+ ) " Y " IN L E T " Y " O U T L E T P R O D U T P o s itiv e P o la rity = ATION TRANSFER MEMBRANE 169

4 ADVANES IN EDR MEMBRANES The first membranes used for electrodialysis were anion and cation exchange membranes made from crushed ion-exchange resins in an inert matrix. These membranes have been replaced by homogeneous membranes which typically have lower resistance and are generally more resistant to scaling. In the 1980 s acrylic-based anion exchange membranes were broadly introduced which exhibited the advantages of substantially increased resistance towards organic fouling and an enhanced degree of chlorine tolerance. The difference in membrane organic fouling observed was somewhat similar to that seen between the analogous organic resistant acrylic resins versus the standard styrene-divinylbenzene resins. The increased chlorine tolerance allowed the EDR systems to be operated with up to 0.5 mg/l free chlorine residual thus offering the benefits of a continuous disinfection residual throughout the treatment process. It also allowed the EDR system to be chemically cleaned with up to 20 mg/l of free chlorine resulting in better cleaning for difficult biological or organic contaminants. More recent advances in membrane technology have allowed the one step machine manufacture of ion exchange membranes reducing the cost and resulting in lower membrane resistivity. Various groups have also developed and manufactured more nitrate selective membranes. While enhanced membrane selectivity has some advantages, the removal by electrodialysis of nitrate in the dilute solutions is generally limited more by spacer hydrodynamics than by membrane properties. Simply put, the nitrate can not be removed unless it reaches the anion membrane surface. ost effective design generally dictates that the EDR membrane stack is operated at close to the concentration polarization point. At this point the concentration of nitrate at the anion membrane boundary layer is depleted and the removal rate of nitrate is more a function of the ionic mobility of the nitrate ion than membrane selectivity. As a result, improvements to the spacer that allow better transport of nitrate to the membrane surface have more of an economic impact on the system as discussed below. ADVANES IN EDR SYSTEMS A new high-performance spacer that promotes greater turbulent flow in an electrodialysis stack, has been developed, and is now in operation (4). The maximum demineralization that can be obtained in a single hydraulic stage has been increased in comparison to a hydraulic stage of conventional stacks. These high-performance spacers are incorporated into a new membrane stack, designated a Mark IV stack. The Mark IV stack has 38% more usable membrane area than a conventional Mark III stack. The increased area is available because the spacers are thinner so more cell pairs can be included in one stack, and because the usable membrane area per cell pair is greater. The increased available membrane area results in higher demineralization per stack; therefore fewer membrane stacks are needed to demineralize a given volume of water to a specific TDS level. 170

5 Recent advances in EDR system design were made with the introduction of the Ionics EDR 2020, using Mark IV stacks, variable frequency drives to minimize pumping power and 4- way valves to maximize recovery (5). The pumping power requirement to an EDR plant frequently varies as cartridge filters fouled or if temperature drops so that the stack pressure drop increases. Use of the variable frequency drive reduced the pumping power by approximately 25 % over a conventional Ionics EDR system, while maintaining a system design that had the pumping capacity to allow for an increase in feed pressure. The variable frequency drives also make the system easy to start up and shut down as the water demand varies. The 4-way valves reduce the piping required to reverse the flow and reduce the amount of time the system operates off-spec at polarity reversal. onventional EDR systems with a 15 minute reversal time are off-spec for about 36 seconds per reversal, or 4 % of the time. In some large EDR plants, use of the 4-way valve reduced reversal times to less than 15 seconds or 1.7 % of the product flow. Since the off-spec is usually treated as part of the EDR system waste, this extra product flow translates into approximately 2 % higher water recovery with the new system than was possible with the conventional system. EDR PERFORMANE IN NITRATE REMOVAL A previous paper (6) has discussed nitrate removal at water plants installed by Ionics Incorporated in Bermuda, Delaware and Italy. These plants have demonstrated reliable operation now for over 8 years, at nitrate removal rates of 69% to 92.6 % along with TDS removal rates of 53% to 88%. Performance data is summarized in the table below. These plants consist of two to three stages and do not include the newer high performance spacers. EDR Plant Data Nitrate Removal Applications Bermuda Delaware Italy Production, M 3 /hr Recovery, % Desalting Stages Feed Nitrate, mg/l NO Product Nitrate, mg/l NO Feed TDS, mg/l Product TDS, mg/l NO 3 Removal, % 86.7 % 92.6 % 69.2 % TDS Removal, % 81 % 88 % 53 % For many nitrate removal applications high recovery is essential. EDR has demonstrated recoveries of 94% for over 10 years now at a 593 m 3 /hr plant drinking water plant for Suffolk, 171

6 Virginia, USA (7). Although this plant is primarily removing TDS and Fluoride, it demonstrates high recovery operation at a large scale over a long time period A new plant requiring high recovery, employing the new MKIV stack design and the other Ionics EDR 2020 features, was commissioned recently in Safaria, Israel for EMS- Mekorot. It produces 2208 M 3 /D of drinking water before blending. The nitrate contaminated source water comes from a network of 80 to 100 different wells with a blended nitrate content of 115 to 135 mg/l. The plant has been in operation since May 1999 at 94% water recovery, achieving 52% nitrate reduction in a single stage at nearly 30% increased capacity per line as compared to previous designs. The plant is designed with 172

7 the provision for the blending of product water with feed to increase production and recovery. The initial blend ratio is low due to feed nitrate levels of 15% to 30% greater than design. Provision has also been made to increase capacity expansion by adding a future second stage thus increasing the nitrate removal to 90%. Data from this plant is included below. EMS- Mekorot EDR Design Parameters Feed Flow, M 3 /hr 97.9 Product Flow, M 3 /hr 92.0 Waste Flow, M 3 /hr 5.9 Water Recovery, % 94.0 Number of lines 3 Number of Stages 1 Feed Nitrate, mg/l as NO Product Nitrate, mg/l as NO Feed TDS, mg/l 1149 Product TDS, mg/l 596 E D R N IT R A T E R E M O V A L F e e d NITRATE ONN.- mg/l P r o d u c t M a y 3 - J u n J u n J u l 2 - A u g A u g D A T E 173

8 N I T R A T E R E M O V A L O F E M S - M E K O R O T P L A N T E D R D A T E R a w W a t e r E D R P r o d u c t R e j e c t i o n m g / l a s N O 3 m g / l a s N O M a y % M a y % M a y % M a y % 6 - J u n % J u n % J u n % J u n % J u n % J u n % J u n % 3 - J u l % 5 - J u l % J u l % J u l % J u l % J u l % J u l % J u l % J u l % 2 - A u g % A v e r a g e % OVERVIEW OF REVERSE OSMOSIS Principles Of Operation Reverse Osmosis a process for the removal of dissolved ions from water in which pressure is used to force the water through a semipermeable membrane element which will pass the water but reject most of the dissolved materials. To understand reverse osmosis, one must first understand the priciple of osmosis. Osmosis can be defined as the spontaneous passage of a liquid solvent (water) from a dilute solution across a semipermeable membrane to a more concentrated solution. The driving force for this flow is called osmotic pressure, and is shown in Figure RO-1 as the difference in height of the water columns on either side of the membrane. If no other force is applied to the system, the pressure (height of the column) on the concentrate side of the membrane will rise until the solution concentration on both sides of the membrane is nearly equal. The flow of solvent from low concentration to high can be halted by applying a pressure equal to the osmotic pressure on the more concentrated solution side. If this external pressure is increased further, the flow of water will be reversed from its natural flowing direction and towards the more dilute solution. The reversing of the flow is the process of reverse osmosis. 174

9 For example, if a variable pressure P were applied on the more concentrated solution side of a semipermeable membrane element, the following conditions could be realized as illustrated below: (1) P is less than the osmotic pressure of the solution. The solvent flows from the more dilute solution to the more convcentrated solution. This condition, as shown in Fig. RO-1, represents the phenomenon of osmosis. (2) P is greater than the osmotic pressure of the solution. Solvent flows from the more concentrated solution to the pure solvent side of the membrane. This condition, as shown in Fig. RO-2, represents the phenomenon of reverse osmosis. 175 RO Membrane Rejection Figure RO-1 Figure RO-2 The ideal situation would be that of a truly semipermeable membrane which would allow only water to pass across it (100% salt rejection), but this is not the case in reality, as there is always a very small amount of salt which passes across the membrane. Nitrates are not as well rejected by most reverse osmosis membranes as is chlorides or sulfates. For comparison, a typical rejection for Nal is 98% as compared to 93% for NaNO 3. This is generally not a significant problem due to the very high overall rejection properties of most RO elements and the typical requirement for permeate blending. Reverse Osmosis Plant Design onsiderations High water recovery in RO sysems is desirable for a number of reasons. These include the need to reduce the size of any filtration equipment, to economize on the pipeline sizes and to minimize the amount of water drawn from the aquifer (in the case of brackish water sources). Another compelling reason in many countries for the reduction in waste quantity is the high cost of disposing of the water. A restriction on high recovery is the potential for scale formation. Since the concentrate stream flowing across the RO membrane surface is being continually more concentrated as it flows down the series of elements in the pressure vessel it can become saturated in certain salts. These salts can then precipitat from solution and be deposited on the membrane surface. Because the membranes are not perfectly flat sheets, but have many millions of pores which are relatively large compared to the salt crystal which is deposited, it may be difficult to remove the

10 salt from the membrane. This is especially true in the case of scaling salts such as calcium carbonate, calcium sulfate, barium and strontium salts.the surface area of the membrane would therefore become smaller, reducing the efficiency of the process, and the salt would become a seed for crystallization. In order to prevent this from occurring, there are two possibilities, either reduce recovery or use chemicals (scale inhibitors or acid) which will retard or prevent scale formation. In most plants recovery is first limited by alcium arbonate precipitation (using Langelier Saturaturation Index or Stiff and Davis Index) which may be controlled by acid or antiscalant addition. Other precipitationg salts such as alcium Sulfate, Silica, Barium or Strontium Sulfate, and alcium Fluoride can only be controlled with antiscalants or reduced recovery. Because RO is not a reversing process the achievable recovery is less than that from EDR plants. Another concern in the design of RO systems is the potential for biofouling. This is not generally as much of a problem for wellwaters as it is for surface waters. Biological activity in the feed water (biofouling) is often controlled by disinfection of the feed water, usually with chlorine or hypochlorite. One drawback with this methodfor RO is that the chlorine needs to be removed before the water reaches the membrane since most membranes show very low chlorine tolerance.. In most cases, it is beneficial to remove all treaces of chlorine upstream of the membranes either by dosing with a reducing sgent (such as sodium bisulfite) or by contacting with activated carbon. The potential for colloidal fouling of the RO membrane is usually evaluated by measuring a silt density index (SDI) at the water source. lean wellwaters usually have low values ranging from SDI 15 of 0 to 3. Poorly drilled wells or water sources subject to contamination may yield much higher SDI s and will then require suitable pretreatment such as in-line coagulation and filtration upstream of the RO plant. RO PERFORMANE IN NITRATE REMOVAL In 1995 Ionics Italian subsidiary commissioned a series of reverse osmosis plants for nitrate removal in the greater Milan area. The source water for these plants is a series of wells with nitrate concentrations in the range of 50 to 60 mg/l and all having silt density indexes (SDI) of less than 1. The objective of these plants is to produce and deliver to the distributing net drinking water at nitrate concentration lower then 40 mg/l, while releasing to the sewer a waste stream at nitrate concentration lower then 132 mg/l. As summarised in the table below, the project consists of a network of 13 small to medium sized plants in 9 locations. The plants are sized to produce from 7 to 58 M 3 /hr of permeate which is blended with feed at each site at a ratio of roughly 79% blend water to 29% permeate. The RO recovery for each plant averages 59% but with blending the overall recovery ranges from 77 to 88%. Over the last five years of successful operation, the plants have used 176

11 antiscalant at a dosage rate of 2.0 to 3.5 mg/l and chemical cleanings (IP) are performed every 18 months. The system has central control and data logging from Ionics Milan office allowing the system to be operated and maintained by a staff of three technicians. These plants offer a good illustration of design characteristic favorable towards reverse osmosis technology for nitrate removal. First of all, the source waters are all from low SDI wellwater sources and thus requires minimal pretreatment. The low IP frequency is indicative of the high quality of the well supplies. Secondly the waste nitrate limit essentially limits the overall system recovery thus giving no incentive towards high recovery. In addition the low recovery generally allows simpler RO design with only one stage generally required. Finally, the small capacity (<25 M 3 /hr) of many of the plants is most economically accomplished by simple RO plants, which have less electrical and hydraulic complexity than EDR and other technologies. Design Data from Milan Area Reverse Osmosis Nitrate Removal Plants Location Aicurzio Bellusco Ornago Sulbiate Bernareg Mezzago Barlassin esano Maderno Lainate Varedo gio a Via Rimembra nze Via Dolomiti ascina Savina Via Solferino Pozzo Bellaria Via Tommase o Well water in m 3 /h By-pass m 3 /h R.O. feed m 3 /h R.O. Permeate m 3 /h R.O. brine m 3 /h Delivered drinking water m 3 /h R.O. Recovery % Overall recovery % Well water NO 3 ppm Water conductivity@20 S/cm Assumed permeate NO 3 ppm Brine NO 3 ppm Pressure Vessels Membranes per Vessel R.O. feed pressure bar ommissioned EDR VS. RO SELETION RITERIA FOR NITRATE REMOVAL Most of the aspects that touch on the selection of either EDR or RO technology for nitrate removal applications have been mentioned in the previous discussion. The 94% recovery achieved by the Safaria EDR plant would not be possible with good RO design. On the other hand, the high quality, low recovery, and low capacity of the Milan area wells favor more economical RO plants. In general one can say that high recovery, elevated SDI, and potential 177

12 for biofouling are preferential to EDR technology, while lower recovery, low SDI, and low plant capacity are preferential towards RO. These criteria are summarized in the table below. OMPARISON OF EDR AND RO FOR NITRATE REMOVAL- Typical Values Parameter EDR RO Recovery 94% 60%-85% Rejection 50%-85% 95%-98% SDI Limit hlorine Tolerance 0.5 mg/l free <0.1 mg/l free ost small sys <40 M 3 /hr higher on clean waters lower ost - larger systems osts are similar and depend on site details ONLUSIONS The basic conclusions of this paper may be summarized as follows: 1. The membrane processes of Electrodialysis Reversal and Reverse Osmosis have proven over time and at many locations to be highly effective in solving nitrate contamination problems in drinking water. 2. Advances in EDR technology have allowed nitrate removal plants to achieve 94% recovery while also reducing salinity and hardness. 3. New technology has also reduced the capital and operating cost of EDR nitrate removal by increasing the hydraulic efficiency of the EDR stacks and pumping system. 4. Membrane improvements have made EDR more organic resistant and chlorine tolerant, thus allowing better control over biological contamination. 5. High recovery requirements, elevated silt density indices, and potential for biofouling tend to favor EDR technology. 6. High rejection, low recovery, very low silt density indices, and low capacity tend to favor RO technology. References 1. United States Environmental Protection Agency, Office of Water, 305(b) Report to ongress 1996/ World Health Organization, Guidelines for drinking-water quality, 2nd. ed. Vol. 2 Health criteria and other supporting information, 1996 (pp ).. 3. Siwak, L.R., Here s How Electrodialysis Reverses and Why EDR Works, Int l Desalination & Water Reuse Quarterly, Vol. 2/4, von Gottberg, A. J. M., New High Performance Spacers in Electrodialysis Reversal Systems, Proceedings, American Water Works Assoc. Annual onf., Dallas, TX, von Gottberg, A. J. M. & L. R. Siwak, Re-Engineering of the Electrodialysis Reversal Process, Int l Desalination & Water Reuse Quarterly, Vol. 7/4, Feb./Mar Prato, T. A. & R.G. Parent, Nitrate and Nitrite Removal from Municipal Drinking Water Supplies with Electrodialysis Reversal, Proceedings, American Water Works Assoc. Membrane onference, Werner, T.E., Five Billion Gallons Later-Operating Experience at ity of Suffolk EDR Plant, American Desalting Association 1998 North American Biennial onference and Exposition. 178