TP1072EN.docx Mar-10 nitrate and nitrite removal from municipal ... enteric bacteria convert the elevated levels of nitrate to nitrite. The nitrite then competes with ...
Water Technologies & Solutions technical paper
nitrate and nitrite removal from municipal drinking water supplies with electrodialysis reversal Authors: Ted Prato and Richard G. Parent, Ionics Reprinted from Proceedings of 1993 AWWA Membrane Conference, by permission. Note: SUEZ purchased Ionics in 2005.
summary Nitrate contamination of drinking water is a widespread problem. It has long been known that levels of nitrates exceeding the 10 ppm (mg/l) (as Nnitrogen) limit are associated with certain health problems. Although high nitrogen concentrations in drinking water are found mainly in regions of intensive agricultural use, there are sources of nitrate contamination other than agricultural. Nitrates and nitrites are removed efficiently and economically using electrodialysis reversal (EDR), a process where overall demineralization takes place via transfer of ions through anion and cation-selective membranes by application of a direct current (DC) electric field. Data are presented demonstrating excellent long-term nitrate removal using EDR.
human health problems The best known and documented human health risk associated with high levels of nitrates in drinking water is methemoglobenemia, also known as asphyxia or “blue-baby syndrome,” which can affect infants under six months of age. In methemoglobenemia, enteric bacteria convert the elevated levels of nitrate to nitrite. The nitrite then competes with oxygen for active sites on hemoglobin resulting in oxygen deprivation. Infants are predisposed to this effect due to a number of physiological factors including “higher fluid intake per body weight, a higher percentage of fetal hemoglobin and higher stomach pH, permitting survival of reducing-type” bacteria. 1
2
Other potential health effects related to high nitrate levels include the risk of cancer and birth defects. These risks are less well-known and documented than methemoglobenemia. Nitrates combine with amines to form nitrosamines, suspected of being potential human carcinogens. Other reactions include the formation of nitrosamides which can also produce tumors in humans. In general, the long-term effect of consumption of high nitrate drinking water is unknown, but some potential for health problems exists from nitrite reactivity.
sources of nitrates in drinking water Fertilizer runoff, farm animal wastes, and septic tank discharge all percolate through the soil into groundwater aquifers and ultimately into water supplies. Agricultural sources of nitrates are by far the most common. Regions of the country where corn is grown experience peak levels of nitrates in groundwater from heavy fertilization. Other sources of nitrate and nitrite contamination are natural and industrial in origin. Industrial sources include chemical manufacturing operations and nitrate-containing cutting oils. Natural sources include atmospheric precipitation (as NH ) and local mineral deposits such as potassium nitrate (saltpeter). Decomposing plant materials contribute as well via nitrogen-fixing bacteria. The overall contribution of natural sources is small in relation to the contribution from human activities. 3
allowable limits of contaminants Standards for maximum levels of nitrates in drinking water have been established by the Federal Government in 1975 with passage of the Safe Drinking Water Act (SDWA). As of the May 1990
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SDWA regulations, some major allowable inorganic contaminants are as follows: Contaminant
MCL or SMCL
Chloride
250 ppm (mg/l)
Fluoride
2 ppm (mg/l)
Nitrate (as N)
10 ppm (mg/l)
prevent passage. See Figures 1 and 2, which illustrate the EDR process. 4
1
(as NO ) 45 ppm (mg/l) 3
Nitrite (as N)
1 ppm (mg/l)
(as NO ) 3.3 ppm (mg/l) 2
TDS
Figure 1: Electrodialysis Diagram
500 ppm (mg/l)
Maximum contaminant level or secondary maximum contaminant level 1
Prior to the SDWA, there was no requirement or a practical, affordable method to remove nitrates from drinking water. Since that time a number of demineralization technologies have been given a best available technology (BAT) status for nitrate removal. These BAT processes include EDR, reverse osmosis (RO) and ion exchange (IE). Effective removal constitutes reducing the level of nitrates to the maximum contaminant level (MCL) or lower. 3
The EDR process can effectively reduce nitrate concentrations to the MCL or lower in public water supplies. The reduction in nitrite concentration is directly related to the design demineralization rate of the EDR system.
Figure 2: Electrodialysis Reversal
A number of EDR plant designs are available to treat contaminated water. Figure 3 presents a diagram of a typical three-stage plant. In general, a two stage plant removes 50% of the influent minerals and nitrates, a three-stage plant removes about 75%, a four-stage plant removes about 83%, and so on.
what is EDR? Electrodialysis (ED) is a process which uses ion exchange resin in sheet form assembled into a stack of components. ED is widely used for total dissolved solids (TDS) removal in a number of different types of plants and applications, including many public and municipal drinking water supplies. EDR uses a reversible DC field applied across a stack of components to remove dissolved ions: the components consist of ion-selective membranes which transport either positive or negative ions, flowdirecting spacers, and electrodes at each end of the stack. Water flows in a thin sheet between membranes. Under the influence of the applied DC electric field, ions migrate toward the electrode opposite in charge, passing through membranes which also have fixed oppositely-charged groups bonded to a polymer surface. Membranes with the same charge as the ionic species repel the ions and Page 2
Figure 3: EDR Process Flow Diagram
In the course of removing charged ions in solution, EDR also removes nitrate and nitrite ions. In this next section, operating data are presented for several EDR plants used for nitrate removal.
nitrate removal using the EDR process Operating data illustrate the practicality of the EDR demineralization process for removing nitrates and nitrites as well as TDS. The examples represent a variety of EDR plants and include three public drinking water installations and one industrial TP1072EN.docx
application. Descriptions of the four plants follow. All of the feed waters contain high levels of nitrate, and the industrial feedwater contains exceptionally high levels of nitrite. Bermuda: 3 desalting stages, 600,000 gpd (2,271 m /day) total production 3
In Bermuda, there are two Aquamite* XX units capable of producing a total of 600,000 gpd (2271 m /day) of demineralized drinking-quality water from a brackish well. The plants were installed to reduce hardness in the existing water supply and to cut back on the amount of purchased water. The brackish water lens under the island is contaminated from septic tank leach fields, making nitrate removal important. The EDR plants yield 81% demineralization and reduce the nitrate concentration from 66 ppm to 8.8 ppm. 3
Delaware: 3 desalting stages, 400,000 gpd (1,514 m /day) total production 3
This plant was installed to improve operating economics (by predemineralizing the IE feedwater to a power plant boiler makeup system) and to reduce NO in the agriculturally-contaminated drinking water supply. This three-stage EDR plant yields 88% demineralization and reduces the nitrate concentration from 60 to 4.5 ppm (mg/l).
Table 1: EDR Plant Data - Bermuda/Delaware Plant Specifications
Model
Bermuda
Delaware
Two Aquamite XX
Aquamite XX Aquamite X
Production
300,000 gpd (1,136 m /day) 600,000 gpd (2,271 m /day) 3
3
300,000 gpd (1,136 m /day) 300,000 gpd (1,136 m /day) 400,000 gpd (1,514 m /day) 3
3
3
Recovery Product Purity
Raw Water
90%
90%
278 ppm TDS
11 ppm TDS
8.8 ppm NO
4.5 ppm NO
3
3
1,614 ppm TDS
114 ppm TDS
66 ppm NO
61 ppm NO
3
Desalting Stages
3
3
Percent Removal
81% TDS
88% TDS
3
3
Industrial: 3 desalting total production
stages,
100,000
86.7% NO
Italy: 2 desalting stages, 300,000 gpd (1,136 m /day) total production 3
Both plants were installed specifically to remove nitrates from municipal wells in agriculture-intensive regions of Italy. These two-stage Aquamite XX plants yield 53% demineralization with reductions in TDS from 1,012 to 474 ppm (mg/l) and nitrate from 120 to 37 ppm (mg/l). The approach here is not only to remove nitrate but also to comply with EC standards in overall TDS reduction.
92.6% NO
3
Water Quality
gpd
This plant was installed as a roughing demineralizer. Nitrite concentration is also quite high. Here an Aquamite X produces 100,000 gpd with 66% demineralization. Nitrate concentration is reduced from 655 ppm to 128 ppm, and Nitrite concentration is reduced from 64 ppm to 21 ppm.
3
Bermuda
Delaware
Feed (ppm [mg/l])
Product (ppm [mg/l])
Feed (ppm [mg/l])
Product (ppm [mg/l])
Sodium
349
72
12
1.6
Calcium
138
13
9
0.5
Magnesium
40
4
8
0.6
Potassium
19
2
-
-
Chloride
656
92
15
1.2
Bicarbonate
259
75
9
2.4
Sulfate
85
10
-
-
Nitrate
66
8.8
61
4.5
pH
7.9
7.0
6.2
5.4
TDS
1,614
278
114
11
Constituent
Tables 1 and 2 present operating data on all of the various EDR units.
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concentration to below the MCL as established by both the SDWA and the World Health Organization.
Table 2: EDR Plant Data - Industrial/Italian Plant Specifications
Model Production
Industrial
Italian
Two Aquamite X
Aquamite XX
100,000 gpd (379 m /day)
300,000 gpd (1,136 m /day)
80%
90%
3
Recovery Product Purity
534 ppm TDS 128 ppm NO 21 ppm NO
Raw Water
3
3
ppm TDS 37 ppm NO
3
3
1,753 ppm TDS 655 ppm NO 64 ppm NO
Desalting Stages
3
Percent Removal
66% TDS
3
3
2
80.4% NO3 67.2% NO3
All nitrate product levels for the public water supply applications are within the Safe Drinking Water Act MCL requirement. Average percent removal of nitrate from the three-stage plants is 86.6% with an average percent TDS removal of 78.3%. Percent removal from the two-stage plant is 69.2% for nitrate and 53% for TDS. Nitrite removal in the industrial application is 67.2% with a 68% TDS removal. In addition, graphs of three of the four plants show analytical data for feed and product salinity and nitrate concentrations over time (Figures 4 and 5). The small variations in the product quality reflect changes in feedwater quality or operating conditions, such as seasonal temperature changes or increases in feedwater TDS. These EDR units consistently meet or exceed SDWA MCL standards for nitrate removal from public water supplies.
53% TDS 69.2% NO
3
Water Quality Industrial
Italian
Feed (ppm [mg/l])
Product (ppm [mg/l])
Feed (ppm [mg/l])
Product (ppm [mg/l])
Sodium
24
14
73
49
Calcium
141
28
127
63
Magnesium
34
8
34
13
Potassium
-
-
4
1.7
Chloride
35
11
120
44
Bicarbonate
514
235
449
240
Sulfate
113
23
85
25
Nitrate
655
128
120
37
Nitrate
64
21
7.3
7.1
pH
7.3
7.0
7.3
7.1
TDS
1,753
534
1,012
474
Constituent
Figure 4: Nitrate and TDS Removal
All of these plants have three desalting stages with the exception of the Italian municipal plant which requires only two desalting stages to reduce the nitrate Page 4
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references
Figure 5: Nitrate, TDS and NO3 Removal
conclusions EDR effectively removes nitrates and nitrites from feed waters, as demonstrated by the plant data. EDR plants are easily designed and operated to yield product water quality to meet SDWA MCLs or SMCLs or lower. Other EDR process advantages include: •
Self-cleaning membrane process
•
Low operating and maintenance costs
•
Long membrane life
•
High recovery operation
•
No chemical feeds
•
Low chemical usage
•
No regenerant wastes
•
Reduction in TDS in addition to nitrate and nitrite
1.
O’Brien, W.J., “Control Options for Nitrates and Fluorides”, Water/Engineering and Management, July, 1983.
2.
Bouchard, D.C., Surampalli, R.Y., Williams, M.K., “Nitrate Contamination of Groundwater: Sources and Potential Health Effects”, Journal AWWA, Management and Operations, September, 1992.
3.
Kartinen, E.O., Jr., “Nitrate Removal from Municipal Water Supplies”, AWWA Conference at Grand Island, Nebraska, November, 1991.
4.
Meller, F. H., “Electrodialysis & Electrodialysis Reversal Technology”, March, 1984.
Although there are only a few very specific health problems identified as a result of high nitrate levels in drinking water, stricter regulations are driving public water suppliers to provide suitable technology for reducing nitrate and nitrite concentrations in the interest of addressing avoidable health risks.
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