ELECTRODIALYSIS FOR NITRATE REMOVAL IN DRINKING WATER

Due to the intensive utilization of fertilizers in agriculture in many parts of the world, the concentration of nitrates in surface and underground water has steadily increased over the last decades. As a result, this nitrate concentration has largely exceeded the required EU (and WHO) norm of 50 ppm for uses as drinking water.

In the last few years, Electrodialysis Reversal (EDR) has become an attractive technique to remove nitrates from potable water thanks to the availability of monovalent-selective anion-exchange membranes, as well as improvements in our spacers. As can be seen on Table1 below, compared to the traditional techniques for nitrate removal (biological processes, ion exchange resins, and reverse osmosis), EDR has many advantages making it one of the most competitive processes for this application. Eurodia / Ameridia has already supplied several plants for nitrate removal, some where membrane life has reached 10 years and some as large as to produce 3500 m3/day of drinking water.

TABLE 1 : COMPARISON OF AVAILABLE NITRATE REMOVAL TECHNIQUES

Description of the ED process for nitrates removal:

As in all EDR stacks, under the influence of a direct current, the nitrates in the feed water migrate towards the anode. They leave the compartments that become the diluate compartment, in this case the drinking water product, move through the anion-exchange membranes but are stopped by the next cation-exchange membranes in the "concentrated" or "brine" compartment. Sodium and calcium are the main cations that are transported through the cation-exchange membranes. The brine circulates in a close loop, while the drinking water passes directly through the ED stack(s).

PROCESS SCHEMATICS : 80 m³/h

Monovalent-selective membranes:

The main difference with other EDR processes is the use of monovalent-selective anion-exchange membranes that remove preferably the nitrates over other anions. If non selective membranes are used, the EDR process would not only remove the nitrates, but also the sulfates, chlorides, and bicarbonates: this would result in a "deionized" water. In addition, power and membranes are used to remove ions that do not have to be removed. It is thus necessary to selectively eliminate nitrates and preserve the initial characteristics of the treated water, mainly its calco-carbonic balance. The NEOSEPTA^® selective anion-exchange membranes used in the Eurodia / Ameridia result in an excellent nitrate selectivity, while sulfates and bicarbonates are not removed as much and chlorides are removed. The specific ion transfer depends on the equivalent fraction of the corresponding ion in the raw water, the mobility of the ion, the overall TDS reduction, and the membranes selection. As an example, for a 20 % TDS reduction in a sample of water, the removal rate could be 50 % for nitrates, 40 % for chlorides, 20 % for bicarbonates, and 0 % for sulfates.

Improvement in spacer and stack design:

Improvements in the Eurodia spacer technology has also allowed to increase the nitrate removal efficiency by ~30%. Over the last six years, we have continuously invested to improve our spacer technology for the removal of nitrates. A new netting structure was made available to increase the limiting current and thus the removal efficiency.

We have also developed a new stack EUR 40 for which the cell length is double than for the original EUR 20: 100 vs. 50 cm. Despite this length, the specific design of the spacer has reduced the TDS variation between its inlet and outlet. Therefore, it is possible to keep the same current density without any current limiting problems. In this case, a nitrate removal rate of 90 % can be reached.

Industrial Plant Performance:

Our first commercial plant was commissioned in 1991 in Italy and the membranes have not been yet been replaced. Since 1996, we have installed four more nitrate removal EDR plants through water treatment engineering companies in various European countries. Their characteristics are shown on Table 3 below. The interest for the EDR process to remove nitrates has dramatically increased in the last years.

TABLE 2 : EURODIA REFERENCES FOR NITRATE REMOVAL IN DRINKING WATER
(as of mid-2000)

Client

Country

Engineering Company

Number of Stacks

Delivery  Date

Daily Flow Rate (m³/d)

Christ

EUR20B-640

2

1991

1000

-

EUR20B-340

2

1995

250

Christ

EUR20B-730

2

1996

1200

AE&E

EUR20B-255

1

1996

120

Frame

EUR20B-850

1

1996

600

AE&E

EUR40B-560

3

1997

3500

SITE

MONTEFANO

IMOLA

KLEYLEHOF

Capacity (m³/d)

1000

600

3500

Stack model

2 EUR 20B-640

1EUR 20B-850

3 EUR 40B-560

Spacer type

EUR 20 Standard

EUR 20-W

EUR 40-W

Water source

Well

Well

Well

T°C

10-12

7-10

12

Design       Operation

NO₃ inlet (ppm)

91.8

52

85-90            120

NO₃ outlet (ppm)

34.1

8.9

10                  41

Removal rate (%)

63

83

89                  66

Waste stream

1

1.2

3.5

Water recovery: 93-98 %

Economical evaluation:

The following estimate gives an idea of the operating and investment costs for a complete EDR unit to remove 69 % of the nitrates in 80 m³/h of drinking water. It can be seen that the operating costs are less than U$ 0.05/m³. For 85 % nitrate removal, these costs will be less than $ 0.1/m³, depending on the starting nitrate level. The investment per m³ of water decreases when the capacity increases.

TABLE 4 : NITRATE REMOVAL FOR DRINKING WATER BY ELECTRODIALYSIS
PLANT ECONOMICS FOR 80 m³/h (21000 gal/h)

Note: With such a unit, it is possible to produce up to 144 m³/h of water that would meet the European Norms (50 mg/l).

Full-scale pilot results:

A full-scale EUR20B electrodialysis pilot unit with a capacity of 11 m³/h was installed at the municipality of Vouillé (Vienne, France) and ran for about five weeks in the summer of 1999.  The pilot unit was equipped with:

• 1 electrodialysis stack:      EUR20B – 300

• Number of stages:              2

• Flow / cell:                              70 L /h

• Membrane:                            NEOSEPTA ACS / CMX

The pilot unit was directly fed by the well pump at a flow of 11.5 m³/h and a temperature of 13°C. The raw water was filtrated in a cartidge filter of 10 µm. To dilute the concentrated loop, 0.6 m³/h of feed water wee used. The product flow was 10.9 m³/hr of drinking water with a nitrate level of  23 ppmm or a 69 % removal rate and 95 % water recovery. The pilot unit was equipped with an automatic current and hydraulic inversion. Every 20 minutes, the current and flows were switched to avoid any eventual electrodialysis membrane fouling.

The power consumption was approximately 100 Wh per m³ of treated water.

The average analyses of raw and treated water for the period are shown on the following table. The nitrate removal was 69 % and the hardness was reduced by 42 %. The pH has been reduced by maximum of 0.3 units. The various parameters recorded during the tests (TAC, TH, pH, cation and anion removal rates, conductivity, etc.) have remained stable.

TABLE 5 : VOUILLE PILOT PROGRAM – AVERAGE WATER ANALYSES

RAW WATER
(mg/l)

TREATED WATER
(mg/l)

Removal Rate (%)

Ca⁺⁺

108

61

44

Mg⁺⁺

3

2

35

Na⁺

13

12

8

K⁺

1

1

0

HCO3^-

  250

171

32

Cl^-

25

12

52

SO4⁼

18

17

6

NO3^-

75

23

69

TAC

20

14

32

TH

29

17

42

PH

7.9

7.7

3

TDS

492

298

39

Cond.

554 (mS/cm)

343

40

Note: Average of water analyses from June 20th to July 29^th, 1999