Water Technologies & Solutions technical paper

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trihalomethanes (THM) precursor reduction of surface water by EDR technology Abrera Drinking Water Treatment Plant, Barcelona Spain

challenge

Elevated levels of salinity in the Llobregat River area result of naturally-occurring calcium sulfate, sodium chloride and potassium chloride deposits in the river basin geology. Tailings for the potash mining industry are also found in the Llobregat River. Both of these sources contribute to the elevated levels of undesirable salt (Na, K, CL and Br.)

The drinking water treatment plant in Abrera, Spain, which serves the greater Barcelona area (about 4.5 million people), is owned by Aigües del Ter-Llobregat (ATLL), a public company as part of the Regional Government of Catalonia. The bromide levels along with the dissolved organic matter create a precursor for chlorination disinfection byproducts, called trihalomethanes (THMs). THMs have been shown to be carcinogenic, and need to be reduced so the water from the ATLL plant meets Spanish drinking water standard RD 140/2003, where the THM limit is currently 100 µg/L at the end of pipe use.

Attempts were made by ATLL to reduce the THM formation during treatment and distribution by reducing the chlorine addition at the plant. This was successful within the plant, but as chlorine was added through the regional distribution system, this additional chlorine and the contact time created THMs that were above the acceptable level.

Since THMs themselves are very difficult to remove, the idea at ATLL was to reduce the precursors like bromide and the organic matter prior to chlorination.

The original treatment plant was first built in 1980, and it consists of oxidation by potassium permanganate, flocculation/sedimentation, chlorine

dioxide oxidation, sand filtration, granular activated carbon, and post treatment chlorination.

However, these processes did not reduce the dissolved solids like bromide and the naturally occurring organic matter responsible for the THM formation potential. As a result, THMs were created in excess of the allowable drinking water standards. These individual THMs of concern at this plant are: CHBr3, CHBr2Cl, CHBrCl2 and CHCL3.

In addition, ATLL goal was to improve the global water quality of the Llobregat River to similar values of the Ter River, the main water source in the metropolitan area.

developing a solution

A solution was needed to reduce the THM potential while being compatible with the current treatment scheme. The lack of rainfall, the continued growing demand for water, and implementation of Spanish drinking water regulation RD 140 led to the pursuit of a treatment strategy that both met the quality goals, but also high water treatment efficiency. In addition, the THM solution must deal with the quality and quality of the existing treatment works.

Initially, electrodialysis reversal (EDR) and reverse osmosis (RO) were considered to be possible solutions. It was determined that a pilot program was required to validate the projected THM precursor removal, understand the compatibility existing works effluent with these solutions, and use real data to design a plant that effectively and efficiently met the water quality.

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Pilot Plant

The largest unknowns in the THM precursor solution were: Will the solution remove the THM precursors? Will the solution be compatible with the water from the existing works? The purpose of the pilot plant was to provide a means of generating performance data to both select and optimize the design of a solution. Table 1 shows the river source water characteristics.

Table 1: Llobregat River Facts

Characteristic Amount

Conductivity range: 500 to 2,500 µS/cm

Bromide 0.5 – 1.2 mg/L

Chloride 150 – 1,300 mg/L

Ba 30 – 190 µg/L

Sr 1,100 – 2,200 µg/L

TOC > 5.0 mg/L

Both reverse osmosis and electrodialysis reversal were piloted, starting in 1999. The EDR pilot program, initiated in April 2004, tracked performance of the EDR unit as variations in bromide, organic matter, salinity, and temperature were experienced.

The RO pilot lasted six months, and during this time, the following challenges were observed:

• The RO water recovery rate was lower than desirable due to the high levels of scaling minerals like sulfates, barium, calcium and alumina

• Biological fouling was also a challenge to the RO membranes and chlorine could not be used on RO membranes

• The turbidity and SDI resulting from the existing pretreatment was too high and variable for an RO system and frequent cleanings were required.

• The variability in the river quality, including salinity, turbidity, and chemical pollution was challenging to adapt the RO operations without

any additional pretreatment after the existing plant, like ultrafiltration (UF).

The SUEZ EDR pilot performed well with a length of 28-month demonstration.

The biggest problem with THMs formation occurs in the summer months where the development of THMs happens faster at warmer water temperatures. This formation happens somewhat upon initial chlorination, but also as the finished water flows through the distribution network.

Fortunately, EDR removal efficiency is also greater on warmer waters, compensating for the higher THM formation potential of this water in the summer. The EDR pilot showed that the THM formation potential (THM-FP) before the EDR was 160+ / -40 µg/L, and after the EDR it was 64+ / -16 µg/L.

The SUEZ EDR pilot validated that:

• The SUEZ EDR removed a sufficient amount of the THM precursors, so the treated river water was below the THM limit.

• No additional pretreatment to EDR was required on the existing works effluent.

• The EDR met its performance measurements even during seasonal variations in river water salinity and temperature.

• The SUEZ EDR pilot demonstrated conditions of a 90 percent water recovery plant.

• EDR had lower remineralization and power costs than RO.

The pilot was also an opportunity to provide a large amount of data to optimize the design of a full-scale plant. Experiments included comparing results of single versus double stage EDR design, temperature variation, flow variation and electrical power optimization.

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The SUEZ EDR uses a Polarity Reversal feature to prevent the accumulation of organic and inorganic foulants and scale. In addition, the SUEZ EDR membranes are compatible with a free chlorine residual, so cleaning the EDR with a solution with free chlorine is an element of flexibility that effectively and inexpensively reduces organic foulants. These features contribute to long membrane life, low maintenance costs and high reliability even on this challenging water under dynamic conditions.

integration of a solution

Based on favorable piloting results, the SUEZ EDR technology was selected as the THM precursor reduction solution. The project, consisting of EDR and other plant upgrades, cost 61 million Euros, and has a capacity of 200,000 m3 per day. The EDR plant, consisting of nine EDR units, began the commissioning in February 2008. It took about one year, as the existing plant works had to deliver water to the population. This facility is currently the largest EDR plant in the world, and one of the largest brackish water desalination plants of any technology type. Due to the water scarcity condition in Spain, the plant is designed to operate at 90% water recovery. (See Figure 1.)

Effluent from the existing carbon filters is sent to a 3,000 m3 feed tank, and 9+3 EDR feed pumps pressurize the water and transport it from the tank, through the cartridge filters (2 per EDR unit, having 270 cartridges at 5 micron nominal rating) and 576 SUEZ EDR Mark 4-2 membrane stacks, 64 per unit (Figures 2 and 3). The stacks are energized with DC power using DC drive technology that is enclosed in a group of electrical cabinets in the electrical room. The membrane stacks are composed of 600 cell pairs, each consisting of:

• Cation exchange membrane

• Flow spacer

• Anion exchange membrane

• Flow spacer

• • Figure 2: SUEZ EDR Mark 4-2 Cell Pair

• • Figure 3: SUEZ EDR Mark 4-2 Membrane Stack

Llobre-gat River

Max 4 m3/s

Figure 1: Treatment diagram

KMnO4

DEC

Coagulant

Flocculant

SAND FIL-TER

GAC FIL-TER

edr

2.3 m3/s

Main flow

rem 263,000m3

1.7 m3/s

Complementary flow

Chlorine dioxide

Chlorine

Chlorine

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• The membranes are homogeneous in composition for low electrical resistance. The flow spacers, which govern the flow of the water across the membrane surface, consist of a screen mesh material to induce turbulence. The two electrodes in each membrane stack are platinum coated titanium for long life and the need to operate as both a cathode and an anode during normal operation.

• Each of the nine EDR hydraulic skids also has 1+1 redundancy concentrate recirculation pumps. The concentrate water from each of the EDR units is collected and sent via pipeline to the Mediterranean at the mouth of the Llobregat River, 30 kilometers (18.6 miles) away from the plant.

• • Figure 4: 200,000 m3/day SUEZ EDR at ATLL site

• Each EDR hydraulic skid has provisions for scale control using antiscalant and hydrochloric acid, and a clean in place system, which can use sodium hypochlorite, salt solution or hydrochloric acid as, needed.

• Post treatment of the EDR effluent consists of a remineralization step to meet Spanish regulation RD 140/2003 for stabilization and corrosion potential in transmission pipes. This will be used when required and is accomplished using lime and CO2 addition to increase the Langelier Saturation Index of the water prior to distribution.

• Consolidated text prepared by William Harvey and Juan Carlos de Armas.

references:

1. Fernando Valero, Juan C. García, Santiago González, MªEugenia Medina, Juan Carlos de Armas, Manuel Hernández, José J. Rodríguez, “Control of THMs at the Llobregat DWTP (NE, Spain) using Electrodialisys Reversal (EDR)”, International Desalination Association, October 2007.

2. Mario Ferrer Arasa, Fernando Valero Cervera, Domingo Zarzo Martínez, Xavier Vives Rifé, “Abrera (Barcelona, Spain) Drinking WTP Upgrade through Electrodialysis Reversal (200,000 m3/day)”, International Desalination Association, October 2007.

3. Mario Ferrer Arasa, Domingo Zarzo Martínez, Raúl Lemes, “Startup and Operation of The World’s largest EDR water Treatment Plant”, International Desalination Association, November 2009.

4. Fernando Valero, Ramón Arbós. “Desalination of brackish river water using Electrodialysis Reversal (EDR). Control of the THMs formation in the Barcelona (NE Spain) area”. Elsevier, 2009.