HIGH RECOVERY DESALINATION IN DEVELOPING COUNTRIES

Malynda Cappelle, UTEP Center for Inland Desalination Systems

500 W University Ave/Kelly 208/El Paso, TX 79902

Email: macappelle@utep.edu Phone: 915-747-8953

Thomas A. Davis, UTEP Center for Inland Desalination Systems, El Paso, Texas

W. Shane Walker, UTEP Civil Engineering Department, El Paso, Texas

Abstract

Fresh water scarcity is a global problem. Rivers, aquifers, and other fresh water resources are

becoming more strained because of population increases, climate change, and overallocation.

According to the U.S. Agency for International Development (USAID), global water demand is

expected to increase by 55% by 2050, and much of this increase is related to food production.

The USAID and the Bureau of Reclamation, in partnership with the Swedish International

Development Cooperation Agency and the Ministry of Foreign Affairs of the Kingdom of The

Netherlands, sponsored the Desal Prize in April 2015. This competition was designed to spur

innovation in efficient desalination technology development for small, rural farmers in

developing countries. The University of Texas at El Paso (UTEP) was one of five finalists to

compete. Teams were required to produce 250 liters of drinking water with less than 600 mg/L

total dissolved solids (TDS) and 8000 liters of irrigation water with no more than 550 mg/L TDS

with appropriate calcium, magnesium, and sodium concentrations suitable for crop irrigation.

The desalination systems were powered completely by renewable energy.

UTEP’s process is a PV-powered hybrid membrane process called Zero Discharge Desalination

(ZDD). ZDD can achieve greater than 95% recovery and is able to recover (1) a solid byproduct

(mostly gypsum) which can be used to improve soil conditions and (2) a liquid stream (mostly

NaCl) which is used in the EDM process. This paper describes the ZDD design process and

optimization, as well as plans for pilot testing in Honduras in 2016.

Background

Water scarcity is an issue for people, the global economy, and the environment. According to the

UN, groundwater is the sole source of water for an estimated 2.5 billion people globally. Seven

hundred million people experience water scarcity; this figure is estimated to increase to

1.8 billion by the year 2025 (United Nations, 2013). These figures include physical (lack of

available usable water) and economic water scarcity (lack of adequate and affordable treatment

systems, storage, distribution, access of usable water). Compounding this problem is the need for

water from the agriculture sectors around the world. Globally, about 70% of the world’s fresh

water is used for agriculture (United Nations, 2015).

Using data from The World Bank for freshwater withdrawals (includes desalination) and GDP,

some interesting observations can be made. First, if all countries’ data are graphed (Figure 1),

there is a fair amount of scatter. In general, increasing water withdrawals are associated with

increased GDP, but the correlation is weak (R2=0.45). If the same dataset is graphed by

geographic location and income levels (Figure 2), the correlation between freshwater extraction

and GDP is more apparent (R2=0.58). In general, as more freshwater is extracted, GDP increases.

The UN estimates that 850 million people in rural communities lack access to reliable irrigation

water for agriculture, their primary source of income (United Nations, 2015).

Honduras is a striking example of a country with a potential for growth with more sustainable

water resources. In part of the country, they have lost 70% of their crops due to drought. In other

parts, farmers’ wells have gone brackish and the water is unusable without severe losses in crop

yield. Desalination would be able to supplement or replace a portion of the water supply in many

cities and regions, but conventional approaches like reverse osmosis are usually limited to 75-

85% recovery. Sustainable concentrate management is key to the implementation of desalination

in inland areas. ZDD offers up to a 33% improvement in recovery compared to conventional

approaches and is able to provide useful byproducts instead of a liquid waste stream.

Figure 1. Global water extractions and economy (by country)

Sources: The World Bank, 2015a and 2015b

Figure 2. Global water extractions and economy (by category)1

Sources: The World Bank, 2015a and 2015b

Remote Desalination with Renewable Energy

Desalination can be grouped into two categories: phase change (thermal desalination) and

processes that utilize membranes. Thermal desalination processes include multi-stage flash

(MSF), multiple effect distillation (MED), vapor compression (VC), freezing,

humidification/dehumidification and solar stills (Charcosset, 2009). Membrane desalination

systems include reverse osmosis (and nanofiltration), electrodialysis (ED), and membrane

distillation. Of all desalination systems powered by renewable energy, reverse osmosis accounts

for the vast majority (62%), followed by MED (14%), MSF (10%), VC (5%), ED (5%), and

others (4%) (Ghaffour et al., 2015). Ghaffour also summarized the distribution of installed

capacity with respect to specific combinations of renewable energy and desalination, as shown in

Table 1.

1 OECD stands for Organisation for Economic Co-operation and Development

Table 1. Distribution of renewable energy desalination by source

Source: Modified from Ghaffour et al., 2015

Renewable Energy Source Desalination Type Installed Capacity (%)

Photovoltaic Reverse Osmosis 32

Wind Reverse Osmosis 19

Solar Thermal MED 13

Photovoltaic Electrodialysis 6

Solar Thermal MSF 6

Wind Vapor Compression 5

Others 19

Tradeoffs & Cost Comparison

In general, thermal desalination processes require substantially more energy than membrane

processes, but pairing renewable energy sources may allow for more implementation. Thermal

processes may be more forgiving in terms of biological fouling and scale formation. Membrane

processes typically offer higher recovery, but are typically more susceptible to fouling and

scaling.

The cost of desalination is related to many factors, some of which are in competition with one

another – especially when considering remote, inland, arid locations. The salinity, pre-treatment

requirements, temperature, and recovery (product water/feed water) will impact the cost of the

desalination system and will impact the size (and type) of the renewable energy system (Table

2).

Table 2. Cost tradeoffs in desalination design

Capital Cost Operating Cost

↑Salinity ↑ Cost ↑Salinity ↑ Energy Cost

↑Pre-treatment ↑ Cost ↑Temperature ↓ Energy Cost

↑Recovery ↑ Cost ↑Pre-treatment ↑ Chemical Cost

↑Energy Intensity ↑ Cost ↑Recovery ↑ Energy Cost

↑Post-treatment ↑ Cost ↑Post-treatment ↑ Chemical Cost

↑Product Water Quality (↓TDS) ↑Cost ↑Salt Recovery ↓ Operating Cost

(potentially)

Many researchers have evaluated desalination systems that utilize renewable energy, and good

summaries are provided by Ghaffour et al. (2015) and Charcosset (2009). Again, many factors

affect the cost of desalination systems, but some general trends are observed when comparing

systems, as shown in Figure 3. The thermal approaches appear to have a lower cost per unit of

product water ($/m3); however, they generally have very low flux (≤ 5-10 LMH) and low

recovery (<40%). Many of the sites where pilot testing has occurred, or where renewable/remote

desalination systems have been installed, intentionally target low recovery so that the concentrate

can be used for other non-potable purposes such as toilet flushing, showering, and feed stock

(Werner & Schafer, 2007) or can be disposed of with less environmental impact. One must

optimize the water recovery with capital and operating costs so as to maximize the amount of

water produced from brackish groundwater while minimizing the impact of concentrate disposal.

Figure 3. Cost comparison of desalination with renewable energy

Source: (Ghaffour et al., 2015)

The Desal Prize

In 2014, the Securing Water For Food (SWFF) program initiated a call for innovation in

desalination for small farms in the developing world (Securing Water for Food, n.d.). The

program was aimed to encourage technology development and demonstration that would enable

environmentally sustainable small-scale brackish water desalination systems powered only by

renewable energy. Five semi-finalists competed at the Brackish Groundwater National

Desalination Research Facility (U.S. Bureau of Reclamation) in Alamogordo, New Mexico, in

April 2015. The performance criteria were designed to challenge the competitors to innovate

with respect to renewable energy, high recovery, and high quality product water. Over two

separate 24-hour periods, each team was required to produce 8,000 L of agriculture water with

TDS < 550 mg/L, pH < 8, B ≤ 0.5 mg/L, SAR < 3, and Ca:Mg > 1, as well as 250 L of drinking

water with less than 600 mg/L TDS that also meets the drinking water guidelines from the World

Health Organization. If the technological approach was clear and met the contest rules and if the

water quantity and quality criteria were met, then teams would be judged on a numerical scale

including five additional criteria (Table 3). Since high water recovery and salt recovery could

potentially account for 50% of the score, UTEP competed using a photovoltaic-powered Zero

Discharge Desalination (ZDD) system. The MIT team won first place with their PV-powered

electrodialysis reversal (EDR) system, and the UTEP team won second place. These top two

teams were invited to submit proposals to demonstrate their technology at an international

location (in a USAID mission region).

Table 3. Judging Criteria for 2015 USAID Desal Prize Competition

Performance Criteria Scale Weight

Technological Approach Yes/No --

Water Quantity & Water Quality Yes/No --

Powered Solely by Renewable Energy Yes/No --

System Water Recovery 1-4 30%

Chemical Treatment 1-4 15%

Concentrate Minimization/Concentrate Disposal Process 1-4 20%

Durability, Reliability, and Practicality 1-4 15%

Life Cycle Cost Analysis 1-4 20%

Planned Pilot: Honduras

The UTEP team has partnered with a Honduran university, Universidad Politécnica de

Ingenierías (UPi), the national water service of Honduras, Servicio Autonomo Nacional de

Acueductos y Alcantarillados (SANAA), and an American company, JCI Industries, to

demonstrate ZDD in Honduras. UPi is a relatively new private university that specializes in

engineering and environmental sciences. They will assist with training activities. SANAA has

assisted UTEP in locating potential pilot sites and providing preliminary water quality analysis.

JCI Industries supplies and services water & wastewater equipment for both the industrial and

municipal markets.

Piloting activities are expected to begin in early 2016 in the region of Valle, Honduras

(southwest of Tegucigalpa) or another site. SANAA, in coordination with the other national

partnering agencies, has identified three potential brackish water well sites in Valle and another

potential site in Tegucigalpa. All brackish water wells are located on 3-5 acre farms of plantains

and watermelons in the in Ojustal Costa de Amates in the County of Alianza in the Department

of Valle in southern Honduras. These wells are active, but their brackish condition limits the

usage of their water. The pilot demonstration will last at least one month and will involve

transferring knowledge and technology to farmers and regional trainers (professors, agricultural

extension agents, etc.). Local and regional suppliers will be identified for equipment and

consumables necessary for the pilot and future installations. A suitable concentrate management

strategy, with possible salt recovery, will also be investigated. The proposed work will address

the following issues requested by USAID:

Simplification of the operational process to include control set points through

automation

Development of an enhanced evaporative process for reduction of concentrate volume

and recovery of salts

Demonstrate the recycle of NaCl recovered from the concentrate as feed in the

process (i.e., closing the NaCl loop)

Market study to assess and target the unit costs and plan to reduce cost to achieve

market access in Honduras

Detailed description of plans to work with local agriculture extension agents to

provide farmer education and outreach

State of drought in Honduras

Valle is located in one of the most arid areas in Honduras. The Vice President of the Honduran

Association of Farmers (ASOHAGRI), stated that in 2014, the farmers in Valle and Choluteca

(east of Valle) lost approximately 70% of their crops as a result of the drought. Average rainfall

decreases towards the south and west in Honduras. The average annual precipitation is 29.6

inches (75 cm), and Figure 4 shows the average monthly rainfall for Valle (World Weather

Online, n.d.).

Figure 4. Average monthly rainfall in Valle, Honduras

The drought conditions in Valle result in critical impacts on water availability for human

consumption and especially for crop irrigation. The impact on agriculture by the yearly droughts

in southern Honduras, especially in Valle, Choluteca and El Paraiso has implications ranging

from the reduction of cultivated areas and the consequent drop in production, resulting in

unemployment. The multiplier effect from this condition means serious economic, social and

environmental consequences for the citizens of these regions. The environmental consequences

caused by the drought can be seen in the transformation of the landscape by the abandonment of

farmland resulting in additional arid territory.

The chief of the Ministry of Agriculture and Livestock (SAG), has stated that measures need to

be implemented to ensure food security and a steady income for families in the region. However,

the erratic and low rainfall in the south of Honduras makes this mandate difficult.

According to a 2013 USAID report (USAID, 2013), fish and shellfish, which are harvested from

the Gulf of Fonseca, are important ecosystem products that support livelihoods in Honduras.

Additionally maize and beans, which are grown on small-scale subsistence farms, are the

“foundation for food security in the area” and depend on the reduction of soil erosion and

maintaining soil fertility. Sugar, melons, and shrimp generate a “very important fraction of the

employment and all income that drives the economy of the region.” Reliable fresh water is

necessary for this to occur. “All current livelihoods (subsistence to agro-industrial) are sensitive

to changes in ecosystem products and services and will need to adapt to potential climate-driven

changes.”

Equipment Description

ZDD has been described in other papers and sources (Cappelle, et al., 2015), (Cappelle & Davis,

2016 (in press)) and is briefly summarized here. As illustrated in Figure 5, brackish water is fed

to the primary desalter, which will be fitted with reverse osmosis (RO) and/or nanofiltration (NF)

membranes. A small portion of the permeate will be sent to a second NF to produce high quality

process water needed for salt solution preparation, stream dilutions, and system shutdown

flushes. The concentrate from both the primary RO/NF and process NF will be sent to an

electrodialysis metathesis (EDM) system where approximately 50% of the ions will be removed,

and the treated stream from the EDM (diluate) will be fed back to the original NF for additional

water recovery. The only chemical feed for the EDM is salt (NaCl). Cations in the NF

concentrate will combine with the Cl- from NaCl to form a highly soluble mixture in a highly

concentrated waste stream called mixed chloride. Anions in the NF concentrate will combine

with Na+ from the NaCl to form a highly soluble mixture in a highly concentrated waste stream

called mixed sodium. These waste streams, one which in calcium and the other rich in sulfate,

can be combined to precipitate calcium and magnesium salts (mostly gypsum and calcium

carbonate) that can be used to improve irrigation water quality. The supernatant liquid will

mostly contain NaCl, which can be recycled as feed for the EDM. The estimated hydraulic

recovery of the entire treatment system is 93-95%.

Figure 5. Process flow schematic for proposed ZDD system in Honduras

Design Optimization

The irrigation water quality objectives for The Desal Prize are required for the pilot

demonstration and will dictate the maximum recovery achievable by the ZDD system. These

standards were designed to provide minimal impact on water infiltration and crop yield. Several

sources provide guidance for irrigation water and salinity management (Grattan, 2002; Ayers &

Westcot, 1994; Fipps, 2003; Yadav & Massoud, 1988). Water quality parameters and their

effects on agriculture include:

Salinity: each plant has a different tolerance for water salinity. Some can be irrigated for

a short period of time with saline water, however, salts will build up in the soil and will

need to be flushed.

Sodium content: a high concentration of sodium relative to calcium and magnesium

(calculated using the sodium adsorption ratio, or SAR2) will lead to reduced infiltration.

Toxicity of certain constituents: boron, selenium, and chloride can cause severe damage

to crops and recommended limits should be implemented for the best yield. Some heavy

metals, like arsenic, can be taken up by crops (e.g., rice) and could present health

concerns for those eating them.

Figure 6 summarizes how salinity and SAR impact crop yield and water infiltration. Table 4

summarizes salt tolerance for certain crops as well as recommended maximum concentrations for

certain contaminants. Desalinated water from membrane processes will typically have a higher

SAR than the feed, because membranes have a higher rejection of large, charged species than

smaller (or uncharged) species. The product from both membrane and thermal processes will

likely require blending or other methods to boost the calcium, magnesium, and other nutrients

needed by plants.

2 SAR is calculated using the following equation. 𝑆𝐴𝑅 =

[𝑁𝑎]

√([𝐶𝑎]+[𝑀𝑔])/2, [x] are concentrations in meq/L

Figure 6. Effects of conductivity and SAR on (a) crop yield and (b) infiltration rate

Sources: Dow ROSA and GE Winflows RO design programs; water quality guidelines from

Ayers & Westcot, 1994

Table 4. Crop sensitivity to salinity, chloride, and boron

Sources: Grattan, 2002; Ayers & Westcot, 1994; Fipps, 2003; Yadav & Massoud, 1988

Crop Max. water salinity

(conductivity)

(S/cm)

Max. chloride

concentration

(mg/L)

Max. boron

concentration

(mg/L)

Beans 700 350 --

Corn 1,100 525 2-4

Potato 1,100 525 1-2

Spinach 1,300 700 --

Zucchini 3,100 1,575 2-4

Cotton 5,100 1,625 6-15

Wheat 4,000 2,100 0.75-1

Sorghum 2,700 2,450 4-6

Barley 5,000 2,800 0.75-1

At the time of this writing, a full water quality analysis was not available. However, several

membrane configurations were evaluated using the most complete water analysis in hand for a

well called “Los Almendros” (feed water in Table 6). The Los Almendros well is a brackish well

that is available for the municipal drinking water system and is somewhat representative to other

brackish wells in the region. The choice of membrane for the primary desalter will be based

whether silica is present in the brackish water and the permeate TDS and SAR. Silica is rejected

very well by most RO and NF membranes but is not removed by the EDM. Without a purge

stream, silica will concentrate in the recycle loop between the RO/NF concentrate and EDM

feed/diluate (silica rejection is associated with salt rejection). Dow NF270 and GE HL2540FM

membranes are well-suited for allowing silica passage, which is a desirable feature in the design

of this ZDD system. Table 5 compares the membranes under consideration. Interestingly, the GE

AK Series membrane is predicted to produce a better (lower TDS) permeate and pass more

silica. GE’s H-series product sheet lists a molecular weight cutoff (MWCO) of 150-300 Daltons

for uncharged organic molecules (GE Water & Power, 2013). Dow’s website suggests the

MWCO for the NF270 membranes is around 400 daltons (Dow, 2015); this might explain how

the GE membranes allow more passage of silica while still producing better quality permeate.

Table 5. Comparison of select 2.5-inch membranes for primary RO/NF design

Source: Product sheets found on GE and Dow websites (GE, n.d.) (Dow, n.d.)

Membrane Dow

XLE

Dow

NF90

Dow

NF270

GE

AK Series

GE

HL Series

Salt Rejection >99% >97% >97% >96% (99%

average)

>95% (98%

average)

Challenge Salt 200 ppm

NaCl

2,000 ppm

MgSO4

2000 ppm

MgSO4

500 ppm

NaCl

2000 ppm

MgSO4

Active area (ft2) 28 28 28 27 27

Permeate Flow (gpd) 850 700 850 710 780

Max Pressure (psi) 600 600 600 400 450

Max Pressure Drop

(psi)

13 13 12 12

A combination of low-pressure RO and lower salt rejection membranes will likely produce the

best quality permeate while also allowing enough silica to pass through into the RO permeate to

minimize the volume of purge water needed to prevent silica scale formation (with antiscalant

addition). A spreadsheet model was used to evaluate the ZDD system with different RO/NF

membrane combinations. This model is able to estimate the steady state silica for each design.

Table 6 summarizes the predicted RO/NF process stream compositions at steady state. The GE

membranes produce permeate with a higher SAR and lower Ca:Mg ratio. This can be improved

with the recovered CaSO4 product from the EDM concentrate streams and still meet the required

TDS limit.

Table 6. Comparison of product water quality from select RO membranes

Brackish

Feed (Los

Almendros) XLE/NF270 GE AK/HL

GE AK/HL +

150 mg/L

CaSO4

pH 7.6 7.1 7.1 7.1

TDS 3064 543 362 512

SAR 2.8 2.0 3.8 2.4

Ca:Mg 2.0 1.2 0.4 1.0

Na 276.0 76.0 76.6 76.6

Mg 144.0 16.3 4.3 4.3

Ca 480.0 79.9 23.4 67.4

HCO3 383.8 84.3 85.7 85.7

NO3 2.56 1.63 1.11 1.11

Cl 712 243 112 112

SO4 1032.0 20.4 30.5 136.1

SiO2 30.0 20.9 28.9 28.9

Feed P (psi) 93 98

Recovery (%) 94 98

Concentrate SiO2 125 80

As described earlier, solid calcium salts will be recovered from the EDM concentrate streams.

This solid can be added to the water or soil, depending on the actual type of calcium present

(Fipps, 2003). If the byproduct has too much calcium carbonate, it will be added directly to the

soil as an amendment. If it is mostly (or contains enough) calcium sulfate, it can be added to the

water.

Photovoltaic System

The photovoltaic (PV) system used for the Desal Prize competition (Figure 7) was designed for

maximum reliability and performance. Since the competition required discharging batteries to

50% charge and April weather can be variable, a large capacity system was designed. The PV

system comprised: (a) a 10.3 kWp array (42 Canadian Solar 255 Watt panels), (b) a 4 kWp

inverter (Shneider Electric Conext 4024), (c) four charge controllers (Xantrex XW MPPT), and

(d) a 24-VDC battery system (40 Trojan L16H 6 V batteries, each with a capacity of 435 AH for

20 hrs, for a total of 104 kWh). The average electrical load of the entire desalination process

during the April 2015 competition was approximately 3.1 kW. The desalination container to be

transported to Honduras will have a different load depending on the actual flowrate and salinity.

The optimized design minimizes the size of the RO/NF pump, shifts as much equipment from

AC to DC power (saves energy by eliminating need for inversion), and reduces redundant pumps

and other systems. However, the batteries will now be installed inside the equipment trailer,

which will require venting, and minimal cooling is necessary for long term operation.

Figure 7. Photovoltaic system with (a) solar panels, (b) batteries, (c) power inverter, and (d)

charge controllers

Conclusions

ZDD is a commercially viable process that has been evaluated in Texas, New Mexico, Colorado,

California, and Florida. ZDD is a hybrid process which uses reverse osmosis or nanofiltration as

the primary desalter for the production of drinking and irrigation water. The concentrate from the

primary desalter is sent to a process called electrodialysis metathesis or EDM, which acts as a

kidney by removing troublesome salts. The EDM’s product is returned to the primary desalter

for recovery of additional water. Useful byproducts can be recovered from the EDM waste

streams, namely gypsum (which can improve soil quality) and NaCl (which is needed in the

EDM). The waste management strategy employs low cost collection, mixing, and concentration

techniques and is expected to achieve at least 94% water recovery with zero liquid discharge.

The primary goal with the planned pilot is to transfer what has been learned in research projects

into processes to purify water for farmers around the world. With funding from USAID and other

sources, we are planning a pilot demonstration in Honduras in early 2016. We are partnering

with UPI, a university in the capital city, Tegucigalpa, for this endeavor. The pilot will

demonstrate our full concept of ZDD for rural applications, including the potential for salt

recovery. The system will be powered entirely by the sun.

Acknowledgments

This paper is made possible by the generous support of the American people through the United

States Agency for International Development (USAID). The contents are the responsibility of the

authors and do not necessarily reflect the views of USAID or the United States Government.

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