So, what is our technology? It is called Zero Discharge Desalination (ZDD), and it was invented by Dr. Tom Davis (a member of our team) after being inspired by learning about the challenges facing Alamogordo, New Mexico, in their desire to design a desalination plant. Alamogordo's water quality is very challenging from a desalination standpoint because it has many dissolved constituents which cannot be concentrated very high without causing problems in the membranes. The "Los Almendros" brackish water source we are using in Tegucigalpa has similar characteristics: high calcium and sulfate (dissolved gypsum) and silica (think sandy beaches). It also has an additional bonus challenge: it has arsenic. ZDD is unlike other desalination technologies in that it removes the troublesome species instead of concentrating them and is able to produce useful solid and liquid byproducts.
How does it work? ZDD is a hybrid membrane process that combines two types of membrane technologies. The first process utilizes pressure to push water molecules through semi-permeable reverse osmosis and nanofiltration (I'll call this RO/NF) membranes to produce water that can be used for irrigation. Since there is arsenic in the water, we use a smaller reverse osmosis and ultrafiltration system to treat the RO/NF product and produce water that would be suitable for drinking water. If this system were to be used without further treatment, it would be limited to 75-80% efficiency. However, in ZDD, the RO/NF brine stream (called concentrate) is fed to a second system called electrodialysis metathesis (EDM). The EDM acts like a kidney, in that it removes troublesome salts from the concentrate. The EDM's desalinated product (called diluate) is returned to the front of the RO/NF to recover additional water. The combination of RO/NF and EDM is capable of achieving up to 98% efficiency. The remaining EDM streams can be combined to recover solid gypsum, which is useful for agriculture, and salt, which is needed in the ZDD process. The combination of ZDD with salt recovery will allow us to achieve zero liquid discharge - all while using power from the sun!
Where has it been tested? ZDD has been tested by UTEP and other researchers in Texas, New Mexico, Colorado, California, and Florida. It is a niche technology that excels in desalinating brackish groundwater with high levels of hardness ions (calcium, magnesium) and sulfate.
Our goals in our demonstration project are to:
- Demonstrate that high efficiency and zero liquid discharge are possible using ZDD powered by renewable energy
- Transfer knowledge to our partners at UPi
- Provide training for Honduras farmers about water quality and desalination technology (including ZDD, of course!)
- Assess commercial potential for ZDD in Honduras
That's a lot to accomplish in two months, but we are a hard working and determined group of folks with a common purpose.
So, what have we been up to? I love pictures, I hope that you do too.
The first step in starting any pilot system, especially after an overseas journey, is to fill each system and check for leaks and loose connections. Our system was built by two skillful men (believe me, if I had remembered to get their permission, their names would be posted!) at Industrial Water Services in El Paso, Texas, who worked with us to have a system that would build upon our years of pilot testing AND be repairable if something got damaged during shipment. Of course, when you plan for things to break, Murphy's Law states tat nothing breaks. In actuality, we only had one cracked fitting and two loose fittings - this is pretty amazing. IWS has supported our research efforts at UTEP and have even been available for assistance while we are in Honduras.
After the leak tests, we started building our EDM stack. This involves placing membranes and spacer materials in a specific order and making sure that they are aligned so that the stack performs optimally. We built the stack in the garden of our apartment (lovely location!) and it took a few tries since we had some new pieces that seemed to be causing leaks.
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| EDM Stack building. We have four different types of membranes and spacers that are placed in between each membrane in a specific order. |
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| Our completed EDM stack in our apartment garden. I'm a kid from the desert, so the plants and trees here are especially lovely. |
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| Third time's a charm? I didn't want to share that we had to rebuild the stack a few times, but it is reality that we make mistakes or have to make adjustments in the field sometimes. (photo credit to Victoria Amador, check out her post here) |
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| EDM System in operation! |
Finally, we turned on all systems and checked to see if alarm setpoints and shutdowns are working and to get a preliminary glimpse at the water quality produced. So far, we making good quality water from a salinity standpoint. Water quality analyses are needed to determine whether our design calculations were accurate. Based on our preliminary test run, we expect to be able to meet the water quality targets set by the Desal Prize and USAID.
The main irrigation water quality goals:
- Total dissolved solids (measure of salinity) of less than 550 ppm (parts per million)
- Sodium Absorption Ratio (measures the relative amounts of calcium and magnesium relative to sodium; important for irrigation) less than 3
The main drinking water quality goals:
- Total dissolved solids less than 600 ppm
- Arsenic less than 10 ppb (parts per billion)
And one final accomplishment, thanks to UPi. We have stairs! We had been using ladders to get in and out of the container until this week. We even got Ingeniero Luis Eveline involved (he sanded the handrails!). Entering and exiting our container is much easier now. Just in time for training sessions! One thing that I really like about these stairs is that they are built with used wood. I'm always a fan of recycling and upcycling, so this is a great example of that in practice.
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| Clara Borrego & Victor Zepeda showing off our new stairs (and cooling off in the process) |
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