Understanding The Salt Problem

Overview

The #salt or #salinity problem can be defined as the increasing salinization of our soils and drinking water. This is a global problem. Along the coast where I live, aggressive farming and water use coupled with frequent droughts and have enabled seawater to penetrate miles inland. Once the seawater reaches your drinnking well, you are in trouble. What to do? On a larger scale, what happens to farmers when their wells accumulate too much salt? Like many things in the natural world, man-made disturbances can alter these balances and over time cause enormous financial and food security problems. This problem is already acute in the central valley of California.

The Main Idea

Salt, what salt? A salt is basically a molecule that contains cations (-+charged) and anions (+=charge). You use a salt probably everyday in your diet such as sodium (+) chloride (-). So whats the problem? The problem is that salts in small quantities are necessary for your diet. Salt in higher concentrations can damage plants and crops and cause human health issues. If we engage in open loop processes, where the output has no effect on the input, salts can easily accumulate. Couple this with a climate change driven (#climatechange) drought which drives aggressive groundwater pumping and you a prescription for slow moving disaster. Existing processes for water treatment only exacerbate this problem as they separate salt from water but then create secondard problem- large quantites of very saline brine that cannot be use- or can it?

#Chloride

Many crops grown for food have extreme sensitivity to chloride at levels around 100 mg/L. Unfortunately, chloride is not a criteria water pollutant. There is a recommended concentration (about 250 mg/L). The problem we have is that much of ground water we are pumping is already above 100 mg/L. For crops like avocados (got to have that guac!), yield loss almost immediate at concentration above 100 mg/L

In drinking water, we can separate chloride from the water by forcing the water at very high pressure across a membrane. The #permeate is chloride and sodium free but the #concentrate, often 50% of the incoming water, now contains 2X the concentration of salt. Where do you put that concentrated salt solution or brine? If you take the approach that nature is your bathroom you just pump the water into a local field and let the water percolate back into the ground water with the salt. If you get frequent rains, no problem as long the aquifer can "recharge". If you don't, are you not drawing down the available groundwater and may simply make the groundwater continuously more saline? Then what?

#Sulfate

Sulfate is other anion of concern. Sulfate is applied in very large quantities in agriculture and is an essential nutrient for crop growth. What happens to the sulfate not used by the crop? It passes through the field and ends up in our groundwater and surface water. Unlike chloride, sulfate can react over time with other chemicals to cause a wide variety of problems. Like the chloride problem, excess quantities of sulfate in irrigation water can negatively affect plant growth and yield.

Some inland agricultural areas have resorted to using us very large quantities of ground water filtered by reverse osmosis. The resulting brine is not drinkable and cannot be used to irrigate plants that are sensitive to salts. What do we do this? Filtering again may reduce the volume but the concentration of salts goes up an 2X (approximately) making the brine even more damaging.

Possible Solutions

We've studied both existing, emerging, and some novel solutions to the many problems associated with salinity. Man-made technologies are good at separating salt from water but all create secondary problems. For example:

• membrane filtration- creates a concentrate brine

• ion exchange- creates a a concentrated brine

• forward osmosis- creates a concentrated brine

• electrodialysis- creates a concentrated brine

The literature on the latter two (forward osmosis and electrodialysis) is full of claims about how they can improve RO or lower the energy requirement. They say nothing about the fate of the "ions" (think sodium, sulfate, chloride, etc). You will see references to evaporation to recover the water and create a very concentrate brine that is also very corrosive or deep well injection. What? We create this problem (brine) then our solution is to bury it underground somewhere? When did engineers decide that waste streams can simply be put somewhere where no one can see them and declare victory? And what is waste? There is not such thing as waste (see my other blog post(s) on this topic).

One More Possible Solution

The one area that is not seeing much activity is biology and specifically the use of some of the more exquisite plants that can can remove salt from water or water from salt. These plants are called #halophytes or "salt loving". How can halphytes do what man-made technology cannot? Go to my my company's website and find out. We are launching a major effort to make hydroponic halophytes as a potential solution.

Summary

Increasing salinity is already causing billions of dollars of damage to crops and infrastructure. Climate change driven continuous drought is making the problem much worse every year. Biological solutions such as halophytes are a very underutilized tool in salinity management. Could halophytes be grown in a way that can literally pump the salt out of the water at extremely low (unmeasureable) cost using existing sodium and chloride pathways? Tailwater is going to try.