Salinity and Hydroponics (Part 2)
By Dr. Nichols and Bruce Christy
Another strategy would be to purify existing water supplies
This will be the subject this particular article which is being continue from the September/October issue.
Reverse osmosis is probably the most efficient method currently available and the main question to be decided is how low a level of salinity do we wish to achieve given that we will wish to minimize the dumping of the nutrient solution. A reduction in salinity will greatly extend the interval between dumping old nutrient solution, but at a cost.
We also need to be well aware that the effluent from the reverse osmosis should be collected and not allowed to enter the environment downstream as that will only make the problem worse for your neigbours. An adequate evaporation tank should be used to produce a dried end product of salt.
Dealing with saline waste water
RO systems produce a waste that can be recycled but normally runs to waste. Old spent /out of spec. nutrient solution may contain nutrients and contaminants like sodium chloride plus some useful water if it can be recovered economically. It is an additional expense if energy has to be used to separate the salts from the water, however energy from the sun can be used for this purpose. Solar evaporation has been used since 1872 in Chile where a solar still covering 0.4ha provided fresh water in arid areas. More recently scientists at the Weizmann Institute of Science in Rehevoth, Israel have refined the solar still to produce freshwater. Evaporation of water is limited by the incoming solar radiation and temperature. The water temperature can be raised by using a relatively shallow layer of water and increasing the air temperature by a glass covering. Water condenses on the slightly cooler cover over the water and runs down to a collection channel at the base of the cover. Plastic has been used but the droplets of condensate on the plastic tend to fall rather than running-off as they do on a glass surface. The resulting condensate will be high quality water and virtually free from contaminants if handled sensibly to avoid pollutants. The dried salts collected after evaporation can be collected and taken to an approved collection place.
To understand the nature of different water standard it is useful to be aware of the contaminants that may be found in water.
We all know that water is an excellent solvent and is essential to plant and animal life as we know it. The solvent properties of water also enable water to accumulate a range of contaminants that have the potential to affect plant and animal life.
There are five basic contaminants found in water:
Dissolved ionized solids and gases
Dissolved non-ionized solids and gases
Particulate matter
Micro organisms
Pyrogens
As pure water is one of the best solvents it actively accumulates contaminants from everything it passes over or through. Dissolved ionized salts such as sodium, calcium and chloride are stripped from rock and soil. However, pollutants from agriculture and industry may also be significant sources of ionized impurities. The main dissolved gas is carbon dioxide.
These contaminants are present in varying concentrations in water and are a potential source of problems to hydroponic growers depending upon their application.
These contaminants are of general importance because they affect the hardness and alkalinity of water, but are highly significant to horticulturists as they alter the conductivity and osmotic potential of the water. Dissolved salts are relatively easy to monitor because we can detect them based on the electrical conductivity/resistivity of the water.
When an electrical current is passed through water, ions are used as the charge carrier by the electrical current. With few ions in solution the passage of electrical current is more difficult and the resistivity high (conductivity is low), as more salts are dissolved the resistivity becomes lower. Most organic compounds and bacteria do not contribute appreciably to the resistivity of water as they do not ionise in water.
Organic compounds are the largest class of solid non-ionized contaminants derived from water’s exposure to ecological contaminants such as decaying vegetation, fish and animal products. These materials maybe present as a difficult-to-remove colloidal suspension. The most important nonionized gas present is oxygen.
Particulate materials form the submicron matter invisible to the naked eye comprising very fine colloidal clay particles through to larger more visible particles that sediment out on standing such as sand, silt, and dirt exist in all water supplies.
Microbial contamination includes protozoa, bacterial, fungal spores and virus particles are usually found in all types of untreated surface water. Most municipal purification processes will remove micro-organisms by the use of chlorination and filtration. Since most types of secondary water purification remove chlorine, bacteria multiply in treated water and become a problem in stored purified water.
Pyrogens, are the end products of bacterial degradation. They are most often lipopolysaccharides and are recognisable as part of the cell wall from gram negative bacteria. Pyrogens are of most concern where the water is for human consumption but may also influence plant growth if they were phytotoxic.
Methods to make more efficient use of existing water resources
Prevent unnecessary evaporation from nutrient solution
Keep all liquids covered with a plastic sheet to reduce opportunity for water loss from tanks and gullies.
Reduce light intensity and temperature to reduce water use by plants. Growing plants in a management regime that promotes controlled environmental stress will promote increased water economy. Growing in sealed shaded greenhouses has been tried successfully by researchers in Cyprus to produce crops with improved water economy.
Methods that can be
used to purify water for reuse
Distillation
Ion exchange
Electrodialysis
Reverse osmosis
All of these methods require pre-filtration of the feed supply to minimise the particulate components of the water supply fouling the purification equipment.
Distillation
When water is heated to boiling point it is changed into a gas leaving behind the non-volatile impurities. When the water vapour is condensed it changes back to liquid water free of the non-volatile impurities. This method can produce potable water from seawater if necessary and remains the classical method of desalination of water for well over 200 years. The method is not without its drawbacks as it requires a large amount of energy to vaporise the water and a large volume of water to condense the water vapour and power to circulate the cooling water. The cost of water produced by distillation is too expensive for most growers to consider as a serious method of water purification.
Ion exchange
The monopoly of distillation in production of high quality water was broken in the 1940’s by the introduction of ion exchange resins for water purification. Ion exchange resins are, for the most part, synthetic polymers with several ion exchange sites attached to the surface. Two basic types of ion exchange resins are used in most water purification systems: Cation removal resins have several hydrogen ions (H+) attached to their surface, each capable of removing one positively charged ion; Anion resins have several hydroxyl groups (OH-) attached to their surface, each capable of removing one negatively charged ion.
Dissolved ionized solids and dissolved ionized gases are removed using ion exchange resins stripping ions from water, replacing them with H+ and OH- ions which ultimately join to form water.
In a two-bed cartridge, these reactions occur separately with the cation removal resin being used first, followed by anion removal resin.
A two-bed cartridge is used to remove the bulk of ionic contaminants, because when the two resins are separated, the cartridge has a higher effective capacity for ionic molecules. But a two-bed cartridge cannot fully remove all ionic contamination because of sodium leakage which occurs.
To achieve the highest purity water, the two resins are mixed together, eliminating sodium leakage. In a mixed bed system, the exchange of anions and cations occurs simultaneously, preventing the passage of cations. The resins have a limited capacity to remove unwanted ions from solution so must be regenerated by washing in acid or base solutions to remove the attached ions.
This system is best used in combination with another purification technique like reverse osmosis that will remove most of the impurities.
Electrodialysis
The first membrane method developed in 1946 for water desalination and is still in use today. In eletrodialysis the ionised impurities are pulled out of the impure inlet water across a membrane by an electric field. The membranes are ion exchange materials that allow either positive or negative ions to pass through whereas oppositely charged ions are bound to the membrane. The salt content of the water is reduced by electricity passed between stacks of cells comprising pairs of membranes causing anions and cations to move in opposite directions through the membranes enclosing the cell. As waters of low salt content become poor electrical conductors it is difficult to produce water that is totally deionised. The energy requirements for water purification by electrodialysis are totally dependent on the concentration of salts in the supply water. Electrodialysis is most suitable for converting brackish water (<2000ppm ) to potable water <500ppm).
Reverse osmosis
Although the process of osmosis, where water moves through a semi-permeable membrane has long been known, the opposite process where osmosis is driven in reverse to produce a purified output is a relatively recent commercial development in water purification.
The first commercial membranes were developed in the early 1960’s and in the years that have followed, membrane technology has grown a great deal, recent developments have been in the area of membranes that are more robust and less prone to fouling by pollutants.
The reverse osmosis membrane system of purification is finding applications in many areas including waste metal reclamation, molecular sieves to separate closely related substances in a solution and also to water purification and desalination.
To appreciate the simplicity of this technology it is useful to understand the concept of normal osmosis where water flows from a less concentrated solution through a semi-permeable membrane, to a more concentrated solution. Reverse osmosis uses hydraulic pressure to reverse the normal direction of osmotic flow, therefore in reverse osmosis water flows from a more concentrated solution across a semi-permeable membrane to a less concentrated solution.
The mechanism employed by the membrane to remove contaminants depends on a combination of processes. There is a thin porous layer on the surface of the membrane which contains many very fine pores. The interaction between the solution and the membrane ideally allows only water to pass through the membrane and the salts are rejected. Some non ionised organic compounds are removed, but in most cases this is dependent on the molecular weight of the organic molecule.
The feed water to the reverse osmosis system flows over the surface of the membrane. The pressure forces a percentage of the water through the membrane, while the remaining water now heavy in contaminants, is run to waste.
Water movement through the membrane is constantly cleaning the surface of the membrane, preventing the build-up of contaminants that could potentially damage the membrane. Modern membranes have a relatively long life but in the end will need replacement as their efficiency will decrease over time.
Reverse osmosis effectively removes a wide range of ionic impurities, the more highly charged he ion the more readily it can be excluded by the membrane. Due to the limited removal capabilities of reverse osmosis, its primary use in water treatment is in the pretreatment of water being fed to a deionizer where very clean water is required.
The factors that most often affect membrane material are as follows: pH, temperature, bacteria, free chlorine, and Langlier saturation index. Most of the factors listed above should be easily understood with the exception of Langlier saturation index. This is the measure of the scaling tendency of a particular water source. In most cases, langlier index is calculated and considered positive or negative. When calculating the Langlier index, the following water components must be measured: water temperature, total ionized solids, calcium hardness, alkalinity, and pH. If the index is positive, the feed water is considered to show a high potential or scaling and subsequent membrane damage.
Resulting from the feed water limitations of reverse osmosis membrane materials, a percentage of the systems require some form of pretreatment to maximize the useful life of the membrane. The most common form of pretreatment is softening. By exchanging the hardness ions in a particular water supply with sodium, the scaling tendencies of water are reduced, thus correcting for a positive langlier index.
Up to this point, we have discussed how the chemical make-up of water affects the reverse osmosis membrane. Now we’ll explain briefly how physical properties, most notably temperature and pressure, affect the reverse osmosis system operation. Feed water temperature has a marked effect on the quantity of water a particular membrane is capable of producing. Membrane performance is based on a feed water temperature of 25C (77F). For every 1 C below 25, a 3% reduction in quantity of the water the membrane will produce will be realized. For this reason, temperature adjustment of the feed water is often recommended. Temperatures above 35C (95F) will damage most membranes.
Incoming water pressure affects both the quality and quantity of water produced by the reverse osmosis system. Although pressures up to 400 psig and beyond will not damage membranes, low operating pressures will reduce the effectiveness of a membrane to remove impurities. Reverse osmosis systems operating at 200 psig will improve the quality 5 - 10%, as compared to operating at 60 psig. Below 50 psig, the quality is more drastically affected. The quantity of water produced will also be affected by pressure. Simply put, the lower the pressure, the lower the amount of product water produced.
Reverse osmosis is a percent removal technology. A typical reverse osmosis system rejects 90-95% of the impurities found in most potable water supplies. As membrane only removes a percentage of the contaminants in a given water supply, it is impractical to predict the purity of water by this technology. Due to the mode of purification, certain contaminants are removed more effectively than others. Polyvalant ions have better removal capabilities than mono-valent ions. Large molecular weight organics (greater than 200) are effectively removed where smaller organics pass through the membrane. Gases readily pass through the reverse osmosis system and will affect the purity of the product water.
What of the future?
In the final analysis the bulk of the water absorbed by the root system is transpired and only about one percent actually remains in the plant as structure etc. Therefore, if some method of recycling the transpiration water could be developed it would be a superb source of “pure” water for replacing the hydroponic system. This is the strategy that will be used for growing plants in space for the Mars project. This technology has been trialled in arid climatic zones on the earth where water is a precious commodity and perhaps could be applied more widely.
So what is the
answer to Joe’s problem?
Our suggestion is that he should approach the solution in the following manner.
1) He should build a reservoir adjoining the greenhouse, and use this as a “temporary” storage for rain water from the greenhouse roof.
2) He should avoid using any secret magic potions, or if he still insists on using them, he should ensure that they have a low or zero sodium content.
3) He should ensure that ALL the water he uses in the hydroponic system is initially treated with ozone (to remove pathogens and some minerals), and then filtered.
4) He should then use reverse flow osmosis on this water to remove the bulk of the mineral ions.
5) The spare land not used for the reservoir should be used for evaporation pans to ensure that the highly saline water from the reverse flow system and from the dumped hydroponic solutions does not damage the environment.
6) If considered necessary an ion exchanger should be used after the reverse flow osmosis system. This may, however be an unnecessary extravagance.
References:
Lorch, W. 1981 Handbook of Water Purification
The Barnstead Water Book 1989