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BOTANICAL CONSERVATORY REFERENCE

- WATER QUALITY

WATER PROBLEMS

Water quality has become an increasingly important element in conservatory cultural considerations. In a survey in 1986, no conservatory reported water quality concerns while this 1995 survey reveals that fully half of the respondents have water concerns. Problems include excess salts and dissolved solids, water treatment chemicals, hardness and pH, and inadequate supplies. This section takes a look at these problems and discusses the possible solutions.

Dissolved Solids. Most of the reported water problems occur in the form of excess salts and other dissolved solids. Conservatories reported dissolved solids as high as 400 parts per million. As water reclamation and reuse programs are instituted, salt problems will probably become more prevalent.

Excess salts can collect in the root zone interrupting proper absorption of water by the roots. This is intensified by high levels of fertilizer salts and/or poor drainage conditions. As the roots are damaged by the salts, the plant stops growth, and consequently the use of fertilizer, resulting in still higher salt concentrations. If the situation continues, plant damage and finally death will result. In addition, the resultant imbalances in soil minerals can block availability of other nutrients needed for plant growth.

Mild cases of soil salt accumulations can be remedied by heavy leaching of the planting areas as long as the drainage conditions are good. Where drainage is poor, the addition of washed sand will improve drainage, and working in soil amendments with low mineral content will increase soil bulk and moderate salt concentrations. The soil can then be leached by applying large amounts of water in a period of a few hours.

Reverse Osmosis or ionization is the prominent method used to reduce the amount of dissolved salts in water supplies. One conservatory is using captured rainwater and more are either in planning or considering this resource.

Water Treatment Chemicals Almost 20% of the conservatories surveyed reported problems with high levels of water treatment chemicals. Chlorine was the worst offender with fluoride as a factor in at least 2 cases.

Chlorine . The amount of chlorine in water supplies is usually very small, but high concentrations of this chemical can cause foliage and root burn to sensitive plants and damage to some equipment. Carbon filters can remove chlorine from water supplies, but must be changed out often. Since chlorine is volatile, it can be dissipated from water by aeration.

Fluoride is added to water is in such small quantities (3 to 5 ppm), that there is no direct danger to the plants. It is when this chemical builds up in the soil that problems can occur (see discussion under Soils: Aggregates). The most sensitive plants are thin-leafed, variegated tropicals. The same methods used for chlorine reduction can be used to reduce fluoride in the water supply. Avoid using perlite and German peat which contain high levels of fluoride.

Water Hardness and pH. Water pH problems were reported in about a third of the surveys. High pH was the usual problem, although one garden reported a problem with low pH. Hard water is caused by soluble solids such as calcium or magnesium bicarbonate. These elements are then deposited in soils, or on foliage and other surfaces.

Acidifying hard water will change the pH of the water but will not solve the problem. Acid injection also presents the danger of damage to equipment and plumbing. It is better to remove the solids through water treatment systems. Water softened by a sodium chloride water softener should never be used for irrigation as the sodium would probably be more detrimental than the original solids. Use sodium softened water only if the sodium is removed by another process before being used for irrigation.

If water is only slightly hard, the use of acidifying fertilizers and soil amendments may well be enough to correct the problem. If not, water filtration and treatment systems may be necessary. Soft water and low pH is less common and easier to correct. It can be treated by injecting fertilizers with an alkaline reaction such as calcium nitrate. The soils can also be amended or top dressed with calcium and magnesium bicarbonate or dolomitic limestone. If a faster adjustment is required, hydrated lime increases pH rapidly.

Water Temperature. Irrigation water temperature can have a great impact on plant growth. Cold water can cause a drastic drop in the temperature in the root zone. In addition, cold water on foliage can sometimes cause permanent spotting on sensitive plants. Water temperature was controlled in 63% of the conservatories in the survey.

Most irrigation water heating systems use blending valves to mix hot water with feed water to adjust temperatures. Some have supply pipes running through temperature controlled spaces to moderate the temperature.

Storage tanks like those required by reverse osmosis, rain collection, and recovery systems offer opportunities for temperature adjustment. Simply locating the tanks in temperature controlled spaces can make a difference.

Fog Systems. Conservatories using fog or mist for cooling and humidification experience the most difficulty due to soluble solids building up on the leaf surfaces and accumulating in the soil. Plants can take on a ghostly appearance - glazed like donuts from layer upon layer of solids left when the mist evaporates. Soil accumulations of calcium have been reported as high as 32,000 parts per million, effectively locking up most of the elements needed for plant nutrition. Another consequence is the damage to glass, structure and fog equipment as these solids erode or plug the systems.

At the very least, sediment filters should be used to prevent abrasion of the high pressure pumps, nozzle orifices and impact pins. Conservatories with high soluble solids in the feed water will need one of the water purification methods described above. If reverse osmosis or deionization treatment is used, you must use non-corroding materials for pumps, plumbing and nozzles.

A more comprehensive discussion of fog and mist systems, electrolysis, and galvanic action will follow in the chapter on climate control.

TREATMENT AND FILTRATION

Of the surveyed conservatories, 85% have tested their water supplies. Half of these found water problems and are using some sort of filtering or treatment system. A wide range of treatment methods have been used. For moderate water quality problems, cartridge or flushable filters, carbon filters and in some cases, acid injection (the latter requiring special equipment and plumbing) have helped to correct the problems. In the worst cases, water treatment systems have been installed and affected soils amended or entirely replaced. All these options are discussed in the following section.

Reverse osmosis water treatment is capable of removing virtually all organic contaminants and as much as 99% of the dissolved salts from water supplies. It can even be used to create usable water from sea water. The efficiency of this method of purification is dependent on the quality of the feed water, the size of the unit, and the amount of product required.

Reverse osmosis uses a plastic membrane to filter contaminants from the water supply. Cellulose acetate membranes can produce up to 50% product water where polymide composite membranes can provide up to 80% recovery. Feed water under high pressure passes through the membrane, leaving contaminants behind. The product water is then stored in a tank and fed through a repressurization pump for irrigation. The storage tank size is usually based on the amount of water required for a 24 hour period. The system size is also based on this amount adding 10 to 20% for variations and decreased efficiency as the system ages before the cartridges and membranes are replaced.

The feed water can be pretreated to increase the life and efficiency of the osmotic membranes. A carbon prefilter removes some of the organic material, volatile organic chemicals and chlorine. Chlorine is particularly corrosive to these membranes. Water softeners may be used to reduce the load of dissolved solids, and a micron filter is used to further decrease materials that might foul the membranes.

Reverse osmosis water is very pure and is not recommended for constant use except for special crops or for remedial purposes. Long term use can deplete too much of the soils minerals, creating deficiencies. This water is aggressive in itself and requires special plumbing, either PVC or stainless steel pipe and equipment. Reverse osmosis water can be re-blended with irrigation water to create an acceptable product, increasing the efficient use of this water and minimizing these problems.

The major disadvantage of reverse osmosis systems is the resulting contaminated water left behind the membrane after treatment. This can be from 25% to 90% in the case of sea water. Reverse osmosis systems are relatively expensive to install and require regular maintenance. Replacement of prefilters and membranes varies according to the quality of the feed water and the amount of product water required.

Most conservatories use treated water only for sensitive plants or fog systems, although some use it for all irrigation.

Deionization systems use ionic exchange principals to remove soluble salts from water supplies. This type of system uses one resin bed (tank) to remove calcium, magnesium, sodium, and other cations and replaces them with hydrogen ions. The second bed exchanges chloride, sulfate and alkaline anions for hydroxide ions. The hydrogen ions combine with the hydroxide ions to produce water as a by product. This process usually works on demand using line pressure and therefore does not require a storage tank and repressurization system.

Many of the same caveats apply to deionized water as to reverse osmosis water in pure form. This water is very aggressive and will attack copper, brass, galvanized and other metals. In two cases, deionized water reportedly removed copper from the conservatories' plumbing systems and deposited it on the plants and in the soils in quantities high enough to damage and destroy the plants. A sign of this type of action is turquoise-blue stains on walls, plants, or equipment where water passes.

Deionized water systems are less expensive to install than reverse osmosis systems and do not have the waste concentrate. The resin beds must be changed out regularly and taken to the dealer for regeneration. The special plumbing requirements are the same as for reverse osmosis.

Acid Injection is sometimes used to correct minor water pH problems. It requires special injection equipment and careful monitoring. To be safe, treated water should flow through a special mixing chamber, then pass through a monitor capable of shutting down the injector or water flow if there is a problem. Some computerized climate systems offer this type of control.

Although acid injection can lower water pH, it cannot solve the problem of hard water (see discussion under Water pH and Hardness).

Cartridge filters are commonly used for pretreatment of feed water to fog and mist equipment and water treatment systems. They are usually made from layers of paper or fabric materials and come in grades from micron size to little better than screens. In areas with good water quality, they work well to trap particulates like rust or sand, protecting equipment from the abrasive effects of these materials. These filters are designed to work under medium water pressures and low-flow demands. Most are disposable and must be changed on a regular basis.

Flushable filters are similar to the paper cartridge filters in use but are flushed and reused. Some are made of a series of serrated rings of plastic material around a central collection pipe. As water is forced between these rings, sediment and particulate material is trapped while the filtered water passes through to the pipe inside. These rings must be flushed when water flow is significantly reduced.

Another version is a screen filter, using a fine screen to filter out larger particulate material. There may be a valve at the end of this type of filter that may be opened to flush out the collected material.

Activated Carbon filters are used for a variety of filter and pretreatment uses. Activated carbon filters can remove chlorine, organics, radon, volatile materials like pesticides, benzene, trichloroethylene, and carbon tetrachloride, as well as some metals. These materials are attracted to or absorbed into the large surface area of the carbon particles. The more activated carbon in the filter, the greater the amount of contaminants absorbed.

The size of a carbon filter is based on the quality of the feed water and the target contaminants to be removed. As an example, chlorine requires little contact time for absorption while volatile organics require longer. Flow rates will obviously influence contact time and thus filter size as well.

It is important that this type of filter be replaced on an established schedule. As the carbon's capacity for absorption diminishes, the carbon bed should be replaced or a new filter installed, even though flow rates have not decreased due to clogging. The use of cartridge prefilters will increase the life of carbon filters by decreasing clogging from rust, sand, and other sediments.

WATER RECLAMATION

A Threatened Resource. Botanical Gardens involved in conservation are aware of the difficulty in management and protection of our natural resources, especially water. Water has become a critical issue in much of the world. Water quality, conservation, and the recapture and reuse of industrial and irrigation runoff are subjects of increased research and development. Botanical gardens have an opportunity to serve as leaders in the development and employment of these technologies for ourselves and our communities.

Alternative water supplies are becoming more feasable as the cost and availability of fresh water becomes more restrictive. Even ground water use is being restricted by local and federal law in some areas. Rainwater harvest and recapture of irrigation water are two promising alternatives to conventional fresh water sources.

Recapture of Runoff. Agricultural irrigation runoff has been the target of increasing scrutiny. In some areas, recapture of irrigation runoff is mandatory by law.

Many commercial nurseries have tanks, ponds or lakes created for the specific purpose of runoff storage for reuse in irrigation. In some cases, water quality might be high enough for this type of storage facility to be incorporated into a garden landscape as an aesthetic element.

Many gardens and conservatories have elaborate planting bed and surface drainage collection systems already in place that might be modified for recapture and reuse. One of the treatments mentioned in this chapter may be needed to return the water to a usable quality, but the primary problem remains in finding adequate storage facilities for this captured water. It may have to be protected from freezing, bacterial, algal, or fungal growth, or reused rapidly enough to avoid these problems.

At least one conservatory is in the process of modifying an existing drainage collection system for direct use in outdoor turf and tree irrigation.

Rainwater Harvest. Rainwater has always been a perfect source of water for plants. It is clean, usually with a moderate pH, and it is free. It presents none of the problems inherent in waters high in soluable solids or treatment chemicals, and is less likely than ground or surface sources to carry pollutants damaging to plants.

Although rainwater would probably not need treatment as recaptured irrigation runoff may, it still demands huge storage facilities in most regions. Ideally, storage capacity would be based on the amount of water necessary to sustain a supply through the period of lowest average rainfall. Therefore, when assesing feasibility of a rainwater harvest system, consider these variables as well as if and how operations can be modified to optimize these variables.

Once it has been determined that, based on previous years precipitation records, rainfall is sufficient to meet irrigation demands, potential collection area must be evaluated. Conservatory glazing, greenhouse roof planes, buildings with smooth roofing materials, and paved areas not subjected to pollutants are examples of potential collection surfaces.

A general idea of yearly collection potential (in gallons) can be calculated by multiplying the square footage of collection area by 0.6 and then multiplying that by the average annual rainfall in inches. Remember to use the area of the flat plane occupied by the collection surface, not the actual square footage of slopes or arcs. Storage capacity can then be based on one fourth of this amount, or storage for a four month period. Compare this to the amount needed to sustain a supply through the period of lowest average rainfall to see if it will be sufficient.

Cost esimates for storage in tanks at present range from 35 to 60 cents per gallon, depending on size and construction material. Other cost considerations include pressurization pump and tank, filtration, and collection systems. Collection systems do not have to be elaborate and can sometimes use existing guttering and drain systems. A good example of this is modifiying gutter connected greenhouse drainage systems.

The collection of rainwater from impermeable surfaces also reduces the surface runoff normally routed into drainage systems. This can reduce the cost of surface runoff systems for new construction projects and reduce the load on storm sewers and other drainage elements.

WHATS IN THE FUTURE?

When asked about thoughts on future needs or trends in water use, the majority of the conservatory staffs seemed to believe that conservation and protection was a pressing if not immediate priority.

Some were already using as much as 95% reuse water and some said that the local regulation does or will soon require 100% recapture of irrigation runoff. High volume irrigation systems would be replaced by drip, trickle systems, and computer controlled pulses to prevent runoff and encourage deep watering. The use of wetting agents and water gels to maximize soil water holding capacities was also mentioned.

Half of the surveys expressed the need for decreased use of quick release fertilizers in favor of composts, slow release and organic fertilizers. Some foresee collection ponds and tanks to hold water for treatment and reuse.

At least one garden is installing rainwater harvest systems on their support buildings and greenhouses and a recapture system on the conservatory drainage systems. Some communities have installed, or are in the process of installing community-wide greywater systems to be used for irrigation and recreation.

This is a part of the first chapter of the Botanical Conservatories Compendium. Other planned chapters include conservatory climate control, management, mission, staffing, biological pest control, support facilities, budget planning, construction considerations, etc. This will be available through American Association of Botanical Gardens & Arboreta resource center, or email Don Pylant.