|Purple gold: a contemporary view of recycled water irrigation|
By Dr. M. Ali Harivandi
Most of us are familiar with the term “Black Gold” as another name for oil, and we may have heard of “Blue Gold” used in some quarters in reference to water. World news increasingly delivers stories of water being “privatized” or of political fights between those who would commoditize it and those who believe access to clean water is a basic human right.
Without question, humanity is polluting and wasting water even as its need for water grows with increasing population. Thus, just as it appears that wars today are fought over oil, future wars may be fought over water (for an excellent treatment of this topic, see Blue Gold: World Water Wars, a documentary film, as well as a book, by Barlow and Clarke).
Agronomists generally do not play the roles of economist, diplomat, or soldier. We can, however, try to educate, and sometimes the topics we broach have large consequences. The use of recycled or reclaimed water is, I believe, such a topic. And considering the critical water needs of today’s world, I would like to assign to recycled water the term “Purple Gold,” after the colour officially used to designate all equipment contacting it.
Having worked with this resource for more than 30 years, evaluating its potential for turfgrass and landscape irrigation, I have witnessed its quality increase significantly. As quality has increased, both the value and the use of recycled water have also risen dramatically.
I believe recycled water, already deserving of the name Purple Gold, will be recognized as such societywide in the near future. Already, in the face of increasingly common drought, habitat erosion, and the escalating cost of potable water, recycled water is the Purple Gold of urban landscape irrigation. In light of recycled water’s importance, a review of its qualities and of the management practices needed to use it successfully is in order.
Although three-quarters of the earth’s surface is covered with water, only a minute fraction of all the water on earth is both readily available and of sufficient quality to be suitable for human use, including irrigation of agricultural crops and landscape plants. In fact, it is estimated that only 0.02 per cent of all water on earth is fresh and immediately available—i.e. could be used with relative ease and with minimal energy input and expense.
That small fraction of earth’s water includes rain and snow-melt stored in lakes and reservoirs, as well as water available in rivers. More than 99 per cent of earth’s water is in its oceans or locked in polar ice caps and glaciers. Converting water from these sources to potable form is highly energy-dependent and expensive. Yet fresh surface and ground water together are being rapidly depleted due to industrial and agricultural use and direct human consumption.
Population growth accelerates and exacerbates the potable water scarcity. Also, human activities continue to pollute much of earth’s waters, contributing to potable water scarcity. It is estimated that by the year 2025, earth’s population will pass eight billion, with the great majority of the population living in large metropolitan areas.
Most of the world’s turfgrass (and other landscape plantings) is also in urban centres, where it competes with human consumption and food production for access to high-quality irrigation water. Another piece of the world’s water puzzle, drought is a serious and increasing problem in much of the world. In the United States and elsewhere over the past two decades, significant drought conditions occurred in various regions. During the same period, Americans migrated in large numbers to “desert” states.
Housing developments in these arid regions, along with their attendant landscape sites (golf courses especially) have significantly increased the demand for water. In most cases, turf and landscape irrigation is not a priority for municipalities during droughts. Severe restrictions on turfgrass and landscape irrigation during droughts are common, including complete shutdown of golf course or park irrigation.
Irrigation with recycled water is therefore a viable means of coping with drought, water shortages, and/or the rising cost of potable water. Currently, large volumes of recycled water are used to irrigate golf courses, parks, roadsides, landscapes, cemeteries, athletic fields, sod production farms, and other landscape sites. Interest in recycled water irrigation also increases as more and better-quality treated sewage water becomes available.
Sewage treatment has become more effective at eliminating potential human pathogens. Historically, treated sewage water was used to irrigate crops not consumed directly by humans (e.g., pasture, fodder, fibre, and seed crops), fruits borne high enough on trees that they did not come into contact with irrigation water, and crops grown for processing (e.g. grapes for wine, tomatoes for ketchup, or cucumbers for pickling).
Today, most sewage treatment plants produce high-quality recycled water suitable (as far as human-pathogen content is concerned) for additional uses such as golf courses, parks, athletic fields, and other urban landscape sites.
In certain southwest desert areas of the United States, most golf courses (and associated landscapes) may use only recycled (or other degraded-quality) water for irrigation. In a larger context, recycled water is now the irrigation source for approximately 15 per cent of U.S. golf courses and close to 35 per cent of courses in southwestern states. These figures are rapidly increasing, as are those for all other commercial, institutional, and industrial sites irrigated with recycled water.
How water is recycled
“Recycled water” refers to water that has undergone one cycle of (human) use and then received significant treatment at a sewage treatment plant to be made suitable for various reuse purposes, including turfgrass irrigation.
Several other terms are also used for recycled water, among them: reclaimed water, reuse wastewater, effluent water, and treated sewage water. Depending on degree of treatment, recycled water is referred to as primary, secondary, or advanced (tertiary) treated municipal or industrial wastewater.
Primary treatment is generally a screening or settling process that removes organic and inorganic solids from wastewater.
Secondary treatment is a biological process in which complex organic matter is broken down to less-complex organic material, which is then metabolized by simple organisms that are later removed from the wastewater.
Advanced wastewater treatment consists of processes that are similar to potable water treatment, such as chemical coagulation and flocculation, sedimentation, filtration, or adsorption of compounds by a bed of activated charcoal. Advanced treatment is often referred to as “tertiary treatment.”
Secondary and tertiary processes significantly reduce suspended matter and pathogenic organisms contained in effluent water. Urban sewage treatment employs sophisticated procedures and equipment to remove human-disease-causing organisms.
Figure 1 presents a simplified schematic of the three levels of treatment. Sewage treatment takes raw sewage with all of its suspended matter and pathogenic organisms and converts it into clear, reclaimed water that looks as good to the human eye as any potable water. In almost all cases, recycled water is thoroughly disinfected before leaving the treatment plant. Disinfection greatly reduces (or entirely eliminates) the human disease-causing organisms and expands the irrigation uses of recycled water.
However, “dissolved” solids (salts) still remain and are of concern if the water is to be used for irrigation. It is technically possible to remove all of the dissolved salts from sewage water, using techniques such as reverse osmosis. Reverse osmosis, in fact, is used on a small scale at a few golf courses to remove almost all dissolved solids from water. However, the expense is such that very few treatment plants in the world currently use it. Therefore, most of the recycled water available for irrigation is only tertiary treated and may contain high concentration of salts.
Turfgrass is particularly well suited to irrigation with recycled water. Among landscape plants, turfgrasses can absorb relatively large amounts of nitrogen and other nutrients often found in elevated quantities in recycled water, a characteristic that may greatly decrease the odds of groundwater contamination by recycled water.
Equally important, turgrass plantings are generally permanent and their growth is continuous, providing a stable need for continuously produced recycled water. Presently, most of the turfgrass irrigated with recycled water grows on golf courses. However, recycled water irrigation is increasing on sports fields, in parks, on many industrial and institutional landscapes, and on sod production farms.
Most municipalities require signage (usually coloured purple) to inform the public of the presence of recycled water. These efforts are intended to prevent anyone from ingesting or otherwise using the water directly, to avoid any risk, however slight, of contact with human pathogens. The colour purple is now broadly accepted as the official colour for recycled water conveyance equipment. Almost all irrigation system components are now available in purple, including pipes, sprinkler heads, valves, and irrigation boxes.
Despite sound reasons for using recycled water for turfgrass irrigation, there are legitimate concerns about possible injury to turfgrass and other landscape plants due to the salt content and other characteristics of reclaimed water. During irrigation, dissolved salts and other chemical constituents move with water into the plant rootzone. Recognizing the problems that may arise from this and understanding their remedies allow turfgrass managers to make use of this valuable irrigation resource, the Purple Gold, in spite of potential challenges.
Recycled waters usually contain higher amounts of dissolved salts than most other irrigation water sources. Salt accumulation in the soil is the most common concern. Ordinarily, a long period of irrigation passes before salt builds up in the soil enough to actually injure plants. Besides saline irrigation water, insufficient natural precipitation, inadequate irrigation, and poor drainage all increase the likelihood of creating saline soil conditions.
Generally, salinity becomes a problem for turfgrass when the total quantity of soluble salt in the rootzone is high. The rate at which salts accumulate to these levels in a soil depends on their concentration in the irrigation water, the amount of water applied annually, annual precipitation, and the soil’s physical and chemical characteristics.
Once rootzone salinity builds to harmful levels, several problems may occur. Salinity may inhibit water absorption by plant roots (due to the high osmotic potential of the soil water solution) and cause plants to appear drought stressed despite the presence of adequate water within the rootzone.
For such osmotic stress symptoms, the term physiological drought is often used. High salinity can also cause some ions (e.g. sodium) to be absorbed by the plant in high enough quantities to cause tissue burn or to compete with other essential elements, creating nutritional imbalances. In most cases, injury caused by high water/soil salinity is due to a combination of these factors.
If the amount of water applied to turf (irrigation plus precipitation) is higher than evapotranspiration and drainage is provided, then salt movement is downward. Conversely, salt movement is upward if evapotranspiration exceeds water applied. In the latter case, salt drawn to the surface gradually accumulates to levels toxic to turfgrasses and other plants. Diagnosing water/soil salinity problems always begins with chemical analysis of the irrigation water and soil.
Water salinity is reported differently by different laboratories. It may be reported as electrical conductivity (ECw) in terms of deci Siemens per metre (dS/m), or as Total Dissolved Solids (TDS) in either parts per million (ppm) or milligrams per liter (mg/L).
Generally, waters of acceptable quality for turfgrass irrigation have electrical conductivities of less than 0.7 dS/m (Table 1). Waters with soluble salt levels above 3 dS/m may injure turfgrass and are not recommended for irrigation. Recycled irrigation water with salt levels up to 3 dS/m may be tolerated by some turfgrass species, but only on soils with good permeability and subsoil drainage, which allow a turfgrass manager to leach excessive salt from the rootzone by periodic heavy irrigations.
For agronomic purposes, in addition to salinity, recycled waters must also be evaluated for their sodium, chloride, boron, bicarbonate, and nutrient content, as well as pH and suspended matter. Each of these elements affects plant growth. Managers can request that labs test their samples for the specific elements they know are likely to cause injury to plants. With test results in hand, managers use published guidelines to determine if their conditions are problematic and, if so, in what way.
Sodium content is as important to recycled water quality as salinity. Although sodium can be directly toxic to plants, its most frequent deleterious effects on plant growth are indirect through its effect on soil structure. The high sodium content common to recycled water can cause deflocculation (dispersion) of soil clay particles or breakdown of soil structure, reducing soil aeration and water infiltration and percolation. Waterlogging and soil compaction are common results of excess sodium. In such conditions, direct sodium toxicity may also eventually occur. The sodium (Na) hazard of recycled water is measured by the Sodium Adsorption Ratio (SAR).
Because calcium (Ca) and magnesium (Mg) flocculate clay particles, while sodium disperses them, the ratio of these elements to each other in irrigation water provides a measure of likely soil permeability resulting from irrigation with a particular water. That said, the effect of sodium on soil particle dispersion (i.e. permeability) is counteracted by high electrolyte (soluble salts). Thus, the likely effect of a particular irrigation water on soil permeability is best gauged by assessing the water’s SAR in combination with its ECw. Note that for recycled waters high in bicarbonate, some laboratories “adjust” the calculation of SAR (yielding a number called “adjusted SAR” or “Adj. SAR”) because soil calcium and magnesium concentrations are affected by a water’s bicarbonate. In simplest terms, Adj. SAR reflects the water content of calcium, magnesium, sodium, and bicarbonate, as well as the water’s total salinity. The combined effect of water ECw and SAR on soil permeability is shown in Table 1. The table provides general guidelines only, since soil properties, irrigation, climate, species salt tolerance, and cultural practices all interact with water quality and plant growth. In general, water with an SAR below 3 is safe for turf and other ornamental plants.
Recycled waters with an SAR above 9 can cause severe permeability problems when applied to fine-textured (i.e. clay) soils over a period of time. Coarsetextured (i.e., sandy) soils experience less severe permeability problems and can tolerate an SAR of this magnitude. Golf course greens and sports fields with high-sand-content rootzone mixes, for example, can be irrigated successfully with high-SAR water because their drainage is good.
Reprinted with permission by the USGA Green Section Record, Vol. 49 (45), Dec. 16, 2011.
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