ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT
VOLUME 4
RECLAMATI ON RECYCLI NG & REUSE OFEFFLUENTS
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4.
RECLAMATION , RECYCLING & REUSE 4.1 PUBLIC HEALTH & ENVIRONMENTAL ISSUES 4.1.1 Guidelines and Regulations 4.2 WATER RECLAMATION TECHNOLOGIES 4.3 AGRICULTURAL IRRIGATION 4.3.1 Irrigation Methods 4.4 LANDSCAPE IRRIGATION 4.5 INDUSTRIAL RECYCLING & REUSE 4.6 GROUNDWATER RECHARGE 4.7 RECREATION/ENVIRONMENTAL USES 4.8 NONPOTABLE URBAN USES 4.9 INDIRECT POTABLE REUSE 4.10 CALCULATIONS 4.11 GLOSSARY 4.12 QUESTIONS & ANSWERS
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4. RECLAMATION , RECYCLING & REUSE The inclusion of planned water reclamation, recycling, and reuse in water resource systems reflects the increasing scarcity of water sources to meet social demands, technological advancements, increased public acceptance, and improved understanding of public health risks. The major pathways of water reuse include irrigation, industrial use, surface water replenishment, and groundwater recharge. Surface water replenishment and groundwater recharge also occur through natural drainage and through infiltration of irrigation and stormwater runoff.
The role of engineered treatment, reclamation, and reuse facilities in the cycling of water through the hydrologic cycle
(Eddy, 1999)
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Wastewater reclamation,recycling & reuse graph
The seven principal categories of municipal wastewater reuse are listed bellow in descending order of projected volume of use : • • • • •
Agricultural irrigation Landscape irrigation Industrial recycling and reuse Groundwater recharge Recreational/environmental uses
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• •
Nonpotable urban uses Indirect Potable reuse
4.1 Public Health & Environmental issues Despite the existence of technically proven advanced wastewater treatment processes, as discussed in Chap. 2.4, long-term safety of reclaimed water and the impact on the environment are still difficult to quantify. Giventhe fact that it is possible to produce water of almost any quality, public health and environmental issues that now must be addressed are: what constituents are found in municipal wastewater, which of them must be removed and to what extent must they be removed?
Classification of typical constituents found in wastewater Classification Constituent Conventional
Total suspended solids Colloidal solids Biochemical oxygen demand Chemical oxygen demand Total organic carbon Ammonia Nitrate Nitrite Total nitrogen Phosphorus Bacteria Protozoan cysts and oocysts Viruses
Nonconventional
Refractory organics Volatile organic compounds Surfactants Metals Total dissolved solids
Emerging
Prescription and nonprescription drugs Home care products Veterinary and human antibiotics Industrial and household products Sex and steroidal hormones Other endocrine disrupters (Eddy, 1999)
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5 The term conventional is used to define those constituents measured in mg/L that have served as the basis for the design of most conventional wastewater treatment plants. Nonconventional applies to those constituents that may have to be removed or reduced using advanced wastewater treatment processes before the water can be used beneficially. The term emerging is applied to those classes of compounds measured in the micro- or nanogram/L range that may pose long-term health concerns and environmental problems as more is known about the compounds. In some cases, these compounds cannot be removed effectively, even with advanced treatment processes. For most of the emerging compounds listed above, there is little or no information concerning health or environmental effects. Unfortunately, some of the compounds that have been identified in reclaimed water are known to have both acute and chronic health effects, depending on their concentrations and exposure pathways.
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4.1.1 Guidelines and Regulations Today in Europe, no common legislation, guidelines or regulations for all countries-members of E.U. can be found. Instead, regulations or guidelines adopted by individual countries, is the practice used until today, due to the fact of differences among countries-members of E.U. such as the environment, land uses, economy direction and culture.
An indicative summarry of US EPA suggested guidelines for water reuse, is given in the following table : EPA suggested guidelines for water reuse
(Eddy, 1999) It should be noted though, that above guidelines suggested by US EPA, may not be applicable in detail for other countries worldwide. Thus, a correct order of magnitude for water reuse , depending on the level of treatment - is given in the above table, which can lead us to safe results when planning a reclamation & reuse system.
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MORE Other important guidelines that exist for wastewater reuse are the ones published by the World Health Organization (WHO), and are mainly focused on the needs of developing countries. WHO guidelines specify the microbiological quality and the treatment method required to achieve this quality, which is limited to the use of stabilisation ponds since it is cheaper, simpler and ensure removal of parasites which is the most infectious agent in the developing world. WHO guidelines are presented in following table. Guidelines for the use of treated wastewater in agriculture a Category
Reuse conditions
Exposed
Intestinal nematode
b
Faecal coliforms
Wastewater treatment expected
group
(arithmetic mean no.
(geometric mean
to achieve the required
eggs per litre) Irrigation Α
of
crops
likely to be eaten uncooked,
Irrigation Β
crops,
of
no. per 100ml)
microbiological guideline
≤1000
A series of stabilization ponds
consumers,
designed
public
achieve
the
or equivalent treatment ≤1
Workers
industrial
No standard
Retention in stabilization ponds
recommended
for 8-10 days or equivalent
crops, fodder crops,
helminth and faecal coliform
e
Localized irrigation of
to
microbiological quality indicated,
d
cereal
pasture and trees
C
c
≤1
Workers,
sports
fields, public parks
c
removal None
Not applicable
Not applicable
Pretreatment as required by
crops in category B if
irrigation technology, but not
exposure to workers
less than primary sedimentation
and the public does not occur a In specific cases, local epidemiological, sociocultural and environmental factors should be taken into account and the guidelines modified accordingly. b Ascaris and Trichuris species and hookworms. c During the irrigation period. d A more stringent guideline (200 faecal coliforms per 100 ml) is appropriate for public lawns, such as hotel lawns, with which the public may cone into direct contact. e In the case of fruit trees, irrigation should cease two weeks before fruit is picket, and no fruit should be picked off the ground. Sprinkler irrigation should be used.
(World Health Organization, 1989)
To understand better the above table, let’s elaborate on Category A. The WHO has recommended that irrigation of crops likely to be eaten uncooked, sports fields, and public parks should be irrigated with wastewater treated by a series of stabilization ponds. The ponds are designed to achieve a microbiological quality of less than or equal to 1 intestinal nematode per liter and faecal coliforms less than or equal to 1000 per 100ml.
The main features of the WHO (1989) guidelines for wastewater reuse in agriculture are therefore as follows:
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•
Wastewater is considered as a resource to be used, but used safely.
•
The aim of the guidelines is to protect exposed populations (consumers, farm workers, populations living near irrigated fields) against excess infection.
•
Faecal coliforms and intestinal nematode eggs are used as pathogen indicators.
•
Nematodes are included in the guidelines since infectious diseases in developing countries are mainly due to the presence of parasites which are more resistant to treatment.
•
Measures comprising good reuse management practice are proposed alongside wastewater quality and treatment goals; restrictions on crops to be irrigated with wastewater; selection of irrigation methods providing increased health protection, and observation of good personal hygiene (including the use of protective clothing).
4.2 Water reclamation technologies As noted previously, the constituents in wastewater subject to treatment may be classified as conventional, nonconventional, and emerging. Conventional constituents are removed by the conventional treatment technologies. Advanced treatment technologies are used most commonly for the removal of nonconventional constituents. The removal of emerging constituents occurs in both conventional and advanced treatment processes, but the levels to which individual constituents are removed are not well defined. Typical performance data for selected treatment process combinations are presented in the following Table :
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9 Treatment levels achievable for conventional constituents , with various combinations of unit operations and processes used for wastewater reclamation
(Eddy, 1999)
As reported in the above Table, it is clear that for conventional constituents a very high treatment efficacy can be attained using a variety of treatment process flow diagrams.
Corresponding data for the removal of conventional & nonconventional constituents are reported in the next Table for complete treatment plus reverse osmosis.
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NIREAS VOLUME 4 Removal of wastewater conventional & nonconventional constituents in a water reclamation facility (all units are mg/L)a
(Eddy, 1999)
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Corresponding data for the removal of conventional, nonconventional and emerging constituents are reported in the next Table for complete treatment plus reverse osmosis.
Typical range of effluent quality after various levels of treatment for conventional, nonconventional and emerging constituents in a water reclamation facility
(Eddy, 1999)
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Limited data for emerging constituents are reported in the following Table. The impact of the constituents that remain after various treatment processes is of profound importance with respect to the long-term protection of public health and the environment. The emerging constituents were unable to be measured in the ppb or ppt range until just a few years ago, and their impacts are mostly unknown at present. For example previous research for 17β- Estradiol, has shown that it can cause genetical disturbances aquatic life when found in high concentrations.
Reported removal ranges for selected emerging constituents of concern
(Eddy, 1999)
4.3 Agricultural irrigation The first category, agricultural irrigation, is the largest current use of reclaimed water worldwide. Agricultural irrigation includes crop irrigation, and commercial nurseries.
This reuse category
offers significant future opportunities for water reuse.
Potential constraints and issues for agricultural irrigation are given bellow :
Issues/constraints • Surface and groundwater contamination if not managed properly • Marketability of crops and public acceptance • Effect of water quality, particularly salts, on soils and crops • Public health concerns related to pathogens (e.g., bacteria, viruses, and parasites) • Use area control including buffer zone may result in high user costs
The physical and chemical characteristics of irrigation water are of particular importance in arid zones where extremes of temperature and low humidity result in high rates of evapotranspiration
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(ET). Evapotranspiration refers to the water lost through evaporation from the soil and surface water bodies and transpiration from plants. Water used for irrigation can vary greatly in quality depending upon type and quantity of dissolved salts. The consequence of evapotranspiration is salt deposition from the applied water, which tends to accumulate in the soil profile. The physical and mechanical properties of the soil, such as degree of dispersion of the soil particles, stability of aggregates, soil structure, and permeability, are sensitive to the types of exchangeable ions present in irrigation water. Thus, when irrigation with reclaimed water is being planned, not only is the crop yield important, but also soil properties must be taken into consideration. The problems, however, are no different from those caused by salinity or trace elements in any water supply and are of concern only if they restrict the use of the water or require special management to maintain acceptable crop yields.
Irrigation water quality guidelines applicable to both freshwater and reclaimed water are given in the following table :
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Guidelines for interpretations of water quality for irrigationa
(Eddy, 1999)
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15 Four main categories of potential management problems associated with water quality in irrigation are: (1) salinity (2) specific ion toxicity (3) water infiltration rate (4) other problems
Salinity
Salinity of an irrigation water is determined by measuring its electrical conductivity and is the most important parameter in determining the suitability of a water for irrigation. The electrical conductivity (EC) of a water is used as a surrogate measure of total dissolved solids (TDS) concentration. The electrical conductivity is expressed as decisiemens per meter (dS/m) or mmho/cm. It should be noted that one dS/m is equivalent to one mmho/cm. Values for salinity are also reported as TDS in mg/L. For most agricultural irrigation purposes, the values for EC and TDS are related to each other and can be converted within an accuracy of about 10 percent using following Eq.
TDS (mg/L) = EC (dS/m or mmho/cm) x 640
The presence of salts affects plant growth in three ways: (1) osmotic effects, caused by the total dissolved salt concentration in the soil water; (2) specific ion toxicity, caused by the concentration of individual ions; (3) soil particle dispersion, caused by high sodium and low salinity. With increasing soil salinity in the root zone, plants expend more of their available energy on adjusting the salt concentration within the tissue (osmotic adjustment) to obtain needed water from the soil. Consequently, less energy is available for plant growth.
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16 Specific ion toxicity If the decline of crop growth is due to excessive concentrations of specific ions, rather than osmotic effects alone, it is referred to as “specific ion toxicity.� The ions of most concern in wastewater are sodium, chloride, and boron. The most prevalent toxicity from the use of reclaimed water is from boron. The source of boron is usually household detergents or discharges from industrial plants. The quantities of chloride and sodium also increase as a result of domestic usage, especially where water softeners are used. For sensitive crops, specific ion toxicity is difficult to correct short of changing the crop or the water source. The problem is also accentuated by hot and dry climatic conditions due to high evapotranspiration rates. The suggested maximum trace element concentrations for irrigation waters are reported in the following Table. In severe cases, these elements tend to accumulate in plants and soils, which could result in human and animal health hazards or cause phytotoxicity in plants.
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17 Recommended maximum concentrations of trace elements in irrigation waters
(Eddy, 1999)
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18 Water Infiltration Rate In addition to the sodium toxicity discussed above, another indirect effect of high sodium content is the deterioration of the physical condition of a soil (formation of crusts, waterlogging, reduced soil permeability). If the infiltration rate is greatly reduced, it may be impossible to supply the crop or landscape plant with enough water for good growth. In addition, reclaimed water irrigation systems are often located on less desirable soils or soils already having permeability and management problems. It may be necessary, in these cases, to modify soil profiles by excavating and rearranging the affected land. The water infiltration problem occurs within the top few centimeters of the soil and is mainly related to the structural stability of the surface soil. To predict a potential infiltration problem, the sodium adsorption ratio (SAR) is often used.
MORE
where the cation concentrations are expressed in meq/L.
The adjusted sodium adsorption ratio (adj RNa) is a modification of above Eq., which takes into account changes in calcium solubility in the soil water.
where Na+ and Mg2+ concentrations are expressed in meq/L, and the value of Cax2+, also expressed in meq/L, is obtained from following Table:
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Values of Cax2+ as a function of the HCO3- / Ca2+ ratio and solubilitya
(Eddy, 1999)
Use of the adj RNa value is preferred in irrigation applications with reclaimed water because it reflects the changes in calcium in the soil water more accurately. At a given sodium adsorption ratio, the infiltration rate increases as salinity increases or decreases as salinity decreases. Therefore, SAR, or adj RNa and electrical conductivity (ECw) of irrigation water should be used in combination to evaluate the potential permeability problem.
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20 Nutrients The nutrients in reclaimed water provide fertilizer value for crop or landscape production. However, in certain instances, when nutrients are in excess of plant needs they can cause problems. Nutrients that are important to agriculture and landscape management include N, P, and occasionally K, Zn, B, and S. The most beneficial and the most frequently excessive nutrient in reclaimed water is nitrogen. The nitrogen in reclaimed water can replace equal amounts of commercial fertilizer during the early to midseason crop-growing period. Excessive nitrogen in the latter part of the growing period may be detrimental to many crops, causing excessive vegetative growth, delayed or uneven maturity, or reduced crop quality. If alternate low-nitrogen water is available, a switch in water supplies or blending of reclaimed water with other water supplies has been used to keep nitrogen under control, otherwise expensive seasonal denitrification would be required.
Other Problems Clogging problems with sprinkler and drip irrigation systems have been reported, particularly with oxidation pond effluent. Biological growth (slimes) in the sprinkler head, emitter orifice, or supply line causes plugging, as do heavy concentrations of algae and suspended solids. The most frequent clogging problems occur with drip irrigation systems. From the standpoint of public health, such systems are often considered ideal, as they are totally enclosed, minimizing the problems of worker exposure to reclaimed water or spray drift. In reclaimed water that is chlorinated, chlorine residuals of less than 1 mg/L do not affect plant foliage, but chlorine residuals in excess of 5 mg/L can cause severe plant damage when reclaimed water is sprayed directly on foliage.
4.3.1 Irrigation Methods Out of various irrigation methods used, modern technology might be of particular interest in irrigating with treated wastewater. Selection of any method, whether conventional or modern, should be handled carefully in order to operate the irrigation system efficiently and safely. Selection of the appropriate irrigation method depends on the quality of the effluent, crops to be grown, farmers tradition, background and skill of the farmers and the potential risk to workers and to public health.
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Conventional Surface Irrigation methods •
Flood irrigation of long borders or large basins, wetting almost the entire land surface.
•
Furrow irrigation for row crops, wetting only part of the ground surface.
•
Small- basin irrigation, whereby water is delivered in sequence to small basins or to individual trees.
These methods are easy to implement, less costly and require no energy for water application at the field level. Such methods, although are practiced and suitable for many developing countries particularly whenever water is plentiful and with relatively flat heavy to medium-textured soils.
The contact risk with these methods is high and implies an advanced level of treatment to the effluent before use.
Modern Irrigation methods These are, in general, pressurized networks including pumps, flow-meters, control valves and piped distribution, they include: •
Sprinkler, including fixed units, hand-movable units, center pivots, side-rolls, or mini-sprinklers, whereby all soil surfaces is wetted.
•
Localized surface systems, including trickle, bubbler systems and drip systems .
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22 1. Flood irrigation The application of irrigation water where the entire surface of the soil is covered by ponded water.
Flood irrigation
Water is applied over the entire field to infiltrate into the soil (e.g. wild flooding, contour flooding, borders, and basins). In flood irrigation, a large amount of water is brought to the field and flows on the ground among the crops. In regions where water is abundant, flood irrigation is the cheapest method of irrigation and this low tech irrigation method is commonly used by societies in developing countries. It should be applied only to flat lands that do not concave or slope downhill so that the water can evenly flow to all parts of the field, yet even so, about 50% of the water is wasted and does not get used by the crops. Some of this wasted water accumulates at the edges of a field and is called run-off. In order to conserve some of this water, growers can trap the run-off in ponds and reuse it during the next round of flood irrigation. However a large part of the wasted water can not be reused due to massive loss via evaporation and transpiration.
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23 One of the main advantages of flood irrigation is its ability to flush salts out of the soil, which is important for many saline intolerant crops. However, the flooding causes an anaerobic environment around the crop which can increase microbial conversion of nitrogen from the soil to atmospheric nitrogen, or denitrification, thus creating low nitrogen soil.
Surge flooding is an
attempt at a more efficient version of conventional flood irrigation in which water is released onto a field at scheduled times, thus reducing excess run-off.
2. Furrow irrigation Furrows are small, parallel channels, made to carry water in order to irrigate crops. The crop is grown on the ridges between the furrows. The method is suitable for a wide range of soil types, crops and land slopes. Water is applied between ridges (e.g. level and graded furrows, contour furrows, corrugations). Water reaches the ridge (where the plant roots are concentrated) by capillary action. A partial surface flooding method of irrigation normally used with clean-tilled crops where water is applied in furrows or rows of sufficient capacity to contain the designed irrigation system.
Furrow irrigation is actually a type of flood irrigation in which the water poured on the field is directed to flow through narrow channels between the rows of crops, instead of distributing the water throughout the whole field evenly. The furrows must all have equal dimensions, in order to guarantee that the water is distributed evenly. Like flood irrigation, furrow irrigation is rather cheap in areas where water is inexpensive.
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24 Furrow irrigation
Furrow irrigation does not wet the entire soil surface, and can reduce crop contamination, because plants are grown on ridges. Complete health protection cannot be guaranteed and the risk of contamination of farm workers is potentially medium to high, depending on the degree of automation of the process. If the treated wastewater is transported through pipes and delivered into individual furrows by means of gated pipes, the risk to irrigation workers is minimum, which may induce the development of disease vectors. Levelling of the land should be carried out carefully and appropriate land gradients should be provided. The following crops can be irrigated with furrow irrigation: •
Row crops such as maize, sunflower, sugarcane, and soybean
•
Crops that would be damaged by inundation (tomatoes, vegetables, potatoes, beans)
•
Fruit trees (citrus, grape)
•
Broadcast crops (wheat)
It is also suited to the growing of tree crops. In the early stages of tree planting , one furrow alongside the tree row may be sufficient but as the trees develop then two or more furrows can be constructed to provide sufficient water. Furrows can be used in most soils types. Soils that crust easily are especially suited to furrow irrigation because the water does not flow over the ridge and so the soil in which the plants grow remains friable.The shape of furrows is influenced by the soil type and the stream size.
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The location of plants in a furrow system is not fixed and depends on natural circumstances: •
In areas with heavy rainfall, the plants should stand on top of the ridge in order to prevent damage (waterlogging)
•
If water is scarce the plants may be put in the furrow itself, to benefit more from the limited water
For winter and early spring crops in colder areas, the seeds may be plant on the sunny side of the ridge. In hotter areas seeds may be planted on the shady side of the ridge to protect them from the sun.
3. Basin Irrigation For basin irrigation, flat areas of land surrounded by low bunds prevent water from flowing to the adjacent fields. Basin irrigation is used for rice grown on flat lands. In general the basin method is suitable for crops that are unaffected by standing in water for long periods. Other crops that can be irrigated are: •
Pastures (alfalfa, clover)
•
Trees (citrus, banana)
•
Crops which are broadcast (cereals)
•
Some extent row crops such as tobacco.
Basin irrigation
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Basin Irrigation is not suited to crops which can not stand in wet or waterlogged conditions for periods longer than 24 hours. These are usually root and crops such as potatoes, cassava, beet and carrots which require loose, well drained soils.
The construct of basins is easier the more flat is the surface of the land. A separation between rice and non-rice or other crops is made.Paddy rice is grown on clayey soils. Rice could also be grown on sandy soils. Many other crops can be grown on clays, loamy soils are preferred for basin irrigation so that waterlogging can be avoided. Coarse sands are not recommended for basin irrigation due to the high infiltration rate of the soil and also soils which form a hard crust when dry are not suitable.The basins shape and size are determined by the land slope, the soil type, the available stream size, the required depth of the irrigation application and farming practices. Basins should be small if the: •
Slope of the land is steep
•
Soil is sandy
•
Stream size to the basin is small
•
Required depth of the irrigation application is small
•
Field preparation is done by hand or animal traction.
Basins can be large if the: •
Slope of the land is gentle or flat
•
Soil is clay
•
Stream size to the basin is large
•
Required depth of the irrigation application is large
•
Field preparation is mechanize
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4. Sprinkler Sprinkler irrigation is similar to natural rainfall. Water is applied in the form of a spray and reaches the soil in much the same way as rain (e.g. portable and solid set sprinklers, travelling sprinklers, spray guns, center-pivot systems). A planned irrigation system in which water is applied by means of perforated pipes or nozzles operated under pressure so as to form a spray pattern.
Sprinkler irrigation
Sprinkler systems are the most common. They work on slopes with up to 30 % grade and are not limited by wastewater quality. The lateral pipes supplying water to the sprinklers should always be laid out along the land whenever possible. This will minimize the pressure changes at the sprinklers and provide a uniform irrigation. All types of crops can be irrigated using sprinkler systems. Is suited for most row, field and tree crops and water can be sprayed over or under the crop.
Large sprinklers are not recommended for irrigation of delicate crops such as lettuce because the large water drops produced by the sprinklers may damage the crop. Solid set sprinkler systems that are most often used in wastewater reuse system are: center pivot, travelling gun, and travelling lateral systems also have applications.
Sprinklers are suited to sandy soils with high infiltration rates but they can be used to most soils. They are not suitable for soils which easily form a crust.
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Some disadvantages to the use of sprinkler systems are the purchase, high placement costs, and large field space for the equipment. Another major limitation of sprinkler systems, especially when wastewater reuse takes place, is spray drift. Setbacks must be included in the field layout to minimize spray drift onto roads and dwellings.
Typical sprinkler system irrigation has the following components : •
Pump unit
•
Mainline and submainlines (sometimes)
•
Laterals
•
Sprinklers
To avoid problems of sprinkler nozzle blockage and spoiling the crop by coating it with sediment the water must be clean and free of suspended sediments.
When water sprays from a sprinkler it brakes up into small drops between 0.5-4.0 mm in size. These drops fall close to the sprinkler and the larger ones fall close to the edge of the wetted circle. Large drops damage delicated crops and soils so it is better to use smaller sprinklers (small diameter nozzles). For good uniformity several sprinklers must be operated close together. This uniformity can be affected by wind and water pressure. For that reason sprinklers must be positioned closely to reduce the effects of wind.
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5. Center-Pivot Center-Pivot is an automated sprinkler irrigation achieved by automatically rotating the sprinkler pipe or boom, supplying water to the sprinkler heads or nozzles, as a radius from the centre of the field to be irrigated. Water is delivered to the centre or pivot point of the system. The pipe is supported above the crop by towers at fixed spacings and propelled by pneumatic, mechanical, hydraulic, or electric power on wheels or skids in fixed circular paths at uniform angular speeds.
Center-Pivot irrigation
Water is applied at a uniform rate by progressive increase of nozzle size from the pivot to the end of the line. The depth of water applied is determined by the rate of travel of the system. Single units can irrigate about a 0,4 km2 circular area.
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30 6. Travelling Gun Travelling Gun is a sprinkler irrigation system consisting of a single large nozzle that rotates and is self-propelled. The name refers to the fact that the base is on wheels and can be moved by the irrigator or affixed to a guide wire.
Travelling Gun
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31 7. Drip systems Drip irrigation is sometimes called trickle irrigation and is the dripping of water onto the soil at very low rates from a system of small diameter plastic pipes fitted with outlets called emitters or drippers. Water is applied close to the plants so a little part of soil is wetted; the one near the roots of the plant. With drip irrigation applications are more frequent and this provides high moisture level in the soil.
While drip irrigation may be the most expensive method of irrigation, it is also the most advanced and efficient method in respect to effective water use. Usually used to irrigate row crops such as soft fruits and vegetables, tree and vine crops where more than one emitters can be applied for each plant. Because is a high cost system is applied only in high value crops. This system consists of perforated pipes that are placed by rows of crops or buried along their root lines and emit water directly onto the crops that need it. As a result, evaporation is drastically reduced and 25% irrigation water is conserved in comparison to flood irrigation. Drip irrigation is adaptable to any farmable slope and is suitable for most soils. On clay soils water must be applied slowly to avoid surface water ponding and runoff. On sandy soils is needed higher emitter discharge.
Water high in salts should be filtered before use because it might clog the emitters and create a local buildup of high salinity soil around the plants if the irrigation water contains soluble salts. If also the water contains algae, fertilizer deposits and dissolved chemicals such as calcium and iron may also cause blockage of the emitters.
A typical drip irrigation system contains the following: •
Pump unit
•
Control head
•
Main and submain lines
•
Laterals
•
Emitters or drippers
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32 Drip irrigation system
In a planned drip irrigation system water is applied directly to the root zone of plants by means of applicators (orifices, emitters, porous tubing, perforated pipe, etc.) operated under low pressure with the applicators being placed either on or below the surface of the ground.
Drip irrigation systems use low-rate emitters to deliver wastewater slowly to the plant. Wastewater must be very low in solids, and disinfection may be required to reduce biofilms that can clog
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emitters. Drip systems can be used on any slope and are well suited to permanent planting, such as landscaping. The equipment and installation costs for drip systems may be high, but they do not create spray drift problems.
When compared with other systems, the main advantages of trickle or drip irrigation are: •
Increased crop growth and yield achieved by optimising the water, nutrients and air regimes in the root zone.
•
High irrigation efficiency because there is no canopy interception, wind drift or conveyance losses, and minimal drainage loss.
•
Minimal contact between farm workers and wastewater.
•
Low energy requirements because the trickle system requires a water pressure of only 100-300 kPa (1-3 bar).
•
Low labour requirements because the trickle system can be easily automated, even to allow combined irrigation and fertilisation
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34 8. Bubbler Irrigation A relatively new technique called "bubbler irrigationÂť that was developed for localised irrigation of tree crops avoids the needs for small orifices. This system requires, therefore, less treatment of the wastewater but needs careful setting for successful application.
Bubbler irrigation
Bubbler systems (bubblers, pipes, valves, trenches, and basins), like drip systems, require routine maintenance. Bubbler systems are not immune to vandalism and wear, particularly at commercial, institutional and multifamily sites. In addition, the basins and trenches need to be kept clean to prevent overflow. Because of higher flows, bubbler systems waste more water when leaks, breaks, or over-scheduling problems occur.
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35 General Code of practice for treated domestic sewage effluent used for irrigation 1. The sewage treatment and disinfection must be kept and maintained continuously in satisfactory and effective operation so long as treated sewage effluent are intended for irrigation, and according to the license issued under the existing legislation. 2. Skilled operators should be employed to attend the treatment and disinfection plant, following formal approval by the appropriate authority that the persons are competent to perform the required duties, necessary to ensure that conditions of (1) are satisfied. 3. The treatment and disinfection plant must be attended every day according to the program issued by the Authority and records to be kept of all operations performed according to the instructions of the appropriate Authority. A copy must be kept for easy access within the treatment facilities. 4. All outlets, taps and valves in the irrigation system must be secured to prevent their use by unauthorized persons. All such outlets must be colored red and clearly labelled so as to warn the public that the water is unsafe for drinking. 5. No cross connections with any pipeline or works conveying potable water, is allowed. All pipelines conveying sewage effluent must be satisfactorily marked with red or other distinctive tape so as to distinguish them from domestic water supply. In unavoidable cases where sewage/effluent and domestic water supply pipelines must be laid close to each other the sewage or effluent pipes should be buried at least 0,5 m below the domestic water pipes. 6. Irrigation methods allowed and conditions of application differ between different plantations as follows: a. Park lawns and ornamental in amenity areas of unlimited access: •
Subsurface irrigation methods
•
Drip irrigation
•
Pop-up, low pressure and high precipitation rate, low angle sprinklers (less than 11 degrees). Sprinkling preferably to be practiced at night and when people are not around.
b. Park lawns and ornamental in amenity areas of limited access, industrial and fodder crops: •
Subsurface irrigation
•
Bubblers
•
Drip irrigation
•
Pop-up Sprinklers
•
Surface irrigation methods
•
Low capacity sprinklers
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Spray or sprinkler irrigation, is allowed with a buffer zone of about 300 meters.
For fodder crops, irrigation is recommended to stop at least one week before harvesting and no milking animals should be allowed to graze on pastures irrigated with sewage. Vetenary Services should be informed. c. Vines: •
Drip irrigation
•
Minisprinklers and sprinklers (in case where crops get wetted, irrigation should stop two weeks before harvesting).
•
Movable irrigation systems are not allowed.
•
No crops should be selected from the ground.
d. Fruit trees •
Drip irrigation
•
Hose basin irrigation
•
Bubblers irrigation
•
Mini sprinklers
No fruits to be collected form the ground except for nut-trees. In case where crops get wetted irrigation should stop one week before harvesting. e. Vegetables •
Subsurface irrigation
•
Drip irrigation
Crops must not come in contact with the ground or the effluents (only vegetables which are supported). Other irrigation methods could also be considered. f. Vegetables eaten cooked. •
Sprinklers
•
Subsurface irrigation
•
Drip irrigation
Other irrigation methods may be allowed after the approval of the appropriate Authority Restrictions may be posed to any method of irrigation by the appropriate authority in order to protect public health or environment. 7.
Suitable facilities for monitoring of the essential quality parameters, should be kept on site of treatment.
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MORE ABOUT AGRICULTURAL IRRIGATION
http://www.fao.org/docrep/T0551E/t0551e07.htm http://www.fao.org/docrep/T0551E/T0551E00.htm
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4.4 Landscape irrigation The second category, landscape irrigation, includes the irrigation of parks; playgrounds; golf courses; freeway medians; landscaped areas around commercial, office, and industrial developments; and landscaped areas around residences. Many landscape irrigation projects involve dual distribution systems — one distribution network for potable water and another for reclaimed water.
Potential constraints and issues for landscape irrigation are given bellow :
Issues/constraints • Surface and groundwater contamination if not managed properly • Effect of water quality, particularly salts, in landscape areas • Public health concerns related to pathogens (e.g., bacteria, viruses, and parasites) • Use area control including buffer zone may result in high user costs
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As for agricultural irrigation, the same four categories of potential management problems associated with water quality in irrigation are:
(1) salinity (2) specific ion toxicity (3) water infiltration rate (4) other problems
Common irrigation methods (as listed in agricultural irrigation), for landscape reuse, are : •
Sprinkler irrigation
•
Travelling gun irrigation
•
Drip irrigation
•
Bubbler irrigation
4.5 Industrial recycling & reuse
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40 The third major use of reclaimed water is in industrial activities, primarily for cooling and process needs. Cooling water is the predominant industrial water reuse and, for either cooling towers or cooling ponds, creates the single largest demand for water in many industries. Industrial uses vary greatly, and to provide adequate water quality, additional treatment is often required beyond conventional secondary wastewater treatment.
Potential constraints and issues for Industrial recycling & reuse are given bellow :
Issues/constraints • Constituents in reclaimed water related to scaling, corrosion, biological growth, and fouling • Public health concerns, particularly aerosol transmission of pathogens in cooling water • Cross connection of potable and reclaimed water lines
Cooling tower operation Among several industrial water reuse applications, cooling tower makeup water represents a significant water use for many industries and is currently the predominant industrial water reuse application. For industries such as electric power generating stations, oil refining, and many other types of manufacturing plants, onequarter to more than one-half of the total water use may be cooling tower makeup water. The basic principle of cooling tower operation is that of evaporative condensation and exchange of sensible heat. The air and water mixture releases latent heat of vaporization. Water exposed to the atmosphere evaporates and as the water changes to vapor, heat is consumed. Under normal operating conditions, the loss of water discharged from the cooling tower to the atmosphere as hot, moist vapor amounts to approximately 1.2 % for each 5.5°C of cooling range. Drift, or water lost from the top of the tower to the wind, is the second mechanism by which water is lost from the cooling system. About 0.005 % of the recirculating water is lost in this way. While evaporation results in a loss of water from the system, the salt concentration is increased because salts are not removed by evaporation. To prevent the formation of precipitates in the resulting moreconcentrated tower water, a portion of the concentrated cooling water is bled off and replaced with low-salt-makeup water to maintain a proper salt balance. This highly saline water bled off from the cooling tower system is called blowdown.
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Definition sketch for salt balance in the recirculating, evaporative cooling tower
(Eddy, 1999)
When the cycles of concentration are on the order of 3 to 7, some of the dissolved solids in the circulating water can exceed their solubility limits and precipitate, causing scale formation in pipes and coolers. To avoid scale formation, sulfuric acid is often used to convert calcium and magnesium carbonates into more soluble sulfate compounds. The amount of acid used must be limited to maintain some residual alkalinity in the system. If the pH of the system is reduced too far below 7, accelerated corrosion can occur.
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Four general water quality problems are encountered in industrial cooling tower operations: (1) scaling, (2) metallic corrosion, (3) biological growths, (4) fouling in heat exchanger and condensers.
Both freshwater and reclaimed water contain constituents that can cause these problems, but their concentrations in reclaimed water are generally higher.
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4.6 Groundwater recharge The fourth reuse application for reclaimed water is groundwater recharge, via either spreading basins or direct injection to groundwater aquifers. Groundwater recharge involves assimilation of reclaimed water for replenishment, storage in groundwater aquifers, or establishing hydraulic barriers against saltwater intrusion in coastal areas.
Potential constraints and issues for groundwater recharge are given bellow :
Issues/constraints • Possible contamination of groundwater aquifer used as a source of potable water • Organic chemicals in reclaimed water and their toxicological effects • Total dissolved solids, nitrates, and pathogens in reclaimed water
There are several advantages to storing water underground:
(1) the cost of artificial recharge may be less than the cost of equivalent surface reservoirs; (2) the aquifer serves as an eventual distribution system and may eliminate the need for surface pipelines or canals; (3) water stored in surface reservoirs is subject to evaporation, to potential taste and odor problems due to algae and other aquatic productivity, and to pollution; (4) suitable sites for surface reservoirs may not be available or environmentally acceptable; (5) the inclusion of groundwater recharge in an indirect potable reuse project may also provide health, psychological, and aesthetic secondary benefits as a result of the transition between reclaimed water and groundwater.
Two methods of groundwater recharge are used commonly with reclaimed water: (1) surface spreading in basins, and (2) direct injection into groundwater aquifers
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Groundwater Recharge by Surface Spreading in basins In surface spreading, recharge waters percolate from the spreading basins through an unsaturated groundwater (vadose) zone. Infiltration basins are the most favored methods of recharge because they allow efficient use of space and require relatively little maintenance.
Where hydrogeological conditions are favorable for groundwater recharge with spreading basins, water reclamation can be implemented relatively simply by the rapid infiltration (also known as the soil-aquifer treatment (SAT) system or geopurification). Because recharged groundwater may be an eventual source of potable water supply, groundwater recharge with reclaimed water often involves treatment beyond the conventional secondary treatment. For surface spreading operations practiced, common wastewater reclamation processes prior to recharge include primary and secondary wastewater treatment, and tertiary granular-medium filtration followed by disinfection with chlorine or UV radiation.
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Typical schematics of soilaquifer treatment (SAT) systems with recovery of renovated water by: (a) subsurface drains, (b) wells surrounding the spreading basins, and (c) wells midway between two parallel strips of basins
(Eddy, 1999)
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There are four types of land treatment systems when surface spreading method is implemented :
1. Slow-Rate Irrigation System 2. Overland Flow System 3. Rapid Infiltration Systems 4. Subsurface Drip Irrigation
1.
Slow-Rate Irrigation System
Slow-rate irrigation systems are the most frequently used land treatment systems. They provide essential nutrients to satisfy the growth requirements of agricultural crops. Pretreated wastewater is applied to land (soil texture ranges from sandy loams to clay loams), by sprinkler or surface distribution, at a relatively slow rate (0.5–6 m/year) (weekly loading rate of 1.3–10 cm) and serves as a source of nutrients for forage (e.g., alfalfa, bermuda grass, ryegrass), or field crops (e.g., corn, cotton, barley).
Some disadvantages of the slow-rate irrigation system are land cost, high operating cost, and transport of wastewater to the treatment site.
Slow-rate irrigation system
(Gabriel Bitton, 2005)
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2.
Overland Flow System
Wastewater is applied at a rate of 3–20 m/year or more and flows down a grass-covered slope (2– 8 %) with a length of 30–60 m. The most suitable soils are clay or clay-loamy soils with low permeability (equal to or less than 0.5 cm/h) to limit wastewater percolation through the soil profile. The treated effluent is captured in a collection channel. Nutrients (N, P, and BOD), suspended solids, and pathogens are removed as wastewater flows down the slope.
Overland flow system
(Gabriel Bitton, 2005)
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3.
Rapid Infiltration Systems
Wastewater is applied intermittently at high loading rates (6–125 m/year) onto a permeable soil (e.g., sands or loamy sands). Most of the applied wastewater flows to groundwater aquifers. The treated wastewater may be collected via recovery wells. The minimum depth to groundwater is 1 m during flooding periods and 1.5–3 m during dry periods. The treatment potential of rapid infiltration systems is lower than in slow-rate systems. Removal of nitrogen is generally low, but may be increased by encouraging denitrification. Denitrification requires adequate carbon levels (as found in primary effluents) and low oxygen levels, necessitating flooding periods as long as 9 days followed by drying periods of about 2 weeks.
4.
Subsurface Drip Irrigation
Subsurface drip irrigation is practiced in arid areas to save water. It consists of dripping water near the root zone (30–50 cm below the soil surface) at a rate that depends on the plant water requirements. This practice helps avoid or minimize biological aerosol production, contamination of aerial parts of crops and wastewater flow to groundwater. From previous researches, it was shown that subsurface drip irrigation of turfgrass can reduce the risk of contamination by potential viral pathogens when compared to sprinkler irrigation. Similarly, as compared to sprinkler irrigation, surface and subsurface drip irrigation were found to reduce vegetable contamination with Cryptosporidium oocysts or Giardia cysts .
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49 Typical Subsurface drip irrigation system installation
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50 Typical Subsurface drip irrigation system schematic
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51 Groundwater Recharge by Direct Injection Direct subsurface recharge is achieved when water is conveyed and injected directly into groundwater aquifer. In direct injection, generally, highly treated reclaimed water is injected directly into the saturated groundwater zone, usually into a well-confined aquifer. Groundwater recharge by direct injection is practiced, in most cases, where groundwater is deep or where the topography or existing land use makes surface spreading impractical or too expensive. This method of groundwater recharge is particularly effective in creating freshwater barriers in coastal aquifers against intrusion of saltwater from the sea.
Particulate constituents including microorganisms in reclaimed water are removed by filtration and retained effectively by the soil matrix.
In addition to the common dissolved mineral constituents, reclaimed water contains many dissolved trace elements. The physical action of filtration does not, however, accomplish the removal of these dissolved inorganic contaminants. For trace metals to be retained in the soil matrix, physical, chemical, or microbiological reactions are required to immobilize the dissolved constituents. In a groundwater recharge system, the impact of microbial activities on the attenuation of inorganic constituents is small. Physical and chemical reactions in the soil that are important with respect to trace metal elements include cation exchange, precipitation, surface adsorption, and chelation and complexation. Although soils do not possess unlimited capability in attenuating inorganic constituents, it has been found in experimental studies that soils do have capacities for retaining large amounts of trace metal elements. Therefore, it is conceivable that a site used for groundwater recharge may be effective in retaining trace metals for extended periods of time.
Biodegradation of easily degradable substances takes place almost exclusively in the first one or 0.6 m of travel. There is increasing evidence that a significant portion of the observed degradation occurs in the biological mat that forms on the infiltrating surface in a spreading basin. The fate of some of the more resistant organic compounds found in recharge water is still poorly understood.
Groundwater contamination by pathogenic microorganisms has not received as much attention as surface water. It has been generally assumed that groundwater is free of pathogenic microorganisms. However, a number of welldocumented disease outbreaks have been traced to
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contaminated groundwater. The fate of bacterial pathogens and viruses in the subsurface environment is determined by their survival characteristics and their retention in the soil matrix. Both survival and retention are largely determined by (1) climate, (2) nature of the soil, and (3) nature of microorganisms. Temperature and rainfall are two important climatic factors that will affect viral and bacterial survival and movement. At higher temperatures, inactivation or natural die-off is fairly rapid. For bacteria, and probably viruses, the die-off rate is approximately doubled with each 10째C rise in temperature between 5 and 30째C. Rainwater, because of its lower pH value, can elute adsorbed virus particles, which may then move with the groundwater. The physical and chemical characteristics of the soil will also play a major role in determining survival and retention of microorganisms. Soil properties influence moisture-holding capacity, pH, and organic matter. All of these factors control the survival of bacteria and viruses in the soil.
Summary of the Main Factors Governing the Transport of Microbial Pathogens Through Soils
(Gabriel Bitton, 2005)
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Summary of the Main Factors Governing the Persistence of Microbial Pathogens in Soils
(Gabriel Bitton, 2005)
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54 Factors to be considered in the formulation of the groundwater recharge guidelines are summarized in the following Table :
Factors to be considered in the formulation of groundwater recharge guidelines
(Eddy, 1999)
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4.7 Recreation/environmental uses A fifth use of reclaimed water, recreational/environmental uses, involves a number of nonpotable uses related to land-based water features such as the development of recreational lakes, marsh enhancement, artificial lakes and ponds, snowmaking and stream-flow augmentation. Reclaimed water impoundments can be incorporated into urban landscape developments. Man-made lakes, golf course storage ponds, and water traps can be supplied with reclaimed water. Reclaimed water has been applied to wetlands for a variety of reasons including creation, restoration, and enhancement of habitat, provision for additional treatment prior to discharge to receiving water, and provision for a wet weather disposal alternative for reclaimed water.
Potential constraints and issues for recreational/environmental uses are given bellow :
Issues/constraints • Health concerns related to presence of bacteria and viruses (e.g., enteric infections and ear, eye, and nose infections) • Eutrophication due to nitrogen and phosphorus in receiving water • Toxicity to aquatic life
4.8 Nonpotable urban uses The sixth reuse category, nonpotable urban uses, includes such uses as fire protection, air conditioning, toilet flushing, construction water, commercial car wash, driveways or tennis court washdown and flushing of sanitary sewers. Typically, for economic reasons, these uses are incidental depending on the location of the wastewater reclamation plant and whether these applications can be coupled with other ongoing reuse applications such as landscape irrigation.
Potential constraints and issues for nonpotable urban uses are given bellow :
Issues/constraints • Public health concerns about pathogens transmitted by aerosols • Effects of water quality on scaling, corrosion, biological growth, and fouling • Cross connection of potable and reclaimed water lines
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4.9 Indirect potable reuse The seventh reuse opportunity involves potable reuse, which could occur by blending in water supply storage reservoirs (surface water or groundwater)
Potential constraints and issues for indirect potable reuse are given bellow :
Issues/constraints • Constituents in reclaimed water, especially endocrine disrupterstrace, organic chemicals and their toxicological effects • Aesthetics and public acceptance • Health concerns about pathogen transmission, particularly enteric viruses
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4.10 Calculations Problem Calculation of Adjusted Sodium Adsorption Ratio and Evaluation of Potential Water Infiltration Problems . The following water quality analyses were reported on an aerated lagoon effluent which will be used for irrigating agricultural land.
Using the reported water quality data estimate following questions: (1) calculate adj RNa (2) determine whether an infiltration problem may develop by using this effluent for irrigation
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Solution
(1) Calculate the adj RNa
•
Convert the concentrations of the related water quality parameters to meq/L Ca2+= 37/20.04 = 1.85 meq/L Mg2+= 46/12.15 = 3.79 meq/L Na+ = 410/23 = 17.83 meq/L HCO3- = 295/61 = 4.84 meq/L
•
Determine the value of Cax2+ using the given water quality data i. Salinity of applied water (ECw) = 2.4 dS/m ii. Ratio of HCO3- / Ca2+= 4.84/1.85 = 2.62 iii. From relative Table, (Values of Cax2+ as a function of the HCO3- / Ca2+ ratio and solubility) the value of Cax2+ = 1.20 meq/L
•
the adj RNa is : = 11.29
(2) Determine whether an infiltration problem may develop by using this effluent for irrigation
Entering relative Table (Guidelines for interpretations of water quality for irrigation) with adj RNa =11.29 and ECw = 2.4 dS/m, no restrictions are indicated for use of this reclaimed wastewater.
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GLOSSARY 4.11 Glossary Beneficial uses - The many ways water can be used, either directly by people, or for their overall benefit. Examples include municipal water supply, agricultural and industrial applications, navigation, fish and wildlife, and water contact recreation Direct potable reuse - A form of reuse by the incorporation of reclaimed water directly into a potable water supply system, often implying the blending of reclaimed water with potable water Direct reuse - The use of reclaimed water which has been transported from the wastewater reclamation plant to the water reuse site without intervening discharge to a natural body of water, including such uses as agricultural and landscape irrigation Dual distribution system - Two sets of pipelines for water delivery, one for potable water and another for reclaimed water Indirect potable reuse - Potable reuse by incorporation of reclaimed water into a raw water supply, allowing mixing and assimilation by discharge into an impoundment or natural body of water, such as in a domestic water supply reservoir or groundwater Indirect reuse - Use of reclaimed water indirectly by passing through a natural body of water or use of groundwater that has been recharged with reclaimed water Nonpotable water reuse - All reuse applications that do not involve either indirect or direct potable use Planned reuse - Deliberate direct or indirect use of reclaimed water, without relinquishing control over the water during its delivery Potable water reuse - An augmentation of drinking water supplies directly or indirectly by reclaimed water that is highly treated to protect public health Reclaimed water - Water that, as a result of wastewater treatment, is suitable for a direct beneficial use or a controlled use that would not otherwise occur Recycled water - Reclaimed water that has been used beneficially. The term recycled water is used synonymously with reclaimed water Unplanned reuse - Reuse of treated wastewater following discharge (control relinquished), such as in the diversion of water from a river downstream of a discharge of treated wastewater Water reclamation - Treatment or processing of wastewater to make it reusable. Also, this term is often used to include delivery of reclaimed water to place of use and its actual use Water recycling - The use of wastewater that is captured and redirected back into the same water use scheme. Recycling is practiced predominantly in industries, such as manufacturing, and normally involves only one industrial plant or one user Water reuse - The use of treated wastewater for a beneficial use, such as agricultural irrigation and industrial cooling
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ASSIGNMENTS SECTION QUESTIONS
1. Mention the first 4 categories of municipal wastewater reuse worldwide, in descending order.
2. In which categories typical constituents found in wastewater are classified?.
3. Which treatment process can achieve the best effluent quality ? 1) Activated sludge + granular medium filtration + carbon adsorption + reverse osmosis 2) Biological nitrogen and phosphorus removal + filtration
4. Which are the four main categories of potential management problems associated with water quality in irrigation ?
5. The electrical conductivity (EC) of a water can be used as a surrogate measure of total dissolved solids (TDS) concentration. True or false?
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61 6. Which are the ions of most concern in wastewater, for agricultural reuse?
7. Excessive nitrogen in reclaimed water is always benefficial for crops. True or false?
8. Chlorine residuals in excess of 5 mg/L can cause severe plant damage when reclaimed water is sprayed directly on foliage. True or false?
9. Mention conventional irrigation methods used.
10. Flood irrigation method must always be applied only to flat lands. True or false?
11. Flood irrigation is the most efficient irrigation method in terms of water economy. True or false?
12. Mention the main advantage of flood irrigation in terms of salt transport to the soil.
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13. Mention the main disadvantage of sprinkler irrigation in when reuse of wastewater takes place
14. Drip irrigation may is the most expensive method of irrigation. True or false?
15. Drip irrigation is the less advanced and inefficient method in respect to water use, because of small flows from driping system. True or false?
16. Mention the more significant industrial water reuse application.
17. Mention the two methods of groundwater recharge.
18. Mention the four types of land treatment systems when surface spreading method is implemented.
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19. When groundwater recharge takes place,
die-off rate of bacteria, and viruses, is
approximately doubled with each 10째C rise in temperature between 5 and 30째C. True or false?
20. When groundwater recharge takes place , the higher the flow rate, the higher the microbial adsorption to soils. True or false?
21. Microbial pathogens show greater survival rates at soil surface. True or false?
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SUGGESTED ANSWARS: 1. • • • • 2. •
Agricultural irrigation Landscape irrigation Industrial recycling and reuse Groundwater recharge
•
Nonconventional
•
Emerging
Conventional
3. 1) 4. (1) salinity (2) specific ion toxicity (3) water infiltration rate (4) other problems 5. True 6. sodium, chloride, and boron 7. False 8. True 9. •
Flood irrigation
•
Furrow irrigation
•
Small- basin irrigation
10. True 11. False 12. Its ability to flush salts out of the soil, which is important for many saline intolerant crops 13. Spray drift 14. True 15. False 16. Cooling tower makeup water 17. (1) surface spreading in basins (2) direct injection into groundwater aquifers 18. •
Slow-Rate Irrigation System
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•
Overland Flow System
•
Rapid Infiltration Systems
•
Subsurface Drip Irrigation
19. True 20. False 21. False
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