1.1Introduction
Growing unprecedented populations and increasing pressure on the development of new water resources have prompted a variety of measures to reclaim, recycle and reuse wastewater over the last two or three decades. As part of this trend, some municipalities have commenced to reuse wastewater for non-potable water needs, such as irrigation of golf courses and parks. In a little but increasing number of municipalities, these measures involve the use of treated wastewater to augment the general water supply. A major catalyst for the development of wastewater reuse, recycling and reclamation has been the need to provide alternative water resources to achieve water quantity requirement for industry, irrigation, urban potable and non-potable water applications. The benefits coupled with reusing treated wastewater for supplemental applications prior to disposal or discharge include environmental protection, preservation of high quality water resources and economic advantages.
These “wastewater reuse” projects are made possible by reliability and effectiveness of wastewater treatment technologies that can turn municipal wastewater into reclaimed wastewater that can serve as a supplemental water resource in addition to meeting standards established by the Safe Water Drinking Act. However, important problems remain regarding the levels of testing, monitoring and treatment needed to ensure human health when reclaimed wastewater is consumed for potable purposes. Some engineering and public health professionals oppose in principle to the reuse of wastewater for potable purposes, because standard public health philosophy and engineering practice call for using the purest source possible for drinking water.1 Others worry that existing techniques might not discover all the chemical and microbial contaminants that may be present in reclaimed wastewater. Several guidelines pertaining to potable reuse of wastewater have been issued, but these guidelines offer conflicting guidance on whether potable is adoptable and, when it is adoptable, what safeguards should be in place.
1.2 The Earth’s Water Resources
Earth is known as the “Blue Planet” because water is discovered in many places on Earth including in the atmosphere, on the surface of the Earth and within rocks below the surface. The total volume of water on the planet is about 1,360,000,000 km3. About 71 percent of Earth’s surface is covered with water, and the oceans hold about 97 percent of all Earth’s water. Figure 1.1 illustrates the approximate distribution of the locations of water on Earth, of which only about 3 percent of the Earth’s water is classified as freshwater and only about 0.91 percent is discovered in freshwater lakes, swamps, rivers and groundwater supplies available for human consumption.
Figure 1.1 Distribution of water in the hydrosphere.
The water cycle or hydrologic cycle describes the continuous movement of water within the hydrosphere. This indicates the cyclic movement of water evaporated from water surfaces, land surfaces and snow fields or evapotranspiration from land plants and animals to the atmosphere. Atmospheric moisture condenses into clouds and precipitated to the earth as rain, snow, hail or in some other form. Once the precipitated water has fallen to Earth, it may percolate through soil strata to form groundwater aquifers or runs off into streams, lakes, ponds and the sea. Groundwater and surface water drain toward the sea for recycling.
Many sub-cycles to the global-scale hydrologic cycle exist, involving the managed transport of water, such as an aqueduct. Wastewater reuse, reclamation and recycling have become important elements of the hydrologic cycle in industrial, agricultural and urban areas. Figure 1.2 illustrates an overview of the cycling of water from ground water and surface water resources to water treatment plants, industrial, irrigation, municipal application, and to wastewater reclamation and reuse facilities.
Figure 1.2 Water reuse application.
1.1Types of Water Reuse
When considering the reuse of treated wastewater for potable purposes, critical distinctions must be made between “indirect” and “direct” potable reuse and between “unplanned” and “planned” potable reuse.
The key distinction between indirect and direct potable reuse is that direct potable reuse does not make use of any environmental barrier. In other words, simply sending treated wastewater from a wastewater treatment facility directly to a potable water-supply distribution system or a potable source treatment facility. This practice is rarely use because of the increased potential risk to public health and the negative public perception.
Indirect potable reuse is that the purified reclaimed water is pumped into a raw water supply, such as an underground aquifer or in potable water storage reservoirs, resulting in mixing, dilution and assimilation, thus providing an environmental buffer.
Indirect potable reuse can be unplanned and planned. Unplanned indirect potable reuse occurs continuously in the environment. This results when a water supply has a natural source that contains unintentional addition of wastewater. Planned indirect potable reuse is common practice to artificially recharge water supply sources with reclaimed water derived from treated wastewater. The water receives additional treatment prior to distribution. The reason that indirect potable reuse is not considered to cause a health risk is that the treated wastewater benefits from natural treatment from storage in surface water and groundwater aquifer before abstraction to ensure good water quality.
1.2Overview of Wastewater Treatment Technology
The problems surrounding wastewater reuse are essentially related to public health. Only in unusual situations do the substances in sewage significantly downgrade the value of water for other purposes. Many diseases are caused by organisms that may be present in wastewater. In addition, there are many toxic and carcinogenic substances present in wastewater at levels that may or may not be adequate to cause disease.
The effective wastewater treatment technology to meet water quality requirements for wastewater reuse applications and to protect public health is a crucial element for wastewater reuse system. Conventional wastewater treatment consists of a combination of physical, chemical and biological processes and operations to eliminate solids, organic matter, pathogens, metals and sometime nutrients from wastewater.2 Common terms used to define different degrees of treatment, in sequence of increasing treatment level are preliminary, primary, secondary, tertiary and/or advanced treatment. In some regions, disinfection step for control pathogenic organisms sometimes follows the last treatment step. Figure 1.3 shows a generalized wastewater treatment diagram.
Figure 1.3 Generalized flow diagram for conventional wastewater treatment
1.1.1 Preliminary Treatment
The purpose of preliminary treatment is the removal of sands, solids and rags that would settle in channel and interfere with treatment processes. Removal of these materials is necessary to protect the operation of subsequent treatment units. Preliminary treatment of wastewater typically includes screening, grinding, grit removal, flotation, equalization and flocculation. Treatment equipment such as bar screens, comminutors and grit chambers are adopted as the wastewater first enters a wastewater treatment plant. In grit chambers, the velocity of wastewater through the chamber is retained sufficiently high, so as to avoid the settling of organic solids. Comminutors are sometimes used to supplement course screening and serve to decrease the size of particles so that they will be removed and disposed of in a landfill.
1.1.2 Primary Treatment
Primary treatment is the second step in treatment and removes organic and inorganic matters from raw sewage by the physical processes. Primary treatment includes screening to trap solid matters, comminution for removal of large solids, grit removal and sedimentation by gravity to remove suspended solids. In general, about one-half of suspended solids and 20 to 50 percent of the biochemical oxygen demand are removed from the wastewater by primary treatment process. Nutrients, pathogenic organisms, trace elements and potentially toxic organic compounds that are associated with solids in wastewater can also be removed by primary treatment processes.
1.1.3 Secondary Treatment
Secondary treatment systems remove the biodegradable dissolved and colloidal matter using an array of biological processes coupled with solid/liquid separation. Biological processes are engineered to provide effective microbiological metabolism of organic substrates dissolved or suspended in wastewater.2 Part of the organic matter is oxidized by the microorganisms, thereby producing carbon dioxide and other end products. The remaining organic matter in wastewater provides the materials and energy needed to sustain the microorganism community. Secondary treatment systems can remove suspended solids and up to 95 percent of the biochemical oxygen demand entering the process, as well as certain organic compounds and significant amount of heavy metals.
1.1.4 Tertiary and/or Advanced Treatment
Tertiary and/or advanced treatment is adopted when specific constituents which cannot be removed by primary and secondary treatment must be removed. In general, tertiary treatment refers to additional removal of suspended material by granular medium filtration and chemical coagulation. In other cases, advanced treatment refers to more complete removal of specific constituents, such as ammonia or nitrate removal by ion exchange or total dissolved solids removal by reverse osmosis.2 These processes essentially remove more than 99 percent of all the pollutants from wastewater, producing an almost drinking water quality.
1.1.5 Disinfection
The objective of disinfection in the wastewater treatment is to destroy all pathogenic microorganisms. The major groups of pathogenic microorganisms include bacteria, viruses, amoebic cysts and protozoa. In general, disinfection can be achieved by chemical or physical method that destroys pathogens. Chemical methods are based on the addition of a strong acid, alcohol or an oxidizing chemical (such as chlorine, ozone, hydrogen peroxide or bromine). Alternatively, physical methods might include heating, incineration and irradiation with ultraviolet radiation. Disinfection is frequently combined with treatment plant design, but not effectively practiced, because of the reduced effectiveness of ultraviolet radiation or the high cost of chlorine where the water is not sufficiently clear or free of particles.
1.2 Types of Contaminants
An important issue for people to understand that there are various types of contaminants that may be in your water. The specific contaminants leading to pollution in water involve a wide spectrum of pathogenic organisms, inorganic chemicals and organic chemical. High concentrations of contaminants can have adverse effects to our health.
1.2.1 Pathogenic Organisms
Bacterium in water, also known as pathogenic organism, is a public health hazard with risk factors in nearly all regions of the world. It is evident from the water purification attempts throughout history that human realized that drinking water could be hazardous. Several other infectious diseases can be transmitted by contaminated water. Bacterial diseases include Typhoid fever, Cholera, Shigellosis and Salmonellosis. Gastroenteritis, Hepatitis A and SARS are examples of viral disease. Parasitic diseases, such as Schistosomiasis, Ascariasis and Taeniasis, are also transmitted via water.
1.2.2 Inorganic Chemicals
Wastewater contains many inorganics that present known or potential health risks if consumed. These contaminants include such compounds as lead, cadmium, chromium, arsenic, nitrate and sulphate. Arsenic and lead are cumulative chemical poisons that can result in cancer, dermal lesions, peripheral neuropathies and vascular effects.
1.2.3 Organic Chemicals
In a 1980 survey, a number of organic chemicals were found in water supplies. The term organic chemicals in this sense mean that they contain carbon atoms, such as chlorinated hydrocarbons, aliphatic compounds, benzenes and phenols, which mean that they are derived from petroleum. Organic chemical can easily combine with human tissue which can cause damage that includes kidney, liver system problems and increased cancer risk.
Chapter 2 Wastewater Reuse Criteria2.1 Introduction
The principal issue of concern for consumer of treated wastewater is the quality of this water includes its physical, biological, chemical and radiological characteristics. These concerns therefore necessitate the formulation of criteria, standards and guidelines that are appropriate for the consumers of this water.3
A first stage in establishing wastewater reuse regulations and guidelines is wastewater reuse criteria. Wastewater reuse criteria are principally directed at health and environmental protection and typically address wastewater treatment, reclaimed water quality, treatment reliability, distribution systems and use area controls.2 Wastewater reuse criteria imply an idea condition without a legal basic.
Regulations and guidelines are different in that regulations are legally enforceable and spell out specific figures that can be used for enforcement and administrative action, which guidelines do not have legal basic and compliance is voluntary. In theUnited States, the Environment Protection Agency issued guidelines in 1992 that are intended to offer guidance to states, which have not developed their own regulations or guidelines. At the international level, the World Health Organization has developed guidelines for wastewater reuse in agriculture and aquaculture. The World Health Organization guidelines are adopted throughout the world and provide all countries with the necessary information to set their own wastewater reuse regulations or guidelines.
2.2 Wastewater Quality for Reuse Applications
Table 2.1 presents general wastewater reuse applications. The types of wastewater reuse may be classified into the following six broad categories include agricultural and landscape irrigation, industrial reuse, groundwater recharge, recreational and environmental, non-potable urban uses and potable reuse. Wastewater reuse can be employed to satisfy the water demand in various fields and contribute to the freshwater resources conservation.
Table 2.1 Categories of Wastewater Reuse and Potential Constraints
Wastewater reuse categoryaPotential constraints Agricultural and landscape irrigation Crop irrigationEffects of salts on soils and crops. Commercial nurseriesPublic health concerns, surface and groundwater pollution, marketability of crops, and public acceptance. Parks School yards Freeway medians Golf courses Cemeteries Greenbelts Residential areas Industrial reuse CoolingScaling, corrosion, biological growth, and fouling; public health concerns. Boiler feed Process water Heavy construction Groundwater recharge Groundwater replenishmentPotential toxicity of chemicals and pathogens. Salt water intrusion Subsidence control Recreational and environment Lakes and pondsHealth concerns and eutrophication. Marsh enhancement Streamflow augmentation Fisheries Snowmaking Non-potable urban uses Fire protectionPublic health, foulinf, scaling, corrosion, and biological growth. Air conditioning Toilet flushing Potable reuse Blending in water supplyPotential toxic chemicals, public health, and public acceptance. Pipe-to-pipe water supplya Arranged in descending order of anticipated volume of use.
From Asano, T.D., et al., Water Environ. Technol., 4, 36, 1992.
2.2.1Wastewater Reuse for Agricultural Irrigation
By far the biggest user of wastewater is agriculture throughout the entire semi-tropical and arid tropical areas of the world. Agriculture receives 67 percent of total water withdrawal and account for 86 percent of consumption in 2000. In Asia and Africa, an estimated 85 to 90 percent of all the freshwater use is for agriculture. By 2025, agriculture is anticipated to increase its water demands by 1.2 times. Therefore, wastewater reuse is important for sustainable water management. The reuse of wastewater for agriculture has some benefits as well as some disbenefits.4, 5
Benefits include the following:
Source of extra irrigation water. Conservation of freshwater for other beneficial uses. Low cost source of a water supply. Alternative way to dispose of wastewater and avoid pollution and sanitary issues. Dependable, continuous water source. Effective use of plant nutrients contained in the wastewater, such as nitrogen and phosphorus. Provides extra treatment of the wastewater before being recharged to the groundwater.Disbenefits include the following:
Wastewater not properly treated can cause potential public health issues. Hazardous chemical contamination of groundwater. Certain soluble constituents in the wastewater could be present at concentrations toxic to plants. The wastewater could contain suspended solids that may plug the capillary pores in the soil as well as block nozzles in the water distribution system. Great investment in equipment and land.Regulation, guideline and criteria have been established for the reuse of wastewater for agriculture and are normally based on several parameters, such as public health protection and concentration of components in the water. These components include salinity, boron, exchangeable ions and trace metals are of particular important.
Table 2.2 presents the details of guidelines for water quality to be used for agricultural irrigation. These guidelines are established by the Food and Agricultural Organization in United Nation.
I. Salinity
As indicated, salinity is the most influential parameter in determining the applicability of water for agricultural irrigation. Salinity refers to the presence of dissolved salts in the soil and water.
Table 2.2 Guidelines for Interpretation of Water Quality for Irrigation
Degree of restriction on use Potential irrigation problemUnitsNoneSlight to moderate Severe Salinity (affects crop water availability)a ECdS/m<0.70.7-3.0 >3.0 or, TDSmg/L<450450-2000 >2000 Infiltration (affects infiltration rate of water into the soil. Evaluation using EC and SAR together)b SAR= 0-3and EC =>0.70.7-0.2 <0.2 = 3-6and EC =>1.21.2-0.3 <0.3 = 6-12and EC =>1.91.9-0.5 <0.5 = 12-20and EC =>2.92.9-1.3 <1.3 = 20-40and EC =>5.05.0-2.9 <2.9 Specific ion toxicity (affects sensitive crops) Sodium (Na)b Surface irrigationSAR<33-9 >9 Sprinkler irrigationmg/L<3<3 Chloride (Cl)c Surface irrigationmg/L<44-10 >10 Sprinkler irrigationmg/L<3>3 Boron (B)mg/L<0.70.7-3.0 >3.0 Trace elements (See Table) Miscellaneous effects (affects susceptible crops) Nitrogen (NO3-N)dmg/L>55-30 >30 Bicarbonate (HCO3) (overhead sprinkling only)mg/L<1.51.5-8.5 >8.5 pHNormal range6.5-8.4a EC = electrical conductivity, a measure of water salinity, report in deciSiemens per meter at 25°C (dS/m) or in units millimhos per centimeter (mmho/cm). Both are equivalent. TDS = total dissolved solids, report in milligram/liter (mg/L).
b SAR = sodium adsorption ratio. At a given SAR, infiltration rate increases as water salinity increases. Evaluate the potential infiltration problem by SAR as modified by EC.
c For surface irrigation, most tree crops and woody plants are sensitive to sodium and chloride; use the values shown. Most annual crops are not sensitive. With overhead sprinkler irrigation and low humidity (<30%) sodium and chloride may be absorbed through the leaves of sensitive crops.
d NO3-N, nitrate nitrogen, reported in terms of elemental nitrogen (NH4-N and organic-N should be included when wastewater is being tested).
From Ayers, R.S. and Westcot, D.W., FAO, 7, 11, 54, 69, 1976.
There are two assessments that characterize the salinity of water involving measuring total dissolved solids and electrical conductivity. Total dissolved solids refers to the material left in a vessel after evaporation of a filtered water sample and subsequent placed in a drying oven at a defined temperature.6 The total dissolved solids concentration relates to the conductivity of the water. The total dissolved solid can be calculated by multiplying conductivity by a factor, but the factor is not a constant. A factor most often used in agricultural is 640.
TDS (mg/L) = EC (mmho/cm or dS/m) ? 640Electrical conductivity is other measurement that more useful than total dissolved solids because it can be made easily and instantaneously by irrigators in the field. Salts that are dissolved in water conduct electricity. Therefore, the salt in the water is related to the electrical conductivity. Table 2.3 presents general guidelines as to the salinity hazard, total dissolved solids (TDS) and electrical conductivity.
Table 2.3 General Guidelines for Salinity in Agricultural Irrigation Watera
Classificationb TDS (mg/L) EC (mmhos/cm)c Water for which no detrimental effects are usually noticed500 0.75 Water that can have detrimental effects on sensitive crops500-1000 0.75-1.50 Water that can have adverse effects on many crops, requiring careful management practices1000-2000 1.50-3.00 Water that can be used for tolerant plants on permeable soils with careful management practices2000-5000 3.00-7.50a Normally only of concern in arid and semiarid parts of the country.
b Crops vary greatly in their tolerance to salinity (TDS or EC).
c EC = electrical conductivity.
Adapted from USEPA, Office of Water Program Operations, EPA-430/9-75-001, 1975.
The adverse impacts of salinity can be augmented by a soil with poor characteristics (such as high evapotranspiration rates and poor drainage) that can indirectly affect the crop. The only way to control salinity hazard is by applying more water that carries off excess salt and leaches throughout the plant’s root zone.
II. Exchangeable Cations
The concentration of exchangeable cations in irrigation water must be considered. The exchangeable cations include sodium, calcium and magnesium. When sodium concentrations are high, the soil permeability is reduced and the soil structure is affected. When calcium is normally the predominant exchangeable cation in soil, the soil tends to have a granular structure which is easily worked and readily permeable.
The sodium adsorption ratio has been developed to assess the degree to which sodium in irrigation water and provide an indicator of its potential deleterious effects on soil structure and permeability. The sodium adsorption ratio (SAR) of water is defined to the equation below:
where: Na+= sodium
Ca2+ = calcium
Mg2+ = magnesium
For irrigation water containing significant values of bicarbonate, the adjusted sodium adsorption ratio is sometimes used. The equation of adjusted sodium adsorption ratio (SARadj) is defined as follow:
where: pK’2 – pK’c = empirical constants
p (Ca2+ + Mg2+) = negative logarithm of the calcium and magnesium ion concentration in moles/liter
p (ALK)= negative logarithm of the total alkalinity in milliequivalents/liter
For general crops, the tolerance value of SAR and adjusted SAR for irrigation water is 8 to 18. In fact, the calculated SAR values in the range are suitable for sensitive crops.
III. Boron
Boron in treated wastewater is a potential hazardous ion for agricultural irrigation at high concentrations of around 1mg/L. The sources of boron in wastewater are normally from household detergents, industrial plants and sewage system where boron fertilizers are used.
However, it must be remembered that boron is essential in crop productivity at low concentrations. Boron is also one of the important micronutrients for crops to obtain a high quality and quantity crop yield.
As indicated, the deleterious effects for boron can happen on crop. Such effects are dependent on crop sensitivity to boron and boron concentrations in soil. A number of crops have been tested by experiment for boron sensitivity. The boron sensitivity of selected crops is listed in Table 2.4.
Table 2.4 Relative Tolerance of Crops and Ornamentals to Boron
TolerantSemitolerantSensitive (4.0 mg/L of Boron)(2.0 mg/L of Boron)(1.0 mg/L of Boron) AthelSunflower, nativePecan AsparagusPotatoWalnut, black and Persian or English PalmCotton, Acala and PinaJerusalem artichoke Date palmTomatoNavy bean SugarbeetSweetpeaAmerican elm MangelRadishPlum Garden beetField peaPear AlfafaRagged-robin roseApple GladilsOliveGrape (Sultanina andMalaga) BroadbeanBarleyKadota fig OnionWheatPersimmon TurnipCornCherry CabbageMiloPeach LettuceOatApricot CarrotZinniaThornless blackberry PumpkinOrange Bell pepperAvocado Sweet potatoGrapefruit Lima beanLemon (2.0 mg/L of boron)(1.0 mg/L of boron)(0.3 mg/L of boron)Note:Relative tolerance is based on the boron concentration in irrigation water at which boron toxicity symptoms were observed when plants were grown in sand culture. It does not necessarily indicate a reduction in yield. Tolerance decreases in descending order in each column.
From Ayers, R.S. and Westcot, D.W., FAO, 7, 11, 54, 69, 1976.
In United Nations, the Food and Agricultural organization issued guidelines for boron concentrations in irrigation water. The guidelines indicate that no issues will occur will occur for crops at boron concentration less than 0.75 mg/L. Between 0.75 and 2.0 mg/L of boron concentrations, increasing problem will exist, and severe problem happen at boron concentration above 2.0 mg/L. Table 2.5 presents the detailed guidelines for the allowable concentration of boron in treated wastewater for agricultural irrigation.
Table 2.5 Limits of Boron in Irrigation Water
Permissible Limits (Boron in miligrams per liter or parts per million) Crop Group Class of waterSensitiveSemitolerantTolerant Excellent<0.33<0.67<1.0 Good0.33-0.670.67-1.331.0-2.0 Permissible0.67-1.01.33-2.02.0-3.0 Doubtful1.0-1.252.0-2.53.0-3.75 Unsuitable>1.25>2.5>3.75From van der Leeden, F., Troise, F.L., and Todd, D.K., The Water Encyclopedia, 2nd ed., Lewis Publishers, Boca Raton, FL, 1990, 466.
Wastewater treatment systems are not efficient at removing boron unless some form of treatment is carried out, such as chemical precipitation. Some management options can also be adopted to degrade the toxicity of boron in treated wastewater and improve yields. These management options are engineered to provide additional nitrogen to maximize fertility of the soil.
IV. Trace Metals or Elements
All wastewater sent to treatment plants contain trace elements. The source of trace element is usually from industrial plant, but wastewater from residences can also have high trace element concentrations. Trace elements normally occur in treated wastewater but at very low concentrations, usually less than a few milligrams per liter with most less than 100 micrograms per liter. Some trace elements are essential for plant and animal growth at low concentrations, but all can exhibit plant toxicity at elevated concentration. The essential trace elements in wastewater include cadmium, chromium, copper, lead, mercury, molybdenum, nickel and zinc.7, 8, 9
The concentrations of trace elements in treated wastewater vary with wastewater treatment processes provided and their sources. Typically, the concentrations of trace elements in treated wastewater are in the range where negative effects are not likely to happen in short term. However, long term application of treated wastewater containing trace elements may lead to accumulation of trace elements in soil and may potentially result in groundwater contamination and plant toxicity. The range and recommended maximum concentrations of the trace elements in treated wastewater for agricultural irrigation are presented in Table 2.6.
Table 2.6 Recommended Limits for Constituents in Reclaimed Water for Irrigation7
Long-Term UseShort-Term Use Constituent(mg/L) (mg/L) Remarks Trace Heavy Metals Aluminium5.0 20.0 Can cause nonproductivity in acid soils, but soils at pH 5.5 to 8.0 will precipitate the ion and eliminate toxicity. Arsenic0.10 2.0 Toxicity to plants varies widely, ranging from 12 mg/L forSudangrass to less than 0.05 mg/L for rice. Beryllium0.10 0.5 Toxicity to plants varies widely, ranging from 5 mg/L for kale to 0.5 mg/L for bush beans. Boron0.75 2.0 Essential to plant growth, with optimum yields for many obtained at a few-tenths mg/L in nutrient solutions. Toxic to many sensitive plants (e.g., citrus) at 1 mg/L. Usually sufficient quantities in reclaimed water to correct soil deficiencies. Most grasses relatively tolerant at 2.0 to 10 mg/L. Cadmium0.01 0.05 Toxic to beans, beets, and turnips at concentrations as low as 0.1 mg/L in nutrient solution. Conservative limits recommended. Chromium0.1 1.0 Not generally recognized as essential growth element. Conservative limits recommended due to lack of knowledge on toxicity to plants. Cobalt0.05 5.0 Toxic to tomato plants at 0.1 mg/L in nutrient solution. Tends to be inactivated by neutral and alkaline soils. Copper0.2 5.0 Toxic to a number of plants at 0.1 to 1.0 mg/L in nutrient solution. Fluoride1.0 15.0 Inactivated by neutral and alkaline soils. Iron5.0 20.0 Not toxic to plants in aerated soils, but can contribute to soil acidification and loss of essential phosphorus and molybdenum. Lead5.0 10.0 Can inhibit plant cell growth at very high concentrations. Lithium2.5 2.5 Tolerated by most crops at up to 5 mg/L; mobile in soil. Toxic at citrus at low doses – recommended limit is 0.075 mg/L.Table 2.6 (continued) Recommended Limits for Constituents in Reclaimed Water for Irrigation
Long-Term UseShort-Term Use Constituent(mg/L) (mg/L)Remarks Trace Heavy Metals Manganese0.2 10 Toxic to a number of crops at a few-tenths to a few mg/L in acid soils. Molybdenum0.01 0.05 Nontoxic to plants at normal concentrations in soil and water. Can be toxic to livestock if forage is grown in soils with high levels of available molybdenum. Nickel0.2 2 Toxic to a number of plants at 0.5 to 1.0 mg/L; reduced toxicity at neutral or alkaline pH. Selenium0.02 0.02 Toxic to plants at low concentrations and to livestock if forage is grown in soils with low levels of added selenium. Tin, Tungsten, & Titanium2 2 Effectively excluded by plants; specific tolerance levels unknown. Vanadium0.1 1 Toxic to many plants at relatively low concentrations. Zinc2 10 Toxic to many plants at widely varying concentrations; reduced toxicity at increased pH (6 or above) and in fine-textured or organic soils. Other Parameters ConstituentRecommended Limit Remarks pH6 Most effects of pH on plant growth are indirect (e.g., pH effects on heavy metals’ toxicity described above). TDS500-2000 mg/L Below 500mg/L, no detrimental effects are usually noticed. Between 500 and 1000 mg/L, TDS in irrigation water can affect many crops and careful management practices should be followed. Above 2000 mg/L, water can be used regularly only for tolerant plants on permeable soils. Free Chlorine Residual<1 mg/LThe secondary treatment processes vary in their effectiveness at the removal of significant trace elements. However, advance treatment process such as carbon adsorption and chemical coagulation can remove over 90 percent of the trace elements from the wastewater.
As indicated, some trace elements are toxic at elevated concentrations. Cadmium, copper and molybdenum can be hazardous to animals at concentration too low to affect crops. Cadmium is of special concern as it can accumulate in the food chain. It does not affect ruminants in the little amounts they ingest. Most beef and milk products are unaffected by livestock ingestion of cadmium as it is stored in the kidneys and liver of the animal rather than the muscle tissues or fat. Copper is not harmful to monogastric animals but can be toxic to ruminants. However, the animal’s tolerance to copper increases as available molybdenum increases. Molybdenum may also be hazardous when available in the absence of copper.
While zinc and nickel are a lesser concern than cadmium, copper and molybdenum. They have negative effects on plants at lower concentrations than the levels harmful to plants and animals. However, zinc and nickel toxicities are decreased as the pH is increased.
2.2.2 Wastewater Reuse for Industrial Use
Treated wastewater can be an important potential source of water for many industries, particularly in water-short regions. The quantity of water used in power generation and manufacturing processes is very large and the availability of unlimited of water was considered as a prerequisite.
Wastewater reuse for industrial use has many potential applications, ranging from common housekeeping options to advanced technology implementation. The reuse of wastewater for industry can be adopted through industrial processes, internal recycling and non- industrial reuse of industrial facility effluent. The major industrial categories that use treated wastewater include:7
Evaporative cooling water, Boiler feedwater, Process water, and Irrigation and maintenance of plant grounds, fire protection, and dust control.Among the various industrial users of treated wastewater, cooling water is the greatest single application. All heat from various industrial processes must be removed and the most efficient coolant is water. The water can be a once-through recirculating cooling system or cascading use of cooling water in other applications.
Water quality requirements for industrial applications are related to four different issues include scaling, corrosion, biological growth and fouling, which may affect industrial process efficacy and integrity, as well as product quality. These concerns are addressed by the options summarized in Table 2.7.
Table 2.7 Industrial Water Reuse: Concerns, Causes, and Treatment Options
ConcernsCausesTreatment options Scalinginorganic compounds,saltsscaling inhibitor, carbon adsorption, filtration, ion exchange,blowdown rate control Corrosiondissolved and suspended solids pH imbalancecorrosion inhibitor,reverse osmosis Biological growthresidual organics, ammonia, phosphorousbiocides, dispersants, filtration Foulingmicrobial growth, phosphates, dissolved and suspended solidscontrol of scaling, corrosion, microbial growth, filtrationchemical and physical dispersantsFrom Asano and Levine, 1998.
Pathogens in treated wastewater used in industrial applications present potential health risks to workers and public from aerosols and windblown spray. Aerosols contain toxic organic compounds and bacteria, such as Legionella pneumophila, which causes Legionnaire’s disease.
In recent years, the net quantity of water used has reduced sharply because water shortages and discharge regulations have made it necessary to treat it before disposing it away. A large quantity of this reduction is achieved by internal reuse.
2.2.3Wastewater Reuse for Recreational Use
The treated wastewater may serve a variety of recreational applications include swimming, boating and fishing. The appearance of treated wastewater is essential when it is used, and treatment for nutrient removal may be adopted. Without nutrient control, there is a potential issue for algae blooms, resulting in odors and eutrophic conditions.
The criteria, regulations and guidelines of treated wastewater for recreational purposes will vary with the potential for human contact, as well as the sources of the secondary pollutants, such as body discharges, air contaminants and sewage. The criteria, regulations and guidelines of treated wastewater to be used for recreational applications can be subdivided into the following three groups.
I. Elementary Body Contact Recreational Water
This group of treated wastewater used in situations where there is intimate contact between the human body and the water and where there is a potential risk of ingesting a large amount of water which may pose a health risk. The treated wastewater used for contact recreational purposes include swimming, waterskiing, bathing, etc.
The methods of transmission of virus may happen due to ingestion of water or via the exposed mucous membranes and skin in protective ski barrier. Swimming pools have been implicated as the adenovirus pharyngitis and conjunctivitis, as well as enterovirus meningitis.10 Some of the diseases transmitted by swimming pool water are listed in Table 2.8.
Table 2.8 Some Diseases Transmitted by Swimming Pool Water
DiseaseCausative agent ConjunctivitisVirus Sinusitis and otitisStreptococci and Staphylococci (propagated by nasal mucus) Certain types of enteritisSome pathogens or certain viruses ingested with water Skin disease: EczemasKoch bacillus GranulomaMycobacterium marinum EpidermophytosisBrought about by the fungus that attaches itself to the skin between the toes and is contracted particularly easily when walking on areas around the pool. Typhoid feverSalmonella typhi DysenteryEntamoeba histolytica, Shigella Infectious hepatitisVirusCompiled from Reference 10 and 11.
Normally, the criteria, regulations and guidelines of treated wastewater used that are adopted for this group are more stringent. For use in recreational applications where full body contact with the water is permitted, the water should be colorless, microbiologically safe and non-irritating eyes or skin.
II. Secondary Body Contact Recreational Water
This group of treated wastewater used includes fishing, boating, canoeing, camping, and golf course and landscape irrigation. Treated wastewater used for this category should not contain high levels of heavy metals or pathogens that accumulate in fish to degrees that pose health threat to the consumers. The recommended water quality criteria for body contact and secondary body contact are presented in Table 2.9.
References1Committee to Evaluate the Viability of Augmenting Potable Water Supplies with Reclaimed Water, Water Science and Technology Board, Commission on Geosciences, Environment, and Resources, National Research Council, Issues in Potable Reuse: The Viability of Augmenting Drinking Water Supplies with Reclaimed Water, National Academy Press, Washington, D.C., 1998.
2Takashi Asano, Wastewater Reclamation and Reuse, Technomic Pub.,Lancaster,Pa., 1998.
3 Donald R. Rowe, Isam Mohammed Abdel-Magid, Handbook of Wastewater Reclamation and Reuse, Lewis Pub., Boca Raton, Fla., 1995.
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