Wastewater reclamation and reuse

Wastewater to be reclaimed can be generated by towns, industries and agriculture in the broad sense (i.e. cattle, forestry, aquiculture). Nevertheless, the most studied reclamation procedures are those with domestic or urban wastewater. In this case, the whole flow can be assimilated to domestic wastewater with respect to quality. As the EU Directive (91/271) on wastewater is theoretically implemented and the water generated has at least secondary treatment, wastewater-related impacts are reduced to a certain acceptable level.

As water is used for many purposes in a developed society (Table 2), it should be considered which of these uses can be filled with reclaimed wastewater. Practically all uses can, theoretically, have reclaimed wastewater as a source. Nevertheless, several of them are usually discarded (e.g. potable uses), although there is a description of reclaimed wastewater being used for tap water in Windhoek, Namibia (Odendaal et al. 1998) for more than 30 years. Full studies of this possibility are described by WEF & AWWA (1998).

Due to the several possible uses of reclaimed water, there is a need to attain a certain quality (of reclaimed water) depending on the type of use. For functional reasons, five groups of reclaimed water use (excluding tap water) are considered:

• agricultural,

• environment and leisure, and

• groundwater recharge.

The main use of reclaimed wastewater at the global scale is in agriculture. This is because agricultural use of water for irrigation is the most usual. Due to the fact that the main production of wastewater is in towns and big cities, which become more and more distant from agricultural areas, there is excess reclaimed water near towns. If this water is needed in the country, there are two main solutions, namely transportation from the production area to the places where it can be used, or the search for uses other than agricultural ones near or in towns.

In the first case, there is the need to build a network for transportation and final distribution. A classic case is in Israel, where reclaimed wastewater from Tel Aviv area is conveyed through a water carrier to the Negev desert (Chikurel et al. 2001).

The second case is being developed through the use of reclaimed water for other purposes, like urban parks and gardens irrigation, industrial use or groundwater recharge. If we consider additional uses, as indicated in Table 2, we can describe, in relation to the coastline, streamflow augmentation and landscape-related uses, which in turn can contribute indirectly to groundwater recharge.

Reclaimed water quality

Reclaimed water quality is nowadays defined by a few analytical parameters, even in regulations, but the need arises to improve the determination of qualitative aspects, perhaps not increasing the number of analyses but implementing complementary tools, like risk assessment or good reuse practices. However, there is a lack of physical-chemical parameters and toxicity-related ones.

Treated wastewater needs to be reclaimed by advanced treatment before reuse. For the specific purposes of recharge, two different qualities need to be established depending on the type of recharge. Direct recharge means that water reaches the aquifer directly, without natural barriers, while indirect recharge is based on the passage through soil and subsoil.

Several rules and regulations could establish the different qualities needed for any specific case, but it is absolutely necessary that water in the aquifer does not reduce its quality. When reclaimed water is used for creating a barrier against seawater intrusion, the quality does not seem at first sight to be so important. Yet, this is not acceptable because of the possibility of reclaimed water reaching wells inland which could be used for obtaining tap water or water for other purposes.

Theoretically, water quality is improved while remaining in the aquifer and for this purpose long water residence times are suggested or mandated. Nevertheless, due to the specific features of aquifers, one should not expect great activity of the solid matrix on the water quality.

Water quality considerations may be classified into biological, chemical and physical. From the biological point of view, the number of water-related pathogens is important (Table 3) but it is impossible to examine the water to detect the presence of them all. Instead, indicators are employed. In this sense, E. coli and the coliforms (total and faecal) are used to establish the microbiological quality of reclaimed water and the efficiency of the treatment. Nevertheless, the mentioned indicators are not at all reliable to guarantee the absence of viruses and parasites. For this reason other indicators are being employed (e.g. nematode eggs) or suggested (e.g. bacteriophage), and several pathogens are determined directly (e.g. cysts and

Table 2. Possible uses of water, observations of quality, and resources adequate for the defined use

Type of use

Specific use

Observations

Useful resource

Urban domestic 'potable'

Urban commercial

Urban fountains

Urban 'general' not for drinking, but related to domestic

Urban not domestic

Industry

Agriculture

Non-agricultural irrigation

Livestock

Drinking Hygiene

Cooking / food-related Drinking, cooking and hygiene in hotels, restaurants, and similar Drinking

Air conditioning Toilet flushing

Fire protection Irrigation of public spaces Irrigation of private spaces Urban cleaning (streets...) Sewerage management

Food-related

Pharmaceuticals and similar Cooling water* Boiler feed Process water Heavy construction

Food crops Non-food crops

Aquaculture

Golf course irrigation

Landscape Forest

Watering and dairy operations Fish farming

Maximum quality (suitable for drinking purposes)

Disinfected, especially for Legionella Disinfected

Disinfected

Secondary treatment if wastewater Tap-water quality

Constituents related to scaling, corrosion, biological growth, and fouling to be controlled

For reclaimed wastewater: rules or regulations. River water and freshwater usually do not have quality-related rules

Specific rules (WHO)

For reclaimed wastewater as indicated by rules or regulations

Freshwater usually don't have quality-related rules

Should be 'potable'

Specific rules (WHO) for the use of reclaimed water

Conventional

Occasionally reclaimed wastewater (if there are no other possibilities) with or without blending

Conventional and non-conventional Conventional and non-conventional

Conventional and non-conventional

Non-conventional if possible Conventional

Conventional and non-conventional

Conventional and non-conventional

Some countries are forcing these facilities to use only non-conventional resources Conventional and non-conventional

Conventional mainly, and non-conventional other than reclaimed water Conventional and non-conventional

ATE E

(Continued ) 1

Table 2. Continued

Type of use

Specific use

Observations

Useful resource

Groundwater recharge

Water-related sports, leisure activities

Stream and water - body recharge

Thermoelectric power use

Direct recharge Indirect recharge Contact allowed

Contact not allowed Snowmaking

Leisure boats maintenance

Habitat wetlands Lakes and ponds Enhancement of marsh and similar Streamflow augmentation

Power generation

Advanced tertiary treatment if wastewater

Pre-potable if other source Through soil formations

Specific rules and regulations if reclaimed wastewater is used

Conventional and non-conventional

Conventional and non-conventional

Toxicity for aquatic and water-related wildlife Conventional and non-conventional

No need of quality rules, but resource management is essential

Usually conventional, but stream augmentation with reclaimed wastewater is possible

From Brissaud et al. (2005). *Disinfection for Legionella.

Table 3. Pathogens potentially present in wastewater and their associated diseases

Pathogen

Associated disease

Bacteria Salmonella typhi

Salmonella paratyphi Salmonella spp. Shigella spp. Vibrio cholera Vibrio parahaemolyticus Campilobacter jejuni Pathogenic Escherichia coli Enterobacter aerogenes Klebsiella pneumoniae Proteus mirabilis Serratia marcescens Haemophilus influenzae Coxiella brunetti Chlamydia psittaci Mycoplasma pneumoniae Staphylococcus aureus Streptococcus pyogenes Enterococcus faecalis Bacillus anthracis Clostridium botulinum Clostridium perfringens Clostridium difficile Listeria monocytogenes Corynebacterium diphtheria Actinomycetes israelli Mycobacterium tuberculosis Mycobacterium avium

Pseudomonas aeruginosa Brucella abortus Brucella melitensis Brucella suis Bordetella pertussis Francisella tularensis Yersinia enterocolitica Legionella pneumophila Leptospira spp.

Viruses Polioviruses

Picornaviruses (animal viruses) Togaviruses (animal viruses) Paramyxoviruses, rhabdoviruses

(animal viruses) Orthomyxoviruses, arenaviruses

(animal viruses) Hepatitis A virus Hepatitis E virus Rotaviruses

Retroviruses (animal viruses) Adenoviruses

Herpesviruses (animal viruses)

Poxvviruses (animal viruses) Papovaviruses (animal viruses) Parvoviruses (animal viruses) Norwalk agent Reoviruses

Typhoid fever Paratyphoid fever Salmonellosis

Shigellosis (bacillary dysentery) Cholera

Gastroenteritis from seafood

Gastroenteritis

Gastroenteritis

Gastroenteritis

Pneumonia

Urinary tract infections Opportunistic infections Meningitis, other pediatric diseases Q fever Psittacosis

Primary, atypical pneumonia Food poisoning, skin infections Pharyngitis, skin infections Opportunistic infections Anthrax

Botulism, food poisoning

Gas gangrene, food poisoning

Gastroenteritis, colitis

Meningitis

Diphtheria

Actinomycosis

Tuberculosis

Pulmonary disease, disseminated disease in immunocompromised Wound, burn, urinary tract infections Brucellosis Brucellosis Brucellosis

Pertussis (whooping cough) Tularemia

Yersiniosis (diarrhoea and septicaemia)

Legionellosis

Leptospirosis

Poliomyelitis

Paralysis, common cold, myocarditis Encephalitis, yellow fever Measles, mumps, rabies

Influenza, haemorrhagic fevers

Infectious hepatitis Hepatitis Gastroenteritis Leukaemia, tumours, AIDs Respiratory diseases

Oral and genital herpes, chickenpox, shingles, mononucleosis Smallpox, cowpox Warts

Roseola in children, aggravates sickle cell anaemia

Gastroenteritis

Gastroenteritis

(Continued)

Table 3. Continued

Pathogen

Associated disease

Astroviruses Caliciviruses Coronaviruses Coxsackie A Coxsackie B

Enteroviruses 68-71

Echoviruses

Protozoa Entamoeba histolytica Naegleria fowleri Acanthamoeba spp.

Giardia intestinalis Cryptosporidium parvum Isospora spp. Balantidium coli Cyclospora spp. Toxoplasma spp. Enterocytozoon bieneusi Encephalitzoon councili Encephalitzoon intestinalis Phyllum Microspora

Helminths Ascaris lumbricoides (N)

Ancylostoma duodenale (N) Necator americanus (N) Clonorchis spp. (T) Taenia spp. (C) Enterobius vermicularis (N) Hymenolepis nana (C) Trichuris trichiura (N) Schistosoma spp. (T) Strongyloides stercoralis (N) Toxocara canis (N) Toxocara cati (N)

Gastroenteritis Gastroenteritis Gastroenteritis

Meningitis, fever, respiratory illness, herpangina Myocarditis, rash, meningitis, fever, respiratory illness, pleurodynia Meningitis, encephalitis, respiratory illness, rash, diarrhoea, fever Meningitis, encephalitis, respiratory illness, rash, diarrhoea, fever

Amoebiasis (Amoebic dysentery) Primary meningoencephalitis (PAM) Meningoencephalitis, eye lesions, respiratory and skin lesions Giardiasis (diarrhoea) Cryptosporidiasis (diarrhoea) Diarrhoea

Balantidiasis (diarrhoea, dysentery) Intestinal diseases Toxoplasmosis Diarrhoea

Disseminated disease of lungs and liver Disseminated disease of lungs and liver Microsporidiosis (intestinal and nervous diseases)

Ascariasis (roundworm infection)

Anaemia, intestinal diseases

Anaemia, intestinal diseases

Clonorchiasis

Taeniasis

Enterobiasis

Hymenolepiasis

Trichurasis

Schistosomiasis (bilharziasis) Diarrhoea, abdominal pain, nausea Fever, abdominal pain Fever, abdominal pain

From: Metcalf & Eddy, Inc. (1991); Rowe & Abdel-Magid (1995); Yates & Gerba (1998); Haas et al. (1999) and Salgot (2002). N, Nematodes; T, trematodes; C, cestodes.

oocysts of Giardia and Cryptosporidium) and are designed as index organisms.

From the physical and chemical points of view, the number of wastewater constituents is huge, and it seems impossible to deal with such a huge number of them. Detail can be found in Tables 4 and 5. For the physical parameters, the number is relatively reduced and several of them can be determined in a continuous way (e.g. temperature, pH and conductivity). It is not possible to find indicators for chemicals, and there is the need to know the origin of the treated wastewater and decide which types of chemicals are most important on a case-by-case basis. For example, when determining volatile organic contaminants, Romero et al. (2003) describe a method to make an initial assessment, without having to determine the whole spectrum of such chemicals.

Rules and regulations

Although almost all the laws governing wastewater reuse refer to its use for agriculture, there are several countries that support groundwater recharge and have established limitations for such purposes. It is necessary to distinguish between regulations, i.e. actual rules that have been passed and are enforceable by government agencies, and guidelines, which are not enforceable but can be used in the development of a reuse programme (Salgot & Angelakis 2001).

Table 4. Chemical agents potentially present in municipal wastewater

Group of chemical

Chemical agent

Effect / observations

Easily biodegradable organic compounds

Hardly biodegradable organic compounds Xenobiotic compounds

Nutrients (macro) Nutrients (micro) Metals

Radioactive compounds Dissolved salts

Other chemical compounds (organic and inorganic)

Proteins, carbohydrates

Greases, phenols, cellulose, lignin and similar Several formulations of synthetic compounds

Nitrogen, phosphorus, potassium

Chlorides, sulphides, nitrates

Pesticides, plaguicides, organic halogens, residual chlorine/disinfection by-products

Loss of dissolved oxygen from aquatic ecosystems (anoxic conditions). Generation of hydrogen sulphide and methane gases Residual COD, loss of dissolved oxygen Bioaccumulation, toxicity, interferences with the life cycle

Eutrophication, loss of dissolved oxygen, toxic effects Plant toxicity

Bioaccumulation, toxic effects (See Table 5)

Effect on agricultural uses, risk for human health (nitrates)

Carcinogenic, teratogenic and/or mutagenic effects x

From Metcalf & Eddy, Inc. (1991); Rowe & Abdel-Magid (1995) and Crook (1998).

Table 5. Physical pollution potentially present in urban wastewater

Physical parameter

Agent

Effect/observations

Radioactive compounds Residual heat (thermal pollution)

Odour

Colour

Taste

Total solids: suspended solids (settleable, non-settleable), filterable solids (colloidal, dissolved)

Radon, radioactive isotopes

Water temperature above normal level

Gases (hydrogen sulphide, mercaptans, cyanide, ammonia)

Natural metallic ions (iron oxides, manganese oxides), humic acids, lignin derivatives Phenols, dissolved salts, disinfection by-products Suspended solids (organic matter, clay, silt)

Filterable solids (organic matter, dissolved salts)

Bioaccumulation, toxic effects

Effects on aquatic life Reduction of dissolved oxygen concentration Nuisance effects on human health

Aesthetic and nuisance effects

Effects on human health (see Table 4) Diverse effects (see Table 4)

From Metcalf & Eddy, Inc. (1991) and Rowe & Abdel-Magid (1995).

Usually, reclaimed wastewater quality is established independently from other considerations, using standards. Standard figures depend on several concepts such as (Salgot & Angelakis 2001): economic and social circumstances, legal capacity from different entities and implicated administrations, human health/hygienic degree (endemic illnesses, parasitism), technological capacity, previously existing rules and/or criteria, crop type, analytical capacity, risk groups possibly affected, technical and scientific opinions and other miscellaneous reasons.

Three types of factors can be distinguished: technological (analytical, treatment methods, and capacity, knowledge); legislative and economical (criteria, socio-economical, legal competence); and health-related (sanitary state, diseases, risk groups). Standards and quality regulations have been a matter of discussion among scientists, health and legislation officers, and engineers, because of the parameters to be controlled. Much controversy has been aroused among research teams and regulating bodies on the quality that reclaimed water must meet before its reuse with an acceptable degree of risk.

Existing wastewater reuse regulations have been based traditionally on biological quality considerations, and only during the last few years have chemical and toxicological concerns appeared.

At the European level, the only reference to reuse is article 12 of the European Wastewater Directive (91/271/EEC): 'Treated wastewater shall be reused whenever appropriate'. In order to make this statement reality, common definitions of what is 'appropriate' are needed. A complete revision of the guidelines and regulations can be found in Salgot & Angelakis (2001), although several modifications appeared after this revision. The most important changes have been the appearance of a new edition of the USEPA Guidelines for Water Reuse in 2004, and the WHO drafts on reuse (2002 and 2005, for Europe and the world, respectively).

The USEPA (2004) guidelines refer to planned groundwater recharge, and indicate the rules in several states, mentioning specifically California, Florida, Hawaii and Washington.

California and Hawaii do not specify required treatment processes and determine requirements on a case-by-case basis. The California and Hawaii health services departments base evaluation on all relevant aspects of each project including treatment provided, effluent quality and quantity, effluent or application spreading area operation, soil characteristics, hydrogeology, residence time and distance to withdrawal. Hawaii does require a groundwater monitoring programme.

Washington has extensive guidelines for the use of reclaimed water for direct groundwater recharge of non-potable aquifers. It requires Class A reclaimed water defined as oxidized, coagulated, filtered and disinfected. Total coliform content is not to exceed 2.2/100 ml as a 7-day median and 23/100 ml in any sample. Weekly average biological oxygen demand (BOD) and total soluble solids (TSS) limits are set at 5 mg/l. Turbidity is not to exceed 2 NTU as monthly average and 5 NTU in any sample. Groundwater monitoring is required and is based on reclaimed water quantity and quality, site-specific soil and hydrogeological characteristics, and other considerations. Washington also specifies that reclaimed water withdrawn for non-potable purposes can be withdrawn at any distance from the point of injection and at any time after direct recharge.

Florida requires that TSS do not exceed 5.0 mg/l in any sample, achieved prior to disinfection, and that the total nitrogen in the reclaimed water be less than 12 mg/l. Florida also requires continuous on-line monitoring of turbidity; however, no limit is specified.

Concerning Europe, Brissaud (2003) describes the criteria for health-related guidelines in ground-water recharge with recycled municipal wastewater. He describes four different cases.

Direct recharge for indirect potable use. If the recharge is direct, then the injected water should be potable. Moreover, the water injected should also be treated to prevent clogging around the injection wells, long-term health risks linked to mineral and organic trace elements and the degradation of injected water quality into the aquifer.

Indirect recharge for indirect potable use. The quality of infiltrated water may be dramatically improved when percolating through the vadose zone, thanks to several processes, which include retention and oxidation processes. These processes affect organic matter, nutrients, micro-organisms, heavy metals and trace organic pollutants, among others. When transfer through the vadose zone is part of the treatment intended to bring injected water up to potable water, a case-by-case approach should be highly recommended.

Direct recharge of non-potable aquifers. The recycled water should have been upgraded to the standards and limits required for the intended applications. A high degree of treatment is also necessary to make the injection sustainable. Suspended solids and organic matter should have been drastically reduced to avoid clogging around the injection wells.

Indirect recharge of non-potable aquifers. This requires a less treated injectant and is easier to implement. SAT is an appropriate treatment to meet the required water quality, provided it is properly designed and managed.

The discussion on how strict the standards must be is still on going and no agreement has been achieved so far among the different points of view. Nevertheless, it seems that the WHO is promoting a new perspective, using DALYs (disability-adjusted life years) instead of numeric standards (G. Kamizoulis pers. comm. 2005). This new way seems promising, but needs a better understanding, focused research and a good communication policy.

There are other ways to progress, like the development of good reuse practices (GRP) or the risk analysis approach, which is connected with the DALY approach.

In several countries, such as Spain, so-called 'autocontrol' procedures - using tools derived from classic industries, mainly the HACCP (hazard analysis and critical control points) approach - are gaining momentum, as described below. Again, this is related to risk developments.

Reclamation technologies

Nowadays, there are no technological problems to reaching any desired water quality when waste-water is used as a raw material. Nevertheless, there is a clear relationship between the degree of treatment needed and the economy of the whole procedure, and additionally the question of who must pay for the treatment. So it appears that the main problem is economic. Another consideration is that there is a frontier between what is a public service (wastewater treatment) and the manufacture of a product (reclaimed water). The end-product (reclaimed water) must compete in an open market (theoretical) with the conventional water resources. Both conditions (economy and technology) must be improved with adequate research, development and innovation (R + D + i). Two main lines of R + D + i are required (Salgot & Verges 2003): (a) new management tools for water resources management which include risk analysis and management; (b) new wastewater reclamation technologies, capable of reducing the treatment costs to a level compatible with the socioeconomic characteristics of the reuse site.

All this raises several questions on the level of reclamation that a country can accept from the economy and risk points of view. There is not a clear solution, and several discussion points arise (Salgot & Verges 2003). First to consider is the socio-economic context in which the reuse takes place: where wastewater treatment is a brand new, or nearly new, element in the anthropic water cycle, the taxation effort supported in order to fulfil the legal requirements for wastewater treatment has usually been important. It seems difficult in this context to implement new taxes to reclaim secondarily treated wastewater.

The second point is that several users obtain a benefit from the reclaimed wastewater. This aspect is included in the main perception that irrigated agriculture is not using water resources efficiently. In many places, water is not paid for, or is paid for at an inadequate price. This low price does not promote the saving or efficient use of water. Other users (e.g. golf courses) have been the subject of heavy criticisms from a number of stakeholders on the grounds of suspected wasteful use of water resources. Nevertheless, such users usually pay for the resources and the comparative economic benefits for society of 'sumptuary' users and the 'classic' users should be examined and discussed.

A third point, true in mid-2005, that should be stated. The water authorities around the Mediterranean promised that wastewater reuse will be promoted and backed up. Yet, this promise has not exactly been fulfilled, with a few honourable exceptions (e.g. Tunisia and Israel).

Finally, the wastewater reclamation technologies usually implemented have mainly been intensive, using large amounts of energy. It seems that from the point of view of the ecological impact, it would be more logical to use, especially for small- and medium-sized facilities, extensive, less energy-consuming technologies.

Brissaud (2003) indicates that it is common agreement that recharge should not lead to additional or supplementary treatment after withdrawal to meet the standards related to the intended water applications. Meeting the standards at the point of use is not enough; qualitative requirements have to be satisfied within the aquifer.

Asano & Cotruvo (2004) indicate that four water quality factors are particularly significant in groundwater recharge with recycled wastewater: microbiological quality, total mineral content (total dissolved solids), presence of heavy metal toxicants, and the concentrations of stable and potentially harmful organic substances.

The same authors establish that some basic questions that need to be addressed include: What treatment processes are available for producing water suitable for groundwater recharge? How do these processes perform in practice at specific sites? How does water quality change during infiltration-percolation and in the groundwater zone? What do infiltration-percolation and ground-water passage contribute to the overall treatment system performance and reliability? What are the important health issues to be resolved? How do these issues influence groundwater recharge regulations at the points of recharge and extraction?

Table 6. Best available technology conditions

Wastewater characteristics

Size/type of the facility

Existing rules and regulations

Available technology

Landscape integration

Economy (town, county, country)

Social acceptance

Centralisation vs. decentralisation

Desired final quality of water (following rules and regulations) Reclaimed water reuse possibilities Political decisions

What benefits, problems and successes have been experienced in practice?

In all cases the 'best available technology' (BAT) concept needs to be applied. The BAT (Table 6) is used considering all possible options with respect to technology, economy and social aspects. It starts with a systematic analysis of the technical, economic, environmental and financial factors necessary to select a cost-effective wastewater management plan (Metcalf & Eddy, Inc. 2003).

BAT procedures describe, at the planning levels, the technology most suitable for a given location or town. Sometimes, this approach is described as BATEA (best available technology economically achievable), BPWTT (best practicable technology currently available) or BPCTCA (best practicable control technology currently available) for industries (Rowe & Abdel-Magid 1995).

Risk-related aspects

When reusing reclaimed wastewater it is obvious that hazards will appear (Salgot 2002). The hazard is due to the presence in the reclaimed wastewater of several contaminants, pathogens and chemicals. Pathogens include protozoa, bacteria and viruses (Table 3), while chemicals include the simplest (e.g. nitrates) to the more complex molecules, e.g. trihalomethanes (Table 4).

Pathogens usually generate infectious illnesses that may or may not be apparent (clinical and non-clinical), and usually appear in the short term, while chemicals are related to toxicity, either acute or long term. It is obvious that the better the reclaimed water quality, the less the risk related to both types of contaminant. When dealing with risk, two steps should be considered: assessment and management. It seems basic to have a guide on how to determine the risk assessment. Nevertheless, this does not exist in full, and is nowadays an object of intensive research (Salgot & Pascual 1996; Verges & Salgot 2002).

Several tools are being developed and becoming available for hazard or risk assessment. The best known is the HACCP system. This system, developed for the food industry, focuses on the detection of relevant control points, whose establishment increases safety while reducing the costs through a better use of analytical work. HACCP is conducted considering seven main points as indicated in Table 7.

HACCP procedures must theoretically be performed for all planned reuses. In specific cases, as for bathing water or shellfish cultivation, it is necessary to establish contacts with other authorities implicated, e.g. health departments. In any case, the risk is related to the presence of a contact between the target (man, animals, plants) and reclaimed wastewater.

The solutions to reduce risks to an acceptable amount are: reduction of contacts between the pathogen and the target organism; and good reclaimed water quality. The reduction of contact could be reached through what is called good reuse practices, intended partly to reduce the possible contacts. While considering that 'contact' in a broad sense could imply either direct or indirect contact (e.g. skin or mucous membrane contact, aerosols entering the respiratory tract), any contact reduction will imply risk reduction. Good reclaimed quality is obtained using standards, which when enforced must guarantee a quality of water theoretically good enough to reduce the risk to an acceptable level.

Groundwater recharge

The definition of artificial recharge is: 'the techniques or operations which have the main objective of allowing a better aquifer management by increasing the water resources and creating reserves, by means of a direct or indirect intervention in the natural water cycle' (Custodio & Llamas 1996). Groundwater recharge with wastewater is always artificial recharge.

Dillon (2005) describes the management of aquifer recharge (MAR) as the intentional banking and treatment of water in aquifers. The same author discusses the meaning of 'artificial recharge' indicating that the adverse connotations of 'artificial', in a society where community participation in water resources management is becoming more prevalent, suggests that it is time for a new name. The old name implied that the water was in some way unnatural. Managing recharge is intentional as opposed to the effects of land clearing, irrigation and installing water mains, where recharge increases are incidental. MAR has also been called enhanced recharge, water banking and sustainable underground storage (Dillon 2005).

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Among the main objectives of recharge, several are related to recharge with reclaimed water: supplement groundwater resources available; reduce or eliminate (even increase) groundwater level drop; compensate natural recharge lost by human activities; improve coastal aquifer situation; use aquifers as a storage medium for water, instead of using surface facilities; improve joint uses of surface and groundwater; avoid movements of bad quality waters inside the aquifer; increase good quality water availability, through blending; increase leaching of salts and other contaminants; use the soil aquifer system for water treatment; reclaim treated wastewater, store it, and complete the treatment using the soil/aquifer system; reduce, mitigate or even eliminate subsidence caused by over-exploitation of groundwater; compensate negative effects of hydraulic and civil works; maintain flows in a stream or levels in lakes during low waters; and use the aquifer as a water transportation media.

There are basically two types of groundwater recharge/application: on-surface (indirect, over the soil); and deep injection (direct injection onto the aquifer). These are several techniques, described in Table 8 and depicted in Figure 1.

Surface or deep works can be employed for recharge, as summarized in Table 9. In both cases, the reduction of the recharge capacity (clogging) can be attained by soil surface alterations, addition of too much suspended matter, or biological activity. The causes of clogging are usually the presence of suspended solids and/or gas bubbles in the recharge water or bacterial growth in the well and surrounding it. Other causes can be chemical precipitation in the water, soil and well, clay swell or dispersion, and soil structure erosion and subsequent aquifer obstruction. Reduction of recharge capacity, social acceptance, pollution of aquifers used for potable water supply, hazard and risk increases and extraction abuses are the main resulting problems.

Once reclaimed water reaches the aquifer, there are several phenomena that can occur, namely: organic matter reduction; water odour and taste correction; and adsorption of some organic matter compounds. Reactions in saturated media are much slow than in non-saturated ones. It should also be noted that the bacterial flora in groundwater is usually scarce.

In general, groundwater recharge presents several disadvantages:

• huge (surface) application areas are needed;

• if reclaimed water is injected, energy is necessary;

• recharge is increasing the groundwater pollution risks;

Table 8. Major techniques of groundwater recharge

Technique

Acronym

Description

Aquifer storage and recovery Aquifer storage transfer and recovery

Bank filtration

Dune filtration

Infiltration-percolation

Infiltration systems

Percolation tanks

Rainwater harvesting Soil aquifer treatment Soil aquifer plant treatment Sand dams

Underground storage

Stream releases

ASR ASTR

SAT SAPT

Injection of water into a well for storage and recovery from the same well

Injection of water into a well for storage and recovery from a different well, generally to provide additional water treatment

Extraction of groundwater from a well or caisson near or under a river or lake to induce infiltration from the surface water body thereby improving and making more consistent the quality of water recovered Infiltration of water from ponds constructed in dunes and extraction from wells or ponds at lower elevation for water quality improvement and to balance supply and demand Infiltration and percolation of partially treated wastewater in sand formations for further treatment. Water may or may not be extracted later. Can be considered as a variant of SAT. Ponds constructed usually off-stream where surface water is diverted and allowed to infiltrate (generally through an unsaturated zone) to the underlying unconfined aquifer A term used in India to describe harvesting of water in storages built in ephemeral wadis where water is diverted and infiltrates through the base to enhance storage in unconfined aquifers and is extracted down-valley for town water supply or irrigation Roof runoff is diverted into a well or a caisson filled with sand or gravel and allowed to percolate to the water-table where it is collected by pumping from a well Reclaimed water is intermittently infiltrated through infiltration ponds to facilitate nutrient and pathogen removal in passage through the unsaturated zone for recovery by wells after residence in the aquifer If plants are implemented in SAT or similar systems, there is an additional improvement of the quality of the water applied because of the action of plants (e.g. further removal of nutrients) Built in wadis in arid areas on low permeability lithology, these trap sediments when flow occurs, and following successive floods the sand dam is raised to create an 'aquifer' which can be tapped by wells in dry seasons

In ephemeral streams where basement highs constrict flows, a trench is constructed across the streambed keyed to the basement and backfilled with low-permeability material to help retain flood flows in saturated alluvium for stock and domestic use Dams on ephemeral streams are used to detain floodwater and uses may include slow release of water into the streambed downstream to match the capacity for infiltration into underlying aquifers, thereby significantly enhancing recharge

Modified from Tuinhof & Heederik (2003).

Percolation Tank Contents
Fig. 1. Major techniques of groundwater recharge (modified from Dillon 2005). ASR, Aquifer Storage and Recovery; ASTR, Aquifer Storage Transfer and Recovery; WWTP, Wastewater Treatment Plant; IP, Infiltration Percolation; SAT, Soil Aquifer Treatment; SAPT, Soil Aquifer Plant Treatment.

Table 9. Works for groundwater recharge

Surface works for recharge

Deep works for direct injection

Lagoons or ponds

Channels, trenches and furrows

Pits

Areas for surface infiltration River bed actuations

Wells

Absorption or diffusion wells

Drains or galleries in the bottom of a well

Trenches filled with gravel reaching the phreatic level

Natural sinks, ravines or fissures in karst areas

• not all water added can be recovered;

• there is great surface demand for the system being operative;

• instant demands cannot be satisfied (low rate answer);

• there are problems with the water legal status.

Direct aquifer recharge with reclaimed water. Direct recharge means that the reclaimed water is introduced directly into the aquifer through injection wells. Rowe & Abel-Magid (1995) make several statements and describe direct recharge indicating that treated wastewater is pumped under pressure directly into the groundwater zone, usually into a well-confined aquifer. WEF & AWWA (1998) also describe recharge into wells by using gravity flow.

Groundwater recharge is practised, 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 seawater.

Rowe & Abel-Magid (1995) also state that locating the extraction wells at as great a distance as possible from the injection wells increases the flow-path length and residence time of the recharged groundwater, as well as mixing the recharged water and the other aquifer contents.

In any case (State of California 1978), it seems clear that wells are the least desirable method of groundwater recharge, largely because of problems of pore clogging, well silting, air entrainment, bacterial and algal growths, and deflocculation caused by reaction.

Indirect aquifer recharge. Indirect recharge is feasible only for unconfined aquifers; the recharged water is spread on the land - over irrigation - or on infiltration basins or by other means, and infiltrates through the vadose zone down to the water table. The unsaturated layer (including the soil) behaves as a filter and a natural reactor, providing an additional treatment and allowing the percolating water quality to greatly improve.

On-surface irrigation has the advantage of employing the treatment capacities of the soil, which constitutes an additional barrier, while direct injection has the main disadvantage of introducing water directly into the aquifer. Direct injection is considered more hazardous than indirect (Table 10).

Another form of indirect groundwater recharge with reclaimed water is the 'collateral' recharge which happens when this water is used for irrigation. The excess water which infiltrates and later percolates through the soil/subsoil can reach the aquifer. For irrigation to be sustainable, the chemicals (especially salts) carried with the reclaimed water must not be allowed to accumulate in the root zone of the soil. Then, an extra amount of water (leaching water) needs to be applied in order to control the salinity.

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