wastewater re-use

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Water reuse if a political and socio-economic is a major issue for the future development of drinking water and wastewater services worldwide. Indeed, the major advantage it has is that an alternative and permanent resource is ensured allowing water shortages to be reduced, natural resources to be better conserved and water shortages caused by climate change to be alleviated. Some countries, states and large cities (Australia, California, Cyprus, Spain, Florida, Israel, Jordan, Malta, Singapore, etc.) already have ambitious objectives to meet 10 to 30%, even as much as 60%, of their water demand through reusing purified wastewater.

Agricultural irrigation was, is and will remain the largest consumer of recycled water with several well recognised advantages and benefits, in particular its contribution to food safety.

The reuse and internal recycling of industrial wastewater have become common practice for many industries with new trends such as the zero liquid discharge objective and the reuse of inter-sector water such as the use of urban wastewater for industrial purposes.

Water reuse in an urban environment is marked by a rapid development due to its crucial role in tomorrow's sustainable city. Moreover, the irrigation of urban green spaces and other applications are gaining importance and include industrial uses (cleaning, fire-fighting, cooling towers, etc.), recycling in buildings and environmental uses for maintaining and restoring water bodies, rivers and wetlands.

Water table recharge, feeding reservoirs for the indirect production of drinking water, even the direct reuse of ultra-pure water to increase water supply have been deployed as sustainable solutions against increasing water shortages in some countries who will face this problem with the next 20 years.

irrigation in agriculture and green spaces

Agriculture is the water's largest water consumer, i.e. around 70% of world demand. In some arid and semi-arid countries, the majority of irrigation water is recycled water. Over and above the usual factor of chronic water shortage, the need for an alternative water resource has accelerated over the last few years due to increasingly severe and repetitive droughts, which not only occur in traditionally arid areas in the United States, the Mediterranean region, the Middle East and South Asia, but also in a certain number of temperate regions, e.g. in Europe and in North America.

The reuse of wastewater in agriculture has been practised for thousands of years. It was developed by ancient civilisations and up until the 20^th century was also used as a system to purify wastewater in fields used for land spreading. Indeed soil represents an efficient filter with up to one or two tonnes of "purifying" micro-organisms per hectare. Not only does wastewater provide water for crops but also contributes to improving the yield by providing a input in nutrients.

Currently, the main interest in reusing wastewater in agriculture is to alleviate in moisture and to increase agricultural production yields by a suitable supply of irrigation water. To maintain food production, technical and research work over the last few years has focused more on efficient irrigation by developing localised irrigation systems (drip irrigation) and/or computer-controlled networks equipped with sensors. However, efficient irrigation alone is not enough to resolve the problems of moisture deficits. Therefore, supply from an alternative resource through the reuse of wastewater is becoming a development priority in many countries and regions.

The underlying principle of water reuse for agricultural purposes is the need for the appropriate treatment of municipal wastewater to achieve a specific quality for a given use. It should be noted that besides the well-known benefits, using recycled water for irrigation purposes may have negative impacts on public health and the environment and which depend on the level of treatment, local conditions and irrigation practices. In all cases, existing scientific knowledge, operational feedback and best practices allow risks to be reduced through the implementation of efficient planning, the selection of appropriate technology and rigorous management of irrigation practices.

The main risks related to the reuse of treated wastewater for irrigation purposes may be divided into three categories:

Health risks,
Agronomic and environmental risks,
Operational risks in the deterioration in the quality of recycled water within distribution systems and clogging of irrigation equipment.

On principle, most of the more recent standards require as a minimum, biological treatment of wastewater intended for reuse as irrigation water. In certain cases, such as Mexico for example, priority may be given to conserving the fertilising value of wastewater by an advanced primary treatment process (e.g. coagulation, flocculation and lamellar settling), followed by filtration and/or disinfection for priority removal of microbiological pollution, whilst keeping carbon and nutrients for irrigated crops.

Additional tertiary treatment is often essential for uses with high sanitary risks such as irrigated market produce eaten raw and the irrigation of green spaces. Additional filtration is also required in order to avoid deposits in the distribution system and/or the risk of clogging of irrigation equipment, especially of spray nozzles and drip irrigation systems.

The development and implementation of new wastewater treatment processes for irrigation has also contributed to improving the aesthetic quality of recycled water  through the removal of odour problems and the colour of recycled water  which were shown to be obstacles for several projects due to the negative perception by users.

controlling health risks

The major public health risk associated with recycled water irrigation is the contamination by pathogenic micro-organisms, namely, viruses, bacteria, parasitic worms (helminths) and protozoa. As highlighted by the World Health Organisation (WHO) in its guidelines on the use of wastewater in agriculture dated 2006, a risk of transmitting infections exists when the following conditions are met:

the population, the agricultural workers or animals are exposed to pathogens in recycled water by direct or indirect contact with them in irrigated areas or by consuming contaminated crops;
the number of pathogens ingested is higher than the infective dose;
the host becomes infected;
the infection leads to an illness and /or transmission of the pathogen to other people or animals.

Consequently, the main objective of these public health protection measures in water projects, is to prevent the first two conditions from occurring. This means that the appropriate practices must be implemented in order to reduce the number of pathogens in recycled water as well as implementing several barriers and other measures to reduce the probability of coming into contact with potentially infectious micro-organisms. The choices in best practices for protecting human and animal health depends on local conditions and economic aspects.

The following four areas of best practice constitute the main measures to control health risks when irrigating with recycled water:

wastewater treatment and quality control during water distribution and storage;
audit on the use of wastewater through the choice of suitable irrigation methods and crop farming practices;
restriction on the types of crops to be irrigated and restricted public access, in particular for irrigating green spaces;
other measures, including controlling human exposure (e.g. measures to protect farm workers such as gloves, masks, etc.), harvesting methods, educating the public concerned and promoting best practices in health.

Treating wastewater is widely recognised as the most effective measure to reduce health risks linked to the use of recycled water for irrigation purposes. For this reason, all national or international regulations set or recommend the quality that wastewater treatment must achieve in accordance with the use envisaged and the level of risk involved. The degree of risk depends on several factors, including local endemic health, farming methods, weather conditions, the risk of direct contact, economic feasibility, etc.

Over the last few years several countries and federal states have included the recycling of water in their national water resources management policies with rules, obligations and financial incentives. In addition to the level of quality in relation to microbiological and physico-chemical indicators, certain standards also recommend additional treatment processes and barriers in order to reduce health risks.

In 2006, based on the approach used by the Australian Controlled Health Risk standards, the WHO reviewed all of its 1989 recommendations on agricultural irrigation using treated wastewater. The guide values were maintained at the same level, thereby allowing health risks to be limited to an acceptable level for the protection of public health, but with new concepts on additional barriers which ensure the economic viability of reuse projects.

The USA is considered as a world leader in wastewater reuse, with the first regulations adopted in California in 1918 followed by several reviews, the last dating from December 2000. At a federal level, the USEPA revised its 2004 recommendations in 2012 by mainly maintaining two recycling water quality guideline levels and recommendations on better monitoring of treatment processes.

In most of the standards and regulations, a precautionary principal is applied for high-risk uses, such as, for example, the irrigation of crops consumed raw and restriction-free irrigation of green spaces in the urban environment, as illustrated by table 36.

controlling agronomic and environmental risks

As a general rule, environmental risks are mainly agronomic chemical risks linked to the potential presence of trace elements, heavy metals and organic micropollutants in recycled water. Preventive measures for protection against health risks are more than sufficient to protect soil, surface water and ground water.

Environmental risks on the pollution of resource water are, in principle, taken into account in regulations protecting drinking water intake zones and at-risk areas.

Consequently, and as demonstrated by state-of-the-art studies, the major agronomic risks are as follows:

excessive salinity of the soil which affects the transpiration and growth of sensitive cultures. The risk of soil becoming saline is high in arid zones and may be assessed by monitoring the concentration levels of dissolved solids, electrical conductivity and chlorides.
excess sodium which deteriorates the structure of clay soil and which may cause its permeability to be reduced. In addition to an excessive quantity of exchangeable sodium, this harmful effect also encourages high pH and low electrical conductivity.
toxic conditions for crops, in particular linked to high levels of boron, sodium and chlorides, and occasionally certain trace elements (including heavy metals), most often coming from industrial effluent. Toxicity affects plant growth, shown by signs of burning and/or leaf loss.
excessive nitrogen can affect a crop's nutrient balance and the quality of surface or ground water.

Considering that wastewater treatment has no effect on chemical compounds, the best solution consists in controlling: saline effluent discharge at source, the intrusion of seawater in wastewater networks, the use of detergents containing boron and industrial discharges.

In order to control agronomic risks, best recycled water irrigation practices consist in combining several preventive or remedial measures, including:

selecting an adequate irrigation method,
choosing the right crops to irrigate, which are sufficiently tolerant to sodium and chlorides,
suitable soil management practices (preparation, soil enhancers, etc.),
sufficient leaching and drainage of soil in order to drain water and evacuate excess of salts,
appropriate schedule to both irrigate and leach,
correct use of fertilisers.

controlling recycled water quality in distribution and irrigation systems

The main problem related to the distribution of purified wastewater is the potential deterioration of microbiological quality, in particular in hot climates and very long distribution networks. For this reason as well as for uses carrying high health risks, it is recommended that a residual chlorine level be maintained in order to prevent bacterial growth. However, the concentration level of residual chlorine must not exceed 0.5-1 mg/L in order to avoid the toxic effects on sensitive plants.

The presence of suspended solids in recycled water leads to risks of deposits building up in the networks, to deterioration in the water's taste and odour qualities and localised fouling of irrigation systems. In order to overcome these drawbacks, best practices in distribution network maintenance includes regular hydraulic purges, even the mechanical cleaning of significant deposits.

It should be noted that drip irrigation systems often require additional filtration following storage and specific cleaning protocols using chemicals.

industrial uses

One of the greatest potentials for water reuse is to add to or to replace the use of drinking water and/or natural resources in industry. Industry is the second biggest water consumer after agriculture and accounts for around 25% of world's demand.

In general, the reduction of water demand in industry, up to and including the end of industrial water cycle, includes three water-saving strategies and the minimisation of wastewater discharge:

(1) Cascading reuse, involving direct reuse with little or not treatment;
(2) Recycling of water following suitable treatment;
(3) Wastewater reduction at source by reducing the water requirements of industrial processes.

Industrial wastewater reuse and internal recycling are well-established practices. Their development potential should increase in the future with the growth in water deficits and the supply costs of fresh water as well as increasingly stringent regulatory requirements regarding discharges.

Water reuse has been traditionally practised for years in the oil and gas, textile, car, paper and pulp and energy production industries and more recently in the electronic and food industries.

Although existing types of industrial water reuse are wide, the main uses are:

Open or closed circuit cooling systems,
Cleaning water,
Boiler feed water,
Process water,
Various other uses such as fire-fighting, cleaning, etc.

The requirements and areas of application of industrial water recycling depend upon the industry, specific industrial processes as well as their efficiency targets. For this reason, it is impossible to generalise quality requirements for recycled water used as process water.

cooling water

When using recycled water in cooling towers, the major health concern is the risk of inhaling spray containing potentially pathogenic agents For this reason, the majority of regulations require that water be treated with an additional wastewater disinfection process.

Despite the very low concentration levels of micro-organisms in treated water, the risk of biological growth in cooling systems is very high due to the presence of nutrients and to high temperatures. In addition to health risks, bacterial development may produce unwelcome biofilms and deposits, which can alter the transfer of heat and cause microbiological corrosion and/or clogging of heat exchangers and spray nozzles. The method the most used to limit bacterial growth in cooling systems is the addition of biocides.

Another, potentially negative impact when using recycled water as cooling water is scaling. The scaling potential depends on concentration levels in calcium, magnesium, sulphate, phosphate, silica and fluoride as well as the level of alkalinity. The type of scaling most commonly found is caused by calcium phosphate, followed by silica and calcium sulphate (quite frequent).

boiler water

The use of recycled water for feeding boilers is not very different from the use of conventional drinking water because a demineralisation pretreatment step is always necessary. For this usage, the quality of recycled water depends on the boiler's operating pressure, with those with the highest pressure requiring better quality water.

Accumulation of scale and the corrosion of equipment are the major risks for this type of use. To limit these risks, in addition to eliminating water hardness, alkalinity and dissolved solids, the concentration levels of calcium, magnesium and silica are also limited and controlled.

diversification of uses and reuse of urban wastewater in industry

Significant progress in wastewater treatment technologies, in particular the development of membrane processes, has encouraged industrial uses and accelerated work between sectors. Several urban wastewater reuse projects have been implemented over the last ten years, essentially for supplying cooling towers and boilers in the petrochemical industry and energy production sector. Since 2000 for example, several refineries in California have adopted the use of urban recycled water as their main source of boiler and cooling water.

The West Basin water recycling plant to the south of Los Angeles is one of the biggest reuse plants in the world (capacity of 220,000m³/day in 2015) and a world leader in the diversification in the uses of recycled water. For the first time ever, this plant deployed the 'fit for purpose" water production and "designer water" concept, distributing five different water qualities, three of which are specifically intended for industries (1) nitrified cooling water, (2) demineralised water for low pressure boilers and (3) very highly quality demineralised water for high pressure boilers.

Recycled water production for supplying cooling towers is performed on-site by biofiltering recycled water, a satellite treatment, which is distributed for irrigation purposes and other urban uses. Compact and high productivity Biofor^® bioreactors are used for the total nitrification of water using residual ammonia concentration levels below the detection limit of 0.1 mgN/L (average input concentration level of 40 mgN/L).

Boiler feed water in refineries requires a much higher level of demineralisation (table 37), which is performed by a reverse osmosis treatment process following microfiltration pretreatment. For example, the dissolved solids ( TDS ) in feed water are reduced from 800-900 ppm to around 50ppm for low pressure boilers. For high pressure boiler feed water, a second passage through the reverse osmosis stage is performed in order to reduce dissolved solids to below 5 ppm.

urban and municipal uses

Non-potable water distribution in the urban environment is quite an old activity (e.g. Los Angeles, Paris, San Francisco, etc.) in which particular interest has been shown over the last few years. In response to increasingly frequent droughts and water shortages, several cities (e.g. in Australia, China, Spain, United States, etc.) have developed a dual, recycled water distribution network for irrigation and other urban uses. The main water reuse categories in the urban environment are as follows:

the irrigation of green spaces, which is the most common usage and which includes the irrigation of public and private parks, sports grounds, greenbelt land, golf courses as well as residential areas and private gardens.
other examples of urban uses include street cleaning, carwashes, fire-fighting, air-conditioning, toilet flush water and other commercial uses.
recycling within a building which is essentially water recycling in high-rise buildings, including offices, shopping malls and private residential blocks.
improving the environmental and recreational uses for the reconstitution and the supply of water bodies, including those used for swimming (with or without physical contact), leisure pursuits or fishing.

Generally, water reuse in the urban environment requires a suitable infrastructure and in particular a dual distribution network. Dual distribution and plumbing systems are relatively easy and economically viable to install in new urban areas and/or new buildings.

Protecting public health is the most important requirement for this type of use due to risks of direct contact with recycled water. Due to this, effluent disinfection requirements are the amongst the most stringent for non-drinking water uses and are comparable to those for restriction-free irrigation for parks and green spaces open to the public. In order to achieve such a treatment level with near-total disinfection, additional filtration and disinfection treatment processes are necessary following biological treatment. In addition to monitoring recycled water quality, stringent and regular control of the distribution systems is recommended, in particular in order to avoid all risks of interconnecting with the drinking water the network.

potable uses

Technological progress in the domain of water treatment enable excellent quality recycling water to be produced from urban wastewater, which may even be of better quality than the drinking water from springs. Several scientific studies have demonstrated that there are no pertinent objections for reusing purified wastewater as drinking water following appropriate treatment. However, the main constraints in this type of use are psychological and cultural, with a negative perception of wastewater as being unhealthy and irreversibly contaminated.

Although not publicly recognised, several cities around world are supplied by unplanned wastewater reuse schemes, with the water diluted a little or less by surface water for indirect drinking water production. Compared to this existing and widespread situation, planned reuse of wastewater to increase drinking water resources have a certain number of advantages which allow health risks to be reduced and the profitability of water and wastewater treatment to be improved.

The history of the success of reusing purified wastewater for producing drinking water started in the 1960s with an acquifer recharge project at Montebello Forebay, California (1962) by infiltration of recycled water and first direct reuse as drinking water at Windhoek in Namibia (1968). Since then, the use of these projects experiences and those of several other projects implemented since, has demonstrated the feasibility of this type of reuse and the absence of negative effects on public health.

The most common practices in indirect reuse of purified water for drinking water production includes:

indirect (via infiltration tanks) or direct (via injection well) replenishment of aquifers used for drinking water production or as a barrier against the intrusion of seawater or polluted water (e.g. river bank filtration),
replenishment of surface reservoirs used for drinking water production.

Due to the high health risks related to possible microbiological or chemical pollution, treatment processes are based on the "multiple barrier" concept in order to achieve and to ensure a very high degree of effectiveness and reliability. Very stringent safety requirements for public health are not only met through adequate treatment but also by regularly and continually controlling water quality as well as each individual process' reliability and by deploying best project management practices.

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