In this article the hydrological cycle on large (global), medium (regional) and small (local) scales is examined, and the anthropogenic threats to water budgets and water quality are outlined. Humans use and re-use just a fraction of 1% of the global water budget. With surface water residence times typically of the order of weeks-to-months, any water contamination from this use can impact relatively quickly on the environment for a duration prolonged enough to damage the ecosystem significantly.

 

The key abstraction issues and pollutant problems associated with our use of water are identified, and the major policy and technological approaches adopted to minimise these problems are also outlined, which ensure water is not only ecologically fit, but can also be used and reused for industrial and for potable supplies.

 

We also make an assessment of the efficacy of abstraction control policies, catchment management techniques, pollution control measures and technological treatment advances in providing a sustainable interaction with the hydrological cycle.

In practical terms we can envisage the hydrological cycle operating on three scales. On a large (global) scale, about 1.4 million km3 water resides predominantly in the oceans, with extractable and available fresh water comprising less than 0.32% of this global budget much of which (ca. 0.3%) is shallow groundwater (see Figure 1). Average residence times for water molecules in the main global compartments (oceans, lithosphere, and atmosphere) are of the order of a few days in the atmosphere, weeks-to-months in fresh surface waters and millennia in the oceans and groundwater. The enormous scale of the global cycle means that anthropogenic influences tend to be limited here. However there is increasing evidence of measurable anthropogenic impact, primarily related to ice-sheet melting and increased atmospheric water cycling resulting from climate change derived from greenhouse gas emissions.

 

Heavy anthropogenic use and re-use of restricted surface fresh water budgets means a medium-scale (catchment- or region-wide) hydrological scale typically operates. Water inventories are often small enough and water residence times short enough for human activity to alter both water flux and water quality significantly at this scale. Contaminants introduced to this local water cycle can quantitatively change water composition, and water quality deterioration can be prolonged enough to adversely affect humans, habitats and ecology. Many countries are beginning to plan for the potential impacts of climate change at this medium-scale, as water fluxes and budgets may change substantially (either up or downwards) and average temperature changes can, for example, impact on water oxygenation. It is at this medium scale that much attention to water quality protection and water abstraction controls have been focused in many countries. This in turn will form a major focus for this article.

 

A third small-scale cycle can also be envisaged. Much of the legislation relating to water is drawn up such that individual companies, organisations or specific sites must comply. Each organisation must deal with abstraction restrictions, perhaps requiring on-site process water capture, treatment and re-use, or must decontaminate aqueous discharges to a prescribed degree. It is mainly at this scale that technological approaches to minimise water use and treat waste water streams are employed over short time-scales (hours-days) to achieve the water protection objectives.

 

Although threats to the global water cycle have been outlined, there is great uncertainty of both the extent of the impacts of our activities and where these may be felt. Major concerns relate to changes in local drought or flood risk.

 

At a regional scale, excessive water abstraction can restrict down-stream water availability, deplete aquifers and result in the intrusion of salt-water into coastal aquifers contaminating groundwater supplies. Impacts on water quality at this scale derive from long-recognised threats including oxygen-demanding inputs, turbidity, toxic metals (imparting lethal and sub-lethal effects on biota), nutrients (resulting in eutrophication and subsequent water oxygenation issues), pesticides (impacting on aquatic flora or fauna and changing ecological water balances) and pathogens (bacteria and viruses). More recently threats from a greater range of persistent organic pollutants (POPs) and endocrine disrupting chemicals (EDCs) are being recognised by regulators. Contaminant discharge controls have generally been effective in limiting point-source impacts, whilst land management and better procedural activity have begun to reduce the effects of diffuse-source pollution.

Water use and its contamination prior to re-introduction to the environment have caused significant degradation to aquatic habitats historically. Significant improvements have been observed across many countries as national regulators have enforced regulations covering water abstraction and discharge controls to ensure local water budgets are sustainable and that treated waste water released back to the environment is fit to maintain not only human health but also to maintain and, where necessary, improve ecological and habitat conditions.

 

The approach used to achieve this environmental protection is through legislative measures, or by means of enforced or voluntary codes of practice, supported by the publication and promotion of good practice to specific communities such as farmers or town planners. For example, in the EU legally binding discharge consents restrict the introduction to the environment of a large number of dangerous inorganic and organic prescribed substances. In recent years, the implementation of Integrated Pollution Prevention and Control (IPPC) has lead to a review of existing discharge consents to ensure that all necessary hazardous substances used at each location are controlled and that the local environment at each specific location is uniquely assessed with appropriate discharge modeling to ensure the risk of acute or chronic pollution is minimised with due regard to flora and fauna. Importantly, this process has allowed different regulators to liaise to “join-up” legislative initiatives. In England and Wales, IPPC licenses issued by the Environment Agency (EA) were required to give due consideration to impacts under the Habitats Directive, itself regulated largely by English Nature (now Natural England), as well as the health authorities whose concern rests largely with human health protection. Similarly, IPPC legislation ensures the requirements of the Groundwater Regulations are met, overlapping with aspects of Local Authority contaminated land jurisdiction.

 

Recently, the EU Water Framework Directive (WFD) has extended trans-regulator liaison in the UK as River Basin Management Plans (RBMPs) have been drawn up with the key objective of maintaining and improving ecological and habitat health. Described in detail in a prior article (Working with Water, April 2008), implementation of the WFD in the UK has lead to a wide ranging review of point- and diffuse- source pollution on a catchment-wide scale, and incorporates water abstraction licensing (Catchment Abstraction Management Strategies, CAMS) to ensure water budgets are sustainable on a regional scale.

 

As regulators have established tighter controls over harmful emissions to the aquatic environment from point sources, efforts to limit diffuse-source pollution have taken greater prominence. Catchment Sensitive Farming (CSF) aims to reduce the leaching of organics, solids, nitrogen, phosphorus and pesticides to surface waters in part by adopting alternative agricultural practices (see Figure 2). Although legislative controls apply to certain aspects (for example COD or pesticide contamination can often be uniquely attributable to an individual farm), a co-operative rather than confrontational approach is adopted by regulators to educate and encourage farmers to reduce diffuse emissions through changes to planting regimes or other land-use changes, the effects of which are observed only on a wider catchment scale. A similar approach is adopted with town planners to manage flood risk and urban diffuse pollution.

 

The elimination or reduction of use of certain dangerous substance has also played an important part in ensuring surface waters are fit for natural ecology. Newly developed pesticides are evaluated for specificity, solubility and partition coefficient (hence the environmental mobility and bioavailability), and for biodegradability (hence environmental lifetime) prior to obtaining registration and release for use. Periodically prescribed substance lists are reviewed to ensure releases of the more harmful substances are controlled. Identification of suspected EDCs, responsible for the observed inter-sex in an increasing number of species, is of high priority. Known EDCs have a varied array of chemical structures making the activating mechanism difficult to understand clearly. Nonetheless, chemical structural modeling is used to identify EDC potential across a wide range of pharmaceutical, plasticiser and other organic substances, with subsequent in-vitro and in-vivo testing to confirm (or otherwise) endocrine disrupting activity.

Environmental protection from the introduction of hazardous chemicals and the over-abstraction of water from catchments uses a range of legislative and policy approaches to ensure this aim is achieved. Industry often has differing requirements and may adopt additional technological approaches to ensure its own specific requirements are met, as constrained by the regulator requirements outlined above.

 

Industry can be a heavy user of water in a variety of ways. Water might be integral to the industrial process itself (e.g. aspects of the chemical or food and drink sectors), may be used extensively for cooling (e.g. electricity generation), or may be used to maintain site cleanliness, product cleaning or be used for dust control. The quantity of water needed and its required quality varies by sector and by site. Used water will normally require treatment to a suitable degree prior to its re-introduction to the industrial process or, alternatively discharge to sewer or the environment.

 

Water budgets form an increasingly important part of the industrial environmental managers concerns. IPPC requires periodic reporting of water use to the regulator, and water reduction may well have been a requirement of the IPPC improvement program. Alternative water harvesting (e.g. rooftop rainwater capture or drainage diversion) and process water treatment and recycling reduces abstraction requirements and also reduces the quantity of used water discharged; this can often be economically beneficial as the disposal of wastewater is, in part, based on volume.

 

To ensure the quality of the water used for industrial process and that subsequently discharged is fit-for-purpose, a wide range of technologies are available. COD removal through aeration and filtration to reduce solids has long used conventional settling, bubbling, cascading, spraying and flotation technologies. Advances in membrane technology have allowed micro- (< 0.1 µm) and ultra-filtration (< 0.01 µm) to play an important and economically feasible role in many wastewater treatment processes with the ready capability of reducing solids loads (< 1mg/l), and COD (<2 mg/l) with an appropriate bioreactor. The energy costs of these advanced systems, an important aspect of economic feasibility studies, can be dramatically reduced by careful plant design and by non-continuous, influent responsive aeration (e.g. GE Zenon’s Intelligent Aeration). Innovative filtration technologies continue to be developed; for example Fibra’s parallel-flow MF device removes solids in a highly controlled way and can be configured to simultaneously correct effluent pH using carbon dioxide gas.

Metals in wastewater can be removed by means of conventional chemical precipitation or by using newer technologies such as porous adsorptive media. Certain adsorptive media have the capacity to readily remove most metals to sub-ppm levels and can neutralise substantial acidity concurrently. Inoculated adsorptive media have proved successful in the bioremediation of POPs in sludge and contaminated soils, whilst activated charcoal continues to be successfully employed in removing dissolved organics and color from wastewater. Nitrogen removal from wastewater through staged nitrification-denitrification is well established, whilst the potential for anaerobic membrane bioreactors is currently being developed at Imperial College, London.

 

Developers of advanced treatment systems often modularise their equipment and emphasise a small plant footprint to maximise the potential to retrofit and upgrade existing older treatment systems. Low maintenance automated plant is also pre-eminent to enable its employment at remote and unmanned site. Individual users of the available technology have an array of hybrid possibilities at their disposal to reliably treat almost any capacity of wastewater to a well-defined quality.

The means by which industrial users treat wastewater to ensure it is fit-for-purpose are varied. Wastewater treatment systems have become very robust as well as effective, in many cases drastically reducing the incidence of discharge non-compliance. In order to supply fit-for-purpose drinking water supplies, additional treatment steps are required to remove pathogens and, if necessary, to ensure salts are removed to improve taste and color to improve aesthetics. Drinking water quality standards are in place in many countries setting a legal framework around these parameters.

 

A conventional approach to pathogen removal from potable water supply is by means of chemical disinfection. Whilst effective, care must be taken to chemically re-adjust the treated water to ensure it is safe to drink, an expensive and finely-balanced process. Accordingly, alternatives for pathogen removal have increasingly found favor, notably membrane UF, a physical barrier capable of log 4 removal value (i.e. 99.99%) of bacteria, Cryptosporidium and Giardia, together with a similar degree of virus removal under many circumstances. Modern UF systems have throughputs similar to MF systems providing a significant throughput capacity.

Membrane nanofiltration, removing colloids and larger ions, which can provide water softening for a relatively small additional cost to producers, has been introduced in a significant way in Germany. Here one membrane supplier plans to provide 50 Mm3/yr water softening capacity over the next few years. Reverse osmosis (RO) membrane technology provides an effective and proven method for producing potable water from brackish or saline raw water (see Figure 3). RO is often combined with UV treatment for pathogen and color removal. The relatively high production costs of RO-UV, an energy intensive process, need to be balanced against the potentially high costs of water transport and the extensive potential for water re-use that the technology opens, particularly where production plants themselves site and utilise renewable energy sources such as solar, tidal or wind power.

 

The Singapore Water Reclamation Study, commenced by the Singapore Public Utilities Board (PUB) in 1998, utilises MF, RO and UV at designated NEWater treatment plants to produce high quality reused water from secondary treated effluent. Much of this water is re-used by industry, while a proportion (currently ca. 3 MLD) is blended with raw water in reservoirs. This is subsequently treated as normal and provides approximately 1% of the daily potable water demand. The proportion of blended NEWater in reservoirs is set to rise (ca. 2.5% daily supply in 2011) as public confidence in the safety of the system improves. Experience from existing NEWater plants has shown that MF effectively pre-treats influent for the effective long-term use of low fouling composite RO membranes. When the latest NEWater plant at Changi is expected to be operational in 2010, 30% of Singapore’s current water needs will be met by NEWater. The potential offered by treatment technologies for extensive water reuse, as seen at Singapore and previously in the USA and elsewhere, for both industrial and potable supply demonstrate the scope for this approach where water supplies are limited.

Anthropogenic impacts on the global hydrological cycle are uncertain and are likely to be associated with changing the quantity of water we deal with rather than its quality. Given this uncertainty, much of the focus from policy-makers has centered on a catchment-wide or regional scale. As predictive modeling of climate change effects on regional scales become more conclusive, legislation relating to water management (relating to drought or flood risk) and water quality control (relating to potential average water temperature shifts) will require review.

 

Currently, abstraction licensing limits impacts from over-use of water by reducing groundwater depletion rates, limiting salt water intrusion into aquifers and maintaining sustainable surface water base-flows. Older legislation primarily focused on human health protection. More recent legislation aims to protect the natural environment, its flora, fauna and habitats by means of a holistic approach to water quality control from both point and diffuse sources. Regulators have achieved good success in reducing the frequency and severity of point-source pollution incidents through legislation and effective monitoring. Progress is being made with diffuse-pollution control (e.g. falling nitrate levels in surface and groundwater), whilst further improvements are likely as the WFD is fully implemented.

 

Industrial and municipal water users ensure that water protection regulations are adhered to by using a wide range of treatment technologies. Technologies currently exist to effectively treat waste water for the all major categories of recognised pollutants. Wastewater treatment imposes associated costs to operators, so regulatory penalties for non-compliance will need to be increased to keep in line with higher capital and running costs of treatment technologies. Appropriately treated wastewater may be discharged back to the environment in a controlled and safe manner or be reused to reduce the need to abstract raw water resulting in a more sustainable interaction with the hydrological cycle.