Water Treatment

 

Water contamination

There are many sources of contamination to water bodies and a number of pathways through which pollutants can move. Contamination sources are divided to point and non point sources.

A point source is any single identifiable source from which pollutants are discharged, e.g. pipes, ditches, ships or factory smokestacks.

Nonpoint source (NPS) runoff has sources harder to precisely identify, thus the term nonpoint. Because of this uncertainty, NPS contamination is harder to control than point source. The word runoff indicates rain water or snowmelt carried across land to water. Runoff arises from nonpoint sources and can carry oil, grease, dirt, trash, animal waste, microorganisms and chemical pollutants such as metals, pesticides, and fertilizers. Urban nonpoint sources include streets, parking lots, roofs, and construction sites. Rural nonpoint sources include agriculture, logging, and mining sites. Polluted runoff is the most serious water quality problem, and is a major problem worldwide.

Groundwater contamination

More than a quarter of the world’s population depends on groundwater for drinking water. When groundwater is very deep underground, contaminants running off in rainwater may not reach it. However, much of the groundwater used for drinking water is in shallow aquifers and above-ground pollutants may be incompletely removed as they are carried down into groundwater. Once in groundwater then, because of often close connections between shallow groundwater and surface water, the contamination can reach surface water.

The opposite is also true, i.e. surface water contamination sometimes reaches groundwater. Once polluted, groundwater can remain that way for a long time. Especially in slow-moving groundwater, pollutants may persist indefinitely. Oily chemicals with low water solubility pose special problems. Trapped in soil and rock, they may continue to slowly leach into the groundwater, maintaining contamination indefinitely.

Water-soluble surface pollutants infiltrate soil, often reaching groundwater. Shallow groundwater is most easily contaminated. How much pollutant reaches groundwater depends on soil type, pollutant characteristics, and the distance to the groundwater.

Contamination sources (Table 1) include agricultural and urban runoff, chemical spills and landfill leachates.

                        

Land use affects how much contamination reaches groundwater: intensively used agricultural land can leach out nutrients (reactive nitrogen in fertilizers), pesticides, and microbes; a portion of each of these can reach groundwater. Likewise, in urban areas highly contaminated runoff, once it reaches surface soil, can percolate down to groundwater carrying with it a portion of its pollutant load.

Pathogens, especially viruses which are much smaller than most bacteria, reach groundwater too. For example, sewage from improperly installed or maintained septic systems can contaminate groundwater. Improperly protected landfills can leach contaminants, some of which reach groundwater. This is also true for petrochemicals from leaking underground-storage tanks. Groundwater often has detectable levels of pesticides too.

Detectable doesn’t necessarily indicate a problem, but points out the need for contamination prevention because cleaning up groundwater is either very difficult or not possible at all (Hill, 2010).

Overview of methods used to treat water

A variety of methods have been developed as well as new methods are being developed for the treatment of water. In most cases, a combination or sequence of methods is needed depending on the quality of untreated water and the desired quality of treated water. Although treating water is relatively inexpensive on a volumetric basis, there is limited opportunity to modify water quality directly in most natural systems such as streams, lakes and groundwaters because of the large volumes involved. It is common to treat the water used for public water supplies before distribution and to treat wastewater in engineered systems before it is returned to the environment. The constituents in water and wastewater are removed by specific methods which are classified as (Crittenden et al., 2005):

i. Physical unit operations, in which change is brought about through the application of physical forces, include screening, mixing, gas transfer, sedimentation and filtration.

ii. Chemical unit processes, in which the removal or treatment of contaminants is brought about by the addition of chemicals or by chemical reactions. Chemical precipitation and disinfection are two important examples.

iii. Biological unit processes, in which the removal of contaminants is brought about by biological means. Nitrification and denitrification are the best-known biological processes that have been used for water treatment.

Reducing groundwater contamination

Surface water and groundwater are often closely interconnected and runoff can contaminate both. Thus, a holistic approach is necessary. Contamination prevention strategies can help maintain groundwater purity as when regulations specify that only those pesticides with little tendency to migrate into groundwater can be used in a wellhead area, or regulations may specify how to apply pesticides in a way that limits the amount that runs off with rain.

Another contamination prevention approach is regulating how land can be used: prohibiting siting of landfills or gasoline stations over groundwater that feeds into a wellhead; prohibiting large confined-animal operations near vulnerable groundwater or more strictly, prohibiting farmers from grazing livestock there.

Cleaning up polluted groundwater is extremely costly, often not possible with today’s technology. Nonetheless, pump-and-treat is commonly used with the goal of restoring the water to drinking-water quality. The groundwater is pumped to the surface, treated to remove pollutants and then returned to its source. Especially in aquifers with large volumes of water, pump-and-treat may continue for many years without notably reducing contamination.

Sometimes groundwater is treated in place, not removed from the aquifer. One such technique involves digging trenches, and installing tons of iron filings mixed with sand in the path of the groundwater. As contaminated groundwater flows through this permeable barrier, some organic pollutants, such as trichloroethylene will react with the iron and decompose into benign products. Another in place technique being explored is using anaerobic microorganisms that can degrade the contaminants; they need to be anaerobic because groundwater contains little oxygen.

Water Management

Water management is dictated by water use and controlled by water availability and water quality. Today, public policy makers must consider the entire hydrologic system, environmental concerns and social needs when water management decisions are made. Knowledge of aquifer characteristics is required for the management of groundwater systems. In addition, the connection between surface water and groundwater resources is increasingly being recognized as a major management issue.

Knowledge of the recharge to, flow through, and discharge from a groundwater system, including how it interacts with surface water, is needed to manage the system. This knowledge will assist in determining where to locate wells and the quantity of water that can be optimally withdrawn from the groundwater system. Maintaining good groundwater quality requires protecting the aquifer from sources of contamination.

Water conservation is an efficient and effective means of solving many water supply problems. Conservation can help reduce the effects of short-term drought and to some degree eliminate the need for development of new water sources. Water conservation is the responsibility of everyone (Vandas et al., 2002).

References

  • Crittenden J., R.R. Trussell, D.W. Hand, K.J. Howe and G Tsobanoglous (2005). Water Treatment: Principles and Design, 2nd Edition, John Wiley & Sons, Inc., Hoboken, New Jersey, ISBN: 978-0-471-11018-7.
  • Hill M.K. (2010). Understanding Environmental Pollution, Third edition, Cambridge University Press, New York, ISBN-13 978-0-521-73669-5 (paperback).
  • Vandas S.J., T.C. Winter and W.A. Battaglin (2002). Water and the environment. American Geological Institute in cooperation with Bureau of Reclamation, National Park Service, U.S. Army Corps of Engineers, USDA Forest Service, U.S. Geological Survey, ISBN: 0-922152-63-2.