Dalecarlia Water Treatment Plant, Washington, D.C.
Water treatment is any process that improves the quality of water to make it more acceptable for a specific end-use. The end use may be drinking, industrial water supply, irrigation,
river flow maintenance, water recreation or many other uses, including
being safely returned to the environment. Water treatment removes contaminants
and undesirable components, or reduces their concentration so that the
water becomes fit for its desired end-use. This treatment is crucial to
human health and allows humans to benefit from both drinking and
irrigation use.
Drinking water treatment
Treatment for drinking water production involves the removal of contaminants from raw water to produce water that is pure
enough for human consumption without any short term or long term risk
of any adverse health effect. In general terms, the greatest microbial
risks are associated with ingestion of water
that is contaminated with human or animal (including bird) faeces.
Faeces can be a
source of pathogenic bacteria, viruses, protozoa and helminths.
[Guidelines for Drinking-water quality]. Substances that are removed
during the process of drinking water treatment, Disinfection is of
unquestionable importance in the supply of safe drinking-water.
The destruction of microbial pathogens is essential and very commonly
involves the
use of reactive chemical agents such suspended solids, bacteria, algae, viruses, fungi, and minerals such as iron and manganese.
These substances continue to cause great harm to several lower
developed countries who do not have access to water purification.
Measures taken to ensure water quality not only relate to the
treatment of the water, but to its conveyance and distribution after
treatment. It is therefore common practice to keep residual
disinfectants in the treated water to kill bacteriological contamination
during distribution.
Water supplied to domestic properties, for tap water or other uses, may be further treated before use, often using an in-line treatment process. Such treatments can include water softening or ion exchange. Many proprietary systems also claim to remove residual disinfectants and heavy metal ions.
Processes
Empty aeration tank for iron precipitation
The processes involved in removing the contaminants include physical processes such as settling and filtration, chemical processes such as disinfection and coagulation and biological processes such as slow sand filtration.
A combination selected from the following processes is used for municipal drinking water treatment worldwide.
Chemical
Tanks with sand filters to remove precipitated iron (not working at the time)
- Pre-chlorination for algae control and arresting biological growth.
- Aeration along with pre-chlorination for removal of dissolved iron when present with small amounts relatively of manganese.
- Coagulation for flocculation or slow-sand filtration.
- Coagulant aids, also known as polyelectrolytes – to improve coagulation and for more robust floc formation.
- Disinfection for killing bacteria, viruses and other pathogens.
Physical
- Sedimentation for solids separation that is the removal of suspended solids trapped in the floc.
- Filtration to remove particles from water either by passage through a sand bed that can be washed and reused or by passage through a purpose designed filter that may be washable.
Technologies
Technologies
for potable water and other uses are well-developed, and generalized
designs are available from which treatment processes can be selected for
pilot testing on the specific source water. In addition, a number of
private companies provide patented technological solutions for the
treatment of specific contaminants. Automation of water treatment is
common in the developed world. Source water quality through the seasons,
scale, and environmental impact can dictate capital costs and operating
costs. End use of the treated water dictates the necessary quality
monitoring technologies, and locally available skills typically dictate
the level of automation adopted.
Desalination
Saline water can be treated to yield fresh water. Two main processes are used, reverse osmosis or distillation.
Both methods require more energy than water treatment of local surface
waters, and are usually only used in coastal areas or where water such
as groundwater has high salinity.
Portable Water Purification
Living
away from drinking water supplies often requires some form of portable
water treatment process. These can vary in complexity from the simple
addition of a disinfectant tablet in a hiker's water bottle through to
complex multi-stage processes carried by boat or plane to disaster
areas.
| Constituent | Unit Processes |
| Turbidity and particles | Coagulation/ flocculation, sedimentation, granular filtration |
| Major dissolved inorganics | Softening, aeration, membranes |
| Minor dissolved inorganics | Membranes |
| Pathogens | Sedimentation, filtration, disinfection |
| Major dissolved organics | Membranes, adsorption |
Standards
Many developed countries specify standards to be applied in their own country. In Europe, this includes the European Drinking Water Directive and in the United States the United States Environmental Protection Agency (EPA) establishes standards as required by the Safe Drinking Water Act. For countries without a legislative or administrative framework for such standards, the World Health Organization publishes guidelines on the standards that should be achieved. China adopted its own drinking water standard GB3838-2002 (Type II) enacted by Ministry of Environmental Protection in 2002.
Where drinking water quality standards do exist, most are
expressed as guidelines or targets rather than requirements, and very
few water standards have any legal basis or, are subject to enforcement.
Two exceptions are the European Drinking Water Directive and the Safe
Drinking Water Act in the United States, which require legal compliance
with specific standards.
Industrial water treatment
Processes
Two of the main processes of industrial water treatment are boiler water treatment and cooling water treatment.
A large amount of proper water treatment can lead to the reaction of
solids and bacteria within pipe work and boiler housing. Steam boilers
can suffer from scale or corrosion
when left untreated. Scale deposits can lead to weak and dangerous
machinery, while additional fuel is required to heat the same level of
water because of the rise in thermal resistance. Poor quality dirty
water can become a breeding ground for bacteria such as Legionella causing a risk to public health.
Corrosion in low pressure boilers can be caused by dissolved
oxygen, acidity and excessive alkalinity. Water treatment therefore
should remove the dissolved oxygen and maintain the boiler water with
the appropriate pH and alkalinity levels.
Without effective water treatment, a cooling water system can suffer
from scale formation, corrosion and fouling and may become a breeding
ground for harmful bacteria. This reduces efficiency, shortens plant
life and makes operations unreliable and unsafe.
Boiler water treatment
Boiler water treatment is a type of industrial water treatment
focused on removal or chemical modification of substances potentially
damaging to the boiler. Varying types of treatment are used at different
locations to avoid scale, corrosion, or foaming.
External treatment of raw water supplies intended for use within a
boiler is focused on removal of impurities before they reach the boiler.
Internal treatment within the boiler is focused on limiting the
tendency of water to dissolve the boiler, and maintaining impurities in
forms least likely to cause trouble before they can be removed from the
boiler in boiler blowdown.
Cooling water treatment
Water cooling is a method of heat removal from components and industrial equipment. Water may be a more efficient heat transfer fluid where air cooling is ineffective. In most occupied climates water offers the thermal conductivity advantages of a liquid with unusually high specific heat capacity
and the option of evaporative cooling. Low cost often allows rejection
as waste after a single use, but recycling coolant loops may be
pressurized to eliminate evaporative loss and offer greater portability
and improved cleanliness. Unpressurized recycling coolant loops using
evaporative cooling require a blowdown waste stream to remove impurities
concentrated by evaporation. Disadvantages of water cooling systems
include accelerated corrosion and maintenance requirements to prevent heat transfer reductions from biofouling or scale
formation. Chemical additives to reduce these disadvantages may
introduce toxicity to wastewater. Water cooling is commonly used for
cooling automobile internal combustion engines and large industrial facilities such as nuclear and steam electric power plants, hydroelectric generators, petroleum refineries and chemical plants.
Technologies
*Chemical treatment
Chemical
treatments are techniques adopted to make industrial water suitable for
discharge. These include chemical coagulation, chemical precipitation,
chemical disinfection, chemical oxidation, advanced oxidation, ion
exchange, and chemical neutralization.
Developing countries
Appropriate technology options in water treatment include both community-scale and household-scale point-of-use (POU) or self-supply designs. Such designs may employ solar water disinfection
methods, using solar irradiation to inactivate harmful waterborne
microorganisms directly, mainly by the UV-A component of the solar
spectrum, or indirectly through the presence of an oxide photocatalyst, typically supported TiO2 in its anatase or rutile phases. Despite progress in SODIS technology, military surplus water treatment units like the ERDLator are still frequently used in developing countries. Newer military style Reverse Osmosis Water Purification Units (ROWPU) are portable, self-contained water treatment plants are becoming more available for public use.
For waterborne disease reduction to last, water treatment programs that research and development groups start in developing countries
must be sustainable by the citizens of those countries. This can ensure
the efficiency of such programs after the departure of the research
team, as monitoring is difficult because of the remoteness of many
locations.
Energy Consumption: Water treatment plants can be significant
consumers of energy. In California, more than 4% of the state's
electricity consumption goes towards transporting moderate quality water
over long distances, treating that water to a high standard.
In areas with high quality water sources which flow by gravity to the
point of consumption, costs will be much lower.
Much of the energy requirements are in pumping. Processes that avoid the
need for pumping tend to have overall low energy demands. Those water
treatment technologies that have very low energy requirements including trickling filters, slow sand filters, gravity aqueducts.
Regulation
United States
The Safe Drinking Water Act requires the U.S. Environmental Protection Agency (EPA) to set standards for drinking water quality in public water systems (entities that provide water for human consumption to at least 25 people for at least 60 days a year). Enforcement of the standards is mostly carried out by state health agencies. States may set standards that are more stringent than the federal standards.
EPA has set standards for over 90 contaminants organized into six
groups: microorganisms, disinfectants, disinfection byproducts,
inorganic chemicals, organic chemicals and radionuclides.
EPA also identifies and lists unregulated contaminants which may require regulation. The Contaminant Candidate List is published every five years, and EPA is required to decide whether to regulate at least five or more listed contaminants.
Local drinking water utilities may apply for low interest loans,
to make facility improvements, through the Drinking Water State
Revolving Fund
