Water is essential to all aspects of life and as a result, so too is water treatment. Tony Wheatley explains why, and where gases are involved.
Globally it is estimated that 1.8 million deaths were caused by waterborne diseases in 2006, mainly due to inadequate or non-existent public sanitation facilities.
Reliable data to indicate what proportion of wastewater collected in sewers is being adequately treated is not available in many developing countries.
According to public-contributed online encyclopedia Wikipedia, only 15% of the wastewater collected in Latin America goes through a treatment process and in Venezuela, 97% is discharged as raw sewage into the environment.
Meanwhile, most of sub-Saharan Africa survives without water treatment. The increasing amount of uncontrolled wastewater discharged in rapidly developing countries is expected to cause a significant threat to world health in the short term future.
History of water treatment
The first city-wide, municipal water treatment plant was installed in Paisley, Scotland in 1804 to provide filtered water to every household.
This installation employed slow sand filters designed by Robert Thom, an important Scottish scientist.
The term ‘Water Treatment’ includes a range of processes that share the goal of making it fit for the intended use, by reducing the concentration of existing contaminants or removing them entirely.
In the majority of situations water is intended for human consumption, but water treatment is also often required before it is returned to the environment.
Indigenous populations of bacteria feed on organic contaminants and the population of disease-causing micro-organisms are diminished by natural conditions like predation or exposure to the ultraviolet radiation in sunlight.
Various physical, chemical and biological processes have been developed that mimic those found in nature.
In each case, an appropriate sequence of treatment processes can be determined only after determining the contaminants present and the seasonal variability of water flow.
Potable water purification
Around 70% of the Earth’s surface consists of water and there many sources where water may be obtained.
Groundwater and surface water are the two major sources of water supply, with surface water in lakes, rivers and reservoirs being preferred for large volume systems.
The wells from which groundwater is pumped for smaller requirements range from 15 to 300 metres deep and very often provide water that satisfies quality standards without any treatment at all.
Surface water, being exposed to the atmosphere and wet weather run-off, often contains contaminants including bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, as well as synthetic pollutants like chemicals, pesticides and fertilizers.
Municipalities around the world apply various combinations of the following processes in the production of drinking water.
Before water can be passed into the public supply, it is necessary to remove all potentially pathogenic micro-organisms by disinfection with ozone or chlorine. Further treatment to remove components such as nitrates and trace organics may also be required.
Sewage is created by households, hospitals, institutions, commercial and industrial organisations and often inadvertently contains many toxic organic and inorganic compounds.
The objective of sewage treatment is to produce a stream of waste water and a solid waste output, both of which are suitable for reuse or discharge back into the environment.
The processes used in conventional sewage treatment are typically applied in three stages, with distinct objectives.
Primary stage processes
The primary treatment stage separates solids from the influent first, by screening to prevent physical damage or clogging of downstream equipment and sedimentation.
Primary sedimentation aims to remove the sludge that can then be separately treated or processed, while leaving behind a generally homogenous liquid that can be biologically treated.
Secondary stage processes
Secondary treatment processes are designed to substantially degrade the organic content of the liquid sewage derived from human waste, food waste, soaps and detergent and to progressively convert the dissolved matter into a solid mass, by encouraging the activity of indigenous water-borne microorganisms.
Most treatment plants apply aerobic biological processes to the settled sewage liquor, in which bacteria and protozoa consume the biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc.
Various systems have been developed to provide both oxygen and a substrate for the biota to live on and these are classified as either fixed-film or suspended-growth systems.
Fixed-film systems include trickling filters and rotating biological contactors but these are typically found in older facilities.
Suspended-growth systems include surface aerated basins but because of the superior performance of activated sludge systems, these have recently been further developed to include high purity oxygen enrichment.
The final step in conventional secondary treatment is sedimentation to settle out the biological floc and produce sewage water containing very low levels of organic material and suspended matter.
Tertiary stage processes
The objective of processes in the tertiary stage of conventional sewage treatment is to raise the quality of effluent before it is discharged to the receiving environment.
Filtration through sand removes residual suspended matter and over-activated carbon removes residual toxins.
It is important that any remaining concentration of nitrogen and phosphorous in treated wastewater should be removed.
If these are released in excessive quantities, their accumulation (called eutrophication) will encourage the rapid growth of weeds, algae and cyanobacteria (blue-green algae), often resulting in an algal bloom and serious depletion of the dissolved oxygen level.
Nitrification is a two step aerobic process that converts ammonia to nitrate and denitrification requires anoxic conditions to reduce nitrate to nitrogen gas that is released into the atmosphere.
Disinfection is always the final step prior to release and may be applied chemically using ozone or chlorine, physically using ultraviolet light or by lagooning or microfiltration.
New hybrid processes
Membrane bioreactors combine activated sludge treatment with a membrane solid-liquid separation process. Considerably higher mixed liquor suspended solid concentrations can be processed than with the conventional activated sludge process, and problems of poor settling are effectively overcome.
Although more expensive to build, these systems require a smaller footprint and have been widely accepted.
The sequencing batch reactor is typical of hybrid treatment plants developed to deal with intermittent flow, limited space or difficult to treat waste. Designed to achieve the objective of all three stages in one combined process, these plants are being widely deployed.
Municipal authorities in developing countries challenged with rapid population growth and urbanization are often chronically under-funded.
Low tariffs, low billing efficiency, poverty and poor administration are among the reasons and many systems are ineffective due to inefficient operating practice, and non-enforcement of environmental standards.
But there’s no escaping the fact that water is fundamental for life and health and is something of a basic human right.
As developing countries continue to modernize and provide the best possible service for this human right, municipal authorities are likely to be driving the implementation of waste water treatment systems and therefore, driving the consumption of gases, however slight.