Drinking Water Treatment
Drinking water treatment aims
Access to drinking water is a human right meaning that through water treatment there are many aims in order to deliver an acceptable product that is consumable at the tap. As such, there are many objectives that must be met during the water treatment process to meet the high standards required for drinking water, these include a product that is:
Free of pathogens and toxic substance (Safe to be drank)
Low organic content (preventing biological growth of harmful products in pipes and tanks)
Palatable (no destinct bad taste)
Colourless (no dissolved materials within the water that can affect colour and clarity)
Odourless
Reasonable Soft allowing for use in washing machines (low concentrations of Calcium and Magnesium)
Non-corrosive (preventing deterioration to pipe networks and storage tanks)
In order to meet these objectives the techniques that are implemented often mimic natural processes but on a more intense basis with the main aims being to meet the standards that are in place by removing harmful products via binding and concentration, transforming the water through oxidation process and deactivating harmful biological activity as well as the addition of require compounds to meet standards via chemical dosing. The actions that take place vary as a result of the source of drinking water with different sources often outlining different properties, this is most notable when comparing Surface water and Groundwater.
Surface water often has a high level of organic content with high dissolved oxygen levels within it as a result of the natural flow of the water providing aeration to the water. Groundwater is located deep within the Earth’s surface located in aquifers where there is little to no oxygen within the water as well as concentrations of elements and compounds that are not suitable for untreated consumption such as Iron, Manganese, Ammonia, Hydrogen sulfide, Methane, Leached anthropogenic chemicals such as pesticides as well as harmful products such as arsenic and flouride. unlike the surface water, however, in deep or protected groundwater sources there are no pathogens present.
As with all water sources in order to reach the standards that are set on both an international and national scale treatment is required.
Treatment of Groundwater
As was stated previously, Groundwater contains products and characteristics making it not suitable for direct consumption. Therefore to meet the standards for water potability, groundwater goes through many treatment steps. treatment also varies depending on wether the groundwater is aerobic or anaerobic with aerobic water requiring less treatment compared to anaerobic water as there is oxygen within the water. In terms of unpolluted aerobic groundwater following the initial abstraction the water is conditioned to meet potability standards before being stored for use, while Anerobic ground water requires further steps to meet the standards.
Step one: Aeration
The first step on the way to potable water from groundwater is aeration, in this step there is an addition of Oxygen into the water increasing the dissolved oxygen content of the water. Aeration is conducted through the use of aerators such as cascade aerators, water pour over the edge of a surface into a collection tank being aerated as it falls into the tank, or through the use of spray nozzels, groundwater is sprayed in fine droplets within a collection area aerating the water. This process also removes carbon dioxide, methane and hydrogen sulfide allowing for these compounds to escape. Aeration can also lead to the precipitation of metals and compounds such as Iron, Manganese and Ammonium via oxidation processes, these precipitates can then be removed via filtration.
Step two: Filtration
The second step of groundwater treatment is filtration, this process can be done using many methods including sand filtration as well as the use of filtration membranes and processes such as reverse osmosis, activated carbon filtrations. Filtration is undertaken to remove metal oxides created during the aeration phase, particle removal, the transformation of ammonium (NH4) to Nitrate (NO3) and the reduction of nartural organic matter that can impact the colour of the water.
Step three: Disinfection
The final step in treating groundwater before the water is disinfection the methods that are used in this period to provide safe drinking water often vary depending on national legislation with some countries prefering the use of chemical dosing to disinfect the water such as chlorine, however, this is not a globally recognised practice. Other regions impliment technology such as UV disinfection, using UV light to reduce colony counts of bacteria within the water.
Following the process of disinfection the water is then distributed as drinking water.
Treatment of Surface Water
The treatment of surface water often requires more steps than that of groundwater as unlike many of the groundwater aquifers that are abstracted from, surface water is often contaminated to some degree, with high concentrations of organic material, geological contaminants, chemicals and pathogens that enter the system as a result of both natural and anthropogenic sources as a result of surface water. This occurs as it is extremly hard to protect surface water systems from external influences. As such, treatment techniques that are used in surface water treatment for drinking water are highly focused on the removal of these substances following abstraction
Step one: Coagulation and Flocculation
In this initial step of surface water treatment chemicals with a possitive chathe are added to the abstracted water neutralising the degative charge of sediments and other dissolved particles within the water these particles bind creating floc, larger material that easily removable relative to the pre-coagulated surface water.
Step two: Sedimentation
Following the initial step the flocs that are created are removed through the process of sedimentation, this is often done in sedimentation takes with the contant minimal movement of water within a sedimaentation basin allowing for the heavier floc to sink to the bottom of the tank where it is later removed. Water that does not contain any floc then flows to the next step of water treatment.
Step three: Filtration
The water following the sedimentation phase passes through filters designed to remove disolved particles, pathogens, bacteria and chemicals. To do this the filters that are used vary in composition from sand and gravel filters to active carbon filtration and more advanced filtration technology including reverse osmosis and membrane filtration technology. This step produces a sludge of harmful components that are removed making the water almost ready for potable use.
Step four: Disinfection
Disinfection occurs following the filtration phase of treatment and is the final treatment step before the water is distributed for use. In order to complete the disinfection phase multiple techniques are used dependant on the regulation that are in place for drinking water. Often chemical disinfection is used, with the addition of chlorine into the water to fully disinfect the water and make it consumable to acceptable standards, other methods of disinfection include UV disinfection to reduce the biological activity within the drinking water.
Following these steps the treated drinking water is distributed for use.
Meeting Drinking Water Demands
In many arid areas of the world the issue of safe and reliable drinking water is a major issue with surface water sources being few and far between as well as an increasing pressure on groundwater resources, with over abstraction in many of these arid regions especially in coastal zones leading to salinisation of freshwater resources. It can also be noted that some regions have very minimal access to freshwater resources and as such they are looking to other methods to create safe and clean drinking water to sustain the water needs of the region.
Desalination
The desalination of sea water is a method of removing the salt from the sea water creating a clean source of drinking water. Desalination is an extremly energy intensive process with many methods being designed to improve the sustainablility of this practice. Multiple methods are currently in operation across the world to desalinise sea water these include:
Flash desalination - the repeated heating of seawater before capturing the vapour/steam produced, this steam is condenced before being heated again. This pattern is repeated until a usable product is created, as such the process itself requires a high level of energy for the repeated heating of the water.
Reverse Osmosis membrane filtration - by increasing the pressire on the salt side of the reverse osmosis filter and passing the water across the membrane the salts from the water remain while the fresh water passes through creating a source of pure water removing between 95 and 99% of total dissolved salts. This process requires both a high level of energy to achieve the pressures required to allow for reverse osmosis to occur but also a high cost of maintainance as the membranes are quite costly. As this process removes almost all disolve material from the water it also requires treatment with the addition of key minerals natural found in freshwater.
Low energy solar desalination - through the use of solar radiation and structures designed best for the capture of the treated water, often circular pyramid constructions, captured sea water is heated naturally usting the process of solar distilation to heat up salt water the steam produced condences along the walls of the structre and drains away to collection points for storage. This process is very sustainable in terms of energy requirements with minimal external energy used, as such this whole process is reliant on the presense of the Sun therefore it is not the most reliable method to meet water demands.
Rainwater Capture
Another method of capturing freshwater is the use of rainwater capture this is often seen on a smaller scale for personal drinking water needs in regions with little freshwater as well as to improve personal sustainablity levels, with the use of surfaces such as rooftops being used as a capture area. During precipitation events the rainwater is captured and drained into storage tanks where it can be used as and when it is needed, due to the potential for contamination from bird dropping and other organic matter this water is often used for matters such as flushing toilets, reducing water footprints.
Mist and Vapour Harvesting
A more novel approach to meeting the water requirements of a region, mist/vapour harvesting is a method of condensing the natural moisture within the are using a large surface area of a permiable material, allowing for moisture to enter before condensing and being captured and stored for usage. This method of water capture is highly dependant on local climatic conditions with a high requirement of moist air, mist/vapour harvesting also has a low energy requirement improving the sustainablility of this method of water capture.