Clean Water To Drink

Water by all means have its role in our daily lives and water is surely important in the sense of water provides us with natural solvent for all uses as well as hygienic purposes just to name the main roles. Water sources comes just from everywhere where rain befalls and the rain is contained. The most natural water source that is available to the earth is the groundwater. The water emerging from some deep ground water may have fallen as rain many decades, hundreds, thousands or in some cases millions of years ago. Soil and rock layers naturally filter the water from the ground to high degree of clarity before it is pumped for treatment in treatment plantation. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep ground water is generally of very high bacteriological quality (i.e., pathogenic bacteria or the pathogenic protozoa are typically absent), but the water typically is rich in dissolved solids, especially carbonates and sulfates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bicarbonate. There may be a requirement to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection may also be required. Where groundwater recharge is practised; a process in which river water is injected into an aquifer to store the water in times of plenty so that it is available in times of drought; it is equivalent to lowland surface waters for treatment purposes.

Other water sources include upland lakes and reservoirs. These reservoirs are generally located in the headwaters of river systems usually at high lands. Bacteria and pathogen levels are low usually but certain bacterias, protozoans or algae are still present as members of the ecology. Uplands waters are usually forested or peaty, that is why acids such as humic acids sometimes colour the water. Many upland sources have low pH which require adjustment.

On the other hand, low land reservoirs or rivers have a significant bacterial load and may also contain algae, suspended solids and a variety of dissolved constituents.

New technology in the informative world also provides us with new means of producing water sources that can be drink as well as used. Atmospheric water generation is one example where high quality drinking water can be produced by extracting water from the air by cooling the air and thus condensing water vapor. Rain harvesting or desalination of seawater (reverse osmosis) are other primitive methods used to produce clean water (methods are still considered modern but primitive compared to atmospheric water generation technique).

The topic of clean water certainly must include ways of treatment of water that ensure the water we have everyday is clean and healthy. Therefore, from here onwards, discussion of pre-treatment and main treatment of water in generally the water purification plant. Treatments mentioned below are not necessarily used by all purification plants and the treatments might differ at different scale depending on various factors such as quality of water and the plant resources.

Generally there are few pre-treatments done before the 'real' treatment:

1. Pumping and containment - The majority of water must be pumped from its source or directed into pipes or holding tanks. To avoid adding contaminants to the water, this physical infrastructure must be made from appropriate materials and constructed so that accidental contamination does not occur.


2. Screening (see also screen filter) - The first step in purifying surface water is to remove large debris such as sticks, leaves, trash and other large particles which may interfere with subsequent purification steps. Most deep groundwater does not need screening before other purification steps.


3. Storage - Water from rivers may also be stored in bank side reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters. Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during transitory pollution incidents in the source river.


4. Pre-conditioning - Many waters rich in hardness salts are treated with soda-ash (Sodium carbonate) to precipitate Calcium carbonate out utilising the common-ion effect.
   Pre-chlorination - In many plants the incoming water was chlorinated to minimise the growth of fouling organisms on the pipe-work and tanks. Because of the potential adverse quality effects (see chlorine below), this has largely been discontinued.
   

Pre-treatments differ from one place to another as treatment to filter large particles are many more that are not listed above as well as depending on the water quality.

After all the pre-treatments, the main treatment begins. The main treatments mentioned here are the ones that are commonly used and not all methods are listed here.

The most common treatment used is certainly by simply adjusting the pH of the water as we know that water has a neutral pH of approximately 7. If the water is acidic (lower than 7), lime, soda ash, or sodium hydroxide is added to raise the pH. For somewhat acidic, alkaline waters (lower than 6.5), forced draft degassifiers are the cheapest way to lower the pH, as the process raises the pH by stripping dissolved carbon dioxide (carbonic acid) from the water. Lime is commonly used for pH adjustment for municipal water, or at the start of a treatment plant for process water, as it is cheap, but it also increases the ionic load by raising the water hardness. Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively and also helps to minimize the risk of lead being dissolved from lead pipes and lead solder in pipe fittings. Acid (HCl or H2SO4) may be added to alkaline waters in some circumstances to lower the pH. Having an alkaline water does not necessarily mean that lead or copper from the plumbing system will not be dissolved into the water but as a generality, water with a pH above 7 is much less likely to dissolve heavy metals than a water with a pH below 7.

Treatments done after pH adjustments are namely flocculation and sedimentation. I will start with flocculation first. Flocculation is a process which clarifies the water. Clarifying means removing any turbidity or colour so that the water is clear and colourless. Clarification is done by causing a precipitate to form in the water which can be removed using simple physical methods. Initially the precipitate forms as very small particles but as the water is gently stirred, these particles stick together to form bigger particles - this process is sometimes called flocculation. Many of the small particles that were originally present in the raw water absorb onto the surface of these small precipitate particles and so get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of the suspended matter out of the water and is then filtered off, generally by passing the mixture through a coarse sand filter or sometimes through a mixture of sand and granulated anthracite (high carbon and low volatiles coal).

Coagulants / flocculation agents that may be used include iron (III) hydroxide. This is formed by adding a solution of an iron (III) compound such as iron(III) chloride to pre-treated water with a pH of 7 or greater. Iron (III) hydroxide is extremely insoluble and forms even at a pH as low as 7. Commercial formulations of iron salts were traditionally marketed in the UK under the name Cuprus. Furthermore, aluminium hydroxide is also widely used as the precipitation of the flocculation process although there have been concerns about possible health impacts and mis-handling led to a severe poisoning incident in 1988 at Camelford in south-west UK when the coagulant was introduced directly into the holding reservoir of final treated water. Not only that, polyDADMAC is an artificially produced polymer and is one of a class of synthetic polymers that are now widely used. These polymers have a high molecular weight and form very stable and readily removed flocs, but tend to be more expensive in use compared to inorganic materials.

I will now continue to the process of sedimentation which is a section where water then enters after undergoing the flocculation process. The sedimentation process which is indicated by a sedimentation basin is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins can be in the shape of a rectangle, where water flows from end to end, or circular where flow is from the centre outward. Sedimentation basin outflow is typically over a weir so only a thin top layer - furthest from the sediment - exits.The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. The retention time of the water must therefore be balanced against the cost of a larger basin. The minimum clarifier retention time is normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles bump into and integrate smaller particles as they settle. In effect, large particles sweep vertically through the basin and clean out smaller particles on their way to the bottom.

Water which goes through sedimentation then enters a process commonly called filtration. This process can be considered as the final process of the water treatment process to remove remaining suspended particles and unsettled floc. The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odor. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog.

As a extra note, there are also another kind of filtration techniques used that are capable of removing ions and other dissolved substances which methods mentioned above cannot. This process uses an ultra filtration membranes use polymer membranes with chemically formed microscopic pores that can be used to filter out dissolved substances avoiding the use of coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes of micro-organisms can be filtered out. Ions on the other hand are removed using an ion exchange system which utilizes ion exchange resin- or zeolites-packed columns to replace unwanted ions. The most common case is water softening consisting of removal of Ca2+ and Mg2+ ions replacing them with benign (soap friendly) Na+ or K+ ions. Ion exchange resins also used to remove toxic ions such as nitrate, nitrite, lead, mercury, arsenic and many others. Water in this process also goes through electrode ionization by passing between a positive electrode and a negative electrode. Ion exchange membranes allow only positive ions to migrate from the treated water toward the negative electrode and only negative ions toward the positive electrode. High purity deionized water is produced with a little worse degree of purification in comparison with ion exchange treatment. Complete removal of ions from water is regarded as electrodialysis. The water is often pre-treated with a reverse osmosis unit to remove non-ionic organic contaminants.

Certain disinfection methods are used also in the modern world as diseases are widespread around the globe. Methods of treatments mentioned above are not exact description of what happens in a water purification plantation but close enough to depict what actually happens. So, efforts took by countries to produce just clean water are certainly tough jobs that we must treasure and appreciate as we living today have clean water to drink.