Water and Waste Water Treatment
Disinfection of water streams originating from potable water plants is key in protecting environmental health.
Briefly describe the type of microorganisms that are targeted for removal and why, and then describe the process design and application, along with potential processing advantages, limitations and disadvantages of replacing chlorination with: (i) UV light; (ii) ozonation, and (iii) crossflow membranes to disinfect potable water from a municipal treatment plant. Information in your report should be fully referenced within the text, including any pictures or diagrams copied and used. It must include an Abstract at the beginning (no more than 8 lines).
The Abstract should be followed by an Introduction, then four individual Sections (one per disinfection method). Follow these sections with a separate section that discusses when a combination of methods is needed.
Finally, include a Conclusion that provides an overall discussion of the key points arising from the entire report. The Conclusion should be no more than one page. Also make sure you include at the end of the report a list of all References used in the text. For a guide on how to use references in the text and compile a reference list, refer to the separate Assignment Guidelines loaded up on D2L and/or an article in a refereed engineering journal. On-line encyclopedias, such as Wikipedia, are not acceptable as a reference – you must quote the original source (e.g., journal article, book, technical report, company website etc.).
Water and Waste Water Treatment
This report seeks to investigate the process of disinfecting potable water to protect the environment. A brief outlook on the importance of disinfecting water is given as well as some of the pathogens that are targeted by the process. The discussion then centers on the four processes that are mostly used in disinfecting water; chlorination, UV light, ozonation and crossflow membranes. The report examines the processes, advantages, and disadvantages of using these methods. The report concludes that it is vital to use the most efficient way to disinfect water in an environmentally friendly way.
The purpose of this report is to examine the current state of purifying potable water and the various methods that are used to do it. Water that is used for human consumption, such as cooking and drinking, originates from a wide range of sources. Rivers, streams, lakes, and other groundwater sources are used to provide water for daily use. However, water from these sources is contaminated by pathogens that cause diseases like typhoid fever, cholera, and dysentery if ingested. Most of the bacteria found in potable water sources come from contact with animal or human feces. These pathogens may include viruses, bacteria, and parasites such as salmonella, campylobacter, and cryptosporidium.
Much municipal treatment plants have the mandate to ensure that water is safe for human use. Chlorination is one of the methods widely used to kill pathogens in drinking water (Water Treatment 2003). Chlorination is an effective means of disinfecting water by killing pathogens, but it does not eliminate all of them. Other methods are used, and they don’t involve the introduction of chemicals into the water. This paper will look at chlorination as well as other methods used in disinfecting water to come up with a clear explanation.
Chlorine is a cheap and highly effective disinfectant and hence it is the method that is most widely used around the world (2016). Several chemicals are dissolved in water as well as microorganisms, odors, colors, tastes and even plant matter. When chlorine is introduced into the water, external elements like feelings, manganese, and iron get oxidized, and hence the water becomes more palatable. Chlorine also kills water-borne bacteria damaging the cell membrane and entering the cell to disrupt cell respiration and DNA activity. When this happens, the cells are unable to survive anymore since their standard functions are disrupted (2016). Hydrogen sulfide that may exist in the water is expelled as well as any color. Chlorination occurs in primary and secondary stages.
The disinfectant is introduced into the drinking water at the treatment plant. The water is mixed with chlorine, and the reaction is timed to ensure that the process is efficient. The time it takes for chlorine too reacts in the water is known as contact hours. Chlorination may happen at various stages during primary disinfecting:
The chlorine is applied immediately the water enters the plant so as to eliminate organisms such as algae early on. When this occurs, the latter stages become easier to control without the algae. It has been discovered that pre-chlorination also helps to eliminate odors and tastes as well as controlling any biological growth throughout the system (Zyara et al. 2016). The filtration media and sedimentation tanks remain free of algae since the chlorine was introduced early on.
After sedimentation before filtration
It controls organic growth while removing tastes and odors from the water. It also oxidizes manganese and prevents the growth of algae.
It is the stage that is very crucial on how chlorine is added to the water. Chlorination should disinfect the water while maintaining adequate residuals in the water as it moves through the distribution system. It is an economical means since chlorine is used at only one centralized location after all other elements have been removed by filtration and sedimentation. It results in a shorter contact time as well as lesser amounts of chlorine than other stages.
It is applied as the water leaves the plant and into the distribution system. It may also happen at specific chlorination areas that help to maintain the chlorine residue throughout the system. The chlorination process limits the growth of biofilm which may bring about taste and odor issues in the distribution.
Advantages of Chlorination
- Inexpensive- chlorine
is not very costly to procure for the municipal authorities
- Effective- it has been observed that chlorine is effective in eliminating bacteria and other inorganic compounds such as manganese and hydrogen sulfide.
- Non-toxic- when in the form of free chlorine.
- Reduces taste and odor issues brought about by algae and other chemical compounds.
- There is residual disinfection since the chlorine remains in the water throughout and hence the process is continuous.
Disadvantages of Chlorination
- Some chemical by-products may be produced by the reaction of chlorine with other compounds. These by-products such as chlorophenols may cause diseases and other health complications.
- The chemical by-products may also alter the taste and odor of the water if in large amounts.
- UV Light
UV rays originate from the Sun, and their spectrum is of a higher frequency than visible light but lower than x-rays (Zyara et al. 2016). UV rays purify water since they have a great ability to deactivate the cells of microorganisms in the water. When the UV-C rays penetrate the cell membrane of microorganisms, they are absorbed into the pathogen’s DNA. Once there, they proceed to inactivate the cell from within thus leading to its death and it is unable to reproduce anymore. The organisms in the water become inactivated but are not removed from the water. The effectiveness of the UV disinfection process depends on other factors such as intensity and exposure time of the rays and also the condition of the water. regionsUV rays may be unable to penetrate dark areas or regions with obstructions since they only travel in a straight line (Zyara et al. 2016). The exposure time recommended by the U.S. Department of Health and Human Services is 16,000 µwatt-sec/cm^2 for disinfection systems.
Several bacteria are destroyed at varying levels of light intensity. UV light is effective against a host of bacteria and protozoa. Due to the varying intensity of the lamps used, some organisms like Cryptosporidium may survive due to their thick cell membrane (Water Treatment 2003). Low light intensity cannot penetrate such membranes. Lamps should be replaced with time since their power depreciates so as to ensure adequate disinfection continues. The water should also pass through filtration and sedimentation before being subjected to UV so as to improve the effectiveness of the process.
Destruction of micro-organisms in the water depends on:
- The intensity of the
lamp- if the energy is sufficiently high, all micro-organisms will be
inactivated by the UV rays.
- Contact time- the time should be sufficient enough to ensure that all pathogens are effectively inactivated.
- The quality of the water- if the water contains sediments and dirt particles, they absorb or reflect the UV rays and hence pathogens may survive.
- Maintenance of the equipment
There are various benefits of using UV rays to disinfect water:
- It is environmental friendly since no external chemicals are introduced into the drinking water. The chemistry of the water remains intact.
- The treatment process of water is instantaneous and does not require the use of many holding tanks.
- It is easy to install since it requires only two water connections and a power source.
- It is easy to maintain since there is limited wear and tear apart from lamp replacement
- There is no residual disinfection ability. The disinfection by UV light only occurs at one point, and hence any further contamination down the distribution system is likely to go unchecked.
- The presence of other materials such as sediments, manganese and iron will either reflect the UV rays or absorb them thus limiting their capacity to inactivate bacteria.
- UV radiation cannot improve the taste and odor of the water if used as the only method to disinfect
Ozone comprises of three oxygen atoms, and it is highly unstable meaning that it reverts to oxygen very readily (2016). When ozone reverts to oxygen, a free radical molecule is left, and it is highly reactive for the short period it exists. Oxygen is converted to ozone requires the use of energy such as an electricity field or UV radiation. Chemical and electrolytic reactions can also be used to produce ozone in sufficient amounts. Ozonation involves passing clean, dry air via a high voltage electric discharge (corona discharge) so as to increase a concentration of ozone (2016).
The water that requires disinfecting is then squeezed through a venturi throat to create a vacuum that allows air to bubble upwards through the water. The ozone reacts with the metals such as manganese, sulfur, and iron to form metal oxides that are insoluble and hence post-filtration is a necessity. Ozone has a higher ability to disinfect against bacteria compared to chlorination.
- Ozone is effective at various pH levels, and it reacts rapidly with bacteria and protozoans to eliminate them compared to chlorination.
- No chemicals are introduced into the water during the process.
- Ozone can eliminate taste and odor problems.
- Removal of inorganic matter from the water.
- The equipment and cost of maintenance are high, and experts in the process are hard to find in most areas.
- There is no residual disinfection to limit regrowth.
- The insoluble by-product of ozonation has been said to have certain carcinogenic properties.
- Pre-treatment of the water may be required so as to reduce hardness.
- The solubility of ozone in water is way lower than that of chlorine, and this calls for special mixing techniques.
- Ozone gas is a fire hazard.
- Crossflow Membranes
In the case of crossflow filtration, an incoming stream is let to flow across the surface of the membrane resulting in the generation of two exiting streams. A particular portion of the liquid will pass through the membrane, and it is known as the permeate. The permeate is not completely pure since it contains some impurities from the original stream that managed to pass through the membrane.
The water flows over the membrane from the inlet. Transmembrane pressure is applied on the water so as to extract the permeate through the membrane and the retentate out the other side (Edzwald 2012). The tangential placement of the inflow limits the build-up of filter cake since the water flows over the membrane rather than directly into it. A substantial amount of the filter cake is washed away hence particles cannot coagulate to block the screen. Membrane filtration involves:
- Microfiltration- a low-pressure process that retains suspended material larger than 0.01 microns. 5-45 psi of pressure. Bacteria like cryptosporidium are extracted.
- Ultrafiltration- medium pressure is applied, and proteins and other biological matter are retained. 7-150 psi of pressure.
- Nanofiltration- monovalent ions and water will pass through the membrane while divalent ions get retained. 120-00 psi of pressure.
- Reverse osmosis- highest pressure process where water and individual organic components pass through the membrane.
- The removal rate of the liquid is high since there is no dirt build-up on the surface of the membrane.
- The water that filters out still contains some particles which were so small that they passed through the membrane.
The four methods, chlorination, UV light, ozonation and crossflow membranes, can be used to purify water on various capacities. Chlorination is the only method that involves the addition of chemical components into the water but it has been found to have certain levels of contamination due to the formation of chemical compounds. The compounds formed are few and largely ineffective, though. The other methods don’t introduce any chemicals into the water, but they lack the ability to retain disinfecting qualities throughout the distribution system. A method like UV radiation can be used alongside chlorination. UV eliminates more bacteria compared to chlorination, and the residual chlorine enables the elimination of microorganisms over an extended period. It is effective to use UV treatment and chlorination together because the two have greater power to eliminate microorganisms in the water than if they were used separately (Water Treatment 2003).
The report shows that chlorination is still
the best method to use in the process of purification of drinking water. One of
the areas where it is effective is in the cost constraints. It is relatively
inexpensive to use chlorination as compared to other means such as ozonation.
It is also easier to train personnel to use chlorine compared to the most
technical methods that include production and mixing of ozone. UV light is an
effective way, but it is limited by the fact that it only functions in debris
free water (Water Treatment 2003). In that case, filtration must be done
before the UV lamps can be used. Ozonation results in the formation of certain
insoluble metal oxides that could be harmful upon consumption and hence the
water must be filtered after disinfection. Crossflow membranes are mainly used
to strain liquids to get a filtrate and a substrate and hence it could be
uneconomical to use it as a method of water purification. It is easy to use
chlorine, and the effect of the chemical compounds formed from the reactions
are not severe enough to cause major harm to the consumers.
Edzwald, James K. 2012. Water Quality & Treatment. New York: McGraw-Hill.
Water Treatment. 2003. Denver, CO: American Water Works Association.
Zyara, Alaya, Eila Torvinen, Anna-Maria Veijalainen, and Helvi Heinonen-Tanski. 2016. “The Effect Of UV And Combined Chlorine/UV Treatment On Coliphages In Drinking Water Disinfection.” Water 8 (4): 130. doi:10.3390/w8040130.