Water and Wastewater Treatment

Water obtained from rivers, wells, boreholes, and lakes contains micro-organisms. While some organisms are not disease causing, some of these diseases cause diseases in humans. The disease causing organisms are called pathogens. These pathogens can be distributed through a drinking water transmission system thereby infecting those who consume it with water borne diseases. Different water treatment methods can be employed in the treatment of water to remove disease causing pathogens. Such methods include chlorination, UV light, ozonation and crossflow membranes.

Chlorination

Chlorination is the use of chlorine containing substances as well as chlorine itself for the disinfection and oxidation of the prospective potable water source. Chlorination has been in use since it was discovered in Sweden in 1974 (Glaze, Kang & Chapin, 1987). It was initially used to remove water odors. It was later discovered to be an effective disinfectant. As a disinfectant, it has been in use in many countries for over one hundred years (O’Connor, O’Connor & Twait, 2009).

Process design and application

Chlorination is done by the addition of chlorine into water either by the addition of chlorine itself on by addition of some of its compounds. Chlorination can be done at any point in the water purification process. It will however serve a different concern in the purification process (Glaze, Kang & Chapin, 1987).

Pre-chlorination is the addition of chlorine to water at the point of entry. At this step, chlorine is added to the raw water directly or added by the use of a flash mixer. The added chlorine helps in eliminating all forms of life in the water so that they do not cause issues in the later phases of water treatment. The pre-chlorination process assists in the removal of growths in the later phases of the treatment. It, for example, prevents growth in the sedimentation reservoirs where the solids are distanced from the water by the process of gravity settling as well as in the filtration media. Pre-chlorination also serves to oxidize any iron that may be in the water hence facilitating its removal in the sedimentation and filtration phases. Chlorination may also be employed as a step between filtration and sedimentation. In this case, it serves to remove iron and magnesium, biological growth, bad tastes and odors and water color. It does not eliminate growth in the sedimentation tanks (Glaze, Kang & Chapin, 1987).

Chlorination may also be made the last step of water treatment. This is the most applied method. This addition primarily serves to disinfect the water as well as remain in the water as it is transmitted to the consumers through the distribution channel. The method is also more economical as it requires less concentration and contact time for it to be effective (O’Connor, O’Connor & Twait, 2009).

potential processing advantages,

• Chlorination is widely used hence advanced an advanced technology
• Chlorination is cheaper to do than other methods except when it must be used according to fire code requirements
• The remaining residue serves to prolong the process of disinfection beyond the plant disinfection process
• Can be relied to disinfect water against a wide spectrum of pathogens
• Dosage is flexible. This allows for differentiated dosage over time since the quality of wastewater differs from time to time
• It also serves to eliminate odors during disinfecting (Glaze, Kang & Chapin, 1987).

Limitations

• Chlorine is highly toxic and even a small amounts may require de-chlorination if it is to be used with aquatic life, it is to be used with aquatic life
• Chlorine in all its forms is highly toxic and corrosive and poses a hazard during handling, shipping and storage
• Chlorine’s oxidation property sometimes leads to the formation of compounds that are harmful to the environment and to humans.
• The process increases the content of chlorides in wastewater
• It is ineffective in the removal of certain micro-organisms
• The impacts of chlorine in the environment in the long-term are unknown (Glaze, Kang & Chapin, 1987).

Ozonation

Ozonation was first used as a water treatment method in the late 1800s. It has a more prevalent usage in Asia and Europe than in the US. Ozonation is the use of a gas known as ozone which contains three oxygen molecules. It has been known to be more effective than chlorination (Binnie, Kimber, Smethurst & Smethurst, 2002). When added to water, ozone gas breaks down to oxygen gas and a nascent atom of oxygen. The nascent oxygen is very reactive and reacts with the disease causing organisms thereby eliminating them. Ozone also reacts with substances such as sulfur, iron and manganese forming insoluble oxides which are then filtered off. Ozone does not last long in the water and degrades to oxygen within a period of between a few seconds and thirty minutes (O’Connor, O’Connor & Twait, 2009).

Process design and application,

Ozone is formed by the use of energy. The process of formation of ozone is done by an electric discharge field or in the presence of UV radiation. Ozone may also be produced by the use of chemical and electrolytic reactions. Ozone is then mixed with the raw water. UV radiation is most often applied for small scale ozonation while other large scale ozone production methods like electrolysis and using a discharge field are employed for large scale ozonation (Glaze, Kang & Chapin, 1987).

Potential processing advantages

• Ozone is effective over a wide range of pH and removes bacteria, protozoans and viruses. It is also more effective with the removal of germs than chlorination. Its oxidizing power is very high and its reaction time low
• The process does not involve the addition of chemicals into the water
• Ozone is good for the removal of various microbiological, organic and inorganic problems as well as improving the taste and odor of water (Binnie, Kimber, Smethurst & Smethurst, 2002).

Limitations

• The operational and equipment cost is higher and personnel with proficiency in ozonation are rare
• The process lacks a residual and hence does not inhibit regrowth
• The by-products of ozonation have not been satisfactorily evaluated. It is possible that some of them are carcinogenic
• Pretreatment may be required to remove hardness. The formation of the carbonate scale is prevented by the addition of polyphosphates
• In comparison to chlorine, ozone’s solubility is lower in water. It therefore requires the application of special mixing techniques
• Ozone generation is associated with issues of toxicity and fire hazards (O’Connor, O’Connor & Twait, 2009).

UV Light

When UV light is passed through water, it can be used in the removal of micro-organisms. The UV light strikes the cells of the microorganisms and sterilizes it preventing further reproduction (Environmental Protection Agency, 2006). The impact of UV light is largely dependent on the dose of UV light applied to the water. The dose is a product of intensity of UV radiation and the time of exposure. Different microorganisms require different doses of UV radiation to remove (Edstrom Industries Inc, n.d.).

Process design and application,

The treatment of water using UV light involves the use of unique low-pressure mercury lamps which produce UV light with a wavelength of 254 nm (Environmental Protection Agency, 2006). This is the optimal wavelength for water treatment. These lamps never come into direct contact with the water. Instead, theu are housed in glass chambers inside the water or they are placed externally. This method may also be used to reduce the total organic carbon in water (Binnie, Kimber, Smethurst & Smethurst, 2002).

The success of this process depends on a number of variables.

Quality of water

In case the water contains contaminants, they could lower the effectiveness of the UV light. They may prevent the UV light from reaching some other parts of the water. Contaminants that prevent UV from being effective include fulvic and humic acid, iron and turbidity (Edstrom Industries Inc, n.d.). Suspended particles pose a threat to the process as the organisms that are located deep in the water are shielded from the rays of UV light. This leads to a situation where some organisms are unaffected as they pass through the unit. The process is most effective when the water is clear (Environmental Protection Agency, 2006).

Flow rate

The flow rate determines the time of exposure. If the flow rate is too high, the micro-organisms may not get sufficient exposure to UV light. If the flow rate is very low, the temperatures may build up and thereby destroy the lamp (Edstrom Industries Inc, n.d.).

Advantages of UV light

• The method does not involve the addition of chemicals. This eliminates the chances of poisoning
• The process also does not have any disinfection byproducts. The chances that they could be disease causing in the long-run is inexistent
• The method is cheap to manage and maintain (Environmental Protection Agency, 2006)
• The process is also fast and its effect
• The method is trusted by its users and widely tested
• The number of micro-organisms that are affected by the method is bigger than that which is affected by chlorine
• The equipments are easy to maintain (Binnie, Kimber, Smethurst & Smethurst, 2002).

Disadvantages

• UV light does not remove inorganic contaminants like metals
• Certain water characteristics like its cloudiness could impact the effectiveness of the method
• The method does to have a residual effect and only works within the precincts of the light (Binnie, Kimber, Smethurst & Smethurst, 2002).

crossflow membranes

Crossflow membranes are another method of removing water impurities. It works by introducing a physical barrier preventing bacteria, viruses, solids and other unwanted molecules. The method employs different membranes for different purposes. There are membranes that are used for water softening, others for desalination, organic removal, disinfection and softening. The method can be installed in the form or mechanized, compacted, modular units (Edstrom Industries Inc, n.d.).  

The advances in technology have reduced the cost associated with the erection of a crossflow membrane system. The cost is reduces massively as membranes are today able to produce large amounts of water and remove more impurities at relatively low energy consumption (Glaze, Kang & Chapin, 1987).

Process design and application,

By creating a physical barrier on the path of the water, crossflow membranes only allow the passage of particles that are of a certain character, shape and size. There are three forms of processes that employ the crossflow technology

Ultrafiltration

This is a process that is driven by pressure and necessitates the removal of large molecular weight materials like suspended solids, dispersed materials, emulsions, colloids, metal hydroxides and emulsified oils (Binnie, Kimber, Smethurst & Smethurst, 2002). The membranes are differentiated by their cut-off using molecular weight. UF is best used for the removal of suspended solids, viruses and bacteria. Sometimes they are also used in the food industry (Lobo, Cambiella, Benito, Pazos & Coca, 2006).

Reverse Osmosis

Reverse osmosis has the smallest size of membranes. It is used in removing dissolved molecules from water. Its effectiveness is dependent on the ionic diffusion of the substances involved. It is commonly used in the desalination of brackish and sea water (O’Connor, O’Connor & Twait, 2009).

Nanofiltration

Nanofiltration is very similar to reverse osmosis. It is however different in that it is only used in the removal of those ions that are either divalent or larger (Binnie, Kimber, Smethurst & Smethurst, 2002). For this reason, it is mostly employed in desalting. Nanofiltration is mostly used in water filtration to remove water hardness, eliminate pesticides and reduce water color.

Microfiltration

These systems are applied at very low pressures. They are also created with their purpose in mind. They are mainly used for obtaining the solid residue of a certain size or larger (Al-Malack & Anderson, 1997).

Advantages

• The solid waste is less and commonly digestible
• The method is closer to absolute filtration and leads to the formation of higher purity product
• The method can be erected in a fully-automated manner
• The method has been widely studied hence providing it with a variety of source to back one up in case of difficulty using it (O’Connor, O’Connor & Twait, 2009).

Disadvantages

• Requires a large footprint area for installation. It also costs very expensively to erect
• It is indiscriminate and may remove substances that were otherwise meant to be left alone.
• If the system fails due to lack of adequate care, the equipment could be expensive.
• Its applicability is limited as the method does not work well in cases of high viscosity and compressible solids
• Is not effective in handling changing influent circumstances
• The membrane has a short life of from 3-5 years and which is expensive to replace.
• Waste disposal problems may arise if there are regulations on concentrate disposal
• Often involve the use of toxic materials like caustic and oxidizing agents and acids (O’Connor, O’Connor & Twait, 2009).

Combined water treatment methods

Different methods of water treatment work best in certain situations. Combining any two methods makes them stronger. A combination of, chlorination, and UV light could, for example, increase the number pathogens that are removed (CDC, 2014). Chlorination also serves in the removal of certain solid compounds. Combining chlorination with UV gives the UV radiation method clearer water to work with since chlorination facilitates sedimentation. It also takes advantage of the UV rays capacity to eliminate a wider range of pathogens.

While no one method is capable of removing all types of impurities, combining more than one method creates a superior water treatment method.  CDC (2014) suggests the use of a combination of one filtration method with a water disinfection method. In this case, disinfection will help in the removal of the smaller pathogens while filtration removes what was too big or could not be killed by the substance. The two methods will complement each other hence eliminating some of their disadvantages.

Conclusion

Water treatment is important in the control of disease. It helps in the removal of disease-causing pathogens. Various methods are in use today. Ozonation and chlorination have been in use for over one hundred years. Ozonation is preferred in Europe and in the East while Americans prefer chlorination. Each of the methods has its own advantages and disadvantages. Selecting the most suitable method would require the comparison of the advantages and disadvantages of each of the methods to ensure the best solution is reached.

One factor that should be put in mind is the impact of a method on the environment. Certain methods are more sustainable than others. Ozonation for example causes a risk of fires. Chlorination requires the use of various chemicals that are both toxic and corrosive. Handling such chemicals, causes a serious threat.

References

Al-Malack, M., & Anderson, G. (1997). Crossflow microfiltration with dynamic membranes. Water Research, 31(8), 1969–1979.

Binnie, C., Kimber, M., Smethurst, G., & Smethurst, G. (2002). Basic water treatment. London: Thomas Telford.

CDC,. (2014). CDC – A Guide to Drinking Water Treatment and Sanitation for Backcountry and Travel Use – Camping, Hiking, Travel – Drinking Water – Healthy Water. Retrieved 19 October 2014, from http://www.cdc.gov/healthywater/drinking/travel/backcountry_water_treatment.html

Edstrom Industries Inc,. Ultraviolet Disinfection.

Environmental Protection Agency,. (2006). ULTRAVIOLET DISINFECTION GUIDANCE MANUAL FOR THE FINAL LONG TERM 2 ENHANCED SURFACE WATER TREATMENT RULE. Washington.

Glaze, W., Kang, J., & Chapin, D. (1987). The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Taylor \& Francis.

Lobo, A., Cambiella, \., Benito, J., Pazos, C., & Coca, J. (2006). Ultrafiltration of oil-in-water emulsions with ceramic membranes: Influence of pH and crossflow velocity. Journal Of Membrane Science, 278(1), 328–334.

O’Connor, J., O’Connor, T., & Twait, R. (2009). Water treatment plant performance evaluations and operations. Hoboken, N.J.: Wiley.