It is an unassailable fact that water is the most valuable and vital natural resource on the planet in a way that its absence for only few days will definately lead to fatal consequences in human life and a nearly complete shutdown of many industries. However, it has been well established that waterborne diseases have always been associated with increasing death rate and growing health problems in different areas around the world throughout history (Tebbutt, 1998).
Since one of the most significant factors in keeping a good health standard in each community is the availability of a reliable and clean water supply, various water treatment process and different technologies have been developed to achieve the required quality of the water supply that is fit for human consumption (potable) and maintain this quality from the source to the tap. Never the less, as the purity and the quality of the water is mainly determined by its supply source, i.e. ground water quality is thought to be of higher quality and cleaner than surface water sources and therefore needs less treatment processes while surface water is more vulnerable to different kinds of contaminations and requires a more sophisticated treatment system. Similarly the end use purpose of the treated water is another major player in deciding to what level the water needs to be treated in order to reach the quality standards set by regulators, i.e. (industrial water supply, agricultural irrigation, fish propagation, wildlife, recreation water course, etc.) each requires a different water quality standard not necessarily similar to the potable water quality (Hamer and Hamer, J. R., 2008).
However, in this report the treatment process and technologies used to purify water from different sources in order to reach the required quality standard (potable) are evaluated and the economic and environmental implications of each technology are discussed.
Coagulation and flocculation, clarification, filtration, adsorption, disinfection, organic and inorganic removals are some of those processes that drinking water undergoes before reaching consumers.
It's worth mentioning that primary applications of some of these processes go back in history as early as 1500BC. Coagulation which is in principle the agglomeration of colloidal particles in the form of flocs in water has been used by the Egyptians around 1500BC. Clarification which is settling down of the suspended particles in the water under the influence of gravity and filtration process which is retaining most of the remaining contaminants in the water by passing it through a granular bed (porous media) have initially been used by the Romans and are still regarded as two of the most critical stages in each modern water treatment work plant (Parsons and Jefferson, 2006).
- Critical evaluation of water treatment technologies.
- Principles and rationales behind these technologies.
- Influence and implications of these technologies in relation to the environment and economic issues.
Water quality and regulations:
A fair knowledge of chemistry, hydraulics and biology sciences is fundamental to achieve a grip on all the aspects of potable water quality and its treatment processes. Turbidity (suspended particles in water), micro -organisms including specific bacteria and protozoa, Algae, pesticides, non-organic matters, disinfection by-products, pharmaceuticals and arsenic are the most common contaminants that ought to be removed from drinking water in a multi stage water treatment process before reaches consumers to meet the quality standards required by water quality regulators (Parsons and Jefferson, 2006). As it's indicated in (table 1) the parametric values set by European Directive (Council Directive 98/83/EC, 1998) for microorganisms is zero and for turbidity should be to a level that doesn't cause any abnormal changes and is acceptable to consumers while table 2 shows all the values that are proposed for chemicals in the drinking water according to the (WHO Guidelines for Drinking Water Quality, 2004). It's mainly due to meeting these prescribed standards through different water treatment technologies that the water related diseases are rare in many developed countries and the life expectancy has almost doubled since 19th century while In the developing countries the resulted death toll from water-related illnesses is over 5 million including 2 million children and %80 of all diseases are water related (Tebbutt, 1998).
Water treatment processes and technologies:
The complex water treatment operations can be classified in two three major groups of physical process that mostly depend on the physical behaviours of the impurities (particle size), chemical processes that makes us of the chemical properties of the added chemical substance (coagulant) and the biological processes which mainly utilize biochemical reactions to eradicate organic impurities.
Coagulation and Flocculation:
This simple but very significant process in many water treatment works is the destabilization of the suspended particles in water with or without adding chemical substances (coagulant) and some time prompt development of small agglomerates which they later be encouraged to collide with each other (flocculation) by stirring or passing through a reactor to create larger flocs that precipitate faster.
The aim behind developing of this process is releasing water from the turbidity (suspended particles), natural organic matter and soluble pollutants.
This process can be defined as the downward movement of suspended particles which have lower density than the surrounding medium (water) and accelerate due to gravity force until a point that this dragging force balances the weight of the particles (terminal velocity), from which the particles descend in a constant speed. However, it's fundamental to differentiate between discrete particles with constant size, shape and mass and flocculent particles which their characteristics change continuously. However, the presence of discrete particles is assumed in sedimentation theory (Tebbutt, 1998).
Horizontal flow clarifiers (horizontal and radial configuration), Lamella plates, vertical flow (sludge blanket) clarifiers and high rate systems are some common technologies available to perform the sedimentation of suspended particles of concern, however, each technology has its own economic and environmental issues that ought to be addressed when put in action.
Lamella plate technology:
It is a developed version of the horizontal clarifiers in which a batch of tubes or plates is added to the horizontal sedimentation tanks. The proprietary designs of these inserted tubes depend on their geometrical shapes (circular, rectangular or hexagonal), however, in many cases their length are 1-2 m and spacing between the tubes are 50mm. The angle of which the plates are pitched to the tank is between 50o - 70o to facilitate the self cleaning function.
The principle benefit behind developing this technology is increasing the surface area. Thus, the issue of the size of the tank to generate the required area can be overcome. It also allows the loading water into the system in a higher rate without deterioration of the concentration of the effluent.
The environmental and economic issues associated with this technology can be summarized in few points:
- Low solid content scum which means large volumes of sludge.
- Large sedimentation tanks compare to the clarification that are used in air flotation technology
- High total cost because of the restriction on overflow rate in the settling tank not to exceed 2 m/h.
Filtration in simple term is the process in which water flows down through a granular media after chemical coagulation and sedimentation process. The collision of the particles and their attachment to the surface of the granular media through a multi-stage mechanism of diffusion, interception and sedimentation will finally result in capturing the suspended particles in the granular media. Cyst protection and robust impact of filtration on minimising the presence of micro-organisms have highly contributed in the control of waterborne diseases caused by Giardia lamblia and Cryptospordium parvum (Parsons and Jefferson, 2006).
Technology options developed to perform this process are:
- Rapid granular bed filtration.
- Slow sand filtration.
- Direct filtration:
- Low capital costs (no floatation or sedimentation).
- Low operating cost
- Les chemical dose and less power is used
- Less sludge produced.
- Ultra violet radiation
- Chlorine is widely available.
- Its high solubility make the application easier
- It has memory, i.e. its protection action against contamination doesn't disappear immediately and stays in the distribution system after application.
- And it's cheap
- The potential hazards of produced disinfection by products (DBPs) that is associated with undesirable taste and odour and forming trihalomethanes (THMs)which is understood to be carcinogenic at high doses in animal.
- The risk of a gas leak from the treatment plant or during transportation of gaseous form of chlorine. An increase in nitrate concentration through the growth of nitrifying bacteria with attended health risks due to the presence of ammonia in the distribution system (Parsons and Jefferson, 2006). 4.4.2. Ultraviolet Radiation (UV): Since 1970s when the association between trihalomethane (by-product of chlorination) and cancer potential hazard was understood, serious efforts started to find alternatives for chlorine disinfection technology. This technology uses the strong biocidal property of the high frequency band of ultra violet radiation around 254nm (Lund and Hongve, 1993). The inactivation of micro-organism is achieved through the penetration of ultra violet light into the cell and disrupts the DNA results in causing the DNA to fuse and prevent the bacteria from reproduction. However, the efficiency of this technology is highly determined by the turbidity level of the water and the dose of the applied radiation. Xenon lamps, Mercury vapour and Antimony are the raw materials used in the commercial production of ultra violet radiation (Parsons and Jefferson, 2006). The absence of disinfection by-products (DBPs), persistence of the inhibitory effect on hetereotrophic bacteria after radiation of water containing humic substances, no formation of taste or ordure are some advantages in applying (UV) as a water treatment technology (Lund and Hungove, 1993). There is no sufficient evidence suggesting the association of this technology to any environmental or economic concerns. It has minimal health risk; safety of and acceptance by the operators and the public. No large tank required and minimal contact time is needed. Simplicity of operation and maintenance makes the running cost very low. However, it's not used in many places due to its inability to control formation of biofilm and its requirement of low turbidity water. 4.5. Adsorption process: The essence in this process is the separation of a material (adsorbate) from the solution, and the concentration at the surface of a substance called (adsorbent). It can be physical or chemical and it is mainly the dependant on the temperature, pressure and the pH. 4.5.1. Activated carbon: The development of activated carbon adsorption as a water treatment technology is to remove; natural organic matter and colure, pesticides, taste and odour and algal toxins. There two kinds of activated carbon used in the treatment process of public water: granular activated carbon and powdered activated carbon. The activation of carbon increases the internal surface area to enhance its absorbance capability. Powdered activated carbon is used when occasional traces of organics is required to be removed as an intermittent need while in the case of regular removal using granular carbon is more effective as the surface area is1000 m2/g (Tebbutt, 1998). In terms of environmental implications, using activated carbon technology in water treatment is generally seen as more acceptable than some disinfection processes as no by-product materials is produced. The low cost of adsorption unit process can be achieved from the reactivation of the coal based granular activated carbon. 6. Conclusion: 7. References: Hammer , M. J. and Hammer, J. R. (2008). Water and Wastewater Technology. Six edition. Pearson Education International. New Jersey, USA. Parsons, S. A. and Jefferson, B. (2006). Introduction to Potable Water Treatment Processes. Blackwell Publishing Ltd. Oxford, UK. Tebbutt, T. H. Y. (1998). Principles of Water Quality Control. Fifth edition. Butterworth-Heinemann. Oxford, UK. Lund, V. And Hongve, D. (1993). Ultraviolet Irradiated Water Containing Humic Substances Inhibits Bacterial Metabolism. Journal of Water Research, 28 (5): 1111-1116. Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Off J Eur Commun, 5. 12. 98, L330/32 - L330/53. WHO (2004). Guidelines for Drinking-Water Quality. World Health Organisation, Geneva. 1
This technology is mainly a one primary process of filtering the coagulated/flocculated water through a porous media which requires creating pin point flocs that are filterable rather than higher dense flocs for sedimentation. It's called direct filtration as the coagulated water is directly sent on to the media and no other processes happen ahead of it.
As it's pointed out by (Parsons and Jeffersons, 2006),The economic and environmental issues regarding this technology are:
This is in most occasions the final stage of water treatment processes by which the municipal water supply is supposed to have been cleaned to a maximum capacity from any undesirable constituent so that it's healthily fit to be consumed and is palatable.
The main reason behind applying this process is the fact of reaching the target of zero tolerance which has been set by regulators for the presence of harmful micro-organisms like; Giardia lamblia, Cryptosporidium species, enteric viruses and legionella in public water supply due to their well established role in different type of water-related diseases. Inactivation and removal of these species in particular ensures the removal of all other pathogens as they are the most persistent pathogens (Hammer and Hammer J. R., 2008).
There are many disinfection technologies that are used to remove harmful contaminants during the course of water treatment process:
Chlorine is a heavier-than air, greenish-yellow coloured, toxic gas. Chlorine existing in water as hypochlorous acid and hypochlorite ion is defined as free residual available chlorine and dependant on the pH, temperature, time and initial chlorine/ammonia ratio can readily reacts with ammonia in water to form chloramines (Combined available residual chlorine):
HOCl + NH3 = H2O + NH2Cl (Monochloramine)
HOCl + NH2Cl = H2O + NHCl2 (dicloramine)
HOCl + NHCl2 = H2O + NCl3 (tricholarmine)
The disinfection affect of chlorination is represented by chemical reactions which happen between HOCl and the microbial cell structure which leads to inactivating its required life processes (Hammer and Hammer J. R., 2008).
The main positive points of chlorination technology according to (Tebbutt, 1998) are:
However, there are some environmental and health concerns around chlorinating public water supplies, such as: