Wastewater treatment is the process of taking wastewater and making it suitable for discharge back into the environment. Wastewater can be formed by a variety of activities, including washing, bathing, and using the toilet. Rainwater runoff is also considered wastewater. No matter where it comes from, this water is full of bacteria, chemicals, and other contaminants. Wastewater treatment reduces the contaminants to acceptable levels so as to be safe for discharge into the environment.
In general use, there are two types of wastewater treatment systems: a biological treatment plant and a physical/chemical treatment plant. Most households and businesses create waste that can be broken down by natural means. Biological treatment plants use bacteria and other biological matter to break down waste. Industrial wastewater can contain chemicals that can harm the environment, so a chemical plant is needed to treat this waste. Physical/chemical wastewater treatment plants use both physical processes and chemical reactions to treat wastewater.
A biological wastewater treatment plant, such as a municipal treatment plant, uses several tanks to treat the wastewater that comes into it. First, wastewater is screened to eliminate easily removed objects, some of which could ruin the treatment plant's machinery. Next the wastewater is taken to a primary settling basin where matter can float or sink in the tank. The remaining water is then sent to the secondary treatment tank where biological matter, such as bacteria, removes much of the remaining suspended matter.
The substances that are removed during water treatment are called sludge. This sludge is treated and can be used as fertilizer or in land reclamation, or will be sent for land filling or incineration. To treat sludge, waste management professionals may use anaerobic digestion, aerobic digestion, or composting. The difference between anaerobic and aerobic digestion is that aerobic digestions happens in the presence of oxygen where as anaerobic digestion does not. In composting, sludge is mixed with carbon before being introduced to the bacteria for digesting.
After the secondary treatment, water is then sent through tertiary treatment. This treatment is the last stage before water can be released into the environment and usually ends with a disinfecting step. This step is actually a chemical treatment in a biological treatment setting. The treated water, called effluent, is then disposed of in the environment. This reclaimed water can also be used in fountains and to water lawns.
Physical/chemical wastewater treatment starts with removing suspended solids from the wastewater. The water is pumped into large tanks where matter settles or sinks, just like in a biological treatment plant. Just like in a biological treatment plant, this process may be aided by stirrers that mix the water up causing small particles to join into bigger particles. In a physical/chemical plant, this process can also be further aided by the addition of flocculants, a chemical that forms larger particles. Dissolved air may also be used to aid in the removal of particles.
Chemical processes include added chemicals to precipitate dissolved materials. Chemicals like chlorine may also be used to convert cyanides into carbon dioxide and nitrogen. Organic chemicals can also be oxidized by adding ozone or hydrogen peroxide. Chemicals in wastewater can also be broken down by using ultraviolet light. Municipal treatment plants may also use chemical treatments to remove materials such as phosphorus from wastewater.
At a typical wastewater plant, several million liters of wastewater flow through each day, approximately 50 to 100 liters for every person using the system. The amount of wastewater handled by the treatment plant varies with the time of day and with the season of the year. In some areas, particularly communities without separate sewer systems for wastewater and runoff from rainfall, flow during particularly heavy rains or snowmelts can be much higher than normal.
What happens in a wastewater treatment plant is essentially the same as what occurs naturally in a lake or stream. The function of a wastewater treatment plant is to speed up the process by which water cleanses (purifies) itself.
WHY WASTEWATER TREATMENT IS NEEDED
Planet Earth contains an enormous amount of water, even though there is a lot of water on Earth, only about 2.5% is fresh water, and because most of that water is stored as glaciers or deep groundwater, only about 0.01% of the world's total freshwater is readily available to terrestrial life. Water is essential and very important to our everyday life. Water is a renewable resource and cycles globally, water & wastewater treatments are a very important part of global water cycle. Unlike oil, water circulates, forming closed hydrologic cycles. The amount of water will not diminish on shorter than geological time scales. Every site on earth is causing pollution, including industries and households. Water gets polluted by pathogenic microbes, chemical contaminants and metals. Wastewater need to be treated to environmental acceptable level before is released into the environment.
HISTORY OF WASTEWATER TREATMENT
Figure 1: The early years of the development of sanitary sewers.Wastewater treatment and collection has followed paths of both historic and scientific discoveries. From a historic perspective, as communities have grown, so has the need for quality water. The need to supply safe water, remove wastes from water, and to protect public health, have been the endeavors and concern of many generations. Scientifically, as public health issues and the understanding of what causes outbreaks of disease such as cholera and dysentery have been discovered, the building of infrastructure and development of processes that can be used to end these issues has followed.
In 80 A.D. the Roman Statesman Frontinus, in his treatise on the "Water Supply of Rome" quotes a Roman Ordinance:
"I desire that nobody shall conduct away any excess of water without having received my permission or that of my representative; for it is necessary that a part of the supply flowing from the delivery tanks shall be utilized not only for cleaning our city, but also for flushing the sewers."
Homes at this time were not connected to a collection system that removed wastewater from individual households, but instead the streets themselves were the collection point for waste materials that were washed out to open sewers.
In 1684, Dutch scientist, Antony van Leeuwenhoek, published sketches of common forms of bacteria that he observed under his simple microscope.
In 1800's the development of sewers began in London, introducing the solution to public health problems created by unsanitary conditions. In 1842 the sewerage system of Hamburg Germany was laid out by the English engineer Lindley. In 1850s the engineer Chesborough designed the first comprehensive sewage system in Chicago. A London typhoid epidemic is believed to have been caused by bad water. An 1855 cholera outbreak in London was found to be caused by sewage contamination of a pump well, known as the Broad Street Pump Affair.
The discovery of bacteria and the cause of many "water-borne" diseases such as typhoid fever, cholera, and dysentery led to great strides in public sanitation efforts. Wastewater treatment today with its various types of treatment processes, utilize microorganisms to convert the organic substances in the sewage into harmless materials.
Wastewater treatment duplicates the naturally occurring activities of soil and water microorganisms, concentrating these organisms that use the organics in the sewage as its food source. The resulting treated water is then returned to the river or water body for use by downstream communities.
Since that time, the practice of wastewater collection and treatment has been developed and perfected, using some of the most technically sound biological, physical, chemical, and mechanical techniques available. As a result, public health and water quality are protected better today than ever before.
The modern sewer system is an engineering marvel. Homes, businesses, industries, and institutions throughout the modern world are connected to a network of below-ground pipes which transport wastewater to treatment plants before it is released to the environment. Wastewater is the flow of used water from a community. As the name implies, it is mostly water; a very small portion is waste material.
WASTEWATER TREATMENT TECHNOLOGIES
Sanitary practices appeared since ancient times. Pre modern societies (Romans, Aztecas, others ????) used sewers (channels and pipes) to transport wastewater out from buildings or even cities centers. The ancient trend was to discharge wastewater directly to water bodies or land without treatment citation. With the growth of population concentrated in cities and the advances of the industrial revolution the need to treat wastewater aroused. From the late 19th century until the early 1970s the main objectives of treatment regarded the neutralization of pathogens, removal of floatable, suspended and colloidal matter and processing of biodegradable organics (Metcalf & Eddy 2003).
These main objectives applied to most of municipal wastewater treatment plants, where the bulk of the wastewater stream used to be domestic wastewater. However, as industrial processes changed the compounds discharged and the wastewater characteristics also changed. This fact highlighted the importance of effective industrial pretreatment prior the discharge to municipal networks and treatment plant. At the same time, analytical techniques have improved over the years allowing more accurate measurement of chemical and biological features on water. Advances on science and engineering practice also increased the understanding of physical, chemical and biological processes, which allowed the development of consistent treatment technology.
As a result of the convergence of several disciplines (civil, chemical, mechanical, environmental engineering, chemistry, biology, etc.) in the wastewater treatment field, different authors have grouped wastewater treatment methods on different ways. Some authors classified treatment methods based on the force or nature of the processes. Physical methods where physical forces lead the treatment are known as unit operations or just physical methods (Cheremisinoff 2002; Metcalf & Eddy 2003). When the removal of pollutants is lead to chemical or biological processes the methods are consistent named chemical methods or biological methods, all grouped and known as unit processes (Metcalf & Eddy 2003). Some authors make the difference on energy intensive technologies to highlight the energy consuming feature of some methods (Cheremisinoff 2002). It is also common to find classifications based on the function of methods through the treatment process (sedimentation, clarification, organic matter removal, nutrients removal, disinfection, etc.)
Furthermore, methods are grouped according the level of treatment they provide (preliminary, primary, secondary, tertiary, advanced and other sub levels) (Metcalf & Eddy 2003). The level of treatment classification system results convenient for the operation of wastewater treatment facilities. A certain level of treatment gives a general idea of the quality of the effluent produced. However, sometimes the boundaries between the different levels and sublevels are not very clear because of the lack of an international unified classification system.
When treating wastewater the general aim is to eliminate or to neutralize the constituents that polluted the water. The presence of such constituents increases dangers to the public and environmental health downstream. During the pretreatment physical (mechanical) methods are applied to remove large objects, rags, sticks, rubbish, floatables, grit, grease and other suspended solids. This step prepares the wastewater for the former levels of treatment and protects the next units from the damage that the objects removed could cause. The primary treatment uses physical methods (flocculation, sedimentation, clarification) to remove the settle able fraction of the suspended solids and particulate organic matter. Enhanced removal of such fraction can be achieved by addition of chemicals (chemical precipitation) or filtration.
A secondary treatment level corresponds to removal of particulate and dissolved biodegradable organic matter and suspended solids. This is often accomplished by biological methods and it is common that includes nutrients (N and P) removal (Metcalf & Eddy 2003). Secondary treatment has been objective of intensive research and developed in the last X decades. Different biological aerobic and/or anaerobic types of technology are now available to accomplish these aims.
Activated sludge, which is based on the oxidation of organic matter by bacteria, is one of the most popular. (Gernaey, van Loosdrecht et al. 2004) presented a review on the modeling of activated sludge, where different models alternatives (white-box, black-box, stochastic grey-box, hybrid models and artificial intelligence), interactions and potential applications are described. The kinetics of nutrients removal have been subject of research and modeling as referred by (Hu, Wentzel et al. 2003), who pointed the importance of phosphorous accumulating organisms and their interaction with the nitrification-denitrification biota. (Blackall, Crocetti et al. 2002) described the ecological interaction and competition between the different members of the microbiological community of these systems. (Seviour, Mino et al. 2003) reviewed the enhanced biological phosphorous removal highlighting the progress in the filed due the application of rRNA-based methods, but recognizing the gaps on the understanding of such systems.
There are several modifications and versions of activated sludge processes. Sequencing batch reactor is one of them, it operates sequences of fill and draw cycles in an open single tank (aerobic), its flexibility, ease design and automatation have given it an increasing popularity (Mace and Mata-Alvarez 2002). Anaerobic versions of such approach have been used on industrial applications. To make the discontinuous process anaerobically feasible were necessary: optimization of the operating procedures and bioreactor designs with inert support media for biomass immobilization (Zaiat, Rodrigues et al. 2001). Anaerobic technologies became practical for industrial wastewater treatment because their cost reduction advantages as lowest energy requirement (or even energy production as biogas), low microbial excess cell (sludge) production and low space demand (Speece 1983).
Particle-based biofilm reactor technology is now used on several full-scale industrial and municipal applications (Nicolella, van Loosdrecht et al. 2000). In the last three decades have been developed, constructed and operated different types of reactors, including Upflow Sludge Blanket (USB), Biofilm Fluidized Bed (BFB), Expanded Granular Sludge Bed (EGSB), Biofilm Airlift Suspension (BAS) and Internal Circulation (IC) reactors (Nicolella, van Loosdrecht et al. 2000). Particle-based biofilm reactors perform compact and high rate processes which allow increase the volumetric conversion capacity saving space, a key advantage for industrial applications (Nicolella, van Loosdrecht et al. 2000). Some disadvantages of this technology are: long start-up times due to large time of biofilm stabilization, difficult biofilm thickness control, biofilm's overgrowth leads to particles elutriation, fluidization difficulties in large applications, biomass' detachment mechanism and biofilm's morphology and structure are not fully understood (Nicolella, van Loosdrecht et al. 2000).
Anaerobic granular sludge technology, specifically Upflow Anaerobic Sludge Blanket (UASB), is a worldwide used wastewater treatment technology. The granular sludge formed by anaerobic microorganisms through self-immobilization of bacteria cells (Liu, Xu et al. 2002). The UASB system performance depends on the granulation process and the wastewater-biomass contact provided by the turbulence cased by the inflow and the biogas production (Seghezzo, Zeeman et al. 1998). Complex interactions in the reactor's concentrated microorganisms community make possible waste degradation in a relative small volume (Liu, Xu et al. 2002). Large granules size and its high density simplify the biomass separation from the treated effluent, through rapid settlement (Liu, Xu et al. 2002). The biomass wash out avoidance related to low sludge production and the high grade energy, as biogas, production are two competitive advantages of the UASB reactor (Seghezzo, Zeeman et al. 1998). The Expanded Granular Sludge Bed (EGSB) reactor is a modification of the UASB reactor developed as a consequence of its suboptimal internal mixing for temperatures between 4 and 20C (Seghezzo, Zeeman et al. 1998). Compared to UASB reactors EGSB have higher height/diameter ratio, upflow velocity, organic load and biogas production. Recirculation is also a distinguishable feature of EGSB. There is no need to apply recirculation for dilute wastewater. However, due to the mixing pattern, some flocculent sludge is wash out form the EGSB reactor decreasing its suspended solid and colloidal matter removal efficiency (Seghezzo, Zeeman et al. 1998).
Long start-up times, low pathogen and nutrient removal and bad odor production are the most highlighted disadvantages of anaerobic granular sludge technology. Its, simplicity, flexibility, low space, chemicals and energy requirements, low sludge production and high organic matter removal efficiency are its stronger advantages (Seghezzo, Zeeman et al. 1998). The use of this technology (specifically UASB reactors complemented by pre and post treatment) is recommended by (Aiyuk, Forrez et al. 2006) for the treatment of sewage in hot climate regions because its stability competitive advantage in such conditions. Variations of anaerobic granular and biofilm reactors are widely used in the industrial wastewater treatment, specially for high strength organic effluents as is referred by (Rajeshwari, Balakrishnan et al. 2000).
An alternative to film and upflow granular reactors is the Anaerobic Baffled Reactor (ABR) developed since the early 1980's. The treatment principle of the system is the same than other high rate anaerobic reactors, which is to achieve a high reaction rate per unit reactor volume independently of the hydraulic residence time. (Barber and Stuckey 1999) suggest the ABR has the following advantages over the UASB and biofilm technologies: simple design and low capital costs, low sludge yield, high solid retention time, no special gas or sludge separation required, high stability to hydraulic and organic shock loads, intermittent and long term operation without sludge washing, partially separation of anaerobic catabolism which confers protection from toxic materials in the inflow and environmental conditions (pH, temperature) changes.
Other aerobic and anaerobic technologies variations found in the alternatives menu is thermophilic aerobic biological. It has the advantage of rapid biodegradation rates, low sludge yields and excellent process stability. Which make it particularly favorable for high-strenght wastewaters (i.e. paper and pulp and livestock industry) that content the necessary energy for autothermal operation. The thermophilic bacteria difficulty to aggregate makes the biomass separation from the treated effluent a design parameter (LaPara and Alleman 1999).
Wet air oxidation is a chemical treatment technology based on aquous waste oxidation at high tempreratures (400 - 573 K) and high pressures (0.5 - 2.0 MPa) in rich oxygen ambient (usually air). Such severe operating conditions implicate safety issues, which together with the high capital costs constitute the main disadvantages. Catalysts and other techniques are in development to ameliorate the technology limitations (Kolaczkowski, Plucinski et al. 1999).
Tertiary treatment is the name given to the level of treatment in which residual suspended solids and nutrients can be removed from the wastewater through biological or physicochemical processes. Disinfection (removal or neutralization of pathogens) is often included in the definition of tertiary treatment (Metcalf & Eddy 2003). On the other hand, advance treatment is the term used to refer to further removal of dissolved and suspended remaining materials or removal of specific compounds (metals, synthetic organics) for different water reuse or disposal purposes (Metcalf & Eddy 2003). A diverse range of techniques is applied for tertiary and advanced treatment including: filtration, membrane's processes, carbon adsorption, oxidative techniques, ion exchange, chemical processes, ultraviolet photoxidation, ultra-sound, natural or constructed lagoons and wetlands, etc. Many variations of such technologies can be found in the practice and more over in the literature, however just a few of them are mentioned here to show such diversity.
Heterotrophic (bacterial) nitrifaction-denitrification (sequence reduction of ammonium N to nitrate, nitrate to nitrite and finally to N2 gas), is one of the most common methods used to remove N and partially P (Metcalf & Eddy 2003). There are variations of the process, for instance (Ghafari, Hasan et al. 2008) reviewed about the potential of bio-electro reactors in N removal. They conclude that both heterotrophic and autotrophic organisms are able to denitrify, but autotrophs are more effective. (Morse, Brett et al. 1998) highlighted in their survey, that P removal and recovery from wastewater have been a matter of interest since the terrestrial limited availability of P compared to the relative N abundance in the air.
Other techniques can be used as advanced treatment to enhanced the removal of refractory compounds to biological traditional treatment. This newer technology degrade such refractory compounds into smaller molecules that later can be oxidized by biological methods. Gogate and Padit reviewed these technologies in two papers (Gogate and Pandit 2004; Gogate and Pandit 2004). They highlighted the advantages of oxidation at ambient condition by cavitation, photocatalysis (using UV radiation), Fenton's chemistry (hydrogen peroxide in the presence of a ferrous salt), ozonation and use of hydrogen peroxide. Photocatalityc oxidation is the most developed of this group of technologies in large scale. Hybrid methods as UV/H2O2, Ozone/H2O2, Photo-Fenton processes and the use ultrasounds in combination with the former are promising technologies for wide treatment (Gogate and Pandit 2004), as well as the synergy effects of photocatalysis and ozonation have been pointed by (Agustina, Ang et al. 2005). The combination of ozone with other technologies was reviewed by (Rice 1997) remarking its advantages for industrial application. However, most of these hybrid methods suppose high capital and operational associated costs which difficult large-scale use.
Tertiary lagoons, or maturation ponds, have been commonly used to improve wastewater quality by nutrients and pathogens removal, specially in developing counties du to their relative low capital, maintenance and specialized skills requirements (Maynard, Ouki et al. 1999). Although it is known that such lagoons can function in aerobic, anaerobic or facultative way, ample variations in their performance suggest that removal mechanisms are not fully understood and consequently design criteria currently in use are far from optimum (Maynard, Ouki et al. 1999).
There had been multiple attempts to use autotrophic aquatic organisms for wastewater treatment. (Mallick 2002) pointed out this fact in his review about nutrients and metals removal by immobilized algae. Nonetheless, the use of wetlands is one of the technologies which popularity have increased in the late decades and is based in macrophytes instead of algae growth (Verhoeven and Meuleman 1999). The treatment active principle of wetlands is simple: retention and removal of organic matter and nutrients by plants through the wastewater flow (Kivaisi 2001). Constructed wetlands modeling and performance was reviewed for horizontal subsurface flow by (Rousseau, Vanrolleghem et al. 2004) and for vertical-flow and hybrid reed bed by (Cooper and Griffin 1999). Due the wetlands treatment efficiency dependence of plant biomass productivity, adequate species must be selected and wetland design must be engineered according to climate (Kivaisi 2001). (Werker, Dougherty et al. 2002) presented a survey for wetland application on cold climates, while (Kivaisi 2001) presented a review regarding wetland technology for developing countries related to hot and tropical climates. According to (Verhoeven and Meuleman 1999) constructed wetlands in general can be designed to achieve BOD and COD removal around 90%, however nutrient removal stay closer to 50% in most cases.
In wastewater treatment context, adsorption is the mass transfer of constitutes from the liquid phase to the solid phase (Metcalf & Eddy 2003). In this operation a adsorbent, carbon activated the most common used, is the medium where the adsorbate, e.g. suspended particles, synthetic organics (Metcalf & Eddy 2003), refractory organic pollutants (Park, Heo et al. 2010), metals (Wan Ngah and Hanafiah 2008), arsenic (Mohan and Pittman 2007) ore other are accumulated. There are variations for the activated carbon technology, including granular, powdered in suspended, supported and fluidized beds configurations (Sutton and Mishra 1994). Granular activated carbon based biological fluidized bed is the most applied for treatment of contaminated streams according to (Sutton and Mishra 1994) who reviewed the topic. Activated carbon have been used successfully for advanced treatment of paper and pulp effluents, where the high temperatures enhanced the adsorption efficiency, although high costs are related to the carbon regenaration (Temmink and Grolle 2005). Other materials can also be used as adsorbents, synthetic polymeric and silica-based have been not often used for wastewater treatment purpose due their high cost (Metcalf & Eddy 2003). Nevertheless, recently other low cost and recycled materials as treated slags, agriculture waste (char carbons and coconut husk carbon), orange juice residues, chemically modified plant waste have showed good results as adsorbents for heavy metals (Wan Ngah and Hanafiah 2008) and arsenic (Mohan and Pittman 2007) treatment.
Traditionally the use of membranes has been more extended for drinking water than wastewater treatment (Cheremisinoff 2002). However, with the current water scarcity and water reclamation trend, specially from industrial wastewater as the tanning industry (Bodalo, Gómez et al. 2005), the implementation of pressure-driven membrane processes arises within the alternatives (Van Der Bruggen, Vandecasteele et al. 2003). This technology uses the driving force of pressure applied on the wastewater solution to separate it into a permeate on one side of a membrane and a retentate on the other side (Van Der Bruggen, Vandecasteele et al. 2003). The pressure applied is proportional to the membrane's pore size and the quality of the permeate (effluent) (Metcalf & Eddy 2003). The pore size varies from 100-10,000 nm for microfiltration that retains suspended particles, through 2-100 nm for ultrafiltration that retains macromolecules and multivalent ions, 0.5-2 nm for nanofiltration that retains small organic compounds and less than 0.5 nm for reverse osmosis which retains monovalent ions. The applied pressure varies from 0.1-2 bar for microfiltration to 5-120 bar for reverse osmosis (Van Der Bruggen, Vandecasteele et al. 2003). The use of high pressure integrated membranes systems (nanofiltration + reverse osmosis) can achieve water recovery rates of more than 95% and promises an almost zero discharge process (Hilal, Al-Zoubi et al. 2004). Electrodialysis is an alternative process to reverse osmosis where ion components of the wastewater aqueous solution are separated using ion-selective membranes (Metcalf & Eddy 2003). Although membrane technologies produce high quality effluent they have some disadvantages that difficult their extensive use: high energy-pressure associated costs, continue washing or replacement need due to fouling and the related pretreatment and concentrated disposal waste streams post-treatment (Cheremisinoff 2002; Metcalf & Eddy 2003; Van Der Bruggen, Vandecasteele et al. 2003; Bodalo, Gómez et al. 2005).
A key step on the advanced levels of treatment is disinfection. The objective of disinfection is preserving public health by neutralizing or partially destructing the organisms contained in the wastewater stream that can cause disease to human beings. Disinfection is different to sterilization, which is the total destruction of all the organisms (Metcalf & Eddy 2003). The predominant disinfection technique in most aqueous applications has been chlorine and chlorine based compounds (Kuo and Smith 1996). The extensive use of chlorination variations for water and wastewater disinfection is due to its effectiveness, low cost and the amount of existing information and specialized skills to apply it (Rossi, Antonelli et al. 2007). However, chlorination has disadvantages that have promoted development of other techniques (Blatchley III, Bastian et al. 1996; Acher, Fischer et al. 1997). The main inconvenience of chlorine is the generation of toxic by products due its reaction with remaining organic matter and formation of carcinogenic trihalomethanes and other compounds (Lazarova, Savoye et al. 1999). Other disadvantages of chlorination are: residual chlorine toxicity harmful for receiving water ecosystems, weak inactivation of some kinds of microorganisms (e.g. spores, cysts, viruses), high safety costs related to operation, the need of dechlorination to ameliorate residual chlorine which increase costs (Blatchley III, Bastian et al. 1996; Lazarova, Savoye et al. 1999; Gomez, Plaza et al. 2007).
The state of the art of disinfection techniques was reviewed by (Kuo and Smith 1996) and (Lazarova, Savoye et al. 1999) who listed and described the technologies used in the wastewater field including: chlorination/dechlorination, ultraviolet irradiation (UV), ozonation, peracetic acid (PAA), titanium dioxide catalyzed disinfection, membranes and filtration technologies (already addressed above). UV, PAA and ozonation have been subject of research and comparisons because their feasibility to become large-scale alternatives to chlorination for wastewater disinfection (Lazarova, Janex et al. 1998; Rossi, Antonelli et al. 2007). Although their several differences these methods have the active principle of generation of very reactive free radicals (e.g. hydroxyl radicals) that act as initiators of oxidative degradation, so they are know as Advanced Oxidation Processes (AOP) (Legrini, Oliveros et al. 1993). Bases and examples of these technologies are described: for UV by (Legrini, Oliveros et al. 1993; Moreno, Goni et al. 1997; Xie, Gomez et al. 2007); for PAA by (Kitis 2004; Koivunen and Heinonen-Tanski 2005; Rossi, Antonelli et al. 2007); for ozonation by (Tyrrell, Rippey et al. 1995; Rice 1997; Xu, Janex et al. 2002).
(Collivignarelli, Bertanza et al. 2000) compared experimentally UV, PAA and ozonation the different technologies concluding that further their already proven germicide effects other factors as investment and operative costs, plant complexity and impact on organic matter water quality for reuse. Their results suggest highest investment cost for ozonation and UV, lower operation costs of UV for medium-large facilities even periodic lamp susbstitution is required, the simplicity and cheap costs of PAA make it attractive specially for small applications, meanwhile ozonation represent a suitable solution for large scale facilities in cases of water reuse because the improvement of water quality (COD, color and UV absorbance). Disinfection efficiency of PAA, UV and O3 after enhanced primary treatment of municipal wastewaters was assessed, using four microorganism as indicators, by (Gehr, Wagner et al. 2003) who suggest that a single indicator organism might not be suitable and the necessary dose of disinfectant results economically not feasible, so upstream modifications to the treatment plant should apply. The fact that long term biodegradability should be considered in the total disinfection efficiency was pointed by (Lazarova, Janex et al. 1998).
(Blatchley III, Bastian et al. 1996) found UV performance superior to chlorination/dechlorination to treat municipal wastewater (technical and economic assessment). They suggest fouling of lamp jackets as UV's major limitation, which according them can be slowed by air sparging. According to (Gomez, Plaza et al. 2007), compared to membrane processes the efficiency of disinfection with UV results dependent to the quality of the influent, particularly regarding transmittance. In their analysis they found that the macrofiltration-ultrafiltration process showed slightly better effluent quality which was related to higher installation costs. (Liberti and Notarnicola 1999) looking for municipal wastewater reuse in agriculture found long inactivation values for UV, PAA and ozonation, with no toxic by-products for UV and PAA, while limited formation of aldehydes with O3. Later (Liberti, Notarnicola et al. 2003) confirm the positive outcome of using UV which affected parasites like Giardia lamblia and Cryptosporidium parvum oocysts without formation of undesirable by-products.
The potential of enhanced disinfection by combined advanced oxidation processes (AOP) to achieve further wastewater effluent quality has been suggested (Legrini, Oliveros et al. 1993). Some examples of combined applications are provided for PAA and UV by (Caretti and Lubello 2003) who related the higher efficiency achieved to the formation of highly oxidizing free radicals due PAA photolysis in the presence of UV. Other combined methods recently studied include: addition of silver and cooper to hydrogen peroxide and PAA (de Velasquez, Yanez-Noguez et al. 2008) and combination of ultrasound and UV (Blume, Martínez et al. 2002). Particular techniques have been also suggested for wastewater disinfection, (Klyuzhin, Symonds et al. 2008) purposed a method based on the particle-exclusion phenomenon, while (Acher, Fischer et al. 1997) recommended the use of sunlight irradiation as photochemical wastewater disinfectant.
The decision of the final disinfection technology to use between the diversity of methods will depends on several factors, the influent quality, final effluent desired quality depending on final disposal or use, the existing standards, the capacity of the wastewater treatment works and many other local factors (Lazarova, Savoye et al. 1999). This remark can be extrapolated to the decision on the choice of technology for every level or step in the complete treatment processes. As it have been described above, currently there is a wide range of methods available to employ. Determining the optimal configuration of wastewater treatment facilities is a major task; this work contributes toward such goal.
