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Since water is such an imporatant resource and ther is strong likelihood that the world will face a fresh water shortage in the near future, we must find ways to more efficiently manage our limited sources of water. One of these ways is to reuse and recycle water by purifying wastewater. Wastewater is an inevitable end-product of water use for different human and industrial activities. It needs to be treated before being discharged into the ecosystems. Wastewater treatment encompasses two principal processes - physical-chemical and biological. Physical-chemical processes are generally based on mechanical/physical treatments and chemical reactions while biological processes comprise use of microorganisms to convert wastewater contaminants into innocuous forms. In order to remove different levels of contaminant, the wastewater treatment is carried out in different stages, classified as primary, secondary and tertiary waste-water treatment.
Source: Metcalf and Eddy, Inc., Wastewater Engineering, Treatment and Reuse, fourth edition (New York: McGraw-Hill, 2003)
The importance of treating wastewater
Wastewater is treated to remove the inorganic solids like sand and gravel, and to reduce the amount of organic material before it is out to the City's rivers. Treated wastewater is 90-95 percent free of organic material present in sewers (as measured by the standard 5-day carbonaceous Biochemical Oxygen Demand (CBOD5) analysis). In many parts of the world, it is found that the sewage treatment is low or absent, which leads to eutrophication of the coastal zone. It will also cause the toxic algae blooms and eventually reduce the ability of coral to recover from bleaching. These organic materials when untreated also reduce the oxygen levels in the rivers, thereby reducing the healthy fish and other populations, give unpleasant odour and create public health concern.
Ratio of wastewater treatment
Fig 2: This ratio explains worldwide wastewater treated and untreated that reaches the coastal water. The untreated wastewater reaching the coast of East Asia and Southern Asia is found to be 90 - 95%.
Waste water flow is found to fluctuate with variations in water usage and the community size. The various factors like climate, water supply, cost of water and industrialization also found to affects the flow of wastewater. Flow is observed to be typically low when water consumption is lowest during the early morning hours and when the base flow consists of infiltration-inflow and small quantities of sanitary wastewater. The first peak of flow is observed to be during the late morning hours when wastewater from the peak morning water use reaches the treatment plant and the second peak flow was found to be usually during the evening hours. The time and relative magnitude of these peaks was found to vary from country to country and with the size of the community and the length of the sewers. It is found that the small communities with small sewer systems have a much higher ratio of peak flow than the large or average communities. The day to day variations in the flow make it ineffectual to apply the effluent directly from the treatment plant. To provide a relatively constant supply of reclaimed water for efficient irrigation some form of flow equalization or short-term storage of treated effluent is necessary although additional benefits result from storage.
Wastewater Treatment Methods
The process of removing pollutants from the water coming out of industries, from agricultural and municipal uses is generally referred to as wastewater treatment. Biological, chemical and physical techniques are applied to remove the pollutants and these different techniques are applied through the many stages of wastewater treatment. The wastewater treatment is carried out in different steps staring from the preliminary treatment, primary, secondary and tertiary and the last stage is solid separation and incineration.
Fig 3: Wastewater treatment unit operations and processes
Source: Wastewater treatment technologies, New York,2003.
The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Primary treatment usually includes the removal of large solids from the wastewater via physical settling or filtration and screening is usually the first step in primary treatment. Secondary treatment typically removes the smaller solids and particles remaining in the wastewater through fine filtration aided by the use of membranes or through the use of microbes, which utilize organics as an energy source.
A common first step in the secondary treatment process is to send the waste to an aeration tank. Tertiary treatment involves the disinfection of the wastewater through chemical or energetic means. Increasing the number of steps in a wastewater treatment process may insure higher quality of effluent; however employing additional technologies may incur increased costs of construction, operation, and maintenance.
Fig 4: Typical processes diagram for wastewater treatment
Source: Recent Patents on Chemical Engineering, 2008, Vol.1
Chemical Vs Biological
The chemical process is usually carried out during the primary treatment of wastewater. The phosphates, organics and heavy metals dissolved in the sewage are precipitated. The polymer is added at this stage to cause flocculation of precipitated particles as suspended solids. This assists in settling of the solids to the bottom of the tanks, where they are removed as settled sludge. Then the ammonia which, at high levels, is toxic to fish in the river systems is removed. The effluent, after primary sedimentation, enters the biological reactors tanks where the micro-organisms convert the ammonia to nitrates in a process known as nitrification.
In most cases, secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes. Aerobic biological treatment is performed in the presence of oxygen by aerobic microorganisms. Air is pumped in through porous diffusers in order to mix the sewage and activated sludge and to provide oxygen for the micro-organisms. Generally Bacteria for this purpose, that metabolize the organic matter present in the wastewater producing more microorganisms and inorganic end-products such as CO2, NH3, and H2O. There are several aerobic biological processes which are used for secondary treatment and they differ primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter.
Chemical processes are used in treatment to bring some form of change by means of chemical reactions. The main applications of chemical process are the complete secondary treatment of untreated wastewater, including the removal of either nitrogen or phosphorus or both, to remove phosphorus by chemical precipitation and to be used in conjunction with biological treatment. Physical- chemical process may be of various types based on treatment efficiencies and modes, common being - dissolved-air-flotation, precipitation, carbon adsorption, reverse osmosis, filtration and disinfection. The main disadvantage of chemical process in general compared to physical operations in that they are additive process. The principle chemical unit processes are chemical precipitation, chemical oxidation and neutralization and the different operations are adsorption, disinfection and dechlorination. Chemical oxidation is carried for treating wastes remaining after biological treatment. Ferrous chloride and lime are added to the sewage before it enters the grit tanks, to improve the subsequent chemical treatment process. Ferrous chloride precipitates phosphorous. Removing phosphorous is crucial as it contributes significantly to the growth of toxic algae in rivers and reservoirs. Lime is added to raise the pH level which helps FeCl2 to remove phosphorous and other materials from the solution.
Physical-chemical systems are commonly employed to treat industrial wastewaters, while biological processes are generally used for municipal wastewater treatment. Meanwhile, biological treatments also are of two types - attached growth and suspended growth, each having its own advantages. Some frequently used attached growth processes are rotating biological contactors and trickling filters and suspended growth processes are activated sludge, sequencing batch reactors, lagoons and wetlands. Biological processes, due to their cost-effectiveness, have advantages over physical-chemical processes, on the other hand biological processes may suffer from failures if toxic wastes are encountered which may not affect the physical chemical processes.
High-rate biological treatment processes typically remove 85 % of the BOD5 and SS and also the heavy metals that is present in the raw wastewater when combined with primary sedimentation. Activated sludge generally produces an effluent of slightly higher quality, in terms of these constituents, than biofilters or RBCs.
Fig 5: Typical diagram showing the chemical process and biological process during the wastewater treatment.
Conventional vs. new methods
Conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter and, sometimes, nutrients from wastewater. The conventional method starts with screening of wastewater which then passed to the primary clarifier; the activated sludge particles are flocculated to form larger particles which are heavier and more easily settled out and sent to aeration basins where oxygen is provided for the growth of bacteria and flocculation is carried over. The mixture is then passed on to the secondary clarifier where a solids-liquid separation process removes the micro-organisms from the wastewater. The effluent is collected and chlorination/dechlorination step is carried over to remove any remaining microbial pathogens. Tertiary treatment is done when viruses are believed to be inhibited by suspended and colloidal solids in the water and therefore these solids must be removed by advanced treatment before the disinfection step.
The conventional wastewater treatment systems with primary sedimentation tank and activated sludge treatment may not be able to meet the treatment needs to meet effluent discharge standards. New and advanced treatment methods have been discovered in recent years to treat wastewater more efficiently. New technologies now in use include vortex separators, high rate clarification, membrane bioreactors and membrane filtration.
Membrane Bio-Reactor is a combination of two basic processes - biological degradation and membrane separation - into a single process where suspended solids and microorganisms responsible for biodegradation are separated from the treated water by membrane filtration unit. The Membrane Bioreactor method in general is a simple, very effective process which combines both the activated sludge treatment process and the membrane filtration process. It is nothing but the activated sludge aeration basin with sets of micro- or ultra-filtration membrane filtration modules submerged in the aeration basins.
In this method the treatment process starts with usual preliminary and primary treatment processes.
In traditional activated sludge facilities a secondary clarifier is used which then follows the aeration basin which allows microbes to settle to the bottom of the tank, and effluent leaves the clarifier. While the MBR process obviously does not use a secondary clarifier, as the effluent is far cleaner than that which would be produced by a secondary clarifier. The nominal pore size is 0.04 µm. The fibre diameters used in MBR process are 0.9mm inside and outside 1.9mm.
The method has many advantages compared to the conventional treatment process. The effluent produced in this method is of high quality, very low in BOD (less than 5 mg/l), very low in turbidity and suspended solids. It produces high purity of water and but the operation needs trained people while the conventional process is comparatively very easy. The simple filtering mechanism significantly reduces the disinfection process by creating a physical disinfection barrier.
The treatment process has no secondary clarifiers or tertiary filters while the conventional method uses both to achieve similar water quality results. The MBR process produces less waste activated sludge than a simple conventional system. This process produces a consistent, high water quality discharge. The water can be reused and used in wide range of applications like irrigation, non-root edible crops, highway median strip and golf course irrigation, and cooling water re-charge when disinfected. The membrane reactors can be used when reverse osmosis is required for the treatment.
It should be taken care to have a method of controlling the dissolved oxygen concentration in each cell with an automatic air balance in each aeration cell, so that electrical energy is optimized and not wasted. It is also necessary to have a method of controlling the treatment process flow rate between banks or trains since the loading rate varies for different treatment trains by automatically controlling valves, or gates.
Other conventional methods that are discovered to improve the efficiency of wastewater treatment are: sewage treatment system with a floating filter medium to separate solid components in the sewage (Funakoshi et al.).
The sewage flows upwardly in the treatment tank and is filtered through the floating filter media layer made of cylindrical mesh floating filter media which have a smaller specific gravity than the sewage.
Fig. 7: Sewage treatment system with floating filter, US 5578200, 1996.
Kaltchev developed a clarifier for liquids containing suspended matter
The clarifier separates the suspended in two stages: by flotation--natural flotation (if the density of suspended solids density is lower than that of the liquid), or in combination with the dissolved air flotation technique; and then by filtration on a back washable filter medium.
Fig. 9: combined flotation and clarification system, US 6312592, 2001.
Many new technologies are available for wastewater treatment. The major determining factor for using those methods will be cost in most developing countries. Now that the costs of membrane modules are declining, the membrane systems are more likely to be used especially for large capacity systems. The effluent and sludge produced can also be reused and used in agricultural lands seeing as they possess high fertilizing value. With the increasing demand for drinking water and requirements for improved quality, more strict regulations for effluent discharge limits, and environmental awareness for water quality impacts, the research and development in water and wastewater technologies will increase in the coming years. The recent trends indicate that there will be new requirements to monitor and regulate the emerging groups of contaminants which are not currently regulated for example, the micro constituents which originate from over the counter drugs entering wastewater systems. Now the technologies available are aligned with the anticipated requirements to improve water quality. With the advancements in materials science, nano technology, and information technology; it is likely that there will be new developments in the area of membranes filtration and disinfection/ oxidation methods.