Reviewing Aerobic Technologies for the Treatment of Wastewater

Wastewater treatment has become essential these days, because it is so interconnected with the other uses of water. This wastewater is treated and then converted to effluent to discharge it to the natural water streams. If un-treated water discharged in river streams, leave its effects on the wildlife habitats thriving in oceans, rivers and marshes, migratory birds using these areas for breeding, resting, and nesting, fisheries which have direct impact upon human consumption. Thus this effluent must be treated before it goes back to the environment to have minimum impact on the environment, or can be directly reused. This reclaimed water can be used for purposes other than consumption. As disposal or reuse is the ultimate aim of the treatment of wastewater, treatment is decided accordingly to create minimum impact to the river streams and low sludge for landfills.Wastewater is treated in three stages primary, secondary and tertiary. Primary treatment includes filtering the insoluble solids, grit, suspended matter etc. from water, the sand filter is the most commonly used filter. The oil and grease found on the surface of some wastewater can also be removed easily through this method. Next is secondary treatment, the collected filtered water is then treated to reduce its toxicity, decrease oxygen demand and disinfect. This treatment includes oxidation and chemical treatment. Oxidation can be aerobic or anaerobic involved in treatment and the tertiary treatment involves further disinfection like absorption, advanced oxidation, disinfection depending upon quality of wastewater.The oxidation is the most important and integral part of the wastewater treatment processes, it defines the efficacy of the treatment plant. Thus, secondary treatment should be efficient as well as it should be economical. Oxidation means decomposition of organic or inorganic matters present in effluent. The secondary treatment involves biological oxidation and chemical oxidation. Biological oxidation processes can be aerobic and anaerobic. Rely on microbial decomposition to treat wastewater, the key difference between anaerobic and aerobic treatment is that aerobic systems require oxygen, while anaerobic systems do not. Most people consider bacteria and other microorganisms to be undesirable components of wastewater. In fact, only a small fraction of the microbes found in wastewater are truly pathogenic. Aerobic wastewater treatment encourages the growth of naturally-occurring aerobic microorganisms as a means of renovating wastewater. Such microbes are the engines of wastewater treatment plants. Organic compounds are high-energy forms of carbon. The oxidation of organic compounds to the low-energy form (carbon dioxide) is the fuel that powers these engines..The biological aerobic treatment uses bacteria or microbes to clean water systems that degrade the organic matter in presence of oxygen. The oxygen required for the decomposition of organic matter by these microbes is often measured in biological oxygen demand or BOD, which refers to the amount of dissolved oxygen needed by aerobic organisms to break down organic matter into smaller molecules. Provided that oxygen and food, in the form of settled wastewater are supplied to the microorganisms, the biological oxidation process of dissolved organic matter will be maintained. The bioconversion of dissolved organic matter into thick bacterial biomass can fundamentally purify the wastewater. Subsequently, it is crucial to separate the microbial biomass from the treated wastewater through sedimentation. This secondary sedimentation is basically similar to primary sedimentation except that the sludge contains bacterial cells rather than fecal solids. The biological removal of organic matter from settled wastewater is conducted by microorganisms, mainly heterotrophic bacteria but also occasionally fungi. The microorganisms are able to decompose the organic matter through two different biological processes: biological oxidation and biosynthesis. The biological oxidation forms some end-products, such as minerals that remain in the solution and are discharged with the effluent. The biosynthesis transforms the colloidal and dissolved organic matter into new cells that form in turn the dense biomass that can be then removed by sedimentation.Oxidation:COHNS + O2 + Bacteria → CO2 + NH3 + Other end products(Organic matter) +EnergyBiosynthesis:COHNS + O2 + Bacteria → C5H7NO2(Organic matter) (New cells).High levels of BOD indicate an elevated concentration of biodegradable material present in the wastewater and low levels of dissolved oxygen and can be caused by the introduction of pollutants such as industrial discharges, domestic fecal wastes or fertilizer runoff. High BOD levels means low oxygen levels thus low or slow decomposition of organic matter resulting into reducing efficacy of treatment plant.Thus to treat these water oxygen incorporation is necessary. According to the amount of organic matter and requirement of effluence, wastewater treatment might involve different processes and different types of microbes required to decompose organic matter. They also require particular operational procedures that will differ that are subject to the environment norms needed to keep biomass growth rates optimal for the specific microbial populations. It is also required to monitor aeration process to maintain a consistent dissolved oxygen level to keep the system’s bacteria multiplying at the appropriate rate to meet discharge requirements. Not only Dissolved oxygen but other parameters like pH, nutrients temperature and ratio of organic matter to microbes are also important to monitor as they affect the treatment process rate of decomposition.Following are examples of some common types of aerobic biological wastewater treatment systems, including a brief description of how they function within an industrial wastewater treatment procedure to give you an idea of the types of technologies and systems that might benefit your industrial facility.1. Activated SludgeActivated sludge is a very common type of suspended growth aerobic method. In this method the sewage is treated using aeration and aerobic microbes composed of protozoa and other aerobic bacteria. This method is based on biological oxidations with active sludge aiming to remove the organic matter from wastewater. In addition to organic matter removal in this method also aids the removal of nitrogen and phosphorus to some extent.The active sludge consists of aerobic active micro-organisms with sufficient oxygen supplied so that these micro-organisms sufficiently oxidize the organic matter present in the wastewater to CO2 and water. Biological floc is an ecosystem of living biota subsisting on nutrients from the inflowing primary clarifier effluent. These mostly carbonaceous dissolved solids undergo aeration to be broken down and either biologically oxidized to carbon dioxide or converted to additional biological floc of reproducing micro-organisms. Overflow from the activated sludge mixing chamber is sent to a secondary clarifier where the suspended biological floc settles out while the treated water moves into tertiary treatment or disinfection. Settled floc is returned to the mixing basin to continue growing in primary effluent..There are two types of implementation of Active sludge System one is conventional type of sludge system and other is Sequencing Batch Reactor. Conventional type consists of aeration tank and a sedimentation tank. Aeration tank used for biological oxidation and sedimentation tank is where the sludge is separated from treated water and moved to tertiary treatment. The aeration basins are sometimes preceded by a mixing tank (selector), where the influent is intensively mixed with sludge. The aim of this is to prevent the growth of thread-forming bacteria whereas, in the Sequencing Batch Reactor the purification processes of aeration, sedimentation and discharge are performed sequentially in same basin. Thus, Sequencing Batch Reactor is best suitable for the plants where waste is received in batches. An SBR can be used to consecutively perform various biological processes, like nitrification and denitrification. An SBR system is better at preventing thread-forming bacteria because the system acts as a selector during the supply phase.Review:a. Active sludge systems are flexible, robust and cost-effective. A wide range of influent concentrations can be treatedb. These systems are effective to attain purification in influents that vary little in terms of composition or supplyc. A buffer tank is needed to control the high variance where the wastewater characteristics vary in terms of contamination and volumed. The process can be optimized according to the wastewater, also needs a continuous supervision, but have very limited maintenancee. For the wastewater to be effectively undergoing biological degradation, it can be tested in lab for small samplesf. This system requires a relatively large system due to long retention times in the tank, the relatively low sludge content and the large sedimentation surfaceg. The temperature of wastewater is normally between 15°C and 35°Ch. A wide range of COD values can be treatedi. Highly acidic or alkaline waters must be corrected so that a pH between 6.5 and 8.5 can be implemented within the system.j. Active sludge systems are relatively insensitive, but can be inhibited by high concentrations of salts and specific chemicals.2. Membrane BioreactorsIn membrane bioreactors technology is advanced biological wastewater treatment technology that combines the conventional suspended growth aerobic method to membrane filtration rather than sedimentation. Membrane bioreactor can produce high quality effluent enough to discharge to costal area, sea, brackish channels or to be reclaimed and reuse for irrigation processes. Membrane bioreactors are an activated sludge system that uses a membrane for liquid-solid phase separation process. The membrane component uses low pressure microfiltration membranes and eliminates the need for a secondary clarifier or filtration. The membranes thus, replace the sedimentation basin in common biological purification and help to separate the sludge from the effluent. This helps to ensure that all floating matter is retained, whereby sedimentation is no longer a restrictive factor for sludge concentration.Membrane bioreactors can be used to reduce the footprint of an activated sludge sewage treatment system by removing some of the liquid component of the mixed liquor.There are two Membrane Bioreactor configurations: internal/submerged, where the membranes are immersed in and integral to the biological reactor; and external/side stream, where membranes are a separate unit process requiring an intermediate pumping step. The internal/submerged the membrane is installed in either the main reactor or in separate tank. These membranes can incorporate and online backwash system to prevent the membrane surface fouling; by pumping membrane infiltrate back through the membrane. Whereas in External/side stream; the filtration components are fixed externally to the reactor. The biomass is pumped through membrane module in series and back to the bioreactor. Cleaning and soaking of the membranes can be undertaken in place. .Review:a. Advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plantsb. It is possible to operate Membrane Bioreactor processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional sedimentation systems, thus reducing the reactor volume to achieve the same loading ratec. Discharge is possible in vulnerable areasd. Levy costs are reducede. Direct use as process water is possible in various applicationsf. Direct post-purification is possible via reverse osmosis for the removal of salts or recalcitrant organic compoundsg. The quality of the MBR permeate is greatly determined by the quality of the influenth. Dissolved substances, primarily high calcium contents and aluminum salts can also cause damage to the membranesi. Maintenance cleaning with higher chemical concentration and intensive chemical cleaningj. In comparison to the conventional activated sludge process (ASP) which typically achieves 95 percent, COD removal can be increased to 96 to 99 percent in MBRs.3. Trickling FilterTrickling filters are advanced biological wastewater treatment technologies used for efficiently treating wastewaters with high to extremely high organic contamination levels. Of all biological treatment systems, these can hold the most contaminant-eating microbes in the smallest area, which makes them space-saving and energy-efficient technologies ideal for treating wastewaters with medium to very high BOD. It consists of a fixed bed of rocks, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic over which sewage or other wastewater flows downward and causes a layer of microbial slime (biofilm) to grow, covering the bed of media. Aerobic conditions are maintained by splashing, diffusion, and either by forced-air flowing through the bed or natural convection of air if the filter medium is porous..The wastewater collected after primary treatment flows into a dosing device, often a tipping bucket which delivers flow to the arms of the filter. The trickles of water flow through the arm and exist through the series of holes pointing in an angle downwards. This arm distributes the wastewater by propelling over the surface of filter media. The basin can be covered or open to air, if covered, the air is pumped along with influent in the basin. The removal of pollutants from the wastewater stream involves both absorption and adsorption of organic compounds and some inorganic species such as nitrite and nitrate ions by the layer of microbial bio film.The filter media is typically chosen to provide a very high surface area to volume. The materials are often porous and have considerable high surface area in addition to the outer surface of the medium. Passage of the wastewater through the media provides dissolved oxygen which the bio-film layer containing microbes that requires Dissolved Oxygen for the oxidation of the organic matter and releases carbon dioxide gas, water and other oxidized end products. As the bio film layer thickens, it eventually marshes off into the liquid flow and consequently forms part of the secondary sludge. A trickling filter is followed by a clarifier or sedimentation tank for the separation and removal of the film.Review:a. Simple, reliable, biological processb. Suitable in areas where large tracts of land are not available for land intensive treatment systemsc. Effective in treating high concentrations of organics depending on the type of medium usedd. Moderate level of skill and technical expertise needed to manage and operate the systeme. High effluent quality in terms of BOD and suspended solids removal; in combination with a primary and tertiary treatment also in terms of pathogensf. Requires regular operator attentiong. Some residential systems require forced aeration units which will increase maintenance and operational costsh. Efficient nitrification (ammonium oxidation)i. Resistant to shock loadsj. Incidence of clogging is relatively high.4. Moving Bed Biofilm ReactorMoving bed Biofilm reactors are more advanced technology to the conventional trickling filter. In this process the aeration tanks are filled with small moving porous biofilm carriers held in the basin by media retention sieves. These plastic biofilm carriers are typically half- to one-inch diameter cylinders or cubes and are designed to be suspended with their immobilized biofilm throughout the bioreactor by aeration or mechanical mixing. Because of the suspended moving bio-film carriers, Moving Bed Biofilm Reactor allow high BOD wastewaters to be treated in a smaller area with no plugging.These solutions significantly increase the capacity and efficiency of existing wastewater treatment plants, while minimizing the size of new plant deployments. This method makes it possible to attain good efficiency results of disposal with low energy consumption. This process is used for the removal of organic substances, nitrification and denitrification.The Moving Bed Biofilm Reactor system consists of an activated sludge aeration system where the sludge is collected on recycled plastic carriers. These carriers have an internal large surface for optimal contact water, air and bacteria.The bacteria grow on the internal surface of the carriers. The bacteria break down the organic matter from the wastewater. The aeration system keeps the carriers with activated sludge in motion. Only when there is extra amount of bacteria growth, the excess sludge will come separate from the carriers and will flow with the treated water towards the final separator.The system can consist of a one stage or more stage system, depending on the specific demands. The specific bacteria remain in their own duty tank because of the fact that the carriers remain in only 1 tank protected by screens..Review:a. Higher effective sludge retention time (SRT) which is favorable for nitrificationb. Responds to load fluctuations without operator interventionc. Lower sludge productiond. Less area requirede. Resilient to toxic shockf. Process performance independent of secondary clarifierg. Maintenance-friendlyThese are multitudes of aerobic biological treatment processes and technologies in literature and practice. Biological treatment using aerobic activated sludge process has been in practice for well over a century. Increasing pressure to meet more stringent discharge standards or not being allowed to discharge treated effluent has led to implementation of a variety of advanced biological treatment processes in recent years. It is very critical to select the suitable treatment process that will be cost effective as well as effective in high quality effluent production. 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