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INTRODUCTION: WHY WORRY ABOUT WASTEWATER TREATMENT? The goal of all wastewater treatment is to change harmful pollutants in waste products into non-polluting products that can be safely released back into the environment. While waste treatment has a long history, there have been major advancements in the forms of treatment, allowing ever increasing numbers of people to be serviced by reliable, quali ty systems. The waste in wastewater is actually a rich food source for many species of bacteria and protozoa. While it is not a population that is normally encouraged to develop in our modern environment, these microscopic organisms are the true janitors of all larger life forms. Bacteria are able to use the waste products found in wastewater as energy sources to grow and reproduce. In this food chain, the waste products of the bacteria (carbon dioxide, water and other nutrients) become food for plants, which then rejoin the wider environmental life cycle. In domestic wastewater, the primary contaminants are carbonaceous products that are rich in nutrients and energy. The problem is these products demand oxygen from their surroundings. If wastewater is allowed to enter the environment untreated, say to a stream or lake, the pollutants exert an oxygen demand on the water. Fish, plants and other aquatic species require oxygen in the water in order to breathe, so a significant release of raw wastes may deplete the oxygen in the water. The animals in the water then suffocate and die. A corresponding problem may be identified with both nitrogen and phosphorous wastes. Both of these elements are limited in the natural world and often they are added to agricultural lands to encourage plant growth. However in aquatic environments, the addition of excess nutrients encourages the rapid growth of algae, called eutrophication. When algae blooms form, they block out sunlight and consume large quantities of available oxygen in the water. The effect is twofold, the oxygen supply diminishes as the algae respire, and bottom dwelling plants die for lack of available light. The dead biomass (fish, plants and algae) settles to the bottom and again demands more oxygen from the water in decomposition. Algae blooms are common in many waters located nearby human habitation, as a direct result of pollutant runoff. When wastewater is discharged directly to land without treatment, similar deleterious effects can be seen. While there are no fish to kill, the wastewater still requires an adequate supply of oxygen to decompose. If the wastewater pools on the ground, there will be a noticeable odour associated with the waste. The smell is due to the formation of reduction byproducts, usually hydrogen sulfide and methane gas, both of which are potentially toxic. As the raw waste is so rich in nutrients, its direct application can kill almost any plant life currently existing on the ground. Plants simply ‘burn’ from the excesses of nitrogen compounds. The largest risk to direct disposal of wastewater however remains the threat of microbial contamination. If wastewater is dispersed onto the ground, all of the microorganisms that come with it are also dispersed. This can mean the release of infectious bacteria, viruses and protozoa that can easily contaminate both people and animals alike. Because water always flows towards the lowest point available, wastewater discharges often end up contaminating some water body if it hasn’t been treated. This may include groundwater used for drinking supplies. MECHANISMS OF WASTEWATER TREATMENT Treatment Stages Three levels of sewage or wastewater treatment are typically defined. These are primary, secondary, and tertiary (or advanced) wastewater treatment. Primary treatment removes the majority of settleable or bulk solids, from the waste stream. Well-designed systems also trap greases, oils, and other floatable solids, preventing them from fouling mechanical equipment downstream. Primary treatment provides a clarified raw sewage but does not remove dissolved organic materials or other pollutants from the wastewater stream; it is a minimal treatment stage, useful only when followed by subsequent treatment or storage. Secondary treatment removes dissolved organic materials through either chemical or biological mechanisms, and combined with clarification further removes suspended solids. Secondary treatment processes are generally considered to remove greater than 85% of the BOD and suspended solids from the wastewater stream, but only provide limited removal of the nutrients nitrogen (N) and phosphorus (P). Secondary treatment is sufficient in cases where the effluent is to be directed to land use where the N and P are beneficial to living plants, or where the nutrient levels are low in the effluent. Tertiary wastewater treatment provides an extra stage for the removal of phosphorous and nitrogen nutrients and even further removal of BOD and suspended solids. Generally, advanced wastewater treatment processes can provide removals of greater than 95% for BOD and suspended solids and high levels of nitrogen and phosphorus elimination in the final effluent. Tertiary treatment is necessary when receiving bodies of water would be adversely affected by the influx of the extra nutrients, and could result in eutrophication of watercourses. The fundamental processes used in all wastewater treatment systems to achieve these levels of treatment are defined as physical, chemical and/or biological unit processes. Most treatment systems include combinations of all these process types. Treatment Processes The most common physical treatment processes include sedimentation, screening, and filtration, all of which are solid liquid separation stages. Sedimentation is the settling of dense solids in a quiescent tank under the effects of gravity. This stage is normally accomplished at both the start and end of the treatment process train (where it is called clarification), first to remove non-biodegradable solids, finally as a polishing stage for the treated effluent. Sedimentation stages also remove solids that are less dense than water from the waste stream through flotation and skimming. Screening removes particles from wastewater that are larger than the internal diameter of the screen openings. Filtration uses a porous media such as sand or fabric to remove suspended materials by straining the wastewater - the size of particles removed depends ultimately upon the pore size of the filtration medium. Chemical treatment processes utilize a chemical reaction to alter the state of wastewater constituents so they are more easily removed from the wastewater stream. For example, to remove phosphorus from wastewater, ferric chloride is added to react with dissolved phosphate to create a dense solid, which can then be physically removed by sedimentation as a sludge. Chemical treatment processes are combined with physical process for optimal treatment, and the term physical/chemical process has evolved to describe these methods. Ion exchange, adsorption, and reverse osmosis are examples of physical/chemical treatment processes. Biological processes utilize microorganisms to alter the state of wastewater constituents. Microbes remove organic materials and nutrients in wastewater by breaking down these materials into harmless end products or incorporating the nutrients into new cell tissues. A gross measure of the amount of biodegradable organic materials in wastewater is biochemical oxygen demand, or BOD. BOD is the milligrams of oxygen utilized by microbes per litre of wastewater of while breaking down the organic materials in the liquid, and is a simple measure of the organic strength of the wastewater. Biological treatment that removes BOD are classified as either suspended-growth or attached-growth processes. Suspended-growth mechanisms maintain a culture of organisms in suspension in the wastewater liquid, either by vigorously aerating the liquid or by mechanical mixing. Aerobic suspended growth systems are known as activated sludge processes. In attached-growth biological processes organisms responsible for treatment are attached to fixed media surfaces, such as rocks, sand, plastic or other specially designed materials. Wastewater flows over the media surface and the attached organisms consume the organic materials in the wastewater as it passes. Biological treatment processes may be further divided into either aerobic or anaerobic classes. Aerobic biological processes utilize organisms that require a steady supply of fresh oxygen for respiration and thus are normally accompanied by compressors or surface aerators. The end products of aerobic biological activity are carbon dioxide and water. Conversely, anaerobic biological processes use organisms that do not require oxygen, thus aeration and the associated equipment is not required. However, anaerobic processes result in the formation of certain amounts of methane and hydrogen sulfide, both of which are hazardous. While anaerobic treatment requires less equipment, aerobic processes operate much more rapidly, require less time and volume for the treatment process to occur, and do not form explosive gasses. Anaerobic treatment is suitable when the total volume of wastewater is low or there is significant room for containment ponds. KINDS OF TREATMENT Biological plants are more commonly used to treat domestic or combined domestic and industrial wastewater from a municipality. They use basically the same processes that would occur naturally in the receiving water, but give them a place to happen under controlled conditions, so that the cleansing reactions are completed before the water is discharged into the environment. Physical/chemical plants are more often used to treat industrial wastewaters directly, because they often contain pollutants which cannot be removed efficiently by microorganisms, although industries that deal with biodegradable materials, such as food processing, dairies, breweries, and even paper, plastics and petrochemicals, may use biological treatment. Biological plants generally use some physical and chemical processes, too. Primary Some wastewater facilities engage only in the removal of solids rather than in the reduction of hazardous pollutants. This type of treatment is found commonly in developing nations where the technical and financial expertise is not available for more complete treatment. Primary treatment is also commonly found in municipalities along marine environments where dilution with salt water is felt to neutralize any pathogenic organisms. While there is evidence to suggest certain harmful bacteria and viruses are inactivated when mixed with seawater, the organic constituents are not changed in any significant way and still exert an oxygen demand on the surroundings. The city of Victoria, BC, is one large center using only primary screening as its treatment method. This method of treatment requires very little mechanical or technical expertise to implement and is inexpensive in terms of land use. The simplicity makes the system easily adaptable to any size of city or town. However, receiving water bodies may be adversely affected by the undigested sewage discharges. Lagoons Lagoons are large holding basins designed to retain wastewater effluent for periods of up to several years. The purpose of a lagoon is to allow sufficient time for natural processes to degrade pollutants before discharge. As most lagoons are uncovered, weather events encourage oxygen transport, at least in the upper treatment layers. Below a meter or so oxygen becomes limited and anoxic conditions prevail. This can lead to pungent odours. Lagoons are very effective treatment methods in warmer climates where bacterial activity is constant year-round. In colder locations, such as Canada, lagoons only operate when the water temperature is warm. This requires the lagoon to be quite large to contain winter wastewater storage. Lagoons may be aerated to speed up the process and reduce the size of the holding basin to several months’ capacity, rather than several years. Because of the land requirements for lagoons, they are typically found only in rural areas where space is not a constraint. Lagoons require extensive quantities of land but little complex machinery to operate. The sheer volume of contained waste gives the lagoon system the benefit of biological momentum, small volumes of hazardous chemicals can easily be neutralized or diluted to non-toxic levels. The land requirements mean that larger population centers cannot reasonably accommodate lagoons. Activated Sludge/Extended Aeration Activated sludge is a form of extended aeration where biomass formed in the process is allowed to settle in the aeration tank when the air supply is turned off. The sludge is then pumped out and mixed with incoming raw sewage to increase active biomass. Excess sludge is removed and subjected to further treatment prior to land disposal. Activated sludge plants are usually complex, tightly monitored facilities serving large municipalities. The cost of such facilities discourages smaller populations from adopting this very effective style of treatment. Activated sludge differs from straight extended aeration in that sludge must be pumped from one chamber to another and that it is finally removed from the system for additional treatment. The extra volume of air provided in extended aeration facilities allows decomposition of the formed sludge, though at some additional cost. Economies of scale allow large activated sludge plants to remove biosolids and dispose of it at a lower unit cost than paying for additional extra electrical and compressor facilities to reach complete digestion. Extended aeration is suitable for smaller facilities while activated sludge is more cost effective for large town and cities. These processes require extensive mechanical equipment but relatively low land use. They can be scaled to treat any amount of wastewater. Mechanical, Physical and Chemical Treatment A physical process usually treats suspended, rather than dissolved pollutants. It may be a passive process, such as simply allowing suspended pollutants to settle out or float to the top naturally, depending on whether the particles are more or less dense than water. The process may be aided mechanically by gently stirring the water to cause more small particles to bump into each other and stick together, forming larger particles that settle or rise faster, a process known as flocculation. In municipal treatment plants chemical flocculants in the form of aluminum or iron salts form larger particles and also remove phosphorus by precipitation. Polymeric additives are used to thicken and condition the sludge produced. As a final treatment stage filtration can produce very high quality effluent water. Ultrafiltration, nanofiltration, and reverse osmosis are processes that force water through membranes to remove suspended, colloidal and even some dissolved matter. Adsorption on activated charcoal is a physical process that removes dissolved chemicals. Air or steam stripping can be used to remove pollutants that are gasses or volatile liquids from water. The removed vapours are often passed through beds of activated charcoal to prevent air pollution. These last processes are used mostly in industrial treatment plants, though activated charcoal is common in municipal plants, as well, for odour control. Chemical treatment can also remove oxygen demand by treating the waste with aggressive oxidizing chemicals such as ozone and potassium permanganate. Virtually all the organic matter can be removed through the application of strong oxidants, unlike biological removal which often doesn’t affect industrial pollutants. Chlorine or ozone (or ultraviolet light) may be used to kill microorganisms before the final discharge of the wastewater. These processes require extensive mechanical equipment but relatively low land use. They are better suited to larger municipalities where the accumulated sludge can be disposed of efficiently. Rotating Biological Contactors (RBCs) Rotating biological contactors, or RBCs, are comprised of large porous disks that are rotated alternately between the wastewater and air. The disks, and there may be hundreds on any single axle, provide a fixed surface for bacterial growth and attachment. As the disk rotates through the wastewater, the biofilm absorbs nutrients. Then the disk rotates into the air and the bacteria decompose the sewage products. Aeration is provided by the rotation of the disks rather than through active compressors and pumps, though supplementary aeration can be added. The treatment capacity of the system is primarily related to the surface area of the rotating disks and bacterial attachment sites. As the RBC rotates, the relatively high velocity of the rotating surfaces will cause the attached biofilm to shear off, continually re-exposing fresh layers underneath. The relatively large flocs removed from the RBC settle rapidly, accumulating as sludge. This sludge can be digested aerobically if aeration is provided, or anaerobically if septic conditions are allowed to persist. RBCs need extensive mechanical equipment but low land requirements. They are suitable for smaller installations, as activated sludge plants are more cost effective at larger scales. FUNDAMENTALS OF AERATION Aerated wastewater systems have provided secondary treatment to many small to medium sized communities for years. The attributes of these processes have been attractive as cost effective options for the treatment of municipal wastewater. The aeration segment in these systems is the critical component; the ability to aerate incoming sewage has a direct impact on the level of wastewater treatment achieved. An ample oxygen supply in aerobic wastewater systems is the key to rapid and effective wastewater treatment. Bacteria combine oxygen with carbon based compounds to form carbon dioxide and water as harmless end products. Without sufficient oxygen, bacteria are not able to quickly biodegrade the incoming organic matter and some of the pollutants will pass through the treatment stage unchanged. In the absence of dissolved oxygen, decomposition must occur under septic conditions that are slow, odorous and yield incomplete conversions of pollutants. With septic conditions, some of the carbon compounds will be combine with hydrogen and sulfur to form hydrogen sulfide and methane. Other carbon will be converted to organic acids that create low pH conditions, making the water more difficult to treat. The biodegradation of organic matter in the absence of oxygen is a very slow kinetic process. Sewage systems must take into account the final fate of cellular material, called sludge that eventually settles out in the treatment process. Both anaerobic and mechanical treatment mechanisms produce large quantities of sludge that requires regular removal and disposal. One advantage of using aerobic completely mixed systems to digest wastewater is the lack of sludge production that requires further treatment. Unlike conventional wastewater treatment plants that must remove excess sludges daily, aerated systems can go long periods without ever needing cleaning. In the presence of sufficient oxygen, bacterial cells that settle out are eventually biodegraded into carbon dioxide and inert materials by scavenging bacteria and protozoa. Since a large portion of municipal wastewater consists of biodegradable organic carbon matter, the remaining active bacteria can quickly decompose most of the settled sludge. In systems without sufficient dissolved oxygen, the sludge layer will accumulate faster than it can be biodegraded. When this occurs, the sludge must be removed, at some expense, for disposal. The provision of adequate aeration is also an important element in keeping the system contents mixed and in suspension. With adequate mixing, incoming pollutants and wastewater are better distributed throughout the entire bioreactor. This results in more uniform and efficient treatment. In addition, solids that settle can be re-suspended by the aerator's mixing action and brought back into contact with the microbial population floating throughout the bioreactor. Poor mixing has the effect of creating thick solids deposits that fall to the bottom before proper treatment has occurred. If enough large, unmixed zones exist, sludge can accumulate and create pockets of septic conditions that pull available oxygen out of the upper process layers, reducing treatment effectiveness. Proper aeration and mixing is critical if the system is to properly treat influent wastewater pollutants. A poorly designed system will often result in a lot of money being spent on electrical power for aeration purposes with little benefit to show for the expenditure. Deep accumulations of partially treated sludge are the end result. Ammonia (NH3) has been found to be toxic to aquatic organisms, thus proper oxygenation also has important implications for its removal. While raw sewage contains little ammonia it forms as a natural consequence of protein degradation in wastewater, typically reaching concentrations of 30 mg/l. In the presence of adequate free dissolved oxygen, ammonia oxidizes to form non-toxic nitrates (NO3-), a valuable plant fertilizer. Oxygen Transport Factors to Consider There are many factors that will act to hinder the transfer of oxygen to wastewater, these must be considered to ensure that sufficient air is added for the process. Some of these factors include:
While it is not necessary to keep the bioreactor saturated with oxygen, biological activity is optimized when a minimum concentration of 2 mg/l dissolved oxygen is maintained at all times. Aeration equipment is sized on this basis. In certain cases, atmospheric pressure at the treatment plant site is an important factor in determining how much oxygen can be transferred. It is more difficult to transfer oxygen at higher elevations (low ambient pressures) than at sea level (standard pressure). Thus sites at high altitudes may require extra aeration equipment. The above factors must be considered to properly size aeration equipment for any treatment system. The actual oxygen supply requirement needed to meet field conditions is always larger than the minimum value calculated based simply upon BOD needs. The reason being most of the air added to a system bubbles to the surface and is lost to the atmosphere. While it would be cost prohibitive to attempt to capture all of the added air, there are many designs that improve overall oxygen transport, resulting in compressor efficiencies and lower power costs. The amount of oxygen that needs to be added under full design loading, full ammonia conversion, and worst case temperature conditions is often ten times that required by the active bacteria alone. Having determined the oxygen requirement of a system, it is next important to consider how much air volume in l/s will be needed to deliver the required mass of oxygen. Each cubic metre of air contains about 270 g of oxygen at standard temperature and pressure. The overall oxygen transfer efficiency (OTE) of a diffuser is a function of its depth in the ponds, in addition to the specific water conditions. Typically, an OTE of about 5% per meter of depth is found for fine bubble diffusers in wastewater. For a two-meter deep lagoon a transfer efficiency of about 10% is expected. This means that 90% of the oxygen in the air added at a depth of two meters will bubble to the surface and be lost. This is about the best a simple diffuser can manage without significantly increasing costs. Finally, aeration basin and aerator layout geometry can dramatically alter oxygen transfer efficiency. Aerator layouts should be designed to maximize air and wastewater contact and there are a number of options available. Full floor aerator coverage is theoretically the best possible aerator arrangement, however it is costly and complicated to even attempt to meet the ideal goal. Full floor coverage is not normally needed to meet the biological aeration demands in any case. A uniform distribution of diffusers is commonly applied in treatment as the mechanism is simple to design, relatively inexpensive and requires little technical planning. However when the design is not implemented correctly, this style can also create many difficulties. Placement of diffusers too far apart creates quiescent zones in the aeration basin where anoxic conditions quickly develop. While the surface of the aerator appears to be violently stirred, the upwelling air fails to mix solids in the bottom section of the tank, creating sludge buildup. Aeration planners know the area of influence around the diffuser heads and use this value to estimate the required number of inlets to completely mix the tank. When applied without some knowledge, the uniform diffuser arrangement can be a disaster. An improvement on uniform distribution is the air curtain style of diffuser arrangement. The idea is to provide air-delimited channels within the aeration basin through which all wastewater must flow in a defined pattern. The diffuser heads themselves are horizontal tubular models arranged perpendicular to the overall wastewater flow direction. When air is released from the diffuser it rises upward in a thin curtain blocking the water from crossing the barrier. Water must flow along the length of the diffuser rather than over top of it. The rising bubbles trap water near the base of the diffuser head and transport it to the surface creating a rolling convection current between separate diffusers. The design eliminates short-circuiting of sewage, meaning that no sewage can enter the tank and head directly towards the outlet without first being mixed in the curtain. There are two competing types of diffusers available, coarse and fine bubble diffusers. Coarse systems produce large bubbles from many heads to aid mixing and oxygen transport. There is a low head loss through any one diffuser and the size of the orifice significantly reduces the chance of clogging. Fine bubble diffusers require a lower volume of air to be supplied by improving transfer efficiency. Smaller bubbles are more easily mixed in the wastewater, however more head loss is generated in the fine bubble systems and there is a much greater chance of fouling the diffuser orifices. Either coarse or fine bubble diffusers will suffice if the planner appreciates the design difficulties of each. MICROORGANISMS Lagoons and ponds are similar to activated sludge systems in function, however, the mass of biological solids is much lower. Lagoons and ponds typically have 50-200 mg/L dry weight biomass compared to activated sludge systems which typically have 1000-5000 mg/L. The overall mass of biological solids determines the rate of biological reaction in a waste treatment system. Lagoons and ponds function 10-20 times slower than activated sludge systems because they have less biomass. The microorganisms responsible for biological treatment are varied and interrelated. In any given system there may be aerobic, anaerobic, and facultative bacteria, in addition to larger protozoa, and even algae. Wastewater entering a system contains organic materials (comprised primarily of carbon), nitrogen, phosphorous and trace micro-nutrients, which bacteria use as an energy source. Bacteria decompose the organic material and convert it into new cells, carbon dioxide and water. Other species graze on the bacteria and cellular wastes. Throughout the operation of a treatment system, the distribution of flora and fauna in the bioreactor will change to meet the particular wastewater conditions. This adaptability has created microbes capable of digesting solid fats, polymers and even heavy metal laden wastes. Organisms that use free dissolved oxygen are called aerobes and participate in what is called aerobic respiration. Most of the microorganisms in aerated systems convert food to energy in the presence of free dissolved oxygen. Anaerobes are able to obtain energy from chemically bound oxygen compounds such as nitrate and sulfate. This process is called anaerobic respiration. Facultative organisms use either free dissolved oxygen or chemically bound oxygen and are able to switch between either source freely. Organisms that get their carbon from organic material are called heterotrophic. Organisms that get their carbon form inorganic sources are called autotrophic. Life Forms in a Bioreactor Bacteria are tiny (0.05 microns by 1.0 - 5.0 microns), single-celled organisms that receive carbon from either organic or inorganic materials. They are responsible for the majority of the activity in a biological treatment system. Freely dispersed, floc-forming and filamentous bacteria are found in activated sludge, lagoon and pond systems. Many of these bacteria are motile, that is, they can move using their own power. Aerobic bacteria oxidize organic carbon to produce carbon dioxide, water and new bacterial cells. Typically, 20-40 percent of the applied organic wastewater load is directly incorporated into new bacterial cell material, becoming sludge. The remainder is lost to the air in the form of carbon dioxide or other waste gasses. Protozoa are single-celled organisms. They are larger than bacteria (10 - 200 microns) and more complex. Protozoa are strict aerobes and are usually identified on the basis of locomotion. Amoebae have no defined shape and move using finger or foot-like protrusions, called pseudopodia (false feet). Free-swimming ciliates propel themselves through the liquid using a rapid rhythmic movement of cilia The third group have flagella rather than cilia. Fungi are multicellular, non-photosynthetic, heterotrophic protista. They are strict aerobes. Rotifers and nematodes (worms) are animals found primarily in lagoon systems with open access to the environment rather than in enclosed extended aeration plants. Rotifers are multicellular animals and are generally strict aerobes. Anaerobic bacteria found in biological treatment systems include methane bacteria and sulfur bacteria. Two different groups of anaerobic bacteria convert organic material to methane in a two-step process. Acid-forming bacteria convert organic materials to simple alcohols and acetic, propionic and butyric acids. Acid-forming bacteria are hardy and function over a wide pH range. Methane-forming bacteria convert acetic acid to methane. Methane-forming bacteria are environmentally sensitive and function in a narrow pH range of 6.8-7.4. These anaerobes are not present in sufficient quantities in a well-designed aerobic digester to produce any appreciable quantity of methane. The anaerobic sulfur bacteria oxidize reduced sulfur compounds using light energy to produce sulfur and sulfate. An excess of these bacteria results in a lagoon with a pink or red color. These bacteria are of no consequence in covered treatment units where photosynthesis does not take place. Algae are aerobic organisms that use light as an energy source and grow with simple inorganic compounds such as carbon dioxide, nitrate, nitrite and phosphate. Algae are found in lagoons or open treatment systems only where there is a strong light source directly impinging on the wastewater. Bacterial Respiration and Sewage Consumption Respiration is a physiological process in which organic compounds are oxidized mainly to carbon dioxide and water. However, respiration does not only lead to the production of carbon dioxide, but to the synthesis of cell material as well. Respiration is an orderly process, catalyzed by enzymes and consisting of many integrated step-reactions terminating in the reduction of oxygen to water. Aerobic respiration common to species of bacteria, protozoa and higher animals, may be represented by the following simple equation, with the waste modeled as a simple sugar:
Controlling Factors Temperature has a tremendous effect on the rate of cell growth. An increase in temperature of 10oC (within the range of temperature that bacteria can grow) doubles the rate of microbial growth. Aerobic BOD removal works well at temperatures from 3-4oC to 60-70oC (thermophilic bacteria replace mesophilic bacteria at temperatures above 35oC) and declines rapidly below 3-4oC and ceases at 1-2oC. The type and number of microorganisms in a treatment system respond to the quantity and quality of food present. The growth of bacteria can be represented by five distinct phases on a growth curve. The adaptation or lag phase represents the time required for the organisms to acclimate themselves to the organic material present in the wastewater. Once the bacteria have adapted, the rate of growth becomes logarithmic until food becomes limiting. At this point, the rate of growth declines and eventually becomes stationary. When the supply of food becomes insufficient to maintain bacterial mass, the microorganisms use organic matter within their own cells as a source of energy. This is known as endogenous respiration. Incoming wastewater contains settleable and dissolved organic matter. Some of the organic matter settles and becomes sludge while most of the dissolved organic matter is digested by heterotrophic bacteria using free oxygen to form new cells and release carbon dioxide. Organic nitrogen is converted to ammonia. Autotrophic bacteria convert ammonia to nitrite and nitrate. Anaerobic reactions occur in the sludge layer, which release hydrogen sulfide and methane. Denitrification (an anoxic reaction) occurs in the sludge layer and releases nitrogen gas. In open systems, algae are able to use sunlight, carbon dioxide, nitrogen, and phosphorus to generate new algal cells and release oxygen during the daytime. Reaction Kinetics BOD removal for mixed, aerated systems can be estimated using a complete mix hydraulic model and first order reaction kinetics: Insert image here Where, Cn = effluent BOD Co = influent BOD k = reaction rate T = total hydraulic detention time n = number of treatment stages (cells) in series Where, insert image here t = temperature of pond at operating temperature ft = temperature factor k = reaction rate at design temperature k20 =reaction rate at 20 Degrees Celsius The expression suggests the effluent BOD is a function of the influent concentration, the lagoon operating temperature, the number of treatment stages (cells), and the overall detention time. The effluent concentration will decrease as the influent concentration decreases, given the same operating temperature, number of cells and detention time. As the detention time increases, the effluent concentration decreases. As the temperature increases, the effluent concentration decreases. More cells operated in series will produce a lower effluent concentration given the same overall detention time and operating temperature. Nitrification The forms of nitrogen most often found in wastewater are ammonia (as NH4+), nitrate (as N03-) and organic nitrogen in the form of amines and other nitrogenated compounds (proteins). Most of the organic nitrogen is converted to ammonia by ammonifying bacteria. Ammonia is removed by stripping to the atmosphere (this can be significant at pH above 8), assimilation into bacterial cells, and bacterial nitrification (which may be followed by denitrification). Nitrification is a naturally occurring, two-step aerobic biological process through which autotrophic bacteria oxidize the ammonium ion to nitrite or nitrate. In the first step, ammonia is oxidized to nitrite by Nitrosomonas bacteria. Step 1 - Ammonia is oxidized to nitrite by Nitrosomonas
Step 2 - Nitrite is oxidized to nitrate by Nitrobacter bacteria.
The overall energy reaction is:
Along with obtaining energy, however, some of the ammonium ion is assimilated into cell tissue. The nitrification process requires a significant amount of oxygen, produces a small amount of biomass, and may result in a change in pH if there is a lack of alkalinity in the wastewater (this is normally not a problem in domestic wastes). Factors that influence nitrification include waste characteristics, dissolved oxygen concentration, BOD loading, detention time, pH, alkalinity, temperature, nitrifier mass, and toxins. Wastewater high in ammonia tends to nitrify to a greater degree. As nitrification consumes large amounts of oxygen, uninhibited nitrification requires an operating dissolved oxygen level of 2.0 mg/L or more. Nitrifying bacteria do not compete well against heterotrophic bacteria for oxygen and nutrients; before nitrification can take place the soluble BOD must be sufficiently reduced to eliminate competition (generally below 20-30 mg/L). Activated sludge plants are able to nitrify in 6 to 48 hours. Most lagoons and ponds have detention times of 30 days or longer. Temperature changes influence the rate of nitrification. Above 20 degrees Celsius and up to about 35 degrees Celsius, nitrification is enhanced. Nitrification slows down dramatically or may stop altogether at around 5 degrees C. The operating temperature of a wastewater facility will greatly influence the quality of effluent, with discharge high in ammonia having a noticeable odour. A very important factor is that a sufficient population of nitrifying bacteria must be present in order to nitrify. These bacteria are attached growth organisms, meaning that they must attach themselves to the surface of an object. During initial startup conditions of a waste treatment unit, the population of nitrifying bacteria is low and several weeks may be required for full reactor development. "Incipient nitrification" occurs when the nitrifying bacteria just begin to establish themselves, but the detention time is not adequate for detectable ammonia reduction. In an activated sludge plant, the surface is a floc particle. In a trickling filter or RBC, the surface is the artificial media. In lagoons and ponds, it is believed that nitrifiers may attach to sideslopes, baffles and algal particles. Nitrifying bacteria are more sensitive to inhibitory compounds, such as heavy metals, than are the BOD reducing bacteria, thus the nitrifying bacteria would be the first ones to die off if strong chemicals are discharged into the treatment system. TEST PARAMETERS BOD vs. CBOD BOD has been the standard method for measuring influent and effluent sewage strength, however it is important to understand what is actually being measured by this parameter and the problems associated with using it. BOD5 is a measure of the uptake of oxygen by respiring bacteria over a five-day period, this value includes the amount of oxygen required to consume both carbonaceous and nitrogenous wastes in the water. CBOD testing is similar to BOD testing with the exception that a nitrification inhibitor is added at the start of the process to eliminate nitrifying bacteria from the sample, thus only the carbonaceous demand is measured. In spite of the fact that effluent BOD5 is a key parameter in many discharge permits, it is the most misleading. Effluent BOD5 data are flawed as the result of being inflated by nitrification that occurs in the BOD5 test itself. Up to 60 percent of the BOD5 violations in the U.S. may have been caused by nitrification in the BOD5 test rather than by improper design or operation. Consequently money may be spent needlessly on new treatment facilities when the problem is the test. The total BOD of a wastewater is composed of two components – a carbonaceous oxygen demand and a nitrogenous oxygen demand. Because of the slow growth rates of nitrifying organisms that exert the nitrogenous oxygen demand, it has been assumed that no nitrogenous demand is exerted during the BOD5 test. This assumption appears to be valid when the test is performed on raw, untreated municipal wastewater where organic loading is high and the nitrifying bacteria can’t compete, it does not appear to be valid when performed on secondary effluents, especially those following bacterial treatment. The BOD5 of effluents from the latter are almost always inflated by a nitrogenous component. Unlike carbonaceous demand, which is proportional to the concentration of the biodegradable carbon constituents in the effluent, the nitrogenous demand exerted during the 5-day test is proportional to the number of nitrifying organisms that happen to be caught in the sample being tested. Thus the BOD test doesn’t provide a true insight into potential downstream impacts of discharge. This discrepancy is particularly important in treatment facilities where land disposal is considered. The nitrogen components in the effluent could provide a net benefit to vegetation but disposal may not be permitted if a BOD5 test result shows an inflated oxygen demand, implying a substandard treatment level. Solids Solids in wastewater are measured as either suspended or dissolved. Suspended solids are particles that can be trapped on a filter paper of a standard pore size. These are physically separable particles that may be examined under a microscope, for example. These solids may or may not settle out of the bulk solution. Suspended solids influence the clarity of a liquid by scattering light, giving a hazy or dirty impression. The total allowable mg/l of suspended solids (or TSS) in discharge effluent is normally contained in operating permits. Dissolved solids are those that form a part of the liquid matrix, being in the same phase and cannot be removed by filtration. Bacteria and other organisms in the liquid can directly utilize these solids. BOD and CBOD measurements approximate the mg/l of dissolved solids by indicating the oxygen demand exerted by the compounds. Thus operating permits don’t normally require dissolved solids tests as the oxygen demand accounts for this value. |
Table Of Contents Introduction: Why Worry About Wastewater Treatment? Mechanisms of Kinds of Treatment: Fundamentals of Aeration: Microorganisms Test Parameters
Actual Samples Bottom to Top is: |
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