30-08-2014, 10:35 AM
WASTEWATER TREATMENT USING ANAEROBIC PROCESS
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1. INTRODUCTION
1.1. What Is Wastewater?
Wastewater is the used water or liquid generated by the community due to its various activities, and contains the impurities in excess of the permitted/regulated statutory limits. Technically, wastewater can be defined as any water or liquid that contains impurities or pollutants in the form of solids,liquids or gases or their combinations in such a concentration that is harmful if disposed into the environment[1,62]. A wastewater impurity contains solids which may be organic and inorganic in nature and may be presented in suspended, colloidal, dissolved or in the various forms of their combinations. Wastewater also contains nutrients which stimulates the growth of aquatic plants and also contains toxic compounds which harms the human intestine tract. Hence it is necessary to treat wastewater[3,4].
1.2. Wastewater Treatment
Wastewater treatment consists of applying known technology to improve or upgrade the quality of a wastewater. Treatment involves collecting the wastewater in a central, segregated location (the Wastewater Treatment Plant) and subjecting the wastewater to various treatment processes. A complete treatment system may consist of the application of a number of physical, chemical and biological processes to the wastewater[5].
Physical treatment consists of physical operations to improve the quality of water. Some of physical treatments are Sedimentation (Clarification), Screening, Aeration, Filtration,Flotation and Skimming, Degasification,Equalization.
Chemical treatment consists of using some chemical reaction or reactions to improve the water quality. Some of physical treatments are Chlorination, Ozonation, Neutralization, Coagulation, Adsorption, Ion Exchange.
Biological: Biological treatment methods use microorganisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products. More microorganisms, or sludge, are formed and a portion of the waste is converted to carbon dioxide, water and other end products. Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on availability of dissolved oxygen.
Aerobic: Activated Sludge Treatment Methods, Trickling Filtration, Oxidation, Ponds, Lagoons, Aerobic Digestion
Anaerobic: Anaerobic Digestion, Septic Tanks,Lagoons
The purpose of wastewater treatment is generally to remove from the wastewater enough solids to permit the remainder to be discharged to receiving water without interfering with its best or proper use. The solids which are removed are primarily organic but may also include inorganic solids. Treatment must also be provided for the solids and liquids which are removed as sludge. Finally, treatment to control odours, to retard biological activity, or destroy pathogenic organisms may also be needed.
While the devices used in wastewater treatment are numerous and will probably combine physical, chemical and biological methods, they may all be generally grouped under four methods:
1.2.1.Preliminary treatment:At most plants preliminary treatment is used to protect pumping equipment and facilitate subsequent treatment processes. Preliminary devices are designed to remove or cut up the larger suspended and floating solids, to remove the heavy inorganic solids, and to remove excessive amounts of oils or greases.
1.2.2.Primary Treatment:In this treatment, most of the settleable solids are separated or removed from the wastewater by the physical process of sedimentation. When certain chemicals are used with primary sedimentation tanks, some of the colloidal solids are also removed. Biological activity of the wastewater in primary treatment is of negligible importance. The purpose of primary treatment is to reduce the velocity of the wastewater sufficiently to permit solids to settle and floatable material to surface. Therefore, primary devices may consist of settling tanks, clarifiers or sedimentation tanks.
1.2.3.Secondary Treatment:Secondary treatment depends primarily upon aerobic organisms which biochemically decompose the organic solids to inorganic or stable organic solids. It is comparable to the zone of recovery in the self-purification of a stream.
Secondary treatment is classified furthered classified into
1. Suspended growth processes:The microorganisms responsible for treatment are maintained suspension by appropriate mixing methods.
2.Attached – growth processes: The microorganisms are attached to some inert medium, such as rock, slag or specially designed ceramic or plastic materials. (also called Fixed-film processes)[6,7]
1.2.4.Tertiary Treatment:The terms "primary" and "secondary" treatment have been used to generally describe a degree of treatment; for example, settling and biological wastewater treatment. Since the early 1970's "tertiary" treatment has come into use to describe additional treatment following secondary treatment. Quite often this merely indicates the use of intermittent sand filters for increased removal of suspended solids from the wastewater. In other cases, tertiary treatment has been used to describe processes which remove plant nutrients, primarily nitrogen and phosphorous, from wastewater.
Improvement and upgrading of wastewater treatment units as well as the need to minimize environmental effects has led to the increased use of tertiary treatment[8,9].
1.3. Why Anaerobic Process?
1. Less energy requirement, because noaeration is needed.
2. Energy generation in the form of methane gas.
3. Less biomass (sludge) generation.
4. Less nutrients (N & P) requirement becauseof low biomass.
5. Higher organic loading rate.
6. Space saving due to high organic loading[10,11
2.1. Comparison between Aerobic and Anaerobic Process:
The advantages of anaerobic treatment can best be indicated by comparing this process with aerobic treatment. In aerobic treatment, as represented by the activated sludge and trickling filter processes, the waste is mixed with large quantities of microorganisms and air.
Microorganisms use the organic waste for food, and use the oxygen in the air to burn a portion of this food to carbon dioxide and water for energy. Since these organisms obtain much energy from this oxidation, their growth is rapid and a large portion of the organic waste is converted into new cells. The portion converted to cells is not actually stabilized, but is simply engaged in form. Although these cells can be removed from the waste stream, the biological sludge they produce still presents a significant disposal problem.
In anaerobic treatment, the waste is also mixed with large quantities of microorganisms, but here, air is excluded. Under these conditions,bacteria grow which are capable of converting the organic waste to carbon dioxide and methane gas.
Unlike aerobic oxidation, the anaerobic conversion to methane gas yields relatively little energy to the micro-organisms. Thus, their rate of growth is slow and only a small portion of the waste is converted to new cells, the major portion of the degradable waste being converted to methane gas. Such conversion to methane gas represents waste stabilization since this gas is insoluble and escapes from the stages.Each stage represents the culmination of growth of a population of methane formers capable of fermenting one particular group of compounds. The process is not completely operational until all the groups of methane formers are finally established. This may take several weeks if the process is started without the benefit of "seed" sludge containing the methane formers required for the specific acids present.
While there are many different methane forming bacteria, there are also many different acid forming bacteria. Waste utilization requiresa balance among all these organisms. The establishmentand maintenance of this balance is normally indicated by one of the most important control tests, that for the concentration of volatile acids. The volatile acids are the short chain organic acids indicated in table 1.
The acids shown are the major intermediates produced by the first stage conversion. They represent the intermediate compounds of most importance in anaerobic treatment, and most of the methane formed from this process results from fermentation of these acids by the methane bacteria.
When the system is in balance, the methane bacteria use the acid intermediates as rapidly as they appear. However, if the methane bacteria are not present in suitable numbers, or are being slowed down by unfavorable environmental conditions, they will not use the acids as rapidly as they are produced by the acid formers, and the volatile acids will increase in concentration . Thus, an increase in acid concentrationindicates the methane formers are not in balance with the acid formers. An analysis for the individual acids present will indicate the particular methane bacteria not carrying out their portion of the treatment. Unfortunately, the volatile acids analysis does not indicate an unbalance in the acid forming organisms. At present, no satisfactory method is available to determine the relative populations of the bacteria specifically responsible for production of certain acids [13].
2.2.Methane Formation:
The conversion of organic matter into methane proceeds through a long series of complex biochemical steps .
Although almost nothing is known of the individual steps involved, tracer studies have indicated the major sources of methane as shown in fig.2. One source of methane is the direct cleavage of acetic acid into methane and carbon dioxide. This acid is one of the most important volatile acids formed from the decomposition of complex organics and is the source of most methane in anaerobic treatment.
The methyl carbon of acetic acid marked with an asterisk in fig.2. ,together with its three hydrogen atoms, are converted intact into methane gas. The carbonyl carbon, shown without an asterisk, is converted to carbon dioxide.
Most of the remaining methane in anaerobic treatment is formed from the reduction of carbon dioxide.Here, hydrogen, which is removed from organic compounds by enzymes, reduces carbon dioxide to methane gas. The carbon dioxide here functions as a hydrogen or electron acceptor, just as oxygen in aerobic treatment. There is always a large excess of carbon dioxideavailable in anaerobic treatment, and thus the availability of carbon dioxide for this reduction is never a limiting factor in treatmentof complex materials [14].
2.3.Volatile Acid Intermediates:
The two major volatile acid intermediates formed in anaerobic treatment are acetic acid and propionic acid The importance of these two acids as precursors of methane is indicated in Fig.3. which shows the pathways by which mixed complex organic materials are converted to methane gas . The percentages shown are based on COD conversion and are for methane fermentation of complex materials such as municipal waste sludge or other wastes of similar composition . The percentages would be different for other wastes . The complete methane fermentation of complex wastes has been compared to a factory assembly line operation 8 in that the processing of raw waste material to the final methane product requires the help of several different workers . The raw material must be worked on by each group of organisms to prepare it for handling by the next.
As much as 80 to 90 percent of the degradable organic portion of a waste can be stabilized in anaerobic treatment by conversion to methane gas, even in highly loaded systems. This is in contrast to aerobic systems, where only about 50 percent of the waste is actually stabilized.
Since only a small portion of the waste is converted to cells, the problem of disposal of excess sludge is greatly minimized.Also, the requirements for the nutrients, nitrogen and phosphorus, are proportionately reduced. This is especially important in the treatment of industrial wastes which lack these materials. The sludge produced is quite stable and will not present a nuisance problem.
Since anaerobic treatment does not require oxygen, treatment rates are not limited by oxygen transfer .The absence of a need for oxygen also reduces power requirements for treatment. In contrast, the methane gas produced by anaerobic treatment is a good source of fuel energy and is frequently used for heating buildings, tuning engines, or producing electricity.
The anaerobic treatment process does have some disadvantages which may limit the use of this process for certain industrial wastes./ The major disadvantage is that relatively high temperatures are required for optimum operation ; temperatures in the range from 850F to
950F are preferred. Dilute wastes may not produce sufficient methane for waste heating and this may represent a major limitation. This limitation suggests a need for more research
on low temperature anaerobic treatment, as there are indications that much lower temperatures can be used if the systems are adequately designed.
Another disadvantage of anaerobic treatment is related to the slow rate of growth of the methane producing bacteria.Because of it, longer periods of time are required for starting the process. This slow rate of growth also limits the rate at which the process can adjust to changing waste loads, temperatures, or other environmental conditions .The advantages of anaerobic treatment are quite significant, while the disadvantages are relatively few. The advantages normally far outweigh the disadvantages for more concentrated wastes, withBOD values greater than 10,000 mg/L. For less concentrated wastes, the disadvantages become more important, and may limit the use of this process. A noted exception is the successful anaerobic treatment of meat packing wastes with BOD concentrations as low as 1,000 mg/L. These wastes are fairly warm and the temperature requirement does not present a limitation[15].
2.4.Process Description
In anaerobic treatment, there are two basically different process designs. One is the "conventional process" most widely used for the treatment of concentrated wastes such as primary and secondary sludges at municipal treatment plants. The other process is one designed to handle more dilute waste and has been termed the "anaerobic contact process."2 Schematic diagrams of each process are shown in Fig.3. The conventional anaerobic treatment process consists of a heated digestion tank containing waste and bacteria responsible for anaerobic treatment. Raw waste is introduced either periodically or continuously and is preferably mixed with the digester contents. The mixed treated waste and microorganisms are usually removed together for final disposal. Sometimes this mixture is introduced into a second tank where the suspended material is allowed to settle and concentrate for more efficient disposal.
As the detention time in the conventional process is reduced, an increased percentage of bacteria are removed from the tank each day with the effluent. The limiting detention time is reached when the bacteria are being removed from the system faster than they can reproduce themselves, occurring after about three to five days at temperatures of operation of 950F. For practicalcontrol and reliable treatment,a detention time much above this, orabout ten to thirty days, is normally used. With dilute wastes, hydraulic detention times should be very short if the process is to be economical.
These are possible in the anaerobic contact process. Here, the bacteria are not lost with the effluent, but are maintained in the system. In this case, a digester is used. However, it is followed by a settling tank which removes the active biological suspended solids from the effluent stream for recycle back to the digester. This system is similar in operation to the activated sludge process and permits the maintenance ofa high biological population for rapid decomposition, while operating at a relatively low hydraulic detention time. Such a system has been found economical with wastes having BOD concentrations of about 1,000 mg/L and detention times of less than 6 to 12 hours. The gas produced in anaerobic treatment makes the suspended particles buoyant and difficult to settle. Therefore, a degasifier is frequently required between the digester and the settling tank in the contact process to permit proper settling of the suspended solids. A flotation process making use of the large quantities of dissolved gases to float and concentrate the solids for return to their 1g ester also appears feasible.