Types of Biological Wastewater Treatment Methods!
Due to rapid change in industrialisation, huge volume of liquid effluent is generating which is a matter of concern. Liquid effluent treatment methods were developed in response to the concern for public health and the adverse conditions caused by the discharge of liquid effluents to the environment.
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To accelerate the forces of nature under controlled conditions in such facilities of comparatively small size that was the purpose of treatment of such waste. There is a wide range of biological treatment systems in use presently for the purification of liquid effluents based on the apparently simple processes by which mixed populations of micro-organisms degrade organic material, using it as a source of nutrients.
Biological waste treatment processes the process by which any waste treated biologically includes micro-organism for final discharge within the permissible limit specified by World Health Organisation (WHO).
There are three basic categories of biological waste treatment:
1. Aerobic
2. Anaerobic and Anoxic.
1. Aerobic Treatment Process:
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Aerobic treatment may follow some form of pretreatment such as oil removal, involves contacting wastewater with microbes and oxygen in a reactor to optimise the growth and efficiency of the biomass.
The micro-organism act to catalyse the oxidation of biodegradable organics and other contaminants such as NH3, generating CO2, H2O and excess biomass. This excess biomass is known as sludge.
2. Anaerobic (Without Oxygen) and Anoxic (Oxygen Deficient) Treatment Process:
Anaerobic (without oxygen) and anoxic (oxygen deficient) treatments are similar to aerobic treatment, but use of micro-organism that do not require the addition of oxygen these micro-organisms use the compounds other than oxygen to catalyse the oxidation of biodegradable organics and other contaminants.
Bioreactor is accomplished in one of two general ways:
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i. Fixed Film Process:
In fixed film process micro-organisms are held on a surface of film, the fixed film, which may be mobile or stationary with wastewater flowing past the surface of film media. This process is designed to actively contact the biofilm with the wastewater and is mixed with oxygen when needed. Fixed film based treatment processes are bio towers (trickling filters), rotating biological contactors, and submerged biological contactors.
ii. Suspended Growth Process:
Suspended growth process is the process in which biomass is freely suspended in the wastewater. In this process, components of liquid are properly mixed and can be aerated by number of devices that transfer oxygen to the bioreactor contents.
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Different aeration methods can be used in this growth process. Aeration methods for suspended growth processes are diffused aeration, surface aeration.
3. Micro-Organisms Involved in Aerobic Digestion Process:
Different types of aerobic micro-organisms are involved in this biological treatment process.
These are as follows:
i. Bacteria:
Bacteria are the most common type of micro-organisms and their number may be more than 1012 cell/ml. They are responsible for the removal of about 85-90% of the BOD remaining after primary treatment of liquid waste.
1. Bacterium ‘Zoogloea ramigera’ secretes a mucous like polysaccharide which is involved in the attachment of various bacterial species to the filter or disc surface. They degrade carbohydrates, proteins, lipids in to CO2, NO3–, SO4-2 and PO4-3.
2. Many heterotrophic bacteria are also responsible for aerobic oxidation- Saricina, Pseudomonas, Eschorichia, Stophylococaes, Streptococcus, Aerobactor, Shigella Salmonella.
3. Ammonium released from protein is toxic to fish if present in river water. Nitrifying bacteria – Nitrosomonas and Nitrobactor convert toxicity (NH4+ N03–) present in wastewater.
ii. Fungi:
Fungi usually applicable on surface of biofilm in filter beds. They may help in removal of nitrogen and phosphorus present in wastewater.
iii. Protozoa:
These are represented in wastewater by flagellates, ciliates and amoebae forms. The ciliate ‘vorticella’ is often used in activated sludge process.
4. Micro-Organisms in Anaerobic Treatment Process:
Bacteria responsible for anaerobic digestion may be divided in to three groups.
i. Group I – Hydrolytic Bacteria:
This group of bacteria is responsible for hydrolytic degradation process of macromolecules in to soluble products such as sugars, amino acids, and fatty acids. These bacteria belong to genera – Actinonyces, Aerobactorbacter, Escherichia, Klebsiell, Lactobacillus, Pseudomonos, Streptococcus, Streptomyces, etc.
These bacteria degrade polymeric substrates by use of enzymes such as amylases, cellulases, lipases, pectinases and proteases. These are responsible for hydrolysis process of proteins, lipids and polysaccharides.
ii. Group II – Acetogenic Bacteria:
In the second group the acetogenic bacteria ferment the end products of the first stage butyrate, propionate, caproate, glucose, amino acids, acetate, H2 and CO2.
1. Soluble carbohydrates, starch are fermented by micro-organism like species of ‘Clostridium’ resulting in the formation of acetic acid, butyric acid, CO2 and H2.
2. Glucose is utilised by other Clostridium species like ‘Clostridium thermoaceticum’ and produce acetate.
3. Homoacetogenic bacteria are unique in that they can convert 1 mol of glucose to 3 mols of acetate. Out of these 3 mols, 2 mols are formed by fermentation of glucose and one mole is formed by fixation of CO2.
4. Methanol fermentation also takes place by these organisms as follows-
iii. Group III – Methanogenic Bacteria:
Methanogenic bacteria are involved in the third stage of bioconversion of organic substrates in to methane. These belong to group ‘Archaebacteria’.
They are found in water logged soils, guts of animals, sewage sludge rotting vegetable and aquatic sediments.
These orders of strict obligate methanogens have been recognised as these are the methane bacteria including some species of Methanobacterium and Methanomicroblates including species of Methanomicrobium, Methanoganium Methanospirillum and Methanosarcina.
1. Methanogenes are non-motile and strict anaerobe.
2. Substrates such as CO2 and H2 have proved to be most ideal for the growth of methanogens.
3. Energy for their growth is derived from the reduction of CO2 to methane.
Some examples of methanogenic bacteria are- Methanobacterium bryantii, M. formicicum, M. Sochngenii, M. Thermoautotrophicum, Methanococcu vannielii, Methanococcus voltae, Methanogenium aggregans, Methanogenium marisnigri, Methanomicrobium mobile, Methanosarcina barkeri, and Methanobrevibacter smithii (Gut of human).
The two best described pathways involve the use of CO2 and CH3COOH as terminal electron acceptors and convert CO2 and CH3COOH in to CH4
5. Principle of Biological Treatment:
Basic principle behind the biological treatment process is that, waste containing complex organic material which having high BOD and COD and considered as highly polluted will be converted in to simple waste. By using selective biological treatment process, high BOD and COD will be reduced and complex organic material will be simplified in presence of microorganisms.
Different components as carbon source, nitrogen source and sulphur present in wastewater, will be converted in to respective products depending upon methods used for treatment either aerobic or anaerobic.
6. Aerobic Digestion/Treatment Process:
Aerobic digestion of waste is the natural biological degradation and purification process in which bacteria in presence of oxygen, breaks down and digested the waste. During oxidation process, pollutants are broken down in the CO2, H2O, nitrates, sulphates and biomass (micro-organisms). By operating the oxygen supply with aerators, the process can be significantly accelerated. It is most widespread process is used throughout the world.
7. Anaerobic Treatment/Digestion Process:
Anaerobic digestion is a complex biochemical reaction carried out in a number of steps using several types of micro-organisms that require little or no oxygen to survive. CH4 and CO2 are produced as end product during this process.
Aerobic digestion proceeds in 4 steps:
i. Hydrolysis:
Complex organic matter is decomposed in to simple soluble organic molecules using water to split the chemical bonds between the substances.
ii. Acidogenesis:
The decomposition of carbohydrate by enzymes, bacteria, yeasts or molds in the absence of oxygen.
iii. Acetogenesis:
The fermentative products are converted in to acetate, hydrogen and CO2 by acetogenic bacteria.
iv. Methanogenesis:
CH4 is formed from acetate and hydrogen/CO2 by methogenic bacteria.
Hydrolysis:
Complex materials such as lipids, proteins, and carbohydrates are primarily hydrolysed by extracellular, hydrolases, excreted by microbes present in stage-1. Hydrolytic enzymes, (lipases, proteases, cellulases, amylases, etc.), hydrolyse their respective polymers into smaller molecules, primarily monomelic units, which are then consumed by microbes.
In methane fermentation of wastewaters containing high concentrations of organic polymers, the hydrolytic activity relevant to each polymer is of paramount significance, in that polymer hydrolysis may become a rate-limiting step for the production of simpler bacterial substrates to be used in subsequent degradation steps.
Acidogenesis:
In this step, lipids will be to long-chain fatty acids using lipase enzyme. Population of lipase producing micro-organism should be in appropriate number. A population density of 104 – 105 lipolytic bacteria per ml of digester fluid has been reported. Clostridia and the micrococci appear to be potent extracellular lipase producers. The long-chain fatty acids present are further degraded by p-oxidation to produce acetyl CoA.
Protease enzyme hydrolyses proteins to amino acids. Micro-organisms like Bacteroides, Butyrivibrio, Clostridium, Fusobacterium, Selenomonas, and Streptococcus may be used for availability of protease enzyme. The amino acids produced during these steps are then degraded to fatty acids such as acetate, propionate, and butyrate.
Another component of the feed is polysaccharide such as cellulose, starch, and pectin. These polysaccharides are hydrolysed by cellulases, amylases, and pectinases enzymes.
The cellulases are of three types-
(i) Endo-glucanases,
(ii) Exo-glucanases,
(iii) Cellobiase or p-glucosidase.
These three enzymes act simultaneously on cellulose and hydrolysing its crystal structure, to produce glucose.
Microbial hydrolysis of starch requires amylolytic activity, which consist of five type amylase enzymes-
(i) α-amylases,
(ii) p-amylases,
(iii) Amyloglucosidases,
(iv) Debranching enzymes, and
(v) Maltase that acts on maltose liberating glucose.
Pectinase enzyme is required for pectin degradation. These includes pectinesterases and depolymerases. Five carbon sugar and six carbon sugars will be converted to C2 and C3 intermediates and to reduce electron carriers (e.g., NADH), via., common pathways.
Most anaerobic bacteria undergo hexose metabolism, via., the Emden-Meyerhof-Parnas pathway that produces pyruvate as an intermediate product along with NADH. The pyruvate and NADH are transformed into fermentation end products such as lactate, propionate, acetate, and ethanol by other enzymatic activities produced microbial species.
Acetogenesis and Dehydrogenation:
Acetate and H2 are directly produced by acidogenic fermentation of sugars, and amino acids, both products are primarily derived from the acetogenesis and dehydrogenation of higher volatile fatty acids. H2-producing acetogenic bacteria are potent to produce acetate and H2 from higher fatty acids. These are obligate micro-organisms.
The use of co-culture techniques incorporating H2 consumers such as methanogens and sulphate-reducing bacteria may therefore facilitate elucidation of the biochemical breakdown of fatty acids. The overall breakdown reactions for long-chain fatty acids are presented in.
Due to high free energy requirements, H2 production using acetogens is generally energetically unfavourable. Co-culture systems provide favourable conditions for the decomposition of fatty acids to acetate and CH4 or H2S. In addition to the decomposition of long-chain fatty acids, ethanol and lactate are also converted to acetate and H2 by acetogenic bacteria along with Clostridium formicoaceticum.
This produced hydrogen creates pressure in reaction vessel. The effect of the partial pressure of H2 will be on the free energy associated with the conversion of ethanol, propionate, acetate, and hydrogen or carbon dioxide during methane fermentation. Low partial pressure of H2 (10-5 atm) appears to be a significant factor in propionate degradation to CH4.
Methanogenesis:
Methanogens are physiologically united as methane producers in anaerobic digestion. In this anaerobic digestion, acetate and H2 or CO2 are the major substrates present in the natural environment. There are other substrate like formate, methanol, methylamines, and CO. These are also converted to CH4.
Methanogenic micro-organisms are obligate anaerobes and they require a redox potential of less than -300 mV for their growth. Isolation and cultivation of these microbes are tough task due to technical difficulties encountered in handling them under completely O2-free conditions. By using improved methanogen isolation techniques more than 40 strains of pure methanogens have now been isolated.
There are two major groups of methanogens-
(i) H2 and CO2 consumers, and
(ii) Acetate consumers.
Some of the H2 and CO2 consumers are also capable of utilising formate. Limited number of strains can consume acetate, such, as Methanosarcina species and Methanothrix species. These are not capable of consuming formate.
As a large quantity of acetate is produced in the natural environment, Methanosarcina and Methanothrix play an important role in completion of anaerobic digestion and in accumulating H2, which inhibits acetogens and methanogens.
H2-consuming methanogens are also important to reduce the levels of atmospheric H2. Methanogens reduce CO2 as an electron acceptor after consumption of H2 and CO2 via. the formyl, methenyl, and methyl levels through association with unusual coenzymes, to finally produce CH4.
The overall reaction can be expressed as:
CH3COOH → CH4 + CO2
In this reaction, a small part of the CO2 is also formed from carbon derived from the methyl group; it is suspected that the reduced potential produced from the methyl group may reduce CO2 to CH4.
Microbiological and Biochemical Aspects of Anaerobic Digestion:
The degradation of organic matter to produce methane is based on the complex interaction of several different groups of bacteria (consortia). For best performance of digester operation, it is require that these bacterial groups be in dynamic and harmonious equilibrium in the reaction vessel or digester.
Changes in environmental conditions can affect this equilibrium and result in the synthesis of intermediates which may affect the overall digestion process. It is important to understand the basic microbiological and biochemical pathways, in order to maximise digestion and production of biogas. Anaerobic digestion of complex organic material is summarised hereafter.
Fermentations of complex materials occurred through oxidation reduction reactions to produce hydrogen, carbon dioxide and acetic acid.
It has been reported the formation of methane from hydrogen and carbon dioxide:
4H2 + CO2 = CH4 + 2H2O
In above mentioned reaction, hydrogen reacted with carbon dioxide to form methane. It has been also assumed that the acetic acid produced was simply decarboxylated to form methane and carbon dioxide. Today the importance of maintaining a correct balance between the two phases is well recognised, and the two-phase concept is widely used in the control of the anaerobic process.
The four metabolic groups of micro-organisms, involved in anaerobic digestion include:
i. Hydrolytic and Fermentative Bacteria:
These bacterial groups digest and convert a variety of complex organic molecules (i.e., polysacharides, lipid and proteins) into acetic acid, H2 or CO2, monocarbon compounds, organic fatty acids larger than acetic, and neutral compounds larger than methanol, a broad spectrum of end products.
ii. Hydrogen-Producing Acetogenic Bacteria:
This group of bacteria includes both obligate and facultative species that can convert the products of the first group bacteria. For example, the organic acids larger than acetic acid (e.g., butyrate, propionate) and neutral compounds larger than methanol (e.g., ethanol, propanol) to hydrogen and acetate are the products.
iii. Homoacetogenic Bacteria:
This group of bacteria can convert very wide spectrum of multi or monocarbon compounds to acetic acid.
iv. Methanogenic Bacteria:
Methanogenic bacteria convert H2 or CO2, monocarbon compounds (i.e., methanol, CO, methylamine) and acetate into methane. These are also involved in decarboxylation of acetate and form methane.
8. Microbial Metabolism in Anaerobic Digestion:
Biodegradation of organic wastes into methane using mixed culture requires the co-ordinated metabolic activities of different microbial populations.
The key control parameters should be under control which influences the rate of organic degradation, the yield of reduced metabolites, and thermodynamic efficiency in the anaerobic digestion process. Effective digestion of organic matter requires the combined and co-ordinated metabolism of different kinds of carbon catabolising, anaerobic bacteria.
Four different types of bacteria group have been identified and isolated from anaerobic digesters and their function in anaerobic digestion. The methanogenic bacteria perform an important role in anaerobic digestion because their unique metabolism controls the rate of organic degradation. They also direct the flow of carbon and electrons, by removing toxic intermediary metabolites, and by enhancing thermodynamic efficiency of interspecies metabolism.
In order to understand the intermediate metabolism of anaerobic digestion, examination is needed of the metabolic factors. These metabolic factors control the rate of organic degradation, the flow of carbon and electrons, thermodynamic efficiency in pure and mixed culture, and the bacteria associated with biogas production. This fundamental examination also helps to identifies several control parameters that can be engineered to improve methanogenesis and anaerobic digestion processes.
9. Effluent Reuse:
Water shortages are becoming a serious problem in the developing countries due to rapid growth of population, urbanisation and industrialisation coupled with the introduction of modern intensive agricultural techniques causing increasingly heavy demand on water resources. Thus, the reuse of wastewater has become an attractive option for increasing water resources. Effluents from biological wastewater treatment plants may lack the characteristics required for direct reuse.
However, the most important uses of the reclaimed water are agricultural irrigation, fish rearing and ground-water recharge. The design of biological treatment facilities must ensure that the characteristics of the effluent are those required for reuse. Effluent will more often than not fail to meet these stringent standards and would require upgrading before reuse.
However, it should be noted that, for many places, these standards will be unnecessarily restrictive and they should be relaxed wherever feasible. Moreover, many effluent quality improvement methods are not appropriate for use in developing countries and are too costly.
The reuse of wastewater for agricultural purposes is an age-old and common practice. The quality of the reuse water is important both for the health of the workers and for the particular application for which it is used. Trace elements toxic to crops may be a problem; for example, boron, a component of many commercial laundry powders and one that is not removed by conventional treatment process, is well known as a toxicant in citrus fruit crops.
Effluent from biological treatment plants has been used for rearing of fish. The most popular fish which have been successfully reared in effluents include carp and tilapia. The silver carp and the big head are capable of direct feeding on the plankton. The carp’s popularity is largely due to its rapid growth rate and consequently high productivity under pond condition.
In Europe carp productivities from sewage-fed ponds are reported to range from 400 to 900 kg/ha year whereas through careful feeding and mixing of fish species, productivities as high as 5000-7000 kg/ha/year have been achieved. Pond productivities of about 1000 kg/ha year are not uncommon in Asia where night soil is applied and the pond is well maintained. Industrial wastes have also been used in fish culture.
Effluents can also be used for the purposeful recharge of groundwater. Such effluents should be free from heavy metals, with nitrogen and phosphorus contents of less than 50 and 10 mg/litre respectively and faecal coliforms always less than 1000/100 ml.
Domestic wastewater, fully treated in WSPs, has been successfully used in growing vegetables in gravels rather than soil according to the horticultural practice known as hydroponics. While fish and ducks are reared in the fish ponds, the overflow from these ponds is used for hydroponics and finally the overflow from the hydroponics basin is allowed to seep into the soil.
Such an arrangement is worth following in rural areas of developing countries. Hydroponics have a great future in countries like the Gulf and the Middle East where water is scarce and demand for vegetables can be met through hydroponics.
In municipal practice the effluent can be reused for road washing, arboriculture (along roads) and watering of lawns and parks. Effluents to be used for watering golf courses, street flushing, lawns, etc., should be chlorinated to keep the coliform count below 1000/100 ml.
Water Hyacinth:
Water hyacinth has been used successfully to upgrade effluents, particularly for the removal of algae. Water hyacinth is able to take up large amounts of nutrients, i.e., nitrogen and phosphorus and heavy metals.
At the same time its roots provide support for a gelatinous biomass which further stabilise organic matter, producing CO2, inorganic substances and other materials. Bacteria and other organisms adhere to the gelatin-covered paste. When the hyacinth is harvested, all these substances are removed from the water. Hyacinth grows very rapidly in hot climates, doubling its mass in about 6 days.
One hectare of a hyacinth-covered pond can produce more than 4 T wet weight of plants or 200 kg of dry solids per day, production of more than 290 kg/ha/day has been reported. Reductions of 80 per cent nitrogen and 44 per cent total phosphorus have been achieved by 0.55 ha of a hyacinth pond 0.6 m deep with a detention time of 24-48 hours, and fed with 1000 m3/day of facultative WSPs. Very low concentrations of ammonia nitrogen are present in water hyacinth ponds, which is important for fish rearing and clear, low- BOD effluents are produced.