Depending on the availability of oxygen the processes involving growth of microorganisms can be classified as aerobic and anaerobic. Though these two are important, there is another process called anoxic which is a biological environment that is deficient in molecular oxygen, but may contain chemically bound oxygen,such as nitrates and nitrites is called anoxic. Biological waste treatment processes are also classified as suspended growth and attached growth processes.
Suspended Growth Processes: In suspended growth processes, the microorganisms, which are responsible for the conversion of organic or the other matter in the wastewater to gases and cell tissues are maintained in suspension with in the liquid.
Attached Growth Processes: In attached growth process, the microorganisms,which are responsible from the conversion of organic or the other matter in the wastewater to gases and cell tissue, are attached to sum inert material.
Sludge Treatment: Sludge produced in biological treatment processes of liquid effluents needs further treatment. The sludge from the various stages of treatment is collected in thickening tanks to which chemicals are added to facilitate further aggregation of the solid particles. To further break down organic matter and to reduce foul-smelling substances, the sludge is eventually pumped into a digester,where the organic substances are broken down under anaerobic conditions into carbon dioxide and methane are the main components of digester, which can be utilized as fuel for heating. Digester sludge is a homogeneous, practically odourless, dark-coloured substance which still has high moisture content of 94-97%. It is therefore dewatered most effectively in a decanter centrifuge which discharges a solid phase of about one-eighth of the original volume. The dewatered sludge can then be utilized as fertilizer or landfill or simply deposited as waste.
Aerobic Biological Treatment : Biological means can be utilized to rapidly and efficiently remove bio-degradable organic materials from waste water if a suitable environment is provided. Treatment processes that occur in the presence of oxygen are called aerobic process. Supply of air or oxygen is important in these processes. Aeration and mixing are the two high energy demanding operations in aerobic processes. Optimal conditions of food, oxygen, nutrients and pH, and mixed culture of microbes are the essential factors for proper aerobic systems. pH ranges between 5 and 9. The waste water should not contain toxic substances. With prior removal of fat, dairy effluent can be easily made suitable for aerobic biological treatment system. In the aerobic biological treatment of dairy waste, three distinct phases have been identified:
Rapid initial incorporation of the milk constituents, both dissolved and suspended, into the biomass block. This may be due to an adsorption phenomenon or possibly formation of a complex between the bacterial cells in the flock and the protein and lipids. The extent of this rapid incorporation or adsorption depends on the organic loading ratio (mass of BOD 5 in the waste water per unit mass of suspended cells) and the ecological condition of the organic matter.Oxidation of the organic material, both adsorbed and in solution, by bacterial assimilation. The organic material in the dairy waste is partially synthesized to new cell material and partially oxidized to supply the energy needed for the growth of new cell material. The rate of assimilation (oxidation) is only about 10% of the adsorption in the first stage. It is reported that protein in milk waste is not utilized until all the lactose has been degraded in this phase.
Near exhaustion of organic matter in waste water results in death of some microbes due deficiency of nutrients. Endogenous respiration in which the living cells feed on the substances of the dead bacterial cells. These are oxidized, with the production of simple compounds such as water, ammonia and carbon dioxide.
The rate of oxygen consumption in this phase is 10% of that required during the assimilation phase.Anaerobic Biological Treatment: The decomposition by microorganisms of organic matter in wastewater in the absence of dissolved oxygen is classed as anaerobic digestion. Anaerobic digestion is a process, which occurs widely in nature. Anaerobic digestion is generally considered to take place in two stages - acid formation and gas formation. In the first stage a group of microorganisms breaks down carbohydrates, fats and proteins into simpler substances such as alcohols and volatile fatty acids. Some of the substances are used as food by the organisms and some persist in the sludge, but most of the lower volatile fatty acids (acetic, propionic, and butyric acids) will be converted to methane and carbon dioxide by a different group of organisms. This operation constitutes the second stage of the digestion process.
The actual processes are in fact much more complex than this.
The anaerobic microorganisms are very sensitive to pH and other environmental conditions. In anaerobic conditions, the growth of microorganisms is slow. While the first factor results in loss of biomass, the second factor inhibits the replenishment lost biomass. Various types of reactors have been developed in an attempt to improve the treatment efficiencies and to overcome the loss of anaerobic microbes.Among the important factors believed to affect the treatment efficiencies are the type of reactors used, the characteristics of wastewater to be treated, the hydraulic regime in the reactor, the concentration of microbes and types of microbes in the reactor. High rate anaerobic reactors are becoming popular for the treatment of various types of wastewaters because of their low initial and operational cost,smaller space requirement, high organic removal efficiency and low sludge production, combined with a net energy benefit through the production of biogas. The term ‘high-rate’ was once used for the designs of sewage sludge digesters, but it is now widely used to refer to anaerobic treatment systems meeting at least the following two conditions:
a) High retention of viable sludge under high loading conditions, and
b) Proper contact between incoming wastewater and retained sludge. Among the various anaerobic reactors developed so far, the Upward Anaerobic Sludge
Blanket (UASB) Reactors have been found to be relatively superior because it is simpler and more economical and it neither requires added substratum as in anaerobic filters, nor effluent recirculation as in fluidized bed reactors. Also, solids retention times can be maintained at a high level even at a low hydraulic retention time (HRT) by the development of a granular sludge bed.
The biological treatment systems generally used in dairy wastewater treatment include aerated lagoon, trickling filters, rotating biological contactors, Upward Anaerobic Sludge Blanket (UASB) and activated sludge treatment. These will be discussed in the following sections.
Suspended Growth Processes: In suspended growth processes, the microorganisms, which are responsible for the conversion of organic or the other matter in the wastewater to gases and cell tissues are maintained in suspension with in the liquid.
Attached Growth Processes: In attached growth process, the microorganisms,which are responsible from the conversion of organic or the other matter in the wastewater to gases and cell tissue, are attached to sum inert material.
Sludge Treatment: Sludge produced in biological treatment processes of liquid effluents needs further treatment. The sludge from the various stages of treatment is collected in thickening tanks to which chemicals are added to facilitate further aggregation of the solid particles. To further break down organic matter and to reduce foul-smelling substances, the sludge is eventually pumped into a digester,where the organic substances are broken down under anaerobic conditions into carbon dioxide and methane are the main components of digester, which can be utilized as fuel for heating. Digester sludge is a homogeneous, practically odourless, dark-coloured substance which still has high moisture content of 94-97%. It is therefore dewatered most effectively in a decanter centrifuge which discharges a solid phase of about one-eighth of the original volume. The dewatered sludge can then be utilized as fertilizer or landfill or simply deposited as waste.
Aerobic Biological Treatment : Biological means can be utilized to rapidly and efficiently remove bio-degradable organic materials from waste water if a suitable environment is provided. Treatment processes that occur in the presence of oxygen are called aerobic process. Supply of air or oxygen is important in these processes. Aeration and mixing are the two high energy demanding operations in aerobic processes. Optimal conditions of food, oxygen, nutrients and pH, and mixed culture of microbes are the essential factors for proper aerobic systems. pH ranges between 5 and 9. The waste water should not contain toxic substances. With prior removal of fat, dairy effluent can be easily made suitable for aerobic biological treatment system. In the aerobic biological treatment of dairy waste, three distinct phases have been identified:
Rapid initial incorporation of the milk constituents, both dissolved and suspended, into the biomass block. This may be due to an adsorption phenomenon or possibly formation of a complex between the bacterial cells in the flock and the protein and lipids. The extent of this rapid incorporation or adsorption depends on the organic loading ratio (mass of BOD 5 in the waste water per unit mass of suspended cells) and the ecological condition of the organic matter.Oxidation of the organic material, both adsorbed and in solution, by bacterial assimilation. The organic material in the dairy waste is partially synthesized to new cell material and partially oxidized to supply the energy needed for the growth of new cell material. The rate of assimilation (oxidation) is only about 10% of the adsorption in the first stage. It is reported that protein in milk waste is not utilized until all the lactose has been degraded in this phase.
Near exhaustion of organic matter in waste water results in death of some microbes due deficiency of nutrients. Endogenous respiration in which the living cells feed on the substances of the dead bacterial cells. These are oxidized, with the production of simple compounds such as water, ammonia and carbon dioxide.
The rate of oxygen consumption in this phase is 10% of that required during the assimilation phase.Anaerobic Biological Treatment: The decomposition by microorganisms of organic matter in wastewater in the absence of dissolved oxygen is classed as anaerobic digestion. Anaerobic digestion is a process, which occurs widely in nature. Anaerobic digestion is generally considered to take place in two stages - acid formation and gas formation. In the first stage a group of microorganisms breaks down carbohydrates, fats and proteins into simpler substances such as alcohols and volatile fatty acids. Some of the substances are used as food by the organisms and some persist in the sludge, but most of the lower volatile fatty acids (acetic, propionic, and butyric acids) will be converted to methane and carbon dioxide by a different group of organisms. This operation constitutes the second stage of the digestion process.
The actual processes are in fact much more complex than this.
The anaerobic microorganisms are very sensitive to pH and other environmental conditions. In anaerobic conditions, the growth of microorganisms is slow. While the first factor results in loss of biomass, the second factor inhibits the replenishment lost biomass. Various types of reactors have been developed in an attempt to improve the treatment efficiencies and to overcome the loss of anaerobic microbes.Among the important factors believed to affect the treatment efficiencies are the type of reactors used, the characteristics of wastewater to be treated, the hydraulic regime in the reactor, the concentration of microbes and types of microbes in the reactor. High rate anaerobic reactors are becoming popular for the treatment of various types of wastewaters because of their low initial and operational cost,smaller space requirement, high organic removal efficiency and low sludge production, combined with a net energy benefit through the production of biogas. The term ‘high-rate’ was once used for the designs of sewage sludge digesters, but it is now widely used to refer to anaerobic treatment systems meeting at least the following two conditions:
a) High retention of viable sludge under high loading conditions, and
b) Proper contact between incoming wastewater and retained sludge. Among the various anaerobic reactors developed so far, the Upward Anaerobic Sludge
Blanket (UASB) Reactors have been found to be relatively superior because it is simpler and more economical and it neither requires added substratum as in anaerobic filters, nor effluent recirculation as in fluidized bed reactors. Also, solids retention times can be maintained at a high level even at a low hydraulic retention time (HRT) by the development of a granular sludge bed.
The biological treatment systems generally used in dairy wastewater treatment include aerated lagoon, trickling filters, rotating biological contactors, Upward Anaerobic Sludge Blanket (UASB) and activated sludge treatment. These will be discussed in the following sections.
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