i. Treatment Methods
The treatment methods can be categorized as physical, chemical and biological treatment. Since the major part of dairy wastewater are biodegradable organic substances, the biological treatment forms the backbone waste treatment. To increase the efficiency of biological treatment, the load of organic substances should be decreased as much as possible by using physical and chemical methods in pretreatment. Pretreatment of wastewater generally consists of screens, grit chamber, oil and grease trap, chemical precipitation, neutralization, and air flotation (to remove fats and solids) and flow equalization.
ii. Screening and Removal of Oil and Fat
Screens help in retaining bigger particles, pieces of damaged packaging material, rags, and other large debris. These will clog the pipe lines and damage pumps if screens are not installed. The matter is continuously scraped off the screen and disposed off separately, usually as landfill. After screening, the effluents are led to a grit chamber. Grit chamber is a basin in which coarse separation takes place. It is dimensioned and operated in such a way that sand and other heavy particulates have time to settle down to the bottom. The effluents from grit chamber flow to oil and grease trap. Being hydrophobic material and lighter than water, free floating oil and grease will accumulate at the top. Provision will be there in oil and grease trap to skim the top layer. Fat removal is done before any pumping or chemical addition for pH correction, since de-emulsified fats are removed easily at lower pH.Lime addition raises the pH and again emulsifies the fats and also pumping has a churning effect, which emulsifies the fats.
iii. Flow Equalization
The wastewater is then directed into flow equalization tank. The flow rate and the organic matter content of the wastewater is not constant and varies with time during a day. The purpose of flow equalization tank is to reduce these variations.Two tanks are generally provided, so that even when cleaning of one of the tanks is in progress, the equalization of effluent does not suffer. It is remarkable that in case of dairy effluents a holding time of 10-12 hours is sufficient, more holding of this effluent may lead to septic conditions and release foul odours. Equalization tank of waste water containing biodegradable organic matter is generally provided withaeration mixing in order to promote aerobic micro organisms thus reducing the chance of these nuisance odours.
iv. Chemical Treatment
If the pH of wastewater is expected to vary too much, then provision should be made to adjust pH by addition of acid or alkali. This is important because all the treatment processes are affected by pH. Limestone is generally used for neutralization of acidic waste water and sulphuric acid is commonly used for basic waste water.
v. Chemical Precipitation
Another optional chemical treatment is chemical precipitation. Addition of chemicals leads to the alteration of physical state of dissolved and suspended solids which facilitates their easy settling in sedimentation tanks. The principal purpose of this chemical treatment is to remove the phosphorus and other settleable organic matter from wastewater. The precipitation stage starts with flocculation tanks where the flocculants are added and vigorously mixed into the water by agitators. This results in precipitation of insoluble phosphates and organic matter, initially in the form of very fine particles which, however, gradually aggregates into larger flocs. The flocs settle out in pre-sedimentation basins from which a clear effluent overflows into the basin for biological treatment. Pre-sedimentation is the final step in the combined physical and chemical treatment. The water is allowed to flow slowly through one or more basins where the finer particles gradually settle to the bottom as primary sludge. The sedimentation basins are equipped with devices that continuously scrape the sediment into a sump and transverse gutters that carry off water from the clarified surface layers. The key to success of this treatment is the correct dose of lime and coagulant. Excellent results can be obtained and are being achieved using the combination of like alum, Lime-FeSO 4 in the correct doses. About 30% of organic matter is removed by this treatment. The chemical treatment requires a constant feed back from the laboratory for dose optimization based on application of varied doses of lime and alum to the effluent from the equalization tank. Once the optimum dose is established and the flow rate of effluent is known, the proportionate chemical flow rate to achieve a desired dose can be easily worked out.
The treatment methods can be categorized as physical, chemical and biological treatment. Since the major part of dairy wastewater are biodegradable organic substances, the biological treatment forms the backbone waste treatment. To increase the efficiency of biological treatment, the load of organic substances should be decreased as much as possible by using physical and chemical methods in pretreatment. Pretreatment of wastewater generally consists of screens, grit chamber, oil and grease trap, chemical precipitation, neutralization, and air flotation (to remove fats and solids) and flow equalization.
ii. Screening and Removal of Oil and Fat
Screens help in retaining bigger particles, pieces of damaged packaging material, rags, and other large debris. These will clog the pipe lines and damage pumps if screens are not installed. The matter is continuously scraped off the screen and disposed off separately, usually as landfill. After screening, the effluents are led to a grit chamber. Grit chamber is a basin in which coarse separation takes place. It is dimensioned and operated in such a way that sand and other heavy particulates have time to settle down to the bottom. The effluents from grit chamber flow to oil and grease trap. Being hydrophobic material and lighter than water, free floating oil and grease will accumulate at the top. Provision will be there in oil and grease trap to skim the top layer. Fat removal is done before any pumping or chemical addition for pH correction, since de-emulsified fats are removed easily at lower pH.Lime addition raises the pH and again emulsifies the fats and also pumping has a churning effect, which emulsifies the fats.
iii. Flow Equalization
The wastewater is then directed into flow equalization tank. The flow rate and the organic matter content of the wastewater is not constant and varies with time during a day. The purpose of flow equalization tank is to reduce these variations.Two tanks are generally provided, so that even when cleaning of one of the tanks is in progress, the equalization of effluent does not suffer. It is remarkable that in case of dairy effluents a holding time of 10-12 hours is sufficient, more holding of this effluent may lead to septic conditions and release foul odours. Equalization tank of waste water containing biodegradable organic matter is generally provided withaeration mixing in order to promote aerobic micro organisms thus reducing the chance of these nuisance odours.
iv. Chemical Treatment
If the pH of wastewater is expected to vary too much, then provision should be made to adjust pH by addition of acid or alkali. This is important because all the treatment processes are affected by pH. Limestone is generally used for neutralization of acidic waste water and sulphuric acid is commonly used for basic waste water.
v. Chemical Precipitation
Another optional chemical treatment is chemical precipitation. Addition of chemicals leads to the alteration of physical state of dissolved and suspended solids which facilitates their easy settling in sedimentation tanks. The principal purpose of this chemical treatment is to remove the phosphorus and other settleable organic matter from wastewater. The precipitation stage starts with flocculation tanks where the flocculants are added and vigorously mixed into the water by agitators. This results in precipitation of insoluble phosphates and organic matter, initially in the form of very fine particles which, however, gradually aggregates into larger flocs. The flocs settle out in pre-sedimentation basins from which a clear effluent overflows into the basin for biological treatment. Pre-sedimentation is the final step in the combined physical and chemical treatment. The water is allowed to flow slowly through one or more basins where the finer particles gradually settle to the bottom as primary sludge. The sedimentation basins are equipped with devices that continuously scrape the sediment into a sump and transverse gutters that carry off water from the clarified surface layers. The key to success of this treatment is the correct dose of lime and coagulant. Excellent results can be obtained and are being achieved using the combination of like alum, Lime-FeSO 4 in the correct doses. About 30% of organic matter is removed by this treatment. The chemical treatment requires a constant feed back from the laboratory for dose optimization based on application of varied doses of lime and alum to the effluent from the equalization tank. Once the optimum dose is established and the flow rate of effluent is known, the proportionate chemical flow rate to achieve a desired dose can be easily worked out.
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