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| Design Basis |
The impact of the plausible pollution prevention measures including waste segregation measures have to be assessed based on which characteristics of the combined waste water will have to be evaluated. Site characteristics and wastewater characteristics form an integral part of design basis. Pre-treatment standards for waters entering the collection system serving the CETP and treatment standards for effluents discharged from CETP also are significant design considerations. |
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| Site characteristics :- |
| Characteristics such as topography, soils, geology, hydrology, climate and land use are to be considered while designing a sewer network and a CETP. Topography and depth to bedrock effect the cost of sewer installation, for example elevation distributions that allow gravity flow and adequate depth for burial of pipe are most desirable. Soil thickness and soil characteristics like clay content, sand content, permeability etc. play a major role while deciding on certain treatment options such as land and lagoon treatment or granular media filtration etc. Climatic factors such as precipitation is important when inflow is a problem with sewers and evaporation is important when treatment processes being considered rely on evaporation of treated waste water. |
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| Wastewater characteristics :- |
| Key characteristics that must be considered in designing CETP are flow and physical and chemical characteristics of the wastewater. |
| Flow (m3/day or MLD)- It is important in determining the size of CETP. Minimum and maximum flows should be computed as they decide the hydraulic computations and the size of distribution pipes. Anticipated future increase should also be incorporated. Temporal flow variations require use of equalisation ponds to allow a constant flow rate through downstream processes. Mixing of waste water with lower concentration such as addition of sewage helps in reducing toxic shock on treatment processes. |
| Physical characteristics :- |
• Solids- Solids in the form of floating debris, grease and oil slicks indicate a highly polluted stream and suspended solids contribute to turbidity and silt load and require sedimentation or filtration for removal.
• Temperature- It is an important criterion as it affects chemical and biological reactions and solubility of gases such as oxygen. For example high temperatures increase reaction rates and solubility to a certain extent.
• Colour and odour- These serve as indicators of the degree of pollution of a waste stream and there presence in waste water indicate inadequate pre-treatment prior to discharge.
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| Chemical characteristics :- |
| Significant chemical characteristics include organics, inorganics in solution and gases. These are indicated by :- |
• BOD (mg/l)- Biological oxygen demand provides an indicator of the amount of organic substances of biological origin such as proteins, carbohydrates, fats and oils and biodegradable synthetic organic chemicals in water.
• COD (mg/l)- Chemical oxygen demand measures non-biodegradable as well as biodegradable organics. The ratio between BOD and COD provides an indicator of the ease of biological treatment.
• Pre-treatment standards- Wastewater from industrial processes requires some form of pre-treatment prior to discharge to CETP. This is mainly required 1) when waste water is carried through sewer lines to minimise corrosion and clogging of sewer lines and 2) to prevent reductions in biological treatment process efficiency by toxic effects from toxic concentration of organic and inorganic substances. Pre treatment standards for sulphides, sulphates and pH are concerned with preventing corrosion of concrete parts in sewers and limits to discharge of oil, grease, grit and heavy sediments prevent clogging of sewers. Limits to heavy metals and toxic organics ensure proper performance of biological treatment and minimise accumulation of contaminants in residual sludge.
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