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Definition of Water Consumption (W.C) It is the amount of water consumed by a community in one day. It is usual to express water consumption in litres/capita/day. Average water consumption is usually 150 to 600 lpcd. Classification of W.C according to Use No. Purpose Use Quantity 1 Domestic Sanitary, Drinking, Washing, Bathing, Cooking, Gardening etc. ~ 50-250 lpcd 2 Commercial and Industrial Markets, Office buildings, Dental clinics, Private Schools, Garages, Workshops etc. ~ 12.2 cube m/1000 sq. m of floor area/day 3 Public Use Public buildings i.e. Tower halls, Jails, Schools, Street washing, public toilets, gardens + Fire fighting ~ 10-20% of total water supply 4 Unaccounted Loss of water through leaks, Unauthorized connections ~ 10-15% of total water supply Terms related to Water Consumption There are some important terms related to water consumption which are listed below, Average Daily Consumption Maximum Daily Consumption Peak Hourly Consumption 1 - Average Daily Consumption It is the average amount of water consumed by a community in one day divided by the number of people served. 2 - Maximum Daily Consumption It is the maximum water consumption during any one day in the year. It is about 150 to 180% of average day consumption. 3 - Peak Hourly Consumption: The peak consumption during any hour of the year excluding fire demand is called Peak Hourly Consumption. It is around 150% of the maximum daily consumption. Ratios of water consumption used in Design Most common ratios that are being used in the design of water distribution system for a certain community are as follows, Maximum Day : Average Day = 1.5 : 1 Peak Hour : Maximum Day = 1.5 : 1 Peak Hour : Average Day = 2.25 : 1 The above ratios can be different depending upon the specific needs of the community and design criteria.
Sewer design requires prior knowledge of soil and site conditions to determine overburden loads that will be placed on buried pipes. Total Load = Backfill Load + Live Load Backfill Load (W): It depends upon following factors: Trench Width (B) Depth of Burial i.e, depth of fill above pipe (H) Unit Weight of Fill Material (w) Frictional Characteristics of Backfill Live Load : Live Loads on the Surface rarely influence design of sanitary sewer because of their Great Depth and Small Size. Backfill Load on Sewers Backfill load on buried pipes can be calculated using Marston's equation. W = CwB2 W = Load on the pipe per unit length, Kg/m w = Weight of the backfill material per unit volume, Kg/m3 B = Width of trench, m B = 1.5 D + 0.3 m (as minimum) D= Diameter of the sewer in m C = a coefficient depending upon depth of fill on top of the pipe and character of construction fill materials. μ = Coefficient of sliding friction k = Ratio of active lateral pressure to vertical pressure kμ = 0.1 to 0.16 for most soils (0.11 for saturated clay) H = Depth of fill above the top of the pipe, m B = Width of the trench, m For structural stability, strength of sewer as determined by 3-Edge Bearing test should be greater than backfill load on the Sewer.
What is Activated Sludge? It is a FLOC (i.e body of micro-organisms gathered in a crowd) produced in a raw or settled sewage by the growth of bacteria and other organisms in the presence of dissolved oxygen and accumulated in sufficient concentrations by returning floc previously formed. Activated Sludge Process Activated Sludge Process is the biological method for treatment of wastewater. It was devised by Arden and Lockett in 1914. In this process a mixture of sewage and activated sludge is agitated and aerated in an Aeration Tank. Bacteria present in the activated sludge aerobically metabolize the organic matter present in the influent. The organic matter is oxidized to CO2, H2O, NH3 etc. and a portion of it is converted into new bacterial cells. The activated sludge is subsequently separated from the Mixed Liquor (mixture of sewage and activated sludge in the aeration tank) by sedimentation in the Final clarifier and wasted or returned to the aeration tank as needed. The treated effluent overflows the final clarifier. Sludge Settleability The degree of treatment in ASP depends upon the settleability of sludge in the final clarifier. The biological floc settles by gravity and leaves a clear supernatant for disposal. However, if filamentous micro-organisms grow in the aeration tank, they do not separate by gravity and contribute to BOD and Suspended Solids in the effluent. Excessive carry over of FLOC, resulting in the inefficient operation of final clarifier is referred to as Sludge Bulking. Conditions Promoting Growth of Filamentous Micro-Organisms Insufficient Aeration - Dissolved Oxygen (DO) Level Lack of Nutrients -Nitrogen (N), Phosphorous (P) Presence of toxic substances Low pH - Promotes fungal growth Over Loading i.e. High Food : Micro-Organisms (F:M) Ratio F : M Ratio F:M ratio is expressed in terms of kg of BOD applied per day per kg of MLSS. If Q is the sewage flow in cu meter per day and it has a BOD expressed in mg per liter then: If V is the volume of aeration tank in cu meter and it has MLSS concentration expressed in mg per liter then: Where t is the Aeration Time in days Sludge Volume Index (SVI) SVI Indicates sludge settling characteristics. It is the volume in ml occupied by one gram of settled suspended solids. An SVI from 50 to 150 indicates good settling characteristics. Advantages of ASP High BOD removals (greater than 95%) Low land areas required Odour free operation Treats industrial wastewater well Disadvantages of ASP Extremely sensitive and sophisticated Skilled operation is needed Sludge bulking problem High operating costs
The following points should be kept in mind while selecting pipe for a certain water supply system, Carrying capacity. Durability. Fire cost. Maintenance cost. Type of water to be conveyed. 1 - Cast Iron Pipes (C.I) Most widely used for the city water supplies. Average life is 100 years. Corrosion my reduce its capacity by 70%. Must be lined with cement or bitumen. C = 130 for new pipe. C = 100 for old pipe (Selected for Design). "C" is the Hazen Williams Coefficient known as HWC. It is the important term used in the design of water distribution system. 2 - Steel Pipes Contains less carbon than Cast Iron pipes. Frequently used for trunk mains. Difficult to make connections hence seldom used for water distribution systems. Much Stronger and lighter than Cast Iron pipes. Cheaper than Cast Iron pipes. Cannot withstand vacuum, hence collapse. Highly susceptible to corrosion, hence high maintenance charges are required. 3 - Ductile Pipes Similar to Cast Iron pipes except increased ductility. Ductile iron is produced by adding a controlled amount of Mg into molten iron of low sulphur and phosphorous content. Stronger, tougher and elastic than Cast Iron pipes. More expensive than Cast Iron pipes. 4 - Galvanized Iron (G.I) Pipes Manufactured by dipping Cast Iron pipe in molten zinc. Resistant to corrosion. Mainly used for plumbing. 5 - Concrete Pipes Usual size of Reinforced Cement Concrete pipe is 400mm dia. and above. Not subjected to corrosion. Manufactured at or near site. Average life is 75 years. C = 138 to 152. 6 - Asbestos Cement Pipes (A.C) Sizes are 100mm to 600mm dia. Average life is 30 years. Immune to actions of acids, salts, soil and corrosion. Less cost for laying and jointing. Less plumbing cost due to less friction. C = 140. Asbestos Cement pipes are economical and are generally preferred to use in the design of water supply systems. 7 - Poly Vinyl Chloride Pipes (PVC) Mainly used for domestic plumbing. Easy to install and easy to handle. Cheaper in material cost Weak to sustain load. Only available 350mm dia size. Expected life is 25 years.