WASTEWATER (Chapter 22) Sewer – Sanitary waste Generated from buildings –Liquid discharged from plumbing fixtures Carried by sanitary sewer – Storm water Consists of rainwater, surface water, condensates, cooling water. Carried by storm sewer
Municipal sewage treatment Sedimentation Aeration Chlorination Dechlorination
Septic tank systems Construction of septic systems – Septic tank Provides primary treatment of waste through sedimentation Aerobic decomposition of sediments –Aerobic bacteria survives in the presence of oxygen – Seepage pit Provides secondary treatment of waste through leaching or filtration – Drainfield Provides secondary treatment of waste through leaching or filtration Percolation test
Sizing of septic tank Sized according to the number of bedrooms in residences According to the total number of fixtures for non- residential uses Sizing is also determined by the soil condition Once the capacity of the tank is determined in terms of gallons, it can be converted into cubic feet ( 1 cubic foot = 7.48 gallons)
Sizing a seepage pit Sized by determining the required leaching area Leaching area is the pit surface (wall area of the pit) required for seepage of effluent from the septic tank Wall area of a circular pit = * diameter of the pit * depth of the pit Assume an office building with 20 employees Daily sewage from the building = 20*20 = 400 gallons Minimum surface area required for seepage (assuming sandy loam soil condition): 40 sq.ft./100 gallons Required pit surface for 400 gallons = 400*40/100 = 160 sq.ft. Assuming the diameter of the pit to be 6 ft., depth of the pit = 160/( *6) = 8.5 ft.
Sizing a drainfield Sized by determining the required leaching area Leaching area of a rectangular drainfield trench per lineal foot of its length = (1/2*Depth of the trench)*2 + Width of the trench Assume an office building with 20 employees Daily sewage from the building = 20*20 = 400 gallons Minimum surface area required for seepage (assuming sandy loam soil condition): 40 sq.ft./100 gallons Required pit surface for 400 gallons = 400*40/100 = 160 sq.ft. Leaching area per foot of a drainfield (when trench section is 2’*2’ = (1/2*2)*2 + 2 = 4 sq. ft. Total trench length = 160/4 = 40 ft.
Sizing a drainfield Minimum number of trench lines required = 3 Length of each trench line = 40/3 = 13.33 ft. Maintain an o/c distance of 6 ft. between the trenches Provide at least 5 ft. on all sides Size of drainfield = (5+6*2+5)*(5+13.33+5) = 513.26 (say, 515 sq. ft.)
Lagoons, storm water Percolation test – Determines the absorption capacity of soil – Digging of holes, 4” dia 14” deep – 2” of course sand or gravel placed at bottom – Filling the holes with water, keeping it for at least 4 hrs. – Filled with 6 inches of water after the above soaking – Drop in water level measured every 10 minutes for one hour – Rate measured as drop in inches per minute Lagoons – Shallow ponds – Aerobic decomposition of waste – Relatively impervious sides and floors – Seven-day retention Storm water – Separate pipes for carrying storm water – Cross-connection with sanitary sewer to be avoided