Waste treatmentWaste treatment

SUPPLY

TREATMENT

WASTE

TREATMENT

SOURCESUPPLY &

WASTE CYCLE

WASTEWATER (Chapter 22)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 treatmentMunicipal sewage treatment

Sedimentation Aeration Chlorination Dechlorination

Septic tank systemsSeptic 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 tankSizing 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 pitSizing 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 drainfieldSizing 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 drainfieldSizing 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 waterLagoons, 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