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sewage treatment

Sewage Treatment

Contents of Entire Course

Methods of treating sewage
Sources of Household sewage
Composition of Household Wastewater
Septic Tank System

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University

Methods of treating sewage

• Primary Treatment
• Removing solids and some organic materials. (These are then dumped or landfilled somewhere).
• Secondary Treatment
• The main purpose of Secondary treatment is to reduce BOD (Biochemical Oxygen Demand)
• Sludge dominantly made up of bacteria is a by-product.
• Many plants use the activated sludge process in which sludge is added to more effectively reduce BOD
• This form of treatment is standard now.
• Tertiary Treatment
• Major purpose is to reduce nitrogen and phosphates.
• Nitrogen and phosphates act as fertilizers, and can cause serious problems when they get into our streams, ponds, lakes, oceans and sounds by enhancing the growth of algael blooms which results in eutrophication.
• This form of treatment is still somewhat rare, but this is changing.
• Tertiary treatment uses bacteria to denitrify  nitrates in the water producing nitrogen gas which is then released into the atmosphere(NH3 --> NO3 --> N2).

Household sewage is comes from a variety of sources. 

Sources of Household sewage

 

Toilets	40%
Bathing	30%
Laundry	15%
Kitchen	10%
Miscellaneous	5%

Excluding about 1% solids, household sewage is 99.9% water. 

Composition of Household Wastewater

 

Constituent Materials	Concentrations (ppm)
Dissolved Solids	500
Suspended Solids	200
Settleable Solids	(1%)
Biochemical Oxygen Demand (BOD) material	200
Chemical Oxygen Demand (COD) material	500
Nitrogen	40
Organic	15
Free Amonia	25
Nitrite	0
Nitrate	0
Phophorus	10
Organic	3
Inorganic	7
Chloride	50
Alkalines	100
Grease	100

Septic Tank System
 
In many areas the main type of sewage disposal is a septic tank system.

In the diagram,  sewage travels to a leaching field after it goes through the septic tank. This is needed in places where they do not have good drainage.

Sandy sediments are necessary in order to oxidize the sewage water, and filter out the bacteria from it.

   

Ammonium that travels through the unsaturated zone is oxidized and converted to nitrate by the time it reaches the water table. Thus, areas with high populations that rely on septic tank systems have to be concerned about the possibility of the groundwater achieving  high nitrate concentrations.