adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Methods of treating sewage
Removing solids and some organic materials. (These are then dumped
or landfilled somewhere).
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.
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
This form of treatment is still somewhat rare, but this is
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
Excluding about 1% solids, household sewage is 99.9% water.
Biochemical Oxygen Demand
Chemical Oxygen Demand
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
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