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).
Sources of Household sewage
Excluding about 1% solids, household sewage is 99.9% water.
Composition of Household Wastewater
|Constituent Materials||Concentrations (ppm)|
Septic Tank System
In many areas the main type of sewage disposal is a septic tank system.
Ammonium that travels
through the unsaturated zone is oxidized and converted to nitrate by
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.