Quds University Small Scale Waste Water Treatment Plants in West ...

After this step, the water is discharged into the receiving body, unless a further, advanced
treatment process step is required (Metcalf & Eddy, 1991).
1.11.5 Tertiary treatments
Tertiary (or advanced) treatment may be defined as any treatment process in which unit
operations are added to the secondary treatment. These treatments are necessary to
remove nitrogen, phosphorus, additional suspended solids, refractory organics,
heavy metals and dissolved solids (Metcalf & Eddy, 2003).
1- Filtering
Filtering is a tertiary treatment particularly suitable when it is necessary to
reduce significatively the suspended solids, for example in wastewater reuse. This
treatment is also applied for water recycling in industrial processes. In case of disk filters,
the water flows by gravity into the segments of the filter from the drum set at the center.
The solids are separated from the water by the fine filter mounted on the two sides of the
segments (generally filtration degree >10-20 micron depending on specific aim). When the
screen stops, the suck back cycle starts and the solids are delivered to the collection tank
(Erbe, 2002).
2- Nutrient removal
In wastewater treatment, nutrient removal generally refers to compounds of nitrogen and
phosphorus. Excessive levels of these substances in the outflow is the primary
cause of eutrophication in surface waters. The scope of the nutrient removal stage is then
to reduce the concentrations of phosphorus and nitrogen in the wastewater so as to prevent
algal blooms and excessive aquatic plant growth in the receiving waters.
The long-term effects of over-enrichment include low dissolved oxygen, fish kills, and
depletion of desirable flora and fauna. In the course of biological treatment only a small
amount of nutrients are taken up by microorganisms so that it could be necessary add a
further treatment unit to remove the residual load. Nutrients can be removed from
wastewater using physical-chemical or biological processes. Phosphorus removal is usually
achieved by chemical precipitation with salts of iron (e.g. ferric chloride), aluminum
(e.g. alum), or lime to form chemical flocs. These flocs are then settled out to remove
phosphorus from the wastewater. Chemical phosphorus removal requires significantly
smaller equipment than biological removal, is easier to operate and is often more reliable
16