About half of the particles and a third of the BOD are removed from raw wastewater by primary treatment of wastewater. Secondary treatment is the process of removing biodegradable organic compounds (in solution or suspension), and suspended particles.
In addition to being often used in the treatment of municipal wastewater, secondary treatment is also appropriate for a variety of agricultural and industrial wastewaters. Now, let’s see the kind of pollutants removed by secondary treatment in wastewater treatment plants.
Which pollutants are removed by secondary treatment in wastewater treatment plants?
In a wastewater treatment facility, the objective is to reach a certain level of wastewater quality that is suitable for disposal or reusing.
In a "first treatment" technique that typically comes before secondary treatment, physical phase separation is performed to remove settleable elements.
In secondary treatment, biological processes are used to remove dissolved and suspended organic contaminants, which is measured as the biochemical oxygen demand (BOD). The following contaminants are among those that secondary treatment removes:
1. Suspended solids,
2. Colloidal solids,
3. Dissolved solids,
5. Biological organic matters such as:
a) Spermophyta – Water weeds.
b) Bryophyta – Mosses and lever words.
c) Pteridophyta – Ferns and horsetails.
d) Thallophyta – Algae
e) Worms – Aquatic earthworms, thread worms, rotifer.
f) Metazoa –Hydra, polyzoa & protozoa – Endameba histolytica etc.
g) Molds (or fungi), bacteria, viruses
Biochemical Oxygen Demand (BOD)
The biochemical oxygen demand (BOD) is a measurement of the quantity of oxygen, necessary for wastewater-derived microbes to oxidize the organic matter in a sample. It is the organic contamination parameter that is most frequently utilized in both wastewater and surface water.
Thus, the BOD is the quantity of oxygen necessary for microorganisms to decompose dissolved solids, or organic matter in wastewater biologically under aerobic circumstances at room temperature.
Processes that help in removing organic and dissolved pollutants through secondary treatment!
· Activated Sludge
It is a common secondary treatment method. Dissolved oxygen is used in activated sludge facilities to encourage the production of biological floc, which considerably reduces organic waste.
· Sequencing Batch Reactors
A system that combines secondary treatment and settling is a cyclic activated sludge (CASSBR) or sequencing batch reactor (SBR). Raw incoming sewage is frequently mixed with activated sludge, before being blended and aerated.
· Membrane Bioreactors
A liquid-solid phase separation membrane is used in membrane bioreactors, which are activated sludge systems. The membrane component uses low-pressure microfiltration or ultrafiltration membranes, to eliminate the need for a secondary clarifier or filtration.
Since, CAS systems are constrained by sludge settling, the approach enables bioreactors to function at much higher, mixed liquor suspended solids (MLSS) concentrations than CAS systems.
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