As the given situation describes, eutrophication is a result of the presence of nitrogen and phosphorus in river since no protection methods are carried out to end the release of waste into the water bodies. Such rivers are commonly known as sensitive rivers. These nutrients excite the production of algae providing food for their growth under the accurate circumstances thus causing eutrophication. Eutrophication can harm the ecosystem causing unwanted alteration in aquatic population, extreme reduction of oxygen, health issues to human-beings and animals, expensive water treatment costs and, interfering with recreational significance of water (Kuba et al., 1997). Thus the wastewater must be maintained from nitrogen and phosphorus before disposing into water bodies. The removal of such harmful nutrients must be carried out efficiently in the secondary treatment itself.
Wastewater treatment includes two procedures as standard: Primary Treatment (elimination of rock-solid materials) and Secondary Treatment that uses microorganisms to eradicate organic wastes by disintegration. Tertiary treatment may also be mandatory in case of removing phosphorus and nitrogen when wastewater is discharged into sensitive rivers.
The biological secondary treatment is the major type of treatment for elimination of the organic materials from wastewater (up to 90% elimination). The two usual procedures used in the treatment are:
• Attached Growth Method (Fixed film system)
• Suspended Growth Method (Free-film system).
Each of the above types has been described within the subsequent section.
3.2 Activated Sludge Treatment (AST) – Free Film System
This is the most familiar suspended development method used for wastewater treatment to eliminate BOD and other suspended solid materials. It consists of the subsequent steps:
(a) An aeration tank, in which microorganisms are held in reserve with wastewater.
(b) Solid – liquid partition and release of the clarified effluent
(c) Elimination of surplus biomass (sludge)
(d) Return of a fraction of the sludge to the aeration tank using a sludge reuse method.
In this process the added sewage from the primary treatment is left in the aerator for a particular duration throughout which micro-organisms utilizes the organic material as food to generate steady solids (flocs) that stays suspended in the activated sludge and is detached in the settling tank. Some portion of the sludge are recycled to the aeration tank to shape a Mixed Liquor Suspended Solids (MLSS) and the surplus sludge is separated at equal intervals which might otherwise diminish performance and break out to runoff tank.
Fig. 2.2.(a). Activated Sludge Wastewater Treatment Flow Diagram
Some of the merits are
(a) BOD, COD and nutrients consisting nitrates and phosphates are effectively eliminated.
(b) Effluent is of superior value.
(c) Setting up expense, property requirement and head loss...