Activated Sludge Based Waste Water Treatment -STP

Aeration is used to operate activated sludge process units and is perhaps the most frequently used process to remove biochemical oxygen demand (BOD) from wastewater. Successful BOD removal in an activated sludge process depends on studying and controlling some basics – such as wastewater sources and quantities, wastewater characteristics, and needed execution in any required preliminary and primary treatment. After performing these operations, the primary treated wastewater entering the activated sludge system achieves BOD removal based on the details below.

 

Biological basics

The activated sludge process is a biological wastewater treatment operation. This means that treatment occurs through many different microorganisms using pollutants as a food source. It is a suspended growth process – since the organisms are suspended in wastewater rather than attached to a medium, as they are in a trickling filter or r biological contact process.

 

The microorganisms

The activated sludge process relies on harvesting a population of millions of microorganisms with different characteristics – mostly aerobic, facultative and heterotrophic bacteria suspended in the wastewater – as the wastewater travels through a reactor known as an aeration tank. This suspension, referred to as the mixed liquor or mixed liquor suspended solid, is supplied oxygen and kept mixed by bubbling air through the entire aeration tank.

These are naturally occurring organisms. They do not need to be supplied from an outside source. As the organisms feed on the organic pollutants in the wastewater, the pollutants are converted to more organisms known as biomass and some spin-offs.

 

Three steps

Transfer

First, food from the wastewater is transferred to the cell. Adequate mixing and treatment (detention) time are essential to ensure that the organisms contact the nutrient source.

Conversion

Conversion occurs as the organisms metabolize the food supply, converting it into new cells. For this to occur, the food supply must be a usable type and in a usable form. Some compounds are easily degraded by the bacteria, while others are metabolized more slowly. Some pollutants may not be metabolized until the organisms become acclimated to the pollutants by producing the proper enzymes. A proper DO environment must be present. Aerobic organisms will not efficiently remove pollutants in an anaerobic environment. The nutrients must also be properly balanced for conversion to occur. Like other life forms, the organism needs nitrogen and phosphorus, among other minor nutrients, to metabolize food and grow new cells.

 

Flocculation & separation

During this phase, the microorganisms stick together to form bigger particles that will fall out of the treated wastewater in the secondary clarifier. Flocculation occurs when mixing allows the organisms to contact one another but does not cause conditions so turbulent that the flocculated material is pulled apart. The settling and compaction of the floc particles depend on their density, size and configuration as well as the efficiency of the clarifier.

Settled solids

The settled biomass is returned to the treatment process to provide organisms that continue to remove pollutants. This returned biomass is referred to as return activated sludge.

Since this is a living and maturing process, the biomass will reach a stage at which it has too much volume. The quantity of biomass in the process is restrained by removing, known as wasting, a determined amount of the biomass each day. This removed biomass is waste activated sludge.