Secondary Treatment

Secondary wastewater treatment refines sewage after primary treatment by using biological
processes to reduce pollution, aiming to make it less harmful to the environment.

Biological Processes

In secondary wastewater treatment, biological processes are essential for further purifying
sewage or wastewater. These processes harness the power of microorganisms to break down
organic matter and remove dissolved and suspended contaminants. Here's a brief elaboration
on the main biological methods used:

● Activated Sludge - Activated sludge uses microorganisms to consume organic

pollutants in wastewater, transforming them into less harmful substances through
biological digestion.

● Trickling Filters - Wastewater is dispersed over a surface with microorganisms

forming a biofilm. As it flows over the biofilm, these microorganisms cleanse the
water by breaking down organic matter and removing impurities.

● Other Biological Treatment Methods - Different biological methods, such as

rotating biological contactors, sequencing batch reactors, and constructed
wetlands, use microorganisms to lower BOD and suspended solids in
wastewater, ensuring its environmental safety or potential reuse.

Biological treatment is a sustainable and effective way to treat wastewater, as it relies on

natural processes to improve water quality.

Biological Process Dynamics Applied to the Activated Sludge Sytem

● Microorganisms consume organic matter in aeration tanks.

● Nutrient and oxygen balance is crucial for their efficiency.
● Proper aeration and sludge recycling are essential for system dynamics.

Nutrient Removal

Nutrient removal in secondary treatment typically focuses on two key nutrients: nitrogen and
phosphorus. Here's how it works:

1. Nitrogen Removal
● Nitrification - Ammonia in the wastewater is converted into nitrate ions
through a biological process. This is typically achieved in the aeration tank where
nitrifying bacteria play a vital role.
 ● Denitrification - In a separate tank, denitrifying bacteria convert nitrate and
nitrite ions back into nitrogen gas, which is released into the atmosphere. This
reduces the nitrogen content in the treated water.

2. Phosphorus Removal
● Chemical Precipitation - Chemicals like aluminum or iron salts are added to
the wastewater to form insoluble compounds with phosphorus. These
compounds settle as sludge, which is then removed from the treatment process.

● Biological Phosphorus Removal - Some treatment plants use specialized

bacteria to accumulate and remove phosphorus as part of the activated sludge
process.

These nutrient removal processes are crucial for maintaining water quality and environmental
standards, as excess nitrogen and phosphorus in wastewater can lead to water pollution and
harm aquatic ecosystems.

Disinfection Methods

● Chlorination
Chlorination disinfects water by adding chlorine to inactivate microorganisms, ensuring
safe drinking water through careful control of chlorine levels and ongoing monitoring.

● UV ( Ultraviolet ) Disinfection
UV disinfection uses ultraviolet light to quickly inactivate microorganisms by damaging
their genetic material. It's a chemical-free and efficient method commonly used in water
treatment, but it's less effective in turbid water and doesn't remove non-living
contaminants. Regular lamp maintenance is necessary to ensure continued
effectiveness.
● Ozonation
Ozonation disinfection uses ozone (O3) to rapidly eliminate contaminants from water,
including bacteria and organic compounds, leaving no harmful residues. It's a preferred
method for water treatment.
● Chlorine Dioxide
Chlorine dioxide is a powerful disinfectant used in water treatment and sanitation
processes. It is highly effective in destroying a wide range of pathogens, including
bacteria, viruses, and algae. Unlike chlorine, chlorine dioxide doesn't typically produce
harmful disinfection by-products, making it a preferred choice for ensuring safe and
clean drinking water.
● Membrane Filtration
Membrane filtration is a disinfection method that involves passing water through a fine
membrane, which acts as a physical barrier to capture and remove microorganisms,
particulate matter, and pathogens. This process effectively eliminates bacteria, viruses,
 and larger particles from the water supply, ensuring a high level of disinfection and
producing safe, clean drinking water without the use of chemicals.

Removal of Remaining Suspended Solids and Organic Matter

In order to ensure that the treated water meets acceptable quality criteria before it is released
into the environment or reused, the removal of leftover suspended particles and organic matter
is a crucial component of secondary wastewater treatment.

These are the ways on how can we achieve it:

● Final Sedimentation
● Filtration
● Advanced Treatment Processes
● Polishing ponds or lagoons
Tertiary Treatment

What is tertiary treatment?

The third and most sophisticated stage of the wastewater treatment process, tertiary
treatment seeks to further enhance the quality of the waste before it is released into the
environment or recycled. Tertiary treatment concentrates on removing leftover contaminants to
meet strict water quality standards or to prepare the water for specific reuse purposes after
primary and secondary treatment methods have eliminated a sizable portion of pollutants.

Tertiary Treatment Methods

● Micro Pollutant Removal

Micropollutant removal tackles trace-level contaminants in wastewater, including

pharmaceuticals, personal care products, pesticides, hormones, and other potentially
harmful chemicals. These substances, even in tiny quantities, can impact the
environment and human health.

● Effluent Reuse

Reusing treated wastewater, also known as effluent in various non-potable uses is

referred to as effluent reuse. In order to manage water resources sustainably, this
strategy reduces the demand for freshwater resources.

● Advanced Oxidation Processes

Advanced Oxidation Processes (AOPs use reactive oxygen species to rapidly degrade
persistent organic pollutants, making them essential in wastewater treatment, particularly
tertiary treatment. AOPs improve effluent quality for safe reuse and drive research in
eco-friendly treatment technologies.

● Membrane Filtration

Membrane filtration selectively removes solids, microbes, and dissolved compounds

from water in wastewater treatment. It employs various filtration stages like reverse
osmosis, microfiltration, ultrafiltration, and nanofiltration. This versatile method produces
high-quality effluent for diverse uses, including water reuse and environmental discharge
in tertiary treatment.
In summary, tertiary treatment is a crucial component of wastewater management and
embodies a paramount significance that goes above and beyond standard treatment stages.
Tertiary treatment ensures that residual pollutants are effectively eliminated, pathogens are
neutralised, and water quality reaches unmatched refinement through cutting-edge techniques
like micro-pollutant removal, nutrient reduction, disinfection, and innovative technologies like
membrane filtration and advanced oxidation processes. In addition to safeguarding aquatic
ecosystems and promoting public health, this procedure also helps to conserve resources by
allowing safe water reuse. Tertiary treatment supports the preservation of our most priceless
resource, enabling a future where water scarcity is reduced, ecosystems flourish, and
communities prosper. It serves as a symbol of environmental protection and sustainable water
management.