What Chemicals Does an STP (Sewage Treatment Plant) Use?

Overview of the STP Process

Most sewage treatment plants use a combination of the following three stages to treat wastewater:

1. Primary Treatment: This involves the removal of large suspended solids and sediments through screening, sedimentation, or grit removal. While this stage does not primarily rely on chemicals, coagulants may sometimes be added to assist in solid separation.

2. Secondary Treatment: Here, biological processes break down organic matter in the wastewater. Beneficial microorganisms are used to degrade pollutants such as human waste, food particles, and pathogens.

3. Tertiary Treatment: To meet stringent discharge standards or for reusability, this stage involves advanced disinfection and removal of specific contaminants. Many chemicals are used at this stage to ensure compliance with environmental guidelines.

Chemicals Used in STP Processes

While the use of chemicals can vary from plant to plant depending on the type of wastewater, treatment scale, and local requirements, here are the most common chemicals and their roles in sewage treatment:

1. Coagulants and Flocculants

Coagulation and flocculation are used in the primary and secondary treatment stages to aggregate suspended particles into clumps, making them easier to remove from water.

• Common Coagulants:

  • Aluminum sulfate (Alum)
  • Ferric chloride
  • Ferric sulfate
  • Polyaluminum chloride (PAC)

• Flocculants:

  • Polymers such as polyacrylamide are used to enhance the effectiveness of the coagulation process.

2. pH Adjusters

Adjusting the pH is a critical aspect of sewage treatment, especially in biological processes where microorganisms require a specific pH range to function optimally.

• Acidic Chemicals to Lower pH:

  • Sulfuric acid
  • Hydrochloric acid

• Basic/Alkaline Chemicals to Increase pH:

  • Sodium hydroxide
  • Calcium carbonate (lime)

3. Disinfectants and Oxidizing Agents

At the final stages of treatment, disinfectants are used to kill harmful bacteria, viruses, and pathogens to ensure the treated water is safe. These agents are particularly important in tertiary treatment.

• Common Disinfectants:

  • Chlorine gas, sodium hypochlorite (bleach), or calcium hypochlorite (solid chlorine) are widely used as chlorination agents.
  • Ozone is another powerful oxidizing agent used for disinfection and removing organic pollutants.

• Dechlorination Agents:

  • Sodium bisulfite or sulfur dioxide is used to remove excess chlorine before discharging treated effluent to the environment.

4. Anti-Foaming Agents

Foam generation can occur in wastewater systems as a byproduct of biological decomposition or chemical reactions. Anti-foaming agents are added to suppress foam formation.

• Examples:
  • Silicone-based defoamers
  • Polyol esters

5. Activated Carbon

While not a “chemical,” powdered or granular activated carbon (PAC/GAC) is often used in tertiary treatment to absorb dissolved organic compounds, heavy metals, and odors.

6. Phosphorus Removal Chemicals

Excess phosphorus from detergents, fertilizers, and industrial waste can cause eutrophication (overgrowth of algae) in natural water bodies. Phosphorus removal is often achieved chemically.

• Common Phosphorus Removal Agents:
  • Aluminum sulfate (Alum)
  • Ferric chloride
  • Ferrous sulfate

7. Nutrient Enhancers

In biological treatment processes, nutrients such as nitrogen and phosphorus may need to be added to maintain the health of microorganisms breaking down organic waste.

• Examples:
  • Urea (Nitrogen source)
  • Diammonium phosphate (Nitrogen and phosphorus source)

8. Sludge Treatment Chemicals

Sludge is an inevitable byproduct of sewage treatment and has to be handled carefully. Chemicals are added to stabilize, dewater, and