Waste Water

Waste Water Generation

The generation of sewage is multifaceted, the main sources include domestic sewage, industrial sewage, agricultural sewage, stormwater runoff and so on. The pollutants in these sewage are diverse, including organic matter, inorganic matter, pathogenic microorganisms, heavy metals and so on.

The characteristics and pollutant content of various types of sewage are different, and the sewage treatment process usually requires customized treatment according to the specific sewage type and pollutant characteristics to ensure that the quality of the water body is discharged or reused.

Sewage treatment process flow chart

Here’s an overview of the main stages in sewage treatment and the key parameters that need to be monitored at each stage.

1. Pre-treatment Stage

The main goal of pre-treatment is to remove larger solid particles and reduce the load on subsequent treatment stages. It typically includes processes like screening and grit removal.

Key Processes:

• Screening: Removes large solids such as plastics, paper, and other debris.
• Grit removal: Removes heavier particles such as sand, gravel, and other inorganic solids.

2. Primary Treatment Stage

Primary treatment is a physical process that removes a portion of the suspended solids and organic material from wastewater. It typically involves sedimentation (primary settling).

Key Processes:

Primary Sedimentation: In a primary settling tank, suspended solids settle to the bottom, forming sludge, while the remaining liquid flows to secondary treatment.

3. Secondary Treatment Stage

Secondary treatment is the heart of the wastewater treatment process. It primarily uses biological methods, such as activated sludge, to remove dissolved and suspended organic matter, nitrogen, and phosphorus.

Key Processes:

• Activated sludge: Microorganisms break down organic pollutants into simpler compounds, including carbon dioxide and water.
• Contact stabilization: Microbial organisms oxidize organic matter, and the treated water is then separated from the microbial mass.
• Biological filters: A microbial community on the surface of a filter medium helps break down pollutants.

Parameters to Monitor:

• Chemical Oxygen Demand (COD): To track the reduction of organic matter.
• Biochemical Oxygen Demand (BOD): To evaluate the biological oxygen demand of the water, indicating the level of biodegradable organic material.
• Total Nitrogen (TN): Nitrogen is one of the key nutrients to be removed in secondary treatment.
• Total Phosphorus (TP): Phosphorus removal is crucial to prevent eutrophication in water bodies.
• Dissolved Oxygen (DO): This is crucial for aerobic biological processes. Low DO levels can inhibit microbial activity and organic matter breakdown.

4. Tertiary Treatment Stage

Tertiary treatment is used to further purify the wastewater, often targeting specific pollutants such as nitrogen, phosphorus, and pathogens. Methods such as chemical precipitation, filtration, and disinfection are commonly used.

Key Processes:

• Chemical precipitation: Chemicals like alum or iron salts are added to precipitate phosphorus from the water.
• Filtration: Sand or other filters further remove suspended particles and bacteria.
• Membrane filtration: Techniques like reverse osmosis or nanofiltration remove dissolved contaminants.
• Ultraviolet (UV) disinfection: UV light kills pathogens like bacteria and viruses.

Parameters to Monitor:

• Total Phosphorus (TP): To ensure that phosphorus is effectively removed.
• Total Nitrogen (TN): Nitrogen levels should be further reduced.
• Pathogens: The presence of harmful microorganisms such as E. coli or coliform bacteria should be monitored to ensure water safety.
• Turbidity: Measures the clarity of the water after filtration, indicating the effectiveness of the removal process.
• Microbial activity: Ensures that disinfection is effective and that there are no remaining pathogens.

5. Sludge Treatment

During the sewage treatment process, sludge is generated. Sludge treatment involves reducing its volume and making it suitable for disposal or further use, such as for energy production or composting.

Key Processes:

• Thickening: The sludge is concentrated to reduce its volume by allowing solid particles to settle.
• Anaerobic digestion: Microorganisms break down organic matter in the absence of oxygen, producing methane gas.
• Dewatering: Mechanical processes such as centrifuges or belt presses remove water from the sludge.
• Drying: The remaining water is evaporated, reducing the volume of the sludge.

Parameters to Monitor:

• Sludge moisture content: To assess the water content in the sludge.
• Sludge volume: To measure the amount of sludge produced during the treatment process.
• pH: To monitor the acidity or alkalinity of the sludge, which can affect its stability and potential for reuse.
• Volatile solids (VS): This measures the organic material in the sludge, helping to assess the treatment process and microbial breakdown.

Summary

Sewage treatment is a multi-stage process designed to remove contaminants from wastewater. Each stage has specific goals, such as removing large solids, organic matter, nutrients, and pathogens. The key parameters monitored throughout the process include suspended solids, COD, BOD, nitrogen, phosphorus, dissolved oxygen, pathogens, and sludge characteristics. Monitoring these parameters ensures that the treatment process is functioning effectively and that the treated water meets environmental and health standards.