Unit 2: Sewage/Waste Treatment and Management

Sewage/Waste Treatment

Composition of Sewage

Sewage (or wastewater) is water that has been used by a community (homes, businesses, industries) and contains waste products. Its composition is complex:

• Water (99.9%): The primary component, which acts as the carrier.
• Solids (0.1%):
  • Organic Matter: Human waste (feces, urine), food scraps, paper, soaps, detergents. This is the main source of pollution.
  • Inorganic Matter: Silt, grit, salts, and heavy metals (from industrial discharge).
• Microorganisms: A vast population of bacteria (many are harmless decomposers), viruses, and protozoa. Includes harmful pathogens.
• Nutrients: High levels of nitrogen (from urine) and phosphorus (from detergents), which can cause eutrophication if released untreated.

Treatment of Municipal Waste (Sewage Treatment)

Municipal sewage treatment is a multi-step process designed to remove pollutants and kill pathogens before the water (effluent) is returned to the environment.

1. Primary Treatment (Physical Process)

• Goal: To remove large floating and settleable solids.
• Steps:
  1. Screening: Water passes through screens to remove large objects (rags, sticks, grit).
  2. Sedimentation: Water is held in a large primary clarifier (settling tank). Solids (feces, food) settle to the bottom as primary sludge. Scum (grease, oil) floats and is skimmed off.
• Result: Removes ~60% of suspended solids, but very little dissolved organic matter.

2. Secondary Treatment (Biological Process)

• Goal: To remove dissolved organic matter using microorganisms.
• Process: This is an aerobic process. Microbes are "fed" the organic waste and oxygen.
• Common Methods:
  • Activated Sludge Process: Sewage is pumped into an aeration tank and mixed with a microbe-rich "activated sludge." Air is bubbled through, and the microbes consume the organic matter.
  • Trickling Filters: Sewage is trickled over a bed of rocks or plastic media coated with a "biofilm" of microbes that consume the waste.
• Result: The water then goes to a secondary clarifier where the microbes (now heavy) settle out as secondary sludge. This removes ~90-95% of organic matter and pathogens.

3. Tertiary Treatment (Chemical/Advanced Process)

• Goal: To "polish" the effluent for specific purposes (e.g., reuse) or to protect sensitive ecosystems.
• Steps:
  • Nutrient Removal: Specific biological or chemical processes to remove nitrogen and phosphorus.
  • Disinfection: Killing remaining pathogens using chlorine, UV light, or ozone.

What is Sludge?

The solid material removed during treatment (primary and secondary sludge) must also be treated. It is often put into an anaerobic digester (see Biogas, Unit 1) to reduce its volume, kill pathogens, and produce methane. The final product is called biosolids, which can be used as fertilizer.

Treatment of Industrial Effluents

Industrial wastewater is highly variable and often contains specific, toxic pollutants that cannot be handled by municipal plants. Treatment must be tailored to the industry.

• Examples:
  • Tanneries: High levels of chromium, sulfides, and organic matter.
  • Textile Industry: Toxic dyes, high pH, heavy metals.
  • Electroplating: High concentrations of heavy metals (cyanide, cadmium, nickel).
• Treatment Methods: Often involve physical-chemical processes like chemical precipitation (to remove metals), reverse osmosis, and adsorption (using activated carbon) *before* any biological treatment.

Waste Management and Energy Production

This section focuses on managing solid organic waste (food scraps, yard waste, manure) in a way that is beneficial and can generate energy.

Composting

• Definition: A controlled aerobic (oxygen-requiring) process where microorganisms (bacteria, fungi) decompose organic matter into a stable, nutrient-rich, soil-like material called compost or humus.
• Key Requirements:
  1. Organic Matter: A balance of "Greens" (nitrogen-rich, e.g., food scraps) and "Browns" (carbon-rich, e.g., dry leaves).
  2. Oxygen: Achieved by turning or aerating the pile.
  3. Moisture: Must be kept damp, like a wrung-out sponge.
• Process: The microbial activity generates significant heat (55-65°C), which kills pathogens and weed seeds.
• Benefit: Reduces landfill waste and creates a valuable soil conditioner. It is waste management, but does not *produce* usable energy (like fuel).

Vermicomposting

• Definition: A type of composting that uses specific species of earthworms (e.g., *Eisenia fetida* or "red wigglers") to enhance the decomposition of organic waste.
• Process: The worms consume the organic matter, and their digestive process, along with microbial activity, breaks it down. Their castings (worm manure) are exceptionally rich in nutrients.
• Benefits:
  • Faster than traditional composting.
  • Produces a higher-quality end product (vermicompost).
  • Operates at cooler temperatures (it is not a thermophilic process).

Biogas Production

• Definition: An anaerobic (oxygen-free) process of waste management that produces usable energy.
• Process: As covered in Unit 1, organic waste (sewage sludge, animal manure, food waste) is placed in an anaerobic digester. Methanogenic archaea break it down, producing biogas (methane) for fuel and digestate (a liquid fertilizer).