Environmental Engineering: Wastewater Treatment

• Environmental engineering protects the environment and human health using engineering solutions.
• It includes water and wastewater treatment, air pollution control, solid waste management, and contaminated site remediation.

Wastewater Treatment Overview

• Wastewater treatment removes contaminants from wastewater.
• This process makes water safe for discharge or reuse.
• The main aim is reducing pollutants like organic matter, solids, nutrients, and pathogens.
• Wastewater treatment plants (WWTPs) use physical, chemical, and biological processes.

Preliminary Treatment

• Preliminary treatment removes large objects to prevent equipment damage.
• Screening removes large solids like rags, sticks, and plastics.
• Grit removal removes sand, gravel, and heavy inorganic materials.
• Flow equalization basins dampen flow rate variations.

Primary Treatment

• Primary treatment physically removes settleable solids and floating materials.
• Sedimentation tanks allow solids to settle as sludge.
• Skimming removes floating grease, oil, and scum.
• Primary treatment removes 50-60% of suspended solids (SS) and 30-40% of biochemical oxygen demand (BOD).

Secondary Treatment

• Secondary treatment biologically removes dissolved and suspended organic matter.
• Microorganisms, mainly bacteria, consume organic pollutants.
• The activated sludge process mixes wastewater with a microbial culture in an aeration tank.
• Air is supplied to the aeration tank to provide oxygen for microorganisms.
• Afterward the mixture flows to a secondary clarifier for sludge settling.
• Some sludge is recycled to the aeration tank to maintain microorganism concentration.
• Trickling filters spray wastewater over media covered with a microbial film.
• Microorganisms consume organic matter as wastewater trickles down.
• Treated water is collected and sent to a secondary clarifier.
• Oxidation ponds (or lagoons) treat wastewater through natural processes with sunlight, algae, and bacteria.
• Constructed wetlands mimic natural wetlands to treat wastewater.
• Plants, soil, and microorganisms remove pollutants in constructed wetlands.

Tertiary Treatment (Advanced Treatment)

• Tertiary treatment removes pollutants not removed by secondary treatment.
• Nutrient removal, specifically nitrogen and phosphorus, is a common objective.
• Nitrogen can be removed through nitrification/denitrification.
• Nitrification converts ammonia to nitrate.
• Denitrification converts nitrate to nitrogen gas.
• Phosphorus can be removed through chemical precipitation using metal salts like aluminum or iron.
• Filtration removes remaining suspended solids.
• Disinfection kills pathogens.
• Chlorination, UV radiation, and ozonation are common disinfection methods.

Sludge Treatment and Disposal

• Sludge is the solid material removed during wastewater treatment.
• Sludge treatment reduces volume, stabilizes sludge, and removes pathogens.
• Thickening increases the solids concentration of the sludge.
• Anaerobic digestion breaks down organic matter without oxygen.
• It reduces sludge volume and produces biogas.
• Biogas can be used as a renewable energy source.
• Dewatering removes water from the sludge to reduce its volume.
• Belt filter presses, centrifuges, and drying beds are common dewatering methods.
• Sludge disposal options include land application, incineration, and landfilling.

Wastewater Reuse

• Treated wastewater can be reused for irrigation, industrial cooling, and toilet flushing.
• Reclaimed water must meet specific water quality standards.
• Dual piping systems separate reclaimed water from potable water.

Regulations and Standards

• Wastewater treatment is regulated by environmental agencies.
• Regulations protect water quality and public health.
• Effluent limits define the maximum pollutant concentrations in treated wastewater.
• Regulations vary by location and receiving water body.