11 Waste Water Treatment

Objectives of Wastewater Treatment

• Make human and industrial liquid wastes safe for the environment

• Potential for wastewater to be reused for specific purposes

• Treatment systems replicate natural purification processes in a controlled environment

• Ringsend Wastewater Treatment Plant (WWTP) as an example

Wastewater Effluents

• Definition of effluent: waste liquids from various processes

  • Sources include:

    • Rainwater

    • Sewage

    • Livestock wastes

    • Dairy/Food processing wastes

    • Silage/Paper mill wastes

• Effluents contain organic matter and nutrients, with some containing pathogens

• Treatment designed to eliminate microbes, nutrients, and pathogens

Types of Treatment Systems

• Septic Tanks

  • Used for small volumes of waste

  • Approximately 26 million systems in use in the USA

  • Suitability influenced by soil type and depth to water table

  • Common failures due to poor soil drainage

  • Potential contaminants include bacteria, heavy metals, nutrients, and synthetic organic chemicals

• Biogas Plants

• Wastewater Treatment Plants (WWTPs)

  • Treat larger volumes of municipal or industrial waste

Sewage Treatment Process

Primary Treatment (Physical Process)

• Removal of large objects using grates and screens

• Settling and removal of suspended solids leading to primary sludge

  • Flocculating chemicals enhance sedimentation

• Outputs sent to secondary treatment

Secondary Treatment (Microbial Process)

• Primary effluent has high levels of Biological Oxygen Demand (BOD)

• Aeration stimulates aerobic degradation

• Two main methods:

  • Activated Sludge Reactor

  • Trickling Filter Reactor

• Process reduces organic carbon to CO2

Aerobic Treatment Benefits

• Aerobic activity lowers BOD effectively

  • Complete mineralization of organic matter produces carbonate, ammonia, nitrate, sulfate, and phosphate

  • BOD reduction efficacy varies with treatment system

  • Aerobic sludge residence time can be up to 30 days, achieving >95% of BOD removal

Trickling Filter System

• Bed made of 2m crushed rock or sheet materials

• Liquid waste sprayed over the bed for treatment

• Capable of removing up to 90% of BOD present

• Output is recycled to complete the oxidation process

Pathogen Removal in Activated Sludge

• Over 90% effectiveness in destroying E.coli and Salmonella

• Pathogens removed through various mechanisms

  • Inactivation, grazing by protozoa, adsorption to sludge solids

  • Viruses removed mainly by adsorption

Activated Sludge Process Details

• Waste mixed and aerated, increasing dissolved oxygen for biodegradation

• Slime-forming organisms create flocs for microbial attachment

• 70-90% BOD removed within 5-10 hours

• Settling flocs inoculate incoming waste batches

• Treated sludge can be composed or used as a soil conditioner

Final Settlement and Sludge Treatment

• Flocs settle as sludge, clearing liquid on top

• Liquid can continue to a trickling filter system for further treatment

• Clean effluent discharged to bodies such as the River Shannon after meeting Environmental Protection Agency standards

Anaerobic Digestion Process

• Sludges from primary and secondary treatment are pumped into digesters

• Anaerobic bacteria convert sludge to methane and carbon dioxide

• Involves complex reactions breaking down polymers to simpler substances

• Effective reduction of BOD up to 75%

Tertiary Treatment Process

• Applied as physicochemical steps to further refine wastewater

  • Removing phosphates to prevent eutrophication

  • Ammonium-sulfate generation for ammonia removal

  • pH neutralization and odor removal

  • Optional chlorination for disinfection before distribution

Wastewater Treatment in Ireland (2017 - 2019)

• 2% did not receive treatment or only preliminary treatment

• 1% received primary treatment

• 67% received secondary treatment

• 30% received nutrient reduction in addition to secondary treatment