Nitrate and Phosphate removal

Biological Nutrient Removal From Municipal Wastewater

1. Introduction

• Main problem: Rising concentration of nutrients in municipal wastewater.

• Primary causes:

  • Nitrogen

  • Phosphorus

• Manifestations of eutrophication:

  • Algal blooms

  • Symptoms include:

    • Low dissolved oxygen levels

    • Fish kills

    • Murky water

    • Depletion of aquatic flora and fauna

2. Biological Nutrient Removal (BNR)

• Definition: BNR is a process used to remove nitrogen and phosphorus using microorganisms.

• Components of BNR processes:

  • Biological Nitrogen Removal (BNR - Nitrogen)

  • Biological Phosphorus Removal (BNR - Phosphorus)

3. BNR Processes

• Nitrogen Removal:

  • Involves nitrification and denitrification processes.

    • Objective: Remove ammonia, nitrate, and nitrogenous compounds.

    • Importance: Key steps in the nitrogen cycle.

• Phosphorus Removal:

  • Targets soluble and particulate phosphorus

  • Involves the growth of phosphorus-accumulating organisms (PAOs).

4. Effluent Limits, Mechanisms, and Technologies

• Total Nitrogen (TN) and Total Phosphorus (TP) limits and removal mechanisms:

  • Total Nitrogen:

    • Nitrification limit: <0.5 mg/L

    • Denitrification limit: 1-2 mg/L

    • Particulate Organic-N: <1.0 mg/L

  • Total Phosphorus:

    • Microbial uptake: 0.1 mg/L

    • Chemical precipitation and solids removal: <0.05 mg/L

5. Nitrogen Cycle Overview

• The nitrogen cycle includes four key processes:

  1. Nitrogen fixation

  2. Ammonification

  3. Nitrification

  4. Denitrification

• Nitrogen significance:

  • Largest atmospheric gas (78% N2).

  • Building block of DNA, RNA, and proteins.

• Most organisms need nitrogen in a usable form (NH4+, NO3-).

6. Nitrification Process

• Nitrification comprises two steps:

  1. Ammonium Oxidation:

     • Involves ammonia-oxidizing bacteria (e.g. Nitrosomonas).

     • Converts ammonium (NH4+) to nitrites (NO2-).

  2. Nitrite Oxidation:

     • Involves nitrite-oxidizing bacteria (e.g. Nitrobacter).

     • Converts nitrites (NO2-) to nitrates (NO3-).

7. Denitrification Process

• Definition: Reduction process converting nitrates (NO3-) into nitrogen gases (N2, N2O).

• Occurs under anaerobic conditions (e.g. waterlogged soils like rice fields).

• Two forms of denitrification:

  • Assimilatory Denitrification: Converts NO3- to NH4+ (preserved).

  • Dissimilatory Denitrification: NO3- → NO2- → NO → N2O & N2 (lost).

• Common bacteria involved:

  • Pseudomonas, Bacillus

  • Thiobacillus denitrificans (autotrophs).

8. Ecological Conditions for Denitrification

• Requirements:

  • Anaerobic conditions

  • Temperature: Optimum at 37°C

  • Nitrate as a primary electron acceptor

  • Carbon as an energy source

  • pH between 5-9 (optimum at pH 7)

9. Biological Phosphorus Removal

• Processes:

  • Involves PAOs converting organic matter to polyhydroxyalkanoates (PHAs).

  • Enhanced Biological Phosphorus Removal (EBPR) for phosphate accumulation.

• Treatment phases:

  • Anaerobic Phase:

    • PAOs uptake organic carbon, releasing phosphorus into water.

  • Aerobic Phase:

    • PAOs take up phosphorus, storing it as polyphosphate intracellularly.

    • Settling and removal of phosphorus-rich sludge from the treatment system.

10. Common Treatment Processes

• Integrated Fixed Film Activated Sludge (IFAS) Process

• Sequential Batch Reactor (SBR) Process

• Oxidation Ditch Process

• Membrane Biological Reactor (MBR) Process

• Moving Bed Biofilm Reactor (MBBR) Process

• Step Feed Process

11. Kinetics for Nitrate & Phosphate Removal

• Overview of the kinetics involved in the processes for effective nutrient removal from wastewater.