Anaerobic Baffled Reactor (ABR): Design and Function

This article is based on a lecture from the course Design of Wastewater Treatment Plants for Onsite Projects. The course teaches you how to design full-scale treatment systems for small to medium projects.

Here, we explore the Anaerobic Baffled Reactor (ABR): what it is, how it works, and its design principles.

What is an Anaerobic Baffled Reactor?

The Anaerobic Baffled Reactor (ABR) is essentially an improved septic tank used in Decentralized Wastewater Treatment System (DEWATS). While a septic tank provides primary treatment, the ABR enhances performance by forcing wastewater to flow through a series of vertical baffles (partitions).

  • Each compartment acts as a small anaerobic digester.
  • Compared to a septic tank, an ABR achieves 30–50% higher BOD removal and significantly reduces total suspended solids (TSS).
  • This makes it an ideal pretreatment step before constructed wetlands or other secondary systems, protecting them from clogging.

ABRs are typically used in decentralized wastewater treatment projects, ranging from households to institutions and small communities.

Anearobic Baffled Reactor ABR DEWATS wastewater treatment

How Does an ABR Work?

  1. Settler (Septic Tank Section)
    • Wastewater first enters a settler (1–2 chambers), where heavy solids settle to form sludge and fats/oils/grease (FOG) float to form a scum layer.
  2. Baffled Chambers
    • Pre-settled water then flows through long inlet pipes into the baffled compartments.
    • Wastewater enters at the bottom of each chamber to maximize contact with the anaerobic sludge blanket.
    • Microorganisms digest organic material, reducing BOD, COD, and TSS.
  3. Stepwise Treatment
    • The process repeats in each chamber, progressively improving treatment as water flows upwards and overflows into the next baffle.
  4. Effluent Discharge
    • Treated wastewater leaves the ABR with reduced organic load, ready for secondary treatment such as wetlands or ponds.

Typical Performance

  • Flow range: 2–200 m³/day.
  • COD removal: 65–90%.
  • BOD removal: 70–95%.
  • TSS removal: up to 90%.
  • Pathogen reduction: Low → requires tertiary treatment (disinfection with chlorine or UV).

💡 The ABR is most effective when paired with a constructed wetland as secondary treatment.

Advantages of ABRs

  • ✅ No energy required (passive process).
  • ✅ Low operating costs.
  • ✅ Long service life.
  • ✅ Low sludge production, with sludge stabilized in the process.
  • ✅ Simple to operate (no blowers, no mechanical equipment).

Limitations

  • ❌ Requires expert design and proper construction.
  • ❌ Long startup phase (anaerobic biomass grows slowly, ~6–7 months).
  • ❌ Low pathogen/nutrient removal → requires tertiary treatment.

Design Considerations

When designing an ABR, several parameters are important:

  • Number of baffles/compartments: 4–6.
  • Width-to-length ratio (R): 3–4.
  • Water depth (D): 1–3 m.
  • Freeboard: 20–30 cm.
  • Hydraulic Retention Time (HRT): 24–48 hours.

Maintenance

  • Similar to septic tanks: monitor sludge and scum levels.
  • Desludging frequency: every 1–3 years.
  • ⚠️ Important: Never remove all sludge—always leave some active sludge in each compartment to maintain treatment efficiency.