Conventional UASB or EGSB Digesters use gas hoods or similar gas/liquid/solids separators and overflow weirs in the digester top section to (a) separate biogas and biomass and (b) settle and return biomass to the sludge bed. These separating devices are prone to the following operational deficiencies:

  • Clogging of gas hoods with fat/oil/grease
  • Clogging of weir notches
  • Poor leveling of overflow weirs and consequently uneven flow distribution
  • Corrosion near the air/liquid interface
  • Odour generation beneath the digester covers


Furthermore, poor mixing of feed and biomass in the digester bottom section leads to reduced sludge activity in UASB Digesters. To avoid the above stated deficiencies a novel digester type based on UASB technology was developed  with the following main features:

  • Main compartment accommodating sludge bed and feed pipework
  • Settling compartment with sludge non return valve and self cleaning overflow weir
  • Gas lift device connecting main and settling compartment
  • Gas mixing valve for sludge return and sludge bed mixing by gravity
  • Sludge return valve – check valve preventing sludge displacement from main to settling compartment

The digester is divided into a main and a settling compartment connected by a gas lift devices. Feed enters the main compartment continuously at the bottom and passes through the sludge bed where organic matter is converted to biogas. Depending on the mixing valve position biogas is discharged from the digester (valve open, Phase I)) or accumulates in the main compartment head space (valve closed).

Phase I: liquid level equilibrium while mixing valve is kept open

Upon closing the mixing valve pressure builds up in the main compartment head space and liquid is displaced via the gas lift device to the settling compartment, hence leading to a level increase until the overflow weir is reached (Phase II). At this point effluent is discharged from the digester by gravity over a water seal.

When the liquid level in the main compartment reaches the lower end of the gas lift vertical pipe, the gas lift device starts to operate: gas/sludge mixture is lifted into the degassing pot where biogas is separated from the sludge - sludge flows into the settling compartment by gravity (Pase III).

In the settling compartment well settling sludge settles to the bottom whereas floating particles rise to the top and exit the digester via the effluent weirs. Settled solids are flushed back to the main compartment due to a pressure gradient induced by the gas lift device. This ensures gentle sludge return with a minimum of floc/granule destruction (Phase III).

Phase II: pressure build up in main compartment while mixing valve is closed

Normal operation with mixing valve closed and gas lift device operating

Phase III: normal operation with mixing valve closed and gas lift device operating

By opening the mixing valve at set intervals the settling compartment liquid volume is flushed back into the main compartment to ensure proper mixing of feed and sludge bed and to avoid dead spaces in the sludge bed (Phase IV).

Back flushing breaks up any scum layers and cleans weirs notches. After back flushing the mixing valve is closed again, pressure builds up again in the main compartment repeating the cycle (see Phase II). Frequency of back flushing is controlled by opening and closing the mixing valve.

Phase IV: back flushing with mixing valve open

Main advantages:

- No installation of gas hoods
- No risk of clogging gas hoods with fibre or fat/oil/grease
- No risk of clogging overflow weirs with fibre or fat/oil/grease
- No maintenance for scum removal around overflow weirs
- No off-gas and odour from digester
- No air/liquid interface inside digester and therefore reduced corrosion potential
- No separate recirculation tank
- No dead spaces in the digester sludge bed due to back flushing



Biogas Plant Berglandmilch, Austria

- 180 m³ whey per day 
- 180 m³ of highly poluted wastewater per day 
- 11.000 kg COD per day
- COD removal > 90 %
- Electrical power: 12 MWh per day
- Thermal power: 13,5 MWh per day
- Start-up April 2006

Energy globe, Austria awarded 2006