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Sizing of UASB (Upflow Anaerobic Pond) for Wastewater Treatment Plant

I think this is my first time I learnt about Upflow Anaerobic Sludge Blanket (UASB). I’ve ever learnt about anaerobic wastewater treatment, however I don’t have any idea about UASB and what the differences are. Since I am responsible as engineer in wastewater treatment package, I need to learn about UASB (there is a possibility that this system will be included in the package).

What is Upflow Anaerobic Sludge Blanket?

Upflow anaerobic sludge blanket, or UASB, is a form of anaerobic (oxygen-free) digester used in wastewater treatment. It is a methane-producing digester which uses an anaerobic process and forming a blanket of granular sludge and is processed by anaerobic microorganism.

UASB was developed in 1970s by Letinga in the Netherlands. It is essentially a suspended growth system in which proper hydraulic retention time (HRT) and organic loading rate (OLR) is maintained in order to facilitate the dense biomass aggregation known as granulation. (To learn some of design criteria of wastewater treatment, follow this link).

Several considerations need to be taken into account in the operation of UASB in wastewater treatment unit.

  • Initial seeding of some well digested anaerobic sludge blanket could be used. Minimum seeding is 10% of total volume
  • Provide optimum pH and enough alkalinity
  • Supplement nutrients and trace metals if needed. Provide N & P at a rate of COD : N : P = 400 : 7 : 1 (conservative estimate)
  • Addition of Ca2+ 200 mg/L to promote granulation

How to Design UASB Pond

About a week ago, I asked the appointed vendor who will design and supply wastewater treatment unit about design calculation of UASB pond. The design of UASB pond is a function of:

  • COD input of wastewater
  • Flow of wastewater
  • Organic loading rate
  • Hydraulic retention time

Organic loading rate is a measure of the biological conversion capacity of the anaerobic digestion system. It is expressed in kg Chemical Oxygen Demand (COD) or Volatile Solids (VS) per cubic meter of reactor. A relatively high organic loading rate facilitated the formation of anaeronic granules in UASB systems.

In general, there are two ways to design an UASB reactor.

  1. If input COD is 5000 – 15000 mg/L or more, the design method should be used based on Organic Loading Rate
  2. If input COD less than 5000 mg/L, the design method should be based on velocity

Let’s take an example.


Wastewater input flow rate (Q) = 62.5 m3/day

COD = 15,000 mg/L = 15 kg/m3

Organic loading rate = 5.5 kg COD/ (I found that organic loading rate is sometimes state of art in wastewater treatment design. Some vendors may have already had the data)

Volume of tank = Q x C /OLR = 62.5 x 15 / 5.5 = 170 m3

Note: the above volume is the actual volume. You need to add more volume (as void volume).

The second method of calculation can be found in this link.


  1. Anaerobic water treatment technologies.
  2. An Introduction to Anaeronic Digestion of Organic Wastes.
  3. Advanced Biological Treatment Processes. 
  4. Methods for UASB Reactor Design.

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