Aerobic Biological Treatment

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As one of the leading industrial water and effluent treatment consultants, ARL Consulting are ideally placed to assist in the development of any aerobic biological treatment projects being considered. ARL are particularly experienced in the design and management of Membrane Bioreactor (MBR) technologies.

Membrane Bioreactors (MBRs) have now become one of the main go to technologies for aerobic effluent treatment. Recent technological advantages in membrane technologies, plus reduced cost, have meant that MBRs have, over the past 10 years or so, overtaken the more conventional activated sludge process in terms of numbers of installed units and site area required.


The advantages of the MBR process include:


Very high quality of treated effluent, in terms of both COD and suspended solids. The recent changes to legislation require us to further consider treated effluent recycling, or discharge direct to water course (referred to by the EA as receiving water).

Higher values of Mixed Liquor Suspended Solids (MLSS) are achievable when compared with the equivalent process utilising the more conventional secondary clarifier, allowing reduced F:M ratios within the biological reactor.

Less concern as to the detrimental effects of filamentous bacteria , frequently the main cause of operational failure with the more conventional activated sludge process when using settlement clarifiers.

MBRs are generally available in two distinct formats, as follows:

(a) Internal membranes, usually flat sheet membranes. (Hollow fibres membrane cassettes are also available). These submersible membranes are manufactured as cassettes or separate banks of membranes, designed to provide a high surface area. They are installed either in the main reactor aeration tank itself or occasionally in a side membrane tank, to allow ease of maintenance. The hydraulic head of the main aeration tank above the membranes is often sufficient to drive the clean effluent through the membrane, or supplemented with the negative pressure from a treated effluent transfer pump.

(b) External tubular membrane modules. (Less commonly spiral wound membranes may also be used). With this arrangement the effluent is pumped through external banks of membranes, usually arranged with a number of tubular membranes housed in cylindrical casings arranged in either parallel or series. The motive force to overcome fluid flow frictional pressure drop and the driving force to overcome transmembrane pressure drop is provided by high pressure pumps. Again this arrangement allows maintenance and cleaning without having an impact on the reactor itself.

Other commonly encountered aerobic processes that may be encountered include:

  • Sequencing Batch Reactors (SBR)
  • Moving Bed Bioreactors (MBBR)
  • Percolating Filters
  • Biotowers
  • Submerged Aerobic Filters (SAF)
  • Biological Aerated Flooded Filter (BAFF)
  • Conventional Activated Sludge (utilising clarifiers for sludge separation)

Conventional Activated Sludge systems, MBRs and SBRs are referred to as suspended growth systems, whereas MBBRs, percolating filters, bio-towers, SAFs and BAFFs are referred to as attached growth systems.

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Because aerobic polish is often the last step before discharge to a natural receiving water, it is essential to get it right. For all enquiries and further information please use the "contact us" form.

Latest Aerobic Biological Treatment Project

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Glenmorangie Distillery

AD plant design and project management

ARL Consulting were commissioned to evaluate technical and commercial options for effluent treatment at both of the Glenmorangie whisky distilleries at Tain and Ardbeg, on the Isle of Islay.

This initial feasibility study resulted in the selection of the Tain distillery to be the site for a state of the art anaerobic digestion plant.

ARL Consulting undertook detailed design for the effluent plant and prepared a detailed specification and contract documentation, allowing the project to be tendered on the open market.

ARL undertook detailed negotiation with SEPA to determine the required effluent quality to allow discharge of high quality treated effluent to the Dornoch Firth. The AD plant included beneficial use of the methane in a dedicated biogas boiler to allow displacement of heavy fuel oil.

To view the full Glenmorangie Distillery project details please Click Here.

Aerobic Biological Treatment FAQ's

One of the key advantages is that there is a physical barrier - usually a UF membrane - between the activated sludge process and the final effluent.

The barrier ensures a very high quality of final effluent, it also avoids problems often associated with more conventional secondary clarifiers, such as solids loss due to filamentous bacteria or gas entrapment in the biological floc. Avoiding a secondary clarifier ensures space requirements  optimised, and allows for higher reactor MLSS values compared with conventional clarification.


Of course. There is still a need for the more traditional designs, such as conventional activated sludge or Sequencing Batch Reactors (SBRs)


That depends on the overall design. Consideration should be given to surface aeration (fixed or floating/tethered), sub-surface air diffuser, and also venturi type 'jet' mixers. Method of Aeration is an important operational consideration as over 70% of the blower energy  can be wasted, due to the high proportion of nitrogen gas in air.

There is definitely a place for utilising pure oxygen as opposed to aeration.

There is the inevitable saving in power by utilising oxygen, due to avoiding the need for blowing nitrogen into the reactor, as well as the ability to operate at higher MLSS values. The downside is that pure oxygen comes at a price, but it is always worth considering within the overall mix.


Very much so. 

Anaerobic processes cannot normally achieve the treated effluent quality (in terms of COD, BOD and/or ammonia) required for direct discharge to natural receiving waters. In this instance, an aerobic polish will easily achieve the required quality, the aeration capacity required, compared with a stand alone aerobic process, is significantly reduced as the anaerobic process has already removed a large proportion of the organic load.