In 1918, Ardern and Lockett in Manchester obtained a patent entitled ‘Sewage

Treatment Without Filters’ for a technology developed at Davyhulme STW. Since

then, the activated sludge process has come to dominate wastewater treatment.

Despite the long history and a reputation as a well-understood and reliable

process, most surveys of activated-sludge plants show that at least half of

the facilities have some form of operational deficiency, mostly associated with

the clarifier – loss of solids through insufficient sludge settleability, bulking

or foaming.

Low-profile STWs

The BAF process is thus much more compact than conventional treatment technologies,

making it easier to integrate it into small sites, which is especially useful

for plant upgrading and extension. But the small footprint also allowed a new

concept in wastewater treatment, locating plants underground or, for sensitive

locations, in small buildings not readily recognisable as STWs. All environmental

nuisances could be contained.

The first large-scale use of the BAF process was in Soissons, France in 1982.

Since then around ten companies have developed full-scale BAF technologies,

with designs which differ in a number of areas:

  • direction of water flow – up-flow versus down-flow,
  • media density – higher or lower than water, differing grain size and surface


  • direction of backwash – counter or co-current, use of air/water, rates and


  • differing hydraulic and volumetric loading rates,
  • process air flow rate, distribution and control.

The first generation BAF, named Biocarbone, used heavy media in a down-flow

configuration and was mainly used in France for BOD oxidation. As demand shifted

towards nitrogen removal, this process was slowly phased out. In ten years of

application at 100 plants worldwide the process treated a population equivalent

(PE) of 1M.

A competing system, Biofor, developed in France, also used a sunken media but

in an up-flow mode. It is widely used today after having passed the 100 references

mark, mainly in much larger plants in Scandinavia, Germany and the UK. The process

has served a PE of more than 10M, with peak flows of up to 600,000m³/d

at the largest plants in Koeln-Stammheim and Frankfurt.

More recently, BAFs have been developed specifically for nitrogen removal,

where the principle of fixed bacteria caters especially well with the slow-growing

nitrifying organisms. Two new developments made these reactors better suited

to the high velocities often associated with low-nitrogen residuals as a result

of nitrate recycling or tertiary applications:

  • floating filters using lightweight plastic media, first demonstrated in the

    late 1980s,

  • larger media or open structures used in the later half of the 1990s.

The Biostyr process, introduced in 1990, uses expanded polystyrene beads as

filter media. About 50 Biostyr plants are either in operation, under construction,

being designed or firmly on the order books. Recent contracts for large plants

of more than 500,000 PE in cities such as Rome, Lisbon, Paris, Toulouse, Geneva,

Bern and Helsinki, have pushed the total population served to more than 10M.

Ironically, the largest plant now operating is Manchester-Davyhulme STW, bringing

sewage treatment with filters back to the site where Ardern and Locketts first

went ‘filter-less’ 80 years ago. The first stone of a similar sized plant has

just been laid in Syracuse, New York, where a peak flow close to 500,000m³/d

is expected.

Another upflow floating filter, Biobead, was launched in the early 1990s in

the UK. The process uses recycled plastic media, slightly less dense than water,

allowing simplified backwash by bed agitation. Today, a PE of more than 1M is

treated by 30 plants using these filters, the largest being in Yorkshire, which

is designed for a peak flow close to 150,000m³/d.

Also located in Yorkshire, with a peak flow of 100,000m³/d, is the largest

in a series of 35 STWs serving a population of almost 4M. This reference list

was recently consolidated by the Austrian Biopur conglomerate which combines

a German technology using granular sunken media and a Swiss process using open

plastic structures.

The latest addition to the BAF family reverts to the traditional trickling

filter principle, using larger media but aerating and submersing the bed and

combining it with a tertiary downstream biofilter. While the filter is used

in more than 50 plants for denitrification, the submerged aerated filter currently

has only a handful of applications. Yet, because of its simplicity, without

backwash, this process is rapidly gaining acceptance for larger plants. Again

in Yorkshire, a plant with a peak flow above 120,000m³/d has been operating for

more than a year.

As at the beginning of the century, the north of England is today a focal point

for the application of modern wastewater technologies. Large BAF reactors have

proliferated there, with Manchester and Liverpool boasting some of the largest

works and Yorkshire offering the base for three different types of technology.

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