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Innovative treatment trialled

The expanded bed biofilm reactor, an innovative process technology for sustainable wastewater treatment, has been trialled by a Manchester Metropolitan University spin-out company. Mike Dempsey, managing director of Advanced Bioprocess Development explains

Prototype expanded bed biofilm reactor package plant for tertiary nitrification. Expanded bed diameter 1.5.m; expanded bed depth 5m; overall height 7mPrototype expanded bed biofilm reactor package plant for tertiary nitrification. Expanded bed diameter 1.5.m; expanded bed depth 5m; overall height 7m

The expanded bed biofilm reactor (EBBR) is a fixed-film technology that has been developed by Advanced Bioprocess Development (ABD) of Manchester. It is considered to be sustainable because it has been designed with minimum energy use in mind.

For example, the full-scale, prototype package plant ABD has designed for tertiary nitrification can treat the secondary effluent of up to 4,000 population equivalent (PE), depending on the effectiveness of the upstream processes, and yet the pump for fluidising the particles and expanding the bed only draws 1kW. Furthermore, the aeration system is designed on the counter-current principle, which provides high efficiency oxygen transfer that uses only half the air compared to co-current aeration in biological aerated flooded filter (BAFF) plants.

A further advantage of the EBBR technology is the large, specific surface area of active biofilm, which results in a compact plant of low capital cost. For example, the expanded bed column of the package plant is only 1.5m in diameter for up to 4,000 PE.

Moreover, by using media particles of approximately 1mm diameter, ABD’s technology is able to provide a biofilm surface area of up to 2,800m2 m-3, which is achieved using a novel biomass support media called ABDite. Because ABDite is a porous carbon foam manufactured from a readily available mineral, bituminous coal, it is also cheaper than media manufactured from plastics, costing less than £150 per m3 of expanded bed volume.

Colonisation

The media particles become colonised with a mixed community of bacteria and other microorganisms, forming a biofilm that completely coats them.

These biofilm-coated particles are referred to as bioparticles and, because they are fluidised, they are completely surrounded by upward flowing wastewater that supplies nutrients (the pollutants in wastewater) and dissolved oxygen, and removes waste products (mineralised nutrients). In this way, there are no issues with channelling or blockage.

The fact that channelling or blockage cannot occur in an expanded bed means that backflushing is never required and therefore the plant is much simpler than processes that do require this procedure.

Furthermore, the lack of backflushing allows a more complex microbial community to develop, which includes protozoa, rotifers, nematode and oligochaete worms, which are responsible for removing suspended solids contained in the upstream process effluent or generated within the expanded bed when excess biofilm is removed.

The pilot plant study where effluent from an activated sludge process was nitrified also demonstrated that the EBBR was able to reduce the suspended solids concentration by 62%. Furthermore, when operated to achieve a low ammonia discharge consent, the pilot scale EBBR was able to produce an effluent containing less than 1mg/L, as well as reducing the soluble organic matter (carbonaceous biochemical oxygen demand [CBOD]) by 56% and coliform bacteria by about 80%.

Another innovative aspect of ABD’s EBBR is its system for control of biofilm thickness. Because biofilms grow continuously, their thickness must be controlled, otherwise cells deep
within them become starved of nutrients or dissolved oxygen.

For example, we have found that cells deeper than about 0.4mm are inactive, owing to diffusional limitation (Akhidime and Dempsey, 2009).

The EBBR is therefore designed to control the biofilm thickness, and hence the expanded bed volume, completely automatically, but without the need for sensors, actuators, or process control software.

This control is achieved by allowing the bioparticles with the thickest biofilm to be removed and recycled to the base of the bed, using an eductor pump that only uses about 5% of the fluidising pump energy.

Because excess biofilm is removed at the base of the bed, stripped biomass has to pass through the whole bed, where the protozoa and metazoa mentioned before can consume the removed cells by filter feeding or grazing.

The full-scale package plant is currently under trial at a UK trickling filter works and the pilot plant is being moved from the activated sludge plant to another trickling filter works for further trials. ABD now has a licensee for manufacture of package plants for tertiary nitrification of domestic and municipal wastewater.

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