Expanded Bed Technology for Biological Wastewater Treatment
Dempsey, M., Advanced Bioprocess Development Ltd and Manchester Metropolitan University, UK
(free)
We have developed a low-cost, high-rate, generic technology for biological wastewater treatment, based
on the expanded bed principle. For tertiary nitrification, this achieves a treatment rate of up to 2.3 kg
NH3-N m-3 expanded bed d -1
- Because this technology is pseudo-plug flow, we have found that the rate-limiting effluent NH3-N concentration is 1 mg L-1
- Therefore, it can achieve excellent effluent quality whilst maintaining a high rate of treatment.
In a recent comparison with leading technologies for tertiary nitrification of municipal wastewater
(McQuarrie et al. 2007), our expanded bed bioreactor (EBBR) was estimated to have a 10-year cost of
ownership that was 50% less than for an equivalent BAFF plant. Furthermore, although comparable to
MBBR in capital cost, our EBBR was estimated to use 26% less energy. In terms of complete process
footprint, EBBR and MBBR were similar but BAFF was 41% larger.
Our nitrification process has been developed to pilot-scale (0.5 m diameter x 3.5 m height) for ammonia
removal from activated sludge final effluent (5-25 mg NH3-N L-1) at Davyhulme Wastewater Treatment
Works, Manchester, UK. The process has also been used at technical-scale for complete nitrification of
sludge dewatering liquor (reject water; 1,000 mg NH3-N L-1) from filter presses at Shell Green, Runcorn, UK. For sludge liquor nitrification, the process removed up to 3.5 kg NH3-N m-3
expanded bed d -1, which is higher than for tertiary nitrification because it was operated with pH control. Owing to its high volumetric treatment rate, our technology is compact and therefore has a small footprint and a low capital cost.
Our EBBR technology uses small particles of biomass support media (glassy coke, manufactured from
bituminous coal) that are about 1 mm nominal diameter, which provides a specific surface area of about
3,600 m2 m-3 if they are considered as solid spheres. However, coke is not solid but has interconnecting
pores, which means that, unlike sand, coke is ideal for microbial attachment and subsequent biofilm
formation. This technology is therefore a fixed film system, with all the advantages that these have plus a
few more that are specific to expanded bed operation.
To operate an expanded bed, wastewater is pumped up through a bed of particles at sufficient velocity to
impart a drag force that counteracts gravity. In this way, the particles become suspended (fluidized) and
the bed expands. With increasing upward velocity, the particles move further apart, so that the critical
inter-particle flow velocity is maintained such that the drag force continues to balance gravity.
Consequently, increased upward velocity increases the distance between particles and thus causes
additional bed expansion.
