Black and Veach's Frank Rogalla looks at issues surrounding membrane bioreactors
The first membrane bioreactors (MBRs) for wastewater treatment were developed
for industrial and commercial applications, in an attempt to generate high-quality
water for non-potable reuse. In the industrialised nations of Europe, North
America and Asia, increasingly stringent environmental regulations are imposed
to protect water resources and human health. They require high levels of treatment
with respect to nutrients as well as bacteria and viruses, but in small footprint,
Currently there are a limited number of MBR installations in municipal treatment
applications. Because of high capital and operation and maintenance (O&M)
costs, mostly proportional to flow, MBR installations have mainly been used
in small plants of less than 5000m³/d: within the US, there are about 24
municipal MBRs, all of which use Zenon hollow fiber membranes, in addition to
about ten installations in Canada. The largest plant under construction has
been announced in Italy, with 38,000m³/d. In the UK, a plant in Swanage
on the southern coast, built and operated by Wessex Water with Kubota plate
and frame membranes, has a peak flow of 12,500m³/d, and has been operating
for nearly a year.
Vivendi Water/US Filter recently introduced its MBR design, and has a facility
under construction in California, which combines the technologies of the Memcor
CMF-S membrane, used in drinking water, with Jet Tech aeration and pumping equipment.
Similarily, Mitsubishi Rayon, which has the largest number of small units in
operation in Japan, has submitted its equipment for evaluation at the Aqua 2000
testing facility in San Diego, California.
It is accepted that fine screening and grit removal is required to protect the
membranes. Fine screening, of 3mm opening or lower, is preferred to prevent
particles from embedding in the membranes. Mixed liquor suspended solids (MLSS),
solids retention time (SRT), and membrane flux rates are key criteria for the
design of the bioreactor. Initial industrial applications indicated design conditions
using SRTs of 25 days or greater as well as MLSS concentrations up to 20,000mg/l.
However, it has been determined through various pilot-scale facilities that
operating at such high MLSS adversely impacts the flux rates and oxygen transfer.
As a result, the suggested MLSS concentration has been reduced to 10,000 – 12,000mg/l
for most applications, which is still three to four-times higher than conventional
Operating at 25 days or greater SRT provides a stable biological system, better
suited for unattended operation at remote facilities. Long sludge age also minimises
solids production, albeit not below that of a conventional extended aeration
process. Yet, the desire to minimise facility footprint and to implement biological
nutrient removal (BNR) has driven research to evaluate operations at shorter
SRTs. On the other hand, reducing the SRT can increase fouling of the membranes,
potentially raising the number of membranes to accommodate a loss in flux rate,
leading to higher capital cost.
The selection of the appropriate membrane flux rate is critical. An aggressive
flux rate can limit plant capacity, needing a high cleaning frequency, which
continuously takes membranes out of service. The flux rate is influenced by
temperature and biomass concentration, therefore a universal flux rate cannot
be given. One manufacturer suggests an average flux rate of approximately 25
l/m²/h at 20ºC, allowing for occasional peak flux rates around twice
the average. Other suppliers contend that flux rates should not exceed approximately
17 l/m²/h at any time in order to minimize fouling and maintain a reasonable
chemical cleaning frequency. In order to accommodate the lower flux rates economically,
upstream equalization is recommended. Satellite ‘scalping’ plants for water
reclamation, operating at constant flow rates, are therefore a favourable application.
The O&M issue of greatest importance is the membrane cleaning operation.
With the high concentration of solids within the basin, there is a need for
a high intensity scouring of the membranes to maintain operating flux rates.
Membrane scouring is provided using aeration, resulting in intensive energy
consumption and power requirements, as well as high residual dissolved oxygen
levels, complicating the design for denitrification. In addition to continuous
air scouring, the membranes must be removed from service at an interval between
one to four months for chemical cleaning, depending on operating conditions
and wastewater characteristics. Chemical cleaning is either achieved by lifting
the membranes out of the basin and dipping them into a chemical bath of a chlorine
or citric acid solution, or by isolating some parts of the membrane system for
chemical injection. This process can be labour intensive and poses specific
safety issues for a large facility, membrane cleaning would be required on a
There is a bright future for MBRs for municipal wastewater where very high
quality effluent is needed from a small footprint. Key issues in the design
of the MBR systems which need further research are:
- SRT optimisation, i.e. how low can it go without sacrificing flux rate?
- determination of BNR design criteria and configuration to consistently achieve
low effluent phosphorus and nitrogen,
- improvements in the membrane cleaning operation to reduce air scouring and
minimise labour and residuals during chemical cleaning.
As MBR design criteria become better established, and as more players enter
the field, the economics will also improve. Currently, even with the trade-off
of significantly less facilities, i.e. no final clarifiers, no filtration and
less disinfection, MBRs are not cost-effective, unless a microfiltration quality
water is required for reuse. With cost reductions and improvements in membrane
cleaning, the technology becomes realistic even for larger facilities.
Black & Veatch is the parent company of Paterson Candy