Technically speaking

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,

low-impact facilities.

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.

Key criteria

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

aeration basins.

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

continuous basis.

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