The energy and space-saving characteristics of submerged membrane bioreactors have prompted many to view them as the most efficient and cost effective wastewater treatment technology. But there are equally viable alternatives, argues Ronald van ‘t Oever of Netherlands-based X-Flow.
A membrane bioreactor (MBR) is a compact-built purification system combining the biological degradation step with a membrane separation step, instead of with a clarification tank. The influent is fed into the aerated bioreactor where the organic components are oxidised by the activated sludge.
The aqueous activated sludge solution then passes through a micro or ultrafiltration membrane filtration unit, separating the water from the sludge. The latter returns to the bioreactor, while permeate is discharged or re-used as particle-free effluent.
The first (industrial) MBR-systems were based on the cross-flow mode due to the relatively high solids content. The advantage was better control of the cake layer build-up resulting in a more constant flux; however this created a more complex system with higher energy costs. The application of the MBR for municipal wastewater was not attractive due to the high flow, with relatively low solid contents to be treated.
This technique, however, has become more attractive for higher flows with the introduction of systems where the refreshing of the feed along the membrane is achieved by pneumatics (aeration) rather than hydraulics; a significant reduction in energy costs can be obtained if the membranes are cleaned by means of air scouring, rather than by cross-flowing of the feed solution.
Presently, two basic MBR systems can be distinguished using the following principles (see figure 2).
• Side-stream: the membranes are placed outside the bioreactor. The membranes are horizontally or vertically placed tubular membranes, fed at the inside. The system operates under overpressure in cross-flow mode
• Submerged: the membranes are submerged in a separate part of the bioreactor. The membranes are either horizontally or vertically placed fibres, or vertically placed flat sheets; all the systems are aerated at the bottom side, while permeate is withdrawn by means of suction at the permeate side.The next generation of MBRs?
These days there is a tendency to think that MBRs are bioreactors with submerged membranes only. These submerged membranes are considered the second generation of MBRs, after the initial generation that used cross flow tubular membranes – with the membranes next to a bioreactor – to separate the sludge from the clean water.
The second generation – now established in the market – are those systems where the membranes are submerged in the reactor, and have specific claims of low energy consumption and low footprint, as well as being easily adaptable, as the membranes can be put in any available reactor.
But now a third generation has arrived, with companies like X-Flow looking at the advantages and benefits of taking membranes back outside of the bioreactor.
X-Flow's ‘out of the box' design
The principle of X-Flow's technology (called the AirLift MBR) is based on the same threory as used for the cross flow principle, but the turbulence within the tubular shaped membranes is achieved by sparging air into the vertically mounted membranes.
The recycling flow propelled from the activated sludge tank at approximately 0.5 m/s is enhanced in turbulence by adding air underneath the module with an additional 0.3-0.5 m/s.
For the controlled permeate output, a simple control valve can be used and ‘gravity flow control' adapted with a dedicated pump. A regular backpulse is executed to maintain the membranes' performance in a steady state.
The biological reactor can be fully designed to its own optimum requirements. Typical optimisation can be done without interference of the ‘clarifying' step (the other part of the total system.) The Ultrafiltration (UF) unit can be connected to this biological reactor. The UF is designed for its best performing configuration, again optimised for the specific task it has to perform. This allows the UF unit to be applied as a ‘clamp on' unit to existing reactors, for the purpose of upgrading to a MBR.
Ready for the black box?
In this article, membrane bioreactor applications have been presented as ‘black box' solutions, and with a little further thinking ‘outside the box' solutions can be offered that are beneficial for the use of membranes.
Putting membranes in a side stream to the bioreactor has been shown to be beneficial, the biological box can be used as such, and the UF can be handled as a separate unit – remaining clean, safe and efficient.
A new generation of thinking on this subject is growing, which advocates moving away from the principle of submerging the membranes in the reactor. New possibilities and improved performances are now within reach by applying this ‘out of the box' principle.