Membrane processes and operation for wastewater treatment and reuse

In most cases waste water is cleaned in conventional active sludge (CAS) systems.
These systems work well but the effluent quality still contains a lot of substances like micro-organisms and in the case of heavy rainfall sludge is discharged in the receiving canals, lakes and rivers because there is no settling in the secondary clarifiers.
In membrane bioreactors (MBR) these settlers are replaced by membranes with the advantages of improved effluent quality with lower concentrations of pathogens and micro-pollutants, better reuse options, a smaller footprint and the possibility to maintain large concentrations of micro-organisms in the system, even under extreme conditions.

The main bottleneck in MBR systems is membrane fouling which requires frequent cleanings with chemicals, a larger installed membrane area and higher energy consumption.
Also shock loads can give rise to severe problems.
One research project identifies the membrane fouling trends against shocks in order to make MBRs more robust, less energy demanding and more economical.

Aerobic waste water treatment processes consume a lot of oxygen/energy for the breakdown of all the dissolved organic substances. Moreover, a lot of sludge is produced. In case the solutes can be flocculated and concentrated with membranes before mineralization takes place, anaerobic digestion can be used to produce biogas. This makes the high loaded MBR process much more efficient.

Permeate from an MBR is cleaner in comparison with the effluent from a CAS system.
This makes the MBR permeate a good source for water reuse after tertiary effluent polishing.
This tertiary treatment can be done with Nanofiltration (NF) or Reverse Osmosis (RO) membranes. However, these processes produce also a concentrate stream which must be discharged. One possibility is to return this concentrate to the MBR but this may result in scaling on top of the membranes and toxicity problems for the biology. Positive point is that the NF/RO concentrate may contain phosphates etc. which may be reused as fertilizer in case they can be separated.

Special water sources for reuse are available in the oil and gas industry. Large amounts of water are produced during the production of oil and gas and also petrochemical waste waters from refineries and chemical plants can be reused. These waters contain many different hydrocarbons and other organic substances that may have an effect on the biology and give rise to special membrane fouling and aging problems.

Wetsus’ research in these waste water treatment fields combines know-how on biochemistry, biotechnology, membrane technology and process technology. This multidisciplinary approach is expected to provide innovative solutions for the complex problems in membrane bioreactors.