MBRs for municipal wastewater treatment

The vast majority of the wastewater treatment capacity provided by MBRs worldwide is for municipal wastewater treatment, since the total flow of sewage is greater than that of industrial effluent. Similarly most of the research into MBRs, focusing largely on membrane fouling, relates to municipal wastewater treatment. Municipal plants also tend to be larger.

Municipal wastewater is generally treated to meet statutory requirements for discharge to environmental waters, though there is increasing implementation of wastewater reuse schemes. The key contaminants demanding removal from municipal wastewaters, roughly in order of importance, comprise:

  • Suspended solids
  • Organic matter
  • Ammonia
  • Nitrates
  • Phosphate
  • Pathogenic bacteria, and
  • Micropollutants.

Removal or inactivation of pathogenic bacteria is required for either discharge to bathing areas or reuse applications such as irrigation. MBRs provide a high level of disinfection and also clarify the water so as to increase the effectiveness of a downstream UV irradiation disinfection process.

The onerous nature of micropollutants  contaminants impacting on the environment even at very low concentrations  has only become apparent since around the turn of the millennium. The future requirement for the removal of micropollutants, and in particular trace organic removal, is unclear. 

These contaminants are wide-ranging in chemistry and include heavy metals, pesticides and herbicides, fire retardant additives and pharmaceutical products. 

Sewage contains suspended organic and inorganic solids, nutrient compounds (containing nitrogen or phosphorus) and pathogenic bacteria from faeces. The organic component is predominantly readily biodegradable, with a BOD:COD ratio normally in the 0.40.5 range. It is therefore almost always treated biologically, with almost all large municipal wastewater treatment works (WwTWs) employing the conventional activated sludge (CAS) process or some variant of it.

Classical treatment process

The classical sewage treatment process proceeds via the screening of gross solids (possibly combined with grit removal), then sedimentation of settlable solids followed by a biological process. A second sedimentation process (secondary clarifier) then separates the biological solids (or ‘flocs’) from the water.

The biological process normally includes recycling the nitrate-rich sludge from the CAS to some point upstream of the aerobic process where anoxic conditions then prevail, which achieves nitrate removal. Various configurations that include a preliminary anaerobic zone to assist biological phosphorus removal (BPR) are also available.

Figure 1.  Classical municipal wastewater treatment | About Mbrs Simon Judd Municipal Mbrs 1
Figure 1. Classical municipal wastewater treatment

Aerobic MBRs can be configured similarly to these modified CAS processes, in essence, the biological function remains unaltered by the membrane: the membrane simply replaces the secondary clarifier.

Municipal MBRs are often implemented as a retrofit to an existing CAS process. Adapting the CAS to an MBR permits an increased flow through the process as well as an improved treated water quality, which is often of practical significance when consents/permits for discharged nutrients in the wastewater are tightened by the regulators.

Increased global capacity

The advantages offered by the MBR process have led to its increasing implementation, with ever larger installations. Whereas the largest installed municipal plant at the turn of the millennium had a capacity of 13 MLD, by 2004 there were two plants of more than 40 MLD. The first 100 MLD plant was commissioned in 2007. By 2016, there were more than thirty 100 MLD capacity plants installed worldwide, mainly in China.

The global capacity provided by MBR technology at the time of writing in 2018 is probably somewhere between 17 and 20 GLD (gigalitres per day). Given that around 1000 GLD of sewage is being generated globally, and that perhaps only half of this is treated, it can be estimated that MBR technology provides 3−4% of the world’s sewage treatment capacity.

Acknowledgements

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