One of the most interesting topics for me at the 2016 WateReuse Symposium in Tampa, September 10-13 was the research into getting pathogen log removal credits for Membrane Bioreactors (MBRs). If granted, this could potentially be a major disruption to the use of Microfiltration/Ultrafiltration in many Advanced Purified Water (APW) projects and a significant boon for the use of MBRs.
In
California, for secondary treated wastewater to be approved by the EPA State
Water Resources Control Board Division of Drinking Water (DDW) for indirect
potable reuse, subsequent treatment must achieve 10 log removals of
cryptosporidium and giardia using treatment processes with pre-approved log
removal credits. Currently, MBRs do not receive any log removal credits for
pathogen removal and even though the MF/UF filtered effluent from MBRs is
suitable for feeding a Reverse Osmosis system, if the objective is to produce
APW for direct or indirect potable reuse, a separate MF/UF treatment step is
required to obtain the necessary log removal credits. A typical APW treatment
train using membranes therefore must include MF/UF [4 credits], RO [1-2
credits] and Advanced Oxidation Processes (AOP) [6 credits] providing 11-12 log
removal credits for crypto and giardia. While the membranes used in MBRs will
certainly provide removal of pathogens, the lack of an approved on-site test method
to quantify the removal is preventing this process from obtaining removal
credits from the regulatory authorities.
Presentations from Erdal (1) and Salveson (2) described studies that showed MBRs can provide log removals of protozoa of at least 3 which would be enough to achieve the required 10 log removals for IPR in combination with RO and AOP. These removals were based on actual analyses for pathogens in the influent and effluent of existing MBRs. The problem is coming up with an integrity test to verify the membranes are providing a given log removal. Pressure decay tests used for DITs on potable MF/UF systems do not work for most MBR configurations which are designed for lower operating pressures. Turbidity or particle counts appear to provide the best correlation with log removals but it probably will take some time before the regulatory authorities approve an integrity test method to allow MBRs to get formal log removal credits for pathogens.
With a lot
less new surface water drinking water projects these days, APW is probably
the biggest market for MF/UF systems, at least in the thirsty Western US. The
implications of MBRs obtaining a log removal credit of at least 3.0 are pretty
obvious. For APW projects where MBR effluent is the feed water, MF/UF treatment
will not be required. For new waste water treatment plants that will also
provide APW, MBRs would allow replacement of the secondary clarifiers from a
conventional activated sludge (CAS) plant as well as the MF/UF treatment step
of the APW process.
For most large
APW projects, secondary wastewater is sourced from existing CAS treatment plants,
so there is no need for MBRs, and this will still be a big market for MF/UF in the future. In
some cases, Tertiary MBRs may be used for nutrient removal on CAS effluent, so
in these cases MF/UF would not be needed. Overall though, I don’t think log removal
credits for MBRs will significantly reduce the market potential for MF/UF in APW
projects due to many of the larger projects treating effluent from existing CAS
WWTPs, but for projects where additional
biological treatment is required, which will typically be smaller projects,
MF/UF will not be required.
- Erdal, U; “Can MBR Replace MF/UF in a Potable Reuse Train – Implementation Concerns?”; 31st Annual WateReuse Symposium, Tampa, FL, September 10-13, 2017.
- Salveson, A; Fontaine, N; “Resolving the Final Hurdles to MBRs for Potable Water Reuse”, 31st Annual WateReuse Symposium, Tampa, FL, September 10-13, 2017.
