At the recent WateReuse Research Conference in Denver, I had expected much of the discussion to be around the use of membranes (low and high pressure) for wastewater reuse applications, but was surprised that much of the research focus was on biological filtration. Perhaps since there are a lot of full scale membrane systems already installed and in the pipeline for water reuse, this is starting to be considered as a more mature technology and not needing as much research. If that is the reason I think it is mistaken, but I will come back to that later in this post.
I haven’t been keeping up with recent developments in
biofiltration, so I was very interested in what was presented at this
conference. Around 10-15 years ago when I was working in ion exchange I recall
there was interest in looking at biofiltration for nitrate and perchlorate
removal from drinking water as an alternative to IX but this treatment
generally wasn’t seriously considered for full-scale systems, due to the perhaps
misguided concern about using bugs to treat the water when you were also trying
to pull them out to protect public health. This did not take into account that
many existing filters were probably operating in biological mode to some degree
anyway.
Wastewater Reuse Helps Acceptance of BiofiltrationToday the landscape has changed significantly with the current drought on the west coast and the recent drought in Texas helping the public to be more accepting of direct and indirect potable reuse of wastewater and with reuse becoming an essential component of the water supply. If the public will accept potable reuse of biologically treated wastewater, acceptance of biological filtration should be a walk in the park!
Wastewater reuse has also opened up new opportunities for
biological filtration, where micropollutants (MPs) not removed in wastewater
plants need to be removed if the treated water is to be used for drinking
directly or indirectly. In combination with pre-oxidation to break down the MPs
to biologically assimable components, biological filtration can be an
attractive alternative to high pressure membranes where it has a significantly lower
waste volume and does not produce a saline waste, which is a considerable
benefit in non-coastal areas where NF/RO waste disposal options are limited
and/or expensive.
Wastewaters Ain’t
Wastewaters!
There are plenty of technical articles today about potable
reuse, so you don’t need to read another one from me. I do think it is important
to mention that I believe the use of membranes for wastewater reuse is not yet
a mature technology and is worthy of further research. We have all read a lot
about Orange County Water District’s Groundwater Replenishment System that has
been using membranes for decades for treating wastewater to drinking water
standards and maybe some think you can just throw membranes at the end of any
secondary wastewater treatment plant and start making high quality recycled
water….but wastewaters ain’t wastewaters and membrane treatment is not that
simple…. The quality of secondary effluent can vary significantly, from inland
WWTPs with stringent nutrient removal requirements to ocean discharge WWTPs with
less stringent requirements. This can result in widely varying cleaning
requirements which also varies by membrane type. While there have been a lot of
pilot studies on different secondary effluent sources, I haven’t yet seen a dedicated
study evaluating the impact of specific secondary effluent constituents and the
impact on low pressure membrane permeability and cleaning effectiveness.
Perhaps with all of the recent reuse pilot
studies, this information will start to come together. Or maybe the key results
will be kept from the public as a competitive advantage for those with the
pilot experience, which is understandable when OEMs and/or engineering firms
have generated this knowledge. This is where no-strings-attached research can
help the reuse industry. After being involved in several recent pilot studies
my take-away is that wastewaters ain’t wastewaters and don’t assume you can
simply take the performance of a membrane at one WWTP and apply it to another.