2.5 Biological filtration
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2.5 Biological filtration


Today’s topic is Biological filtration,
still in the filtration stage of HWTS. The most widely known form of biological filtration, is called slow sand
filtration. And it has been used for centuries to
purify drinking water. In the past few decades, slow sand
filtration has been adapted for use at the household level, where it is most
commonly known as bio-sand filtration. While slow sand filtration is
superficially similar to rapid sand filtration, it’s actually
quite different. We saw in the membrane filtration lecture,
that sand filtration alone can remove large
particles through size exclusion, but that pathogens, and even relatively
large protozoa, are still smaller than the pore spaces and can
pass through. Some pathogens will attach to larger
particles, which are removed, and some pathogens will absorb onto the surfaces of
the sand, due to electrostatic effects, but rapid sand filtration has
only a small impact on pathogen loads. Biological filtration includes these same size exclusion processes and electrostatic
ones. But critically it also includes biological
activity. In biological filtration, a biofilm
develops on the surface of the filtration sand. It’s not a thick, green, slimy layer that
you can see, it’s a very thin biologically
active layer. And then organisms in this active layer,
this biofilm, can actively consume pathogens that are present in
the untreated water. And this gives a much greater pathogen
reduction than filtration alone. Biological filtration is also much slower
than rapid sand filtration. And there also tends to be a long
residence time within a filter where some of the pathogens will die-off naturally,
due to lack of food or oxygen. Or the temperature is not convenient. Because they’re mostly adapted for living
in the intestines, rather than in sand filter in
someone’s house. Let’s look at some design parameters for
conventional slow sand filtration. Slow sand filtration consists of two main
elements. One is a bed of packed sand, and the other
is the water column above it. And both of those tend to be around one
meter in depth. Where the water column might be from 60 to
120 centimeters, and the sand bed again, from 80 to 120 centimeters,
maybe as low as 50 centimeters. The type of sand that’s used in the bed is
important. It shouldn’t be too big or too small. It should also be fairly uniform. And often people aim for an effective size
of around .15 to .35 millimeters. Where the effective size is the D10, or
the size which ten percent of the particles
are smaller than. And a guideline for uniformity is that the uniformity coefficient should be around
two or three. Where the uniformity coefficient is the
ration of the D90, the size of the 90th percentile particle
to the D10. Slow sand filtration is conventionally
operated in a continuous flow mode from up to down, with filtration rates of around ten to 30
centimeters per hour, occasionally up to 50
centimeters per hour. It’s important that the biologically
active layer the bio film, should never be allowed to dry out because
it would die. So, slow sand filters normally have some
kind of flow control, a hydraulic control, so that he water outlet is higher
than the level of the sand bed. And it can never run dry that way. Another thing about slow sand filtration
is that when putting a new filter online it takes some time for
the biofilm to develop. About a month usually, it’s called a
ripening period, and then the layer that forms is called a
“Schmutzdecke”, or the dirty layer. Even though that’s actually where a lot of the important pathogen removal
processes take place, biosand filtration is basically slow sand
filtration, scaled down for use at the household
level. There are many different designs of
biosand filters available. This picture is one promoted by CAWST, The
Center for Affordable Water and Sanitation Technology, and this
is their version Ten biosand filter. In this version, it has the same elements as a conventional slope sand filtration: a
water layer. Here it’s five centimeters deep. Which they say is optimal for allowing
oxygen to diffuse through. And then a sand bed, here at 55 cm deep. And the sand that’s used in the household
biosand filter can be a little bit larger than a conventional
flow sand filter. CAWST recommends that the sand should pass
through a .7 ml sieve. But one of the biggest differences is that
the household biosand filters operated intermittently with
batches of 12 liters in this design. And also, the pour volume in the sand
layer has a volume of 12 liters. Now, this model also has a hydraulic
control so that the outlet is here, which is at the five centimeters
level, so the water will never run dry. And with a cross-sectional area of about
600 square centimeters and a design rate of 40
centimeters per hour. That translates to about 400 milliliters
per minute, which can be treated, or 24 liters per
hour. So that means one batch of 12 liters
should take about half an hour to be completely
treated. Biosand filters are conventionally
available with a concrete housing such as the one shown
here. And in recent years plastic housings have
been developed that are much more lightweight and easy to
transport. In both conventional slow sand filtration
and biosand filtration, a new filter needs some time, about a month to develop the “Schmutzdecke”, the biologically
active layer. Now like membrane filters or ceramic
filters, a new filter has a pretty high flux rate,
which slowly decreases over time as the filter
media becomes fouled, as particles get trapped
in poor spaces. In membranes or some ceramics flux can be
restored by backwashing. But that’s not done in biosand filtration. What’s done instead is in a conventional
slow sand filter, the top layer is scraped off and stored for later use,
usually about one to three centimeters. Because this includes a lot of the
“Schmutzdecke”, then there’s an additional ripening period thus needed of
about seven to ten days. And in a typical slow sand filter, this
type of cleaning is done every 20 to 60 days;
depending on the water characteristics and the flow rates
For biosand filters, a slightly different approach is used, which
CAWST calls swirl and dump. It’s what it sounds like. A bunch of water is added to the inlet and
then manually swirled around, breaking up any dirt that’s in the first
few centimeters of sand. And then that dirty water is dumped and
removed. The process may be repeated if the sand’s
very dirty. Because much of the biological
“Schmutzdecke” remains in the sand, the sand isn’t removed, the ripening period is less, and CAWST
recommends just a few days of ripening. The frequency of such cleaning at a
household level really depends on the quality of the water and the amount of
water that’s treated. There’s some good operating procedures
which can help to get the best performance out of a biosand
filter. One of these is to use a consistent water
source. The “Schmutzdecke” is a collection of
microbes and they get habituated to a certain kind of
water. So if the raw water is changing from
surface water to ground water or rain water, these are very different and
the biofilm won’t be happy with that. It’s also good if the raw water is not
very turbid, say less than 50 NTU, because those particles
can clog the filter. It’s important to use the filter each day,
at least one time and perhaps three or four
times. When the filter’s not used, the dissolved
oxygen in the water can be consumed. And this could be bad for the microbes in
the filter, including in the “Schmutzdecke” which are important for good performance of the biosand
filter. Similarly, it’s important to make sure
that the biological layer is always wet. A “Schmutzdecke” will die if it dries out. And many of the systems have a hydraulic control so that the outlet is above the
sand layer, but still it’s important to check
for leaks and make sure that that sand is always
wet. Likewise, it’s important to check the flow
rate, if the flow rate is too fast, treatment may
not be effective and if the flow rate is too
slow it might be time to do a cleaning. Finally, biosand filtration doesn’t give
any residual disinfectant, so it’s important
to collect the water safely in a safe storage container and manage it hygienically after
treatment. So how well do biosand filters do at
removing pathogens? Well they work best for the larger pathogens the helminths and
protozoa, because they’re easier to strain out physically or
to absorb onto the, the sand layers. So typically you’ll see more than two log
removal of helminths and protozoa. And remember, LRV is in this table stand
for log reduction values. Bacteria slip through a little bit more. You typically see one to two LRV in a
biosand filter. And viruses are typically not terribly
well removed with maybe about 70% removal. Being typical, which is less than one LRV. Some physical and chemical components are
also removed through biosand filters. Turbidity is a nicely removed with about
90% removal, 85 to 95%. Iron is well removed because it oxidizes
within the filter and precipitates out on the sand coatings, so
you might see 90 to 95% removal there. A typical biosand filter will not remove
arsenic. But there is an adapted version which
includes an iron source, usually iron nails, which is much more
effective at arsenic removal. But arsenic removal is complex, and
competing ions, such as phosphorous and silicon can very highly
effect performance. Finally the filters don’t remove nitrate
or nitrite but they can actually increase nitrite by converting ammonia into
the oxidized form, nitrite. This can cause treated water to exceed
health based guideline values. So it’s always a good idea to keep an eye
on the nitrogen chemistry. Look for ammonia, as well as nitrite in waters that have been treated with biosand
filtration. Now let’s give an example of a field application of biosand filters in
Cambodia. One of the first organizations to work on biosand filters in Cambodia was
Samaritan’s Purse from Canada, an NGO, in partnership with two local NGOs, Hagar Cambodia and
CGA. Together, these NGOs have implemented over
100,000 biosand filters since 1999, with a capacity of more than 25,000
per year. In 2007, an evaluation was made, which was
published in 2010. This was sponsored by the World Bank’s water and sanitation program and was a
very rigorous epidemiological study that looked
at 105 intervention households from the two NGOs’
working areas. With 102 matched controls, so they could
tell if any changes were due to the treatment and not just to
changes over time. The evaluation found high use of the
biosand filters. 88% of them were in use at the time of
visit. And of the 12% that weren’t in use, the
most common reason was dissatisfaction with the taste or the odor
or the color of the treated water. The filters had been, in place for a
median of two years, and in one case, had been in use
for eight years. Two thirds of the survey respondents reported receiving some training from the NGO
workers. And among those who did receive training,
they were twice as likely to use the filters as those who
didn’t receive training. Now according to the NGO programs, a 100% of people were trained during
distribution. So either there has been some turnover of people, or people don’t recall having
received the training. One important finding was that the treated water was always stored at the household
level. And you can see from this design that you need to collect the water in some kind of
storage. However, about half of people were storing
it in used, open containers like this one. And about 80% of people were taking water
from the container using a dipper or other instrument that
could introduce contamination. The evaluation found that E. Coli was reduced,
somewhat by the biosand filters. If we first think about high risk waters,
which are considered to be those having at least 100 CFU E. coli per 100 mls. Well, the raw water, 73%
of samples were in this high risk class. And only 13% of treated water was
considered high risk. And that was a significant difference. However, only four percent of the treated
water’s samples had no detectable E. coli, and then would meet
health-based targets. Storage introduced re-contamination or
allowed regrowth. And almost one third of samples were in that high risk category in the storage
container. So on average, there was about one to two
log reduction values, or about 95% E. Coli removal, and
this was highly variable. It’s quite complicated to calculate log
reduction values because of the large storage volume in the filter,
so the sample you collect is not exactly the, same water that you’ve just poured into
the filter. And again, stored water did increase E.
coli levels by about 0.8 LRV. The filters did significantly reduce the
turbidity of the raw water by about 80%. Well let’s consider then some of the advantages and challenges of biosand
filtration. First, operation is quite simple. You pour water in and you get filtered
water out. But it does need regular cleaning, this
swirl and dump method, and the filter should be left to ripen after cleaning, so
that that is can be a challenge. The filter in general doesn’t change the
taste or the odor of the water but it does leave filtered water
with a high risk of recontamination. Especially when safe storage isn’t
applied. Users really appreciate the great
reductions in turbidity, but you should not expect that the filter has much of an
effect on chemicals, except perhaps iron. And in fact, nitrogen might be made worse. Ammonia could be converted to nitrites. Biosand filters can be manufactured on
site. The concrete models at least, but then
those concrete models are very heavy more than 150 kilograms
when they’re full. So once they’re put in place it’s very
difficult to move them around. It’s also difficult to transport them to the
household level if they’re collected at a central
location. There is evidence that the filters are
used for years consistently. But the same studies have shown that the effectiveness in removing pathogens is
rather limited. This has been just a brief introduction to
biosand filters. And I’d like to give you some places where
you could find additional information. First, Manz Water Filters. David Manz was one of the pioneers in
developing biosand filters in Canada. And the website has a lot of useful
information. CAWST, we’ve used some of their graphics
on this lecture and they have quite a number of documents and training materials, construction manuals
on their website. Samaritan’s Purse, which supported the
work in Cambodia, as well as many other countries, has some nice documentation on their website,
including a really nice animation of water passing through
biosand filters. And then biosandfilter.org itself is
another good repository of information. I’d like to point you also to two general portals about water including
household water treatment. This is sswm.info, and AKVO, that, both
have good, general information on biosand filters, as
well as other HWTS processes. So in summary, we’ve learned about biosand
filtration and how it’s an adaptation of slow sand filtration for
use at the household level. We’ve looked at some of similarities and
differences between biosand filtration and
conventional slow sand filtration. And have gone over the operation and
maintenance, including this important step of cleaning
the filter when the flow gets too low, and allowing sometime for ripening to re-establish the
efficiency of treatment. We’ve gone over some of the advantages and
challenges for biosand filters. And one of the significant challenges is
definitely only moderate pathogen removal, one or two log
removal for bacteria. Less than one log removal, typically, for
viruses. So, it’s best to think of biosand filtration as a filtration technology, but
not a disinfection one, and one that should
always be followed with safe storage, and ideally
disinfection.

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