Bio-sand filters overiew

Increase utility rate of bio-sand water filters in villages.

Bio-sand filters overiew

Postby ciardubain » Wed Nov 10, 2010 11:18 am

How does a bio-sand filter work?
A filter is usually an inactive physical structure that gets used up or full. This is not so for a bio-sand filter. The filter itself is an ecosystem that takes time to develop and is affected by what is put into it. The effectiveness of a bio-sand filter is influenced by both physical and biological variables. If maintained properly, the filter should last indefinitely.

Physical Action
The sand in the filter acts as a sieve (trapping large particles at the water-sand interface) and as an adsorption medium (particles adhere to sand grain surfaces) for particulate matter in the source water. The sand grain size is an important factor for the physical aspect of filtration, as larger sand grains have bigger spaces for particles to pass through. The grain size also influences total surface area available for particles to get trapped on. Smaller sand grains are also preferred in this respect. Sand grains that are too small will prevent particulate matter from penetrating the sand surface and the filter will have a very slow or nonexistent flow rate.

Biological Action
When the source water is introduced to a new filter, it takes time for a biological layer to develop at the water-sand interface. This layer is comprised of both harmful and non-harmful organisms including bacteria, algae, protozoa, and small invertebrates. The biological layer is actually a zone with the highest activity at the surface. The non-harmful organisms essentially out-compete and/or prey on the bad organisms, which end up dying. Both the living and non-living organic material introduced from the source water is utilized by the biological zone and reduced to harmless byproducts of metabolism.

Flow Rate
Ideally, the filter would have a fast flow rate to ensure a steady supply of clean water. Conversely, a slow flow rate is necessary for optimal filtration quality in a bio-sand filter. This turns out to be around 6 gallons/hour. On a physical level, a faster flow rate will push particles and oxygen further into the sand bed. This will lead to biological action deeper in the filter, which is not safe unless the sand bed is made deeper, necessitating a taller filter. On a biological level, a long contact time between the water and the sand bed/organisms living there is essential, as biological processes take time to occur.
There are a few variables that predominantly determine the flow rate of a filter. These are: height of water on top of the filter, surface area of water-sand interface, sand grain size and height of sand bed. Some additional factors are: pH of the water source, temperature, and total amount of particulate matter in the source water.

Intermittent Use
The intermittent use of filters is made possible by the filter design. Since the level of water is kept above the sand bed surface (where the biological system lives), constant water flow into the filter is not necessary. The bio-active layer is kept wet and therefore remains alive. However, the bio-active layer needs oxygen and nutrients from the source water, so long periods of non-use (ignoring evaporation) will lead to death of the biological layer. The filter is not forfeit in this instance, as the biological layer will redevelop in time if water is filtered on a somewhat regular basis.

What are some difficulties encountered in using the filters?
I spoke with an individual who worked in Nicaragua with bio-sand filters. His perspectives are listed below:
• Filters are heavy and difficult to deliver
• Incorrect use led to contamination of filtered water.
Same bucket to collect river water and filtered water allows bacteria remaining in bucket to re-multiply in t
filtered water
• Not worth the time if difficult to understand/use the technology
• Flow rate is slow

My thoughts on education and guidelines for use
It would be very difficult to attempt a universal set of guidelines for use of the filters. Things like how often to add more water or how long to wait for the biological layer to form will be extremely dependent on the specific location (climate, source water contaminants, and how much water is filtered daily). Therefore, I think the best investment of effort is to educate the people who will be using the filters. The necessary topics to include in such education would be:
• Connecting illness to microbiology of the source water
• Connecting proper filtration with a “healthy” bio-sand filter
Healthy means kept wet with a sufficient supply of nutrients. The filter does not simply strain out particles. It is
• Connecting contamination of filtered water with continued illness

I think it is worth examining the “buy in” of households who receive the filter for free and are not responsible for maintenance. This setup seems to be counterproductive if education and awareness is the goal. My contact who worked in Nicaragua illustrated a setup where the filters were free in terms of money, but a set number of hours of help delivering filters were required to get a filter. This required some investment by the filter recipients and may have positively impacted the willingness to put effort into using the filter. Sadly, the filters are not a zero-effort convenience.

The information in this document is distilled from and a conversation with a contact who worked in Nicaragua, where biosand filters were used locally.
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Re: Bio-sand filters overiew

Postby H. Timothy Hsiao » Thu Nov 11, 2010 12:58 pm

Hi Jay,
This is GREAT! Thanks so much for your appreciated efforts!
And your suggestion of examining the "buy-in" model is truly inspiring.

A hypothesis based on your thoughts/guidelines would probably be clearly convey the correlation between:
No-filter - sick
Filter - a protection from getting sick (could probably associate "being sick" with the "earning lost/cost for treatment", so the villagers would feel more relevant)

Also, if the filter is available freely, the villagers may not appreciate it, and only dismiss it because of the hassle for maintenance. May be we should divert the image of the filter in the village

"a big inconvenient chunk they tried to make us keep in our household"

"this bio-sand filter costs USD$100, it's "highly valuable", "a true help for my family", I have to earn the right to use it"

However, apart from their time, what else do the villagers have to "pay" for the use of the biosand filters though?
H. Timothy Hsiao
Posts: 46
Joined: Thu Oct 07, 2010 1:10 am

Re: Bio-sand filters overiew

Postby H. Timothy Hsiao » Thu Nov 11, 2010 1:20 pm ... ter_Filter

Hi Jay,

I posted your content on our wiki: ... ter_Filter
I haven't formatted it yet, but would list it up so all could see!

H. Timothy Hsiao
Posts: 46
Joined: Thu Oct 07, 2010 1:10 am

Re: Bio-sand filters overiew

Postby ciardubain » Sun Nov 14, 2010 10:30 am

Hi Tim,
I was driving at the idea that free filters that are maintained for free could very easily turn into "hunks of junk they keep in our house" as you nicely illustrated. As you also put nicely, we want the filters to be viewed as "valuable assets that we keep in our house". I think it is important that filter recipients take some ownership of and investment in the filter for this transformation to occur.

To answer your question about what else the villagers need to pay to use the filters, I think the initial investment of helping with delivery or even construction of the filters is a good way to encourage the perception change we're looking for. You posted previously that Daniel's team makes regular visits for maintenance of the filters. I'm wondering if we could get a list of specific tasks performed at these visits. I have a feeling that this is something the villagers could learn to do and would keep them invested in the filter's operation. With ownership and knowledge of how to maintain the filter, the day to day use of the filter is a natural step from here and that is what we want.

Also, thank you for posting the overview to the wiki page.
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Joined: Tue Oct 12, 2010 10:02 am

Re: Bio-sand filters overiew

Postby H. Timothy Hsiao » Tue Nov 16, 2010 6:42 pm

Found cool content in Team 2's findings:

our wiki: Existing Public Health Education Programs:
"...Programs such as these have been implemented successfully in places like Africa and Asia using strategies that rely heavily on community participation. This makes behavior changes easier to adopt and ensures sustainability, since the people of the community direct the process towards goals that are important to them. It is important to note that improving health is not always the most effective motivator for change. By talking to the community, other motivators can be identified and then used when designing an education strategy. Other motivators include: averted health-related costs and avoidance of time lost from daily activities as a result of illness, improvements in environmental cleanliness, self-esteem and social status..."

lack of awareness of the importance of health may be a big stumbling block for the acceptance of filters,
so public health education in general may be very relevant to our Project 3.

Also, new motivators may be useful...

+ re-brand the filter as a privilege and valuable gifts.

These are some of my thoughts.
H. Timothy Hsiao
Posts: 46
Joined: Thu Oct 07, 2010 1:10 am

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