Tuesday, August 10, 2021

Slow sand filtration

The river in the background is chocolate-brown with sediment and run-off from pig-pens and the feces that washes off the surface at the onset of the wet season.

Slow sand filtration is one of the oldest methods of sanitizing water and cleansing it of (most) pathogenic organisms.

I was skeptical of sand's ability to filter out bacteria and virus. Surely the pores between the grains are hundreds, if not thousands of times greater than the size of the bacteria and virus.

The misapprehension was due to my only looking at the gross, mechanical attributes of the sand bed.

There are other phenomena that come into play.

An analogy

I can physically navigate through city streets. The roads and alleyways and spaces between the houses are such that I can physically squeeze through them and walk from one side of the city to the other.

But suppose I was dressed like a frat-boy or a dandy, what are the odds of my being able to traverse the width of Detroit some July or August evening? Something less than 100%. It seems likely that some of the local toughs might take exception to my presence and terminate my travels.

The same thing happens with the bacteria and virus in water slowly percolating through sand. The surface of every grain of sand is coated with slime. That slime is composed of colonies of bacteria that FEED on organic material in the water.

Those bacteria do not discriminate between dead organic matter and other micro-organisms. To them, organic matter is organic matter. It is all food.

"Schmutzdecke" is the technical jargon for the layer of bio-slime that covers the top of the sand bed.

The prime feeding position is at the interface between the incoming water and the sand, before any of the "food" has been strained out of it. In a short amount of time, a living layer of slime bridges the pores between the grains of sand and the water must filter through that living membrane.

But does it really work?

Let me quote Art Ludwig, the man who took the picture at the top of this post

(Location:) It was 4 m of coarse, highly permeable sand from a river chocolate brown first runoff water containing (the equivalent of) 20,000 buttwipes per swimming pool of fecal coliforms.

(fecal coliforms/100 ml) 0

Discussion: I was astonished at how clean this water was; I've drank untreated spring water with higher levels, without problems. This result suggests that 1) this amount of sand filtration is extremely effective, and 2), the contamination in all the wells in Maruata (Michoacán, Mexico) comes more from above than with the water from below, and that it increases with the age of the well. This was one of the cleanest wells tested, with the only possible explanation being that it was by far the most freshly dug, this factor overriding the close proximity of a huge quantity of very contaminated water.

Shortcomings of Slow Sand Filters

Slow Sand Filters are large for the amount of water they process.

They have to be large because the prime mode of removing pathogens relies on Brownian motion to knock the microbe into the slime that will eat it.

The general rule of thumb for residential usage is to figure 100 gallons per person, per day. In all likelihood, 99 gallons goes to purposes other than drinking water.

City water treatment plants also supply businesses like car washes and manufacturing, all of which use a lot of water that does not necessarily have to be of potable quality.

Consequently, cities quickly abandoned Slow Sand Filters for other methods that were more space efficient.

Another shortcoming of SSFs is that they are biological entities. They are not like electric motors that start with the flip-of-a-switch. They take time for the biological film to develop on the individual grains of sand. The filter's ability to remove pathogens is not 100% as the film develops.

The biological nature also comes into play in that you cannot turn off the filter for a year and expect it to start back up at 100% efficiency. The biological films need to be fed if they are to be maintained.

SSFs require maintenance. The top layer of sand becomes clogged as the biological film on the grains of sand grow in thickness. The filter is renewed by scraping off an inch or two from the top of the bed, letting it rest for 24 hours before restarting.

A deeper reservoir of water above the sand bed is one way to mitigate against incoming water agitating the bed.

SSFs are vulnerable to "washing" from water entering too violently. The film that covers the top of the bed does the lion's share of filtering and, especially when young, is fragile and easily torn if the water enters too violently.

SSFs are vulnerable to clogging quickly when the water source comes from dirty sources. The "dirt" can be either inorganic like silt or organic like algae. It is generally advised that the if clean water is not available that the incoming water be strained before pumping into the SSF. One way to strain surface water is to get perforated drain tile that is encased in a sock. Drain tile encased in a sock is commercially available and is used where the soil is likely to infiltrate and clog the tile.

SSFs become less efficient when it is cold and are vulnerable to freezing if outside.

What are the positives?

They are VERY low tech.

They use materials and parts that are usually very easy to find. That is, the technology is accessible to anybody who can execute farm-yard quality plumbing and use a shovel and buckets.

The are not energy intensive.

They don't HAVE to be huge if the household commits to using potable water only for drinking, cooking and washing hands and face.

A word picture

"Pa" filled two, five gallon buckets of water from the rain-water cistern. Until he had plumbed the cistern he had been dipping the water from the cattle pond 100 yards from the house. He much appreciated the shorter walk and the cleaner water reduced the filter maintenance.

He poured the 8 gallons (some sloshed out as he carried them) into a plastic barrel on top of a short platform. He made two trips every morning.

The barrel on the platform fed the SSF via a hose with a valve. The valve was cracked so the water dripped into the top of the SSF. One of Pa's chores was to check the drip rate to ensure it had not clogged.

The actual SSF was comprised of two, 60 gallon plastic barrels 90% filled with sand Pa had harvested from a sandbar next to the creek. His back-up plan was to wash the silt out of the dirt from west of the barn. That dirt had a high sand content and would do, if washed, in a pinch.

Pa opted for a two filter system because he could do maintenance on one of them while the other supplied the family. He flipped the valve on Monday so each barrel worked one week and "rested" the next.

The water percolated through the filter and then into an in-ground 60 gallon, plastic barrel. Periodically Ma would send a young-un out to run the cistern pump and fill a container to carry into the house to supply the domestic needs.

Most of the other domestic needs were supplied directly from the roof-water cistern. That water was ALMOST potable but there was a tree over the house that provided shade. Unfortunately, the tree was a favorite roosting spot for flocks of starlings which invariably left feces on the roof.

Bonus link: Most of the academic literature on Slow Sand Filters is almost unreadable. That is why I wrote this post...to make the information more readable. This document is almost readable.


  1. Thanks for the education! That's a good fallback plan, especially out here where we have 'plenty' of sand... sigh

  2. Thanks for the usable, readable summary.

  3. Having good reference info is great but if you think you might ever have to rely on a SSF, you should try to make one now. I just bet that a SSF is like growing a garden - you are not likely to have 100% success the first time.

    1. Well, that and the fact that it takes time to cultivate the bacterial slime. Two weeks is what one reference suggested as the amount of time required for the BS to stabilize.

      On the other hand, unless the fall from civilization is catastrophically brittle, there will be time (if only a little) to go through a learning curve.

  4. Thanks for the great break down. Extremely understandable. (And I have a disorder which effects my mental compression.)
    Side note. My father was a College Professor for close to 40 years. I grew up around Teachers. You Sir, rank up there among the best. Your style, your delivery, everything is superlative. Please, keep it up. Thanks for all you do.

  5. ERJ, from the nature of your words I gather sand filters is a new discovery for you. I am somewhat appalled that one as learned and inquisitive as you would be nonplussed by this.
    Nevertheless, glad to have you onboard.

  6. I've had information on sand filters tucked away against need for many years now. Your explanation beats them all to pieces.

    And catfish is correct. Mrs. Freeholder was a teacher, while I could show you how to do something. There is a vast difference, and you're on her side of the road rather than mine.

  7. Question: how do I remove dissolved solids from water in such a simple fashion? Shallow ground water at my house is 2,500ppm. Crystal clear, but poison to everything it touches. Even kills the grass. The water is close to being described as liquid rock.

    1. The only way I know if is a large tank (know as a settling tank) into which you introduce some chemical(s) that bind with the solids and make them heavy enough to settle out.

      Reverse osmosis may also work, but I'm not very familiar with that method.

  8. Sand filters have been on my mind recently. Thank you for this.

  9. They make ceramic filters to do the same thing.

    Not sure if they filter out all the bad stuff though.


Readers who are willing to comment make this a better blog. Civil dialog is a valuable thing.