(This post is an archive from the old blog see the link “about this blog”)
Posted on September 16, 2013 by filter_guy
A new filter has been started here. This is an experiment. I do not expect this filter to work however; there are those who claim a small filter like this will work to purify water. We will give this filter a fair unbiased test, just like all the other filters we have designed, built and tested here. We will note failures and successes. The tests will be done by one of the EPA certified laboratories we used for tests on the other filters, and we will post the results here when we have them. This process will likely take at least 2 months, perhaps more. The buckets are not food grade. The sand was sifted here. We used the 90% retained on 50 mesh sand we ended up with by sifting .35 mm effective size sand from Unimin through a 50 mesh stainless steel sieve. The sand color is white, it is quartz. We are using a light colored sand so we can observe the formation of the Schmutzdecke. Any sand should work as long as it is not contaminated. Know the source of the sand. Sift the sand using a 50 mesh screen.
The most critical parts of the operation of this filter (or really, any slow sand filter) are the speed at which the water flows through the sand and the maintenance of an absolutely undisturbed condition on the top surface of the sand. Also, the sand surface must always be covered by at least 1 inch of water, and water must run through the filter on a regular basis. If the sand dries out or is even exposed to air, the good bugs will die, and the filter will stop working. The beneficial microbes that make a slow sand filter work need to be under water, and have oxygen to survive; they are aquatic and aerobic. In other words, they need water and oxygen to survive. The ideal rate of flow for a full size filter is .2 meters per hour hydraulic loading rate. This means the water moves down through the sand a distance of two tenths of a meter each hour – that’s slightly more than 7 inches per hour. This rate is probably too fast for this small shallow filter. We may need to adjust the flow rate down to about 1/2 cup per minute, or less (about 2 inches per hour). The math to figure all this out is here at our main research web site “Rainwater Harvesting slow sand water filter”.
This filter is intended to be a biological slow sand filter. This means there will always be water covering the top of the sand, and there will be a living biofilm that will form on and in the top 2 or 3 inches of sand within about 3 weeks after first starting to add water to flow through the filter. This biofilm will contain microorganisms that will stay alive by eating harmful (and non-harmful) microorganisms in the water that flows through the filter very slowly on a regular basis. The water will be only a trickle – about 1 and a half cups a minute at the very most – and probably much less as the filter “ripens”. This is the way a “slow sand water filter” works. The difference here is that we are trying to do this with a very shallow bed of sand. Will it work? Maybe. . . . Directly below this introduction will be updates as we observe the operation on a day to day basis.
Update: September 20, 2013;
The 1/16 inch holes in the slip cap on the bottom outside of the top bucket plug up very easily. Some sort of screen on top of the bucket is necessary. We used nylon netting doubled up on top of the bucket.
Update: September 19, 2013;
On the left in this image is the input water, on the right is the output water. This is after 48 hours of operation. Water added every 2 hours using recirculation; filter sits quiet overnight.
Update: September 18, 2013;
So far, the water that has run through the filter is clearing up very nicely, and has a “fresh air after a spring rain” odor. It has taken 3 re-circulation runs through the filter however, to get the water to this point (a 4 gallon bucket filled with raw water that has been re-circulated 3 times through the filter: poured in, collected and poured back in 3 times). All the other filters here use re-circulation. To start with, the input water is somewhat stagnant, yellowish-brown in color, and has a slightly ”swampy” odor (I’ll have more images shortly). The flow rate has slowed to about 1 cup (250 ml) in 70 seconds; .2m per hour; or moving through 8 inches of sand in an hour.
Update: September 17, 2013:
So far the filter has worked to remove “color” from the water. This is raw water straight from the diverter. The same water that goes into filter 1 found on one of the websites associated with this blog.
The flow rate on this filter is absolutely critical. If it flows too fast, the sand breaks right through the pea gravel and fills the drainpipes and shows up in the output.
The flow rate must be restricted at all times to less than 1.5 cups per minute. If four 1/16 inch holes are drilled in the 2 inch slip cap the flow rate stays below 1.5 cups per minute. When you are putting this filter together, be sure to cap the output pipe, otherwise, water and sand will rush through to the drain pipes as you are filling the bucket with sand. Do not put dry sand into the bucket and than add water. Put water into the bucket first and be sure that the sand is completely saturated with water as each handful is added. Be careful when adding sand to begin with do not disturb the gravel layer. Add sand very, slowly at first.
This is unlike any of the other filters running here in that it is made using readily available 5 gallon buckets and pvc fittings, and the sand is only 9.5 inches deep. Usually there is nearly 3 feet of sand in a small slow sand filter. This version uses only slightly more than 50 pounds of sand and several pounds of gravel. This filter can be moved if needed and can be put together in a few hours. The cost is about 15 dollars. The question here is this: Will only 9.5 inches of sand be enough to actually purify water in this small biological slow sand filter? It must be kept in mind that the purpose here is to have this filter work as a biological slow sand filter having a biolayer (schmutzdecke) on top of the sand that removes pathogens from the water that flows through it, in the same fashion as the larger designs that have up to 3 or 4 feet of sand as filtration substance.
This filter will take about 2 months to test. The maximum flow rate is 1.5 cups per minute, the recommended flow rate is 1 cup per minute (this is based on the accepted flow rate range of from .1 to .35 meters per hour. ) Water must move through a biological slow sand filter at a slow rate. All of the math and stuff like that will be posted later. In the mean time below are images describing the filter.
There are 4 holes drilled in the 11/2 inch slip cap that are 1/16 inch diameter and they are evenly spaced around the cap. The cap has 1/2 inch trimmed off of it to fit into the 3 inches of space where the top of the sand meets the water surface.