Introduction: Biosand Filter

Picture of Biosand Filter

A biosand filter is a simple type of water purification system that uses sand, gravel, gravity and some simple engineering to purify water contaminated with biologics and some chemicals. Its structure is generally made of concrete or plastic and is filled with layers of gravel and sand that encourage the growth of good microbes that are naturally present in water, just like in the ground in nature. These good microbes develop into what's called a "biolayer" or a "biofilm," which destroys pathogenic (i.e., disease-causing) microbes to create clean drinking water. The sand also causes pathogenic microbes to become trapped, adsorbed (i.e., stuck to the sand particles) or die from lack of food or oxygen.

The biosand filter constructed in this Instructable was made using materials commonly available at any hardware store (Home Depot in this case). Two were built to test their effectiveness for a real-world application in the community of Chakicherla Pedda Patapu Palem in Andhra Pradesh, India for an Engineers Without Borders project run by the San Diego Professional Chapter's James Harper, PE. Check out this Engineers Without Borders project at the EWB San Diego Professional Chapter website.

Many types of biosand filters are out there, but the standard one that is currently in use all of Africa and other parts of the world is made out of concrete. The Centre for Affordable Water and Sanitation Technology (CAWST) developed a great biosand filter construction guide that has lots of useful information. I've attached a PDF copy of this document to this Instructable for your knowledge and reading pleasure.

A few other interesting biosand filters out there have been made by Surfers Without Borders and the BioAusable Project.

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This project is ongoing and not complete but should be completed within the next two weeks. Please check back soon for more to come!

Step 1: Research Research Research

You should always complete your due diligence (i.e., educate yourself until you can make informed decisions) for every project, small or large, that you start. This gives you a solid foundation on which to design, build, test and optimize everything you make so that your projects work just as you want them to! It's also really cool to see what other people have done; there are some SMART PEOPLE out there; why not learn from them?

Here are some of the better sources of information I've found on biosand filters, from most helpful to least.

CAWST.org* BiosandFilters.info* SlowSandFilter.orgAfropedea.orgHydraid.orgOffTheGridNews.comKnowledgeWeighsNothing.orgH2OHow.comAberAndOut Blog

Step 2: Effectiveness and Limitations

Picture of Effectiveness and Limitations

Effectiveness

The effectiveness of this type of filter varies based on many factors:

1. Was the filter installed more than 30 days ago?

It takes the filter about 30 days to grow an effective biolayer.

2. The filter is used at least once per day using water from the same source every time.

3. A pause period (see Step ?) is used after each filling.

4. The water poured into the biosand filter is clear.

If you only have access to dirty or cloudy water, leave it in a separate bucket until the sediment settles out of it. Then, pour in the clear water on top and discard the sediment.

5. There are no leaks or cracks in the biosand filter container.

6. There is a diffuser on top of the sand.

7. When the water stops running, the water surface is 5 cm (2") above the top of the sand.

8. The top of the sand is flat and level.

9. When the filter is full, the water flow rate is 400 mL per minute or less.

Types of Water

This filter should be effective when using fresh natural water sources that have not been pre-treated (e.g., rivers, lakes, wells). It is important to note that this type of filter will not be effective when using chlorinated water sources, such as the municipal water supplies available in many cities.

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Note that the information reported here refers to the biosand filter described by CAWST in the document linked in the first step of this Instructable and is not specific to the particular biosand filter described in this Instructable. However, the design of the biosand filter described in this Instructable is similar to that described in the CAWST document and should thus have a similar effectiveness.

Step 3: Buy Your Materials

Picture of Buy Your Materials

Here are the materials I used. I specifically chose these materials to make this filter as cheap as possible while still being readily available. All materials should be available at your local hardware store.

  • Three 5 gallon buckets with lids
  • 51 inches (4 feet 3 inches) of 1/2" PVC Schedule 40 pipe
    • A little extra wouldn't hurt to account for errors in measurements
  • One 90 1/2" PVC degree elbow with one threaded female end and one female press fit end
  • One 1/2" PVC sprinkler riser
    • These seemed to screw all the way into the above female elbow's threads so that the o-ring below will seal properly on the inside of the top bucket.
  • One o-ring with a 3/4" inner diameter
    • At Home Depot, I bought a pack of #215 O-rings.
  • One 90 degree 1/2" PVC elbow with one threaded male end and one female press fit end
  • One 1/2" PVC T joint
  • Two 1/2" PVC end caps
  • Silicon caulk
  • PVC cement
  • Super glue
  • Goo Gone
    • To get those stubborn retail stickers off!

...

Note: a press or interference fit is a type of connection where two smooth-walled parts come together tightly to form a solid rigid connection. In this project, threaded PVC and press fit PVC are used in different locations.

Step 4: Get Your Tools

Picture of Get Your Tools

These are the tools I used.

  • Eye protection (100% required whenever using power tools!)
  • Ear protection (99% required whenever using power tools!)
  • Power drill with 7/8" spade bit or hole saw
    • This bit is usually used for wood but works fine here on plastic
  • Dremel with a multi-purpose cutting bit
    • The Dremel is really nice for cutting the middles out of the bottoms of the buckets and the lids, but a keyhole saw or something similar could work as well. My jigsaw was too big to get close enough to the rim around the buckets' bottoms and lids.)
  • Hacksaw
  • 220 grit sand paper
    • Makes all the edges of your cuts and holes really smooth!

...

Step 5: Start Building, Part 1

Picture of Start Building, Part 1

First, some rules.

  • Again, always wear eye protection and almost always ear protection!
  • It is VERY IMPORTANT (and good practice) to clean all cut edges with fine-grit (e.g., 220 grit) sand paper.
    • This makes sure everything works well, fits together and looks good.
  • Remove all UPC labels. They are ugly and could potentially contaminate your water. Goo Gone as needed.

Alright. Here we go!

  1. Drill a ⅞” diameter hole using a spade bit or a hole saw 6” down from the top of what will be your biosand filter's top bucket. This is the hole that will allow the clean water outlet pipe to pass from inside your filter to the outside so that you can gather.

Step 6: Build Build Build, Part 2

Picture of Build Build Build, Part 2

Cut out the bottoms of your top bucket and one other (i.e., your middle bucket) with your Dremel.

  • Be sure to leave a ¼” to ½” wide rib to retain the rigidity of your bucket's walls.

Step 7: Build Build Build, Part 3

Picture of Build Build Build, Part 3

Place a lid on your bottom bucket and stack your middle bucket on top of your bottom bucket's lid. Using the cutout in the bottom of your middle bucket, mark out where to cut out on your bottom bucket's lid.

Repeat this process for the middle and top bucket, respectively.

Clean up all of the plastic debris generated, particularly inside the buckets.

Now, you should be able to stack all of your buckets in the correct order and have a large hole going through from the top bucket to the bottom bucket.

Step 8: Build Build Build, Part 4

Cut the following lengths of 1/2" PVC tubing:

  • 1x 33.5” for your main tube
  • 2x 1.5” for the coupling sections between the T joint and the elbow joints
  • 2x 6.75” for the collection tubes at the bottom of the filter

Step 9: Build Build Build, Part 5

Picture of Build Build Build, Part 5

Drill ⅛” holes in the collection tubes every ¾” or so around their circumferences and along their lengths (i.e., turn your collection tubes into swiss cheese!).

  • These perforations ensure that water will flow throw your filter even if some of these holes get clogged over time.
  • After you finish drilling, try your best to clear out the plastic debris from the inside of each tube. This is the hardest and most tedious part of the project. Do it while watching an episode of your favorite TV show.
  • Be aware: that plastic is messy and goes everywhere! Be sure to vacuum up as much as you can.

Step 10: Assemble!

Picture of Assemble!

Let's put it all together now, shall we? No need for glue yet; just a dry fit for now; gluing comes next. Be sure NOT to seat all PVC connections yet; they can be very hard to get undone even without gluing.

  1. Stack all of your buckets in their proper order, if they aren't already. Leave the top lid off.
  2. Place an end cap on each collection tube.
  3. Connect each collection tube to one of the press fit elbow joints.
  4. Connect these two assemblies to the connecting tubes.
  5. Connect these two assemblies to the two opposing sides of the T joint, creating a single assembly.
  6. Connect the main tube to the remaining side of the T joint.
  7. Connect the press fit side of the half-threaded-half-press-fit elbow to the top of the main tube.
  8. Screw on the female side of the sprinkler riser onto the threaded side of the half-threaded-half-press-fit elbow that is now at the top of the main assembly.
  9. Carefully work one of the o-rings over the threads of the sprinkler riser to the base of the threads. The o-ring should be sitting up against the non-threaded outer portion of the sprinkler riser. You should now have the completed inner assembly for your biosand filter.
  10. Carefully lift the entire assembly into the inside of your stacked buckets. It will be a tight fit, but it should fit!
  11. Turn the assembly so that the sprinkler riser pokes out through the 7/8" hole you made at the beginning. The riser and hole may not line up vertically; if so, adjust how your main tube sits in the T joint and in the half-threaded-half-press-fit elbow; only ~1/4" of the main tube needs to sit in each of these joints in the final assembly. It is critical that the sprinkler riser pokes out easily through the hole; any stress on this connection will only be magnified after sand and gravel is added to the filter. If you have to chose, err on making the main tube too long rather than too short. The collection tubes should be sitting firmly on the bottom of the bottom bucket.
  12. After ensuring proper placement of the sprinkler riser and the assembly as a whole, screw on the final outlet elbow, which should be half-threaded-half-press-fit. Ensure that the o-ring seals well against the inside of the top bucket; the o-ring should be compressed slightly but not smashed to smithereens! Moderately tight is good. :-)

Step 11: Glue It Up

Picture of Glue It Up

After ensuring all lengths of tubing are correct, paying particular attention to how the sprinkler riser exits the outlet hole, disassemble everything and start gluing everything up using PVC cement.

  • A surprisingly thin amount of glue is required in the press fit connections. No big drips and globs are required. Just thinly coat both mating parts and press them firmly together in their correct orientation.

I would suggest checking each connection for fit in your filter as you glue each connection to make sure that it fits into your filter as intended. PVC is very forgiving stuff UNTIL YOU GLUE IT; then, it's not easy to fix and will require additional materials.

This is also a very good time to seal and glue up the buckets! Run a bead of silicon sealant around the lids on top of the bottom and middle buckets so that they seal to the buckets that they mate with. This will make sure your filter doesn't leak.

  • We will be super gluing this connection after the silicon sealant has dried for extra strength.

Step 12: Final Assembly

Picture of Final Assembly

After you glue your final connection and rethread the top two threaded components onto the main tube, replace the assembly inside your buckets and screw on the outlet elbow.

Last thing before you leave your filter to dry: add some super glue from the outside between the lids on top of the bottom and middle buckets and the middle and top buckets, respectively. This will give your filter a bit more strength. I added about 1/4 tsp every 2 inches around the circumference of each lid.

Allow the PVC cement to dry for however long the directions indicate (usually 2 hours). Be sure to leave the top lid off so that the cement may outgas (i.e., give off fumes as it dries) as needed.

Step 13: Get Your Gravel and Sand

This project is still a work in progress, but I will be following the instructions in the CAWST PDF, Part 2, Stages B-D; this document is linked in the Introduction of this Instructable.

Step 14: Wash Your Gravel and Sand

This project is still a work in progress, but I will be following the instructions in the CAWST PDF, Part 2, Stages B-D; this document is linked in the Introduction of this Instructable.

Step 15: Add Your Gravel and Sand to Your Filter

This project is still a work in progress, but I will be following the instructions in the CAWST PDF, Part 2, Stages B-D; this document is linked in the Introduction of this Instructable.

Step 16: Instructions for Use

TBD

Step 17: Testing and Performance

TBD

Step 18: Conclusions

TBD

Step 19: Future Work

It may be possible to increase the effectiveness of this filter even further using a piece of wood in the outlet tube. Check out this Popular Mechanics article on wood xylem.

Comments

Sparkchaser1 (author)2015-03-08

CAWST helped us with our base filter design. It was made using a 55USG drum and followed their concrete filter design. Our flood and drain filters were designed to stack on the base filter, up to three filters high.

Adding water to the F&D filter forces air out of the sand as the water level rises and results in a long still water contact time with the sand. When the water gets high enough, the filter will automaticly start to drain into the next stage of F&D filter. As the water drains out of the sand, air is drawn down into the sand bringing oxygen into contact with the biofilm microbes. Those are what does all the work.

We were adding river water that was very yellow and high in Ecoli and fecal coliforms, bad stuff we could measure. By the time the water reached the top of the base filter, we getting 99% reductions or non detects. And the water was so clear it was hard to see if there was any in the base filter. Had to hit the side of the drum to cause ripples so you could see the water level.

The flood and drain system needs more testing.

jimmypat (author)Sparkchaser12015-03-17

Great feedback! I love to hear that CAWST is such a good company; they certainly seem like it. Do you have a link to your project? I'd love to see how your F&D filter works.

Sparkchaser1 (author)jimmypat2015-03-19

These drawings will show a flood and drain cycle. All the sand is flooded, then exposed to air. All gravity powered, just add water.

jimmypat (author)Sparkchaser12015-03-19

Very interesting. Thanks!

Sparkchaser1 (author)jimmypat2015-03-17

Try these...

Klowder (author)2015-03-06

surferswithoutborders.org has a plan for one made out of a 55 gallon drum. It's ideal for a house filter to filter rainwater and has a charcoal component to filter out chemical contaminants.

http://www.surferswithoutborders.org/Slow_Sand_Fil...

jimmypat (author)Klowder2015-03-06

Fantastic! Looks great. Thanks for your comment.

Sparkchaser1 (author)2015-03-04
I was a 4-H leader when we built a similar Bio-sand filter for the Lemelson MIT Inventeams. We presented our version at EurekaFest 2012 in Cambridge Ma. We were unique because our design used flood and drain technology to build a very active aerobic zone, about 30" deep. This is many times deeper than a regular bio-sand filter can achieve. After running through the flood and drain section, the water flowed into a standard 55USG Bio-sand filter to take advantage of the anaerobic zone in the final stage. Of the 16 projects presented at MIT that year, ours was the only one that could be built by the people needing it, using hand tools and items commonly found in the Philippines. Water testing showed great results, even though the feed water heavily contaminated.

Please see this link about our project.

http://bioausableproject.blogspot.com/

jimmypat (author)Sparkchaser12015-03-05

Fantastic! Great project! I'll definitely take a look into this in more detail. Thank you!

RakeshM21 (author)2016-02-11

Sir can i get more information regarding how to build that bio layer in the filter. I am constructing this bio sand water filter for my project but i dont know how to create this bio layer or what are the good bacteria's present in the bio layer.

Thanks in advance

Regard's

Rakesh G

jimmypat (author)RakeshM212016-02-15

Hi, Rakesh.

Thanks for your question! It is great to hear that you are constructing a biosand filter!

This layer grows naturally when unpurified water is added consistently to the filter over the course of approximately 1 month. If you add chlorinated water into the biosand filter, it will kill off the good layer of bacteria. I don't have any additional information about what type of organisms are in the biofilm layer or how to improve their growth or health.

Please let me know if you find any additional information! I would love to read it.

Cheers,

James

OHorizons made it! (author)2015-11-24

Hey JimmyPat!

Thanks for starting a lively conversation around DIY BioSand Filters! We're late to the party, but have a fascinating addition to the conversation. Our nonprofit organization, OHorizons, has engineered a Wood Mold for the production of concrete BioSand Filters. Our construction manual and appendix are open-source and free to download on our website.

If your EWB Chapter is still looking into doing the water project in Andhra Pradesh, India, we'd highly suggest looking at our solution. It is inexpensive and can be made on site using local materials and labor.

Our design is based on CAWST's Steel Mold Version 10. Unlike others who have attempted to create Wood Molds, we've engineered a sustainable solution. In its life cycle, each Wood Mold can produce over 50 concrete BioSand Filtes! Depending on local prices, a single Wood Mold typically costs between $50-80 to build.

Since 2014, we've done pilot projects in Ecuador, Mali, and Bangladesh. With its low implementation cost, local organizations can afford to produce many Wood Molds, thus constructing more filters and providing their community members with clean water at an exponentially faster rate than is currently economically feasible.

I hope this is helpful. After reviewing our site and information we'd appreciate it if you'd include a link to our page in your article. Please let us know if you have any questions.

Best,

Dylan

jimmypat (author)OHorizons2015-11-25

Thanks, Dylan! Great project. I'l definitely take a look into it.

JackieT5 (author)2015-09-29

Where is the pdf drawing to this project.. Thanx

jimmypat (author)JackieT52015-09-30

I'm sorry, but I have not created a PDF drawing of this project. My design is similar to that published by CAWST that is referenced in this Instructable.

k'suge' (author)2015-09-17

Please, is there any video available for this project? Thank you

jimmypat (author)k'suge'2015-09-17

I'm sorry, but there is currently no video. This project has been put on the back burner currently, but upon its completion, I plan to post a video of it in operation.

k'suge' (author)2015-09-17

Please, is there any video for this project? Thank you

baudeagle (author)2015-04-27

Not related but interesting: A water condensing unit that produces 25 gallons of water a day.

http://www.offgridworld.com/crazy-looking-bamboo-tower-creates-25-gallons-of-drinking-water-per-day-from-thin-air/

jimmypat (author)baudeagle2015-04-27

Yes, the Warka Water harvester is pretty amazing. It does need some pretty specific weather conditions to work though. Thanks for sharing!

AugerDog (author)2015-03-03

Thanks for contributing this. It is so timely. I'm in the process of setting up a rainwater collection system and was thinking of how I'd filter it in case I needed potable water. Something you may also want to consider is painting the buckets. In my experience, the buckets can become very brittle from solar UV exposure. You needn't use anything ultra durable unless you aren't able to do periodic maintenance. Also, I'd collect the plastic shavings/dust and just add it to the top layer of sand. From my experience with outdoor ponds and fish tanks, the plastic makes an excellent matrix for beneficial beasties to live on. I'm looking forward to your future developments.

Sparkchaser1 (author)AugerDog2015-03-19

Auger,

The best idea I have seen is a timed flow diverter valve at the collection point of a roof drain system. It allows you to divert away the first 20 minutes of water flowing off the roof, keeping it out of your collection tank. That water will be contaminated by dirt dust and whatever is there, but will still wash/rinse the roof of bird poop and other stuff you would not want to drink. After 20 minutes the unit will switch to send the now much cleaner water into your tank for the rest of the rain event.

AugerDog (author)Sparkchaser12015-04-08

I like your suggestion. The reason I was considering a BSF was because of roof born contaminates. Living in Southern California with our drought, I need to consider if I can spare the first 20 minutes of rainwater. I live near the coast so not all of our rainfall will be heavy. Perhaps if there will be a succession of rain fronts I could allow for the 20 minute pre-rinse cycle especially when considering potable water. Otherwise, I'm using it for irrigating my yard. I'm also exploring fog moisture catcher and dew collection systems. It keeps me off of the streets and out of the pool halls.

jimmypat (author)AugerDog2015-03-05

Thanks for your comment and idea. I definitely do not recommend placing this type of filter outside due to the issues you mention. They are designed primarily for a household and so would ideally be placed inside. This will also keep them from degrading due to UV damage.

sarahhanahghan (author)2015-04-01

what an amazing design!

alexmac131 (author)2015-03-14

I really like this one - charcoal component I agree with on the other comments by others - that said - very nice. Clean water is the most important item in any kit. Storms can ruin pretty much all local sources of water

jimmypat (author)alexmac1312015-03-17

Thank you. Yes, a charcoal filter after the biosand filter would be able to remove chemicals that the biosand filter cannot and would thus be a good and easy addition.

JohnnyMorales (author)2015-03-04

JohnnyMorales0 seconds agoReply

I built something very similar to this to be a biofilter for a pond I have with one difference, I didn't include the piping to a spigot to the top. Instead I used a spigot I could open and close placed at the bottom.

I wasn't going by any plan, and figured that the action of the water pouring through the layers would compact the sand enough to prevent it from flowing out with the water combined with the placement of a nylon scrub (used to lather soap in the shower normally) on the inside to block the gravel from clogging the spigot would be more than enough to prevent that. I was right for the most part.

Whatever sand is loose enough to flow out goes with the first few pour throughs.

After that to get any sand in the water, I had to jostle the container. Otherwise the compaction was enough to ensure the water was crystal clear and free of any sand.

So you could make your good idea even easier to construct by replacing the piping to deliver the water to the top spigot in order I assume to avoid getting sand in your water and maintain a minimum water level to keep the biofilter alive with a single spigot at the bottom that you can close to do the same thing.

The piping to the top is not to prevent the sand from coming through. It is to keep the bio-layer under water so the good bacteria will live to clean the water.

Also I think the sand comment was more in reference to the likely problem of having a spigot at the bottom, I only mentioned it in reference to your design to the issue within the context of both to make sense of it rather than a reference to why you did what you did. In your design it is not an issue.

Yes I got that, that's why I suggested a spigot you could close. ;)

It's a trade off really. It's easier to construct, but you have to mind it a bit to turn it off to keep enough water in to do that.

jimmypat (author)JohnnyMorales2015-03-05

Thanks for both of your comments. Good discussion!

The main reason for the PVC tubing to the top is actually to ensure that the water flows very slowly through the filter. Yes, it does also prevent the sand from flowing through, but that is not its primary purpose.

A slow water flow rate ensures that the biofilm layer develops to its full potential and has time to kill off pathogenic microbes in the dirty water. If a spigot were used at the bottom of the filter, as you suggest, this slow water flow rate could be replicated but only if the spigot were opened a very small amount. The ideal flow rate for a filter of this type is under 400 mL per minute. So, if you adjusted your spigot to match that flow rate, it should work similarly to the filter I show in this Instructable.

marcintosh (author)2015-03-04

the layer at the top is called a Schmutzdecke. It's a gelatinous layer and it's necessary for the filter to work at it's most effective. Not always attractive though.

My 2¢

Thanks for all the info

M.

jimmypat (author)marcintosh2015-03-05

Yep, not the prettiest thing in the world, but it does the trick! Thanks for your comment.

rvandenberg1 (author)2015-03-04

Thanks so much for all the information that you posted. My wife is an environmental prosecutor here in Brazil and she has many problems finding solutions for the people using well water that is not potable for drinking water. She is very excited about making this project in a larger scale from concrete and the links that you posted gives all the needed information to do so. Thanks again Jimmy.
Robert. ( a Canuck living in Brazil.)


jimmypat (author)rvandenberg12015-03-05

This is great news! I'm so happy to hear that this project could help benefit so many. As I mention in the Intro of this Instructable, I will be using it for an Engineers Without Borders project in India. Keep up the good work and spread the word!

ChickenJo (author)2015-02-28

Cool! You should put it in the apocalypse preparedness contest.

jimmypat (author)ChickenJo2015-03-05

Thanks for your recommendation. I entered the contest; we'll see what happens!

Ironwave (author)2015-02-27

Nice,

Please do finish this, I'm interested in building one.

Thanks for the good info!

jimmypat (author)Ironwave2015-03-05

Thanks! I definitely will.

tomatoskins (author)2015-02-27

This is really cool! I'm excited to see how it turns out.

jimmypat (author)tomatoskins2015-03-05

Thanks! :-)

n5yzv (author)2015-03-03

I was reading on a site, I forget which one, but it has been a research project going on for many years with sand filters. They came to the conclusion, the 5 Gal bucket filters didn't work well enough to justify it's usage. The 55Gal Project did. Google it. Now, if this is all I had, sure. I would use a 5 gal solution.

jimmypat (author)n5yzv2015-03-03

Thanks for your comment. You are correct with regard to filters: the bigger, the better. A larger filter has significantly more volume (cubic relationship) and thus surface area on the sand inside on which good microbes (i.e., biofilm) grow. This increases effectiveness.

As a result, the filter you mentioned that uses one 5-gallon bucket is not particularly effective; they simply do not have enough surface area on the sand within them to grow an effective biofilm layer.

However, this project uses three 5-gallon buckets, which do have enough sand surface area to be effective. Check out the CAWST document I posted in the first step of this Instructable; even though there system is made with concrete, it is the same size as the filter described in this Instructable.

n5yzv (author)jimmypat2015-03-03

That is true. I looked over the construction as saw that 3 buckets were being used instead of one. Again, I have the materials to make a smaller 5 gal bucket solution as a primary filter before I would pass the water through a sawyer filter or such. Also, I have considered adding a layer of activated charcoal in the filter to help remove chemicals (ie pool water). I would imagine it would need to be before, as some chemicals will affect the good bacteria. Possibly ever a complete separate filter. But increasing the volume to 15'ish gallons, not to mention gravity helping you would think would help produce a better result.

jimmypat (author)n5yzv2015-03-03

Yeah, I agree. Your plan sounds like it would work. Thanks for your comments and ideas!

baudeagle (author)2015-03-01

I have a suggestion, You may consider adding a piece of wood xylem in to the discharge port. Take a look at this article about the filtering potential from using a piece of green wood.

http://www.popularmechanics.com/science/environmen...

jimmypat (author)baudeagle2015-03-03

Great idea! I've read about these. I'll mention this is the final step as additions to the system. Thanks!

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Bio: Mechanical engineer and STEM teacher with a smattering of search and rescue, ski patrol, EMT, world traveler, garage mechanic and DIYer.
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