Thailand Slow Sand Water Filter

[chownah]

I clean my well water by running it through a slow sand filter...it is a tank made by stacking 1 metre diametre concrete rings to a height of 3 metres. Roughly the bottom third is storage, the middle third is sand, and the top third is water above the sand.

I know a filter for a pond has different problems from domestic water supply but thought I'd just post this here to show how simple a biofilter can be. With my filter I don't need to be running water through it constantly...I do clean the top 10 cm (approx) of sand about twice per year....this filter takes water that has a noticeably disagreeable odor and makes it drinkable.

Chownah

P.S. Much much larger versions of the sand filter I have are used by municipal water departments in the US to clean river water for domestic consumption.

Chownah

[Maizefarmer]

CHOWNAH exactly what I do -even with the well water - and have added a charcoal canister at the bottom just to "sweeten£ it a bit as ground water can be real hard. To stop the sand particals from runing out at the bottom of the concreate ring I threw in some mosquito netting.

Want to ask you some questions about that:

Is it just normal building sand or is it river sand and what size are the particals? Do you get green agle growth on the top of the sand, and does it ever clog up (how often if so). Quite small particals I would have thought as it sounds like you have an amount of water that "sits" on top of the sand and slowly trickles through - at what rate?

Tim

Edited October 15, 2006 by Maizefarmer

[chownah]

CHOWNAH exactly what I do -even with the well water - and have added a charcoal canister at the bottom just to "sweeten£ it a bit as ground water can be real hard. To stop the sand particals from runing out at the bottom of the concreate ring I threw in some mosquito netting.

Want to ask you some questions about that:

Is it just normal building sand or is it river sand and what size are the particals? Do you get green agle growth on the top of the sand, and does it ever clog up (how often if so). Quite small particals I would have thought as it sounds like you have an amount of water that "sits" on top of the sand and slowly trickles through - at what rate?

Tim

MaizeFarmer,

The sand they use is called white sand and is mostly quartz sand. I don't know where they get it but is is not the usual river sand or building sand....it is not pure quartz sand but has a small percentage other stuff. What you want for the sand is something that is inert. About the worst thing you could get would be a sand with a high lime content as this would harden your water. The sand I used is all different sizes mixed....it is probably just the way the dig it out...I doubt that they screen it. The best thing to use is screened sand where all the particles are the same size. This way the filter will have a uniform porosity which will allow the water to flow evenly all through the filter and will also give you a longer time between cleanings. I used the sand just like the Thais do without screening cause I figured I could remove, screen, and replace if there was a problem...but so far it works fine.

The sand plugs up about every six months. When this happens there are two ways to clean it. The tank is plumbed so you can reverse flow to clean it but I have been simply taking off the top 10 cm. or so of sand and rinsing it by hand in buckets and then replacing it.....takes about two hours of relaxed work to do this. Some people just remove the top 10 cm. of sand and then replace it with new sand. I've had this filter running for about 4 years now and this is all I"ve had to do. It is always the top 10 cm. that gets all plugged up and below that the sand seems to maintain its porosity well....perhaps in the future I'll have to remove all of the sand and replace it but I don't know when and it will be no big deal if that time comes.

I have about 40 cm. of water above the top of the sand. I don't get algae growing in the water but there is green/black stuff that grows around the inner surface of the tank up there and I remove it when I wash the sand.

The water moves through the filter at about whatever rate I am using it....the tank has water entering at the top controled by a float valve and the bottom of the tank has an outlet pipe which is where my house pump is connected.....so the entire setup is like one tall tank with a floor about half way up which holds up sand which reaches to within about 50 cm from the top....the tank is always kept full so the sand is continually submerged. The water storage part at the bottom of the tank has an air vent pipe so that when you pump you will not develop negative pressure in the tank and suck water down through the sand but unless the sand is plugged I can run a garden hose full open for well over an hour and not have to stop.

These are the standard water filters used here in my village. When my wife said that she wanted one I wasn't sure how good they were or even if perhaps they were a health hazard or not so I did some research and was lucky enough to find an old engineering text which talked about municiple water systems and sand filters specifically. I compared what the local guy proposed building against the text and it seemed pretty much like it should work so we did it and it does work fine. I've got a copy of a couple of the chapters from the book and if I can find it in my archives I'll post some data on particle sizes for the sand and filter flow rates etc.

Chownah

[Maizefarmer]

CHOWNAH exactly what I do -even with the well water - and have added a charcoal canister at the bottom just to "sweeten£ it a bit as ground water can be real hard. To stop the sand particals from runing out at the bottom of the concreate ring I threw in some mosquito netting.

Want to ask you some questions about that:

Is it just normal building sand or is it river sand and what size are the particals? Do you get green agle growth on the top of the sand, and does it ever clog up (how often if so). Quite small particals I would have thought as it sounds like you have an amount of water that "sits" on top of the sand and slowly trickles through - at what rate?

Tim

MaizeFarmer,

The sand they use is called white sand and is mostly quartz sand. I don't know where they get it but is is not the usual river sand or building sand....it is not pure quartz sand but has a small percentage other stuff. What you want for the sand is something that is inert. About the worst thing you could get would be a sand with a high lime content as this would harden your water. The sand I used is all different sizes mixed....it is probably just the way the dig it out...I doubt that they screen it. The best thing to use is screened sand where all the particles are the same size. This way the filter will have a uniform porosity which will allow the water to flow evenly all through the filter and will also give you a longer time between cleanings. I used the sand just like the Thais do without screening cause I figured I could remove, screen, and replace if there was a problem...but so far it works fine.

The sand plugs up about every six months. When this happens there are two ways to clean it. The tank is plumbed so you can reverse flow to clean it but I have been simply taking off the top 10 cm. or so of sand and rinsing it by hand in buckets and then replacing it.....takes about two hours of relaxed work to do this. Some people just remove the top 10 cm. of sand and then replace it with new sand. I've had this filter running for about 4 years now and this is all I"ve had to do. It is always the top 10 cm. that gets all plugged up and below that the sand seems to maintain its porosity well....perhaps in the future I'll have to remove all of the sand and replace it but I don't know when and it will be no big deal if that time comes.

I have about 40 cm. of water above the top of the sand. I don't get algae growing in the water but there is green/black stuff that grows around the inner surface of the tank up there and I remove it when I wash the sand.

The water moves through the filter at about whatever rate I am using it....the tank has water entering at the top controled by a float valve and the bottom of the tank has an outlet pipe which is where my house pump is connected.....so the entire setup is like one tall tank with a floor about half way up which holds up sand which reaches to within about 50 cm from the top....the tank is always kept full so the sand is continually submerged. The water storage part at the bottom of the tank has an air vent pipe so that when you pump you will not develop negative pressure in the tank and suck water down through the sand but unless the sand is plugged I can run a garden hose full open for well over an hour and not have to stop.

These are the standard water filters used here in my village. When my wife said that she wanted one I wasn't sure how good they were or even if perhaps they were a health hazard or not so I did some research and was lucky enough to find an old engineering text which talked about municiple water systems and sand filters specifically. I compared what the local guy proposed building against the text and it seemed pretty much like it should work so we did it and it does work fine. I've got a copy of a couple of the chapters from the book and if I can find it in my archives I'll post some data on particle sizes for the sand and filter flow rates etc.

Chownah

I have used river sand - about 50mesh - and yes it is hard (hence the activated charcoal).

Reverse flow - has never worked for me

I find that almost all the dirt/dust/soil whatever else, accumulates in the top 5 - 10cm of sand

Its changed about every year or so and has proved to a cheap reliable filter system.

Nitrates Nitriites and all the other horribles - have never ever had a problem with that from the well water, just a lot of calcium at times - normaly towards the end of the dry season it starts to get so bad that the washinh hangs on the line like a wooden board.

The river water/canal water is another story though - there is defineatly an increase nitrates over the last 5 or so years - which is all down to the increased and poor disciplined use of herbacides and insectacides. Its amazing how they show up in river water - always a lot more as soon as the rice starts growing.

Tim

[chownah]

The river water/canal water is another story though - there is defineatly an increase nitrates over the last 5 or so years - which is all down to the increased and poor disciplined use of herbacides and insectacides. Its amazing how they show up in river water - always a lot more as soon as the rice starts growing.

Tim

What I have read on the internet at many reliable sites is that the usual estimate for rice growing by farmers using the typical regimes for chemical fertilizers (not considered to be over application) is that 50% of the nitrogen is lost to either runoff (to rivers) or infiltration....alot of the infiltration eventually ends up in rivers too, it just takes longer.

In another thread you asked why are some rice farmers in other countries getting better yields. Part of the answer is that in other places the farmers have better irrigation systems and can regulate the water better and have a more releable supply. This allows them to regulate the water levels in their paddy to help reduce runoff and infiltration. In Thailand (at least where I live) the irrigation system does not reliably supply water so farmers tend to keep their paddies as deep in water as possible at all times as a hedge against a period when (for no apparent reason) the water stops coming. This greater than necessary depth creates a greater surcharge pressure on the saturated soil resulting in greater infiltration rates and associated loss of nitrogen to the ground water. Also, keeping the paddy full to the brim is done by allowing (and even encouraging) a constant through flow which causes further loss of nitrates by runoff. In addition, keeping the paddy full to the brim means that if more water comes and can not be stopped or diverted (which often happens when for no apparent reason the water authorities decide to increase flows in the canal....a canal which has inadequate embankments so it leaks water at a high rate) then there is no surplus storage capacity in the paddy and all the water will result in runoff...in effect there is no buffer capacity to hold unexpected surges. The farmers don't take steps to solve these problems in that they don't really see the disadvantage of too much water...to them its good luck to have your paddy full of water because it means that you don't have to go out and do anything to distribute water and you surely will get a crop adequate to provide for your needs for another year....there just doesn't seem to be any incentive to improve. I'm trying to slowly implement some better water handling features in my paddy but it will be some time before I have full implementation and even longer before I can evaluate the results.

Chownah

Edited October 16, 2006 by chownah

[chownah]

I haven't had time to search my archives for the slow sand filter data but I did take a quick look via google searching for "slow sand filter" and right off the bat found two good link.

The first one is from wikkipedia and is a good overview for those who want something less techinical and the second is from a professional water type place so it gets more technical:

http://en.wikipedia.org/wiki/Slow_sand_filter_bed

http://www.nesc.wvu.edu/ndwc/pdf/OT/TB/TB14_slowsand.pdf

Chownah

P.S. MaizeFarmer, the way I get my links into my posts is to go to the website and when it is open to the page I want I copy everything in the address window and then paste it in my post on a seperate line by itself...its pretty much foolfproof.

Chownah

P.P.S. I think that the sizing and flow rates for the filter in the second link is for a cold weather climate where the beneficial bacteria that do the biological filtering act very slowly....here in the tropics/subtropics where it is always warm they are more active and work faster so my flow rate is proporionately higher than their recommendation but works absolutely fine...Chownah

Edited October 16, 2006 by chownah

[Maizefarmer]

Noted - thanks.

Tim

[RamdomChances]

Split from The "pond construction" thread

[lannarebirth]

Hi chownah,

Now that it's getting dry enough to do a few projects, I'm going to construct one of those sand filtration systems for a house I built this year for my workers. I have a couple of questions however.

On the cement ring that has a cment bottom holding the sand, did you drill one or several holes in it to allow for the seepage of the filtered water down to the lower tank? If so, how many and what size? Also, did you set a fine screen under the sand to stop particles from flowing to the lower filtered water section? TIA

Ken

[chownah]

Lannarebirth,

I contracted all of the work of building the filter to an individual and didn't have the time to watch carefully. Around here you can buy a circular piece of concrete that looks like a lid for the rings which already has holes in it....you buy it from the same place where you buy the rings. Check around where you live and see if you can find a place that makes concrete products and see if they have one ready made.

I don't think they used a screen....I think they used a layer of gravel....also I believe that the pipe used to remove water from the bottom (storage section) is placed so the inlet is above the floor of the storage section enough so that any small amount of sand falling through the "floor" of the sand filled section would just settle to the bottom and not get sucked into the house supply.

I'll try to remember to try to find out the exact answers to your questions. If I don't get back to you send me a reminder in a couple of days.

Chownah

Edited October 29, 2006 by chownah

[lannarebirth]

Thanks chownah.

[chownah]

Lannarebirth,

I checked out the floor unit that holds up the sand. It has one hole in the center which is about 6 cm in diameter. This hole is for the incoming water pipe. The pipe that supplies the water enters near the bottom of the tank and then makes a 90 degree bend and goes up through the center of the tank, through the floor holding the sand, through the sand, through the water above the sand and extends about 40 or 50 cm above the top water level. This way when the water is pumped it falls a bit through the air and gets aerated somewhat (I guess).

The floor unit that holds the sand also has two holes that are 3 cm in diameter on opposite sides about half way from the center to the edge of the floor. One of these is used for the air vent pipe to vent the clean water storage portion of the tank which is below the sand floor. This pipe goes from the storage portion, through the sand floor, through the sand, through the water above the sand and extends about 20 or 30 cm above the top water level and its purpose is so that if your pump is pumping faster than the rate that water can move down through the sand then air can enter the lower clean water storage portion and your pump won't be sucking a negative pressure in the tank which might overload your pump or suck sand down through the sand floor into the bottom clean water storage area (I guess). I don't know why they have two of these holes....I guess it never hurts to have a spare!!!!

The floor unit that holds the sand also has about 35 holes that are 2 cm in diameter and are spread all around the sand floor unit so that there is no more than 15 cm distance between any hole and its nearest neighbors. These are the holes that the water trickles through to the bottom clean water storage area.

So, to recap....35 holes +/- of 2 cm diameter spread all around no more than 15 cm apart, two holes 3 cm in diameter for an air vent pipe and a spare, and one hole 6 cm in diameter in the center for the water supply pipe.

No screen is used above the sand floor. The sand is kept back by first placing a layer about 10 ore 15 cm thick of rocks that are about 4 or 5 cm in diameter and on top of this a layer of coarse gravel (mixed sizes with max size about 2 or 3 cm) and above this a layer of smaller gravel (mixed sizes max size about 1 1/2 cm). Then the sand. Best to not use limestone since it makes your water hard. Around here they use river rock for this since it is mostly not limestone and the sand is a white quartz type sand (although not purely quartz but mostly) since limestone sand should be avoided too.

Chownah

P.S. There is some rebar in the lid too but I don't know how much...if I remember I'll ask.

Chownah

Edited October 30, 2006 by chownah

[teletiger]

I'm going to build one for the staff house. I hate those bloody vases.

Regards

[lannarebirth]

Lannarebirth,

I checked out the floor unit that holds up the sand. It has one hole in the center which is about 6 cm in diameter. This hole is for the incoming water pipe. The pipe that supplies the water enters near the bottom of the tank and then makes a 90 degree bend and goes up through the center of the tank, through the floor holding the sand, through the sand, through the water above the sand and extends about 40 or 50 cm above the top water level. This way when the water is pumped it falls a bit through the air and gets aerated somewhat (I guess).

The floor unit that holds the sand also has two holes that are 3 cm in diameter on opposite sides about half way from the center to the edge of the floor. One of these is used for the air vent pipe to vent the clean water storage portion of the tank which is below the sand floor. This pipe goes from the storage portion, through the sand floor, through the sand, through the water above the sand and extends about 20 or 30 cm above the top water level and its purpose is so that if your pump is pumping faster than the rate that water can move down through the sand then air can enter the lower clean water storage portion and your pump won't be sucking a negative pressure in the tank which might overload your pump or suck sand down through the sand floor into the bottom clean water storage area (I guess). I don't know why they have two of these holes....I guess it never hurts to have a spare!!!!

The floor unit that holds the sand also has about 35 holes that are 2 cm in diameter and are spread all around the sand floor unit so that there is no more than 15 cm distance between any hole and its nearest neighbors. These are the holes that the water trickles through to the bottom clean water storage area.

So, to recap....35 holes +/- of 2 cm diameter spread all around no more than 15 cm apart, two holes 3 cm in diameter for an air vent pipe and a spare, and one hole 6 cm in diameter in the center for the water supply pipe.

No screen is used above the sand floor. The sand is kept back by first placing a layer about 10 ore 15 cm thick of rocks that are about 4 or 5 cm in diameter and on top of this a layer of coarse gravel (mixed sizes with max size about 2 or 3 cm) and above this a layer of smaller gravel (mixed sizes max size about 1 1/2 cm). Then the sand. Best to not use limestone since it makes your water hard. Around here they use river rock for this since it is mostly not limestone and the sand is a white quartz type sand (although not purely quartz but mostly) since limestone sand should be avoided too.

Chownah

P.S. There is some rebar in the lid too but I don't know how much...if I remember I'll ask.

Chownah

oooh,

That is very very useful information chownah. Thanks ever so much for checking, as I had a different idea; but this makes much more sense. Khup Khun Mak Khup

[Somtham]

Tilapia,

You need not worry about getting a couple hundred cubic metres per hour from a slow sand filter....they are often used for municipal water supplies so just about any flow rate can be achieved.

Chownah

I had built a sand filter last year for cleaning bore hole water for the ponds and used a rate of 0.5GPM/sq ft of filter and put in a 500 sq ft filter. I once tried to run the settling pond water through it and it plugged up in a day. Thank God the filter literally blew up, no rebar in the concrete, because it was a constant chore washing the top layer of sand. Even our new sand filter for house use and plant watering needs to be cleaned every 6 weeks. But the water coming from it is clear and tasty.

Municipal water supplies are not cleaning fish poop through their sand filters and I would approach that idea with extreme caution.

rgds

Thanks for the reply. Indeed, suspended solids plug sand filters quickly and I agree that approaching the idea of using a slow sand filter should be approached with caution. A couple of questions. When you pumped your settling pond water into your 500 sq. ft. slow sand filter did it have suspended solids?....and was it these suspended solids that plugged the filter? Also, can you describe the construction of your 500 sq. ft. slow sand filter?

I guess that I'm asking about this because municipal water supply slow sand filters are considered to be a very cost effective way to clean water so long as sufficient land is available for constructing the facility. It might be that a fish pond is too small of a system to take advantage of the efficiencies available with a full on slow sand filtering system...I don't know...but would like to work on this. Your help by explaining your experiences is appreciated.

Chownah

Yes, the fish pond water had suspended solids that clogged the filter so I only tried filtering the pond water once. The bore hole water is loaded with iron and although the filter removced all the iron it was still constant maintenance cleaning the sand.

On the design, it's been awhile and I can't find my notes but it was basically a down flow system with rectangular e-block construction, no rebar (error!), above ground with a concrete slab tapered to one end. At the end where the water flowed I put in 5 4" PVC catchment pipes that gravity fed a 25K liter storage tank.

Went to the local sand quarry and bought stones and sand to fill the filter. This is where my notes would help but as I recall I first put down 2" dia stones to cover the ends of the PVC, then 20cm of #3 stone, 20cm #2 stone, 20cm #1 sand and m #0.9 sand.

The water was pumped from the bore to a series of 1.5 inch perforated PVC pipe overhead. The holes were facing up and this oxygenated the water which is necessary to bind the iron to remove it.

I'll try to dig out some pictures if you want.

rgds

[chownah]

Somtham,

No need for pictures I think. I don't know what the grades for the rock and the sand mean....I understand 2" rock but not the # grades for the rock and sand...I'm most interested in the sand; was it graded so all the particles were about the same size or was it a mixture of all sizes and if so about what was the maximum size particle for the two grades. It sounds like you conststructed what would be the typical slow sand filter....except for the omission of rebar!!

Also, when this filter plugged, how did you clean it?

Also, is it important to remove iron from water you use for fish?

Also what was the size (surface area) of your filter...and the approximate flow through it (litres per minute...gallons per hour...whatever)?

Chownah

Edited May 2, 2007 by chownah

[Somtham]

Somtham,

No need for pictures I think. I don't know what the grades for the rock and the sand mean....I understand 2" rock but not the # grades for the rock and sand...I'm most interested in the sand; was it graded so all the particles were about the same size or was it a mixture of all sizes and if so about what was the maximum size particle for the two grades. It sounds like you conststructed what would be the typical slow sand filter....except for the omission of rebar!!

Also, when this filter plugged, how did you clean it?

Also, is it important to remove iron from water you use for fish?

Also what was the size (surface area) of your filter...and the approximate flow through it (litres per minute...gallons per hour...whatever)?

Chownah

I haven't been able to figure out the # system either other than when it drops to below 1 I think it is in mm so the #0.9 I believe is 0.9mm and yes it was all the same size, not mixed. On the new sand filter we built for home water use I added a 20cc layer of charcoal below the #3 rock and a 0.5mm(?) top layer of sand.

In the sand filter for cleaning bore water sent to the ponds I put a drain just above sand level which drained to the settling pond. I would use a rake on the sand while filling the filter to agitate the top layer that would flow to the drain.

Went we started rebuilding the farm we planned on planting Machrobrachium but half way through we decide to switch to catfish. The catfish don't seem to be bothered by the iron at all. It settles out quite quickly in the dirt ponds but is ugly for a few days in the concrete ponds.

The filter surface area was 500 sq ft and I never measured it but guess the flow coming out was in the area of 200-300LPM.

rgds

[mickba]

Lannarebirth,

I checked out the floor unit that holds up the sand. It has one hole in the center which is about 6 cm in diameter. This hole is for the incoming water pipe. The pipe that supplies the water enters near the bottom of the tank and then makes a 90 degree bend and goes up through the center of the tank, through the floor holding the sand, through the sand, through the water above the sand and extends about 40 or 50 cm above the top water level. This way when the water is pumped it falls a bit through the air and gets aerated somewhat (I guess).

Hi - I'm about to replicate your filter and just wodering - earlier you said water in by ball cock at top I think, but here you say in at the bottom, which sounds better, but is there a float switch for level or how?

Thanks

Mick

[matte]

Interesting topic! Having enough iron in the water to supply a small-scale blacksmith, I would love to try this filter together with a Birm filter to see if we can use the product in-house. Would anybody know the proper Thai name for this type of slow sand filter?

[MudDoc]

Guys,

Have read all the posts I can find on this as well as the research items mentioned. Have what may still be a stupid question for you though. If you use the 1 meter concrete well liners to build the filter, how are you preventing water leaks between the rings?? Are you cementing them or using selastic or a rubber gasket between the rings?? Have you set up a "crane" type pully to be able to lift off the top ring for cleaning the sand in the middle.

I need to build one of these at my property in Klaeng - doing some major renovations at present, so still have about a month before needing to build this. Our well water is fairly muddy looking when pumped out, but does clean up when allowed to settle for several hours. I was initially thinking of using a normal house filter, but thinking about the clays in the water and reading this and other threads felt the slow sand filter was the way to go. Any advise welcomed.

[Maizefarmer]

Guys,

Have read all the posts I can find on this as well as the research items mentioned. Have what may still be a stupid question for you though. If you use the 1 meter concrete well liners to build the filter, how are you preventing water leaks between the rings?? Are you cementing them or using selastic or a rubber gasket between the rings?? Have you set up a "crane" type pully to be able to lift off the top ring for cleaning the sand in the middle.

I need to build one of these at my property in Klaeng - doing some major renovations at present, so still have about a month before needing to build this. Our well water is fairly muddy looking when pumped out, but does clean up when allowed to settle for several hours. I was initially thinking of using a normal house filter, but thinking about the clays in the water and reading this and other threads felt the slow sand filter was the way to go. Any advise welcomed.

Nothing stupid about your question ... its why the forum exists - and unless you know where a specific answer is, not withstanding answers can be found using search words, the TV search facility could be better than it is.

Scroll back to Page 8 (on the Farming In Thailand section), where you will find a thread titled: “Irrigation System, advices wanted” – I answer the question you have asked in Post # 15, but I suggest you read the whole thread in any event as there is some valuable advice regards using concreate rings as water tanks.

MF

[David006]

I clean my well water by running it through a slow sand filter...it is a tank made by stacking 1 metre diametre concrete rings to a height of 3 metres. Roughly the bottom third is storage, the middle third is sand, and the top third is water above the sand.

I know a filter for a pond has different problems from domestic water supply but thought I'd just post this here to show how simple a biofilter can be. With my filter I don't need to be running water through it constantly...I do clean the top 10 cm (approx) of sand about twice per year....this filter takes water that has a noticeably disagreeable odor and makes it drinkable.

Chownah

P.S. Much much larger versions of the sand filter I have are used by municipal water departments in the US to clean river water for domestic consumption.

Chownah

And I was thinking of buying a used pool filter to take the surface run off cloudiness from my well water in the wet ( concrete rings not sealed) the well is up on the hill believe or not... so not much chance of pollution.

...never dreamed sand filters are the norm. Thank you so much! Question which may be answered already but cannot find.....

- do you just seal the rings with cement?,

- the dividing concrete disks is it the same ones they use for the loo caps with the drain hole meshed or somesuch to stop the sand?

- Do you just pump water in the top from source and pipe/pump it out the bottom to a storage tank

thanks again

[Maizefarmer]

Cement, fine - but use very fine clean sand and use something like a C30 or C35 mix (theres something for you to do - go Google "C mix") - but don;t use aggregate or stone - replace that portion with fine sand (eqaul amount).

Mix it all up, then through in the water (reduce the amoutn of water the C formula reccomends by anything between 30% - 50% - somewhere between a wet and dry), then throw in some cement/concreate water proofing liquid - you by it from hardware stores in 500ml and 1litre bottle (usualy a milky white colour) - mix it all up well, then trowel it into the gap between the 2 rings.

Get the cement right - can't stress this enough - failure to do this explains why so many of these installations you see around Thailand constantly leak.

You've got a proper/solid base laid have you?

[MudDoc]

Maizefarmer,

Thanks for the look back. I would always look in a post dealing with irrigation when looking for items regarding a slow sand filter!!!!! Was still a little perturbed about sealing the rings with a wooden "gasket" rather than one made from rubber, until this last post by David. That did make things a lot clearer for me.

Just one more question. have you guys installed / made a "crane" for lifting off the lids and concrete rings for cleaning the sand?? Just a steel frame with a wire rope and ratchet to help you, or do you just get 4 or 5 other people to help you lift them off and replace them once cleaning is complete?? Those rings and concrete lids are damned heavy for 2 guys to move around, especially from a height of 3 m. Or do you have a scaffolding around the filter for this?

[Maizefarmer]

Forget trying to do the job with "man power" ... to much potential for injury or broken rings. Make up a scaffholding frame, or get a tractor with a front end loader on it.

With the real big rings (2m diameter upwards), youre going to need a crane - but the guys who make these rings have, or have access to cranes so thats not going to be an issue.

As for lids - get the local tin smith to knock up a galvanised tin/steel lid with a lip on it. Much easier to handle.

Remember - the wood is just used to seperate the rings till you've got the sealing done between them - then you must get rid of them - don;t leave them in.

[jaideeguy]

These slow sand filter rigs sound good for removing solids [mud] and iron amd maybe a few other thingies, but to my knowledge, they don't/can't remove disloved solids such as the calcium that is our biggest nusience,,,,spotting glasses,staining tile and cleaned auto, windows., hardening water for clothes etc.

I was told that a resin filter [water softener] is the only way to get the disolved calcium out of the water,,,,unless someone else knows a secret way??

I bought a 2nd hand resin filter and it is a pain in the butt to recharge it with salt once a week and often forget. Not cheap either.....

and regarding the sand that is available to us here in CM, it has been said that is is simply river sand which is maybe graded, but is rounded by erosion and not effective in filtering. The preferred sand for filters [swimming pool and domestic] is crushed quartz sand that has angles, flat surfaces to mechanically capture the particles that pass thru the sand. I've heard this from a major pool supplier here in CM. Alternatives are costly....crushed glass, zeolite and other siwimming pool filter mediums.

[Maizefarmer]

The problem with all these filters is the relationship between surface area and flow rate - and the only practical way to deal with that is by way of using a disc filter - for which you can get "discs" to filter out everything except disolved solids.

Resins, like activated charcoal, are as you quite rightly note really only good for "sweetening"

Your best option is a disc filter - which should be connected up inline with a reverse flow circuit plumbed in to clean out the dirt once a week or whenever, else a spare disc cartridge, which can be cleaned while the other one is in service.

Pic 1 shows a typical disc filter

Pic 2 shows the cartridge(s) - the different colours are different mesh sizes

The brass fitting at the base allows fro reverse flow, permitting accumulated solid particles to be flushed out without having to take the filter dics out, sepearte and flush them - which you wil have to do occassionaly, especially if algea starts to build u between the discs.

A good quality 2" - 3" disc-filter in thailed wil cost somewhere Baht 25/30K - but it will last you 15 - 20years plus

[slapout]

I saw a filtering system utilized for hard ground water using a similar filter as Maize pictures above. The exception being the old fellow had rigged up a oil filter for a large diesel engine as he allowed the filter was much more economical and available. He replaced it about every 5 to 6 months. The mesh/flow specs would be the determining factor I would guess.