Our Philippine House Project – Septic and Drainage Systems

When we made a porch a bit bigger, we had to relocate a catch basin.  They are heavy!

Our Philippine House project.   Septic system, catch basins and drainage system.

Septic tank excavation

Septic tank excavation

One of our workers has been assigned the job of digging the pit in which the septic tank will be built.  The three chamber septic tank will measure 3.1 meters (10 feet)  by 1.9 meters (6 feet)  and will be 2.1 meters deep (almost 7 feet). The tank will have a concrete slab on the bottom, filled, steel reinforced hollow block walls and a concrete  top with clean-out ports. It will be parged with cement fortified with “Sahara” waterproofing additive.

During typhoons, the considerable water from the roof downspouts, will be collected in a series of concrete catch basins and directed outside the lot. The soil, as is true with most rice fields (that’s why they retain water as “rice paddies”) is extremely heavy clay.  The water table really varies with the season from being more or less at the surface of the land during the wet season to about fifteen feet down during the dry season.

If one had to do a percolation test to check the absorptive capacity of this clay  – well there must be none during the wet season.  During the dry season the clay becomes cracked and very absorptive. There are complications with wastewater disposal in our “neighborhood”.  Being flat, mostly undeveloped farmland there is no municipal or subdivision drainage system into which to discharge.  As is typical, only “black water” (toilet waste) will go into the septic system.  Rain water from the roof gutters, and water from washing, sinks and showers will not be treated but will be collected through a series of concrete catch basins, piped through the wall an discharged into a ditch.

As our neighborhood develops, probably a drainage system or formal ditch will be installed to carry wastewater to the nearest stream bed.  Many local families live along the streams and have their dug wells in or near the streams. Outside of treatment in a septic tank, there is virtually no sewerage treatment in the Philippines.  Boracay is one exception we know of.

When you buy property in a formal subdivision, you’re supposed to have access to a formal drainage system provided by the developer, an advantage.  The waste will just be eventually discharged into a ditch, stream, river or the ocean, but at least it will be carried away from your lot.  I have not heard of on-site leaching systems as are typical in the U.S.

Reinforcing bar framework for septic tank

Reinforcing bar framework for septic tank

In the Philippines, septic tanks are built on-site, not delivered by a truck.

Setting the rebar cage into the septic tank excavation

Setting the rebar cage into the septic tank excavation

Concrete floor of tank and beginning of walls and partitions

Concrete floor of tank and beginning of walls and partitions

Lots of smile, even in a septic tank!

Lots of smiles, even in a septic tank!  We used 6″ block.

Septic tank just about done and ready for 4" reinforced concrete top. 3-3-10

Septic tank just about done and ready for 4″ reinforced concrete top. 3-3-10

Septic tank lid ready to be poured.

Septic tank lid ready to be poured.

Catch Basin under construction

Catch Basin floor and forms

A system of concrete catch basins and drain pipes surround the house, collect water from the gutters and downspouts and gray water from showers, floor drains, lavatories and kitchen sink and discharge all this into a exterior drainage ditch.  Our house has ten catch basins connected by 4″ pipe.  Buy good pipe such as “Atlanta” brand.  You’ll pay twice as much but cheap pipe is a false economy.  Remember you’ll be digging and landscaping.  It’s easy to put a shovel through the cheap pipe and it’s more easily damaged by tree roots and digging animals.

Finished catch basins sans covers

Finished catch basins sans covers

 

TIPIt’s a good idea to take lots of photos during the construction of your house so that later you can refresh you memory as to how things were done and where things are located.

This photo shows three elements of the drainage system.  The upper 4″ PVC orange pipe is leading from the toilet to the septic system.  Only toilet waste goes into the septic system — nothing else.   Don’t forget to make sure your crew maintains a proper slope on the sewer and drain lines.  If the slope is too little the solids will not be carried to the septic tank.  If the pipe is too steep, the solids will be left behind as the water rushes to the septic tank.  The rule is 1/4″ of slope for every one foot run of line or 2.5″ per ten foot section of pipe.  Plumbing codes require that 3″ and smaller drain piping be run at 1/4″ per foot minimum slope. The 1/4″ minimum slope assures sufficient flow velocity for the transport of solids. Two feet per second velocity is the minimum recommended for soil and waste lines. A 3″ drain at 1/8″ per foot slope has a flow velocity of only 1.59 fps. A 3″ drain at 1/4″ per foot slope has a flow velocity of 2.25 fps. This is particularly important where 1.6 gpf water closets are involved due to the limited waste carry of some low flow water closets.

Your workers might have never heard of such a rule.  Mine had not.  Perhaps it’s not generally such as issue in the Philippines because it’s usual for the septic tank to be very close to the house.  Our is quite far from the house.  It takes some serious advance planning to get everything right, so that in the end you’ll end up with the proper slope.  Remember, the sewer pipe exiting the house is fairly fixed at a few inches below the horn of the water closet.  You can go deeper, but then the septic tank has to be deeper.  The input pipe entering the septic tank has to be at the correct level,otherwise the tank will not work properly. This should all be figured out before the finished floor level of the bathroom and the height of the septic tank inlet are set. Once the house and tank are built, there’s little one can do but live with whatever mistakes have been made.

The lower 3″ orange PVC pipe carries downspout water from the roof and wastewater from the kitchen, showers, lavatories and floor drains to a series of catch basins and then through the perimeter wall to a ditch.  In the photo you can see two 2″ PVC pipes coming through the wall and connecting to the 3′ drain pipe.  The one on the right is from the sink in the master bedroom bathroom and the one on the left is from the shower and floor drain.  The 3″ pipe continue to the left and empties into the catch basin system.  We used 2″ pipe to try to avoid clogging.  There are no p-traps, except under the lavatory basin.  Remember that  gray water goes to catch basins, not the septic system.  We have not had odor problems.  We installed two 3″ PVC vent pipes which are hidden in the bathroom walls.  They only serve the water closets, not the sinks, floor drains, or shower.  These vents were intended to extend out through the roof.  We have not been in a hurry to cut holes in our beautiful long span roof.  Our attic is well ventilated and we have not had odor problems.  When we do the vent pipes will go up through the roof.

We mostly used heavier and more expensive Atlanta brand drain pipe.  Orangeburg brand pipe also looks good.  Cheaper pipe is available, but is subject to damage before and after construction — for example from landscape work.  It’s easy to put a shovel through the cheaper pipe but less likely with the heavier Atlanta. As you can see below, good pipe costs more than twice the cost of economy pipe.  I really wish we had not used one length of the cheaper pipe.  Guess what pipe you’ll get if someone else builds your house!  You may be charged for Atlanta and receive National.  It will all be hidden underground.  Of course, if you always choose the best materials, the per square meter cost of your house (excluding land) could end up being significantly higher.  It’s a paradox that old foreigners with a few years to live, at best, often insist on building houses which will last a hundred years!

Here’s a sample of prices we paid, mostly in the last half 0f 2010:

  • Atlanta sanitary pipe 4″x10′ P545  Better quality
  • National sanitary pipe 4″x10′ P238 Standard quality
  • Atlanta sanitary pipe 2″x10′ P192
  • Atlanta sanitary pipe 3″x10′ P410
Downspouts temporarily in place

Downspouts temporarily in place

 

When we made a porch a bit bigger, we had to relocate a catch basin. They are heavy!

Completed catch basin and downspouts

Downspout and catch basin, filled and painted

While our house is not large, the the area of the roof is over 300 square meters.  During tropical rains the amount of water coming off the roof is impressive.  We were forced to temorarily install our downspouts because the water pouring off the roof was eroding the fill and making a muddy mess around the house.  The downspouts were not glued so that they can be removed to allow the finishing of the exterior of the house and then permanently installed.  The plans called for twelve 3″ downspouts to drain the roof.
I never gave much thought to the sysstem of catch basins and drains.  We put them in because they were on the plans.  Now I realize how critically important they are.

While our house is not large, the the area of the roof is over 300 square meters. During tropical rains the amount of water coming off the roof is impressive. We were forced to temorarily install our downspouts because the water pouring off the roof was eroding the fill and making a muddy mess around the house. The downspouts were not glued so that they can be removed to allow the finishing of the exterior of the house and then permanently installed. The plans called for twelve 3″ downspouts to drain the roof.

I never gave much thought to the sysstem of catch basins and drains. We put them in because they were on the plans. Now I realize how critically important they are. Read all about our Philippine House building Project at /building-our-philippine-house-index/ Return to Building Our Philippine House main page

Comments (25) Write a comment

  1. The following company sells polyethylene septic tanks, grease traps and water storage tanks: http://www.weida.com.ph/index.html

    I am not sure about the economics of using a poly septic systems instead of concrete, but we’d be glad to add a poly grease trap and, when our stainless steel water tank give out, to replace it with a polyethylene tank. The stainless tanks look very nice but I am dubious about their longevity.

    Reply

  2. Hi Bob;
    Your post are SO great! They are very indeed informative. I had some fine time reading and learning from them, and I will not stop until I read them all.
    Been planning to help build shelter houses for friends and relatives here in Bohol. I see your site as very useful. I hope you won’t shoved me off. I’d practically enjoyed every information you shared. So nourishing and refreshing; Amazing! I am proud of you. Maraming Salamat po’! :)

    Reply

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  4. Hello again Bob,

    I am doing a new septic tank install at my in-laws in Butuan city, but I am here in Palawan until after the job is done. Responsibilities!

    My question is that you separated grey water from the septic, yes no? Did you run it into the septic outlet area or something?

    With regards to the grease catcher i understand that it is critical to the length of life of the output drain area. I thought I seen some drawing similar to the septic tank where the outlets of the grease catchers were lower inside the box so that all the grease and cooking oils float to the top and do not get released, only needing a clean-out occasionally. Your thoughts?

    Also, why 3 compartments for the septic tank. I was thinking two, solids and liquid overflow. Can you explain?

    I may be starting a full small lot house in Butuan near the end of the year so all good info.

    Thanks again for putting this out on the web. It is appreciated,

    Troy

    Reply

    • Thanks for your kind comments. We’re glad you found the site to be useful.

      I think few people in the Philippines put gray water into the septic tank. They have (as we do) two separate systems for gray water and black water. The gray water all (gutters, sinks, showers, washing) goes into the catch basins an is discharged untreated into whatever drainage system is available. The black water is treated via the septic tank and the septage is also discharged into the community drainage system — municipal, subdivision drainage etc.

      Our plans called for a separate grease trap to serve the kitchen sink. It was never installed. I am sure that this will prove to be a mistake.

      As to the septic tank compartments, our plans called for three, so that’s what we did.

      Good luck with your project!

      Reply

      • Thank you for the reply,

        I have a couple more questions for you.

        Is the third compartment just as an extra backup for the second compartment, to make extra sure that solids don’t get into the municipal drain system?

        Also, I understand that you are not using a septic field, but with my Mother-in-laws place there is no municipal drain infrastructure. So, this means making a small drain tile system verses a hole in the ground with stones; my recommendation. In this regard I was wondering if the sink and floor drain lines could bi-pass the septic tank and be connected up to the same drain field, with a grease catcher tank to catch the grease before getting to the drain field, which is not a large drain field like building codes would require in say North America, but also not just a rock filled hole in the ground.

        Your experiences and advice are appreciated.

        Have a great day,

        Troy

        Reply

        • Troy,

          Not sure about the rationale for the third compartment. We just followed the engineer’s plans.

          It took me awhile to understand that septic drain fields and pits are NOT used here. Our property, our entire area, is flat. During the wet season, the depth to the water table is essentially zero. In well-regulated areas in the U.S. it would take a very expensive raised bed drain field to deal with septic effluent. Here, the septic tank output goes directly into a municipal or subdivision drainage system which carries it to the nearest ditch, stream or river. If there is no drainage system, the septic tank output is discharged out through your perimeter fence. That sounds terrible, but bear in mind that only toilet waste goes into the septic tank, so the volume of septic wastes for a single family house is quite small. During the dry season it’s absorbed into the ground. During the wet season, it mingles with and is diluted by all the other monsoon water. In our neighborhood, there are rumors of a communal drainage system some day. It would collect waste and dump it into a nearby seasonal stream bed — no treatment except in the septic tank.

          Does this help?

          Bob

          Reply

          • I appreciate the advice and details. I am not 100% sure on water table levels in the area that I am considering, but I don’t suspect they are much below ground level in the wet season. I guess the only concern would be to ensure the water table was not allowed to flow back into the septic tank. The drainage field may be okay during the dry season, but better to mix with underground water than straight out into the roadway. More local area knowledge to be gathered I guess. Thank you, Troy

            Reply

  5. Hi Bob,
    Our “catch basins” have the inputs lower than the outputs ???
    The foremen explained that it works best this way. The long pipe inputs have a dip in the centre and of course the basins will only empty when levels are high, can’t quite see the logic for this but he insists that its the right way. …we are only talikng about small level differences.
    Peter

    ps the private subdivisions we have lived on have there drainage systems blocked up and do not have the finance to dig them out. Everyones septic tank will overflow into the street if they fill up. Maybe thats why they are so huge.

    Reply

    • Peter,

      Checked my plans. The catch basin inputs are higher than the outputs, just the same as a septic tank. I am sure my autocad plans are pretty standard engineering. A quick catch basin Google shows the same. Of course there should be no dips in the pipe and they should have the same 1/4″ per foot slope as any other drain pipe.

      Bob

      Reply

  6. Does your septic tank have or require a leaching field? If it does how large does it have to be? I am considering buying a small lot and my main concern is how to deal with the sewage system.

    I look forward to what you were required to do to meet the barangay septic field standards or what they dictated as minimum requirements.

    Reply

    • Troy,

      To the best of my knowledge, leach fields are unknown here. When we built our house, I was trying to figure out how to dispose of the septage as we have seasonal high water – e.g. no depth to ground water. Coming from the NE USA I assumed the answer would be a raised leaching bed. I discussed my ideas with our engineer. She had no idea of what a leaching bed was — never heard of such a thing. Unless there is a municipal or subdivision drainage system, septic systems discharge to whatever stream or ditch is handy. The municipal systems to the same. Maybe other have a different experience, but that’s mine with Philippine leaching fields.

      Bob

      Reply

  7. Dear Bob, Im really enjoying all your documentation . Thank you!

    I am now buying a 500sq meter lot on the beach on Siargao island.
    I want to locate a septic tank in the lower back corner along the highway ditch .I would like to follow your design exactly.
    1. do you have any other drawings or dimensions that are not in this blog.. placement of pipes etc.?
    2. Also, do you happen to remember the entire finished cost.?
    3. And I do have one more big problem and that is alot of earthquake/seismic activity. Im wondering if lining the hole with old car tires and gravel before pouring the concrete box would work to create a tank that could jostle around a bit and not crack.?
    Any ideas ? thanks again! charlie

    Reply

    • Charlie,

      I only have the full size plans for our septic system and we did not completely follow the plans. It’s a bit bigger than the plans called for. You should be able to find septic tank specs online such as from government agencies. The principles are pretty universal.

      Regarding earthquakes, maybe your ideas are good. It should be easy to build a strong tank with good concrete and lots of well-connected rebar.

      Good luck with your project.

      Bob

      Reply

    • Hi Charlie, you may want to consider those septic tanks made out of plastic. They are much cheaper to use and install than concrete. They usually come with a chamber divider and a gravel basket strainer to keep the solids in.

      In my projects, I use 3 units in series (P 6000/ unit @ approx 1.8 cu.m.) – I take out the basket dividers in the first units so that I have a large digestive chamber (2 tanks) for the solids. The last tank will act as the leaching chamber before draining out to the municipal sewer. This unit is mostly used in housing subdivisions as it is virtually indestructible and quick to install.

      I also use the same tanks to store grey water from sinks and showers and rain water then install a submersible pump connected to water sprinklers for the garden. The soap in the water is great against aphids.

      Hope this helps.

      Reply

  8. Is there any way that you could email me the drawings or better specs on your septic tank. Not sure of pipe placements inside of tank.
    Thank you, George

    Reply

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  10. Hi Bob,
    The roof on a single story does collect loads of rainwater so why not put it to good use with incorporating it into a water feature. The source will also be at a convenient height providing such things as water falls. I plan to collect my water in one place and then pipe it over a covered walkway to a pool and then smaller streams under a bridge if you like to return back to the beginning. Once it is back at the start it is easy to put a circulating pump to keep the water moving (avoid mosquitos). Since the water is not hard mineral quality from underground the Algae grow will be minimul, any nasties in the water will be flushed out with the next downpour. If you expose the pipes over the walkway or even over a gulley, the temperature will be high and be pleasant for the evening skinny dips haha.
    Along the waterway you can provide a bed for watercress or other plants that like the medium, they also can give filtration. Also required would be bottom drain to clear out leaves etc, possibly make your own filter with “Scotchbrite” pads.

    Reply

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  12. I’m lucky and fortunate that our new community all the sewage is semi process and discharge as grey water to the local creek.

    There is no need for a septic tank INSTALLATION. Yet, I had to install 2 grease catchers in my house according to the subdivision mandates.

    Reply

  13. My wife’s family have a house in Nabas. They have a septic system, which needs pumping and upgrading. Does anyone know of a service or persons who we could contact?
    regards,
    John

    Reply

  14. Thank you Bob.

    I see the new photos. Thankyou.

    The drawing on this document: http://www.abe.psu.edu/extension/factsheets/f/F161.pdf
    is the way we used to do them here years ago. Building code here will now not accept basic septic tanks and a pumped perculation system is called for.

    Just a note. The baffles are very important in stopping the scum layer from being discharged, and keeps the sludge layer towards the inlet end …..

    Reply

  15. PaddyC,

    Yes, it’s a three chamber tank. It’s ended up a bit bigger than the specs called for. Our digger just kept going!

    Bob

    Reply

  16. Do you have to have a 3-chamber septic tank in your area, Bob?

    Anyway, the nett capacity in my estimate will be 2,327 US Gallons as follows (100mm = dividing walls; 10mm = render coats):

    Length: 3.1M – (100mm + 100mm + 10mm + 10mm + 10mm + 10mm + 10mm + 10 mm) = 2.84M

    Width: 1.9M – (10mm + 10mm) = 1.88M

    Depth: Chamber 1 = 1.80M; Chamber 2 = 1.65M; Chamber 3 = 1.50M: Average = 1.65M

    2.84 x 1.88 x 1.65 = 8.81 cubic metres = 2327 US Gallons

    ————————————————————————

    With a 2-chamber setup I estimate the same tank will hold an extra 209 US Gallons

    Length: 3.1M – (100mm + 10mm + 10mm + 10mm + 10mm) = 2.96M

    Width: 1.9M – (10mm + 10mm) = 1.88M

    Depth: Chamber 1 = 1.8M; Chamber 2 = 1.65M: Average = 1.725M

    2.96 x 1.88 x 1.725 = 9.6 cubic metres = 2536 US Gallons

    Reply

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