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

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

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

Setting the rebar cage into the septic tank excavation

Concrete floor of tank and beginning of walls and partitions

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 lid ready to be poured.

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

TIP: It’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

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. Read all about our Philippine House building Project at /building-our-philippine-house-index/ Return to Building Our Philippine House main page

Related posts:

1. Sewerage/Septic/Drainage Systems in the Philippines
2. Our Philippine House Project: Plumbing
3. Our Philippine House Project – Carport
4. Our Philippine House Project – Roof Design
5. Our Philippine house project – design devolution