Sanitary Drainage System Guidelines and Septic Tank Design

Sanitary Drainage System

A system designed to remove waste and sewage from buildings.

Scouring

The self-cleaning action in the pipe wherein organic wastes are removed by flushing or flooding.

Roughing-in

The installation of pipes and fittings of the plumbing system prior to the installation of fixtures and accessories.

Access & Maintenance

Requirements ensuring that plumbing systems can be easily accessed for cleaning and repairs.

Ventilation Requirements

Specifications that ensure proper air flow within plumbing systems to prevent sewer gas buildup.

Plumbing Code Requirements

Regulations that govern the installation and maintenance of plumbing systems.

Cleanout

A sanitary fitting that serves as an access receptacle in the sanitary piping system that can be opened to permit cleaning and maintenance of the pipeline.

Horizontal Waste Pipe

A pipe that runs horizontally and is used to carry waste away from fixtures.

Change of Direction

A point in the plumbing system where the direction of the pipe changes.

Waste Pipe

A pipe that carries waste from fixtures to the drainage system.

Direct Waste

A waste pipe with its terminal directly connected to the plumbing system.

Indirect Waste

A waste pipe connection with its terminal not directly connected to the plumbing system.

Soil Pipe

Any pipe that receives and conveys discharges of water closets, with or without the discharge coming from other fixtures to the house drain or house sewer.

Soil Stack

Soil pipes installed vertically.

Soil Branch

Soil pipes installed horizontally.

National Plumbing Code

Regulations that provide guidelines for the installation and maintenance of plumbing systems.

Siphoning of the Trap Seal

The process by which the water seal in a trap is removed due to suction.

Back Pressure

Pressure that may force the water seal off the fixture trap.

Soil Stack Vent

A vent that extends full size above the roof to ventilate and dispose of sewer gas.

Diameter Requirement

The minimum size of the soil stack must not be less than 75 mm (3") in diameter.

Height Requirement

The soil stack must extend above the roof not less than 30 centimeters long.

Cesspool

A hole in the ground curbed with stones, bricks, concrete hollow blocks, or other materials laid in such a manner as to allow raw contaminated sewage to leach into the soil.

Privy

A concrete sealed vault with a wooden shelter constructed for the collection of raw sewage.

Septic Tank

A watertight covered receptacle designed and constructed to receive the discharge of sewage from a building sewer, separate solids from the liquid, digest organic matter and store digested solids through a period of detention, and allow the clarified liquids to discharge for final disposal.

Sludge

Solid organic matter that are denser than water and settle at the bottom of the septic tank.

Scum

Lighter organic material that rise to the surface of the water.

Effluent

Liquid content of sewage.

Aerobic Bacteria

Relies on oxygen to survive.

Anaerobic Bacteria

Can survive in places without oxygen.

Septic Tank Process

Sewage that was discharged into the tank is retained and during its retention period, about 60% to 70% of the suspended solid of the sewage is removed largely by sedimentation to form a semi-liquid substance called sludge.

Digestion

The process where both the scum and the sludge are processed by anaerobic bacteria and transformed into liquids and gases.

Construction of Septic Tank

The concrete or masonry septic tank is usually constructed in a rectangular form to retard the even flow of the waste.

Minimum inside dimension of a septic tank

90 centimeters wide by 150 centimeters long.

Effective decomposition depth

A 120 centimeters depth of the liquid content is necessary.

First compartment (Digestive Chamber)

Not less than 2/3 capacity of the total capacity of tank; not less than 2 cum liquid capacity; shall be at least 0.9 m width and 1.5 m long; liquid depth not less than 0.6 m nor more than 1.8 m.

Secondary compartment (Leeching Chamber)

Maximum capacity of 1/3 total capacity of tank; minimum of 1 cum liquid capacity.

Slope of digestion chamber

Maintain a slope of 1:10 at the bottom of the digestion chamber to collect the sludge.

Capacity of septic tanks

Determined by the number of bedrooms or apartment units in dwelling occupancies, by the estimated waste/sewage design flow rate for various building occupancies, or by the number of fixture units of all plumbing fixtures; whichever is greater.

Soil structure classification

The capacity of any one septic tank and its drainage system shall also be limited by the soil structure classification in its drainage field.

Inlet and Outlet Inverts

The inverts are installed in the wall of the tank at least 120 centimeters from its bottom floor equally spaced from both sides.

Invert Extension

The invert is extended down the liquid of the tank not more than 30 centimeters to assure smooth delivery of the incoming sewage below the scum line.

Digestion Chamber

The bottom of the digestion chamber should be sloped to one low point to gather the settled organic materials into one mass to favor the propagation of the anaerobic bacteria.

Manhole

The manhole is extended a few centimeters above the surface of the soil to overcome infiltration of surface water and serves the purpose of cleaning, inspection, and repair of the tank.

Suspended Compartment

Septic tanks for large plumbing installations are provided with suspended compartments attached to the ceiling slab of the tank.

Baffle Plate

The baffle plate is extended down to the bottom of the tank about 40 centimeters below the scum line.

Compartment Manholes

Each compartment of the tank separated by baffle plates is provided with a manhole.

Minimum Manhole Dimensions

At least two (2) manholes, 508 mm in minimum dimension, are required; one over the inlet and the other over the outlet.

Additional Manhole Requirement

Wherever the first compartment exceeds 3.7 m in length, an additional manhole is required over the baffle wall.

Surface Construction

The septic tank should be constructed near the surface of the ground because the correction of the waste depends upon the extent of oxidation and the existence of anaerobic bacteria.

Oxidation Depth

Oxidation of the effluent deeper than 150 centimeters would become extremely difficult.

Liquid Allocation per Person

For residential buildings to serve a larger number of people, allocate 0.14 to 0.17 cu. m. of liquid per person.

Small Residential House Capacity

For small residential houses to serve up to 12 people, the chamber should have a liquid content of not more than 2.00 cu. m.

Maximum Depth of Septic Tank

The maximum depth of the septic tank liquid is 1.50 meters.

Septic Tank Volume for Residential Houses

For small residential houses to serve up to 12 people, the chamber should have a liquid content of not more than 2.00 cu. m.

Septic Tank Volume for Establishments

For school, commercial and industrial establishments, the volume of the septic tank should not be less than 0.057 cu. m. nor more than 0.086 cu. m. per person.

Daily Sewage Flow for Motel

Under motel with toilet and bath, and kitchen, the waste per person per day is 50 gallons. For 100 persons, it is 5,000 gallons a day.

Public Sewer Line Definition

A public sewage system, operated and maintained by the government consisting of a sewage treatment plant that conveys the raw sewage from buildings and houses to a disposal system.

Types of Public Sewer Lines

The public sewer line is classified into three types: 1. The Combination Public Sewer 2. The Sanitary Sewer 3. The Storm Drain.

Combination Public Sewer

The oldest type of public sewer that conveys both storm water and sanitary wastes, now obsolete and no longer allowed by sanitary authorities.

Sanitary Sewer Definition

A public sewer facility that carries regular sanitary wastes only and terminates in a modern sewage dispersal plant.

Types of Sanitary Sewers

The sanitary sewer is classified into two types: 1. The tributary or contributing sewer 2. The intercepting sewer or trunk line.

Tributary / Contributing Sewer

Termination points of individual units or structures, usually round shaped, with diameters between 0.60 to 1.2 meters, made of vitrified clay or cement pipes.

Intercepting Sewer / Trunk Line

A sanitary sewer that conveys sanitary waste to a dispersal plant, commonly made of concrete pipes that vary from 0.60 to 3.00 meters in diameter.

Depth of Intercepting Sewer

Intercepting sewers are placed much lower in the ground, from 4 to 30 meters in depth.

Slope of Intercepting Sewer

Intercepting sewers are sloped at an angle of 1:50 or 2%.

Lifting Stations in Sewers

Lifting stations are placed at certain intervals and pumps or sewage ejectors are used to lift the waste; sewers terminate at the disposal plant.

Storm Drain Definition

Another kind of public sewer line that carries storm water, terminating in a natural drain such as canals, lakes, or rivers.

Manhole Definition

A manhole is classified as a device of the main and storm sewer.

Manhole Construction

It is constructed out of bricks, stone, adobe or concrete at an interval distance from 75 to 150 meters.

Manhole Diameter

The manhole diameter varies from 0.90 to 1.20 meters provided with iron rungs to serve as ladders for the maintenance crew to reach the bottom.

Manhole Cover

It is provided with a well fitted cover on top, leveled with the road surface.

Sewage Ejector

Sewage Ejector refers to the pump that will discharge waste in the sump and transfer it to the house drain installed overhead.

Sewage Ejector Necessity

Sewage ejectors are necessary when the public sewer line is installed at a depth from 2 to 4 meters below the street level.

Building Sewer Size

The minimum size of any building sewer shall be determined on the basis of the total number of fixture units drained by such sewer.

Minimum Building Sewer Diameter

No building sewer shall be smaller than 150 mm diameter nor less in size than the building drain.

Public Sewer Availability

Public sewer may be considered as not being available if it is more than 61 meters from any proposed building or exterior drainage facility.

Building Sewer Slope

Building sewers shall be run in practical alignment at a uniform slope of not less than 2% or 20 mm/m toward the point of disposal.

Single Family Dwellings Exception

Single family dwellings with an existing private sewage disposal system may not be connected to a new public sewer when no hazard, nuisance or unsanitary condition is evident.

Sewer Slope Exception

Sewers 102 mm and 152 mm in diameter may have a slope of not less than 1% (10.5 mm/m) and those 203 mm diameter and larger may have a slope of not less than 0.5% (5.3 mm/m).

Building Sewer Installation Distance

No building sewer shall be installed less than 0.60 M from the outer face of any building foundation, nor less than 0.30 M below the finish surface of the ground.

Trench Requirements for Pipes

Building sewer or drainage pipe of clay or materials which are not approved for use within a building shall not be laid in the same trench as water pipes unless the bottom of the water pipe is 0.30 M above the top of the sewer pipe.

Sizing of Sanitary Pipes

To determine the size of house drain or house sewer, compute for the total fixture units of the building.

Minimum Pipe Size for Water Closet

The minimum size of pipe that receives discharges from a water closet is 110mm or 4".

Trap Seal Loss

Direct effect of the Minus & Plus Pressure inside the system due to inadequate ventilation of traps.

Types of Siphonage

Siphonage is classified into two types: Direct or Self Siphoning and Indirect or Momentum Siphoning.

Evaporation in Traps

This process is considered a minor problem and is less probable to drain the water inside the trap.

Wind Effect

Strong winds through the vent system forcing water out of the trap.

Capillary Action

Draining of water seal caused by foreign objects like thread or string suspended and extended over the outlet arm of the trap.