TLTG "Tanki Septik"

LOW-COST WATER SUPPLY AND SANITATION

“Septic Tanks”

GROUP 7

PUTRI SAFRIA                          (25-2011-016)

SITI HAJAR CAHAYANI           (25-2011-027)

DELTA FITRI SARI                   (25-2011-040)

JURUSAN TEKNIK LINGKUNGAN

FAKULTAS TEKNIS SIPIL DAN PERENCANAAN

ITENAS

BANDUNG

2014

Introduction

A septic tank is a rectangular or cylindrical chamber, a usually located just below ground level, that receives both excreta and flush water from toilets as well as other household watewaters (or sullage). Settleable solids settle to the tank bottom, accumulate, and ate then anaerobically diested. A scum of light-wight materials (including fats and greases) rises to the top. The clarified liquid flows through a subsurface soil absorption system. The effluent from a septic tank is an obnoxious liquid, containing high concentrations of organic matter, nutrients, and enteric microorganisms. It should not be discharged without treatment to surface drains, streams or lakes.

Figure 1 : The Conventional Septic Tank

 

Advantages and Disadvantages

The main advantage of septic tank system is their flexibility and adaptability to a wide variety of individual household waste disposal requirements. Another advantage is that the septic tank has no moving parts and therefore, needs little mechanical maintenance.

A major disadvantage of the system is its high cost. Septic tank are more expensive than other on-site waste treatment system and are generally only found in wealthy areas. The system requires a permeable subsoil structure so the effluent can be distributed. If the subsoil structure is too impermeable the septic tank effluent can contaminate surface or graundwater, creating a public health hazard. Space for drainase fields is also required, and all drinking water must be set away from the septic tank system. As well, septic tank system need piped water.

Design of Septic Tanks

A septic tank should be designed to remove almost all settleable solids and to decompose organic matter anaerobically. To accomplish this, the tank must provide the following :

1)    Proper volume of septic tank to ade quately retain the waste

2)    Proper placement of inlet and outlet devices and adequate sludge and scum storage space to prevent the discharge of sludge and scum in the effluent.

3)    Since the digestion process is anaerobic, requiring no oxygen, no direct ventilation is necessary. However, provision should be made to permit the escape of the gases produced in the tank, through a ventilation pipe.

Construction and Operation of Septic Tanks

Septic tanks must be water-tight, structurally durable and stable. Reinforced concrete and ferrocement meet these requirements, but the tanks should be scaled for water-tightness after installation with bituminous coating or other materials with equivalent properties.

Other materials include polyethylene and fiberglass which are light-weight, easily transported, and resistant to corrosion and decay.

Steel is another material that has been used for septic tanks, however, despite a corrosion-resistant coating, tanks can also become deteriorated at the liquid level.

The inlet and outlet pipes should be scaled with a bonding compound that adheres to both concrete and the pipes.

After installation, the tank should be tested for water-tightness by filling it with water.

The most important installation requirement is that the tank be on a level grade and at a depth that provides adequate gravity flow from the house, matching the invert elevation of the house sewer. It should also be easily accessible to facilitate inspection, maintenance, and sludge pump-out

Treatment of Septic Tank Effluent

There are three general procedures for disposing septic tank effluent.

a)   Absorption Fields and Evapotranspiration Beds

Where site conditions are suitable and do not pose any threat to ground-water quality, subsurface soil absorption is usually the best method for disposing of septic tank effluent. As shown in Figure 2, the effluent flows by gravity from the tank through a closed pipe and a distribution box into perforated pipes in trenches. Figure 3 shows the detailed construction of a trench; it consists of open-jointed agricultural drainage tiles of 10-centimetre diameter laid on a 1-metre depth of crushed rocks or gravel and soil. Bacteria and oxygen in soil help purify the effluent.

 

Figure 2: A Typical Septic Tank System

 

Figure 3: Disposal Trench and Tile Line

 

The performance of a soil-absorption system depends on the ability of the soil to accept liquid, strain out bacteria, absorb viruses, and filter the wastes. A proper site evaluation requires accurate measurements of the degree of slope, the position of groundwater table, the effective soil depth, and the depth of any bedrock or other impermeable materials.

Perharps the most important characteristic of the disposal field is its soil permeability. A percolation test is recommended to give a measure of soil permeability.

To test percolation, drill at least three 150mm diameter holes to the depth of 0 to 5m deep, and across the proposed drainfield. These are then filled with water and left overnight saturating the soil. The next day fill the holes with water again to a depth of 300mm. After 30 and 90 minutes the water levels are measured. The soil is considered to have sufficient percolative capacity if the level in each hole has dropped 15mm/hour.

The design approach of absorption fields can be calculated according to:

Where

L                 = Trench length (m)

N                = Number of users

Q                = Wastewater flow (l/cap/day)

D                = Efective depth of trench (m)

I                  = Design infiltration rate (l/m²/day)

Design infiltration rate for the septic tank is usually taken as 10 l/m²/day. This can be used until a more accurate figure is calculated from local experience.

a)    Evapotranspiration Mounds

In areas where the water table is near the surface or soil percolation is insufficient