Installation of tanks

Drainage maintenance

Drainage systems should by flushed several times a year. Furthermore, once in 20 years it is necessary to carry out maintenance work on the existing drainage system i.e. the gravel backfill should be removed and about 30 cm deep layer of soil and sand should be replaced. The rinsed gravel and cleaned pipes should be placed in the existing trenches and buried like when they were initially laid.

Location of a domestic sewage treatment plant

The following guidelines should be taken into account while selecting location for a domestic sewage treatment plant:

  • distance from groundwater - 1.5 m,

  • distance from water intake points - 70 m,

  • distance from a building - at least 3 m (drain field and the septic tank),

  • distance from a neighbour’s fence – at least 2 m (drain field and the septic tank),

  • distance from trees – at least 3 m (drain field and the septic tank).

Requirements for domestic sewage treatment plant and PE tank installation

Prior to installation both the external and internal surface of the tank should be inspected, especially for possible damage resulting from transport and handling. Transporting and positioning of tanks is allowed only with the use of slings. Should tanks be installed in areas with vehicle traffic, in areas with wet ground conditions, or where swelling clays are present, in areas prone to landslides or in areas adjacent to excavations, in the belt equal to the depth of it, and is loaded, it is necessary to draw up design documentation for the installation of the tank. The documentation should be drawn up by a certified engineer. Location of the tank in the plot is regulated by law, especially in the Regulation of the Minister of Infrastructure of 12 April 2002 on the technical conditions to be met by buildings and their location.

Underground installation

When a septic tank is to be installed in dry or moderately wet soil, the following procedures must be carried out:

  • the sub-base for the settling tank should be of type 1, 2, 3 (see table 9) and compacted to at least 95% MPD (see table 10),

  • set and level the tank along the main manifold,

  • connect the settling tank to the laid pipeline and at least one chamber or tank following it,

  • backfill and compact the area around the settling tank. Compaction should be performed in layers in accordance with applicable regulations and good practice, with the use of material of type 1, 2 or 3 specified in tables 9 and 10. For tanks located in soft landscaped areas, soil should be compacted to at least 85% MPD. For those located in areas with vehicle traffic, soil should be compacted to 95% MPD,

  • while backfilling, the septic tank should be gradually filled with water to fill all the chambers evenly. During installation it is sufficient to fill half the volume of the tank.

TYPE 1 Gravel, crushed stone containing stone particle sizes 4-8.4-16.8-12.8-22 mm. Max. 5-20% particles of 2 mm diameter are acceptable. The best material for inspection chamber installation.
TYPE 2 Coarse sand and gravel of maximum particle size approx. 4 mm and other graded sand and gravel of various particle size containing a small percentage of silt. Max. 5-20% particles of 0.2 mm diameter are acceptable. The material is of good quality.
TYPE 3 Fine-grain sand, clayey gravel, a mixture of fine sand, clayey sand with gravel and loam, silty gravel and mixtures of: gravel - sand - silt, gravel - sand - clay, silty sand – sandy silt. Max. 5% particles of 0.02mm diameter are acceptable. This material is of mediocre quality.

Sub-base aggregate classification for tank installation.

Type of compaction method Weight [kg] Max. layer thickness before compaction [m] Min. protective layer thickness above tank [m] Number of runs at compaction level
gravel, sand clay, loam, silt up to 85% MPD up to 90% MPD
Tamping - 0,10 - - 1 3
Manual compaction 15 0,15 0,10 0,30 1 3
Vibratory rammer 50-100 0,30 0,2-0,25 0,50 1 3
Vibratory plate compactor with removable plate 50-100 0,20 - 0,50 1 4
Vibratory plate compactor - planar 50-100
1 4

Compaction method applied and layer thickness while compacting backfill material.

Special recommendations for loaded areas

If the tank is to be installed in an area with vehicle traffic or heavy weight goods are to be stored on it, a protective reinforced concrete slab should be used with a ring transferring the load away from the tank. When the top fill covering the tank exceeds 100 cm, an additional load diffusing layer should be poured, where the layer should be buried and laid approx. 30 cm above the top of the tank. If the septic tank is to be installed in an area with vehicle traffic, the backfill should be compacted to 95% MPD.

Special recommendations for waterlogged areas

  • prior to the installation of the septic tank, a foundation with anchor bolts should be made for securing the tank, and the weight of the foundation should be sufficient to counterbalance buoyancy force of the empty tank

  • stabilize the tank by attaching it to the foundation with galvanised tie-down straps for resisting the buoyancy force

  • in extreme cases it might be necessary to build a formwork for external concrete-encasement of the tank above its axis

  • if the foundation is to be reinforced, it should be performed prior to the erection of formwork

  • backfilling and compaction of the area around the tank should be performed in layers, in accordance with applicable regulations and good practice. For tanks located in soft landscaped areas, soil should be compacted to at least 90 % MPD. For those located in areas with vehicle traffic, soil should be compacted to 95% MPD,

  • drainage of the excavation, if necessary, should be continued until the tank has been stabilised and backfilled so that the buoyancy force of the ground water can be counterbalanced.

  • the tie-down straps and anchors holding the tank to the ground should be protected against corrosion. The tie-down straps should be made of galvanised steel with the cross section being at least 50x3 mm, whereas the anchors should be threaded rods at least M14 made of galvanised or stainless steel. Technical Approval IOŚ - AT/2007-08-0137/A1Septic tank SZAGRU.

Legal and formal basis

(Any works must be conducted in compliance with applicable standards and building code requirements, according to the design specifications and the manufacturer’s technical brochure).
Technical Approval IBDiM (Road and Bridge Research Institute) No AT/2011-02-2685 for SZAGRU Polyethylene tanks (HDPE).

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