Drainage and Wastewater Treatment Systems

Figure 1: House Drainage System

Image from Slidesharecdn.com

This blog looks at purpose and types of drainage and wastewater systems, and some of the techniques involved in building them.

The objective of a drainage is to provide hygienic and adequate disposal of surface, subsoil, waste and soil water.

• Surface water is natural water, such as rainwater. Surface water from a roof is collected in the gutter and travels down via the down pipe into a gully (see Figure 2).

• Subsoil water is water occurring naturally in the soil.

• Wastewater is water from wash hand basins, baths, sinks, etc.

• Soil water is the discharge from soil appliances such as toilets and urinals.

The objective of a wastewater treatment system is to remove impurities from wastewater before the water is released to waterways or the soil.

Figure 2: Surface Water Drain

Image from smithsofstapleford.co.uk

The Irish Building Regulations require i) that drainage systems provide for the hygienic disposal of foul waste water and for adequate disposal of surface water and ii) that wastewater treatment systems should function to a sufficient standard not to cause harm to health (Technical Guidance Document H – Drainage and Waste Water Disposal, 2016).

The Technical Guidance Documents (Figure 3) outline principals to ensure the hygienic and adequate disposal of water. They include items such as the drainage layout should be as simple and direct as possible, joints must be airtight, watertight and free from internal obstruction and that pipes should be laid to falls that give a self cleansing velocity. (Technical Guidance Document H – Drainage and Waste Water Disposal, 2016). Local authorities have the power to order the removal or repair of any drain which is not up to standard.

Figure 3: TGD H Drainage and Waste Water Disposal

Image from assets.gov.ie

Combined Drainage System versus Separate System

A combined system is where all surface water, wastewater and soil water is collected in a shared drainage system (see Figure 4). Because the drainage is combined, there is a risk of water pollution and combined systems are no longer used in modern buildings.

Figure 4: Combined Sewer System

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A separate drainage system is where two separate sets of pipes lead to two different sewers, one carrying soil and waste water, the other carrying surface water (see Figure 5). This is the system used for modern buildings.

Figure 5: Separate Sewer System

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Rigid and Flexible Pipes

Rigid pipes (Figure 6) are used for surface water and made from fibre cement, vitrified clay, concrete.

Figure 6: Concrete Rigid Pipe

Image from www.jdpipes.co.uk

Flexible pipes (Figure 7) can be used for any water type (soil, waste, surface or subsoil) The advantage of flexible pipes are that they are easier to handle and cut and joints are easily fitted together. This is the most common type of pipe.

Figure 7: Flexible Pipe

Image from Idiggreenacres.com

Water Traps

A water trap is the portion of the pipe designed to trap liquid or gas to prevent unwanted flow in or out of the building. It is shape like a U (see Figure 8) or an S. It is designed to carry the water down the drain and then up again and 'trap' some of the water in the bend. A gully trap is for surface water, a wash hand basin trap is for wastewater and a toilet bowl trap is used for soil water.

Figure 8: Water Trap

Image from bbc.co.uk

Ventilation of Drainage Systems

A vent stack is a vertical pipe attached to a drain, acting as air vent to remove gas and allow fresh air into the drain (Everything you need to know about Plumbing Vents, 2021).

The discharge stack connects to where the wastewater and the soil water are brought together to be discharged into the drainage system (See Figure 9). The discharge stack should be at a height that will not cause a nuisance. If less than 3 metres from the nearest opening (eg a window), it needs to be a minimum of 900mm above the opening.

Figure 9: Discharge Stack

Image from www.metrorod.co.uk

Access Points

An access point provides access to the underground drainage for maintenance and inspections.

Access chambers (Figure 10) and inspection chambers (Figure 11) and are shallow chambers and provide access for maintenance equipment such as a drain rod.

Figure 10: Access Chamber

Image from Pavingexpert.com

Figure 11: Inspection Chamber

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Manholes (Figure 12) are larger chambers, large enough to for a person climb down to carry out maintenance at the drainage level.

Figure 12: Manhole

Image from Pavingexpert.com

Access points should be located:

• at a bend or change in gradient

• on or near the head of each drain run

• at a change of pipe size

• at a junction of two more branches

Access points should have removable non-ventilating covers of durable material, except when they are situated within a building. In buildings, they should have mechanically fixed air-tight covers.

Laying a Drain

Drains are laid as follows:

• Set up side rails to the required gradient.

• Dig the trench deep and wide enough to accommodate the drain, the compacted sand underneath and concrete fill on either side of the drain. Ensure the trench is at the correct fall by using a boning rod or traveller.

• Add the compacted sand or rubble (Figure 13) and lay the drain

• The pipe work must be air and water tight in order that both solid and liquid matter are removed from a building without foul odours escaping. To test the drain for air and water tightness:

  • Air test: a manometer with a rubber stopper is placed at the end of the drain. A wing nut is twisted and to expand the rubber stopper and seal the drain. Air is pumped into the drain until the pressure on the manometer reaches 100-120mm. Leave it for 5-10 minutes. Reduce pressure to 100mm. Leave for 5 minutes. Pressure should not drop below 75mm

  • Water test: block ends of the drain with rubber stoppers. Fill the drain with water and leave for 120 minutes to allow for displacement. Monitor for 30 minutes and top with water. Keep a record of water added.

  • Smoke test: this is not an official test of water tightness, but may be used with non PVCu pipelines to identify leaks or defects that have not made themselves apparent in air or water tests. Each outlet is blocked using rubber stoppers except for the lowest outlet. An oily rage is then lit and the smoke is allowed enter the drain at the lowest point where any leaks can be identified.

• Haunch the drain with concrete

Figure 13: Laying a Drain

Image from Blackanddecker.com

Treatment Plants and Septic Tanks

Treatment plants are used in towns and cities. A treatment plant treats the water with biodegradable chemicals before releasing it into the waterways. Wastewater from Dublin has been treated in the Ringsend Wastewater treatment plant since 1906 (Ringsend Wastewater Treatment Plant Upgrade Project, 2021). See Figure 14.

Figure 14: Ringsend Treatment Plant

Image from Water.ie

Septic tanks (Figure 15) are used in rural areas where individual dwellings have their own septic tank. The septic tanks store and treat soil/waste water by means of a scum or slurry forming in the tank. The scum digests the sewage via bacterial action. It then goes through a filter tank and is slowly released back into the surrounding land via pipes. On-site septic tank systems or domestic wastewater treatment systems have to be registered with local authorities and they can be subject to inspections (Waste Water Legislation for Single Houses, 2021)

Figure 15: Septic Tanks

Image from EPA.ie

Soakaway Pits

A soakaway pit (Figure 16) is a hole in the ground, filled with rubble, that is designed to gather excess surface water and then slowly redistribute it back to the soil. It needs to be located at least 5 metres from the nearest foundation in an area of good permeable ground. The drainpipe is fed into the soakaway pit and a membrane cover is placed on the sides and the top, and then it covered in topsoil.

The formula used to calculate the volume of a 1 metre deep soakaway is surface area being drained multiplied by average rainfall.

Example:

- A shed has a roof 15m2

- Average rain fall of 0.012m, ie 12mm

- 15m2 x 0.12m = 0.180m3

The required volume of the soakaway in this example is 0.180m3. The square root of 0.180m3 = 0.42m, so the measurements of the soakaway should be 0.42m x 0.42m x 1m.

Figure 16: Soakaway Pit

Image from Totalpavingsolution.ie

Draining Subsoil Water

Water may need to be drained from the subsoil to prevent surface flooding and thus improve conditions for building and the stability of the soil. One method is for perforated pipes to be laid in the ground and surrounded by pea gravel. This allows the subsoil water to drain into the pipes and flow away thus drying out the ground.

Figure 17: Slotted and Perforated Pipes

Image from Drainstore.com

REFERENCES:

Everything you need to know about Plumbing Vents (2021) American Home Shield Association [online] Available from <https://www.ahs.com/home-matters/repair-maintenance/learn-about-plumbing-vents/

> [accessed 16 April 2021]

Ringsend Wastewater Treatment Plant Upgrade Project (2021) Irish Water [online] Available from <https://www.water.ie/projects-plans/ringsend/> [accessed 16 April 2021]

Technical Guidance Document H – Drainage and Waste Water Disposal (2016) Government of Ireland [online] Available from <https://www.gov.ie/en/publication/16910-technical-guidance-document-h-drainage-and-waste-water-disposal/> [accessed 16 April 2021]

Wastewater legislation for single houses (2021) Environmental Protection Agency [online] Available from <https://www.epa.ie/water/wastewater/legislation/> [accessed 16 April 2021]