What is a sewer?
A sewer is a mostly underground channel or pipe system used to collect and transport foul water and rainwater. In the Netherlands the public sewer is managed by the municipality; the section on your own land, as far as the inspection chamber at the boundary, belongs to the owner.
Types of system
About three quarters of Dutch drainage is gravity drainage: the water flows away under gravity through pipes laid to a slight fall. Within that there are two main forms:
- Combined system — foul water and rainwater share one pipe to the treatment works. In heavy rain the system is overloaded and a storm overflow discharges diluted sewage to the watercourse
- Separate system — two independent pipes: foul water to the treatment works, and the relatively clean rainwater to a watercourse or to infiltration. New development is now almost always laid out as a separate system
There is also pumped drainage, using pumps and rising mains. It carries foul water only, and is used in rural areas, in hilly terrain, or where a pipe has to pass under a river or a dyke.
| Combined system (gravity) | Foul water and rainwater share one pipe to the treatment works. In heavy rain the system is overloaded and a storm overflow discharges diluted sewage to the watercourse. |
|---|---|
| Separate system (gravity) | Two independent pipes: foul water to the treatment works, relatively clean rainwater to a watercourse or to infiltration. New development is now almost always laid out this way. |
| Pumped drainage | Pumps and rising mains, foul water only. Used in rural areas, in hilly terrain, or where a pipe has to pass under a river or a dyke. |
Materials
- On private land: nearly always PVC, in the orange-brown colour used for below-ground drainage
- Main sewer: concrete or vitrified clay pipes, and increasingly plastic
Fall: the key rule of thumb
For horizontal collection pipes carrying domestic foul water, the Dutch standard NEN 3215 sets a fall of between 5 and 20 mm per metre. In practice 10 mm per metre (1 in 100) is a good middle value, and for pipes larger than 125 mm a fall of 8 mm per metre is often enough.
Note that more fall is not better. Laid too steeply, the water runs ahead of the solids, which are left behind in the pipe — and that itself causes blockages. Laid too flat, sediment settles out.
Practical tips
- Use swept bends. Two 45-degree bends rather than one 90-degree bend prevents blockages and allows rodding
- Fit rodding eyes at every change of direction that you will not be able to reach later
- Vent the system with a soil vent pipe taken above the roof, or traps get siphoned out and the house smells
- Do not disconnect rainwater from a combined sewer unilaterally; many municipalities offer a grant or a rebate for doing it properly
- Record the position on an as-built drawing before the trench is backfilled
Related terms
- PVC
- Infiltration
- Crawl space
- As-built drawing
- Renovation
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.