DWA and HWA: two kinds of water
Drainage specialists distinguish between two flows of water. DWA stands for dry weather flow: the waste water from toilet, shower, kitchen and washing machine, which also flows when it is not raining. In English this is usually called foul water. HWA stands for rainwater drainage: the surface water from roofs, patios and paving. You will see these abbreviations on Dutch building drawings and from contractors, for example on downpipes marked as HWA.
The distinction matters because the two flows are very different. DWA is polluted and must go to the wastewater treatment plant. HWA is relatively clean and can often go back to nature: soak into the ground or flow to a ditch, pond or canal. Mixing them costs treatment capacity and energy, because the plant then has to process large volumes of clean rainwater.
The combined system
In a combined system DWA and HWA flow together through one pipe to the pumping station and the treatment plant. This is the classic layout of many town centres and neighbourhoods built up to around the 1970s. Its advantage is simplicity: one pipe per street and one connection per home, to which toilet, kitchen and downpipes all connect.
During a heavy downpour a lot of rainwater enters the system in a short time. When the pipes fill up, the excess water flows via a combined sewer overflow into surface water. In low-lying homes without a non-return valve, water can also flow back to the lowest drainage point in the house during extreme rainfall, such as a cellar gully or shower channel.
The separate and improved separate system
In a separate system there are two pipes in the street: a foul sewer for DWA and a surface water sewer for HWA. The rainwater goes directly to ditches, ponds or a soakaway, the waste water to the treatment plant. Each home therefore has two connections, and it is important that they are not swapped.
An in-between form is the improved separate system. Here the first, dirtiest flush of rainwater from streets and roofs is still pumped to the treatment plant, and the rest goes to surface water. For you as a resident it works the same as a separate system: DWA and HWA stay separate on your property. In the countryside there are also pressure sewers, where only the waste water is pumped away and the rainwater stays on your own land.
Avoiding misconnections
With a separate system it regularly happens that a drain is connected to the wrong pipe. For example a washing machine in the shed connected to the downpipe, or a new bathroom that ends up on the surface water sewer after a renovation. The waste water then flows untreated into the ditch. The other way round, a downpipe on the foul sewer puts needless load on the treatment plant.
Municipalities sometimes trace misconnections with smoke tests, dye tests or CCTV surveys. So during a renovation, always have someone check which pipe a new drain ends up on, especially for a bathroom in the loft, a kitchen in an extension or a washing machine in the garage. A CCTV drain survey from the new fixture shows where the pipe really ends, so you know for certain that everything is connected correctly.
Disconnecting rainwater
Disconnecting means you detach the downpipes from the combined sewer and drain the rainwater another way. This relieves the sewer during heavy downpours, replenishes the groundwater and keeps more water available for the garden in dry spells. It is important that the soil can absorb the water; on heavy clay that goes more slowly than on sand. There are several ways to do it:
- a water butt or rainwater fence that collects water for the garden;
- an infiltration crate or gravel soakaway in which the water slowly sinks into the ground;
- a swale or lowered part of the garden that temporarily holds water during a downpour;
- a connection to a separate surface water sewer if the municipality offers one.
In many municipalities disconnecting is mandatory for new builds and encouraged for existing homes, sometimes with a subsidy. Check with your municipality for the rules and conditions that apply to your address. When disconnecting, make sure the rainwater does not flow towards the outside wall or the neighbours, and that the old downpipe connection to the sewer is neatly capped. An open connection would otherwise let sewer smells and vermin through.
What this means for maintenance and Bosmans
Whatever system lies in your street, your own connection up to the property boundary needs attention. With a combined system a downpour washes in a lot of sand and leaves, which can settle in sections with little fall. With a separate system it matters that both pipes stay clear and correctly connected. Bosmans Rioolservice uses a CCTV drain survey to show which pipe runs where, cleans fouled pipes with high pressure equipment and clears blockages in both foul water and rainwater drains.