Infiltration insight

However this only holds for the average Dutch situation. In some parts of the country, a lack of rainfall can threaten natural reserves. This is why the Dutch Government has developed a policy to reduce the use of groundwater and promote the use of surface water for public supply.

Drinking water in the Netherlands

In the Netherlands drinking water is supplied to 15.5M consumers by 24 water supply companies. Total production of drinking water amounts to approximately 1300Mm³/year. There are three different sources of raw water:

Groundwater (65%);

Surface water (21%);

Infiltrated pre-treated surface water (14%)

NV Waterleidingmaatschappij Oost-Brabant

NV Waterleidingmaatschappij Oost-Brabant (WOB) is a water supply company in the south of the Netherlands which produces about 120Mm³/year and distributes it to approximately 1.3M consumers. This is achieved by 23 groundwater pumping stations. But, to cover growing water demands and to comply with the national strategy, WOB will soon use surface water from the River Meuse (Maas).

Surface water contains more contaminants which are difficult to remove, so a number of preliminary studies have been carried out to find suitable techniques to overcome this problem. These studies showed that artificial surface infiltration was the best achievable technique to produce drinking water from surface water.

Project Infiltration Maaskant (PIM)

WOB intends to use the infiltration technique as part of a large-scale water supply project – the Project Infiltration Maaskant (PIM). PIM will run to 2001.

Initially the project will be able to deliver 12.5Mm³ of drinking water annually. Later on, production may be extended to 25Mm³/year or even 50Mm³.

In the first phase a pollution incident on the River Meuse will stop the intake of raw river water. To meet demand, the groundwater pumping stations will temporarily produce more drinking water. In the second phase it will be possible to take in raw water from the River Rhine in case of a pollution incident on the Meuse.

Raw river water is taken into a reservoir, with a maximum depth of 25m, which serves several functions:

i. Control function

The intake of raw water will be monitored on a permanent basis. When raw water of poor quality has been taken into the reservoir it can be pumped back to the river.

ii. Quality function

Several processes contribute to the improvement of water quality such as:

Sedimentation of part of the undissolved solids;

Levelling out of raw water quality fluctuations;

Destruction of pollutants through biological activity;

Softening of the water through dosing of lime at the intake.

Pre-treatment consists of flocculation, dissolved air flotation (DAF), rapid filtration and activated carbon filtration. These techniques lower the turbidity, retain algae and prevent pesticides from polluting the infiltration zone.

Pre-treated water meets Dutch and EC standards and will be infiltrated by means of a series of canals (see photograph, right). Applying a 5.5km long border canal around the infiltration zone, also filled with pre-treated water, will isolate the zone from surrounding agricultural areas.

Abstracted, infiltrated water, with typical groundwater characteristics, is then treated by means of aeration, rapid sand filtration, activated carbon filtration and UV disinfection. In this way solids like iron, manganese and ammonium, dissolved during infiltration, are removed.

Part of the drinking water will be directly distributed to consumers. The other part will be distributed to several groundwater pumping stations, which will supply water during peak demand periods.

Infiltration as a surface water treatment technique

Pre-treated water is pumped to the infiltration zone where it infiltrates from canals into the aquifer by gravity. Retention of water underground provides for:

Natural disinfection;

Minimal water quality variations;

Storage capacity to cover emergencies and level out peaks in demand

In the Netherlands, surface infiltration has been applied only on sand dunes. Large parts of Holland and surrounding countries have other soil types (mostly peat and clay) and shallow groundwater levels. These circumstances make surface infiltration difficult. Thus, WOB has developed a new application of surface infiltration.

In order to demonstrate the surface infiltration technique on clay and peat soils WOB carried out a pilot project, subsidised by the EC LIFE programme.

Conclusion

Summarising the major results of the pilot project:

The infiltration technique has proved reliable in the production of drinking water;

The infiltration technique minimises the use of chemicals because of natural disinfection;

Drinking water is safe and has a constant quality and temperature

The overall conclusions of the pilot project were that the objectives were reached successfully. Therefore, the infiltration technique will form the basis of a NLG400M ($200M) water supply project that WOB will realise in the next 4 years.