Just over 2.5 million tonnes of sewage sludge produced at waste water treatment plants in the Netherlands was removed to dump sites in 2004. This is 60 percent less than in 1990, when the majority of sewage sludge was dumped. In 2004 most sewage sludge was incinerated.
When waste water is treated sewage sludge remains as residue. In 1990 6.3 million tonnes of sewage sludge was transported to dump sites. In 2004 this had dropped to 2.5 million tonnes.
Part of the decrease was caused by the sludge being dehydrated after the treatment process, which reduces its volume. This also makes the sludge more suitable for incineration and recycling.
Sewage sludge and average dry matter residue of waste water treatment plants
Less dry matter
In spite of an increase in the volume of waste water treated by the treatment plants, the transport of sewage sludge in terms of dry matter fell by 5 percent from 1990, to 0.53 million tonnes in 2004.
Less dumped, more incinerated
The processing of sewage sludge has changed enormously since 1990. In 1990 nearly half of sewage sludge was removed to a dump site, in 2004 this was less than 7 percent. In 1990 just over 8 percent of sludge was incinerated, in 2004 this is 52 percent.
One of the reasons that more sludge is incinerated is that regulations for the dumping of sewage sludge have become stricter. This also explains why dehydration of the sludge has been improved: it makes it easier to burn. The incineration of sewage sludge generates electricity. The residue after incineration is recycled and used as a filler in the asphalt industry and salt mines.
Sewage sludge by destination
Sludge as fertiliser and compost
Up to the beginning of the nineties, about one quarter of sewage sludge remaining after treatment in urban waste water treatment plants was used as a fertiliser in agriculture. Since 1995, however, stricter norms have been introduced for the agricultural use of sludge because of its high heavy metal content. Since then only 5 percent of sewage sludge is used as a fertiliser.
About 10 percent of sludge is composted. The compost this produces is used as a cover layer for waste dump sites.
Tjerk ter Veen, Ronnie Huwaë and Kees Baas