The Amsterdam canal water has never been so clean. Reason for water manager Waternet to stop flushing the canals a few years ago. But the quality can be further improved.
Surface water has been used as a garbage dump since time immemorial. Before the days of sewage and garbage collection, pollution of city waterways peaked, especially when cities grew rapidly, such as in the late 1800s. Also in Amsterdam waste and faeces were dumped in the canal or it ended up there through pipes and gutters. But those practices are a thing of the past. The canal water in the capital has never been so clean.
“At the moment, the pollution of the water in the Amsterdam canals mainly comes from the 'overflow' of sewage water during heavy rainfall,” says water expert Maarten Ouboter, who works as a chemist at Waternet. “The overflow, spread over hundreds of points in the city, occurs less than ten times a year, but when it does it can be easily measured. After about three days, that pollution has disappeared again, due to dilution and degradation."
Oxygen as parameter
The overflow is kept in check by using salvage settling tanks, explains Ouboter. “These are large tanks that are underground and can collect the temporary large flow of rainwater.” By measuring the oxygen content, Waternet gets a good picture of organic contaminants in the canal water. The organic compounds, which are broken down by micro-organisms that absorb oxygen from the water, do not only come from the overflow, but also from the more than a thousand houseboats that are not yet connected to the sewer.
Water plants
Normally, water contains between 9-14 mg/l oxygen, depending on the temperature. There is an equilibrium between the gas in the liquid and the air. Ouboter:"In city water we see that the concentration varies between 3.5-10 mg/l. In places where aquatic plants grow, the concentration can rise to 20 mg/l. In places where aquatic plants return, we also see fish. appear that like water plants, such as tench and rudd. But fish are everywhere. They travel via the canals between the Noorzeekanaal and Amstelboezem. Everything is in open connection and the fish make use of that.
Hot summer
“During the heat wave at the end of July this year, we were pleasantly surprised that the oxygen level did not drop below 3.5 mg/l,” says Ouboter. The water of the canals, on average about two meters deep, even had a temperature of 25 degrees Celsius at the bottom. Quite warm, therefore, and a condition in which more oxygen is required by the more active soil bacteria. Despite the hot weather, it turned out not to be necessary to flush the canals. “We can do this if we need to, but we've stopped doing it since 2010 due to water quality improvements.”
Salinity
Waternet not only measures the oxygen content and fertilizer concentration in the city water, but also the salt content. This is done on the basis of the conductivity, not with a view to contamination, but to origin of the water to be determined. Because Amsterdam's canal water comes from multiple sources, both fresh and salt (brackish).
“The brackish water comes from the North Sea Canal,” explains Ouboter. "When boats are locked with the Zeesluis in IJmuiden, salty North Sea water flows in. We notice the daily ebb and tidal wave here in the city from a slight rise in water. The fresh water in the Amsterdam waterways comes from the storage water that flows from the polders are pumped into the basin of which the canal system is part.”
Ebb and flow into the city
Ebb and flow are hardly noticeable now, but until the construction of the Oranjesluizen in 1872, Amsterdam made intensive use of ebb and flow to flush the waterways in the city – which served as an 'open sewer'. When the North Sea Canal was dug and the access to the Zuiderzee in the east (at the Oranjesluizen) was closed, the tide of the Zuiderzee could no longer reach the city. Flushing by the tide was replaced by an emergency pumping station in 1872, and from 1878 by the Zeeburg pumping station (a steam paddlewheel pumping station), which pumped water from the Zuiderzee into the city.
Pressure pipe
To make the flushing of the canals with the Zeeburg pumping station more effective in 1872, sewerage systems were installed in the new neighborhoods outside the canal belt. The sewers collected poo and pee in tanks in the street, which were emptied by vacuuming them. This collection with a vacuum sewer became more and more difficult as more and more houses were connected to drinking water and the sewage water became too diluted. Ouboter:“In 1913, therefore, a sewage pumping station was put into operation on the Zeeburgerdijk. The waste water from the city was drained from the city to the Zuiderzee with a pressure pipeline of more than six kilometers”.
Canals clean after 400 years
It took four centuries before the quality of the canal water was as good as it is today. Today, the new Zuivering-West discharges outside the city into the North Sea Canal, and no longer into Amstel and IJ. Within a few years, all houseboats and ships must be connected to the sewer system.
Overflow after heavy rainfall continues, also in other Dutch cities. Although Amsterdam introduced a separate sewer system for the 'new' neighborhoods that were built after 1900, and was ahead of its time, a quarter of the city (mainly the city center) still has a mixed sewer system. Rainwater and domestic wastewater therefore end up in the same drainage system, with the risk of overload and the need for overflow.
Green roofs and innovative sewer pipes
To Amsterdam more waterproof and to relieve the sewerage system, consideration is being given to measures that should result in rainwater being stored longer before it is discharged to the sewage treatment plant. Sympathetic solutions include green roofs and roofs with a dyke, such as the Polderdak Zuidas. Renewal of the sewer system itself is also being sought, such as infiltration sewers (IT sewers), in which the rainwater is not only transported, but can also slowly infiltrate into the soil via the permeable pipes.