Species rich vegetation generally develops on nutrient poor soils. On former agricultural land, in which phosphorus availability levels are high because of years of fertilisation, the development of species rich vegetation is hampered. The excavation of these phosphorus-rich soils is not only a costly measure, but is also not always possible, due to local cultural-historical and/or archaeological values, or because of negative impact on local water management. Alternative measures to excavation are therefore important.
Application and technology development
This research determined the effectiveness of the application of drinking water sludge to phosphorus-rich, former agricultural soils with various soil types and moisture conditions. A machine was also developed which can directly inject the drinking water sludge at a constant dosage beneath the turf.
The application of drinking water sludge does not always offer a good alternative to the excavation of phosphorus-rich soils. We have therefore developed a decision-support tool to assist land managers decide whether or not to use drinking water sludge.
This project has given us insight into the technical and financial feasibility of the application of drinking water sludge (both iron sludge and iron-lime sludge) to phosphorus-rich soils. It is clear that the effectiveness of this application is greatest in drier sandy soils. In soil types such as clay and peat, it is more difficult to mix the sludge into the soil. The soil’s phosphorus availability was sharply reduced following the application of the drinking water sludge. The effects on the vegetation remain less clear, partly because of the project’s relatively short duration. What can be concluded, on the basis of the soil chemistry after the application, is that the development of moderate nutrient richvegetations, such as kingcup meadows or flower-rich grasslands, is possible.
The mixing of the drinking water sludge by tilling significantly disturbs the soil structure. Since this is not always desirable, it gave us an extra reason to develop a machine that can inject drinking water sludge beneath the turf layer in a single step, while ensuring a good soil mix. The idea was to equip a soilinjector with ‘duck feet’. These cut through and lightly till the turf, so that the sludge from the injectors behind the duck feet can flow into the space created. This application method worked well in practice. Naturally, in the case of heavily root-penetrated soils, extra sharp blades are needed to cut through the turf.
This project has also shed light on the legal aspects of this application. Ferrous drinking water sludge is officially considered a by-product. Its use as a phosphorus-binding agent in soils in nature areas falls under the Dutch Nature Conservation Act and Soil Protection Act. Our research has demonstrated that the use of drinking water sludge has no harmful effects on the soil. By including the heavy-metal concentration level as one of the criteria used in the selection of suitable drinking water sludges, one can ensure that the natural background values for these metals are not exceeded. Moreover, the iron in the drinking water sludge, apart from the phosphorus, also binds the metals present in the soil, thus lowering their available concentrations.
Excavation is the most effective means of removing P-rich soils. But it is very costly. In the case of P-rich plots where excavation is too expensive or undesirable, and nature development by means of mining takes too much time, the use of drinking water sludge can offer an affordable alternative.
The costs of this application depend greatly on the choice of the sludge (iron sludge or iron-lime sludge), on whether the sludge needs to be temporarily stored, and on whether the turf needs to be removed. In most cases the sludge has to be delivered from a temporary storage facility, since this allows for the delivery of the desired volumes and specifications. The application of iron-lime sludge – which currently has no good alternative sales market, other than for iron sludge – is also particularly interesting from an economic perspective. The costs of the sludge application (without turf removal) are, in the case of sandy soils, lower than the costs of excavation and comparable to those of mining.
This project focused on the application of drinking water sludge in P-rich, former agriculture soils and/or nature areas. Alternative applications might also be promising and are currently under researched.