Genanvendelse af slam til fremstilling af organisk rig overfladejord

Summary and conclusions

Use of sludge for production of organically rich topsoil

The objective of the present project is to examine whether it is possible to produce a growing medium from waste water sludge and mineral soil. Such a growth medium can be used in areas with poor plant growth conditions, including removed topsoil. In Denmark, such areas would include banks, waste dumps, raw material pits, and erosion affected localities.

In London, sludge has been used for the production of growing medium used in landscaping. The product called blacksoil is based on sludge and mineral soil and is used instead of a natural topsoil. Furthermore, sludge has been applied in mining areas, where plants have been unable to grow. In these areas good results have been achieved in establishing a permanent vegetation. The organic substance content of the sludge, which may amount up to 5% has resulted in improved plant growth conditions by increasing water-binding, ion exchange capacity and nutrients.

Blacksoil production

In this project simple blacksoil products have been manufactured from sand and the most used sludge types: aerobically stabilised activated sludge, anaerobically stabilised primary sludge and activated sludge and finally limed primary sludge and activated sludge. The application of another and more compound mineral soil will result in a better product. For the mixing, a lime sludge mixing plant is applied, which is provided with approximately 1 sand : 1 aerobically stabilised activated sludge (volume : volume), 2 sand : 1 anaerobically stabilised sludge, and finally 2 sand : 1 limed sludge. About 4 m³ of these compounds are added to containers, which collect any percolate and surface water. In the specific cases there were no problems for the two former types regarding performance or odour emission, while the aerobically stabilised sludge smelled less during and after the mixing. It is not possible to generalise about the usefulness of the sludge types. The content of organic substance was high in the case of aerobically stabilised sludge, and the dry solids contents were considerably lower, approx. 50%, compared with 75% of that of the other sludge types. The properties were considerably improved by increasing the dry solids contents adding more sand. The new mixture was more porous, and less smelling.

The black soils are not suitable for plant growth immediately, but must mature and become stabile before application. This is also the case with compost. The result of the maturing process is a decreasing oxygen consumption to an acceptable level, where sufficient oxygen is available for the plant roots.

The maturing process

The biological maturing process starts at the surface and is driven by oxygen availability due to drying and porosity of the surface layer. Eventually a still thicker layer in the stack is matured because the oxygen penetrates further down.

The time needed to produce a stable blacksoil will depend on sludge type and sludge : sand ratio used. Furthermore, the time will depend on the degree of mechanical turning of the mixture, as oxygen consuming areas will be exposed to oxygen. The time to reach a stable product highly depends also on the relation between surface and volume of the stack. Maturing time from three months to years must be expected. A stable blacksoil of the types produced in this project has similar qualities as a sandy mould.

Potential oxygen consumption

The necessary maturing time is important, partly because it is important in economic considerations and partly because the blacksoil must be of a sufficiently good quality suitable for vegetation. Thus a method for evaluation of the maturity has been developed in the project. The blacksoil is stable and ready for planting if the potential oxygen consumption is sufficiently low. Growth tests using stable blacksoil showed good sprouting and growth results.

The potential oxygen consumption of the blacksoil as well as the other results of the project showed that the dry solids of the mixture of sludge and sand must be above 50%, preferably up to 75% to ensure sufficient conditions for the aerobic maturing process.

Hygiene

Neither Salmonella nor Faecal streptococci could be detected in the upper 10 cm of the blacksoil stacks, having high dry solids content, after 5 months of storing. However, 300 faecal streptococci per gram solids were detected in a blacksoil with solids below 50%, and which was not a stable product.

Organic micropollutants to the environment

The risk of percolation of organic micropollutants, e.g. LAS, can be reduced by using sludge qualities, which comply with the current regulation for sludge used as manure on farm land. It is expected that the aerobic decomposition of organic substance, to a large extent, decomposes organic micropollutants in the sludge.

Metals

If sand, basically not-polluted with metals, is mixed with sludge, complying with the criteria for agricultural use, the blacksoil will have metal concentrations, which in most cases will comply with Danish criteria for clean soil.

Nitrogen amounts in manure, blacksoil and mould

The content of nitrogen compounds in the blacksoil will be halved during the maturing process. This is caused by a combination of evaporation of ammonium and biological nitrogen removal.

In case of using blacksoil for landscaping a layer of up to 30 cm should be applied. The amount of nutrients per hectare is then much larger than the amount, which is allowed as fertiliser on farm land. Apparently, a larger risk, by applying blacksoil, is percolation of nitrogen to the ground water.

However, it is possible to evaluate the risk by comparing the content of organic substance and nitrogen in the blacksoil with the content in natural occurring alternatives to blacksoil.

The content of organic substance and nitrogen in blacksoil proved to be of the same size as in soil of oak forests, pine forests and in grass areas. However, the content is higher than in plough land

The risk of nitrate percolation by applying topsoil is similar for blacksoil and other natural occurring topsoil with high organic content.

The limitations of use of fertiliser have been made to reduce percolation of nutrients from repeated use. It is only the intention to use the blacksoil in an area once before planting. This is of importance for e.g. the estimation of the metal impact, including comparison with the impact from repeated use of artificial fertiliser.

The risk of percolation can be minimised by applying blacksoil in the summer, where the ground water production is low. Besides, the blacksoil can be sown in immediately after distribution, absorbing inorganic nitrogen by the sprouting plants.

Applicable as topsoil

Blacksoil will have an positive effect on creating a sustained vegetation due to its content of organic substance and nutrients. The product can be safe for growing medium purposes by controlling constituent and stability.

Legal basis for use of blacksoil

There are no specific rules in the Environmental Protection Act for government of blacksoil application. Therefore an individual judgement must be made in each case of use of blacksoil.

It is a condition in reusing of sludge as blacksoil, that it is less expensive to produce and use than to incinerate the sludge.