Miljøprojekt, 922 – Fytoremidiering af tungmetalholdigt dambrugsslam

Phytoremidiation of Heavy-Metal Containing Fish Farm Sludge

Summary and conclusions

The sludge produced in fish farms is usually applied as agricultural fertiliser at dosages of approximately 30 t/ha. However, in some areas of Denmark, especially Western Jutland, the cadmium and nickel contents of the fish farm sludge exceed the quality criteria for sludge. This means that the sludge cannot be used for agricultural purposes, but has to be deposited or remedied until it meets the criteria. Depositing the sludge is expensive for the fish farmers. They therefore seek an economically sound alternative. For that purpose phytoremediation by energy crops is an appealing solution that may in principle be established by growing willow clones that accumulate cadmium and nickel from the sludge and is then used as fuel under controlled conditions where the heavy metals are collected as easily disposable ashes.

The project "Phytoremediation of heavy metal-containing fish farm sludge" consists of a preliminary study performed in 2000/2001, and a main project performed in 2001/2002.

The aim of the preliminary project was to screen selected willow clones for their ability to take up and tolerate cadmium and nickel. The six selected clones were cultivated in pots under outdoor conditions. Two dosages of sludge were applied, viz. 10 and 30 t/ha, and the sludge was either spread on the pot soil surface or mixed into the soil. Furthermore, all clones were grown under control conditions, i.e. without sludge. Some of the clones were also grown in pure sludge, since a possible remediation method may be to grow the willow in sludge beds with sealed bottom. Willow twigs were planted in the pots in early spring, and the produced biomass was harvested after leaf fall. Growth as well as cadmium and nickel contents were measured. Thereafter, the remediation potential of the single clones was estimated by combining growth and heavy metal content. The results for willow grown in soil with sludge and willow grown in pure sludge differed substantially. Firstly, all willow clones grew to a larger biomass in pure sludge than in soil with sludge, probably due to a higher nutrient availability. Secondly, the relative remediation potential differed between the two situations. Furthermore, one of the delivered clones turned out to be misnamed as it sprouted and grew during summer. Beside the willow clone experiments, the potential toxicity of the sludge to soil-living animals was estimated by conducting a reproduction test with springtails. After three weeks exposure to soil mixed with fish farm sludge at levels corresponding to 0-50 t/ha there were no significant effects of the sludge on springtail reproduction. It therefore seems unlikely that conventional sludge dosages would cause any harm to soil-living animals.

On basis of the results of the preliminary project we decided to continue with the five most promising clones as well as two new clones. The seven clones were planted in pots with pure sludge or half sludge/half soil. At leaf-fall leaves were collected for cadmium and nickel measurements. After leaf-fall, the aboveground biomass was harvested as well as some of the roots. Wood and roots were dried and weighed, and their cadmium and nickel contents were measured. The cadmium content of wood and roots was similar, 1-3 mg/kg, that of leaves was somewhat higher, 2-13 mg/kg. The nickel content of wood was 0.3-0.9 mg/kg, the content in roots 3-21 mg/kg, and the content in leaves 1-4 mg/kg. Considering the biomass and metal contents of the different plant parts, it may be concluded that a substantial part of the metals taken up by the willow plants is recycled to the soil at leaf fall. The uptake of the two metals was larger when the willow grew in pure sludge compared to sludge in combination with soil. The same was true for the remediation potential, defined ad woody biomass multiplied with the measured metal concentrations. Concerning differences between the tested clones, there was a clear trend for cadmium, but only few significant differences. The clones Jodis, Loden and Aage had the highest potential for cadmium uptake. Concerning nickel, the differences were generally smaller, but Salix alba was on top of the list irrespective of how the willow was exposed to sludge.

If the results of the project are extrapolated to a normal cultivation system with repeated wood harvest e.g. every three years, and it is assumed that cadmium and nickel are taken up to the same wood concentrations through all growing years, it is possible to estimate how much Cd and Ni is removed from the sludge in every harvest cycle, and thereby how many years it will take until the sludge is clean enough to meet the quality criteria. From our experiments it may be concluded that a willow plant will never be able to remedy the pool of sludge in which it grows, if it grows in bed of pure sludge. If, on the other hand, the sludge is applied to soil at low dosages like in normal agricultural practice, it seems likely that the method may be optimised so that the cadmium content can be lowered sufficiently during the 20-year period that is usually the lifetime of willows grown as energy crop. However, this implies that no further cadmium-containing sludge or other kinds of cadmium-containing fertilisers are applied. For nickel it seems totally unrealistic to clean fish farm sludge by growing willow. It may be possible to increase the nickel potential by chemically increasing nickel bioavailability in combination with breeding/selection of more potent clones or species, but at the moment the willow model is not a realistic alternative to deposition of nickel-polluted sludge.