Biomass of energy willow from sandy soils fertilized with sewage sludge

Procesowi oczyszczania ścieków zawsze towarzyszy wydzielanie osadów ściekowych. Jedną z metod utylizacji, wykorzystującą własności nawozowe osadów, jest ich przyrodnicze (jeśli tylko spełniają dopuszczalne normy biologiczne i chemiczne) zastosowanie. Istniejące przepisy rygorystycznie wskazują obszary, na których osady nie mogą być wykorzystywane. Celowa i opłacalna wydaje się możliwość wykorzystania nawozowych walorów osadów ściekowych do nawożenia upraw roślin energetycznych, w tym różnych odmian wierzby wiciowej. Uprawy takie często prowadzi się na gruntach ubogich lub zdegradowanych i w takich warunkach wykorzystanie osadów ściekowych wydaje się uzasadnione. Celem badań autorów pracy było określenie ilości i jakości uzyskanej biomasy wierzby wiciowej (Salix viminalis) z gleby piaszczystej, nawożonej wybranymi osadami ściekowymi i obornikiem. Badania prowadzono w warunkach doświadczenia lizymetrycznego przez okres 3 lat. Zastosowano następujące dawki osadów ściekowych i obornika: 0, 10, 50, 100 i 200 Mg·ha–1. Analizowano ilość uzyskanej biomasy i zawartość skumulowanych w niej metali ciężkich.The process of sewage treatment is always accompanied by development of sewage sludge. One of the methods of sewage sludge disposal, which is based on its fertilization capacity, is its use in nature, e.g. in farming (if all the permissible biological and chemical standards are met). One of the problems that often occur during application of sewage sludge in soils is excessive content of heavy metals and sanitary contaminants. The ordinance that controls the use of sewage sludge in nature in Poland determines sanitary conditions and content of heavy metals in the sewage sludge for non-industrial use and in soils where this sludge cannot be used. Legal regulations show specific areas where sewage sludge cannot be used. Utilization of fertilizing capability of sewage sludge in cultivation of energy crops, including various species of Salix viminalis, seems to be purposive and profitable. These crops are often cultivated in low-productive and degraded soils and utilization of sewage sludge in such conditions appears to be justified. The aim of the study was to determine the amount and quality of biomass obtained from Salix viminalis cultivated in sandy soil fertilized with selected types of sewage sludge. The examinations were carried out under conditions of lysimetric experiment for the period of 3 years. The following doses of organic fertilizers were used: 0, 10, 50, 100 and 200 Mg·ha‒1. Sludge from different types of treatment was added to degraded sandy soil. The amount of biomass obtained and heavy metals content were analysed. Statistically significant differences in Salix viminalis yields were found between consecutive years of the experiment. The amount of biomass was declining in the 2nd and 3rd years. The best yielding of Salix viminalis was observed after fertilization with the doses (the majority of sewage sludge) of 10 and 50 Mg·ha‒1. The use of the doses of 100 Mg·ha‒1 caused a decline of biomass of Salix viminalis by ca 55% compared to the content of the plant from control objects. The dose of 200 Mg·ha‒1 caused usually by plant dying (most probable due to excessive salination of the soil). The least useful sludge for fertilizing of Salix viminalis was from Koluszki, Poland (without addition of polyelectrolyte and non-dewatered). This sludge, used with the doses of 50, 100 and 200 Mg·ha‒1 caused plant diseases followed by plant dying. The results of the evaluation of the content of chromium, zinc, lead, copper, cadmium, nickel and mercury in plants demonstrated that the use of sewage sludge with acceptable content of heavy metals for fertilization of sandy soils usually did not cause the contamination of the plants cultivated with these elements. The well-known phenomenon of bioacumulation of heavy metals in plants was intensifying for higher doses of the fertilizers. However, the increase in the content of heavy metals in plants (even after application of greater doses) did not cause an over-normative contamination of plants. The content of heavy metals (apart from zinc) in Salix viminalis biomass was similar to the contents found in the plants from non-contaminated soils