SEWAGE SLUDGE AS NUTRIENT SOURCE FOR BIOENERGY CROPS

Abstract

Sewage sludge is rich in organic matter, and plant nutrients such as nitrogen, phosphorus andpotassium. Nevertheless, using sludge as fertilizer for crops to produce food or feed is limited due to thepresence of trace contaminants such as heavy metals. Therefore, the aim of this study was to investigatethe use of sewage sludge as a nutrient source for bioenergy crops such as fibre hemp, oilseed rape andwhite lupin. Specific attention is paid to the biomass accumulation and the quality of the plant material aswell as the ability of the crops to take up the heavy metals from soils in order to ensure that the amount ofpollutant does not cumulate.Fibre hemp (Cannabis sativa L.) cv. Uso, oilseed rape (Brassica napus L.) cv. Wildcat and whitelupin (Lupinus albus L.) cv. Amiga were sown in 5-L pots in a greenhouse. Three potting mixes wereused: a standard peat-based potting compost with 50% sludge, sludge with peat (50%:50% by weight), or100% sludge, and a constant mass of 666 g potting mix per pot. Dry weight of plants and leaf area weremeasured four times during the growth of fibre hemp and oilseed rape and at maturity of white lupin. Netphotosynthesis and leaf temperature of plants were measured three times. Ash content and elementalanalysis was made from samples collected at harvest.Sewage sludge application significantly affected most parameters measured. In fibre hemp,maximum dry weight, leaf area and photosynthesis values were obtained from the sludge – peat treatment.In oilseed rape, the maximum values of dry weight of plant, leaf area, photosynthesis, number of siliquesper plant and number of seeds per plant were obtained from the 100% sludge treatment. However, thehighest number of branches per plant, weight of siliques per plant, seed weight per plant and whole plantweight were obtained from the sludge – peat treatment. In white lupin, the sludge – peat treatment resultedin highest net photosynthesis, number of pods per plant, number of seeds per plant and whole weight plantat harvest. Highest sludge application resulted in highest heavy metal concentration in plant material offibre hemp and white lupin. However, high heavy metal concentration in oilseed rape were obtained whensludge was mixed with peat.It is concluded that sewage sludge is suitable for use as a nutrient source for bioenergy cropsstudied. In future experiments we will estimate the optimum level of sewage sludge for each one of thesecrops and determine its potential for use in field conditions.