SEWAGE SLUDGE AS NUTRIENT SOURCE FOR BIOENERGY CROPS
Avainsanat:
oilseed rape, white lupin, fibre hemp, sewage sludge, biomass, heavy metalAbstrakti
Sewage sludge is rich in organic matter, and plant nutrients such as nitrogen, phosphorus and
potassium. Nevertheless, using sludge as fertilizer for crops to produce food or feed is limited due to the
presence of trace contaminants such as heavy metals. Therefore, the aim of this study was to investigate
the use of sewage sludge as a nutrient source for bioenergy crops such as fibre hemp, oilseed rape and
white lupin. Specific attention is paid to the biomass accumulation and the quality of the plant material as
well as the ability of the crops to take up the heavy metals from soils in order to ensure that the amount of
pollutant does not cumulate.
Fibre hemp (Cannabis sativa L.) cv. Uso, oilseed rape (Brassica napus L.) cv. Wildcat and white
lupin (Lupinus albus L.) cv. Amiga were sown in 5-L pots in a greenhouse. Three potting mixes were
used: a standard peat-based potting compost with 50% sludge, sludge with peat (50%:50% by weight), or
100% sludge, and a constant mass of 666 g potting mix per pot. Dry weight of plants and leaf area were
measured four times during the growth of fibre hemp and oilseed rape and at maturity of white lupin. Net
photosynthesis and leaf temperature of plants were measured three times. Ash content and elemental
analysis 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 siliques
per plant and number of seeds per plant were obtained from the 100% sludge treatment. However, the
highest number of branches per plant, weight of siliques per plant, seed weight per plant and whole plant
weight were obtained from the sludge – peat treatment. In white lupin, the sludge – peat treatment resulted
in highest net photosynthesis, number of pods per plant, number of seeds per plant and whole weight plant
at harvest. Highest sludge application resulted in highest heavy metal concentration in plant material of
fibre hemp and white lupin. However, high heavy metal concentration in oilseed rape were obtained when
sludge was mixed with peat.
It is concluded that sewage sludge is suitable for use as a nutrient source for bioenergy crops
studied. In future experiments we will estimate the optimum level of sewage sludge for each one of these
crops and determine its potential for use in field conditions.
