Sewage Sludge Increased Lettuce Yields by Releasing Valuable Nutrients While Keeping Heavy Metals in Soil and Plants at Levels Well below International Legislative Limits
Sewage sludge can be used as an organic amendment as long as it is ensured that there is
no risk of environmental contamination or risk to public health. In this study, sewage sludge from
two wastewater treatment plants (WWTPs) subjected to two disinfection and stabilization treatments
[40% (mass/mass), calcium oxide, and calcium hydroxide] and their respective untreated sewage
sludge were used. Three control treatments were also added: conventional farmyard manure (FYM),
a nitrogen (N) mineral fertilizer (ammonium nitrate 34.5% N) applied at a rate of 50 kg N ha−1
(N50) (the same rate of all organic amendments), and an unfertilized control (N0), totaling nine
treatments. Lettuce (Lactuca sativa L.) was cultivated in pots for two growing cycles. The dry matter
yield (DMY) was higher in the N50 treatment (13.5 and 10.6 g plant−1 in the first and second growing
cycles, respectively), followed by sewage sludge (10.8 to 12.4 and 8.4 to 8.7 g plant−1), FYM (8.5 and
7.2 g plant−1), and the control (7.7 and 6.0 g plant−1). The DMY was related to the N provided by the
different treatments, assessed by the N and nitrate concentrations in tissues, N uptake, and apparent
N recovery (ANR). Sewage sludge, due to its high N concentration and low carbon (C)/N ratio,
mineralized rapidly, providing a significant amount of N to plants, as well as other nutrients, such as
phosphorus (P) and boron (B). FYM, with a higher C/N ratio, provided less N to plants, also due to the
short duration of the lettuce growing cycle. Alkalized sewage sludge increased soil pH and calcium
(Ca) availability for plants. Fertilizer treatments minimally influenced cationic micronutrients. Heavy
metals in the initial sewage sludge were below the threshold values established in international
legislation, and the levels in soil and lettuce tissues were generally not higher than those in other
treatments. Both of the sewage sludges used in this study showed high fertilizing value and very
reactive behavior, making nutrients available much more quickly than FYM. This information is
relevant to consider in defining their agricultural use.
