Abstract:
Sewage sludge (biosolids) from wastewater treatment plants (WWTPs) has been widely used
as a soil improver in Europe, United States of America and some developing countries
including South Africa. It has its benefits for farmers as a good source of organic matter and
minerals, however, sludge after treatment still contains pathogenic organisms, heavy metals
and persistent organic pollutants (POPs). The POP and heavy metal contaminants that
accumulate in sludge may transfer through the food chain and cause adverse effects on
human beings.
In this study, a field experiment was carried out on farmland fertilized with sewage sludge
from a wastewater treatment plant (WWTP) that vasically receives domestic wastewater and
storm water. Vegetable spinach (Spinacia oleracea) was used for this study and was planted
on a farmland under controlled conditions. Ten ridges each of dimensions 20 m × 0.3 m was
made and dry sludge weights of 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 kg were applied as
manure on each of the ridges, respectively.
Representative samples of sludge and soil were taken for analysis of heavy metals and POPs.
At maturity, in twelve weeks, the root and leave samples of the vegetable were taken from all
the ridges including the control. The soil, sludge, and vegetable samples were analyzed for
total heavy metal content (Cd, Cr, Cu, Ni, Pb, Co, Zn, Al, Fe, Mn), speciated heavy metal
content and POP (PAH, PCB). Soil and sludge samples were also analyzed for total organic
content, pH, cation exchange capacity (CEC), conductivity and alkalinity. The analysis for
total heavy metals and speciated heavy metal content was carried out using inductively
coupled plasma optical emission spectrophotometer (ICP-OES), and CEC analysis was
carried out using atomic absorption spectrophotometer (AAS). A two-dimensional gas
chromatograph with time of flight mass spectrometry detector (GC X GC TOFMS) was used
for POP measurements. pH measurement was made using a pH meter and conductivity
measurement using a conductivity meter. Alkalinity and total organic content analysis was
performed using titrimetric apparatus. The highest total heavy metal concentration of 378.9
mg / kg was recorded in Fe metal in soil and Leaf sample while the lowest total metal
concentration of 0.0003 mg / kg was in Cu metal in root sample. The highest heavy metal
concentration of 1002 mg / kg in speciated forms was in Mn metal in F1 fraction and the
lowest of 0.0004 mg / kg was in Cd metal in F5 fraction. PAHs were only found in soil
samples and their concentrations ranged from 2.53 mg / kg to 146.5 mg / kg. There were no
PCB detected in all the samples analysed. The results indicated that the trace metals
concentrations found in the exchangeable fraction were higher than those observed in any of
the preceding extractions except in the case of Cd, Cr, Fe and Pb where Fe-Mn oxide and
organic matter fractions predominated and were closely followed by exchangeable fraction.
