Factors influencing the behavior of land applied biosolids

Using biosolids from the Hampton Roads Sanitation District (HRSD), laboratory studies were conducted to provide an insight into the behavior of this material when it is applied on land. The interaction and affinity of the biosolids for heavy metals was determined by packing reactor columns with known amounts of the biosolids and passing through the packed columns aliquots of several heavy metal solutions including, cobalt, copper, lead, iron, manganese, and zinc. The process was repeated until the retention capacity of the columns was reached. Solution parameters were varied to allow investigations of the influence of pH, ionic strength, and inter-elemental effects. The affinity of the biosolids for each metal was determined by calculating the mass of the metal that was adsorbed on the biosolids (removed from the solution) per gram of the biosolids. Using batch and reactor column procedures, samples of the biosolids were fractionated to determine water soluble, exchangeable, adsorbed, organically bound, carbonate, and sulfide fractions and to determine the lability, mobility, and availability of the heavy metal and other contaminants in those fractions. A fractional distribution extraction protocol was specifically conducted for the lead-biosolids complex to provide an insight into the lability and bioavailability of the biosolids-bound lead. The affinity of metal ions for the biosolids was found to increase in the order Pb>Fe>Cu>Zn>Ni>Mn>Co. Correspondingly, synergistic effects of the different metals led to characteristic decreases in the respective binding capacities. A decrease in system pH led to a decrease in the binding capacity, a trend that was also observed when the ionic strength increased. Using lead as a model, fractionation studies revealed that adsorption of lead by biosolids is mainly associated with the carbonate fraction.