Cocomposting of Sludge from Olive Oil Mill Wastewater Mixed with Tree Cuttings

Abstract

The sludge residue from olive oil mill wastewater (OMW) evaporated in natural conditions in an open-air lagoon was cocomposted with tree cuttings (TC) in two mixtures, 48% of OMW sludge + 52% of TC (M1, C/N ratio, 30), and 58% of OMW sludge + 42% of TC (M2, C/N ratio, 26). The evolution and modification of properties occurring in the OMW sludge-TC mixtures at different stages of the composting process were evaluated by chemical analyses, including pH, electrical conductivity and contents of total organic C, total N, total extractable C and humified C fractions. Further, HAs were isolated from the mixtures by a conventional procedure based on alkaline extraction, acidic precipitation to pH 1, purification by repeated alkaline dissolutions and acidic precipitations, water washing, dialysis, and final freeze-drying. The HAs obtained were analyzed for elemental (C, H, N, S, O) and acidic functional group (carboxylic and phenolic) composition, and by ultraviolet/visible, Fourier transform infrared, fluorescence and electron spin resonance spectroscopies. Composting of the OMW sludge-TC mixtures resulted in an increase of pH and total N, ash and humified C contents, whereas electrical conductivity and total organic C, total extractable C and NH4 +−N contents and C/N ratio tended to diminish. At the initial stage of composting, HAs from the OMW sludge-TC mixtures were characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials and partially modified lignin moieties and polysaccharides-like structures, extended molecular heterogeneity, low organic free radical contents and small degrees of aromatic ring polycondensation, polymerization and humification. With increasing the composting time, a loss of aliphatic materials and carbohydrates, and an increase of oxygenation, acidic functional groups, S- and N-containing groups and aromaticity occurred in HA fractions. Chemical and physicochemical analyses of the two OMW sludge-TC mixtures and their HA components at the end of composting indicated that an adequate degree of maturity and stability was achieved by both end products, and especially for the one obtained from mixture M2.