Stabilization process in reed bed systems for sludge treatment

Reed bed systems (RBS) represent an innovative and ecologically sound treatment method for the stabilization of sludge from wastewater treatment plants (WWTPs), which is also able to provide several ancillary ecosystem services. In this study, the performance of sludge stabilization achieved during the operation of an RBS for the stabilization of excess sludge extracted from a WWTP located in Central Italy (“La Fontina” WWTP, 15,000 p.e.) was studied. In order to eval- uate the process of sludge stabilization, parameters representing the biochemical, chemical and chemico-structural prop- erties of organic sludge matter have been tracked during the entire period of operation (6 years). The main aim of this paper is to monitor the stabilization process of sludge organic matter occurring during the main RBS operational stages (commissioning, operation and resting), in order to derive useful rules and parameters for the for- mulation of novel guidelines for RBS design and operation. Each singular operational stage was characterized by the predominance of a specific process of organic matter stabi- lization as follows: 1) During the commissioning phase, the mineralization of fresh organic matter turned out to be the primary process, as highlighted by the values of water soluble carbon (4195 mg C/kg dw) and dehydrogenase activity (25.5 mg INTF/ kg dw h) reached at 24 months; 2) During the operating phase, the mineralization of pseudo-stable organic matter proved to prevail over the humification process, as demonstrated by the decrease of toluene (from 27.5% to 22.9%) derived from chemical-structural organic matter characterization performed by the Py-GC technique; 3) During the resting period, humification and sanitation processes became predominant, thus enabling a final safe reuse of stabilized sludge as biosolids for land application, proved by the absence of Escherichia coli and Salmonella, and by the values of polycyclic aromatic hydrocarbons (1.53 mg/kg dw), di-2-ethylhexyl-phthalate (3.63 mg/kg dw), nonyl-phenols (20.9 mg/kg dw), and linear alkyl benzene sulfonates (4.99 mg/kg dw).