Article pubs.acs.org/EF
Sewage Sludge Carbonization for Biochar Applications. Fate of Heavy Metals Sam Van Wesenbeeck,† Wolter Prins,‡ Frederik Ronsse,‡ and Michael Jerry Antal, Jr.*,† †
Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States ‡ University Ghent, Biosystems Engineering, Coupure Links 653, 9000 Gent, Belgium S Supporting Information *
ABSTRACT: Biochars produced from sewage sludge show promise for use as soil amendments that could both enhance plant growth and sequester carbon. In a previous study, we showed that moist sewage sludge obtained from a rural, residential region of Oahu, Hawaii, could be carbonized, and the heavy-metal content of its biochar did not exceed United States EPA regulations limiting its use as a soil amendment. In this paper, we show that biochars produced from the nearby residential community of Hawaii Kai cannot be used as a soil amendment in Hawaii because their contents of Zn, Mo, and Cr exceed state regulations. Likewise, their contents of Cd, Cu, Ni, and Zn exceed Belgian regulations. These heavy metals are retained in the biochar during carbonization, whereas Hg and (to a lesser extent) As, Cd, and Se are released and thereby depleted in the biochar. Biochar within the carbonizer does not adsorb the heavy metals which are released during pyrolysis; consequently, these leave the carbonizer in its exhaust stream. and fertilizers.7 Despite the increased As concentration in soils of former sugar cane fields in Oahu, no As has been detected in the drinking water system.8 Up to 80% of the heavy metals present in wastewater migrates to the sewage sludge during wastewater treatment.9 These heavy metals are accumulated in different chemical species via biosorption10 on the cell surfaces present in the sewage sludge and are easy to leach out. The Hawaii Kai WWTP employs primary, secondary, and tertiary treatment. Wastewater is screened before entering a primary clarifier. The supernatant of the primary clarifier is aerobically treated before being pumped to a secondary clarifier. Additional sand filtration and chlorine disinfection are accomplished before the treated water is discharged into the Pacific Ocean. The settled sludge of the primary clarifier is pumped into an anaerobic digester, together with mechanically dewatered settled sludge from the secondary clarifier. The digested sludge is sun dried on gravel beds to reduce costs of landfilling that can exceed $91 per ton wet sewage sludge.11 The digested sludge forms the material of interest for carbonization in this study.
1. INTRODUCTION In a previous paper,1 we at the University of Hawaii presented promising results from a preliminary study of Hawaiian sewage sludge carbonization for biochar soil amendment applications. Our findings showed that sewage sludge from the Honouliuli (“Ewa”) wastewater treatment plant (WWTP) with moisture contents as high as 30 wt % can be carbonized and that the heavy-metal content of its biochar product did not exceed United States EPA regulations for use as a soil amendment. The addition of sewage sludge biochar to the soil can enhance plant growth2 and simultaneously sequester carbon;3 however, its heavy-metal content varies from one community to another, and its use as a soil amendment is controlled implicitly by both federal and state regulations concerning the addition of heavy metals to the soil in the USA and similarly by European Union and country regulations in the EU. To gain further insight into the release and uptake of heavy metals during the carbonization of sewage sludge, and to better quantify the variability of the heavy-metal content of the sludge from one residential neighborhood to another, we undertook this study of the carbonization of Hawaii Kai sewage sludge (HK SS). With a population of more than 220,000 people,4 Hawaii Kai is the largest residential community on the eastern side of the island of Oahu, Hawaii. The Hawaii Kai WWTP receives wastewater from the eastern ridge of Kuliouou Valley to the Kalanianole Highway at Sandy Beach. This wastewater is mainly residential in origin, apart from a small amount of commercial wastewater originating from three shopping centers, restaurants, various plant nurseries, and a few other facilities (e.g., schools, a golf course, and a shooting range). Oahu’s wastewater system is separated from its stormwater system;5 therefore, no heavy-metal contamination from road deposits is possible.6 Oahu does not have any major industries; the heavy-metal inputs are mainly from agricultural pesticides © XXXX American Chemical Society
2. APPARATUS AND EXPERIMENTAL PROCEDURES Sewage sludge samples (SS1, SS2, and SS3) were collected from the surface of the gravel bed during the months of June 2011, September 2011, and October 2012, respectively. Table 1 displays ultimate analyses (i.e., elemental analyses) of SS1 and SS2 together with their biochars. In Table 1 the oxygen content is determined by difference; consequently it is subject to the analytical uncertainty of all of the other components. It is not unusual to see a total above 100% for the other materials. In this case, the oxygen content is listed as
