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A bench-scale assessment of ozone pre-treatments for landfill leachates a
Yu Qiao , Anh Do , Daniel Yeh & Michael J. Watts a
Department of Civil and Environmental Engineering, Florida State University, Tallahassee, FL, USA b
Climate Change Research Center, Institute of Meteorology Hydrology and Environment, Hanoi, Vietnam c
Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL, USA Published online: 02 Sep 2013.
To cite this article: Yu Qiao, Anh Do, Daniel Yeh & Michael J. Watts (2014) A bench-scale assessment of ozone pre-treatments for landfill leachates, Environmental Technology, 35:2, 145-153, DOI: 10.1080/09593330.2013.821141 To link to this article: http://dx.doi.org/10.1080/09593330.2013.821141
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Environmental Technology, 2014 Vol. 35, No. 2, 145–153, http://dx.doi.org/10.1080/09593330.2013.821141
A bench-scale assessment of ozone pre-treatments for landﬁll leachates Yu Qiaoa , Anh Dob , Daniel Yehc and Michael J. Wattsa∗† a Department of Civil and Environmental Engineering, Florida State University, Tallahassee, FL, USA; b Climate Change Research Center, Institute of Meteorology Hydrology and Environment, Hanoi, Vietnam; c Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL, USA
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(Received 29 January 2013; accepted 25 June 2013 ) Leachate from stabilized landﬁll can pose unique challenges to conventional biological wastewater treatment. Ozone-based advanced oxidation processes have garnered recent consideration as an option to reduce the organic strength and recalcitrance of aged landﬁll leachate. With a bench-scale investigation, the reported work examines the potential for leachate conditioning for further biological treatment by treatment with low-mg/L doses of ozone (0–7.5 mg/L O3 ). While not suﬃcient for signiﬁcant organics mineralization, the tested ozone doses could potentially produce both selective and non-selective oxidation of recalcitrant leachate organic compounds leaving bio-available products in the pre-treated leachate. Leachate conditioning by O3 or O3 /H2 O2 was assessed via monitoring of three anthropogenic organic leachate contaminants(tris-(2-chloroethyl) phosphate, tris-(butoxyethyl)-phosphate and 17β-estradiol (E2)) with ozonation, and ozonation followed by anaerobic incubation. In addition, chemical oxygen demand (COD) and BOD5 analysis of the ozonated leachate, and methane and total gas formation during the anaerobic incubation were used to assess the degree of leachate conditioning. When treated with O3 alone, 58% removal of E2 was observed with an ozone dose of 4.5–5.4 mg/L. Direct oxidation of the three leachate contaminants was limited with O3 /H2 O2 pre-treatment. However, this pre-treatment was observed to have signiﬁcantly improved degradation of E2 during anaerobic incubation of ozonated leachates (removal rate of E2 was 53.7% with 15 days of incubation), indicating the potential for ozone synthesized co-metabolism. However, overall anaerobic microbial activity was not signiﬁcantly impacted by the applied ozone pre-treatments, as measured by methane formation, total gas formation, and COD removal during incubation. Keywords: advanced oxidation processes; ozone; landﬁll; leachate; anaerobic
Introduction The most common approaches for management of collected landﬁll leachate are discharge to sanitary sewer, or a combination of physical–chemical–biological treatment processes on-site. In either case, recalcitrant organic matter in municipal landﬁll leachates can be a signiﬁcant obstacle for eﬃcient biological treatment. Municipal landﬁll leachates are typically characterized by the aggregate measures chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5 ), the ratio of BOD5 /COD (to describe the relative bioavailability of organic matter), pH, total suspended solids (TSS), total Kjeldahl nitrogen, and alkalinity and heavy metals concentrations. While there are many site-speciﬁc factors aﬀecting the quality of leachate, such as seasonal weather variations, age and structure of landﬁll, and the type and moisture content of the landﬁll waste, the relative age of landﬁll leachate can be characterized by pH (young: 7.5), BOD5 :COD (old: ratio < 0.1 vs. young: 0.5– 1), and type of dissolved organic matter (old: humic and fulvic acids vs. young: up to 80% volatile fatty acids).[1,2] ∗ Corresponding † Current
Paradoxically, on-site treatment is the most common for young municipal landﬁlls where leachate organics can be readily biodegradable and treated with conventional biological wastewater unit processes. Leachates from old landﬁlls that naturally accumulate at a slower rate, require smaller storage, are often sent directly to publicly owned treatment works for treatment. These stabilized leachates remain rich in organic matter containing a higher degree of refractory, heavy molecular weight organic compounds; organic matter that can be diﬃcult for conventional activated sludge treatment plants to fully remove. When leachate represents more than 10% of the total treatment volume, the inhibitory eﬀects on biological wastewater treatment (particularly, BOD removal) have been well demonstrated. Enhancing the biodegradation and bio-availability of recalcitrant leachate organics might be feasible with on-siteadvanced oxidation. Advanced oxidation processes (AOPs) are energized chemical oxidation processes that produce the short-lived, non-selective free radical •OH. State-of-the-art, engineered AOPs for full-scale water treatment include germicidal UV-based processes (e.g. UV/H2 O2 ), and ozona-
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tion (e.g. O3 /H2 O2 ). The scope of research and applications of AOP for leachate treatment is limited. The rich concentration of UV-absorbing constituents (both particulates and dissolved solids) in leachate-waters poses a unique challenge to engineered photo-oxidation of refractory contaminants. A review of investigations into treatment of leachate with the UV-advanced oxidation process, UV/H2 O2 , found that overcoming the relatively poor UV light transmittance of landﬁll leachate required gram-per-litre-levels of H2 O2 (to achieve signiﬁcant COD removal). Ozone AOPs hold signiﬁcant promise for on-site leachate pre-treatment as the ozone-generation technology is scalable to various landﬁll size and leachate collection rates, and can produce O3 from both ambient air and liquid oxygen supplies. O3 and O3 /H2 O2 systems are among the most frequently used AOPs due to the relatively low-energy input required to produce a mole of •OH. Considerable eﬀort has been invested in determining the eﬃciency of ozone mineralization of dissolved carbonaceous leachate contaminants. Wu et al.  observed a signiﬁcant decrease in overall molecular weight of leachate dissolved organic carbon (DOC). The applied ozone dose of 1.2 g/L was enough to shift the ratio of BOD5 /COD from 0.06 to 0.5, indicating a higher degree of biodegradability in the eﬄuent DOC. Adding H2 O2 to leachate before ozone treatment has been shown to signiﬁcantly reduce leachate colour and increase the rate of COD removal. However, it should be noted that the reported doses of ozone for COD removal from landﬁll leachate are an order-of-magnitude higher than doses applied for the disinfection of reclaimed waters (typically 2–30 mg/L). The impracticality of delivering g/L of O3 to leachate is demonstrated by the energy input that can be required to generate ozone from air: 22–26 kWh/kg of O3 . Nevertheless, the practice of dosing g/L of O3 per g/L of COD in leachate has demonstrated not only signiﬁcant colour, BOD5 , and COD removal, but a signiﬁcant shift in the molecular weight distribution of leachate organic matter to smaller organic compounds. It is, as of yet, unknown whether conventional, costeﬀective, low-mg/L doses of O3 could be utilized for onsite pre-conditioning of landﬁll leachate before discharge for biological treatment. Of considerable practical interest are optimum O3 and H2 O2 -and-O3 dosing regimens that can generate shifts in old leachate organic biodegradability, and subsequent biological oxidation of anthropogenic organic contaminants of concern found in municipal solid waste leachates. World-wide, landﬁll leachate is a welldocumented source of pharmaceuticals, personal care products, volatile organic contaminants, plasticizers, and ﬂame retardants in the hydrosphere.[9–12]
OR, 97%), tris-(butoxyethyl)-phosphate (TBEP; TCI America, 95%), and 17β-estradiol (E2; Alfa Aesar, Ward Hill, MA, 99%). Sodium azide (NaN3 ) (99%) was also acquired from Alfa Aesar. Hydrogen peroxide aqueous solution (30–32% H2 O2 ) was acquired from MACRON Chemicals (Center Valley, PA). Dichloromethane (HPLCgrade) was purchased from Sigma-Aldrich (St. Louis, MO). Puriﬁed, laboratory-grade water of Type III (PURELAB S-R 7–15 series, ELGA LabWater, UK) was used for all dilutions.
Experimental To 250 mL aspirator bottles (borosilicate glass), 17.5 mL of combined ozone stock solution and laboratory-grade water were added to 65 mL of leachate sample containing
Reagents The selected analytes were purchased as neat standards: tris(2-chloroethyl)-phosphate (TCEP; TCI America, Portland
Ozone generation Ozone was produced from oxygen using a 4 g/hr ozone generator (AZCO Industries RMU 16–04, Langley, B.C.). Fresh ozone stock solution was generated by bubbling ozone gas into 250 mL of pH 6, 10 mM phosphate buﬀer solution in a gas washing column. To minimize the decay of ozone in the stock solution, the ozone washer was surrounded by ice and water. Temperatures