Treatment of a Mature Landfill Leachate: Comparison between Homogeneous and Heterogeneous Photo-Fenton with Different Pretreatments

Abstract

This study focuses on the treatment of a mature landfill leachate by coagulation and photo-Fenton at different conditions. Optimal coagulation is carried out with ferric chloride in acid conditions; and with alum in near-neutral conditions, to minimize the use of sulphuric acid for pH adjustment (1 g/L vs. 7.2 g/L), the generation of sludge and the increase of conductivity in the final effluent. In both cases, a similar chemical oxygen demand (COD) removal is obtained, higher than 65%, which is high enough for a subsequent photo-Fenton treatment. However, the removal of absorbance at 254 nm (UV-254) was significantly higher with ferric chloride (83% vs. 55%), due to the important removal of humic acids at acid pH. The best results for coagulation are 2 g/L ferric chloride at initial pH = 5 and 5 g/L alum at initial pH = 7. After coagulation with ferric chloride, the final pH (2.8) is adequate for a homogeneous photo-Fenton using the remaining dissolved iron (250 mg/L). At these conditions, using a ratio H2O2/COD = 2.125 and 30 min contact time, the biodegradability increased from 0.03 to 0.51. On the other hand, the neutral pH after alum coagulation (6.7) allows the use of zero valent iron (ZVI) heterogeneous photo-Fenton. In this case, a final biodegradability of 0.32 was obtained, after 150 min, using the same H2O2/COD ratio. Both treatments achieved similar results, with a final COD, UV-254 and color removal greater than 90%. However, the economic assessment shows that the approach of ferric chloride + homogeneous photo-Fenton is much cheaper (6.4 €/m3 vs. 28.4 €/m3). Although the discharge limits are not achieved with the proposed combination of treatments, the significant increase of the pre-treated leachate biodegradability allows achieving the discharge limits after a conventional biological treatment such as sequencing batch reactor, which would slightly increase the total treatment cost.

Assessing an Integral Treatment for Landfill Leachate Reverse Osmosis Concentrate

“…At lower pH values, the efficiency of coagulation in terms of COD removal increased, as it has previously been reported by other authors [34,39]. In particular, it is well-known that the COD of LLROC will be reduced by decreasing the pH in the medium [45] due to the precipitation of humic acids [41,46]. Unfortunately, the high alkalinity of the sampled LLROC (44,125 mg L −1 CaCO 3 , Table 1) required the addition of a relatively large quantity of sulfuric acid, namely: 16, 23, and 24.5 g L −1 of H 2 SO 4 , to reduce the pH from its initial value of 8.13 (Table 1) to 7.0, 6.0, and 5.0, respectively, before performing the coagulation treatment.…”

“…Coagulation with ferric chloride achieved COD removals of 43% at the natural pH value of the LLROC (8.13; Table 1); 78% at pH = 7.0; 76% at pH = 6.0; and 71% at pH = 5.0; respectively (Figure 1). In addition, whereas a 30 g L −1 dose of FeCl 3 were required to optimize coagulation at pH values at or above 7.0, only 5 g L −1 was required at the acidic pH values (i.e., 5.0 and 6.0) because the optimum pH for coagulation with FeCl 3 is about 4-5 [41], and the addition of FeCl 3 produces a significant reduction of pH with the subsequent precipitation of humic acids. In short, decreasing the initial pH value of LLROC to acid values not only favored a higher coagulation efficiency, but it also significantly reduced the optimum coagulant dose to one-sixth of that required at neutral and basic pH values, which results in a significantly lower treatment cost and reduces chloride content.…”

“…The radiated energy in the UV region of the used mercury immersion lamp is reported by the manufacturer to be 15%, whereas new mercury immersion lamps account for 40% radiation emitted in the UV region; so the power consumption of the mercury lamp technology was recalculated, as included in our previous work [41], and proper technological comparison could be made. In short, the application of LED technology resulted in a significant (p < 0.001 ***) monetary saving of the 16.46% in terms of power consumption (as calculated from data on Table 3, which is further discussed in Section 2.4), besides lower costs of investment and disposal (environmental hazard of mercury) shall also be considered; as well as further optimization of the LED technology use might also contribute to even further reduce the final cost of treatment.…”

“…In fact, Rodriguez-Chueca et al (2016) [42] have similarly reported UVA-LED as a promising technology to more efficiently assist the photo-Fenton treatment. Moreover, the combination of coagulation (pH = 5.0; 2 g L −1 of FeCl 3 ) with UVA-LED photo-Fenton oxidation was assessed to cost 1.56 € kg COD −1 in a previous study [41], in which lime precipitation was not considered, denoting the importance of process optimization at lab scale and technology development in the short term.…”

“…High-pressure immersion mercury vapor lamps have been widely used as the UV irradiation source for the photo-Fenton treatment step, although these lamps have a low energy efficiency, with only 20% of the consumed energy actually contributing to the emission of photons within the UV range [41], and the greater rest consumed in infrared (IR) irradiation losses. This is why other radiation sources have gained importance lately, such as light-emitting diode (LED) technology [42] or solar radiation with the aid of compound parabolic collectors (CPC) or thin parabolic collectors [43,44].…”