Landfill leachate treatment

High temperature treatments of landfill leachate, contaminated by PFAS

PFASs (per- and polyfluoroalkyl substances) are a class of about 5,000 synthetic chemical compounds, belonging to the Emerging Contaminants (ECs) category. They are constituted by an alkyl chain of variable length (4 to 16 carbon atoms), fully or partially fluorinated, and they often possess a carboxylic or sulphonic functional group. PFASs are used in a variety of industrial applications (in the tanning, textile, galvanic and photographic industries, in fire-fighting foams, in Teflon polymerisation, etc.), thanks to their impermeabilization properties towards water and oils, their flame retardant characteristics, and their capacity to withstand extreme environmental conditions.

Conversely, there are indications of a severe impact on humans and the environment, which make them polluting agents of grave concern. Despite PFASs having hydrophobic properties, which make them useful for their hydrorepellent characteristics, their functional groups are slightly hydrophilic, and as such these molecules are soluble and persistent in water. They have therefore become ubiquitous contaminants and, in different concentrations, can be found in water sources (both superficial and groundwater), in waste and process streams, and in landfill leachates.

These environmental matrices contaminated by PFASs require the application of removal techniques before being reintroduced into the water cycle. The poor efficacy of traditional treatment methods for PFASs is due to the strength of the C-F bond, cause by the high electronegativity of fluorine.
While many technologies have been investigated to reduce or destroy PFASs in drinkable water (superficial and subterranean) and in wastewater, very few experimentations have been conducted on the treatment of process waters and landfill leachates, and therefore few scientific studies exist in the literature.

Thanks to lab-scale dedicated reactors for high temperature treatments, both with and without a flame, we conduct degradation and mineralisation tests of PFASs-contaminated liquid matrices, with the purpose of:
1. improving the understanding of PFASs stabilisation processes;
2. investigate the mechanisms involved in the degradation process, both from a chemical (i.e., kinetics) and physical (mass and energy transfer) standpoint;
3. obtain useful indications for the design of pilot plants for the continuous thermal treatment of PFASs.

Thanks to a grant provided by the AMGA Foundation through the 2020 Project 4.0 Call, and in collaboration with Acque del Chiampo, K-INN Tech has developed first a flameless reactor and then an an innovative lab scale plant (thermal power: 8kW) capable of treating concentrated leachate contaminated by PFASs, feeding the sample to be treated directly into the flame in a liquid or nebulised state.

The monitoring of the yields of mineralisation, degradation and stabilisation of PFASs is obtained through a cumulated measurement of the hydrofluoric acid (HF) produced during treatment, according to approved methodologies (EPA Method 26 - Determination of hydrogen halide and halogen emissions from stationary sources non-isokinetic method; ISO 15713:2006 - Stationary source emissions - Sampling and determination of gaseous fluoride content).

Tests conducted on synthetic liquid solutions, contaminated by pure PFBA, PFBS or a mixture of the two, confirm very high mineralisation yields of fluorinated compounds into HF, with average values up to 95.5%, obtained with a contact time of 0.5 seconds inside the flame. Tests of actual concentrated leachate, with concentration of PFASs averaging 5 mg/L, are currently underway.

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