Description
This report describes the results of a two‐year study of approaches to mitigation of landfill leachate‐induced UV transmittance issues for leachate disposal at publicly owned treatment works (POTWs).
OBJECTIVES
The objective of the study was to identify the origin and nature of UV‐quenching substances (UVQS) in landfill leachate and develop technically and economically viable on‐site treatment technologies to address the UV transmittance issues at POTWs.
Specifically, Objective 1 was to identify the nature of UVQS from different landfill leachates using a range of analytical techniques. Objective 2 was to conduct initial screening tests for evaluation of different single physical/chemical on‐site treatment technologies for alleviation of leachate UVQS. Objective 3 was to further evaluate the performance of single and combined on‐site treatment technologies coupled with subsequent conventional secondary treatment at POTWs. And Objective 4 was to conduct preliminary cost analysis, review the tested technologies, and make recommendations to the solid waste industry.
RATIONALE
Discharge of municipal landfill leachate to POTWs is a common and preferred leachate management practice in the United States due to the lowest treatment cost and the least management complexity as compared to other strategies. However, the benefits of this option have diminished in many cases because leachate can significantly interfere with their UV disinfection performance due to introduction of UVQS. The emerging concern for municipal solid waste (MSW) disposal seriously challenges the solid waste industry, because a regulatory trend toward less chlorination disinfection byproducts (DBPs), but with the same pathogen inactivation requirement, is forcing POTWs to turn from traditional chlorination to alternative UV disinfection.
Unfortunately, knowledge on the nature of leachate UVQS is very limited. And different treatment technologies for alleviation of leachate UV absorbance are not well tested and optimized. Therefore, it was desired to investigate characteristics of leachate UVQS and develop appropriate treatment technologies for the improvement of UV transmittance in landfill leachate.
METHOD
The study was comprised of four sequential tasks. In Task 1, different traditional and advanced analytical techniques were applied to analyze and characterize UVQS in different mature and young landfill leachates. In Task 2, initial bench‐scale screening tests were performed to evaluate seven physical/chemical treatment processes as a single on‐site treatment for the removal of UVQS at their respective optimal operating conditions. Subsequently, these selected treatments in combination were also tested to examine whether further UVQS removal could be achieved. In the following Task 3, the selected on‐site treatment was coupled with co‐treatment with sewage using a sequencing batch reactor (SBR). The experimental design simulated a real treatment scenario in which landfill leachate is first treated on site, then mixed with sewage, and finally enters into conventional secondary treatment at POTWs. In Task 4, costs were preliminarily analyzed and recommendations were finally made to the solid waste industry.
CONCLUSIONS
Based on the results obtained from our bench scale treatment experiments, an onsite treatment composed of biological treatment and a single or combined physicochemical treatments (three coagulation processes, activated carbon adsorption, and three chemical oxidation technologies in this study) selected in this project could not directly improve UV transmission (UVT) at 65% (the minimum UVT level for effective UV disinfection at POTWs) or greater, though they could effectively remove UVQS, to different degrees. Rather, appropriate on‐site treatment coupled with off‐site biological co‐treatment with sewage at a proper dilution at POTWs might ensure that the effluent UVT of sewage and pre‐treated leachate meets with the minimum UVT requirement.
Three on‐site treatment trains could satisfy the UVT requirement primarily due to sewage dilution and further biological degradation at POTWs. The on‐site treatment trains included SBR FeCl3 coagulation PAC adsorption (Treatment A), SBR FeCl3 coagulation Fenton treatment (Treatment B), and SBR Fenton treatment (Treatment C). To achieve the same UVT improvement goal, the first two treatment trains were relatively economically competitive.
Several specific conclusions were drawn based on this study:
- UVQS exhibited different characteristics between mature and young leachates. Young leachate contained more low MW UVQS than mature leachates. And, mature and young leachates were characterized with high and low specific UV absorbance (SUVA), respectively. Here, SUVA = UV254 absorbance/DOC × 100.
- Leachate SUVA is a useful parameter to characterize the nature of leachate UVQS compounds. A higher SUVA suggests higher aromatic and hydrophobic character as well as more humic matter and high MW molecules. Leachate SUVA increased with the decreased the biodegradability of dissolved organic matter (DOM).
- UV absorbance of mature and young leachates was due to both hydrophobic (HPO) and hydrophilic (HPI) fractions. HPO had a higher SUVA than HPI, indicating that HPO had more abundant chromophores per unit mass of DOC.
- Fluorescence spectra could effectively reflect the presence of the HPO fraction of UVQS, but was not sensitive to the leachate HPI fraction.
- Among the single on‐site physicochemical treatments tested in this study, none could directly achieve a satisfying effluent UVT improvement. The best treatment option was the Fenton treatment in terms of the UVQS removal, likely due to its dual treatment mechanisms (i.e. advanced oxidation and iron coagulation).
- Experimental results in this study showed that FeCl3 coagulation performed better than alum or ACH coagulation for the alleviation of UV absorbance. It was capable of effectively removing high MW UVQS molecules. FeCl3 coagulation may serve as a pre‐treatment prior to other physicochemical treatments.
- PAC adsorption of UVQS followed a Freundlich adsorption isotherm model through immobilization of both hydrophobic and hydrophilic UVQS compounds. Batch studies showed that PAC adsorption could achieve a very high UVQS removal only at an extremely high PAC dose (in this study, 10 g/L PAC adsorption removed 94.2% of leachate UV254 absorbance with an initial level of 12.96 cm‐1), which led to a too high treatment cost.
- The UVQS removal by ozonation or O3/H2O2 was limited, suggesting that chromophores in residual UVQS were very recalcitrant to ozone or •OH oxidation.
- Combined on‐site treatment technologies in this study (SBR FeCl3 coagulation PAC adsorption, Fenton treatment, ozonation, or O3/H2O2) could not directly achieve a sufficiently UVT, though they were capable of highly removing leachate UVQS.
- On‐site leachate treatment combined with co‐treatment of sewage at POTWs is an effective approach to addressing the UV transmittance issue. In this study, three on‐site treatment trains combined with SBR co‐treatment with sewage achieved an effluent UVT ≥ 65%, including SBR coagulation PAC adsorption, SBR coagulation Fenton treatment, and SBR Fenton treatment.
- Preliminary cost analysis (a 50,000 gpd leachate treatment capacity) was made. Among the three on‐site pretreatment that achieved UVT improvement, coagulation + PAC adsorption and coagulation + Fenton were relatively economically competitive related to the Fenton treatment alone (20‐yr expected service lifetime and 3% annual inflation rate) for this study.
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