Effect of hydraulic retention time on landfill leachate and gas characteristics

Previous field studies carried out in order to characterize leachate in Vancouver, B.C. landfills show an effect of water input patterns on leachate characteristics. With high rainfall into the landfills, high volatile fatty acid (VFA) concentrations appear in leachate. It is postulated that with high rainfall there is a reduction in hydraulic retention time (HRT) which results in a reduction in the conversion of VFA to {dollar}CHsb4{dollar} and {dollar}COsb2.{dollar} Coincidence of peak infiltration rates with high dissolved organic carbon concentrations greatly increases the pollutant loadings to the treatment plants.; The objective of the study was to investigate the effects of HRT on landfill leachate and gas characteristics. HRTs from 3 to 200 days were assigned to eighteen lab scale lysimeters during four experimental phases in which the HRT was changed by changing the infiltration rates through the lysimeters. A relationship was established between HRT and infiltration rate in refuse columns and tested using tracer studies.; For unsaturated landfills;{dollar}{dollar}HRT={lcub}Sover Q{rcub}{dollar}{dollar}Where;{dollar}{dollar}eqalign{lcub}rm S = &rm Volumetric water holding capacity of the solid waste incr &rm the landfill Xcr &{lcub}rm Volume of the solid waste in the landfill{rcub} (Lsp3)cr rm Q = &{lcub}rm Flow rate through the landfill{rcub} (Lsp3Tsp{lcub}-1{rcub})cr{rcub}{dollar}{dollar}; Percent {dollar}CHsb4{dollar} concentration in gas decreased with increasing HRT up to 60 days. Lower HRTs showed higher {dollar}CHsb4{dollar} productions and increase in {dollar}CHsb4{dollar} concentration indicating enhanced methanogenesis.; High gas producing columns increased their pH with time from values less than 5.5 irrespective of the changes in HRT. Decrease in HRT helped the columns with low gas production and pH less than 5.3 to increase their pH which is favorable for methanogenesis. When HRT was decreased in columns with high gas production and pH higher than 5.8, a slight decrease in pH was observed with a substantial decrease in {dollar}COsb2{dollar} concentration in gas, but no detrimental effect on gas production was observed. Gas production itself helps in developing favorable conditions for biological degradation; it increases pH, produces alkalinity when VFAs are consumed, responds favorably to changes in HRT, reduces inhibitive concentrations of VFAs and other organics and inorganics in the microenvironments.; The amount of total carbon released to the environment is highly dependent on gas production. High gas producing columns released 50% more carbon than low gas producing columns.; Continuous mobilization of zinc and iron occurred throughout the experiment. Zinc concentrations showed inconsistent correlation to either pH or methane production rate. Correlation of iron concentrations to pH and {dollar}CHsb4{dollar} production rate showed dependence on HRT and {dollar}CHsb4{dollar} production stage of the column.; The high VFA concentrations observed in field studies are not, based on this study likely to be due to the failure of methanogenesis because of high water inputs contrary to the original hypothesis.; During phase I under similar HRT conditions, there was considerable variability in methane production from the lysimeters that could not be explained.