Grahame J. Farquhar

Gas production during refuse decomposition

Gas production in sanitary landfills is a subject of much concern because of the potential hazards of CH4 combustion and of groundwater contamination by CO2. This study investigated the pattern of sanitary landfill gas production and the factors which affect it.A basis for study was prepared by examining factors which influence gas production in soil and sewage sludge digesters. The factors studied included moisture content, temperature, pH, alkalinity, Eh, and nutrition. It was then undertaken to determine whether or not this information was applicable to the landfill.A pattern for landfill gas production was proposed based on the assumption that an anaerobic environment would be achieved and maintained after refuse placement. Four phases were identified: I. Aerobic; II. Anaerobic Non-Methanogenic; III. Anaerobic Methanogenic Unsteady; and IV. Anaerobic Methanogenic Steady. The duration of these phase and the relative amounts of gases produced within each phase were studied.An investigation of information available on factors affecting gas production in sanitary landfills also was made. It was found that, in general, the principles developed from the study of gas production in other media were applicable to the landfill environment. It was found that gas production increases with increased moisture content but that conditions of high infiltration are often conducive to reduction in gas production apparently caused by modifications to the microbial environment. There appears to exist a typical pattern of temperature variation within the landfill with a peak temperature being reached during the initial phase of aerobic decomposition. The magnitude of this peak is related to the refuse temperature at placement. Subsequent temperatures are lower and tend to fluctuate with season. Optimum temperatures for gas production are in the range of from 30°C to 35°C, however, landfill temperatures are often lower than this. Optimum levels of pH and alkalinity exist which maximize gas production rates. The types and amounts of gas produced are influenced by refuse composition.A scheme was proposed to illustrate how the various factors influence landfill gas production and how these may interact. Those factors over which some control may be exerted during landfill design and operation were identified.

Laboratory and controlled field experiments using potassium permanganate to remediate trichloroethylene and perchloroethylene DNAPLs in porous media

Few proven technologies exist that may be used to treat dense non-aqueous phase liquid (DNAPL) contaminants. In-situ chemical flushing is a proposed technology which consists of flushing DNAPL source zones with a reactive solution to degrade the contaminant mass below ground. A laboratory and controlled field experimental program was conducted to assess the potential of potassium permanganate (KMnO4) as a reagent for in-situ DNAPL remediation. The results of laboratory experiments indicated that two common DNAPL contaminants, perchloroethylene (PCE) and trichloroethylene (TCE), were rapidly degraded to chloride and carbon dioxide. Column experiments, using residual PCE flushed with oxidant concentrations as high as 10 g L−1, indicated that chloride could be used as a reaction tracer. From the chloride data, it appeared that the rate of PCE removal from the columns was a complex process dependent upon the kinetics of both dissolution and oxidation. Two experimental applications of in-situ oxidation were conducted in the Borden aquifer isolated within a 7.5 m3 double sheet-pile cell. The cell was fitted with injection and recovery wells through which aqueous solutions of KMnO4 were flushed to oxidize solvent source zones in situ. In the initial experiment, flushing of a 1 L PCE residual source with 10 g L−1 KMnO4 at total flow rates of up to 100 L per day, completely removed the source within 120 days. A second experiment, using an 8 L mixture of PCE and TCE slowly allowed to infiltrate into the cell, was conducted using a system to recycle the oxidant. The oxidant was added at 10 g L−1 with a flow of approximately 50 L per day. After 290 days of flushing, it was concluded from the monitoring data that 62% of the initial source (as equivalent chloride mass) has been oxidized and it was evident that oxidation was continuing in the upper third of the cell. These experiments have suggested that the effectiveness of in-situ chemical oxidation will depend primarily upon the distribution of the DNAPL in the subsurface and its effects upon dissoluttion. In both experiments, spatial variability of chloride measurements appeared to reflect both the DNAPL location and distribution.

Nonpoint source pollution: a distributed water quality modeling approach.

A distributed water quality model for nonpoint source pollution modeling in agricultural watersheds is described in this paper. A water quality component was developed for WATFLOOD (a flood forecast hydrological model) to deal with sediment and nutrient transport. The model uses a distributed group response unit approach for water quantity and quality modeling. Runoff, sediment yield and soluble nutrient concentrations are calculated separately for each land cover class, weighted by area and then routed downstream. With data extracted using Geographical Information Systems (GIS) technology for a local watershed, the model is calibrated for the hydrologic response and validated for the water quality component. The transferability of model parameters to other watersheds, especially those in remote areas without enough data for calibration, is a major problem in diffuse modeling. With the connection to GIS and the group response unit approach used in this paper, model portability increases substantially, which will improve nonpoint source modeling at the watershed-scale level.

Experimental determination of the kinetic rate law for the oxidation of perchloroethylene by potassium permanganate

Flushing soils contaminated with trichloroethylene (TCE) and perchloroethylene (PCE) with a permanganate (MnO4-) solution has been shown to reduce the solvent content of the soil. Experiments were performed to quantify the rate at which KMnO4 oxidizes aqueous solutions of PCE over a range of concentrations. In a series of homogeneous reactors, aqueous phase PCE concentrations were monitored over time in nine experimental trials with excess oxidant concentrations ranging from 5 to 30 g/l. Analysis of the data was performed to quantify the oxidation reaction order with respect to PCE and KMnO4 and the reaction rate constant. The reaction between PCE and KMnO4 was determined to be first-order with respect to both PCE and KMnO4 with an overall specific reaction rate coefficient of 2.45+/-0.65 M(-1) min(-1).

Modeling gas migration through unsaturated soils from waste disposal sites

The movement of gases away from waste disposal sites and hazardous waste spills through soils can result in serious safety and health hazards. As in the analogous problem of contaminant transport in groundwater, mathematical models are useful in predicting future gas excursion distances at existing sites and evaluating gas migration control alternatives. This paper presents a mathematical model for simulating the migration of gases from waste disposal sites through the unsaturated zone. The system equations used to represent gas migration through the unsaturated zone are an amalgam of the traditional groundwater flow-contaminant transport equations with the representation of gaseous flows in molar quantities. The model accounts for gas migration due to gas pressure, concentration and velocity gradients. The system equations are solved with the Galerkin finite element technique. The mathematical model successfully reproduced observed historical gas pressure and concentration data at two landfill sites. These two applications tested the mathematical model for both summer and winter flow conditions and under both natural and forced gas potential gradients.

Temperature effects on wastewater treatment under aerobic and anoxic conditions

Abstract Current evaluations find savings in operating costs of greater than 25% when denitrification is added to a system already achieving nitrification. The economic evaluations performed were based on a sludge reduction of 25% under anoxic conditions compared to aerobic conditions. Conflicting reports of biosolids production for reactors operating under aerobic, anoxic and alternating aerobic conditions exist. This study quantifies and compares biosolids production rates for SBRs achieving carbon removal under aerobic or anoxic conditions as a function of temperature (14°C and 20°C). Proteins were chosen as model substrates because they are a major contributor to the total COD of raw sewage and by virtue of their high molecular weight, require hydrolysis. This study is unique in that the bacterial population was also enumerated using acridine orange direct counts (AODC). Observed yields were 35–52% higher for the anoxic reactors than they were for the aerobic reactors. The observed yield for the aerobic reactor decreased by 4% at a lower temperature whereas the anoxic yield increased by 8%. Further analysis discounted that differences in observed yield were caused by an accumulation of influent proteins. A factorial analysis revealed that the anoxic reactor contained more bacteria than its aerobic counterpart. Microscopic examination of the mixed liquor showed the presence of protozoa in the aerobic reactor while none were noted in the anoxic reactor. Predation is thought to be responsible for differences in the observed sludge production between anoxic and aerobic conditions.

One-dimensional immiscible displacement experiments

Abstract In recent years, a great deal of attention has focused on the development of various methods to predict the fate of immiscible contaminants (NAPLs) in soils. In an attempt to satisfy this requirement, a host of numerical models has been developed. Unfortunately, there exist little experimental data to verify the assumptions used in the derivation of these immiscible flow models. One objective of this paper is to report on a non-destructive measurement technique which was used to capture the relative organic-phase saturation variations in a number of two-phase flow displacement experiments. The data obtained from these experiments were compared to results obtained from a one-dimensional, finite-element based, two-phase flow model. The experiments consisted of five separate trials using three different immiscible liquids (hydraulic oil, kerosene and hexane) in a water-saturated column. Irregular immiscible liquid infiltration fronts were observed in four of the five experiments, indicating that very small-scale heterogeneities control the infiltration of immiscible liquids into soil. Independent of the column experiments, saturation-capillary pressure curves were determined for the various liquids. In general, the simulated NAPL saturation vs. time profiles agreed very well with the observations for all five of the trials.

Gas migration and vent design at landfill sites

AbtractA finite element model has been developed to simulate the migration of gases in soil from a buried source such as a landfill. Using quadratic elements, the diffusion convection equation coupled with the mass conservation equation of a binary mixture of gases is solved under a combination of Dirichlet, Neumann and flux type of boundary conditions. The model is compared with an analytical solution and a set of field measurements. The model is used to display the influence of seasons on the migration of gases. The effectiveness of venting trenches in containing such migrations is examined and a method for determination of trench depth is presented.

An examination of temporal/spatial variations in landfill-generated methane gas

The experimental findings of a data-intensive in situ landfill monitoring study are described. Particular attention is given to the methodology employed in collecting the data, analyses of the spatial/ temporal variability of the gas percentages, by volume and the extent to which the variability in collected results is explained by variations in exogenous independent variables. Measurable gas flow rates from the soil could not be detected through the probe system employed in the field study.

Inhibitory effects of kraft bleachery effluents on methanogenic consortia

Abstract A systematic investigation of the inhibition exerted by kraft pulp mill bleach plant effluents on a methanogenic consortium was completed using anaerobic serum bottle techniques. The inhibition exerted on unacclimated biomass was found to be correlated to the wastewater AOX. A significant portion of the inhibition was determined to be present in the permeate of a 1000 Da ultrafiltration membrane. Treatment of the wastewater with powdered activated carbon prior to biomass exposure removed the inhibitory effects of the wastewater on the biomass. A methanogenic consortium was found to acclimate to a wastewater volumetric fraction of 80% v/v. The wastewater volumetric fraction did not affect the overall time required for acclimation to occur.