On Isolating a Landfill from the Surrounding Water Regime, G.E. Blight Selected Approaches for the Investigation of Microbial Interactions in Landfill Sites, I.A. Watson-Craik and L.R. Jones Mathematical Modeling of the Refuse Methanogenic Fermentation, A. Young Co-Disposal of Industrial Wastewaters and Sludges, I.A. Watson-Craik and K.J. Sinclair Landfill Leachate Treatment, T.J. Britz Landfill-Covering Soils, C.A. du Plessis and J.C. Hughes Revegetation of Landfill Sites, P.J.K. Zacharias Index

Biochar-charcoal used to amend land and sequester carbon-is attracting considerable interest. Its distinctive physical/chemical/biological properties, including high water-holding capacity, large surface area, cation exchange capacity, elemental composition, and pore size/volume/distribution, effect its recognized impacts, especially on microbial communities. These are explored in the context of agriculture, composting, and land remediation/restoration. Considerable focus is given to mycorrhizal associations, which are central to exploitation in environmental technologies involving biochar. The characteristics of biochar, its availability for nutrient cycling, including the beneficial and potentially negative/inhibitory impacts, and the requisite multidisciplinary analysis (physicochemical, microbiological, and molecular) to study these in detail, are explored.

As confidence in gas biofiltration efficacy grows, ever more complex malodorant and toxic molecules are ameliorated. In parallel, for many countries, emission control legislation becomes increasingly stringent to accommodate both public health and climate change imperatives. Effective gas biofiltration in biofilters and biotrickling filters depends on three key bioreactor variables: the support medium; gas molecule solubilization; and the catabolic population. Organic and inorganic support media, singly or in combination, have been employed and their key criteria are considered by critical appraisal of one, char. Catabolic species have included fungal and bacterial monocultures and, to a lesser extent, microbial communities. In the absence of organic support medium (soil, compost, sewage sludge, etc.) inoculum provision, a targeted enrichment and isolation program must be undertaken followed, possibly, by culture efficacy improvement. Microbial community process enhancement can then be gained by comprehensive characterization of the culturable and total populations. For all species, support medium attachment is critical and this is considered prior to filtration optimization by water content, pH, temperature, loadings, and nutrients manipulation. Finally, to negate discharge of fungal spores, and/or archaeal and/or bacterial cells, capture/destruction technologies are required to enable exploitation of the mineralization product CO2.

Diffuse atrazine pollution in German aquifers Wolfgang Tappe, Joost Groeneweg, Barbara Jantsch 3–10 Microbial aspects of atrazine degradation in natural environments T. Komang Ralebitso, Eric Senior, Henk W. van Verseveld 11–19 Resilience of the rhizosphere to anthropogenic disturbance J.M. Lynch 21–27 Effects of agronomical measures on the microbial diversity of soils as related to the suppression of soil-borne plant pathogens Jan Dirk van Elsas, Paolina Garbeva, Joana Salles 29–40

A three-stage continuous culture system was used to segregate the component microbial groups of a methanogenic hexanoate-degrading association enriched from anaerobic refuse. The inhibitory effects of o-cresol concentrations (2-20 mM) on the fermentative, acetogenic, sulphate-reducing and methanogenic bacteria were then assessed in the presence of either 1.4 or 3.5 mM sulphate in the influent medium. The sulphate-reducing bacteria (SRB) in the 1.4 mM sulphate-supplemented systems were the most sensitive to o-cresol, with 29.3 and 56.6% inhibition on supplementation with 4 and 6 mM o-cresol, respectively. With 3.5mM supplementation, inhibition was 4.5 and 19.4%, respectively. Methanogenesis was not inhibited by concentrations < 10 mM o-cresol, and complete inhibition was recorded only at concentrations > or = 10 mM. Both fermentation and acetogenesis were affected by inhibition of the electron sinks. The increase in influent sulphate concentration promoted electron flow to sulphidogenesis, as predicted on thermodynamic criteria, but did not affect the relative sensitivity of the different physiological groups.

A BATCH CULTURE ENRICHMENT PROGRAMme was used to isolate a methanol-oxidizing microbial association from landfill final covering soil. Aerobic closed cultures were also used to determine the critical substrate concentration and the catabolic ranges and optima for pH and temperature. The association was found to be active between 20 and 45degreesC, with an optimum at 35degreesC. The thermal death times at different temperatures were determined by standard methods. The association was active at pH < 8.5, with an optimum at pH 7. The maximum specific growth rate (&mu;(max)) was determined in chemostat culture at 30&DEG;C. The results of this study will be used to assess the potential efficacy of the association to remove methanol vapour and methane gas in a biofilter and, thus, to develop an operational protocol for landfill gas remediation by biofiltration.

The potential toxicity of thes-triazine herbicide atrazine motivates continuous bioremediation-directed research. Several indigenous soilatrazine-catabolizing microbialassociations and monocultures have been enriched/isolated from compromised sites. Of these, Pseudomonas sp. strain ADP has become a reference strain and has been used to elucidate sequences of the catabolic enzymes atzA, atzB, atzCand atzD involvedin one aerobic degradation pathway and develop probes for the genes which encode these enzymes. Despite this, hitherto unknown or novel microorganisms, with unique sequences and different enzyme-mediated operative pathways, warrant continued investigations for effective site bioremediation. Also, the sustained effectiveness of natural attenuation must be demonstrated continually so regular site evaluations and results analyses, despite the limitations of chemical extraction methodologies, are crucial practices. For both directed and intrinsic bioremediation monitoring, traditional microbial association studies must be complemented by more advanced physiological and molecular approaches. The occurrence of catabolic plasmids, in particular, should be probed with DNA hybridization techniques. Also, PCR-DGGEand subsequent new sequenceelucidation should be used prior to developing new primers for DNA sequences encoding novel catabolic enzymes, and for hybridization probe development, to establish the degradative potential of a compromised site, or adoption of FISH to, for example, monitor bioaugmented remediation.

Laboratory microcosms were packed with synthetic refuse and waste activated sewage sludge in two ratios (4.1:1 and 4.1:2 v/v). The sludge was spiked (100 mg·l -1 or 200 mg·l -1 ) with the salts of each of four heavy metals (Ni 2+ ,Cu 2+ , Cr 3+ and Zn 2+ ). Subsequently, the metal concentrations were increased progressively. For all the microcosms, including an unperturbated control, unbalanced fermentations resulted as evidenced by low pH values. Thus, heavy metal presence was not the sole cause. The leached metal concentrations were in a consistent order with high Zn and Ni concentrations recorded compared with immobilised Cr and Cu. After 15 weeks of operation, despite extensive retention, increases in Cr, Ni and Zn were detected in the leachates following elevated loadings. Due to the high redox potentials (+2 to +170 mV), precipitation of the metals as insoluble sulphides was not attainable.

In the presence of different selection pressures, particularly pH and electron donor concentration, indigenous microbial associations which catabolize selected petroleum hydrocarbon components (benzene, toluene and o-, m- and p-xylene (BTX)) were enriched and isolated from a petroleum hydrocarbon-contaminated KwaZulu-Natal sandy soil. Electron microscopy revealed that, numerically, rods constituted the majority of the populations responsible for BTX catabolism. Molecular techniques (polymerase chain reaction (PCR) and 16S rDNA fingerprinting by denaturing-gradient gel electrophoresis (DGGE)) were employed to explore the diversities and analyze the structures of the isolated microbial associations. Pearson product-moment correlation indicated that the different, but chemically similar, petroleum hydrocarbon molecules, effected the isolation of different associations. However, some similar numerically-dominant bands characterized the associations. A 30% similarity was evident between the m- and o-xylene-catabolizing associations regardless of the molecule concentration and the enrichment pH. PCR-DGGE was also used to complement conventional culture-based microbiological procedures for environmental parameter optimization. Band pattern differences indicated profile variations of the isolated associations which possibly accounted for the growth rate changes recorded in response to pH and temperature perturbations.

The successful application of anaerobic digestion technology to the treatment of industrial wastewaters is dependent on the development of high rate bioreactors, such as the ABR. Waste minimisation and the segregation of toxic or high strength organic wastes can only be successfully practiced if there are cost-effective solutions for their safe disposal. In this study, the ABR is seen as a tool for the responsible management of these small but concentrated streams. Laboratory-scale reactors have been operated to investigate start-up; treatment of industrial effluents; and microbial population dynamics. The laboratory-scale tests should facilitate the prediction of the feasibility of treatment of a particular molecule in a full-scale anaerobic digester, with indication of the volumes and concentrations that could be treated effectively.

A leaching column experiment was conducted to investigate the feasibility of soil irrigation with landfill leachate as a decontamination technology. A loamy sand was irrigated with synthetic landfill leachates with and without phenol, zinc and copper and combinations thereof. Irrigation was applied at a rate of one pore volume twice per week for 18 weeks, equivalent to a total application of 1165 mm. Electrical conductivity in all column effluents was lower than in the influent solutions. Changes in effluent pH with increasing numbers of pore volume displacements were influenced by the presence of zinc and phenol and could be adequately modelled using cubic response functions. Copper was totally removed from the leachates by passage through the soil as was zinc during the first few pore volume displacements. Later, however, zinc was detected in the column effluents in concentrations close to or greater than those of the influent solutions. In the presence of phenol zinc mobility was enhanced. Zinc response to number of pore volume displacements was effectively represented by Gompertz functions. The phenol concentrations of the leachates were reduced by 30 to 50% by passage through the soil columns. The results demonstrated that removal of pollutants from landfill leachates by passage through a soil with a low attenuation potential was moderate to high. Therefore, soil irrigation with landfill leachate may be an effective purification technique provided that it is practised with caution, and consideration is given to soil and leachate properties.

Possible nutrient limitations in single (activated sewage sludge) and dual (sludge plus phenol) co-disposals with refuse were examined with the objective of maintaining the fermentation balance and thus minimizing detrimental effects on leachate quality and, subsequent, environmental impacts. Laboratory microcosms, operated in batch mode for 10 months, were supplemented with macronutrients, trace elements or a combination of the two while distilled water was added to the controls. Fermentation progress was monitored by volatile fatty acid (VFA) and phenol concentrations, pH, redox potential and headspace methane content. Perhaps surprisingly, nutrient supplementation proved detrimental to the solid-state refuse methanogenic fermentation in the presence of co-disposals. Low pH values (≤4.5), coincident with elevated VFA concentrations, were consistently recorded while the self-generating redox gradients did not fall below the range of 0 mV to +91 mV. The results have important implications in landfill sites. Unlike co-disposal of anaerobically digested sewage sludge with refuse, which is promoted by nutrient supplementation, no additions were required to optimize the biotechnology. © 1999 Society of Chemical Industry

Refuse microcosms were used to study the dual co-disposal of phenol and anaerobically digested sewage sludge with refuse. Four operating strategies - elution, leachate recycle, single addition (batch), and single addition plus rain permeation - were examined in relation to different ratios of sludge and refuse, fluted microcosms, irrespective of the sludge loading, effected the highest phenol removal rates and these continued for subsequent resupplementations. For the three other operational strategies a 'high' sludge loading was required to effect enhanced phenol removal compared with the refuse control. Thus, optimum sludge loading proved to be specific to the operational regime. The dual co-disposal of sludge and phenol with refuse negated volatile fatty acid (VFA) accumulations compared with phenol-refuse co-disposal. The presence of sludge also promoted methanogenesis. Leachate recycle most effectively promoted refuse fermentation when pH, methane concentration and 'total' VFA concentration were used as the analytical criteria. pH proved to be an important variable for both refuse and phenol catabolism. The addition of mineral salts significantly enhanced microbial activity.

The efficacies of different co-disposal strategies (elution, leachate recycle, single addition (batch), single addition plus rain permeation) to treat phenol (1000 and 2000 mg dm(-3)) were investigated with laboratory microcosms. The elution columns recorded the highest total removals but were coincident with low leachate pH values and high residual phenol concentrations. In contrast, leachate recycle facilitated both increased leachate pH values and methane evolution. Rain permeation proved detrimental to phenol attenuation since the molecule was rapidly displaced together with key methanogenic precursors. All the microcosms were characterised by protracted lag phases prior to phenol catabolism and the nitrate- and sulphate-reducing bacteria were particularly sensitive to the added molecule. (C) 1998 Society of Chemical Industry.

The threshold toxicity concentrations of seven landfill leachate components (acetic, propionic, butyric, iso butyric, valeric, iso valeric and hexanoic acids) were determined with respect to the growth of three ectomycorrhizal fungi ( Laccaria proxima, Hebeloma crustuliniforme and Paxillus involutus ) and to the survival of mycorrhizal and mycorrhizal-free Betula pendula seedlings. Inoculation increased tree seedling tolerance to the acids, with the exception of the susceptibilities of L. proxima and H. crustuliniforme to valerate and iso valerate. Interacting microbial associations catabolically active against the acids were isolated from three different source materials (refuse, subsoil, topsoil). Minimum inhibitory acid concentrations exceeded the threshold toxicity concentrations to B. pendula seedlings, fungal isolates and mycorrhizal associations. Thus, provided that the acid concentrations were not toxic to the microbial associations and the hydraulic loading rates were lower than the maximum specific growth rates of the associations, concentrations should be reduced by microbial intervention during vertical migration. The potential for the routine use of mycorrhizal tree seedlings in landfill site restoration is discussed in the context of site-specific variables.

A multi-stage model, operated with single elution, was used to investigate the effects of organic loadings on the attenuation of a model phenolic wastewater in domestic refuse. Although 100% dissimilation of influent phenol (2-5 mmol dm-3) was recorded at a dilution rate of 0.007 h-1, partial inhibition of both phenol degradation and species competing with methanogens for a common electron donor(s) was apparent at concentrations greater than or equal to 4 mmol dm-3. On extended perfusion with 8 mmol phenol dm-3, the progressive inhibition of phenol dissimilation was not obviated by nutrient supplementation. Simultaneous degradation of the catabolic intermediate, hexanoic acid, and elevated methane release rates suggested that the transformation of phenol to hexanoate was rate limiting.

SUMMARY: Two strains of Pseudomonas putida, S3 and P3, were shown to contain dehalogenase activity against monochloroacetate, dichloroacetate, 2-monochloropropionate and 2,2′-dichloro-propionate but differed markedly in their levels of enzyme activity. Strain S3 had activities of less than 1 μmol substrate converted (mg protein)−1 h−1 and was unable to grow on any of nine chlorinated compounds tested. Strain P3 had enzyme activities 10 to 40 times greater than those of strain S3 but was capable of growth only on 2-monochloropropionate and 2,2′-dichloropropionate. In strain P3, dehalogenase activity was induced by a number of chlorinated compounds other than those that acted as growth substrates. Strain P3 dehalogenase activity dehalogenated C-2 substituted compounds. The evidence of the dehalogenase activity profiles in chemostat cultures and from thermal denaturation experiments suggested that there was more than one dehalogenase enzyme in P. putida strain P3. In crude extract, the enzyme activity was optimal at pH 7·9 to 8·1 and apparent K m values were in the millimolar range for the four major substrates, monochloroacetate, dichloroacetate, 2-monochloropropionate and 2,2′-dichloropropionate.