decomposition. Many sewer undertakers limit maximum ammoniacal nitrogen concentration in their sewers to 250 mg/L to protect sewer maintenance workers, as the WHO's maximum occupational safety limit would be exceeded at above pH 9 to 10, which is often the highest pH allowed in sewer discharges. Many older leachate streams also contained a variety of synthetic organic species and their decomposition products, some of which had the potential to be acutely damaging to the environment. Environmental impact The risks from waste leachate are due to its high organic contaminant concentrations and high concentration of ammonia. Pathogenic microorganisms that might be present in it are often cited as the most important, but pathogenic organism counts reduce rapidly with time in the landfill, so this only applies to the freshest leachate. Toxic substances may, however, be present in variable concentrations, and their presence is related to the nature of the waste deposited. Most landfills containing organic material will produce methane, some of which dissolves in the leachate. This could, in theory, be released in poorly ventilated areas in the treatment plant. All plants in Europe must now be assessed under the EU ATEX Directive and zoned where explosion risks are identified to prevent future accidents. The most important requirement is the prevention of the discharge of dissolved methane from untreated leachate into public sewers, and most sewage treatment authorities limit the permissible discharge concentration of dissolved methane to 0.14 mg/L, or 1/10 of the lower explosive limit. This entails methane stripping from the leachate. The greatest environmental risks occur in the discharges from older sites constructed before modern engineering standards became mandatory and also from sites in the developing world where modern standards have not been applied. There are also substantial risks from illegal sites and ad-hoc sites used by organizations outside the law to dispose of waste materials. Leachate streams running directly into the aquatic environment have both an acute and chronic impact on the environment, which may be very severe and can severely diminish bio-diversity and greatly reduce populations of sensitive species. Where toxic metals and organics are present this can lead to chronic toxin accumulation in both local and far distant populations. Rivers impacted by leachate are often yellow in appearance and often support severe overgrowths of sewage fungus. The contemporary research in the field of assessment techniques and remedial technology of environmental issues originating from landfill leachate has