Modern techniques for purification of landfill leachates using biological filters

Abstract

Increased environmental demands and increased costs for leachate treatment at sewage
plants have stimulated the development of new "in-site" techniques for treatment of
leachates. Such new biologically based techniques include irrigation of leachates in
soil/vegetation systems, root-zone infiltration, treatment in vegetated ponds or wetlands,
and filtration through different types of organic filters.
A major part of the nutrients from landfilled waste will sooner or later turn up in the
leachates. If optimised biological treatment techniques, like e.g. bioreactor cell
fermentation, is used the nutrient fractions will appear in the leachates within a relatively
limited time and in high concentrations. Thus, in order to close the eco-cycle of nutrients,
it is highly desirable that these nutrients extracted from the waste can be re-circulated
back to forestry or agriculture. This can either be as a liquid or as trapped in composted
organic filters or biomass.
Stabilised anaerobic conditions in the landfill result in an effective immobilisation of
heavy metals and other pollutants. In a strict anaerobic environment most heavy metals,
in contrast to nutrients like magnesium, potassium, sodium and calcium, form insoluble
sulphides. Due to these different chemical characteristics most of the nutrients will
appear in the leachates, while most of the heavy metals will be bound up in the
fermentation residue which is left in the landfill. Thus , e.g. the reactor cell or an
advanced landfill with stabilised anaerobic conditions, acts as an anaerobic filter,
enabling a separation of the nutrients from mixed municipal and light commercial and
industrial wastes. If the leachates are used as fertiliser in e.g. energy plantations, forestry,
a.s.o., the nutrients can be brought back to an ecological cycling. This possibility will not
be offered after incineration, where the resulting ashes, due to the high amounts of easily
available heavy metals, are recommended to be landfilled in sealed monofills.
The presentation will include examples from different Swedish leachate treatment
facilities, representing different biological techniques used to catch and use the main flow
of nutrients from the decaying waste in a landfill or biocell.