Removal of heavy metals from biowaste : modelling of heavy metal behaviour and development of removal technologies

In the Netherlands, recycling of solid organic waste streams as compost only becomes possible if the compost complies with the heavy metals standards of the BOOM decree. This dissertation focuses on the removal of heavy metals from biowaste, i.e. the source separated organic fraction of municipal solid waste. Biowaste is referred to as an organic waste stream but surprisingly it was found that a large part of biowaste is composed of inorganic material, i.e. sand, silt and clay minerals. The inorganic part of biowaste originates from the collection of garden waste. Comparison of the natural background content of heavy metals in the original constituents of biowaste with the heavy metal content of biowaste showed that biowaste is not contaminated with heavy metals. Based on these results it can be stated that there is a conflict between two government policies of (1) preventing the accumulation of heavy metals in the soil (BOOM decree) and (2) promoting the recycling and reuse of organic waste streams (National Environmental Policy Plan). It is recommended that the different legislations concerning the dosage of various organic and artificial fertilisers have to be combined in a single legislative framework.

An analytical fractionation scheme was presented to determine the physico-chemical distribution of heavy metals in biowaste. The fractionation scheme was used to assess the application of physical separation and chemical extraction to reduce the heavy metal content of biowaste. On the basis of the results of the physical distribution of heavy metals in biowaste, a physical separation process was designed to valorise biowaste. A pilot plant study showed that physical separation of the biowaste is possible by a combination of physical wet separation units operated under wet conditions. The process results in (1) an organic fraction which can be converted to clean-compost, (2) a sand fraction which can be re-used in road and building construction and (3) a fraction high in humus and lutum which would make an excellent soil improver but which cannot be re-used because the heavy metal levels are too high. From a discussion of the advantages and disadvantages of the chemical extraction by inorganic acids and complexing agents it was concluded that these extracting reagents are not applicable on a practical scale due to the costs of the process and the negative environmental impacts of the discharged solid and liquid waste streams. Citric acid was proposed as an alternative extracting reagent which does not have these drawbacks.

Mechanistic models were developed in order to gain more insight into factors controlling the extraction efficiency and rate of extraction of heavy metals from biowaste. Chemical equilibrium modelling was used to calculate the speciation of heavy metals in biowaste. The NICA-Donnan model taking into account the complex binding characteristics of organic matter, i.e. polyfunctional and polyelectrolytic behaviour was applied to interpret the proton and Cu(II) binding to particle-sized organic particles of biowaste. The results indicate that the humic acid content of biowaste regulates the speciation of heavy metals in biowaste. Moreover, a mechanistic model was developed which describes the course of the acid extraction of heavy metals from solid organic particles of biowaste. The extent of the proton-metal exchange is determined by the competition between the heavy metal ion and the proton for the reactive sites of the organic particles. Diffusion of the ions in the film layer and inside the particles was described by the Nernst-Planck equations. This model is able to give a qualitatively interpretation of the acid extraction of Cu(II) from the isolated particle-sized organic fractions in biowaste and the model can also explain the anomalies observed during the acid extraction of Cd, Cu and Zn from sewage sludge.