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.</p

TEWI-benadering mestbewerking

Aanleiding voor het onderzoek is dat in het verleden mestverwerkingstechnieken op diverse aspecten zijn beschreven en geprioriteerd, echter zonder rekening te houden met de emissie van lachgas en methaan. Omdat bij sommige technieken mogelijk sprake is van een afwentelingmechanisme, waarbij de techniek bijvoorbeeld op het ene gebied de milieubelasting reduceert terwijl de broeikasgasemissies stijgen, is het nuttig ook op dit gebied een prioritering aan te brengen. Het hoofddoel van dit onderzoek is het opstellen van een berekeningsmethode voor het bepalen van het reductiepotentieel voor de emissie van broeikasgassen ten gevolge van de bewerking en verwerking van mest en de inzet van de hiertoe benodigde technieken. Hiertoe is een formulier ontwikkeld dat als separate bijlage is toegevoegd aan deze rapportage. Daarnaast wordt, op basis van de ontwikkelde methodiek, voor een aantal kansrijk geachte behandelingsmethoden voor mest een analyse uitgevoerd naar het reductiepotentieel voor overige broeikasgassen. Op basis van deze waarden is een onderlinge ranking gegeven

Effect of substrate-seed mixing and leachate recirculation on solid state digestion of biowaste

Lab-scale experiments were performed and a mechanistic model was developed to simulate the solid state digestion of biowaste in a batch reactor. Both experiments and model showed that the substrate-seed mixing degree and leachate recirculation rate have a strong effect on the reactor performance. This is due to mass transport limitations of volatile fatty acids (VFA) within the biowaste-seed bed. In that case two regions are developed in the digester, so-called acidogenic and methanogenic pockets. Limitations in mass transport will prevent irreversible acidification during start-up of the reactor because whereas high VFA concentration is met in the fresh waste pockets, the VFA concentration in the methanogenic pockets will remain low. However, accumulation of VFA in the acidogenic pockets will reduce the hydrolysis rate of biowaste due to inhibition by VFA. Moreover, experiments and simulations showed that the reactor performance can be improved by varying the leachate recirculation rate or applying sequential batch operatio

Removal of heavy metals from sewage sludge by extraction with organic acids

Waste water treatment in activated sludge plants results in the production of large amounts of surplus sludge. After composting the sludge can be reused as fertiliser and soil conditioner in agriculture. Compared to landfilling and incineration, utilisation of sludge-compost is a more sustainable treatment because it recycles both nutrients and organic matter. However the high levels of heavy metals in sludge frequently prevent the reuse of sludge compost in agriculture. The extraction of heavy metals from the sludge before composting is therefore a necessary step to achieve a more sustainable sludge treatment. Extraction of heavy metals by inorganic acids and complexing agents has severe drawbacks. Organic acids could be an attractive extracting agent because the extraction can be performed at mildly acidic conditions (pH 3-5) and they are biologically degradable. The extraction was studied for heavy metals Cu and Zn and for competing metals Ca and Fe. The rate of extraction increases for increasing temperature and citric acid concentration. Cu can be extracted for 60-70nd Zn for 90-100␋y citric acid at pH 3-4. A first economic valuation of the extraction and subsequent composting process showed that the total costs of the treatment process are below the costs of incineration

Assessment of heavy metal removal technologies for biowaste by physico-chemical fractionation

In the Netherlands, the heavy metal content of biowaste-compost frequently exceeds the legal standards for heavy metals. In order to assess heavy metal removal technologies, a physico-chemical fractionation scheme was developed to gain insight into the distribution of heavy metals (Cd, Cu, Pb and Zn) over the physical entities of biowaste and to determine the binding strength of heavy metals to these physical entities. The scheme was based on a particle fractionation on size and density followed by sequential chemical extraction of the fractions. The organic fraction >1 mm contributed to 40% of the mass of biowaste and this fraction met the legal standards for compost after composting. About 28% of biowaste was composed of sand, which can be reused in road and building construction as the heavy metal content is very low. The heavy metals were concentrated in the organic particles from 0.05-1 mm and the organo-mineral fraction >Zn>Pb>Cu. The order of extractability for Pb and that for Zn were sometimes reversed most probably because the extraction is not only determined by the adsorption strength but also by the accessibility of adsorbed metals. The results of sequential chemical extraction showed that the amount of heavy metals available for extraction was sufficient. Citric acid turned out to be the best option for the removal of heavy metals from solid organic waste streams

Innovative developments in the selective removal and reuse of heavy metals from wastewaters

Sulphide precipitation of heavy metal containing wastewaters results in low effluent concentrations. However, sulphide precipitation is not widely applied in practice because the dosing of sulphide cannot adequately be controlled. A new process was developed where the combination of a sulphide-selective electrode (pS-electrode) and pH electrode controls the sulphide addition. Precipitation experiments were performed on a laboratory-scale in batch and continuous reactor systems with synthetic wastewaters containing Cd, Cu, Ni, Pb or ZnSulphide precipitation of heavy metal containing wastewaters results in low effluent concentrations. However, sulphide precipitation is not widely applied in practice because the dosing of sulphide cannot adequately be controlled. A new process was developed where the combination of a sulphide-selective electrode (pS-electrode) and pH electrode controls the sulphide addition. Precipitation experiments were performed on a laboratory-scale in batch and continuous reactor systems with synthetic wastewaters containing Cd, Cu, Ni, Pb or Zn. The response of the pS-electrode during precipitation was unique for each heavy metal and was directly related to the solubility product of the corresponding metal sulphide. The metals were removed to level