A laboratory experimental set-up for the study of organic compounds transport through unsaturated soils

P Viotti, M Petrangeli Papini, C Chiulli

The presence of pollutants in soils is one of the major problems for the use of the groundwater and also for future use of the land. The contamination from reactive pollutants can derive from use of fertiliser in agriculture, from industrial processes as well as from illegal landfill. The remediation of these sites is still an open problem due to the different opinion of scientists, and to the different technologies now available on the environmental market. One of the main reason for the uncertainties in the correct choice of the remediation technique is surely that the behaviour of transport and dispersion of pollutants in soil is still not well known. Particularly, the adsorption/desorption phenomena are strictly related to the specific soil and to the specific contaminant. So it can be worth carrying out laboratory experiments (with low costs and with controlled conditions) to evaluate the parameters suitable for the understanding of the involved processes and for the eventual use of models. In the present paper an experimental study of phenol transport and dispersion in soil is proposed. It can be considered as a test on the suitability of laboratory reconstruction of a real situation. The aim is to build a low costs methodology that can give information in controlled situations on the soil response to a contamination. A careful research of sampling instrumentation has enabled us to build a test section larger than those generally described in the literature, in this way it was possible to have a heterogeneous and realistic soil structure . This last aspect can be considered as a step towards "in situ" conditions. A tracer is used to characterise the fluidodynamic part and a solution containing phenol is added to the soil. The aim is to reproduce the circumstances where contaminants in the upper part of the soil are transported by rainfall and add to the pollutant in the deeper part. The soil is completely characterised from the physical and chemical point of view. A soil moisture meter is used to evaluate the wet front in time and space and a tensiometer furnishes the value of the matrix potential in the section. A series of nine micro-lysimeters are used to sample to solution in time.

Sustainable ash pond development in India – a resource for forestry and agriculture

C. Zevenbergen, J.P. Bradley, A.K. Shyam, H.A. Jenner, R.J.P.M. Platenburg
In India, the annual production of coal fly ash is approximately of the order of 70 million tons per year. Most of the production is stored in large ash ponds. Current waste disposal regulations call for the retired ash ponds to be covered with a top soil to prevent leaching of contaminants and erosion of fly ash into the surrounding environment. However, absolute containment of ash ponds on the longer term in a tropical climate is far from feasible: the covering on these ponds dries out and cracks. In addition, the topsoil is stripped off from other fertile areas, away from the site.

Our present understanding of coal fly ash leaching in a disposal environment has largely been obtained from freshly, unreacted ashes and not from ashes, which have been allowed to weather in the open. We showed in a previous paper that alkaline fly ash weathers relatively rapidly into soil-like material when disposed in the open. In this paper, it is argued that fly ash analogous to volcanic ash, has a high potential for large-scale utilisation in forestry and agriculture. A new concept of sustainable ash disposal and utilisation in India is presented.

Behaviour of cement-treated MSWI bottom ash

G. Pecqueur, C. Crignon, B. Quénée

MSWI bottom ash is the residue of combustion. The use of bottom ash in road construction is wide spread. French legislation forbids the disposal of reusable waste in special landfill from 2002. Moreover, " arrêté du 9 mai 1994 " provides environmental criteria (leaching threshold, etc), and evaluates this material according to utilisation in road construction. In such applications, bottom ash is often treated with binder to improve its mechanical features. Nevertheless, bottom ash is subject to chemical problems. These problems induce an expansion, which brings about cracking and finally road destruction. Therefore, it is necessary to estimate the swelling potential of MSWI bottom ash prior utilisation. This is one of the aim of the European contract " Mashroad "(contract BRST CT97-5150).

This study involved four years of work on cement treated MSWI bottom ash. It examined different tests that show the importance of oxidation of aluminium in the swelling reaction and the efficiency of different treatments. Different binders were used in order to have different proportions of clinker. The kinetic aspects of aluminium-binder reaction were also studied. Finally, we present a special cell to measure the swelling pressure of these materials.

The re-utilisation of discarded building materials in cement-stabilised layers of road and airfield pavements

M. Pasetto

Re-utilisation of marginal materials is standard practice in road and airfield pavement construction. However, not all marginal materials have sufficiently well known performance properties or, if they are known, are suitable for a generalised use.

For instance, Construction and Demolition (C&D) wastes are materials that can be used, in accordance with some specific rules, in the construction of road and airfield pavements. A study carried out by the University of Padua (Italy) has proved that these materials can be re-utilised in rigid pavements and has demonstrated the most useful options and criteria for optimising the results (Pasetto, Leeds, 1999).

Further research has confirmed the possibility of using C&D materials, as they are, as well as in cement bound or hydraulic-binder stabilised mixtures, in subgrades, sub-bases and road bases of flexible pavements.

Chemical, by means of leaching tests, and physical-mechanical characterisations by standard laboratory tests (compaction, CBR, freeze-thaw cycles; compression, tensile strength, secant and tangent elastic modulus, dynamic modulus using ultra-sound devices on different aged mixtures) have been done on discarded building materials of different origin and grading. It has been ascertained that if the recycled aggregate seems to be inferior to the natural one, if it is used in cement bound mixtures, it becomes equivalent to the traditional material or even preferable because it is cheaper and harmless to the environment.

Variation in mechanical properties of MSW incinerator bottom ash: Results from triaxial tests

M. Arm

This study deals with laboratory testing of municipal solid waste (MSW) incinerator bottom ash. The aim was to investigate the mechanical properties such as stiffness and stability of the ash for future use in unbound road layers. Especially the effect of the material variation on the mechanical properties was analysed. Specimens of bottom ash from four different plants and four seasons were tested by repeated load triaxial tests. Results so far suggest that there is a significant variation in the mechanical properties, both seasonal fluctuations and differences between incinerator plants. However, the variation is not greater than for studied natural aggregates. It is also shown that the organic content has a limiting effect on the resilient modulus, as expected. If MSWI bottom ash is utilised instead of sand in a capping layer, the same design modulus could be used as for the sand. All tested materials are stable and give reasonable permanent deformations at the stress level that is relevant for a Swedish capping layer.

Self-cementing properties of crushed demolishing concrete in unbound layers: Results from triaxial tests and field tests

M. Arm

A two-year study is underway to evaluate the expected growth in stiffness in layers of crushed concrete from demolishing objects. This growth is said to be a result of self-cementing properties. The study comprises of repeated load triaxial tests on manufactured specimens after different storing time together with falling weight deflectometer, FWD, measurements on test sections. Results so far show a clear increase with time in resilient modulus and in back-calculated layer modulus for all concrete materials. The increase is largest the first months and then diminishes. The field measurements show a more considerable growth in stiffness than the laboratory tests, with a doubled value two years after construction. Comparative investigations on natural aggregates, mostly crushed granite do not show any growth in stiffness, neither in the laboratory nor in the field. Consequences for the choice of design modulus are discussed.

The mechanism of lead (Pb) leaching from incinerator fly ash in monodisposal landfill

K. Miyawaki, T. Shimaoka, M. Hanashima, T. Shinohara, M. Nishigaki

This study considers the leaching behaviour of heavy metals from Municipal Solid Waste Incinerator (MSWI) fly ash contained in a landfill dedicated solely to fly ash disposal. Large scale lysimeters were used to study four materials: untreated fly ash; two types of chemically treated fly ash; and a molten fly ash slag.

The study also considers the effect of highly saline solutions to accelerate the leaching of lead and concludes that the concentration of calcium ions influences the leaching process.

Heavy metal elution characteristics from municipal solid waste scrubber residue by a centrifugation method

T. Shimaoka, K. Miyawaki, M. Hanashima, T. Yoshida, T. Uchida

The objective of this study is to clarify the elution characteristics of heavy metals into scrubber residue solution such as bound water and pore water. The L/S ratio of wet scrubber residue is smaller than that the L/S ratios used in elution tests such as: the one based on Notification No.13 of the Japanese Environment Agency (Japanese official method,JLT13), the Toxicity Characteristics Leaching Procedure (U.S.A. official method) and the Availability test (Dutch official method). A centrifugation method employed to sample soil solution was adopted for use to sample two types of scrubber residue solutions, and to compare the concentrations of heavy metals in the leachates with that obtained from the JLT13 and the Availability test as well as the pH dependency test.

Requirements for a realistic estimate of the source term for heavy metal elution from mineral wastes

J. Mönig, C. Reichelt, T. Brasser

Several toxic high volume mineral wastes which are disposed of in Germany in underground facilities in former salt mines have been subjected to a variety of leaching tests (DEV S4, column elution test, cascade test) with respective geological solutions. The objective was to check the aptitude of these tests to predict the long term leaching behaviour in case of water intrusion into the salt mine. Most insight into the reaction pathway is gained by the cascade test in which solid-fluid ratios of 3 to 8 (kg/kg) can be reached which are representative of underground disposal facilities. As cost would render this test unacceptable as a compliance test, model calculation may bridge the gap since good agreement was observed with experimental data from cascade tests. Indispensable parameters are the elemental contents of the waste and their individual leaching rates which in many cases can be determined by single day elution tests such as the DEV S4.

Disposal options for spent potlining

I. Rustad, K.H. Karstensen, K.E. Ødegård

Steel-shelled electrolytic pots are used by the aluminium industry to produce the pure metal from aluminium oxide. The pots contain two linings, one typically consisting of block insulation to reduce heat losses, and a second consisting primarily of anthracite and pitch. The second liner serves as the cathode of the electrolytic cell. An electrolytic cell is operated until the double lining fails, typically a three-to-ten year period. During the operation of the cell, fluorides, aluminium, cyanides and PAH’s are absorbed into the cell lining. Spent Potliner (SPL) was listed by EPA in 1988 as a hazardous waste, mainly because it contains significant amounts of iron cyanide complexes and free cyanides. The need for an economical and environmental acceptable means for SPL disposal, led the Norwegian aluminium industry, which produces annually 25 000 tons, to support a number of research projects aimed at finding a sustainable use of SPL. This paper discusses three important subjects of SPL disposal: leaching characteristics, challenges in chemical analysis and the possibilities of utilisation of SPL in the cement industry.