Arbejdsrapport fra Miljøstyrelsen, 27/2005 – Mobilt renseanlæg

Mobilt renseanlæg

Summary and conclusions

The Danish Environmental Protection Agency has initiated a number of projects, which will provide the background material for an action plan concerning the future treatment of contaminated sediment from harbours.

The purpose of the project “Mobile Treatment Plant” is to clarify whether it is technically possible to establish a mobile treatment plant, which will constitute a realistic possibility for the treatment of contaminated sediment in small harbours. The described treatment plant represents further development of ongoing considerations and tests in connection with other well-known technologies such as water treatment.

A characteristic feature of contaminated sediment from harbours is that several of the pollution components are primarily linked to the most fine-grained parts of the material and the organic components. The substances that make up the pollution in harbour sludge are typically heavy metals, TBT, oil, PAH, PCB and other substances injurious to the environment. There are great variations in the concentration of the substances depending on the place of origin, the content of organic material and the size of particles.

Separation and purification of contaminated sediment may be an alternative solution in harbours where it is not possible to deposit or utilise the sediment. Many small harbours have no possibility to deposit the sediment, because the necessary space is not available. With the technology of the mobile treatment plant it should be possible after the completion of the separation/purification to characterise the coarse fraction of the material as “non- contaminated” for subsequent dumping, replacement into nature or utilisation as backfilling material etc. The finer fractions of the contaminated sediment will still be polluted and require further treatment, as a minimum dewatering. On the other hand, the volume of materials, which must be deposited, is reduced. Furthermore, with a final treatment of the material it is possible to completely avoid dumping.

The already known experiences and tests with fractionation, purification and utilisation of contaminated sediment show that a wide range of methods has been tested. However, the existing technical literature does not seem to comprise much information on small or mobile plants. Still, a number of small full-scale tests of separation and purification have been carried out.

The main principles of the treatment plant consist of a number of sub-processes: preparatory treatment, separation, dewatering, water treatment and a subsequent final treatment of the material, which may again involve both dewatering and water treatment.

The purpose of the treatment is to separate the various fractions of contaminated sediment from each other in anticipation of concentration of pollution components in a small amount of contaminated sediment or a washout of pollution components to the water phase. A complete process for the treatment of sediment must be established considering the total treatment costs including final disposal of residues, reduction of residues and utilisation of simple and robust equipment, which, if necessary, can be moved to be used at other locations.

The treatment plant primarily consists of sub-processes, which constitute proven technology, possibly combined in a new manner to gain the highest possible efficiency.

For the separation of various sizes of particles a fluidisation principle can be applied. The advantage of applying fluidisation instead of a hydrocyclone or mechanical separation is that the material is exposed in a more efficient way to a large volume of water and the retention time is longer whereby the washing, all other things being equal, becomes better.

The costs of depositing contaminated harbour sediment with the existing methods vary between DKK 1-2,325/m³. Approximately half of the harbours state prices of DKK 50/m³ or less. A rough calculation of the price per m³ for separation and purification, exclusively based on the establishment costs of a mobile plant, shows a price of approximately DKK 274/m³ for a small plant with a capacity of 0.5 m³ of dredge material per hour, and DKK 68/m³ for a large plant with a capacity of 10 m³ of dredge material per hour. The above prices do not include operation and maintenance costs of the plant, nor costs for removal, transportation and possibly final depositing of the material. The annual operation costs are assumed to equal the establishment costs.

The idea behind a mobile plant is to make it possible to move the plant to the pollution and not, as usual, the other way around. A mobile plant can be carried even to small harbours and small amounts of contaminated sediment. However, the mobility of the plant also has an adverse feature, as a number of the plant components are very high and will have to be disassembled for transport. This will mean prolonged erection and disassembling between harbours a thereby less effective operating time.

Alternatively, the plant can be placed onboard a barge/a ship. This will make high treatment capacities possible, but the costs of the barge/the ship will increase the need for investments significantly. At the same time it will be more difficult to service the smallest harbours.

It should be taken into consideration that for each new locality and for each type of sediment a period of some length for commissioning and optimisation is needed, which reduces the effective operating time. These readjustment periods could be minimised with a stationary plant, which includes the possibility of e.g. a large pre-treatment tank, so that a higher degree of homogeneity can be gained.

A number of the chosen sub-processes constitute proven technology, the need for tests of these processes being limited.

Before any decision is made on the technical and financial feasibility of a mobile treatment plant, a number of pilot tests should be carried out. It is primarily a question of testing the following two component processes: the combined suspension/aeration/storage tank and the fluidised separation technique.

At power plants and waste incineration plants it is currently being considered whether to utilise a fluidised separation of residues according to the size of particles and in combination with dissolution of certain salts.

It is being considered by ENERGI E2, I/S Vestforbrænding and I/S Amagerforbrænding together with AV Miljø, DHI Water and Energy and Krüger A/S, to construct a small test plant for this kind of process to be completed by the end of 2001 or the beginning of 2002 depending on the availability of grants and the identification of an eligible location. Co-ordination with this project could be an opportunity to be considered.

In conclusion it should be stated that final clarification regarding the possibility of establishing a mobile treatment plant will have to await full-scale tests of the less known sub-processes.