From Waste in Anodising Plant to New Chemical Product in Sewage Treatment Plants

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Aluminium containing caustic soda used as a pickling agent in the pre-treatment of aluminium before anodising and aluminium containing sulphuric acids used as electrolytes for the formation of oxide layers in anodising plants can be utilised in municipal sewage treatment plants to reduce phosphates and improve precipitation.

From the anodiser's point of view, these products are spent once they have reached a certain level of aluminium content and must be replaced by fresh chemicals. From the sewage plant's point of view, they can be used instead of fresh chemicals for the very reason that they contain sufficient aluminium to absorb excess phosphates from the municipal sewage waters and to improve the flocculation processes.

Some 25 years ago, the Association of Swiss Anodisers, ASA, started to study its members' waste water problems and to find environmentally friendly and economical solutions, preferably by recycling.

The advantages of performing this task within the association were obvious: sharing the costs of consultants and experts, collective transportation, centralised logistics, a stronger position in negotiations with authorities.

Over the years a so called waste-water catalogue was built up, proposing solutions for spent process solutions regularly accumulating in anodising plants. This catalogue was constantly reviewed and instead of waste disposal or simple recycling, the possibility of re-using the spent solutions was increasingly considered as the best solution.

This presentation describes the re-use of aluminous acids and caustic sodas for precipitating phosphates in municipal sewage treatment plants in detail. At present, ASA is also studying the economical and intelligent re-use of aluminous sludge.

As the spent acids and caustic sodas from an anodising plant can generally be used as a flocculant/precipitant if the conditions are right, therefore substituting new chemicals, it would be absurd to destroy them, not least for macroeconomic reasons. Spent process solutions can no longer be used by the anodising plants but are a valuable raw material for other applications.

This insight prompted ASA to look for better recycling possibilities. On account of aluminium's property of precipitating phosphorus as insoluble aluminium phosphate from solutions (1 g Al precipitates 1 g P), or attaching to freshly formed aluminium hydroxide flocs, extensive series of experiments were conducted to devise, test and realise a concept guaranteeing environmentally clean and economically optimum, collective disposal of all the spent acid and caustic soda baths accumulating in the association by recycling them for re-use.

The acids and caustic sodas are used for phosphate precipitation, flocculation and neutralisation in sewage treatment plants in the same state as they accumulate in the anodising plants. However, the caustic sodas must always be pumpable.

Most important steps for realising the concept

Involving the competent authorities

From the outset, ASA engaged recognised waste-water experts to devise solutions that meet the legal requirements and resolve all the problems arising, and to advocate them before the authorities - together with ASA experts. Initial recovery and re-utilization concepts were discussed long before the word 'recycling' came to be a catchword.

Initially, spent acids and caustic sodas were supplied to the paper and pulp industry in agreement with the local authorities, partly for neutralisation in internal waste-water treatment plants, partly for stabilising the ink in waste-paper processing.

Some years ago, it was decided to revert to the (even better) possibility known for more than 20 years of supplying the aluminous products to municipal sewage treatment plants for flocculation and phosphate precipitation. Here again, the authorities were consulted and informed from the very beginning.

Selecting a suitable sewage treatment plant.

Municipal or regional sewage treatment plants vary greatly as regards capacity and the quality of the waste water to be treated. For our purpose, it was necessary to select a sewage treatment plant needing approximately the same quantity of precipitation products as the spent process solutions accumulating in the anodising plants (fluctuations in demand can be levelled out by suitable storage tanks). A sewage treatment plant with at least two processing lines is an advantage because the acids and caustic sodas can then be used separately.

To establish whether the quality of the waste water is suitable, trials must be carried out.

In the first sewage treatment plant chosen by ASA, everything went well: both the phosphate elimination and the sludge values at the plant outlet were perfectly satisfactory. Nevertheless, the trial had to be discontinued because too much hydrogen sulphide was produced, naturally accompanied by an offensive odour. The previously used trivalent iron precipitant managed to prevent this. The waste water from two industrial plants, a bulk butcher and a paperboard mill, were probably the cause of this phenomenon.

In the second trial plant, everything then went according to plan, this time without the above-mentioned side effect.

Performing trials

First of all, the most important data must be recorded before the acids and/or caustic sodas from the anodising plants are used; the main data are the phosphate precipitation values and the quality of the sludge used for agriculture (heavy metal content).

Then the previously used precipitant (e.g. trivalent iron) is discontinued and a suitable dose of aluminous solutions from an anodising plant is added. After a short transition time where both the old and new precipitant are in the sewage treatment plant, the first values for the aluminous process solutions can be determined: initially the phosphate precipitation values and some other parameters of the treatment plant, and then the sludge values later on.

Safety, contractual agreements

ASA, as the party negotiating with the authorities and the contracting party with the sewage treatment plant and the transportation company, assumes the responsibility for handling the recycling perfectly satisfactorily and safely. However, the individual member firm or transportation company remains liable for any damages.

In this connection, there are the following contracts:

ASA with every member supplying acids or caustic sodas for recycling. In the contract, the member undertakes to comply with the heavy metal limits agreed with the sewage treatment plant.

The member also undertakes to make a quick analysis of every load delivered, especially recording the Al content (important for dosing). 3 samples of every load are taken and kept in different places as a reference in case any disorders occur. Furthermore, every plant is obliged to have the supplied process solutions thoroughly analysed in an external, accredited laboratory at least once a year and after every change in the production process in order to determine the decisive heavy metal contents.

ASA with the transportation company using a special vehicle to collect the acids and caustic sodas from the individual plants in separate chambers and deliver them to the sewage treatment plant.

The contract obliges the transportation firm to clean the transportation containers meticulously in every case so that no contaminants enter the sewage treatment plant.
The transportation company also undertakes to obtain the samples and results of the quick analyses required from the supplier plants and to deliver them to ASA and/or the sewage treatment plant.

ASA with the sewage treatment plant using the aluminous solutions from the anodising plants. This contract stipulates quantities and specifications for the precipitation product, in particular the heavy metal limits.

It also lays down the technical procedures and responsibilities.

Economic aspects

For the anodising plant: finding collective solutions in the association make it possible to engage qualified experts but pay only a relatively small portion of the costs.

Delivery of the spent process solutions to the sewage treatment plant entails transportation and administration costs. These are far lower in the Swiss system than any other kind of waste disposal, except for feeding the process solutions directly to a nearby sewage treatment plant in which case the transportation costs are eliminated. Only the construction of special supply pipes must be amortised.

For the sewage treatment plant: eliminating the costs of a phosphate precipitant and flocculant. Technical expenditure perhaps somewhat higher.

For the transportation company: secure permanent order for transporting quantities more or less regularly. Good price in view of the additional services to be rendered (logistics, control, technical aid).

For ASA: the Technical Committee's work, especially in connection with recycling and disposal, is very highly esteemed. Various firms have joined the association chiefly because of this service.

Concluding remarks

The recycling concept that ASA has developed for aluminous acids and caustic sodas from anodising plants is a procedure tailored to local needs and conditions and cannot automatically be applied to other countries or sewage treatment plants.

From country to country, from region to region and from plant to plant, there must be an individual investigation to clarify what is feasible and expedient under the applicable regulations.

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