Normallythe effluents deriving from machines used in the die-casting of aluminium alloys are primarily composed water-based release agents, mineral oils and hydraulic fluids. The water-based release agents are used to prevent metallization and subsequent bonding of the aluminium alloys to the steel mould surfaces. These products are composed basically of mineral oils, polyethylene waxes, silicon oils, esters in emulsion along with traces of anti-oxidant, anti-rust, surfactants and biocide additives. The presence of non-emulsifiable mineral oils is principally attributed to the excessive lubrication of the pistons and consequent drip-loss of these lubricants. The hydraulic fluids on the other hand are derived by accidental losses - the most commonly used are based on blends of glycols and water. Regarding release agent technology and piston lubrication, the evolution of the chemical components used in the die-casting process has led to the development of valid alternatives with formulations based on low-melting point powders or granuals that will continue gradually to become more established in the market. Notwithstanding this, traditional water based release agents will remain in use for many years due to a vast variety of difficulties and demands for the major part of those processes where powder based products cannot be proposed or do not resolve production requirements. The first step in addressing the question of the environmental impact of water based release agents has been the substantial reduction in the formation of spray mist arising at the point of contact between the hot mould surfaces and molten alloy, while today the most significant problem regarding effluent discharges is still the presence of high concentrations of surfactants. Waste disposal of effluents by technique such as distillation, ultra-filtration, flotation and so on permit the management of relatively small quantities in an economic way but not always guaranteeing the attainment of chemical-physical parameters and concentrations of pollutants in an optimum way so as to permit discharge into the sewage system or treated effluent reservoirs.


Objective

The study aims to optimise the efficiency and performance of chemical-physical treatment processes and above all simplify and improve efficiency of biological treatment at the end to obtain an effluent with all chemical, physical, biological and toxicological characteristics ideal for discharge, whilst avoiding the necessity to make substantial structural modifications to existing treatment plants. The study of the management of chemical-physical and biological treatment plants also provides for the insertion of a test to be implemented on the final effluent after the treatment process in order to verify the possible presence of substances harmful to ecological systems.


Legislation

Legislative Decree No. 152 of 11/05/99 "Arrangements for the protection of water from pollution and assimilation of directive 91/271/CEE concerning the treatment of refluent waters and directive 91/271/CEE relative to the protection of waters from pollution provoked by nitrates arising from the agricultural sources" accompanied by the relative notes. We have extracted table 1 of this Legislative Decree (tab. 1). It is important to indicate that individual regional authorities can impose specific restrictions to these values in the above mentioned table. It should be noted that the value in total surfactant (i.e. cationic + anionic + non-ionic) is 4 mg/l for discharge into the sewage system and 2 mg/l in treated effluent reservoirs. The surfactant usually employed in the formation of water-based release agents are principally non-ionic along with inferior quantities of surfactants of ionic nature.


Method

The percolated effluent derived from the die-casting machines is normally subjected to an initial coarse treatment of oil separation by various systems which refer in particular to the system employing oil absorbent strips with simultaneous separation by precipitation of solid particles principally composed of aluminium flakes.

The chemical-physical treatment permits the elimination of all organic substances present in the effluent except the surfactants and, if present, any glycol solutions. These two elements have an impact also on C.O.D. values (Chemical Oxygen Demand). The C.O.D. value of the effluent at the end of its chemical-physical treatment corresponds to 210 mg/l compared to a value of 4170 mg/l prior to treatment.

Biological treatment

The biological treatment plant is designed for the processing of effluents derived from the chemical-physical treatment plants. The techniques behind biological purification of effluents aim to reproduce a similar auto-purification to that existing in the natural environment: The effluent to be processed is exposed in contact with a purifying biomes which is subjected to aeration. The new effluent charge has a pH value of around 7, is clear and transparent in appearance, and is absent of suspended matter. In the biological plant, groups of bacteria are acclimatised and developed, and specifically selected for the breakdown of surfactants.

Based on our experience and experimentation, two most efficient bacterial groups are made up of formulations of saprophytic and non-pathogenic cultured and fixed onto:

a)

inert substrates based on phosphate soya dosed with trace elements in which the concentration of micro-organisms is not less than 109;

b)

inert substrates based on calcium carbonate of marine origin, porous volcanic rock, trace elements in which the concentration of micro-organisms is not less than 106. The bacterial groups indicated above do not produce adverse affects on the environment; they stimulate natural processes of biodegradation and exhibit a tendency to restore a natural biological equilibrium. In order to operate at maximum efficiency it is necessary to continually monitor certain parameters such as the ratio between Carbon, Nitrogen and Phosphorous (theoretically C:N:P = 100:5:1). After biological treatment the effluent is within all parameters presented in the "Legislative Decree No. 152 of 11/05/99" for discharge into the sewage system or treated effluent reservoirs.

Chemical description for untreated effluent (tab. 2) and results obtained after chemical-physical and biological treatment are shown in the following tables: the concentration of reagents (tab. 3), the bacterial nutrition (tab. 4) and the data resulting from the analysis of the treated effluent (tab. 5).

The untreated effluent is made up of an average sample taken during the course of an 8 hour working period based on a semi-synthetic release agent diluted in water at 2 % v/v (dilution rate normally in use). During sampling, care should be taken to avoid loss of hydraulic fluid (water-glycol solutions) due to accidental leakage.

Toxicity test for effluent discharge with algae

In order to further evaluate the biological treatment it is imperative to guarantee the quality from the point of view of the toxicological aspect of the aspect at the point of discharge. The results obtained from a simple and rapid test using particular types of algae (selenastrum capricomutum, scenedesmus, quadricauda, daphnia magna, ceriodaphnia dubia), which permit an evaluation of the toxicity be it more or less acute relating to the waters leaving the biological treatment plant. Moreover this kind of test is very important in as much as it allows the determination of infinitesimal quantities of toxic substances, that common laboratory chemical-physical analyses are not able to determine. All this provides benefits to the eco-system.

The method provides for the determination of the development of oxygen and therefore the proliferation or less of the algae introduced into the effluent sample. This refers to a test demanded by the "Decreto Legislativo 11/05/99 n. 152" for the discharge of effluent in sewage systems or treated effluent reservoirs. The algae used to have the tendency to develop in a manner inversely proportional to the level of concentration of the toxic substances which are present in the effluent.


Conclusion and objectives

The critical part and wide range of variables in this study address specifically the biological treatment which has required a lengthy research to individualise the ideal groups of bacteria for the breakdown of surfactants.

The results obtained have driven us to investigate more deeply into the study of biological treatment such that in our opinion could enable us, at least in part, to substitute the initial chemical-physical treatment. It should be added that the typology of effluents may vary in significant ways from foundry to foundry; it is possible that the chemical-physical parameters of initial values in effluent discharges are drastically higher than those shown from our experiences.

The importance of this experience addresses the implementation of biological treatment with the final objective to treat the effluent for the most part by using bacteria therefore reducing as much as possible the concentration of chemical products actually used in the treatment process such as ferric chloride, sodium hydroxide, and various kinds of acids etc.

The intention of the participants of this study is to continue with such experimentation on an industrial level under real and actual working conditions. The trials already conducted with various customers operating in the die-casting industry have achieved positive results.

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