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Waste Management

On-line prevention of water contamination in a chemical manufacturing plant

By P.Aldrovandi - A Formenti
CIBA GEIGY Spa- Divisione Addittivi - Pontecchio Marconi ( BOLOGNA) - ITALY

INTRODUCTION
In order to detect contamination of water in a chemical industry process and/or prevent environmental contamination, it is necessary to ensure that adequate preventive measures are in place both to identify and react to possible spillages.
Education of production personnel on environmental issues, while extremely important, cannot guarantee that accidental spillages of pollutants, which can compromise the efficiency of the plant, will be eliminated.
Similarly, careful management of the biological process in itself may not necessarily achieve the quality of effluent required for legal compliance.
Two kinds of risks have been identified, both strictly related; accidental leakages upstream of the plant and a non conforming effluent.
We can assume that the use of on-line equipment for effluent monitoring, integrated with an automatic means of diversion will achieve a high level of prevention by storing polluted water and eventually mixing it with low load process waters.
This approach would translate into a high level of investment in instrumentation construction or modification of containment structures.
Nevertheless, economic returns and corporate image were seen to be of greater value than the costs, particularly considering the position of this chemical industry with regard to its environmental impact as perceived by both the regulatory authorities and community alike.

GENERAL BACKGROUND
CIBA has owned the Additives Division plant since 1979. It is located at Pontecchio Marconi near the Appentino Bolognese and the city of Bologna.
The Company is continually expanding, especially in the areas of production and personnel (CIBA currently employs 400 people). Currently, about 90% of plant output is additives for plastic while the remaining 10% is active principles for insecticides.
With regards to environmental protection, the plant has an under-cooling system for recovering solvents, an incinerator for gas emissions treatment, and an activated sludge biological plant for water treatment.
In December 1995 it was awarded environmental certification by CertiEco in compliance with BS7750 (certificate n° 475).

WASTE WATER PLANT
The plant was built between 1985 and 1986; it purifies the contents of the mixed drainage system which collects the low load industrial waters, waste waters and rains.
Process waters which contain the highest organic load, are added during a homogenization phase.
Final treatment is comprised of a two stage treatment plant with activated sludge, a nitrification and a denitrification phase, followed by a filtration on a two stage sand filter following the secondary sedimentation.
The final effluent is discharged into surface waters, thus it must comply with the discharge consents of table A of the Italian law for waste water ( nr. 319/76).
The plant has a 1800 cubic meter emergency tank, which is about double the daily flow, consisting of two different sections.
The smaller (250 cubic meters) is used to store highly polluted waters from flow diversion upstream of the plant (first check-divert point).
The bigger ( 1550 cubic meters) is used to contain the final effluent in the event that discharge compliance cannot be met (second and third check-divert points).
The latter volume allows, if the consumption of water inside the plant is restricted, to retain the waste waters for about 70 hours.

THE SYSTEM
Instrumentation purchase decisions were based on: dependability and robustness of the components, ease of maintenance, support capability of suppliers, and availability of parts and accessories.
Formalization of internal procedures for calibration and general maintenance was deemed to be very important since this would best guarantee the certainty of the analytical data and efficiency of the whole system.
Another fundamental step involved progressively automating the diversion system (figure 1).
The second and third check-divert points are still underway.
This gradual evolution from totally manual to half-automatic and finally to full-automatic, has contributed greatly to a better understanding of all plant personnel to the necessity of each upgrade and to the overall system concept.

FIRST CHECK-DIVERT POINT
The first check-divert point is located downstream of the first tank and consists of a pH-meter and an Isco Model 3500 EZ-TOC analyzer.
Operating conditions for instruments in this area are very harsh for two reasons:

  1. Liquid to be analyzed is the product of a mixed drain and may contain particles, acids, solvents, and product traces or scraps.
  2. Distance from the plant is approximately 700 meters, making it difficult for plant personnel to perform frequent checks.

For these reasons it is necessary to use equipment that can provide maximum efficiency with regards to hydraulic obstructions and corrosion, and, above all, continuously report status.
Speed of response and precision are crucial during this phase.
The first issue will immediatly alert the plant personnel so that the identification and isolation of the spill can begin.
Precision is essential to get reliable outputs of the incoming load which can be combined with process water at a later time
Additionally, the equipment is expected to reliably report the complete cycle of the "spill".
The sequence of events at the first check-divert point is as follows:

The polluted water is later mixed with process waters at other stages to ensure proper treatment.

SECOND CHECK-DIVERT POINT
The second check-divert point concerns the flow from the homogenization tank which feeds the biologic tank.
The equipment used, shared also by the third check-divert point (figure 3), encompasses a much broader range of parameters including: TOC, turbidity and the use of Applikon Analyzer for : ammonia, nitrate, nitrite, and chloride.
This check-point is crucial for correct management of the biologic process. Actually it allows plant personnel to evaluate the effective organic load to which the plant will be submitted and verifies correct dosing of process waters with contents of the first emergency tank.
It also permits the characterization of irregular situations such as excessive levels of nitrogen or chloride substances that are not detectable at the first check- divert point.
In the event that irregularities occur at this check-point corrective measures incorporate partially transferring volumes of water from the homogenization tank to the emergency holding tank.
Volume transferred is sufficient to restore critical parameters to an acceptable level.

THIRD CHECK-DIVERT POINT
The third check-divert point is relevant to the final effluent of the plant. As already underlined, it shares the same equipment as the second point ).
In both cases the sample is relatively simple to handle being low in pollutants and devoid of solids.
The importance of the "on-line" monitors at this phase is to allow for a nearly instantaneous evaluation of overall plant efficiency and to ensure compliance with discharge permit limits.
Internal operating procedures have been devised which require adherence to levels lower than actual permit requirements. These "operating levels" are determined by plant management and serve to create the window of response time plant personnel have to react to spill events and additionally compensate for analytical precision of the various methods involved.
When the internal limit is reached, flow is immediately interrupted and sent to the emergency holding tank.
At the end of the emergency, the stored water will gradually be treated by dilution with process water and in proportion to the level of pollution measured.

CONCLUSIONS
These preventive measures have eliminated most of the pollution problems associated with this chemical plant. Moreover it is clear that the efficiency of the first two stages makes the third almost redundant.
It is retained in case of a temporary mechanical failure in the plant.
This automated system has shown itself to be highly reliable and has allowed plant personnel to concentrate more on the control of the biological process and the internal treatment

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