Special on : TOC

( article : TOC-E-3 )

Waste Water Analysis

On line measurementof Organic Carbon in a "cocktail" of a PetroC hemical waste water.

Maurizio Chioetto

Introduction
The knowledge of the quality of both waste water and purified waters is a fundamental element for the successful operation of the treatment process, as well as any industrial process in the petrochemical industry.
This is possible with the availability of reliable on line analyzers using a proper instrumental technique for the faster determination of the organic loading in a liquid effluent, as opposite of the time consuming laboratory techniques requested by BOD or COD determination made on grab samples.

Organic determination
In the last years , as a normal practice, the organic pollution of a waste water have been more widely expressed as TOC - Total Organic Carbon - values (mg/L of C ) as opposite to COD - Chemical Oxygen Demand ( mg/L of O2 ) or BOD - Biochemical Oxygen Demand ( mg/L O2 ).
This offers several advantages as faster analysis, possible full automation of the monitoring system, better reliability, lower overall costs and at the end better water treatment.
This research have been done in a cocktail of waste water discharged by various processes : cracking, several petrochemical synthesis, polymerization, fibers manufacturing, petrochemicals , oil storage, coal processing etc.
This effluent to the treatment plant, has been pre-treated in a chemical-physical stage, then processed by an active sludge biological treatment.
The measured value of the TOC analysis has been compared with the laboratory COD determination; moreover the performance of different TOC analyzers using different oxidation systems has been examined.
In the previous years different alternative technologies on the same waste water have been evaluated :

a TOD ( Total Oxygen Demand )
analyzer , but it has been proved that this methodology, not anymore accepted by the different guide line and official methodology today used for the waste water analysis, was too much affected by the interference of Nitrogen compounds present in the waste water.
An UV analyzer ( using different absorption A=20 at 240 nm and A=5 at 270 nm) was rejected for the non-existing correlation between the UV absorption and the laboratory COD values and the continuous fouling of the measuring system due to the presence of solids ( 300 mg/L).

In order to fulfill this experiment three different on line analyzer were operated in parallel for two months ( one COD and two TOC).

Analysis on line
The COD analyzer was operating according to the Standard Methods technique ( chemical oxidation at 130°C of the sample , mixed with bicromate, Ag2SO4(catalyst) , HgSO4 ( complexing agent for chlorides) and then a colorimetric detection at 440 nm.
The oxidation time was limited to 20 minutes instead of the 2 hours requested by the official methodology. (Tab.2 shows the results of this tests.)

One of the TOC analyzer was an High Temperature oxidation with a response time of about 10 minutes , with an inorganic removal stages ( acidification and scrubbing).
The second TOC analyzer was a Low temperature UV promoted chemical oxidation , with inorganic removal stages ( acidification and scrubbing) and a response time of 12 minutes.

In both analyzers the Volatile Organics were removed together with the Inorganic removal during the stripping stage.
All analyzer were daily calibrated with a standard solution of Potassium Biphtalate ( KHP) of 500 mg/L and d.i. water.
The same calibration standard was used for the laboratory COD analysis comparison.
The different tables are showing the typical COD trend, compared with the TOC by combustion, and the COD compared with TOC by UV-persulphate.
Both data are giving a positive correlation between the two techniques.

In order to evaluate the difference between the response of the two TOC analyzers, they were connected to the same stream which was containing an average amount of salts of 5 mg/l NaCl.
Both analyzer performance and operating procedures, as well as failures and corrective actions were recordered for each day.
Laboratory COD and BOD analysis were also performed every day in order to achieve a possible correlation factor typical of the inlet stream of the waste water treatment plant.
The sampling was made with the help of a refrigerated sampler located 1 meter far from the intake of the on line analyzers
One table reports data of the COD and TOC correlation factors .
Further tests were also conducted in order to verify the effectiveness of the low temperature UV-persulphate TOC analyzer oxidation system in the oxidation of various organic substances and the removal of inorganic carbon which is always present in the industrial waste waters.
Different solutions of known concentration of organics were analyzed by the UV-persulphate TOC analyzer under standard operating conditions.

Fig.4 reports the trend of zero and span calibrations, its stability over a 20 days period without any maintenance, corrective or cleaning operations.

Conclusions
The management of an effective waste water treatment plant may be carried out with the help of the on line analysis of TOC values.
This methodology has a high correlation with the COD determination, but it should be noted that the COD/TOC correlation factor is different for each effluent.
In several cases we have found that the ratio could be high as 3,5 ( inlet waters) or low as 2,5 ( outlet treated waters ):
Reasons for this difference is the decreasing of the dichromate oxidability of the organic residues after the biological oxidation.
The UV-persulphate TOC oxidation is efficient, complete and in our case compared with the oxidation obtained with High Temperature TOC.
The High Temperature TOC oxidation is a valid technique to be used for laboratory measurement with fairly unclean waters.
In on line measurements of waste water containing limited amount of salts and high amount of corrosive products and solids , the UV persulphate TOC analyzers is the most adequate and reliable.
It allows to overcome troubles, typical of a High Temperature TOC analyzer, associated with high maintenance, corrosion of the reactor, catalyst poisoning etc.
The expected maintenance for a High Temperature TOC analyzer was estimated to be of about 4 - 6 hours a week.
Those action were :

• several need of disassembling ,
• cleaning and replacement of the combustion tube, - change of the catalyst,
• cleaning of the manifolds and of the complete analytical system that does not allow the presence of high particulate,
• continuous clogging of the sampling systems ) .

The UV persulphate analyzer need a replacement of the peristaltic tubing, changing of the reagents and standards solution.
It was programmed for an autocleaning of the tubing on a daily base and autogas and autoliquid calibration every week.
The most critical point of all on line analyzer is the sampling system that should be designed and build with the most accuracy.

( Data obtained from a research done with Mr. R.Basei - Montedison- Italy )

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