Special on : TOC

( article : TOC-E-2 )

Waste Water Analysis

Organic load monitoringin a biological denitrification process, in a plastic fiber’s chemical plant.

Maurizio Chioetto

Introduction
To evaluate the polluting level of an industrial waste water it is important to know the exact content of organic substances.
The evaluation of organic substances is traditionally made by analytical methods based on oxigen consumption in a chemical oxidation reaction (mg/L of O2 as COD) or in a biochemical oxidation reaction (mg/L of O2 BOD) of such organic substances.
More and more widespread are becoming instruments which carry out a direct measure of pollutant charge by means of Total Organic Carbon (TOC) content.
Those analyzers may use different technology, mainly in the oxidation stages, and the one that has been choosen is based on the low temperature oxidation of the organic material promoted by UV light.
This technique has been preferred for it does not allow corrosion or poisoning of the catalyst as the high temperature combustion technique does.

Determination of Organic Material
It is well known that COD measure doesn’t represent the exact organic load of an industrial waste water, since the dichromate oxidation cannot thoroughly oxidize some organic substances such as aromatic hydrocarbon, pyridine etc.
On the contrary TOC measure offers the following advantages:

• very fast analysis and reponse time.
• on site installation
• high precision and accuracy
• low costs per analysis compared with laboratory COD or on line COD analysis.
• absence of dangerous material such as silver, mercury-chrome and iron into laboratory waste waters (these metals are actually present in residual solution of COD method).

Working principle of a TOC analyzer
TOC determination is based upon a chemical oxidation by persulphate and oxygen at a low temperature, catalyzed by UV radiation.
CO2 deriving from the oxidation of the organic carbon in the sample is measured by an appropriate ND-IR analyzer, which will handle all the linearization, autocalibration and autocleaning function.
Waste water sample is introduced in the analyzer by a peristaltic pump that allowes a fast and "on-time" analysis of the water which is present in the waste at that specific time.
The system allows the operator to introduce manually a sample, via a three way valve, in order to allow the batch analysis of a grab sample or a standard solution.
The instrument measures the total organic carbon (TOC) after an acidification with H3PO4 , and a scrubbing with CO2 free comprassed air .
It is possible to measure total carbon (TC= TOC+TIC ) if the acidification stage is bypassed.
Different measuring ranges can be choosen before installing the analyzer, or during its operation , if a new range is required.
This is made with a new configuration of the tubing and pumps, and a new calibration.
The time required to change the measuring ranges is of few hours only.

On line analysis
The plant effluent is a mixture of substances deriving from the polymerization processes occurring during a chemical fibres production (i.e. PA-6, PES and ACN) as well as organic intermediates (i.e. MA and MAS).
The effluent, after pH adjustment, is treated in a biologic denitrification process since it has an high concentration of nitrates.
Biologic denitrification process is based on the following reaction in which NO3- is reduced to N2.:

• 2 N03 - + 4e ==>2 N02
• 2 N02 - + 6e ==>N2
• 2 N03 - + 10e ==>N2

Electrons used for the reduction of nitric-nitrogen in to nitrogen are supplied by the oxidation reactions of organic matter carried out by denitrifying bacteria belonging to these species: Pseudomonas, Micrococcus, Bacillus.
In order to garantee a proper feeding of those microorganisms, an organic substrate (i.e. methanol) is added to the effluent, thus avoiding to have an incomplete denitrification stage.
The whole redox reaction is:

6 N03- + 5CH3OH ==> 3N2 + 5CO2 + 7 H2O + 6OH-

From a stechiometric point of view, 5/6 of the methanol moles are used to reduce one nitrate mole; so the exceeding methanol is added since some is oxidized by dissolved oxigen and some is used by the bacteria.
In order to assure a good bacterial growth it is necessary to have a correct mixture of nutrients containing

carbon / nitrogen / phosphor

in the ratio of

C : N : P = 100 : 5 : 1

The efficiency of the denitrification reaction is not only depending on the organics vs nitrates ratio, but also on pH , on water temperature and the retention time in the activated sludge treatment plant.
For a good monitoring of the plant’s operation, analysis must be performed on different samples , at different points of the plant .
Those points are :

• 1) Inlet waste water into the treatment plant (S12). In this stage the waste water pH is lowered to 4 and passes trought a clarification stage.
• 2) Mixing (Balancing) tank (S22): At this point the methanol and the phosphoric acid are added to the waste water in a measured amount which is related with organic load and the initial nitric load. The addition of chemicals may be fully authomated with the use of the output signal given by the on line analyzer.
• 3) Denitrification tank (S23). The waste water is submitte, in this stage, to biologic denitrification. In this tank denitrifying bacteria metabolize organic substances and reduce the nitrates to elementar nitrogen.
• 4) Outlet (S36): This is the water coming out from the denitrification tank and which flows into the biologic oxidation tank, where the bacteria metabolize the residual organics and where oxigen is added from the aeration turbines.

The aerated water mixture leaves the oxidation tank and flows into the settling tank where the particles are collected on bottom and the clarified water flows away on top of the weir.
Grab samples have been taken with automatic sampling unit and analyzed in laboratory in order to determine the COD values and be able to calculate the correlation factor between COD and TOC analysis for each sampling point.
The sampling point reliability as well as the efficacy of the analyzer oxidation have been tested.

Resaults
In the final table are shown COD and TOC values and the related correlation factors of four samples identified with S12, S22, S23, S36.
Reminding that the analytical correlation factor of COD/TOC results from the theoretical ratio between the value expressed as O2 / C and this is 2,66, we may look at the experimental results

We observe that:

• The correlation factor between COD and TOC of a waste water is specific of the water under test and its characteristic value depends on the type of organic compound present into the water itself, and the ratio existing between Organic material vs other material ( such as N or S compounds that may be oxidized by the COD analysis).
• Correlation factors of samples S12, S23, S36 have given values very similar to the theoretic ratio O2/C and this is due to the lowered oxidability of dichromate on the specific organic matter which is present into the waste water (acrylonitrile, methyl acrylate etc.).
• Sample S22 , in which the organic load is enriched with methanol , has shown a higher correlation factor (3.5) and it corresponds with the theoretic COD/TOC ratio ( and the esperimental one ) that we have found for methanol (i.e. 4).
• The Organic load of an industrial waste waters may be easily monitored on line with the help of a TOC analyzer, and since it is possible to convert the recordered TOC value in to COD using the right correlation factor, we may compare them with all the historical date available for the plant.

 

A solution has been analyzed several times in different moments , and the obtained values were the following :

• 82.5 - 82,2 - 83,2 - 82.6 - 83.2 - 83.4
• 83.6 - 83.1 - 82.3 - 82.7 - 82.6 - 82.5
• 82.8 - 83.4 - 83,3 - 83.5 - 83.1 - 84.0
• 82.9 - 82.3 - 84.1 - 83.5 - 82.5 - 82.9

The average of the different measures is 83 mg/L TOC with a deviation of 0,58
In the previous table are shown the tests done to evaluate the analyzer reproducibility and reliability (s).
It has been calculated on a sample mean value of 83 mg/l of C and it corresponds to 0.58.
To evaluate the oxidation efficacy of TOC method, several tests on standard solutions of organic and inorganic matters, suitably prepared, have been performed.
All the obtained values are shown in the final table .
From these results it is evident the efficacy and completeness of the TOC analysis made with the low temperature UV-persulfate oxidation technology.

Conclusions
It is possible to conclude that TOC measures are comparable with the usual and laboratory made COD measures in order to better evaluate the real organic load present in the plant waste water and consequently its pollution state.
The next step would be to evaluate the possibility for complete automation of the Waste water treatment plant using all data received from the different analyzers, flowmeters and then optimize the process.
It has been estimate that at the end of this project, the running cost of the w.w.t.p. will be reduced.

( Data obtained from a research done with Mr. Bombardieri and Mr. Catucci - Enichem )

 

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