rapid method for the estimation of chemical oxygen demand of dairy wastes

Rapid Method for the Estimation of Chemical Oxygen Demand of Dairy WastesAuthor(s): John PalmerSource: Irish Journal of Food Science and Technology, Vol. 5, No. 2 (1981), pp. 149-155Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25558033 .

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Ir. J. Fd Sci. Technol. 5: 149-155, 1981

RAPID METHOD FOR THE ESTIMATION OF CHEMICAL OXYGEN DEMAND OF DAIRY WASTES

John Palmer An Foras Taluntais, Moorepark Research Centre, Fermoy, Co. Cork

ABSTRACT

A rapid method (15 min) for the estimation of chemical oxygen demand (COD) in untreated dairy wastes is described. A block heater for digestion of the samples in tubes using a sulphuric acid concentration 4<ft

higher than that of the conventional 2 hr digestion methods is a feature of the method. The COD recovery percentage by the rapid method compared with that obtained with the 2 hr method

for typical dairy wastes and for whole milk samples tested in our laboratory were 94.9 ?5.2 and 98.3 ? 3.6. The corresponding values found in two creamery laboratories for a range of dairy wastes were 100.6 ? 5.0 and 99.2 ? 15.0 respectively.

The rapid method should find particular application in measuring effluent strengths over relatively short periods e.g. hourly samples.

INTRODUCTION

Several methods have been proposed for the determination of the chemical oxygen demand (COD) of effluent samples. The more generally recognized or standard methods (1, 2) are similar in that samples are digested for 2h under reflux with 50

percent sulphuric acid in the presence of excess dichromate and silver sulphate catalyst, using mercuric sulphate to suppress interference by chloride. Digestion is followed by titration of the excess dichromate with ferrous ammonium sulphate using ferroin as indicator. More recently micro digestion procedures in sealed tubes have been proposed as replacement methods for macro digestion (3, 4, 5). The main

advantages claimed for these methods are increased throughput of samples and reduced work load. The disadvantages are, the likelihood of less precise results with

samples containing high suspended solids levels and the safety aspects relating to

possible breakage of tubes during digestion. In our laboratory we have scaled down the standard procedure by 50% and used 50 ml tubes for digestion of the samples in a block heater. After digestion, titration of the excess dichromate is carried out in the same tube using an air bubbler for agitation of the sample. While this method increases the throughput of samples tested per day results are not available for at

least 2.5 h after initiating testing as is the case with the previously mentioned COD methods. Shorter digestion time for the method would be of great benefit in

speeding up the analysis and throughput. In this communication we report on a

rapid digestion method which has cut digestion time from 2 h to 15 min.

149



150 IR. J. FD SCI. TECHNOL., VOL. 5, NO. 2

EXPERIMENTAL

Rapid COD method For the rapid method the acid content of the digestion mixture was increased by 4%

to 54%, the potassium dichromate was premixed with the silver sulphate sulphuric acid and digestion was carried out under reflux for 15 min in 50 ml tubes using block

heaters at a block temperature of 180?C. Increasing the acid content of the digestion mixture by 4% raised the digestion temperature by 8?C approx above that obtained

in the 2 hr method. The samples were cooled immediately after digestion for 15 min

and then titrated with ferrous ammonium sulphate. Full details of the method are

given in an Appendix.

Samples tested The samples tested in our laboratory consisted of whole milk, potassium hydrogen

phthalate standards and untreated dairy effluents arising from milk concentrating and drying and cheese manufacture. All samples were tested in duplicate. Samples tested in two creamery laboratories consisted of a range of dairy effluents arising from different processes and included a small proportion of treated effluents.

RESULTS

The mean COD values obtained using the 2 hour and 15 min (both at 50% acid)

digestion time are given in Table 1. The percentage of the 2 hour COD recovered by

digestion for 15 min was over 90% in the case of the milk samples but was less than

90% for the dairy effluents. The percentages of COD recovered for potassium hydrogen phthalate standards

using the 2 hour and 15 min (50% acid) and the 15 min (54% acid) digestion time are

given in Table 2. Recovery values in the region of 94-101 % were obtained using both

15 min tests for standards within the range 800 to 200 mg/1 COD.

TABLE 1: Mean COD values for milk dilutions and dairy waste using 2-hour and 15 min (50% acid) digestion times

COD mg/l

Sample 2 h 15 min *7o of 2 h

Vo milk 0.05 110 104 95.0 0.10 193 203 105.0 0.20 409 396 96.8 0.33 679 634 93.4

Dairy effluents 19 samples 3142 2655 86.0 ?11.4



PALMER: ESTIMATION OF C.O.D. OF DAIRY WASTES 151

TABLE 2: Mean COD values for potassium hydrogen phthalate standards using 2 hour and 15 min (50% acid) and 15 min (54% acid) digestion times

COD mg/l % COD recovered

Phthalate 50% acid 54% acid 50% acid 54% acid

mg/l 2h 15 min 15 min 2h 15 min 15 min

800 790 762 809 98.8 95.3 101.0 600 581 572 600 96.9 95.3 100.0 400 384 389 402 96.0 97.2 100.5

200 195 188 201 97.5 94.0 100.5 100 99.5 108 91 99.5 99.5 91.0

The mean COD values for milk samples and a range of dairy effluents using the 2 hour (50% acid) and the 15 min (54% acid) digestion time are given in Table 3. The

percentage of the 2 hour COD recovered by digestion for 15 min in our laboratory was highest for the milk samples 98.3 ? 3.6% as opposed to 94.9 ? 5.2 for the effluents. The corresponding values obtained in the creamery laboratories were

100.6 ? 5.0 and 99.2 ? 15.0%. The regression equations found for predicting the 2

hour result from the 15 min test for the effluents in the three laboratories were

y = 1.062X?23, 0.981X + 23, and y = 1.009X?62. The correlation coefficients found between the tests for the effluents were 0.996, 0.999 and 0.989 respectively.

TABLE 3: Mean COD values for a range of dairy effluents using 2 hour (50% acid) and 15 min (54% acid) in three laboratories

No. of COD mg/l

Laboratory Effluent type samples 2h 15 min % of 2 h

Moorepark 2% milk 36 3508 3450 98.3 ? 3.6

Moorepark Cheese and milk powder 39 2512 2384 94.9 ? 5.2

Creamery A Range of dairy effluents 35 2051 2063 100.6 ? 5.0

Creamery B Range of dairy effluents 45 3497 3469 99.2 ? 15.0

DISCUSSION AND CONCLUSION

The percentage of the 2-hour COD recovered by the 15 min test (both tests at 50%

acid) for whole milk dilutions is quite high 93-100% whereas for dairy effluents it was only 86.0 ? 11.4%. The 15 min test when used at the 54% acid level gave a COD recovery percentage of 95 to 100% ? 15% for dairy wastes in the three laboratories. It has been reported that increasing the acid concentration to about




60% with a resulting increase in reflux temperature tends to decompose the dichromate resulting in erroneously high COD values (6). No evidence of chromate breakdown was evident in our investigations when using an acid level of 54%. It

appears from our results that the COD of untreated dairy wastes may be adequately determined by digestion for 15 min using 54% acid in the digestion mixture rather than the conventional 50% acid. However, it would be prudent for laboratories

intending to use this rapid method to have a reasonable number of samples of their

particular effluents tested by both methods initially. The method facilitates the

testing of increased numbers of samples within the working day and enables

production staff to respond earlier to excess wastage of product.

ACKNOWLEDGEMENTS

The author wishes to thank Ciaran O'Connor for skilled technical assistance, J.

Hurley, Carbery Milk Products and D. Crowley, Ballyclough Co-op. for their co

operation in supplying test data from their laboratories. I also wish to thank Irish

dairy farmers for financial contributions which made the undertaking of this work

possible.

REFERENCES

1. U.K. Department of the Environment. Methods for the Examination of Waters and Associated Materials. Chemical Oxygen Demand (Dichromate Value) of Polluted and Waste Waters 1977. London H.M.S.O. 1978.

2. American Public Health Association. American Water Works Association and Water Pollution Control Federation. Standard Methods for the Examination of Water and Waste Water. 14th Ed., 1975, p. 550-554.

3. Jirka, A. M. and Carter, M. J. Micro semi-automated analysis of surface and waste waters for chemical oxygen demand. Anal Chem: 47, 1397-1402, 1975.

4. Hach Chemical Company. Oxygen demand, chemical, in Hach Water Handbook. 3rd edition

p. 2-136 to 2-139, 1976. Camlab Laboratory Apparatus Information Sheet, COD Reactor. 5. Best, D. G. and de Casseres, K. E. Determination of COD using a sealed-tube method. Wat. Poltut.

Res. 11, 138-140. 1978. 6. Musselwhite, C. C. An evaluation of the Hach Block-Digestor/Sealed Tube Procedure for the deter

mination of chemical oxygen demand. Water Research Centre, U.K. 1978. Personal communication.

Received January 20, 1981




APPENDIX

RAPID CHEMICAL OXYGEN DEMAND TEST (COD) 15 MIN DIGESTION

Apparatus

(1) 50 ml pyrex tubes MF 24/3 24/29 Quickfit Jobling Catalogue Moloney Bros., Dublin.

(2) Condensers Cl/23 24/29 Quickfit Jobling Catalogue, Moloney Bros., Dublin.

(3) Tecam Dri-Block DB-3H. Unit with 180?C thermostat with 3 blocks to accept 4

tubes/block, holes in blocks 28 mm. P. J. Brennan, Dublin.

(4) Anti-bumping material, glass beads may be used or glass rod fitted to a square of thin PTFE sheet.

(5) Small beaker or dust cap to invert over the top of the condenser while not in use.

Reagents

(1) Silver sulphate/sulphuric acid solution. Dissolve lOg of silver sulphate in 1 1 of analar 98% sulphuric acid. The silver sulphate should be added at least 24 h before the reagent is required as it dissolves very slowly in the acid.

(2) Dichromate-silver sulphate/sulphuric acid solution. Dissove 2.665g potassium dichromate in distilled water and make to 250 ml in volumetric flask. Transfer the dichromate solution to a 1 1 pyrex conical flask and wash the dichromate residue from the 250 ml flask carefully into the conical flask with > 750 ml of the silver sulphate/sulphuric acid solution. When the contents of the flask are cool make to I I in a volumetric flask with additional silver sulphate/sulphuric acid. This solution is stable for 3 months if stored in a brown bottle.

(3) Ferrous ammonium sulphate solution. Add 40 ml of concentrated sulphuric acid to 400 ml of water, mix and cool. Dissolve 39.2g of ferrous ammonium sulphate in the cooled dilute acid and make up to 2 1 (0.05m).

(4) Mercuric sulphate solution. Dissolve 50g of mercuric sulphate in a mixture of 225 ml of water and 25 ml of concentrated sulphuric acid.

(5) Indicator solution 1:10 phenanthroline ferrous sulphate complex (Ferroin indicator).

(6) Standard potassium dichromate solution. Dissolve 12.259g/2 1 (dried at 105?C x 2h (0.125N).

(7) Chromic acid cleaning solution. Add 70 ml of saturated sodium dichromate solution to 2 1 of cone, sulphuric acid.

Standardisation of ferrous ammonium sulphate

Dilute 10.5 ml of concentrated sulphuric acid with about 12 ml of water and cool. Add 5 ml of the standard potassium dichromate solution and titrate with the ferrous ammonium sulphate solution using 1 drop of 'Ferroin* as indicator. The colour

change is from blue-green to red. It is important to add the dichromate after the diluted sulphuric acid has been cooled to avoid the oxidation of any organic contaminants by the dichromate before it can be titrated.




Procedure

Into the 50 ml tube containing the anti-bumping material weigh 5g of sample, add 0.5 ml of mercuric sulphate solution and 11.5 ml of potassium dichromate silver

sulphate/sulphuric acid solution. Assemble the apparatus, mix and reflux for 15 min. Remove the tubes from the digestion block, place in iced water and rinse the condenser from the top with 5-10 ml of water. Titrate the cooled solution in the 50

ml tube with ferrous ammonium sulphate solution using 1 drop of 'Ferroin' indicator and use a minute amount of compressed air or fish tank aerator pump for

agitation of the contents while titrating. Use two blanks per set of determinations. The accuracy obtained for effluents from cheese and milk powder manufacture, with this method was 95?5(7o of the standard 2h procedure.

Treatment of apparatus, samples and reagents

All new apparatus is refluxed for 2 hours with chromic acid, then thoroughly washed with distilled water. The apparatus is kept free from dust by leaving it

assembled at all times except when adding reagents or carrying out the titration. After each use, fill digestion tubes with chromic acid cleaning solution and store at 60?C for 1 h before rinsing 3 times with distilled water, and dry tubes. Keep reagents free from dust. The sample is weighed rather than pipetted to avoid sampling errors

due to coarse suspended solids (pipetting OK where sample is a very fine suspension or solution or which has been homogenised in high speed blender in the case of coarse suspended matter.)

Calculations

12.5 ml FAS = 5 ml potassium dichromate = 5 mg oxygen 1 ml FAS = 0.4 mg oxygen = 80 mg/1 COD with a 5 ml sample

Experimental COD range 80-800= 1 ml-10 ml FAS For untreated dairy effluent use a dilution of 10-20% w/vol

Calculation of COD

COD mg/1 or kg = 80 (Blank titre (ml FAS) minus sample titre (ml FAS). The factor 80 will require altering if the standardisation titre for the FAS is not exactly 12.5 ml in which case

12 5 Factor =

---r--x 80 Standardisation titre (ml FAS)

Example calculation

Standardisation titre for 5 ml dichromate = 12.25 ml FAS Blank titre = 12.5 ml FAS

Sample titre (5g sample) = 7.0 ml FAS




Factor = ? x 80 = 81.6 12.25

COD mg/l or Kg. = (12.5-7.0) x 81.6 = 449

Note:

For convenience and safety and accuracy an automatic pipettor for dispensing the

silver sulphate/sulphuric acid ? dichromate solution is recommended (Oxford labs).

Standard sample for COD: Use 0.34g potassium hydrogen phthalate per litre. Use 5 ml for COD test, COD



rapid method for the estimation of chemical oxygen demand of dairy wastes

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