COLLINS and TENNEY: SYSTEM FOR DETERMINING POLLUTANT PARAMETERS 



COD Reaction 



The oxidation reaction in the COD method fol- 

 lows the usual chemical reaction laws, i.e., the 

 completeness of the reaction is dependent upon 

 the concentration of the reactants (potassium di- 

 chromate and waste). The method uses 25 ml 

 0.25N or 6.25 meq K 2 Cr 2 7 in the reaction flask 

 and 50 ml of effluent. If the effluent is relative- 

 ly strong, most of the dichromate will be ex- 

 pended in the reaction which results in an 

 incomplete reaction and a lower COD value 

 than if the waste were weak, i.e., having a 

 larger excess of dichromate at the completion 

 of the reaction. Moore and Walker (1956) rec- 

 ommended that the size of sample should be 

 selected so that not more than 50^ of the 

 potassium dichromate is used up during the 

 oxidation. To illustrate the relationship be- 

 tween COD and amount of dichromate re- 

 maining (the excess) at the end of the 2-h 

 reflux period, data from six protein prepara- 

 tions were combined and plotted in Figure 3. 

 The equation of the regression line was then 

 used to calculate correction factors so that if 

 the COD were determined at an excess di- 

 chromate level above or below an arbitrary 

 point of 3.5 meq, the value can be corrected 

 to its value at 3.5 meq. These correction fac- 

 tors are listed in Table 3. To correct COD 



TABLE 3. — Multiplication factors to correct COD to 3.5 meq 

 dichromate excess. 



1.400 



z 



UJ 



y 



ta- 

 il. 



uj 



o 



(J 



z 

 o 



o 



o 



- 



o 



o °oo 



cP o ^o 



~®°o% P o°o o 



Y= 138 log X + I.26S 



2 I M S 



POTASSIUM DICHROMATE meq excess 



FIGURE 3. — Relationhship between the protein coefficient and 

 the amount of dichromate remaining at the end of the 2-h 

 reflux period. 



values, determine the excess dichromate 

 (titration value times normality) and multiply 

 the corresponding factor from Table 2 by the 

 COD determined in the usual way. 



Since titration (Jirka and Carter 1975), 

 sample, or reaction errors occur at either end 

 of the curve, we suggest that COD values are 

 valid only between 2 and 5 meq excess. All 

 data for the protein coefficients were deter- 

 mined by obtaining from 10 to 30 COD values 

 at different addition levels (5 to 30 mg pro- 

 tein/50 ml) and plotting the regression line. 

 The coefficient was obtained by substituting 

 the logarithm of 3.5 meq excess into the equa- 

 tion for the regression and solving for COD. 

 In addition, all COD data in Table 1 were cor- 

 rected to 3.5 meq excess dichromate. 



Residue-Ash Correction 



The major components of the total residue that 

 contribute to COD are protein and O&G. In addi- 

 tion, various salts and dirt contribute to TR and 

 possibly to COD. Unfortunately, there is no con- 

 venient method to measure these minor constitu- 

 ents so we estimate them by determining ash and 

 then subtract to give a corrected value for TR. 

 Since the weight of ash obtained after 500°C dry- 

 ing is less than its corresponding weight when 

 dried at 103°C, the TR K value (TR - ash) is accord- 

 ingly greater than it should be. Therefore, the TR K 

 was reduced as follows: To eliminate variability in 

 individual values, the O&G and protein values 

 were predicted using Equations (4) and (5) for the 

 regression lines in Figure 2 and TR K data. The 

 sum of the weight of protein plus O&G was found 

 to be about 37c smaller than TR K , i.e., 



257 



