Estimate the COD of \"weak\" domestic wastewater (Table 12-5) based on the TOC c

Question

Estimate the COD of "weak" domestic wastewater (Table 12-5) based on the TOC concentration (mg C/L) and assuming that the average oxidation state of the carbon in TOC is zero. Compare the value you calculate to the reported concentration. If there are any discrepancies, can you guess why?

Weak Medium Strong Constituent (all mg/L except settleable solids) 100 25 200 Alkalinity (as CaCO3) Ammonia (free) BODs (as O2) Chloride COD (as O2) Total suspended solids (TSS) 10 100 300 100 1,000 250 120 95 25 500 210 160 Volatile (VSS) Fixed 315 85 settleable solids, ml./L Sulfates" Total dissolved solids (TDS) Total Kjeldahl nitrogen (TKN) (as N) Total organic carbon (TOC) (as C) Total phosphorus (as P) 10 20 200 20 75 500 1,000 150 10 300 "To be added to amount in domestic water supply. Chloride is exclusive of contribution from water-softener backwash. Rnewer, tighter collection systems, or where water conservation is practiced, these numbers may be considerably higher Adapted from Metcalf & Eddy, 2003.

Explanation / Answer

C + O2 -------> CO2

1 mole of C requires 1 mole of O2

12 mg/L Of C requires 32 mg/L of O2

from table for weak domestic wastewater TOC is 75 mg of C/L

75 mg/L Of C requires 75^(32/12) mg/L of O2 = 200 mg/L of O2

but from table actual COD or oxygen demand is 250 mg/L

The reason for this discrepancy may be that other forms of oxidisable waste products are present in the wastewater.

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