Alkalinity ( mol L) (1.6.1)
Where the quantities in parenthesis are concentrations in meq/L or mg/L as CaCO3.
( mg L) of X as CaCO3
BIOCHEMICAL OXYGEN DEMAND (BOD)
Stages of Decomposition in the BOD test
BOD – Dilution Method: BOD is the amount of oxygen (Dissolved Oxygen (DO)) required for the biological decomposition of organic matter. The oxygen consumed is related to the amount of biodegradable organics.
When organic substances are broken down in water, oxygen is consumed
Organic Carbon + O2 → CO2
Where, organic carbon in human waste includes protein, carbohydrates, fats, etc
Measure of BOD = Initial oxygen- Final Oxygen after (5 days at 20 °C) or (3 days at 27 °C)
Two standard 300 mL BOD bottles are filled completely with wastewater. The bottles are sealed. Oxygen content (DO) of one bottle is determined immediately. The other bottle is incubated at 20 oC for 5 days or (or at 27 °C for 3 days) in total darkness to prevent algal growth. After which its oxygen content is again measured. The difference between the two DO values is the amount of oxygen consumed by micro-organisms during 5 days and is reported as BOD5. Since the saturated value of DO for water at 20 oC is 9.1 mg/L only and that the oxygen demand for wastewater may be of the order of several hundred mg/L, therefore, wastewater are generally diluted so that the final DO in BOD test is always ≥ 2 mg/L. Precaution is also taken so as to obtain at least 2 mg/L change in DO between initial and final values.
Where, DOi and DOf are initial and final DO concentrations of the diluted sample, respectively. P is called as dilution factor and it is the ratio of sample volume (volume of wastewater) to total volume (wastewater plus dilution water). In the above formula, it was assumed that the diluted wastewater had no oxygen demand of itself and that the dilution wastewater used was pure.
Most of the times, microorganisms are added in the dilution water (seeded water) so as to have enough microorganisms for carrying out biodegradation of organic waste. In this case, the oxygen demand of seeded water is subtracted from the demand of mixed sample of waste and dilution water. In this case,
Where, Bi and Bf are initial and final DO concentrations of the seeded diluted water (blank).
Modeling BOD as first order reaction
Assuming that the rate of decomposition of organic waste is proportional to the waste left in the flask:
Where, Lt is the amount of oxygen demand left after time t and k is the BOD rate constant (time-1). Solving this equation yields
Where, Lo is the ultimate carbonaceous oxygen demand and it is also the amount of O2 demand left initially (at time 0, no DO demand has been exerted, so BOD = 0) At any time, Lo = BODt + Lt (that is the amount of DO demand used up and the amount of DO that could be used up eventually). Assuming that DO depletion is first order:
As temperature increases, metabolism increases, utilization of DO also increases, therefore, k is a function of temperature (T in oC). k at any temperature T (oC) is obtained as:
Where, k 20 is the value of k at 20oC and θ is an empirical constant. θ = 1.135 if T is between 4 - 20oC; θ = 1.056 if T is between 20 - 30oC.
CHEMICAL OXYGEN DEMAND (COD)
This test is carried out on the sewage to determine the extent of readily oxidizable organic matter, which is of two types:
a. Organic matter which can be biologically oxidized is called biologically active
b. Organic matter which cannot be oxidized biologically is called biologically inactive. COD gives the oxygen required for the complete oxidation of both biodegradable and non-biodegradable matter.
Analytical Procedure Organic
Essential differences between BOD and COD