In biological wastewater treatment, oxygen is consumed by microorganisms to treat mainly carbonaceous oxygen demand and nitrogenous oxygen demand. Although some inorganic ions in their reduced form also need oxygen to be oxidized, the amount is negligible comparing to the others.
As illustrated in Fig. 1, carbons convert to heterotrophs and CO2 while consuming O2, which is the most dominant mechanism in carbon-rich wastewater treatment. The nitrogen measured as TKN is oxidized to NO3-N by autotrophs while the autotrophs utilize the the energy from the oxidation reaction to fix CO2. Eventually the carbon in CO2 is used as building blocks of autotrophs. A portion of TKN is also used to build hetrotrophs and autotrophs without being oxidized.
If there is an anoxic stage in the process, a portion of the NO3-N can be used by heterotrophs as an oxygen source. This is so called denitrification, which requires carbons as an energy source. Excess sludge includes both heterotrophs and autotrophs, but typically the amount of autotrophs are not significant relative to heterotrophs due to the low TKN content in wastewater as well as the low autotroph yields. As a result, the production of autotrophs from TKN is typically neglected when biosolids yields, , are estimated unless unless sophisticated equations are used, e.g. IWA’s activated sludge model (ASM).
Fig. 1. Mass flow chart illustrating biological reaction in bioreactor.
Components of oxygen demand
There are three important components in estimating O2 demand: 1) carbonaceous O2 demand, 2) nitrogenous O2 demand, and 3) O2 credit from denitrification.
1) Carbonaceous O2 demand
The carbonaceous O2 demand is estimated based on COD balance. The most common COD measurement methods based on hexavalent chromium represent only carbonaceous O2 demand and are hardly affected by nitrogen because the hexavalent chromium does not oxidize TKN. The total effective carbonaceous O2 demand removed is calculated by subtracting COD stored in excess biosolids from the apparent COD removed as follow.
Carbonaceous O2 demand = COD removed – COD assimilated to excess biosolids
: amount of oxygen used to convert COD to CO2 (kg/m3 or g/L)
: influent flow rate (m3/s)
: COD of influent (kg/m3 or g/L)
: total COD of effluent (kg/m3 or g/L)
: COD/MLSS (g COD/g MLSS)
: observed sludge yield (g MLSS/g COD)
As sludge age (or SRT) increases, decreases due to the accumulation of inorganic materials in biosolids, but it typically ranges between 1.1 and 1.2 in CAS that runs without coagulant dosages. It will be assumed at 1.1 here since MBR runs at substantially higher SRT than CAS. The observed sludge yield can be calculated from the empirical equation that will be discussed in later section.
2) Nitrogenous O2 demand
Only the nitrogen in reduced forms needs oxygen to be oxidized such as ammonium nitrogen (NH4-N), nitrite (NO2–), organic nitrogen that mainly exists in amino acids and proteins, etc. Typically nitrite level is negligible (<1mg/L) comparing to other components and is neglected. Rest of the reduced nitrogen are measured as TKN.
The nitrogen oxidation reaction can be written as follow, where one nitrogen atom in an ammonia molecule needs four oxygen atoms for complete oxidation. By dividing the weight of four oxygen atoms (64 g/mol) by the weight of one nitrogen atom (14 g/mol), a factor of 4.57 g O2/g TKN is obtained as a ration of O2 demand to TKN.
NH3 + 2O2 à H+ + NO3– + H2O
One thing noticeable in above equation is when one mole of TKN is oxidized one mole of acid is produced. Since one mole acid is equivalent to 50 g alkalinity as CaCO3, the alkalinity consumption during nitrification becomes 3.57 g alkalinity/g TKN oxidized.
The nitrogenous O2 demand is calculated as carbonaceous O2 demand, where the amount of TKN embedded in biosolids is subtracted from the apparent TKN removal. One assumption is there is no significant ammonia stripping and this is justified since biological wastewater treatment is performed near neutral pH and the pH rarely exceeds 8. A factor of 4.57 g oxygen/g TKN is multiplied to convert TKN to oxygen demand.
|Nitrogenous O2 demand||= 4.57(Apparent TKN removed– TKN in excess biosolids)|
: amount of oxygen used to convert TKN to NO3-N (kg/m3 or g/L)
: TKN in influent (kg/m3 or g/L)
: TKN in effluent (kg/m3 or g/L)
: TKN content in biosolids (g TKN/g MLSS)
The original in ASM#1 (0.086 g N/g COD in Henze, 1987) is based on the biosolids mass converted to COD. Here, is converted to the value based on MLSS by multiplying the ratio of COD to MLSS (1.1), i.e. 0.095 g N/g MLSS.
3) Oxygen credit from denitrification
If dissolved oxygen is not sufficient and ORP is below -100mV, hetretrophs start to consume the combined oxygens taken from NO3-N since molecular oxygens are scarce. By definition, the amount of the combined oxygen consumed in this process is the amount of COD reduced and the amount of molecular oxygen saved in aeration tank. In the following equation, four nitrogen atoms in four NO3-N create 10 oxygen atoms while they are reduced. The ratio of oxygen to nitrogen is 2.86 g O2/g NO3-N.
4HNO3 à 2N2 + 2H2O + 5O2
It is also noticeable that one mole NO3-N creates one mole alkalinity, which is opposite from nitrification reaction. O2 credit from denitrification is described as the following equation, where the factor of 2.86 is multiplied to convert NO3-N to oxygen credit.
|O2 credit||= 2.86 (Apparent TKN removed – TKN in excess biosolids)|
All of above components can be added up and reorganized to obtain the total O2 demand, (kg/m3 or g/L), for biological wastewater treatment.
Specific oxygen demand (SOD) per volume is calculated as follow. SOD is conceptually a theoretical version of OUR.
One example of using above equations are shown here.
© Seong Hoon Yoon