Denitrification

Effect of ORP

Biological denitrification occurs when molecular oxygen (O2) is not sufficient for microbial respiration. Under this condition, the combined oxygen in nitrate (NO3-N) can be used as an oxygen source for hetrotrophs and the reduced nitrogen escapes as molecular nitrogen (N2) from mixed liquor.  The condition where denitrification occurs is called “anoxic” in order to distinguish it from “anaerobic” condition, where no oxygen sources exist whether it is molecular oxygen or combined oxygen.

Oxidation reduction potential (ORP) is used to monitor and control anoxic and anaerobic conditions. Once oxygen supply stops, dissolved molecular oxygen serves as an electron acceptor until ORP decreases to around +50mV according to Fig. 1. At +50mV, nitrate starts to serve as an electron donor until it depletes at the ORP of around -50mV. If there is no additional nitrate or oxygen supply, sulfate becomes an electron donor and ORP decreases below -50mV.

The exact ORP that triggers denitrification varies quite a bit depending on water chemistry especially pH. In addition, due to the low DO inside biological floc, denitrification can occur at relatively high ORP in bulk. Therefore, the ORP values in Fig. 1 must be considered as guidelines with uncertainties of ± 50-100 mV . For example, the triggering ORP for denitrification can varies between -100mV and 100 mV depending on situation.

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Fig. 1. Relation between ORP and metabolic processes (Goronsky, 1992)

Requirement of readily biodegradable COD/BOD

Denitrification requires electron donors to reduce the combined oxygen in nitrate. If methanol is used, the reaction can be written as follow, where 1.905 mg methanol is required to reduce 1 mg NO3-N (=5×32/6/14). If the methanol requirement is converted to COD requirement, it becomes 2.86 mg COD/mg NO3-N considering the conversion factor of 1.5 mg COD/mg methanol.

 

6 NO3  +  5 CH3OH 3N2 + 5 CO2 + 7 H2O + 6 OH
            Molecular weight    6×14(N only)    5 x 32

The HRT of anoxic tank cannot be prolonged too much due to the adverse effect on microorganisms at low DO/ORP condition. HRT of anoxic tank is typically 0.5-2 hours in municipal wastewater treatment. Due to the short duration of wastewater in anoxic tank, only readily biodegradable portion of COD (or BOD) can be utilized for denitrification. The approximate specific denitrification rates (SNDR) of some of the popular carbon sources are summarized in Table 1. Since denitrification is a fast reaction and the majority of hetrotrophs participate it, the partial activity loss due to low temperature and other non-ideal conditions is not a major concern.

Overall, denitrification rate is mainly controlled by the amount and the quality of readily biodegradable COD supplied to the anoxic tank. In general, BOD/TKN ratio in wastewater needs to be higher than 3, more preferably higher than 4, to obtain a good nitrification in biological nutrient removal (BNR) processes.

Table. 1. Typical denitrification rates for various carbon sources (Tchobanoglous, 2002)

Carbon source SNDR, g NO3-N/g VSS/d Temperature (oC)
Methanol 0.21-0.32 25
Ethanol 0.12-0.90 20
Wastewater 0.03-0.11 15-27
Endogenous metabolism 0.017-0.048 12-20

Following graph (Fig. 2) shows the specific denitrification rate (SNDR) as functions of F/M ratio in anoxic tank and the content of readily biodegradable BOD in feed wastewater to anoxic tank. If F/M ratio is 0.5 g BOD/g MLSS/d in anoxic tank and readily biodegradable BOD takes 30% of the total BOD, SNDR becomes  ~0.125 g NO3-N/g MLSS/d. Following procedure can be used to determine the required anoxic tank volume.

  • Assume HRT of anoxic tank and calculate the tank volume.
  • Calculate SNDR required to denitrify using the nitrogen load to the tank, MLSS, and the anoxic tank volume.
  • Calculate F/M ratio in anoxic tank and find out SNDR from Fig. 2.
  • If the calculated SNDR is less than the SNDR from the graph, the initial HRT assumption is validated. Otherwise, increase HRT and repeat the sequence.

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Fig. 2. Specific denitrification rate as function of F/M ratio and the ratio of readily biodegradable BOD to total BOD (Metcalf & Eddy, 2003).

 

Oxygen credit from denitrification

The oxygen demand in aeration tank decreases as much as the amount of COD treated in anoxic tank. More precisely, the amount of oxygen delivered from nitrate in anoxic tank, which is equal to the amount of COD consumed for denitrification, can be deducted from the total oxygen demand of wastewater. Therefore, if anoxic tank exists, aeration costs tend to decrease as opposed to the system without anoxic tank.

This oxygen credit is theoretically estimated at  2.86 g O2/g NO3-N removed. Since 4.57 mg O2 is consumed when 1 mg NH4-N is oxidized in theory, net oxygen consumption during the removal of 1 mg NH4-N is 1.71 mg O2/mg NH4-N removed assuming no cell production. In practice, however, oxygen demand for nitrification and oxygen credit from denitrification are assumed at 4.3 mg O2/mg NH4-N and 2.4 mg O2/mg NO3-N, respectively, considering the nitrogen loss to cell mass. Overall, net oxygen demand for nitrogen removal becomes around 1.9 mg O2/g NH4-N removed.

 

Alkalinity production from denitrification

Denitrification produces one mole of alkalinity (OH) as one mole of nitrate is removed. Since two moles of alkalinity is consumed to produce one mole of nitrate, net alkalinity consumption during the course of nitrification and denitrification is one mole per one mole of NH4-N (or TKN), where the net one mole alkalinity consumption is in fact the disappearance of NH4+ itself. The alkalinity production during the denitrification can help prevent pH from dropping excessively especially when influent TKN is high and wastewater alkalinity is low.

Table 2 summarizes the oxygen and alkalinity consumption and production during nitrification and denitrification.

 

Table 2. Oxygen and alkalinity consumption/production during nitrification and denitrification

Process O2 consumption Alkalinity consumption
Without cell production With cell production Without cell production With cell production
mol O2

/mol N

g O2

/g N

g O2

/g N

eqv. Alk

/mol N

g CaCO3

/g N

g CaCO3

/g N

Nitrification 2.0 4.57 4.3 2.0 7.14 6.8
Denitrification -1.25 -2.86 -2.4 -1.0 -3.57 -2.9
Overall 0.75 1.71 1.9 1.0 3.57 3.9

 

Simultaneous nitrification and denitrification (SNDN)

Denitrification can occur at oxygen rich environment due to the low DO in the center of floc or in the bottom of biofilm. Since oxygen must diffuse through microbial floc or biofilm while it is consumed by microorganisms, the steady state oxygen concentration near the center of the floc can be low enough to trigger denitrification (Kaempfer, 2000; Daigger, 2007). Fig. 3 shows a dissolved oxygen profile in a floc with 3.6 mm diameter measured by a micro-DO probe. However, growing biological floc to such size is not readily possible due to the turbulence in conventional MBR and the long SRT.

Graphics12345Fig. 3. Dissolved oxygen profile for 3.6 mm floc particle (Daigger, 2007)

A commercial process called NEREDA® is based on the technique that allows forming granular sludge with up to 4 mm diameter. Due to the large floc diameter, the crosssection of the floc is stratified as aerobic, anoxic, and anaerobic toward the center of the floc. Similar to sequencing batch reactor (SBR), NEREDA® is relying on repeated fill and draw, aeration, and settling, where anaerobic conditions develop during fill and draw and settling are . SNDN occurs mainly during aeration stage. Phosphorus accumulating organisms (PAO) growing in floc are enriched while biological floc are repeatedly exposed to aerobic and anaerobic conditions. Due to the high MLSS (10-12 g/L), high food to volume (F/V) ratio (0.8-1.05 g COD/L/d) is possible at low food to microorganisms (F/M) ratio at 0.05-0.06 g BOD/g MLSS/d (Keller, 2010). It has been claimed that 60-80% of nitrogen removal is readily achieved.

Denitr422222Fig. 4. Aerobic granules of NEREDA process (Bruin, 2000)

 

© Seong Hoon Yoon