Nitrogen Removal During Wastewater Treatment

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Nitrogen-containing substances in wastewater are inorganic or organic. Together with phosphates, they represent the main source for eutrophication of surface water. For this reason they must be eliminated together with the organic carbon during wastewater treatment. Whereas phosphates form insoluble precipitates with many heavy metal ions and the precipitates can be separated by sedimentation or flotation, all nitrogen compounds, except for magnesium ammonium phosphate, are easily soluble in water and thus cannot be removed chemically by precipitation. For biological removal of amino nitrogen and of heterocyclic nitrogen compounds, their conversion to ammonia in an aerobic or anaerobic treatment process is the first step. Then ammonia must be nitrified, and the nitrate denitrified to yield nitrogen. Thus, depending on the kind of nitrogen compounds present in wastewater, nitrogen removal requires up to three processes in sequence: ammonification, nitrification, and denitrification.

The major portion of nitrogen compounds in municipal wastewater are reduced nitrogen compounds such as ammonia, urea, amines, amino acids, and proteins. Oxidized nitrogen compounds such as nitrate and nitrite normally are not present at all or not in relevant amounts. Nitrate and nitrite may, however, represent the main nitrogen load in wastewater of certain food or metal industry branches (Gen-sicke et al., 1998; Zayed and Winter, 1998).

Table 1.5 Amount of biogas, biogas composition, and energy content.

Substrate

Amount

Composition

Energy content

(cm3 g-1)

% ch4

% CO2

(kWh x m-3)d

Carbohydrates3

746.7

50

50

4.95

Fatsb

1434

71

29

7.02

Proteinsc

636

60

40

5.93

a Calculated for hexoses.

b Calculated for triglycerides containing glycerol plus 3 mol palmitic acid. c Calculated for polyalanine and reaction of ammonia to (NH4)2CO3. d at standard conditions a Calculated for hexoses.

b Calculated for triglycerides containing glycerol plus 3 mol palmitic acid. c Calculated for polyalanine and reaction of ammonia to (NH4)2CO3. d at standard conditions

The ammonia in raw municipal wastewater is mainly derived from urine and is formed in the sewer system by enzymatic cleavage of urea by ureases (Eq. 13):

The residence time of the wastewater in the sewer system normally is not long enough for a significant contribution of ammonia from other sources, e.g., proteolysis and deamination of amino acids.

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