Dr. Darrin Lew

Breakpoint Chlorination For Removing Ammonia

Chlorination can be used to remove dissolved ammonia and ammonium ion from wastewater by the chemical reactions

Ammonia is converted stoichiometrically to monochloramine (NH2Cl) at a 1 to 1 molar ratio or a 5 to 1 ratio by weight of Cl2 to NH3-N. NHCl2 (dichloramine), and NCl3 (nitrogen trichloride or

TABLE 6.1

Suggested Maximum Parameter Levels in Water Used for Crop Irrigation"

General Problem

Salinity

Parameter Units total dissolved solids (TDS) mg/L specific conductivity mS/cm

Suggested Maximum Value

450 700

Water Infiltration Specific Ion Toxicity

chloride (Cl) mg/L 100

boron (B) mg/L 1-3c Trace Elements'1

manganese (Mn) mg/L 0.20

molybdenum (Mo) mg/L 0.01

a Based on data from "Water Quality for Agriculture," FAO Irrigation and Drainage Paper No. 29, Rev. 1, Food and Agriculture Organization of the United Nations, 1986, and Colorado water quality standards for agricultural uses.

b Depends on salinity. At given SAR, infiltration rate increases as water salinity increases. c Depends on sensitivity of crop.

d Suggested maximum value is for a water application rate consistent with good agricultural practice (about 10,000 m3/year). Toxicity and suggested maximum value depend strongly on the crop. Trace elements normally are not monitored unless a problem is expected. Several trace elements are essential nutrients in low concentrations.

trichloramine) may also be formed, depending on small excesses of chlorine and pH. Further addition of chlorine leads to conversion of chloramines to nitrogen gas. The reaction for conversion of monochloramine is

The overall reaction for complete nitrification of ammonia by chlorine oxidation is

Equation 6.8 is theoretically complete at a molar ratio of 3 to 2 and a weight ratio of 7.6 to 1 of Cl2 to NH3-N. This process is called breakpoint chlorination. The reaction is very fast and both ionized (NH4+) and unionized (NH3) forms of ammonia are removed.

TABLE 6.2

Water Parameter Levels of Potential Concern for Crop Irrigation"

Crop Growing Restrictions

TABLE 6.2

Water Parameter Levels of Potential Concern for Crop Irrigation"

Crop Growing Restrictions

Restriction Cause	Parameter Value	Degree of Restriction
Chloride toxicity (surface irrigation)8	less than 142 mg/L	none
	between 142 and 355 mg/L	moderate
	greater than 355 mg/L	severe
Chloride toxicity (sprinkler irrigation)c	less than 107 mg/L	none
	greater than 107 mg/L	moderate
Sodium toxicity (surface irrigation)'	less than 69 mg/L	none
	between 69 and 207 mg/L	moderate
	greater than 207 mg/L	severe
Sodium toxicity (sprinkler irrigation)c	less than 69 mg/L	none
	greater than 69 mg/L	moderate
Sodium absorption ratiod	SAR less than 3	none
	SAR between 3 and 9	moderate
	SAR greater than 9	severe
Nitratee	less than 5 mg/L	none
	between 5 and 12 mg/L	slight
	between 12 and 30 mg/L	moderate
	greater than 30 mg/L	severe

a Based on data from "Water Quality for Agriculture," FAO Irrigation and Drainage Paper No. 29, Rev. 1, Food and Agriculture Organization of the United Nations, 1986, and Colorado water quality standards for agricultural uses.

b With surface irrigation, sodium and chloride ions are absorbed with water through plant roots. They move with the transpiration stream and accumulate in the leaves where leaf burn and drying may result. Most tree crops and woody plants are sensitive to sodium and chloride toxicity. Most annual plants are not sensitive.

c With sprinkler irrigation, toxic sodium and chloride ions can be absorbed directly into the plant through leaves wetted by the sprinkler water. Direct leaf absorption speeds the rate of accumulation of toxic ions. d SAR values greater than 3.0 may reduce soil permeability and restrict the availability of water to plant roots. e NO3 levels greater than 5 mg/L may cause excessive growth, weakening grain stalks and affecting production of sensitive crops (e.g., sugar beets, grapes, apricots, citrus, avocados, etc.). Grazing animals may be harmed by pasturing where NO3 levels are high.

Rules of Thumb

1. The rate of ammonia removal is most rapid at pH = 8.3.

2. The rate decreases at higher and lower pH. Since the reactions lower the pH, additional alkalinity as lime might be needed if [NH3] > 15 mg/L. Add alkalinity as CaCO3 in a weight ratio of about 11 to 1 of CaCO3 to NH3-N.

3. Rate also decreases at temperatures below 30°C.

4. The chlorine "breakpoint," (see Figure 6.2) occurs theoretically at a Cl2:NH3-N weight ratio of 7.6.

5. In actual practice, ratios of 10:1 to 15:1 may be needed if oxidizable substances other than NH3 are present (such as Fe2+, Mn2+, S2-, and organics).

FIGURE 6.2 Breakpoint chlorination curves showing removal of ammonia from wastewater. Region A: Easily oxidizable substances such as Fe2+, H2S, and organic matter react. Ammonia reacts to form chloramines. Organics react to form chloro-organic compounds. Region B: Adding more chlorine oxidizes chloramines to N2O and N2. At the breakpoint, virtually all chloramines and a large part of chloro-organics have been oxidized. Region C: Further addition of chlorine results in a free residual of HOCl and OCl-.

0123456789 Weight ratio of CI2/NH3-N

FIGURE 6.2 Breakpoint chlorination curves showing removal of ammonia from wastewater. Region A: Easily oxidizable substances such as Fe2+, H2S, and organic matter react. Ammonia reacts to form chloramines. Organics react to form chloro-organic compounds. Region B: Adding more chlorine oxidizes chloramines to N2O and N2. At the breakpoint, virtually all chloramines and a large part of chloro-organics have been oxidized. Region C: Further addition of chlorine results in a free residual of HOCl and OCl-.

Example 6.2: Calculate the Chlorine Needed to Remove Ammonia

A waste treatment plant handles 1,500,000 L/day of sewage that contains an average of 50 mg/L of NH3-N. How many grams of Cl2(aq) must be present daily in the wastewater to remove all of the ammonia?

Answer: By equation 6.8, 3 moles of chlorine are needed for every 2 moles of ammonia nitrogen.

Molecular weights are

Thus, the stoichiometric weight ratio is 213/28 = 7.6 g Cl2 per gram of N (as ammonia). One mole of NH3 contains 14 g of N and 3 g of H. Thus, 50 mg/L of NH3 contains 14/17 x 50 mg/L = 41.2 mg/L of N. In 1,500,000 L there will be

1,500,000 L x 41.2 mg/L = 61,800,000 mg N, or 61,800 g N/day.

The theoretical amount of chlorine required is

7 6 g °2 x 61,800 g N = 470 kg Cl2/day, or about 1036 lb/day. 1 g N

Depending on the quantity of other oxidizable substances in the wastewater, the plant operator should be prepared to use up to twice this amount of chlorine.