Breakpoint Clorination Disertation

http://scholar.lib.vt.edu/theses/available/etd-042299-143911/unrestricted/thesis4.pdf

Research Objectives
Overall Objectives of this study were:
 Determine the process dynamics for real-time, continuous-flow breakpoint operations.
 Define design criteria for a full-scale breakpoint facility.
 Predict VPDES permit compliance.
 Address safety issues.

Phase I
Specific Objectives for Phase I were:
 Determine optimum chlorine to ammonia dose ratio.
 Determine optimum reaction pH for the process
 Determine reaction times and required detention times.
 Investigate analytical methods for reliable measurement of parameters required for control
of the breakpoint process.
 Determine optimum S02:Cl2 dose ratio for dechlorination.
Phase II
Specific Objectives for Phase II were:
 Determine effects of different influent water temperatures (8, 12 and 20 °C) on the
performance of the breakpoint reaction.
 Determine the effect of various influent ammonia concentrations (2.0, 6.0 and 11.0 mg/L
NH3-N) on the breakpoint reaction.
 Determine the effect of increased influent nitrite concentrations (~5.0 mg/L NO2-N) on the
dose ratios required for breakpoint.
 Determine the effect of increased influent organic nitrogen concentrations (~1.0 mg/L) on the
breakpoint reaction.
 Perform a special study to assess the potential for nitrogen trichloride formation during
breakpoint operation.
5
Literature Review
There has been limited research done on the breakpoint chlorination phenomenon. Initial research
occurred in the first half of the century when breakpoint was a hot topic and drew the attention of
researchers. The latter half of this century has seen little done in the way of research into breakpoint
chlorination. With the exception of a few papers, mostly plant specific studies have been performed.
There has, however, been a great deal of research done in the field of chlorine and chlorine-ammonia
chemistry, as well as advances in the fields of chlorine and ammonia analysis. While this chapter will
certainly review all obtainable research on breakpoint chlorination, it will also focus on research done
in the field of chlorine-ammonia chemistry, as it pertains to the breakpoint reaction.
Breakpoint Chlorination
The physical-chemical process of ammonia oxidation with chlorine has been practiced in the water
treatment field for over 50 years. As early as the 1920s superchlorination was used as a successful
means of controlling taste and odors in water treatment plants. In the 1930s an unexplained
phenomenon was being observed at water treatment plants using higher than normal chlorine dosages.
These events prompted research into the chlorination reactions occurring at water treatment plants.
Among the first researchers to explain these chlorine reactions Griffin (16) used the term breakpoint
to describe the point where chlorine and ammonia concentrations were simultaneously minimized.
6
The breakpoint reaction is defined as the chlorination of a water containing ammonia resulting in an
initial increase in combined chlorine residual, followed by a decrease in the combined chlorine residual
along with ammonia concentrations, followed by an increase in free chlorine residual and near
complete removal of ammonia as nitrogen gas. Fig. 1 shows a hypothetical breakpoint curve for a
water with a dose requirement of 9:1 Cl : NH3 (20). Initial research efforts into the mechanism of the
breakpoint reaction are attributed to Calvert (4), and later studies by Griffin and Chamberland (17),
and Rossum (34). Ensuing research by others (32,41,43) has led to an understanding of the
stoichiometry and kinetics associated with the breakpoint process. More recently, a comprehensive
study of the kinetics of breakpoint chlorination was performed by Saunier and Selleck (36). The goal
of their work was to develop a mathematical model, derived from laboratory observations, which
would provide “a rational basis for the design and operation of the breakpoint process in order to
achieve predictable ammonia removal”(36). Unfortunately, past research yielded little or no insight
into the problem of successfully controlling the breakpoint process in a full-scale wastewater
treatment plant. Although Saunier and Selleck (36) performed comprehensive pilot study work, the
results were never incorporated into a full-scale plant application. Pressley et al. (33) performed
extensive pilot study research in order to provide design criteria for a full scale breakpoint operation
at the Blue Plains wastewater treatment plant in Washington, D.C.. Atkins et al. (1) performed an
extensive pre-design pilot study to provide information for full- scale breakpoint operations at the
Owosso wastewater treatment plant in Michigan. The engineering firm of Camp, Dresser & McKee
(5) also performed bench scale
7
Figure 1. Theoretical breakpoint curve. (Zone 1 is associated with the reactions of chlorine and
ammonia to form Monochloramine; Zone 2 is associated with an increase in dichloramine and the
disappearance of NH3; Zone 3 is associated with the appearance of free chlorine after the
breakpoint)(44).
breakpoint testing before construction of full scale facilities at the Lower Potomac Pollution Control
Plant in Lorton Virginia. However, no matter how successful the pilot studies (1,5,33) were, or how
much information they yielded, none have been able to use the pilot information to successfully
control a full scale breakpoint facility.
8
Chlorine Chemistry
Chlorine Hydrolysis
Sodium hypochlorite hydrolyzes rapidly in water according to the following reaction:
NaOCl + H2O ® HOCl + NaOH (1-1)
The formation of HOCl via the above reaction is essential before the initiation of the breakpoint
reaction. Because hypochlorous acid is a weak acid it undergoes only partial dissociation as follows:
HOCl º H+ + OCl– (1-2)
At pH values between 6.5 and 8.5, the above reaction is incomplete and both species are present to
some degree. The extent of the above reaction can be estimated from the following equilibrium
expression:
(1-3)
Dissociation of HOCl in water has been measured by several investigators and has been shown to be
temperature dependant. The following are hydrolysis constant data published by Morris (27):
Table 1 Temperature Dependency of pKa for HOCl
Temperature °

Approach
Physical – chemical conditions which promote the formation of NCl3 during breakpoint are: 1) low
initial system pH and 2) Cl2 : NH3 dose ratios above 10 – 12:1. Under normal breakpoint pilot
operations these conditions are not satisfied and hence, there may not be enough NCl3 present in the
system to measure. In light of these circumstances a special reactor tank (see Figure 10) was used
to create conditions which favor the formation of NCl3. Since NCl3 is mostly insoluble in water, and
it readily volatilizes under turbulent conditions, it is very difficult to detect the compound in that
medium. Since no calibration standards are available for NCl3 it is also difficult to measure
quantitatively. However, the literature suggested a means of qualitative and quantitative analysis of
nitrogen trichloride by means of sample extraction with carbon tetrachloride (CCl4) followed by
sample analysis using UV scanning spectrophotometry (9). The NCl3 was extracted from the head
space of the reactor tank via a PYREX® gas washing bottle filled with a known volume of
spectrophotometric grade CCl4. Once extracted into the CCl4, nitrogen trichloride is in a soluble form
and can be measured with a U.V. spectrophotometer, with a fingerprint of primary and secondary
peaks at 265 & 345 nm respectively. The literature also suggests that quantitative results can be
obtained through a ratio determination of primary and secondary peak absorbance (9).
37
Experimental Methods
1. The pilot reactor was set up using the existing final effluent tank at the pilot study. This
provided the mixer and access ports needed to conduct the experiment. The tank was sealed with
duct tape and removable rubber stoppers to provide access to add chemicals. A sampling port was
provided for a grab analysis to assure operating guidelines (pH, initial ammonia concentration and
Cl2:NH3 dose ratio) were met.
38
2. The water in the reactor tank (Fig.10) was UOSA final filter effluent of a known volume. A
pre-determined mass of ammonium chloride (NH4Cl) was added resulting in a final ammonia
concentration of ~11.0 mg/L NH3-N. Diluted sulfuric acid was added for pH adjustment. Calculated
volumes of 5.25% sodium hypochlorite were added to reach the breakpoint at the varying dose ratios
desired.
3. The reaction proceeded in the sealed tank for approximately 24 hours with constant, vigorous
mixing. During this time the head space of the tank was pumped, via a 12V DC Cole-Parmer Model
#7530-25 diaphragm pump and Tygon® tubing, through a gas scrubber filled with a known volume
of CCl4. This allowed any existing NCl3 in the system to transfer from the aqueous phase (in water)
to the gaseous phase in the head space and finally back to the soluble phase in CCl4. The gas exiting
from the scrubber was recirculated back into the reactor for reprocessing through the system. This
allowed maximum transfer of any existing NCl3 into the CCl4.
4. At the end of the 24 hour period the CCl4 was collected and analyzed using a scanning UV
spectrophotometer at wavelengths between 200 and 400 nm. The primary and secondary peaks of
NCl3 in CCl4 occur at 265 and 345 nm, respectively.
Results and Discussion
Batch testing was performed over a period of several days using several different Cl2:NH3 dose ratios
and different system pHs. While the results (Figure 11 and Table 4) show an increase in NCl3 as the
pH decreased and Cl2:NH3 dose ratio was increased, production of NCl3 appears to be more
dependant on low system pHs (<6.0) than higher dose ratios (>10:1). The most severe NCl3
39
formation appears when operating with both low system pH and a high Cl2:NH3 dose rate.
Operational data can be found in Appendix D.
UOSA currently uses NaOCl for disinfection and will use it for Breakpoint operations, low pH levels
are not a major concern. It is however, recommended that NaOH feed be available for situations
where high ammonia concentrations are encountered, to avoid any possibility of NCl3 generation.
Conclusions
Based on results from the batch testing it is recommended that the breakpoint facility be operated at
a high pH (>7.0 after 30 minutes detention) and low initial Cl2:NH3 dose ratios (<10.0, depending
on influent water quality) to prevent the formation of high concentrations of NCl3.

Phase I. Summary and Conclusions
Pilot Operations
· All pHs provided acceptable operation during steady state operation.
· Slower reaction rates were observed at lower pHs.
· Lower pHs were associated with less stable breakpoint operations.
· Alkalinity consumption was observed close to being stoichiometric.
· Chlorine addition in excess of the Cl2 demand plus the stoichiometric amounts required to reach
the breakpoint resulted in a corresponding decline in ammonia removal efficiencies.
· Data from pilot operation at higher pHs (7.5 & 8.0) showed :
§ lower 30 minute free and total Cl2 residuals,
§ lower effluent NH3 and TKN concentrations,
§ Cl2:NH3 closer to stoichiometric,
§ final effluent pH closer to neutrality (i.e. pH ~7.0).
Laboratory Data
· COD removal was observed through breakpoint.
· Minimal NO3
– production was observed during the breakpoint reaction.
· Total Solids increased through the breakpoint process.
· No Fecal Coliforms were detected after 30 minutes contact time in the pilot.
79
DAS Data
· Loss of breakpoint was observed more frequently at lower operating pHs.
· Loss of breakpoint at higher pHs was not as severe (easier to get back).
· The ability to run at lower free and total chlorine residuals was observed as operating pHs
increased from 7.0 to 8.0.
· Numerous unexplained perturbations occurred at pH 7.0, causing loss of breakpoint.
· A more stable breakpoint process was occurred at pH 8.0.
DBP Data
· Consistent increases in Chloroform through breakpoint, though not to levels that would
jeopardize drinking water sources.
· Creating Total Organic Halides through breakpoint, again, not to levels that would jeopardize
drinking water sources.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: