EVALUATION OF CHLORINE EMISSION POTENTIAL AT THE NEW DEER ISLAND WASTEWATER TREATMENT PLANT

Somnath Basu1, Kenneth A. Shilinski', and Irvine W. Wei'

Presented at the

The Future Uses of Chlorine:

Issues in Education, Research, and Policy Conference

Massachusetts Institute of Technology

Cambridge, Massachusetts, 1996

1. Doctoral Candidate, Dept. of Civil & Environmental Engineering, Northeastern University, Boston, MA . 02115 2. Senior Program Manager, Deer Island Treatment Plant, MWRA, P.O. Box 100, Winthrop, MA. 02152, Tel# (617)539-4164, and the Presenting Author.

3 . Associate Professor', Dept. of Civil & Environmental Engineering, Northeastern University, Boston, MA . 02115

EVALUATION OF CHLORINE EMISSION POTENTIAL AT THE NEW DEER ISLAND TREATMENT PLANT

Somnath Basu\ Kenneth B. Shilinskf, and Irvine W. Wei3

1. Doctoral Candidate, Dept. of Civil & Environmental Engineering, Northeastern University, Boston, MA 02115

2. Senior Program Manager, Deer Island Treatment Plant, MWRA, Winthrop, P.O. Box 100, MA 02152, Tel# (617) 539-4164, and the Presenting Author

3. Associate Professor, Dept. of Civil & Environmental Engineering, Northeastern University, Boston, MA 02115

Abstract

The Massachusetts Water Resources Authority (MWRA) is currently constructing a 1,200 million gallon per day secondary wastewater treatment facility cin Deer Island, in Boston Harbor. Chlorine, in the form of sodium hypochlorite solution, is purchased and barged to Deer Island for use in both effluent disinfection and odor scrubbing. The Process Engineering Department of Deer Island, as part of its on-going cost effectiveness program, is currently evaluating on-site generation of sodium hypochlorite. The technical analysis includes a thorough examination of the chemistry of production and storage, including potential losses to atmosphere. This paper presents an overview of that examination, defines degradation pathways, and quantifies, to the extent necessary, potential emission levels.

Total daily 15% sodium hypochlorite usage, which currently peaks above 40,000 gallons per day (gpd) in the summer, is projected to decrease to a yearly average of about 14,000 gpd at full secondary treatment operation. While the majority of product is used for effluent disinfection, a sigrnficant amount is used to oxidize odorous off-gases from sealed treatment processes. Pollution potential from chlorine entering Boston Harbor will be minimized by the reduction reaction time within the 9.5 mile outfall. In addition, facilities exist to dechlorinate residual chlorine with sodium bisulfite solution. All odor scrubbing chlorine is reacted to salt and returned to the head of the plant.

One of the on-site generation options under study is an integrated caustic-chlorine and sodium hypochlorite process. A membrane cell can be used to produce caustic soda, elemental chlorine, and hydrogen gas from brine. The liquid caustic and chlorine gas are then reacted in a separate hypochlorite forming chamber. The risk of gaseous emissions from this integrated process is small because of the immediate reaction of gaseous chlorine after formation. Emissions from sodium hypochlorite storage are possible from simple dissociation; the majority occurring during tank fills. This quantity, however, was found to be insigniftcant. A larger degradation potential exists if the storage pH were to drop below 6, due to some unknown contamination of tank contents by acid, or by ingress of atmospheric carbon dioxide. Deer Island also experienced storage degradation from metal ion contamination, from wetted instrumentation within the tanks; the principal degradation products being chlorate and chloride ions.

Introduction

The Deer Island Treatment Plant (DITP) of the Massachusetts Water Resources Authority

(MWRA) is currently undergoing a major construction project. A new secondary wastewater

treatment facility is being built that will be coupled with the recently completed primary

treatment phase. The full build-out facility, projected for completion in 1999, will be able to

meet Clean Water Act effluent standards for up to 1.2 billion gallons per day. Deer Island

currently uses in excess of 40,000 gallons of 15% sodium hypochlorite solution for disinfection

of its primary effluent prior to discharge into Boston Harbor. This figure is expected to decrease

to about 15,000 gallons per day when the full beneftt of a secondary effluent is realized. In

addition, Deer Island has three of its odor control sites on-line. Sulfur based off-gases, from

sealed wastewater treatment unit operations, are oxidized in countercurrent wet scrubbers.

Chemicals have a major budgetary impact at wastewater treatment facilities. The Process

Control Engineeting Department at Deer Island, as part of its ongoing cost effectiveness

program, is evaluating on-site generation of sodium hypochlorite verses the current practice of

purchasing barged shipments. The analysis in this paper is part of the technical evaluation on air

emissions of chlorine from both on-site generation and storage of sodium hypochlorite.

Sodium Hypochlorite in Wastewater Industry

Sodium hypochlorite (NaOCl) exists only as an aqueous solution, also referred to as "bleach". It

is a strong oxidant and is commonly used as a disinfectant in the water and wastewater industry.

Hydrolysis and ionization result in two major chemical species, hypochlorous acid (HOC!) and

the hypochlorite ion (OCI"). The use of sodium hypochlorite as a disinfectant is increasing

because of public concerns with respect to the safety of transport and storage of elemental

chlorine liquid/gas. Wastewater plants have historically used hyarolyzed gaseous chlorine as the

base chemical for effluent disinfection. Sodium hypochlorite also lacks the trihalomethane

(TBM) formation potential of elemental chlorine.

Page 1

Concentration of sodium hypochlorite solution is expressed as a Trade Percent. For example,

15% Trade sodium hypochlorite means:

Sodium hypochlorite solution is typically stored on Deer Island in storage tanks. The quantities

required for disinfection are supplied by pumping from the tanks directly to the application

point. Storage in tanks for several weeks between· fresh refills is not uncommon. The most

important chemical property of sodium hypochlorite that concerns a treatment plant operator is

its stability. The stability of sodium hypochlorite solutions, and measures that can enhance its

stability, are discussed in detail in the following section.

Sodium Hypochlorite Dissociation Chemistry

Sodium hypochlorite is an unstable chemical. Shipment and storage are two variables that may

be controllable and are fundamental to any decision on on-site generation. The dissociation

chemistry has been investigated extensively by Gordon et. a! (1995). A brief review of their

work is presented here. Sodium hypochlorite is an unstable chemical whose degradation is time

dependent. Degradation also increases with increasing concentration, increasing temperature,

and decreasing pH. Degradation reactions are catalyzed by dissolved metal ions (e.g. nickel,

copper, iron, etc.) and UV radiation. This results in increased chemical costs as more product is

required to achieve the necessary disinfection level. Under ce1iain circumstances there is even a

remote potential of the release of chlorine gas to the atmosphere.

Page 2

Being the salt of an alkali metal, sodium hypochlorite exists in the almost completely ionized

forms of sodium ions (Na+) and hypochlorite ions (OCI-), in the aqueous phase. In water,

hypochlorite ions exist in equilibrium with hypochlorous acid according to the following

equation:

, OCl- + H\� pK = 7.5 (Morris, 1966) (1)

Figure 1 represents the distribution of hypochlorous acid and hypochlorite ions as a function of

pH. It is evident that above a pH value of 11 almost all chlorine exist as hypochlorite ions.

Below pH 10, reaction (1) starts shifting to the right. Subsequently, hypochlorous acid reacts

fi.Jrther with hypochlorite ions liberating more hydrogen ions. and rendering the solution more

and more acidic with time, according to reaction (2).

2HOCl + OCl- ---> CIO,-+ 2 Cl- +2 H+ (2)

In order to maintain the stability of stored aqueous sodium hypochlorite, it is necessary to

maintain the product above pH 11. This is done by adding an excess amount sodium hydroxide

to the sodium hypochlorite solution.

"<> � fHOCij fOCI-J Ct,OCI o:, "" CT,OCI

1.0 " il 0

.:} 0 0.5 c .2 g

.;: 0 2 4 10 12 14

pH· pK0="'7,5

Figure 1

Distribution diagrams for HOC! and OCI- at 25°C (Snoeyink and Jenkins, 1980).

Page 3

Decomposition also takes place even above pH of 11. Under this condition, the major

decomposition pathway consists of a set of two-step reactions, as follows:

2 oct----> c1o2- + cl-

and ocl- + Cl02- ---> Cl03- + Cl-

overall

(3)

(4)

(5)

The first step is fast while the second one is the rate determining step. Therefore, while the

chlorate (Cl03-) and chloride (Cl-) ions are the major decomposition products, some minor

amount of chlorite (Cl02-) ions also exist in the system. The overall reaction is second order

with respect to hypochlorite ions; The rate of reaction can be expressed as:

d[OCl-]

------------ - k [OCl-]2

dt

(6)

From (6) it is obvious that the rate increases in geometric proportion with concentration .

. Therefore, it is, generally not advisable to store sodium hypochlorite solutions at concentrations

above 15%. However, some amount of concentration is necessary to save storage space. The

optimum is 10%, as determined by US Army (White, 1986). Figure 2 demonstrates the effect of ·concentration of sodium hypochlorite on the rate of degradation of the solution.

The reaction rate constant (k) is a strong function of temperature according to the Arhenius

equation, k = A exp( -E/RT). Thus, the rate of decomposition of sodium hypochlorite increases

exponentially with storage temperature. Avoidance of hot ambient storage conditions is

therefore important in maintaining sodium hypochlorite stability. Care should also be taken

during the treatment plant design stage in deciding the color of the exterior surface of storage

tanks, which should be perfect reflectors, not absorbers, of heat. "

Page 4

E-·? - ,

::,· 160 "' � w z "' 0 -' :r: u w -' "' 3 ::;: �

60 5%

40 20

TRADE% {vol) = <J!L AVAILABLE CHlORINJ.: 10

12.5% HYPO SOLUTION; A RANDOM DEliVERY

CHECK AT A WWTP USING 7400 lb/doy / / AVAILABLE CHLORINE.

V'o / -o/�

// /

CLOROMAT ON·SITE HYPO SO LUi/ON

40 60 80 100 120 140 . 160 DAYS

Figure 2

Decay Rate of Sodium Hypochlorite Solutions (White, 1986)

Along with the reactions described by equations (3), (4), and (5), a very slow side reaction also

takes place following the minor pathway (7). ·

20Cl- ---> 2 Cl- + 02 T (7)

Presence of metals, dissolved or.in direct contact with sodium hypochlorite solutions, catalyze

degradation. At the present time the predominant catalytic mechanism (homogeneous versus

heterogeneous) is unknown. Interestingly, the catalytic kinetics support the reaction (7) as the

major pathway over the reactions represented by equations (3), (4), and (5). Dissolved copper

ions exhibit catalytic activity. Nickel ions are almost ten times as active as copper. While

manganese ions alone do not act as catalyst they exert significant synergistic effect on nickel

ions, when present together. Thus, it is necessary to avoid contamination of sodium hypochlorite

solution by metals in any form during the manufacturing process, or during transport and

storage.

Page 5

Exposure of sodium hypochlorite to UV radiation also catalyzes its decomposition. Reaction (7)

is the dominant pathway for dissociation aided by UV exposure. While it is not conceivable that

stored sodium hypochlorite solution will be exposed to sunlight in a treatment plant, in smaller

scale operations the container material should be opaque, e.g. amber colored glass, etc.

Sodium Hypochlorite Usage at Deer Island

The Deer Island wastewater treatment facility currently serves the northern communities of the

metropolitan Boston area. It is scheduled to receive flow from the southem portion in 1997. f;�<t<f; f"J When completed in 1999, the new Deer Island will treat all of the flow from the MWRA's 2.5

''· million metropolitan Boston customers, to secondary effluent st&ndards.

Deer Island converted to sodium hypochlorite disinfection in August of 1991. Based on a 15%

solution, the usage for disinfection since the beginning of the new primary facility start-up, and

the projected usage through 1999, is outlined in Table 1. Chemical dosage rates will decline as

level of treatment increases.

Table 1

Sodium Hypochlorite (15% Solution) Usage at Deer Island

Start Wastewater Disinfectant DailyNaOCI

Month/Year Flow Dosage Consumption

(MGD) (ppm) (Gallons)

12/94 270 15 28,000

7/96 390 15 40 000

9/96 390 8 21,000

12/99 390 5 14,000

Page 6

One of the fundamental design criteria for the new treatment facility was to minimize process

off-gases for reasons including potential deleterious effects on worker health, concerns of odors

(neighborhoods of the Community of Winthrop are in close proximity to Deer Island), chemical

degradation of plant equipment, and harmful environmental effects of air pollutants. As a result

of this criteria, all off-gas treatment areas are enclosed and the gases treated pri?r to stack

emission. Both odor and combustion emissions are regulated by the Massachusetts Department

of Environmental Protection. Off-gas treatment unit operations include wet scrubbing and/or

activated carbon adsorption.

Wastewater odors are primarily the result of anaerobic biological decomposition of organics.

The most abundant chemical of concern is sulfur, particularly in the reduced form of hydrogen

sulfide gas. In January 1995, along with the new primary treatment facility, Deer Island began

usage of 15% sodium hypochlorite solution in odor processing. Currently three of its eventual

five off-gas processing areas are in operation. All three facilities use countercurrent wet

scrubbers for H2S treatment. The residuals facility also has wet scrubbing for ammonia removal.

H2S scrubbing at Deer Island is a two step process, the first of which is pH adjustment using

sodium hydroxide. Sulfi1r is forced to distribute to the soluble HS· and S2· forms above pH 10, as

indicated in Figure 3.

Figure 3

" Distribution diagram for H2S (Snoeyink and Jenkins, 1980).

1.0 r---

0 0.5 c .2 g

u.

Page 7

The oxidant sodium hypochlorite is added to an adjusted oxidation reduction potential (ORP) of

about + 700 mv. The oxidized sulfur salt is continuously removed by blow-down. When fully

operational, the combined sodium hypochlorite solution ( 15%) usage will be about 1, 600 gpd.

The i\>IWRA currently purchases 15% sodium hypochlorite for delivery to Deer Island by barge.

Barge capacity is approximately 375,000 gallons. Median barge shipping time is two days, but

increases considerably with tnclimate�ea conditions. ' ;, c& {1,"")'-

During the last year of operation, Deer island experienced consumption of 15% Sodium

Hypochlorite solution at a much higher rate than expected. Reduced sulfur off-gases were

higher than projected during the warmer months of 1995, and will result in a redesign and

upgrade of the scrubber chemical feed system. Interim disinfection modes during the

construction phase seem to use more product than steady state operation. This portion is a spike - -1 U/111.1'.1.

that is considered short term until the disinfection basins and new outral�come on-line. The

third unexpected draw on sodium hypochlorite is degradation of the product itself.

Deer Island has experienced decreases in storage concentration over the last year. During the

months of August and September 1995, usage increased at an alanning rate. The Process

Engineering Department of Deer Island Operations tracked down the major cause to metal

contamination, from instrumentation within {f'�nks and to high ambient temperature. As

shown in Figure 4, the concentration dropped from more than 12% to less than 3% over the

· course of three to five days. The wetted instrumentation devices, made of 316 SS and

Hastealloy C, were removed and are now under redesign:

After taking these measures, the degradation decreased significantly as shown in Figure 5.

Degradation from metal ions remains a potential problem because of the small levels of ions

needed for reaction and the many potential sources, from the manufacturing process itself all the

way to product pumping and storage on-site. Deer Island pays for product concentration as

. assayed on debarging. Potential degradations prior to arrival at Deer Island are not addressed in

this paper.

Page 8

..

----------�� 4 ( tvr'!4 --:-:�� J Sodium Hypochlorite"Metal Contamination�radatio�

-20

c .Q 2 15 0 (/) .!" 2 � 10 £ " 0 Q_ >. :r: c " 5 E "

0..

,. Tank #1 (9/1/95-9/6/95)

+Tank #2 (8/16/95-8/18/95)

;.. Tank #2 (8/25/95-8/28/95)

OL_--------------------�--------_j 0 2 4 6

Number of Days Degrading

Figure 5

Sodium Hypochlorite Degradation with Metal Removed

c 0 :.;::; ::J 0 (f) c (j) -<-->

·c 0 ..c 0 0 Q >. I -<--> c (j) 0 ,___ (j) [L

21 -==]1 I

19 � lil Tank #1 (3/15/96-4/11196)

+Tank #2 (3/15/96-4/13/96) m

17 �\ . v��� 1s - 1 \t m. - ��� m �

13 0 10 20 30 Number of Days Degrading

Page9

< <

Estimates of Chlorine Pollution Potential at Deer Island

From Storage Tanks

A recent bleach analysis by the current Deer Island supplier (Kuehne, 1996) yielded the

following sodium hypochlorite composition:

(ocn 2.11M

Wt% NaOCl 13.02

Specific Gravity 1.210 g/mL

[NaOH] 0.0125M

[N<l:2C03) 0.0094M

[en 2.11M

The theoretical annual emission potential from storage tank venting is 42.2 x 10·10 pounds of

chlorine gas to atmosphere (Calculation in Appendix). The sodium hydroxide concentration

corresponds to a pH of 12.1. This is the typical pH maintained for sodium hypochlorite solution

during shipping, storage on site, and to the point of application. Thus, the chlorine emission

potential is suppressed to almost zero by maintaining the pH of the sodium hypochlorite

solution.

From Effluent Disinfection I" �/J!A--The concern over the potential harmful effect ofJree residual sGdium hyf'®ehlemte in the final

blofA- . effluent, on marine b.ielogy, has been properly addressed in the design by including a nine mile

long effluent tunnel into the ocean. A modeling effort, performed during the design phase,

indicated that at that distance there would be enough turbulence in the ocean to effectively

disperse and dilute any residual sg�x:ite. Moreover, during its long residence in the

effluent tunnel, the residual (after disinfection) a��g..sel.J>ti® will react further

with the residual organic chemicals left untreated in the secondary reactors. Th�se measu�e� 'Yill . ·f-et.. �d«� e./J(fr03-____-

ensure that the marine bio� will not be exposed to any shock loads of s.culitt�eehlor-ite.

As a back-up, a dechlorination system has also been provided to neutralize any residual.se4ium--, ()/._em'"'" �ehl®rite at the final point of discharge from the effluent tunnel.

Page 10

From Odor Control Processes

· As mentioned before, the odor control process consists of chemical reactions conducted at high

pH. The addition of sodium hydroxide (caustic soda) maintains the required level of pH. The

reactions result in continuous consumption of both sodium hypochlorite and sodium hydroxide ..

Under the reacting conditions in the scrubbers, the only transformation hypochlorite ions go

through is the conversion into chloride ions, as shown below.

At High pH,

At High pH,

2 NaOH ---> 2 Na+ + 2 OR

H2S ---> 2 H+ + S2·

4 NaOCl --->4 Na++ 4 OCl-

(8)

(9)

(10)

4 OCl- + S2· --->SOt ! + 4 c1·1 (11)

2 W + 2 OR---> 2 H20 (12)

Overall Reaction: 4 NaOCl + H2S+ 2 NaOH ---> 4 NaCl I + Na2S04 I + 2 H20 (13)

When the solubility product is exceeded the chloride ions crystallize in the form of sodium

. chloride (salt) particles. Build-up of excess sodium chloride and sodium sulfate particles in the

scrubber liquid is continuously purged out of the system as blowdown. Thus, the stack gas does

not contain any free chlorine, nor does the liquid discharge from scrubbers (blowdown) consist

of any unreacted sodium hypochlorite. �� Hfti,;._j �

On-Site Generation Option

The option of generating sodium hypochlorite on-site at Deer Island is being investigated by the

Process Control Department at Deer Island. Factors under consideration include the following.

* Cost of purchasing product vs. costs of construction and O&M at Deer Island.

* Savings in degradation cost in storage. On-site storage reduced to 1-2 days.

* Avoidance of overland "topping-off' contracts and their associated shipping risks.

* Greater control over availability of product�

* Reduction of risks associated with the ocean transport of a hazardous chemical,

including the air pollutant potential of gaseous chlorine.

Page 11

Manufacture of Sodium Hypochlorite

The simplest and most inexpensive process is the direct reaction of chlorine with sodium

hydroxide (caustic soda) by the following equation:

Cl2 + 2 NaOH ---> NaOCI + NaCI + H20 (14)

This is accomplished by bubbling gaseous chlorine through an agitated caustic soda solution to

generate an equimolar solution of sodium hypochlorite and sodium chloride. The final pH

should be controlled at 12.0.

The concentration of sodium hypochlorite solution is also expressed in terms of available · chlorine as follows:

Molecular weight of chlorine gas = 71

Molecular weight of sodium hypochlorite = 74 .5

In the above described reaction, one molecule of chlorine produces one molecule of sodium

hypochlorite. Therefore, a 71 1b quantity of chlorine produces 74.5 lbs of sodium hypochlorite .

Thus lib or"sodium hypochlorite is equivalent to 71/74 .5, or 0.95 lbs of available chlorine.

In addition to the ways shown earlier, another way of expressing concentration of sodium

hypochlorite solution is in terms of pounds of available chlorine per gallon of solution. On an

equivalent basis:

15% Trade NaOCl Solution= !50 gms!L of NaOCl Solution

= 150 x 0.95 , or 142.2 gms!L Available Chlorine

= 1.2516 lbs of NaOCl per gallon of Solution

= 1.2516 x 0.95, or (approx.) 1.20 lbs of Available Chlorine per Gallon of Solution.

Page 12

Caustic Soda-· Chlorine Generation Process

The basic process of caustic - chlorine generation is by electrolysis of brine . The stepwise

reactions taking place in the electrolytic cell are:

At Anode,

At Cathode,

In Liquid,

NaCl ---> Na+ + Cl"

HP ---> W + OR

c1· - e ---> 1/2 Cl2

H+ + e ---> 1/2 H2

Na+ +OR---> NaOH (aqueous solution)

(15)

(16)

(17)

(18)

( 19)

The final products are caustic soda solution, chlorine gas, and hydrogen gas.

There are three main types of caustic-chlorine generators. In the mercury cell, mercury acts as

the medium of transport of sodium ions from the anodic to the cathodic region. It is the oldest

method but produces the strongest solution (50%). The mercury cell is an electric energy

intensive process, and causes severe air and water pollution problems due to mercury

contamination.

The most commonly used caustic-chlorine generation process in the United States is the

diaphragm cell, in which anodic and cathodic cell regions are separated by a diaphragm. The '

diaphragm cell is a high consumer of electricity. This method is cost efficient for large

generators, especially the caustic-chlorine industry.

The newest method is the membrane cell which also has separate anodic and cathodic regions. · The membrane cell is also a large consumer of electric power, but its energy efficiency

approaches the theoretical limits and hence is the most energy efficient of the three methods .

This proces�fficiency approaches 100%, as it permits cations (sodium ions) but no anions ,.---·---

(chloride and hydroxide) through the membrane. The membrane cell is non-polluting and allows

for flexibility in production quantities. The most practical option for Deer Island appears to be

the membrane cell . The principle of operation of an ideal membrane cell is demonstrated in

Figure 6 .

Page 13

Principle of Operation of an Ideal Membrane Cell

Cathode H

Gel-

Cation Membrane

OH- 0H+ GNa+ Figure 6 Page 14

. .

Integrated Caustic - Chlorine and Sodium Hypochlorite Generator

The integrated process involves reacting the caustic soda with chlorine gas as they are produced,

according to equation (14), without the need for storing the chlorine gas. The integrated

production process of sodium hypochlorite from brine, via caustic soda, is further demonstrated

in Figure 7. The production process can be classified as either high or low concentration. High

concentration produces sodium hypochlorite solution in the 5 to 15% range, and as such requires

less storage and transportation space than lower concentration product. Because of the high 7 solution concentration, dissociated disinfectants are immediately available. This process needs '

high qu�lity (low contaminant) brine made from food grade sal�. The facilities and equipment of .c/..-,.,� "'"/.e the new Deer Island plant are designed and built to handle high concentration product only.

Low concentration processes typically produce product with less than one percent sodium

hypochlorite. As such, storage facilities and transportation space is large. Given similar

conditions, however, low concentrated solutions are more stable than those of high

concentration. Salt requirements are somewhat less stringent and seawater (from Boston

Harbor) may be used in processing, as can solar salt, which is produced from the evaporation of

sea water. Deer Island has severe space constraints which would make the physical location of

additional buildings difficult. Low concentration generation would also require the resizing of

equipment such as pumps, piping, etc. Physical plant conversion costs make low concentration

generation impractical, even when free sea brine is factored into the cost analysis.

Page 15

, "' "" (1> >-' "'

WATER SUPPLY

-c:=:>-----WATER TREATMENT (FILTRATION+ EXCHANGE)

PURIFIED WATER

SALT

BRINE PREPARATION

CHLORINE GAS

+ /- CAUSTIC COOLER

� CAUSTIC �

E SOLUTION Q::i ELECTROLYZER COOLING

WATER

INTERGRATED CAUSTIC - CHLORINE

-

M L

SODIUM HYPOCHLORITE REP.CTOR

AND SODIUM HYPOCHLORITE GENERATOR

FIGURE 7

HYDROGEN GAS

� HYPOCHLOR!l SOLUTION

E

p- I '"U HYPO SOLUTION TANK

COOLING WATER JN

I COOLING WATER OUT

Conclusions

1. The new Deer Island Treatment Plant, as designed and built, is a facility free from the risk of

chlorine emissions in the air, without compromising the need to fulfill disinfection and odor

control requirements.

2. When fully operational, the residual disinfectant in the liquid effluent stream will be totally

diluted, dispersed, and/or neutralized so that it does not pose any threat to the marine .bielllgy -0Lupnlalli.ill� b /ol?J.- •

3. One of the major degradation products of stored, on-site, sodium hypochlorite solution is the

chlorate ion, which is a known toxic chemical species to living cells. This risk of chlorate

pollution from on-site generation of sodium hypochlorite is lessened due to shorter product

storage time. lJLf /� !L f'·vfc 1- {!f4,__"'-&_ 7 � ,-.i;.,;(Jv(J""'' S:, "'"i<Y).. c.c.-./,_../ ru�:

4. On-site generation of sodium hypochlorite eliminates any risk of aquatic life toxicity due to

transport spillage by barge. Over-the-road transport risk pollution will also be minimized.

Page 17

Ap_pendix

Calculation of Emission Potential from Deer Island Storage Tanks

Development of [Cl,qu,ou.l Equation:

Hc12

Cl2 (gas)j:-HV<==--> Cl2 (aqueous)

K2 t:>-0 '-'---Cl2 (aqueous) <=> HOC!+ H+ + Ct

'

K,

HOC! <---> H+ + OCJ-

From eqn. (A3), K3 = [W][OCI-)/[HOCl]

From eqn. (A2), K2 = [Ir][CI-][HOCI]/Cl2 (aqueous)

[Cl2] (aqueous)= [H+][Cn[HOCI]/K2

From eqns. (A4) and (AS),

[Cl2] (aqueous)= [WY[CI-)[OG]/K,K3

Page Al

eqn. (Al)

eqn. (A2)

eqn. (A3)

eqn. (A4)

eqn. (AS)

eqn. (A6)

.------·

Quantifying example:

[OC!"] 2.11M

Wt% NaOCI 13.02

Speciftc Gravity 1.210 g/mL [NaOH] 0.0125M

[Na,C03] 0.0094M

[Cl"] 2.11M

[NaOH] = [OR] = O.Ol25M

@ 25°C and for dilute solutions, Kw=l.OOE-1 4 moi2/I}=[H·][OR]

[H.]=8E-13M, pH= 12.1

Eqn. (A6), [CI2] (aqueous)=[H']2[CI.][OCI"]/K2K3

@ 25°C, K2=4.0E- 4 moi2/U (Jakowkin, 1899; Connick and Chia, l959)

pK3 = 7. 5 (Morris, 1966)

Hence, K3=3.2E-8 moi!L

[CI,q]={ (8E-13M)\2.11M)(2.11M) }/{ ( 4.0E-4M2)(3 .2E-8M) }::2.23E-13M

[CI2,q]=4.02E-15 molm/molwatcc = Mole Fraction (Approx.)

Vapor Pressure of liquid chlorine at 25°C=98 psig (Chlorine Institute, 1986)

Page A2

pCi2=Partial Pressure of Cl2 in vapor phase

Pc12=(98 psig)( 4.02E-15 mole fraction)=3.94E-13 psig

MF,,"P'"=Air space mole fraction=pc,21P,"�' where P,01�=1 atm.

MF,,.,P'"={(3.94E-13 psig)/(14.7 psig/atm)}/1 atm=2.68E-14

Ideal Gas Law, PV = nRT, n = PV/RT

V = 13. 02% NaOCl consumption=(15/13.02)(15% NaOCl consumption)

V = (15/13.02)(15,600 gallday)(ft3/7. 48 gal)(14 days) (including disinfection and odor control)

V = 33,638 ft3/2 week period; T = 25°C=77"F=537"R

· P = storage tank head-space pressure=14.7psia=1 atm.

R = 10.73 {(#f/in2)(ft3)}/{(#mol)("R)}

' )

n = {(14.7)(33,638})/{(10.73)( 537)}= 85.82 air space #mol

Chlorine in tank air space, N = (85.82 air space #moi)(MF,.,P'")

N = (85.82 air space #mol)(2. 68E-14 #mol Cl,l# mol air space)

N = (2.30£-12 #mol Cl2 vented/2 week period)(70.91 #Cl/#mol Cl2)

N = 1.63E-1 0 pounds of chlorine gas vented to atmosphere every 2 weeks

= 42.2E-1 0 pounds of chlorine gas is vented annually from storage tanks at Deer Island.

Page A3

References

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Jakowkin, A. A., Z. Physik. Chern., 29, 613- 657 (1899).

Kuehne Chemical Co., S. Kearny, N.J., Laboratory Analytical Result of Sodium Hypochlorite

Solution Sample, 1996.

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