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DISINFECTION WITH CHLORINE DIOXIDE
DISINFECTION WITH CHLORINE DIOXIDEa
Gerald Cowley
Market Development Director
Sterling Pulp Chemicals
Toronto, Ontario M9B 6C7
ABSTRACT
A number of methods for controlling disinfection by-products (DBP’s) are
being implemented in North America in order to comply with the new U.S.
EPA Stage 1 Safe Drinking Water Act (SDWA). These include the use of
granulated active carbon (GAC), enhanced coagulation (EC), and also the
use of alternative disinfectants. Due to differences in water quality
across North America, no single approach will fit every location. It is
critical therefore, that Water Utilities have as many options as possible
for addressing the disinfection versus disinfection by-products dilemma.
One of these options is the use of chlorine dioxide (ClO2) . In the past,
principally due to its cost, it was used only for odor and taste problems.
However, recent studies have confirmed that it is a very effective
disinfectant, and that the concerns about its specific DBP’s (chlorite and
chlorate) can and are being avoided.
INTRODUCTION TO CHLORINE DIOXIDE
Chlorine dioxide (ClO2) was first used in North American water treatment
during the 1940’s in Niagara Falls, N.Y. (1). Their water source was
heavily contaminated with chlorophenols, formed during pretreatment with
chlorine (Cl2). Chlorine dioxide has the ability to oxidize them. Its
disinfection properties were also recognized, but the major advantage was
in taste and odor.
In Canada in the late 1940’s, ClO2 was introduced to the Pulp and Paper
Industry by Dr. Howard Rapson of the University of Toronto, to produce
very high strength and high brightness paper, a combination never seen
before. In the 1980’s, Dr. Rapson was also the first to recognize that
ClO2 did not form mutagenic liquid effluent discharges in Pulp Mill
effluents . This has resulted in the almost complete replacement of
chlorine with ClO2 in North America and elsewhere for bleaching. The
effect on decreasing pollution from Pulp Mills can be seen in the
following Graphs 1 through 3:
Graph 1. No. of Revertants in Bleach Filtrate vs Available Chlorine
Graph 2 Yearly Number of Waterbodies Placed under a Dioxin Advisory
The fact that ClO2 oxidizes organics is the fundamental reason why its
by-products are less toxic. Also, ClO2 is less reactive than chlorine or
ozone (O3), with less of its power being consumed by oxidizing fiber (or
other organic and inorganic species). Thus in the last fifty years, ClO2
use has grown from a small specialty chemical with its use measured in
pounds per day worldwide to over a million tonnes per year in 1999.
RECENT DEVELOPMENTS WITH CHLORINE DIOXIDE IN WATER TREATMENT.
Toxicity
Chlorine dioxide is probably the most extensively tested disinfectant in
history. This is because not much was known about the chemical, whereas
experience with chlorine extended in practice for over a hundred years. It
is likely that in the light of this research, ClO2 will be seen as one of
the least hazardous of residual disinfectants. There is growing evidence
that chlorite ion, which is the principal by-product of ClO2, is itself an
adequate post-disinfectant at levels normally used in drinking water (3).
This was previously suggested by Masschelein (4). Even when residual ClO2
had decayed to non-detectable levels, by-product chlorite prevents any
regrowth of microbes within the system. There is growing evidence that
chlorite also reduces biofilms. In 1994 the Chemical Manufacturers
Association (CMA) and the U.S. EPA agreed to CMA's conducting a
two-generation reproduction neurological development study on rats. This
was completed in 1997, and as a result, a No Observable Effect Level
(NOEL) was established for chlorite ion of 0.03mg/kg/day (5). From this,
the Maximum Contaminant Level (MCL) for ClO2 and chlorite are set at
0.8mg/L and 1.0mg/L respectively, in the Safe Drinking Water Act of
November 1998. The Study will be published later this year, but is
available through the CMA. A copy has been sent to Health Canada for
consideration in the new Canadian Regulations expected in the near future.
In 1998, a Swedish Medical Birth Registry and Official data on Drinking
Water Disinfection Method study was conducted in Sweden (6). Data for
approximately 74,000 births were compared. Three areas were studied, the
first using ClO2 , the second, sodium hypochlorite, and the third, no
disinfection .No statistically significant differences were determined
between births in the area using ClO2 and the area where no disinfection
was practiced.
Operations
One of the difficulties experienced by Water Treatment Plants who use ClO2
is the complexity and sensitivity of the method of analysis. The two EPA
approved procedures are Amperometric and DPD Titration methods. Both
require highly skilled analysts and can take upwards of 40-45 minutes per
sample. In fact, in a survey of Water Plants who had tried ClO2and then
abandoned its use, the majority gave this as the principal reason.
Research was carried out in Toronto and in France to find better and
faster methods (7,8). An improved hand held unit is now on the market,
which will give a ClO2 concentration value within 2-3 minutes of obtaining
the sample, and can be used in the field (9). This unit will be field
tested in Ontario this autumn.
Disinfection By-products.
ClO2 does not react with Natural Organic Matter (NOM) to produce either
Tri-Halomethanes (THM’s) or Halo-acetic acids (HAA’s) at doses used in
drinking water (10).
ClO2 does not react with bromides to produce Bromate (10), which is a
known carcinogen, and has an MCL of 10ppb (11). This can be readily
understood by reference to the following table:
Table 1 Oxidation Potentials of Various Water Treatment Agents
Disinfection using Chlorine Dioxide
For disinfection against viruses, ClO2 is not as fast as chlorine, but CT
values are nevertheless practical.
Table 2 C-T Values for Inactivation of Viruses
Table 2 C-T Values for Inactivation of Viruses
Giardia, ClO2 is five times more effective than chlorine
Table 3 C-T Values for the Inactivation of Giardia Cysts @ 10Deg.C and pH
6-9
U.S. EPA calculated the relative costs of compliance with 1998 SDWA Stage
1, showing that ClO2 is very cost effective. The data is shown in the
following graph:
Graph 4 Cost of Compliance with SDWA Stage 1
For disinfection against cryptosporidium, ClO2 has been tested extensively
by Dr. Gordon Finch at the University of Alberta. For 1-Log Kill, CT
values in the range 100-120 are needed at temperatures between 5-15
deg.C.and pH between 6.5-8.5. At very low temperatures, ClO2 is not
effective. However, despite this, ClO2 can be the most economical
disinfection choice depending upon system size, TOC, temperature and
pH.(12).
Table 4. Approximate CT Values for Cryptosporidium Inactivation
Experimental
ConditionsOzoneChlorine Dioxide
Log Units of InactivationLog Units of Inactivation
pHTemp. (◦C)0.51.02.00.51.02.0
8.5151.73.57.03055125
8.552.34.38.545100275
8.5151.73.57.035120225
6.552.34.38.51003501000
Dr. Finch also demonstrated synergistic benefits of ozone/ClO2and
ClO2/chlorine
CHLORINE DIOXIDE GENERATOR DEVELOPMENT
ClO2 is produced on site using a generator. Typically three types of
generators are in general use, using a choice of three different feeds :-
Using chlorine gas and sodium chlorite:
NaClO2 + ½ Cl2 à ClO2 + NaCl + Excess Chlorine (1)
Using hydrochloric acid and sodium chlorite:
5NaClO2 + 4HCl à 4ClO2 + 2H2O + 5NaCl + Excess acid (2)
Using hydrochloric acid, sodium hypochlorite and sodium chlorite:
2NaClO2 + HOCl + HCl à 2ClO2 + 2NaCl + H2O + Excess Chlorine (3)
A typical generator is shown in the following photograph:
Diagram 1 Typical Two Chemical ClO2 Generator
In all of these cases, the product ClO2 is not pure. Excess chlorine is
needed to drive the reaction to completion . This excess also reacts to
form chlorate (ClO3-) via the following side reactions :
ClO2- + HOCl à ClO3- +Cl- + H+ (4)
ClO2- + Cl2 + H2O à ClO3- + 2Cl- + H+ (5)
ClO2 + 1/2Cl2 + H2O à ClO3- + Cl- + 2H+ (6)
A survey of field generators was conducted in 1998-1999, which shows the
dependence of generator efficiency and excess chlorine:
Graph 5 Field Study. Efficiency of Two Chemical ClO2 Generators vs Excess
Chlorine
Although chlorate is not considered toxic, the loss in efficiency can be
significant (13).
These reactions will occur also in the treated water, so that the efficacy
of disinfection is lower. The conclusion is that ClO2 for disinfection
should be as pure as practically possible.
In an effort to solve this problem, single chemical generators have been
developed using an electrochemical cell as the "Reducing Agent", along
with a perstraction membrane to separate pure ClO2. The overall reaction
is as follows :-
NaClO2 + H2O à ClO2 (Anode) + NaOH (Cathode) + 1/2H2 (Cathode) (7)
Diagram 2 Perstraction System for Producing Pure Chlorine Dioxide.
The purity of the product ClO2 (over 99%) can be seen in Graph 5. The
perstraction membrane will only pass components which exert vapor
pressure. This includes water vapor and ClO2, but not chlorite, chlorate
or chloride. The product pH is 6.5, very close to that of pure ClO2 (14).
Diagram 3 ECF Module
This system was developed in the laboratory in Toronto, and will be field
tested in Ontario in the autumn of 1999.
REFERENCES
Gates: The Chlorine Dioxide Handbook AWWA
Cowley,Kaczur,Ranger Lipsztajn."Recent Advances in Chlorine Dioxide
Application Technology." WWW.ClO2.com.
Elf-Atochem. "Chlorine Dioxide By-Products Formation and Reactivity in
the Distribution System.
Masschelein:"Chlorine Dioxide"pp157 .Ann Arbour Science.
Federal Register. 40 CFR Parts 9 141 142, pp 69403, 7. Nov 1998
Kullen. "Drinking Water and Delivery Outcome" Second European Symposium
on Chlorine Dioxide and Disinfection. Paris June 24/25 1999.
Hofman.Univ.Toronto. Env.Technology,vol 19.1998
Lapoire, "Evaluation of a Specific Chlorine Dioxide Method intended to
Onsite Use" Second European Symposium on Chlorine Dioxide and
Disinfection. Paris June 24/25 1999.
Hach Company. http://www.hach.com.
Korn et Al. Univ. Toronto. AWWA Annual Conference, Chicago June1999.
Federal Register. 40 CFR Parts 9 141 142 pp 69405. Nov 1998
Gregory Dean. "Applicability of Chlorine Dioxide for Cryptosporidium
Inactivation. WQTC. San Diego Nov.1998.
Greise et al. AWWA Journ.Nov 1992.
Cowley et al. US Patent 5,932,085 Aug 1999
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International Year of Fresh Water
عنوان : معرفي نرم افزار HYSYS
كلمات كليدي: نرم افزار، شبيه سازي، HYSYS، عمليات واحد
طراحي و شبيه سازي فرايند
در دنياي رقابتي امروز شركتهاي طراحي و توليدي به سرعت و دقت در طراحي بهينه فرايند توليد نياز دارند. اين كار بايد به صورتي انجام گيرد كه حداقل تكرار محاسبات را به دنبال داشته باشد.
شبيه سازي فرايند ابزاري است مفيد و موثر براي دستيابي به اهداف زير:
• انتخاب بهينه و درست تجهيزات مطابق با اهداف طراحي
• ارزيابي صحيح تغييرات خوراك، نوسانات و خارج از سرويس بودن تجهيزات
• ايمني ، قابليت اعتماد و سودمندي در طول عمر مجتمع.
توانايي ها ...
طراحي سريع و قابل تغيير
آيا مي توانيد به سرعت مفيدترين و قابل اعتمادترين طراحي را تعيين كنيد؟
HYSYS.Process به گونه اي برنامه ريزي شده است كه صحت و سرعت شبيه سازي را با سادگي كار شبيه سازي تلفيق مي كند. براي طراحي هاي جديد، HYSYS.Process قادر است به سرعت مدل ها را براي ارزيابي گزينه هاي متعدد بوجود آورد. پس از گزينش چند طرح برتر، مي توان مدل هايي بسيار واقع بينانه بر مبناي آنها ايجاد كرد كه تجهيزات اضافي و جزئيات فرآيند نيز در آنها در نظر گرفته شده باشد.
با HYSYS.Process مي توان عمليات مجتمع را به سرعت بهبود بخشيد و نيز از درستي كاركرد دستگاه ها اطمينان حاصل كرد. براي مثال در تشخيص نقص دستگاه ها مانند ميزان جرم گرفتگي مبدلها و پديده طغيان در برج هاي تقطير مي توان از آن استفاده كرد.
مشخصات فني
ترموديناميك جامع
مجموعه اي معتبر و جامع از خواص مواد خالص در HYSYS.Process قرار داده شده است. در اين نرم افزار معادلات حالات مختلف، معادلات ضرايب اكتيويته و داده هاي كتابخانه اي لازم براي مدل سازي سيستم بخار، فرآيندهاي هيدروكربني و سيستم هاي شيميايي بسيار غيرايده آل نيز وجود دارد. HYSYS.Process روش هاي ترموديناميكي مخصوصي براي سيستم هاي غير ايده آل در فشار زياد دارد و بيش از 15000 ضريب تاثير متقابل دوتايي را در خود جاي داده است.
يك كتابخانه جامع تركيبات خالص در HYSYS.Process وجود دارد كه قابليت افزودن تركيبات اختصاصي با استفاده از داده هاي شخصي و يا تهيه تركيباتي با استفاده از گروه هاي UNIFAC را دارد. همچنين تركيبات نفتي را با استفاده از داده هاي تقطير ASTM استاندارد مي توان به وجود آورد.
محيط محاوره اي كامل
برخلاف بسياري از شبيه سازهاي موجود، محيط HYSYS.Process كاملا محاوره اي است. حتي اگر اطلاعات و داده هاي اوليه ناقص باشند، تا حد ممكن در تمامي نمودار جريان پخش مي شوند. علاوه بر اين، محاسبات واحدها از اول به آخر يا بالعكس نيز انجام مي شود. اين كار باعث حذف محاسبات تكراري شده و لزوم تخمين در بسياري از نمودار جريان ها را از بين مي برد. توانايي انجام محاسبات سودمندي نظير شرايط تشكيل هيدرات، خواص نفتي و محاسبات اندازه يابي دستگاه ها (sizing) بخشي از محيط محاوره اي را تشكيل مي دهد.
عمليات واحد جامع
در HYSYS.Process عمليات واحد متعدد و گوناگوني مانند انواع مبدلهاي حرارتي، تجهيزات دوار مانند پمپ و كمپرسور، جدا كننده ها، برج هاي تقطير، راكتورها، عمليات جداسازي جامدات و عمليات منطقي موجود است. به علاوه عمليات واحد خاص را نيز مي توان از طريق برنامه نويسي به اين نرم افزار اضافه كرد.
واكنشهاي شيميايي يك بار نوشته شده و در طول برنامه در هر جا كه مورد نياز باشند مورد استفاده قرار مي گيرند. واكنشهاي تكي يا گروهي را مي توان در انواع عمليات واحد، نظير برج هاي تقطير به كار گرفت. برجهاي تقطير مي توانند دو فازي يا سه فازي و يا همراه با واكنش شيميايي باشند. با يك اشاره Mouse مي توان ضابطه هاي عملكرد برج را فعال يا غيرفعال كرد. اين ضابطه ها به متغيرهاي خاصي وابستگي ندارند. همچنين اين امكان در HYSYS.Process وجود دارد كه يك برج تقطير پيچيده را در يك نمودار جريان فرعي، به صورت جداگانه حل كرده و سپس محاسبات را در نمودار جريان اصلي ادامه داد.
سازگار با فن آوري اتوماسيون OLE
بهره مندي نرم افزار از فن آوري اتوماسيون OLE در برگيرنده مزاياي زير براي كاربر است :
• به راحتي مي توان نتايج محاسبات را به نرم افزارهاي عمومي نظير Excel و Word منتقل كرد.
• بدون نياز به دانستن متن برنامه (Source Code) به آساني مي توان واحدهايي (نظير مدلهاي خاص رآكتور) را به برنامه اضافه نمود.
• اتصال با ساير برنامه هاي كاربردي براي جمع آوري و تجزيه و تحليل داده ها. اطلاعات را هم مي توان به صورت دستي و هم از طريق ارتباط با ساير برنامه هاي كاربردي پردازش كرد.
امكانات ديگر
به جاي صفحات جدا از هم و فايل هاي متعدد ورودي، در HYSYS.Process يك فايل تمام داده هاي ترموديناميكي متغيرهاي مشخص و نتايج محاسبه را در خود ذخيره مي كند. PFD را مي توان با فرمت DXF براي استفاده در ساير برنامه ها تهيه كرد. مسيرها و محل هاي گوناگون و متنوع براي وارد كردن داده ها و مشاهده نتايج محاسبات، امكانات ترسيمي گسترده براي تجسم بهتر نتايج، امكان دست ورزي در اطلاعات كتابخانه اي برنامه، امكان اجراي برنامه در حالت هاي مختلف و ذخيره سازي نتايج تمام حالت ها، بهينه ساز متغيرهاي عملياتي، صفحه گسترده نظير Excel ، پيش بيني خصوصيات ترموفيزيكي مواد خالص و مخلوط ها به صورت تابعي از فشار و دما و بسياري ظرايف ديگر از جمله امكانات جانبي اين نرم افزار به شما مي باشد.
