Ecological And Hygienic Problems Of Disinfection
ECOLOGICAL AND HYGIENIC PROBLEMS OF DISINFECTION
OF THE WASTEWATER INCOMING INTO ODESSA BAY
B.M. Kats
Physical Research Institute
of I.I. Mechnikov National University of Odessa
In accordance with the sanitary requirements the Black Sea belongs to the second category of water utilization (the one of the cultural-everyday use). Its epidemiological safety index - index LCB (Lactose-positive colon bacilli) must not exceed 5 000 col/l. However according to the data of bacteriological analysis [1], 68.4% from 924 sea water samples taken at the beaches of Odessa in 1999 appeared to be non-standard, while in 1998 this figure was 52.3% and in 1997 - only 48.8%. Furthermore, the LCB index was 5 - 5000 times exceeded.
The main microbiological pollution source of Odessa Bay is the waste water from the biological purification stations (BPS) “Yuzhnaya” and “Severnaya” with the total capacity of throwing-out water of 103 700 000 m3 per year. The waste water from the mentioned BPS flow down into the sea without being decontaminated, so that the LCB index in them is from 110 up to 240 times exceeded. The seawater self-purification capacity in this case is evidently not enough and the high concentrations of organic and surfactants only increase the life period of opportunistic pathogenic bacteria.
All said above convinces us in the necessity of introduction of modern wastewater disinfection methods. Its appropriate to mention that the analogical situation can be faced at other BPS situated on the Ukrainian Black Sea coast from the Danube estuary up to the Strait of Kerch. This fact makes the problem nation-wide important.
Nowadays UV light is widely used in Russia and other countries for the wastewater disinfection [2,3]. This method can provide the necessary effect without any by-products or substances, negatively influencing the sea water living organisms.
Appropriated ultraviolet wavelength is germicidal, it destroys all microorganisms including bacteria, viruses, fungi, yeast and molds, which can
be found in water, on surface and in the air. The main condition of the effective UV technology using is the correct choice of UV dosage, which depends on the sensitivity of this or that microorganism to UV light influence.
Table1.Approximate dosages for 90% inactivation of selected microorganisms by UV light [4]
Microorganisms
Dosage
(μW•s)/cm2
Bacteria
E. coli
3,000
Salmonella typhi
2,500
Pseudomonas aeruginosa
5,500
Salmonella enteritis
4,000
Shigella dysenteriae
2,200
Shigella paradysenteriae
1,700
Shigella flexnery
1,700
Shigella sonnei
3,000
Staphylococcus aureus
4,500
Legionella pneumophila
380
Vibrio cholerae
3,400
Viruses
Poliovirus 1
5,000
Coliphage
3,600
Hepatitis A virus
3,700
Rotavirus SA 11
8,000
Protoozoan cycts
Giardia muris
82,000
Acanthmoeba castellanii
35,000
The comparative characteristics of UV wastewater disinfection method and other ones are given in Table 2.
Table 2 Disinfection methods comparison
Parameter
Ultra violet
Simple chlorination
Chlorination/
Dechlorination
Ozone
Capital cost
Low
Lowest
Medium
High
Operating cost
Lowest
Low
Low
High
Ease of installation
Excellent
Good
Complex
Complex
Ease of maintenance
Excellent
Good
Fair
Poor
Cost of maintenance
Lowest
Low
Medium
High
Frequency of maintenance
Very. Infrequent
Frequent
Frequent
Continuos
Disinfection performance
Excellent
Some regrowth
Possible
Unreliable in effluent
Virucidal effect
Good
Poor
Poor
Good
Personnel hazards
Low to none
High
High
High
Toxic chemicals
No
Yes
Yes
Yes
Effect on water
None
Forms trihalomethanes
Forms
trihalomethanes medium
Toxic by-products
Problems with operating system
Low
Medium
Medium
High
Contact time E
1 – 5 sec
30 – 60 min
30 – 60 min
10 – 20 min
Case of handing varying flow rate
excellent
poor
Poor
good
The comparative analysis of the data given in Table 2 vividly proves the undoubtful advantages of UV method in comparison with the chemical disinfecting methods. Alongside with numerous advantageous parameters the main one should be paid special attention to. It is the complete absence of any toxic by-products, which are ecologically harmful.
The main expediency criteria for UV technology application for waste water disinfection are its ultraviolet transmission and the suspended substances concentrations. In connection with it, in September 1999, Berson Milieutechniek B.V. analyzed the given parameters for «Severnaya» and «Yuzhnaya» BPS wastewater on its equipment in Holland. The obtained data are given in Table 3.
Table 3. The BPS “Severnaya” and “Yuzhnaya” wastewater analyses results
Parameter
BPS “Severnaya”
BPS “Yuzhnaya”
Ultraviolet transmission, %
74
77
Suspended substances concentration, mg/m3
4
10
Index LCB, col/l
70 000 000
130 000 000
In both cases UV transmission is rather high, and the suspended substances concentration is relatively small. It allows us to recommend UV technology for the BPS “Severnaya” and “Yuzhnaya” waste water disinfection. The most rational UV installations to be used in the case are the ones of Berson InLine 25000MLC type, equipped with multiwave middle pressure lamps.
The target of UV disinfection is the genetic material, nucleic acids. Microbes are destroyed by UV if the light penetrates trough the cell and is absorbed by the nucleic acids. The absorption of UV light by nucleic acids causes a rearrangement of the genetic information, which interferes with the cell’s ability to reproduce. A cell that can not reproduce is considered dead since it is unable to multiply to infectious numbers within a host.
Though UV light with wave length 265 nm is the most effective for microorganisms inactivation, the UV radiation in the range of 205-315 nm has the best germicidal effect. Exactly in this range the multiwave lamps work, and their radiation effects not only the DNA but the cell membranes, proteins, lipids and ferments of microorganisms as well. Such a combinatorial influence results not only in the remarkable increase of disinfection effectiveness, but in the practical absence of the well-known photo reactivation lighted microorganisms effect, which can often be noticed after the UV monochromatic low pressure lamps operation was stopped. The mighty modern multiwave UV lamps allowed to decrease remarkably the camera of radiation; and being installed perpendicularly to the decontaminated water flow allow mounting a UV installation into the water pipe easily.
Only 8 Berson InLine 25000 MLC type installations must be used at the BPS “Severnaya” with the flow rate of 24 000 m3/hour, and only 5 these installations are necessary for the BPS “Yuzhnaya” with the flow rate of 14 000 m3/hour. One installation of Berson InLine 25000MLC type is not more than 110011701400 mm and can be easily mounted into a pipe just at the exit of the decontaminated water into the sea. The rate of disinfection in this case will be not less than log 4, i.e. the LCB index will be lowered in no less than 10000 times. It should be stressed that energy consumption will comprise in this case only 15 Wt/m3 of the purified water.
In conclusion it should be noted that the UV Berson Milieutechniek B.V. equipment have been tested in Ukraine as to its sanitary-hygienic norms and can be used for potable and wastewater disinfection in our country.
References
1. Климентьев И.Н., Бабич И.В. Санитарно-гигиенические аспекты состояния прибрежной зоны г. Одессы // Экологические проблемы Чёрного моря: Сб. научн. статей.– Одесса: ОЦНТЭИ. 1999. – С. 112-116.
2. Кац Б.М. Обеззараживание сточных вод прибрежных городов и посёлков с помощью ультрафиолетового излучения. Возможности и перспективы // Там же. – С. 107-111.
3. Васильев С.А., Волков С.В., Костюченко С.В. Обеззараживание воды ультрафиолетовым излучением. Особенности применения // Водоснабжение и санитарная техника.-1998.- №1.-С.28-29.
4. Wolf R.L. Ultraviolet disinfection of potable water // Environmental Science and Technology. – 1990 – V. 24, N 6/ - P. 768-773.
5. Костюченко С.В., Васильев С.А., Ахмадеев В.В., Волков С.В., Якименко А.В. Технологическое обследование очистных сооружений канализации и выбор УФ-оборудования // Водоснабжение и санитарная техника.-1999.-№4.-С.17-19.
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