Document Sample
01_09_2008_Biocide Powered By Docstoc
					                                                                            Indranil Chowdhury
                                                                                January 9, 2008

Table 1. Micorbiological control chemicals
Types of Chemicals                                                      General Characteristics
                Oxidizing  Destroy the microorganism by chemical oxidation
Biocides                      Their capacity to penetrate the cell walls and disturb the metabolism is an essential feature of their
                                 action, which is not selective, and in some cases depends on the pH
                Non-          Attack the micro-organisms by reacting with specific cellular constituents or by interfering with
                oxidizing        metabolic reactions
                              They either destroy the cell membrane or disrupt the biochemical machinery involved in the
                                 production or use of energy
                              Their activity is selective and independent of pH
                              Expensive
                              Show little or no biodegradability
                              Chlorine and other oxidizing biocides are often poor at penetrating and dispersing heavy slimes.
                                 However, many nonoxidizing biocides often have such properties (unlike halogens) and they work
                                 well in conjunction with biodispersants
                Anionic       Tend to be useful for the penetration and cleaning of slimes containing oil and airborne debris
Biodispersants                Compatible with other anionic material and also nonionics
                Nonionic  Can be tailored to provide various degrees of detergency, wetting, and defoaming properties, based
                                 on the degree of problem solving required
                              Tend to be the most versatile and are excellent cleaners
                              Suitable for supporting any biocide
                Cationic      Tend to have the highest degree of surfactant action
                              Cationic nature permits neutralization of the typically anodic biofilm
                              Cationics should not be used with anionic biocides, such as chlorophenols

                                                                                                                  Indranil Chowdhury
                                                                                                                      January 9, 2008

Table 2. Summary of biocides used in cooling water treatment
Chemicals    Role            Use                         Advantages                                                            Disadvantages                                Mechanisms
Oxidizing biocides
Chlorine or Javel                          Can be injected continuously or as a       Cheap                                  Non-selective action                        Readily combines with proteins
water (Cl2 or HOCl)    React with the      shock treatment in either the circuit                                              Consumption of numerous organic              in microbiological protoplasm,
                        organic             or the feed-water                                                                    and inorganic substances and                forming irreversible chlorine-
                        molecules in       Residual free chlorine is usually kept                                               reducing agents with an affinity for        nitrogen bonds, which result in
                        microbial cells     between 0.1 and 0.3 mgL-1                                                            chlorine:                                   loss of enzyme activity and the
                        (glucides and       (depending on national regulations)                                               -NH3 →chloramines                              death of microorganism
                        proteins)          Limited pH range (5 to 7.5)                                                       -nitrites→nitrates                            Also interferes in the
                                                                                                                              -heavy metals (Fe, Mn)                         microbiological respiration
                                                                                                                              -certain polymers                              process and is potentially harmful
                                                                                                                              -H2S → SO42-                                   to all living organism
                                                                                                                               Difficult to maintain
                                                                                                                               Problem of accessibility beneath
                                                                                                                               Ineffective against fungi, little effect
                                                                                                                                 on moulds
                                                                                                                               Sensitive to light
                                                                                                                               Sensitive to pH
                                                                                                                               Enhances degradation of wood
                                                                                                                                 (certain streaming type cooling

Chlorine dioxide                           Used in treating oil field waters, pulp    Effective over a wide range of pH      Must be produced on site                    Chlorine dioxide does not
(ClO2)                 React with the      bleaching, paper mill slime control,        (6 to 10)                              Needs a generator                            hydrolyze in water and, as a
                        organic             odor control, dye and color removal,       Does not react with NH3                Possible problems depending on               result, remains a effective biocide
                        molecules in        and a wide variety of other industrial     Generates little AOX                    method of fabrication (corrosion,            over a wide pH range
                        microbial cells     applications with pH range 6 to 10         Very efficient (low residual of         storage of products)
                        (glucides and                                                   around 0.01 ppm)                       Photochemical decomposition
                        proteins)                                                      Ability to readily oxidize sulfide     expensive
                                                                                       Useful where high levels of organic
                                                                                        process contaminants are present
Di- or                                     Used with surfactants and                  Prolong the presence of chlorine       In the form of powder or granules           Acts as a chlorine stabilizer in
trichloroisocyanic     React with the      biodispersants                              oxidizing oxidizing agent by            difficult to use                             preventing the rapid depletion of
acid                    organic                                                         stabilizing the hypochlorous acid      Possibility of sudden high                   chlorine from the cooling system
                        molecules in                                                    formed →improved sterilization          concentration of free chlorine               by ultraviolet light
                        microbial cells
                        (glucides and
Sodium                                     Usually dispensed two or three times       Cheap                                  When bleach is used, hypochlorous  In water, rapidly hydrolyzes to
hypochlorite           React with the      per week by hand-adding a known            Can act as a lower-cost method of       acid is formed but also caustic soda, form hypochlorous acid which
(bleach)                organic             volume, although the use of a timer-        providing bromine chemistry to          which tends to increase the pH, so    acts as a biocide
                        molecules in        controlled pump is a preferred              large cooling systems                   extra vigilance may be needed if this
                        microbial cells     alternative                                                                         is used in galvanized cooling towers
                        (glucides and      Can also be used in combination                                                    Significant volumes required usually
                        proteins)           with sodium bromide solution to                                                     make bleach suitable for only small
                                            produce hypobromous acid                                                            cooling systems

                                                                                                                  Indranil Chowdhury
                                                                                                                      January 9, 2008

Calcium                                    Packaged in either granule or tablet      Easy to apply                           Produces an increase in both             In water, rapidly hydrolyzes to
hypochlorite          React with the       form with granules usually being          Cheap                                    calcium and alkalinity, which may         form hypochlorous acid which
                       organic              liberally scattered across the surface                                              contribute to a scaling or deposition     acts as a biocide
                       molecules in         of the cooling tower basin and                                                      risk
                       microbial cells      tablets being placed in a floating                                                 Suitable only for smallest cooling
                       (glucides and        plastic basket                                                                      systems
                       proteins)                                                                                               Care should be excercised to ensure
                                                                                                                                that the materials of construction
                                                                                                                                can tolerate this biocide
                                                                                                                               Peak free chlorine reserves can
                                                                                                                                easily exceed 2 to 3 ppm Cl2 unless
                                                                                                                                good control is excercised
Bromine and           In water, bleach    Used alone or in combination with           More powerful biocide than chlorine  Same disadvantages as for chlorine        Same as chlorine
derivatives            rapidly              chlorine at pH > 7.5                         at basic pH levels and in             More expensive than chlorine
(derivatives may be    hydrolyzes to       Can be injected either in the feed           ammoniacal media
Hydantoin,             form                 water or the circulating water              Generate less AOX than chlorine
isocyanurate/sodium    hypochlorous                                                     Kinetics of the microbiological kill
bromide solids, etc)   acid which acts                                                   reaction is more favorable for
                       as a biocide                                                      bromine
                                                                                        Kinetics of decay rates of both free
                                                                                         and combined bromine is more
                                                                                         favorable for bromine which makes
                                                                                         it more environmentally friendly
                                                                                        Bromamines quickly formed in the
                                                                                         presence of nitrogen compounds,
                                                                                         remain as active biocides. As a
                                                                                         result breakpoint bromination is not
Stabilized bromine    A biomimetic        Used for control sessile bacteria           Can penetrate into the biofilm and    Considerably more expensive than         The bromine molecule is
(StarbrexTM from       stabilized                                                        kill sessile bacteria                  traditional halogens                      uncharged, and the stabilization
Nalco)                 bromine                                                                                                                                            of the molecule makes it less
                       produced                                                                                                                                           reactive. Both these attributes
                       naturally in the                                                                                                                                   allow the molecule to penetrate
                       human immune                                                                                                                                       deep into biofilms, which have
                       system                                                                                                                                             areas of positive and negative
                                                                                                                                                                          charge that can repel traditional
                                                                                                                                                                          non-stabilized halogen molecules
                                                                                                                                                                         The increase penetration
                                                                                                                                                                          introduces the uncharged bromine
                                                                                                                                                                          molecule into the heart of the
                                                                                                                                                                          biofilm. Subsequent release of
                                                                                                                                                                          HOBr from the stabilized
                                                                                                                                                                          molecule has two effects: firstly,
                                                                                                                                                                          the biofilm community is
                                                                                                                                                                          profoundly stressed and the
                                                                                                                                                                          equilibrium disrupted, with the
                                                                                                                                                                          result that part of the biofilm is
                                                                                                                                                                          sloughed off into the bulk water;
                                                                                                                                                                          secondly the HOBr is toxic to

                                                                                                                 Indranil Chowdhury
                                                                                                                     January 9, 2008
Iodine                                       Used for many years but there has      Compatible with virtually all scale      Useful only for small comfort            Same as chlorine
                      React with the         until recently been little or no        and corrosion inhibitors and does         cooling systems with fairly constant
                       organic                attention paid to promoting             not contribute to the corrosion of        operating conditions
                       molecules in           commercial iodine-based products        metal surfaces
                       microbial cells        for cooling systems
                       (glucides and
Ozone                                        Little use except in special cases     Most powerful oxidizing biocide          Must be produced on site                 Same as chlorine
                      React with the                                                It does not form trihalomethanes, as     Needs a generator
                       organic                                                        does chlorine                            Very sensitive to water chemistry
                       molecules in                                                                                             (CaCO3, HCO3-, PO43-, etc)
                       microbial cells                                                                                         Capital cost is very high
                       (glucides and                                                                                           Unlikely suitable for cooling
                       proteins)                                                                                                systems with high operating
                                                                                                                                temperatures (over 1000F), high pH
                                                                                                                                (over 8.0), high calcium hardness
                                                                                                                                (over 200 to 300 ppm TH as
                                                                                                                                CaCO3), high chlorides (over 200
                                                                                                                                ppm as Cl), or low recirculating
                                                                                                                                water rates (below 3 to 4 times the
                                                                                                                                system volume per hour)
                                                                                                                               Gaseous ozone is toxic
Peracetic acid                               Has long been available for use in     A strong oxidant                         Corrosive                                Same as chlorine
                      React with the         recirculating cooling water systems                                              Needs to be stored in cool well-
                       organic                but has failed to gain much market                                                ventilated conditions, well away
                       molecules in           share                                                                             from direct sunlight
                       microbial cells                                                                                         Can be effective in relatively simple
                       (glucides and                                                                                            clean cooling systems but is of
                       proteins)                                                                                                limited benefit in hard-working
                                                                                                                                industrial systems
Non-oxidizing Biocides
Guanidine              Non-oxidizing        Periodic injection at low              Excellent dispersal efficiency           Not particularly suitable for fouled     Act as toxicants by virtue of high
derivatives with        biocides              concentrations                         Effective at pH > 8                       cooling systems, but effective in         surface activity, which disrupts
long aliphatic chains  Disperse mineral                                             Effective against algae, bacteria and     maintaining clean conditions              the microorganism’s cell wall and
                        deposits (oxides)                                             fungi                                                                               cytoplasm
                                                                                     Readily biodegradable
Methylene             Non-oxidizing         Periodic injection at low              Effective against algae, sulphate        pH sensitive and hydrolyses at pH        Kill mechanism is to block the
biothiocyanate         biocide                concentrations                          reducing bacteria and fungi               >8 which reduces efficiency               transfer ofelectrons in the
(MBT)                                        Generally used with a polyalkylene                                               Low solubility in water                   microorganism, preventing
(C3H2N2S2)                                    glycol type dispersant                                                           Deactivated by Fe3+ ions                  oxidation/reduction mechanisms
Chlorophenols         Non-oxidizing         No longer used (environmental          Highly efficient                         Toxic towards environment                Attack cell membranes or block
                       biocide                toxicity)                              Good slime controller, especially        Not particularly good bactericide         certain vital enzyme functions
                                                                                      algal slimes                             Not suitable with chlorine or quats
                                                                                     Operates at wide pH range (pH 6 to
                                                                                      9.5) although performance tends to
                                                                                      decrease at high pH
Isothiazolines        Non-oxidizing         Act slowly (bacteriostatic reagents    Effective over a wide range of pH        Effective essentially against aerobic    Kill by inhibiting microbial
                       biocides               needing a long residence time)         Good bactericide and algaecide            bacteria                                  respiration and food transport
                                                                                     Do not foam                              Low efficiency against algae              through the cell wall

                                                                                                         Indranil Chowdhury
                                                                                                             January 9, 2008
                                                                             Quite cheap                              Skin sensitizer which can lead to
                                                                             Generate little AOX                       fairly serious reactions/allergies
                                                                             Effective in the presence of chlorine    expensive
                                                                              and bromine
Gluteraldehyde     Non-oxidizing     High concentrations (> 50 mg/L)       Good bactericide                         Limited effectiveness against algae  Killing mechanism is by cross-
                    biocide                                                  Fast-acting biocide                       and fungi                             linking outer proteins of cell and
                                                                             Nonfoaming and nonionic                  Efficiency reduced in the presence    preventing cell permeability
                                                                             Efficiency increases with pH ( up to      of ammonia (NH3 > 1o mg/L)
                                                                              9)                                       Relatively expensive
                                                                             Compatible with chlorine                 Withdrawn in UK due to HSE EH40
                                                                             Effective against SRBs and biofilms       exposure limits
                                                                             Probably a good biocide where risk
                                                                              of Legionella sp. Exists
                                                                             Readily biodegradable
Acrolein           Non-oxidizing     Seldom used today as is often not     Rapid microbial resistance can           Flammable, volatile, lachrymatory,      Acrolein is a protein and enzyme
                    biocide            worth the gain                         occur                                     and a strong irritant                    poison
                   Aldehyde group
Alkyl sulfonate    Non-oxidizing                                            Good bacterial slime controller for      Limited value as algaecide or
                    biocide                                                   cooling systems with pH below 7.5         fungicide and at alkaline pH
                   Organo-sulfur
BHAP               Non-oxidizing     Useful situations requiring          Good biocide for bacterial slimes
                    biocide            continuous or semicontinuous dosing  Not pH dependent                          Higher dose rates for fungal and
                   Organo-bromine     at low level, such as once-through                                               algal slimes
                    group              cooling systems                                                                 Relatively long half life
Bronopol           Non-oxidizing     General purpose microbiocide,        Good compatibility with other             Suitable for low pH                    
                    biocide            slimicide, and aerobic/anaerobic      biocides and all common cooling
                   Organo-bromine     biocides                              water inhibitors
Quarternary        Non-oxidizing     Used as a routine treatment for       Effective against algae and bacteria     Detergent effect that can cause         Hydrofuge surfaces and vectors
ammonium salts      cationic           remaining trouble-free                Cheap                                     foaming                                  for other biostatic bases
                    dispersants                                              Efficiency impaired in the presence      Cationic products incompatible with
                                                                              of heavy metals                           dispersants and sequestrants based
                                                                                                                        on strong amines (high molecular
                                                                                                                        weight polyacrylic derivatives, etc)

                                                                          Indranil Chowdhury
                                                                              January 9, 2008

Biocides and Biodispersants selection:
    Testing and identification of troublesome microorganisms in cooling systems, followed
       by biocide screening (using the predominant organisms detected) will provide a sure
       route in selecting the optimum combinations of biocides to employ in a biocide program
    Combinations of newer nonoxidizing products (good biocidal potency but expensive)
       with some older materials (good penetrating ability and relatively low cost) may prove
       effective. Also, certain combinations of oxidizing biocides with nonoxidizers may prove
       suitable under many conditions
    Suggested biocide selection for small and large cooling system are summarized in Tables
       3 and 4

Table 3. Suggested biocide selection for comfort cooling system

(Source: ???)
Table 4. Possible biocide selection for large industrial cooling system


                                                                           Indranil Chowdhury
                                                                               January 9, 2008

Microbiological control program:
    Biofilms and biofouling are now recognized to be probably the most damaging and
      efficiency-inhibiting forms of fouling in cooling systems today and yet methods for their
      assessment are poor. As a consequence, protocols for effective microbiological control in
      the field are often equally limited
    Effective biocide programs are often based on some degree of trial and error by
      individual field engineers, coupled with their previous experience and supported by their
      company’s technical department
    Most engineers tend to select the same few biocide products or combination of products
      over and over again because they know how to obtain the desired results from their use
    It is also generally not possible to clean up a microbiologically fouled cooling system
      with only one application of a biocide, as they are never 100% effective at killing or
      dispersing slimes
    As a result, it is generally necessary to evaluate several possible biocides, and then
      propose only programs that both overcome any site-specific limitations and are cost
      effective when applied
    For this reason, the cooling system operator usually has to rely on the expertise of the
      service company’s field engineer, who must therefore establish his or her credibility prior
      to implementation of a biocide program
    In mitigation, it has to be said that for many operators around the world, the difficulties
      and costs associated with providing meaningful and timely diagnostic testing may
      ultimately be even more expensive than the cost of the biocides employed
    Some of the factors that may need to be considered by the service engineer include:

              Biocide application rates and frequency
              Use of biodispersants or other cleaners
              Retention time and water velocity through the system
              Cooling system pH, chlorides, TDS, or other pertinent analytical parameters,
               including any process contaminant load or cooling system nutrients
              Microbiological flora load, including slimes, SRBs, biofilms, etc
              The compatibility and possible effects on other chemical inhibitors
              Cost comparisons of a proposed biocide with other possible starting biocides
              Existing corrosion, deposits, and other reported problems

Types of Microbiological Control Chemicals:

      Chemicals used in microbiological control are of several types which are summarized in
       Table 1


Shared By: