.. a guide to selecting non-oxidising biocides.
A cooling water biocide program must address the problems presented by :
- Gram negative organisms eg Pseudomonads, legionellae etc. and the biofilm slimes they produce or inhabit.
- SRBs and corrosive anaerobic slimes
- Algae & algal biomass and the resultant heavy fouling this can cause.
The crucial role biofilms play in the hazards associated with microbiological fouling of cooling water requires that any program employed possesses good 'slime' penetration & dispersing properties. Many non oxidising biocides (unlike the halogens) have such properties. Indeed biodispersants may be incorporated into a biocide formulation or program to enhance the effectiveness of the chemicals employed.
Values for the MIC (minimum inhibitory concentration) or MKC (minimum kill concentration) of a biocide are usually based on laboratory tests made within a narrow band of operating conditions and using pure cultures. Cooling waters do not contain pure/single culture microbe populations. Also cooling waters encompass a wide range of operating conditions e.g. pH, temperature, degree of fouling etc. Consequently such MIC/MKC figures are of limited value when applied to 'field' conditions and generally undershoot the cooling water concentrations required to be effective.
So selection of the biocide program must take into account not only the range of 'activity' of the biocide itself but also the cooling system operating conditions eg system volume, retention time, cooling water pH etc. Furthermore factors such as the cooling water inhibitor program used, the oxidising biocide employed (if any), effects on the waste water etc must also be taken into consideration.
Dual biocide programs have become increasingly popular not because they minimise the risk of 'resistant strains' developing in the cooling water but because a suitable combination of biocides offers better opportunity to provide biofilm slime penetration & dispersion together with action to kill both algae and bacteria.
Despite the caveats hinted at above the following table is offered as a starting point for biocide selection :
| TT =Techtrol ... TC =Techcide .. | Algae & Algal Biomass | Gram -ve bacteria e.g. Legionella, Pseudomonads & Biofilm | Anaerobes e.g. SRBs & slimes | Optimum pH range | 'Guide' Contact time | Notes - N.B. *= low activity; ** = high activity, ***= very high activity
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| Bronopol - (TC 48) | * | *** | *** | < 8.0 | 'Quick kill' | Compatible with broad range of inhibitors & biocides eg. isothiazolinones & quats. Suitable for low pH 'quick kill' applications as half life <3 hours @ pH8
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| Chlorophenols- (TC 38) | *** | ** | ** | 6.0 - 9.5 | 4 - 6 hrs | Good slime controller. Not compatible with chlorine or quats. Sodium salts have high persistence & environmental toxicity.
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| DBNPA - (TT 555) | * | *** | *** | <8.5 | <1 hour | Hydrolyzes rapidly @ pH >8.0. Short 'half life'. Low environmental persistence. Breakdown products active. Compatible with halogens.
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| Gluteraldehyde- (TC 49) | * | *** | *** | 6.5 - 9.0 | 3 - 4 hours | Non foaming. Non ionic. Compatible with halogens. Short half life - typically 4 - 12 hours.
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| Isothiazolines - (TT515&5153) (TC 32&32) | *** | *** | *** | 6.5 - 9.0 | 5 - 6 hours | Non foaming. Non ionic. Compatible with a wide range of treatment chemicals. Severe skin irritant.
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| Quats (Primary)- (TC 50) | * | * | * | 6.5 - 8.5 | 4 - 6 hours | Deactivated by high hardness, chloride, oil, dirt, debris etc. Tendency to foam @ pH > 8.0 Not compatible with anionics
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| Diamine Quats - (TT 570 & 5712) | *** | ** | ** | 6.5 - 8.5 | 4 - 6 hours | Good dispersion & slime penetration properties. Not compatible with halogens. Slight tendency to foam. Corrosion inhibitor.
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| Thione - (TT586) | - | ** | ** | 7.0 - 8.5 | - | Breakdown products active. 'Stabilised' in alkaline solution. Favoured for closed systems.
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| TTPC - (TT 530) | *** | *** | *** | 6.0 - 10.0 | Very quick | Compatible with oxidising & non ionic broad spectrum biocides. Cationic so care with anionics. High surfactant but low foam.
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