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Problem 1
In two sentences, explain why biological control requires fundamentally different thinking than scale or corrosion control.
Authored answer
Scale and corrosion respond predictably to changes in conditions: adjust pH, temperature, or concentration and the problem slows or stops. Biology adapts: it responds to pressure by reorganizing, building shelter (biofilm), and recovering from partial disruption, meaning that changing conditions alone is rarely sufficient for lasting control.
Read the connected sectionProblem 2
A mature biofilm on a condenser tube is treated with a high dose of chlorine. Planktonic counts drop to near zero within 30 minutes. Two days later, counts are back to pre-treatment levels. Explain what happened using the concepts of EPS, diffusion limitation, and survivor repopulation.
Authored answer
The high chlorine dose cleared the water column of planktonic organisms, which is what the bulk water test measures. However, the established biofilm on the condenser tube was protected by its EPS matrix, which consumed the oxidant through reaction before it could diffuse to the inner cells. Surviving organisms deep in the biofilm structure were never exposed to lethal concentrations. Once chlorine levels dropped, these survivors repopulated the EPS scaffold and shed new planktonic cells into the water column, returning counts to pre-treatment levels within 48 hours. The treatment killed the messengers but left the city intact.
Read the connected sectionProblem 3
Using the oxygen differential cell from the Corrosion chapter, explain how a biofilm creates the same corrosion mechanism as a deposit, but sustains it continuously.
Authored answer
A deposit creates an oxygen differential cell by physically separating aerated bulk water from the oxygen-depleted zone beneath it. The covered metal becomes anodic while the surrounding exposed metal becomes cathodic. A biofilm creates the same differential, but actively maintains it: microbial respiration continuously consumes oxygen at the metal interface, ensuring the anodic zone never re-passivates. Additionally, the biofilm generates metabolic acids that lower pH and, in the case of SRB, produces hydrogen sulfide that directly attacks the metal surface. Unlike a passive deposit, the biofilm regenerates the corrosive conditions around the clock.
Read the connected sectionProblem 4
A tower runs at pH 8.4; a colleague says the chlorine is useless and wants to switch to bromine. Under continuous feed, is he right? What does the biofilm's internal pH do to his argument?
Authored answer
He is not correct that “chlorine is useless.” At pH 8.4, the bulk water contains a much smaller fraction of chlorine as HOCl, so chlorine’s short-contact kill rate is reduced. That is where bromine has a real advantage: short contact time, high pH, and a need for rapid free-halogen activity. But a continuously fed cooling tower is not judged by short-contact speciation alone. If free chlorine residual is maintained for hours, contact time, residual persistence, oxidant demand, and surface access matter as much as the instantaneous HOCl percentage. The biofilm also weakens his argument because the pH inside and near the biofilm is often lower than the bulk water, which shifts more chlorine back toward HOCl where organisms are actually sheltered.
Read the connected sectionProblem 5
A 4-inch branch off a cooling tower recirculation loop has been valved off for 6 months. The water inside is at 85°F. No biocide reaches it. Describe the biological progression you would expect from month 1 through month 6, and explain why this dead leg is a Legionella risk.
Authored answer
Month 1: Planktonic organisms already present in the stagnant water begin attaching to pipe surfaces. Initial EPS production begins. Month 2–3: Without biocide access or flow to disrupt attachment, biofilm matures. Oxygen is consumed at the metal surface, creating anaerobic zones. SRB may begin colonizing the base of the biofilm. Month 4–6: A mature, multi-species community develops with established gradients. The 85°F water temperature is within Legionella’s growth range, though below its optimal growth range (~95–105°F). Legionella replicates inside protozoa (amoebae) that thrive within the biofilm. No biocide reaches the branch. No flow disrupts the community. No thermal treatment exceeds growth temperature. This dead leg is a Legionella reservoir that, if reconnected to the system, could release organisms into the circulating water and potentially into aerosol-generating components.
Read the connected sectionProblem 6
A facility has used the same non-oxidizing biocide (isothiazolone) on a weekly slug-dose schedule for 18 months. Biological counts have been rising steadily for the last 6 months despite consistent dosing. Explain what is likely happening and propose a modified program.
Authored answer
Eighteen months of the same non-oxidizing biocide has created sustained selection pressure. Organisms tolerant to isothiazolone have been preferentially selected. The community has adapted, not disappeared. A modified program should: (a) introduce a different non-oxidizing chemistry (e.g., glutaraldehyde or DBNPA) to disrupt the selection pressure, (b) alternate between at least two non-oxidizing chemistries on a rotating schedule, (c) add a dispersant to expose biofilm-sheltered organisms to chemical treatment, and (d) consider mechanical cleaning of accessible surfaces to physically remove the adapted biofilm community before restarting chemical treatment.
Read the connected sectionProblem 7
After adding a dispersant to a cooling tower, turbidity spikes from 15 to 50 NTU and the ATP reading in the bulk water triples. The operator wants to stop the dispersant because “it made things worse.” Explain what is actually happening and whether this is a problem or a success.
Authored answer
The dispersant is working. The turbidity spike and increased ATP in bulk water indicate that the dispersant is lifting biofilm material from pipe and fill surfaces into the water column. The “worse” numbers reflect what was already present on the surfaces but invisible to bulk water testing. This is a success: the shelter has been disrupted and the previously protected organisms are now exposed to the biocide program. The correct next step is to follow the dispersant application with a biocide treatment to kill the newly exposed organisms, and to increase blowdown or filtration to remove the suspended biological material from the system.
Read the connected sectionProblem 8
On Friday afternoon, a cooling tower's biocide feed pump fails. It won't be repaired until Monday morning. The water is warm. The system keeps recirculating. For 72 hours, nothing is fed. Using the biofilm lifecycle from this chapter, and what you already know about scale and corrosion, trace the likely sequence of events across the weekend. Where does biology go first? What does its growth do to the heat-transfer surfaces, and how does that pull scale and corrosion in behind it?
Authored answer
The order is reliable; the timing is not. A biocide residual suppresses the population, but it never empties the system. Pull it, and the first thing that happens is chemical, not biological: the last oxidant is consumed and nothing replaces it. Only then does biology move, and it moves in a set order. Freed planktonic cells multiply in the warm water, find surfaces, and attach; EPS production ramps up and any suppressed biofilm resumes growth, thickening where it matters most, on the heat-transfer surfaces. That film is an insulator. The surface beneath it runs hotter than the bulk water, and you've met both consequences: calcium carbonate has inverse solubility, so a hotter surface scales more eagerly, while respiration beneath the film consumes the oxygen the metal can't replenish, setting up the oxygen-differential cell from the Corrosion chapter. Under-deposit corrosion begins. How far a system travels in three days depends on temperature, nutrient load, and how much biofilm was already there when the pump failed. A warm, fouled system moves far in a weekend; a cold, clean one barely stirs. The lesson is the direction, not the calendar: biology reaches the surface first, and scale and corrosion follow it in. Killing the organisms Monday won't undo that. The shelter is already built.
Read the connected sectionProblem 9
A hospital cooling tower has excellent bulk water chemistry: free chlorine at 0.5 mg/L, conductivity in range, coupons acceptable, no visible scaling. Legionella culture comes back at 1,000 CFU/mL. Explain how this is possible in a “well-treated” system. Identify at least three potential harboring locations.
Authored answer
Excellent bulk water chemistry does not guarantee clean surfaces. The 0.5 mg/L free chlorine residual maintains the water column but may not penetrate established biofilms on pipe surfaces, fill media, or basin walls. Legionella replicates inside amoebae within biofilms, where it is doubly protected: by the host organism and by the EPS matrix. Three potential harboring locations: (1) Dead legs or low-use branches with stagnant, warm water and no biocide circulation. (2) Fill media surfaces where biofilm accumulates in low-flow zones between distribution nozzles. (3) The basin sump or areas beneath the tower where sediment and biological debris settle, providing shelter and nutrients. The system is “well-treated” by bulk water standards but has not been evaluated by surface standards. The Legionella result is a surface problem, not a water column problem.
Read the connected sectionProblem 10
You are treating a cooling tower with no sidestream filtration, inconsistent biocide feed, and visible biofilm on fill media. The customer asks you to “double the biocide.” Write a 4–6 sentence explanation of why this approach alone will not work, and what additional steps are required. Reference access, shelter, and at least one pillar interaction.
Authored answer
Doubling the biocide will not solve this problem because the limiting factor is access, not potency. Visible biofilm on the fill media means established EPS infrastructure is present. Oxidant will react with the EPS matrix before reaching the organisms inside, and higher doses simply mean more chemistry consumed by the shelter rather than more organisms killed. Without sidestream filtration, suspended solids and biological debris recirculate continuously, providing nutrients and new attachment sites. The correct approach is: (1) mechanical cleaning of the fill and basin to physically remove the biofilm scaffold, (2) addition of a dispersant to lift remaining deposits from surfaces, (3) restoration of consistent biocide feed with rotation between oxidizing and non-oxidizing chemistries, and (4) installation or repair of sidestream filtration to remove suspended biological material and reduce the nutrient load that fuels regrowth. The biofilm is also creating conditions that accelerate corrosion through oxygen differential cells at the metal surface, something biocide alone cannot address.
Read the connected section