The Blueprint of Water

Microbiology: Answer Key

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⟵ 09 Microbiology (Problem Set) · Contents · 10 Suspended Solids ⟶

Microbiology: Answer Key

  1. 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.

  2. 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.

  3. 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.

  4. At pH 8.3, interpolating from the chlorine/pH table, approximately 13–14% of the chlorine is present as HOCl, with 85–88% as the far less effective OCl⁻. At 0.5 mg/L total residual, only about 0.06–0.08 mg/L is present as HOCl. This is marginal at best for effective biological control. The tower needs either pH reduction to shift the equilibrium toward HOCl, a switch to a more pH-stable oxidant such as bromine, or an increase in total residual to compensate for the reduced effectiveness at this pH.

  5. 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.

  6. Eighteen months of the same non-oxidizing biocide has created sustained selection pressure. Organisms tolerant to isothiazolone – through thicker EPS, dormancy responses, or altered uptake pathways – 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 biodispersant 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.

  7. The dispersant is working. The turbidity spike and increased ATP in bulk water indicate that the biodispersant is lifting biofilm material from pipe and fill surfaces into the water column - material that was previously hidden from both monitoring and treatment. 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.

  8. Friday: Biocide residual drops to zero within hours as remaining oxidant is consumed. Planktonic organisms begin reproducing unchecked in the warm recirculating water. Saturday: Planktonic populations multiply rapidly. Cells begin attaching to surfaces - particularly in low-flow zones, on fill media, and on heat-exchanger tubes. Initial EPS production begins. Existing biofilm communities, previously suppressed by biocide, resume active growth and begin expanding. Sunday–Monday: Biofilm thickens on heat-exchange surfaces, adding thermal resistance (fouling – Suspended Solids pillar). The insulating layer raises skin temperature on the condenser tubes, which increases CaCO₃ scaling pressure at the surface (Scale pillar) through inverse solubility. Under-deposit oxygen differential cells begin forming beneath biofilm, initiating localized corrosion (Corrosion pillar). The condenser’s ability to reject heat decreases – approach temperature rises, condensing pressure increases, and compressor efficiency drops (Pillars chapter free body diagram). A 72-hour biocide outage does not create a new problem. It accelerates the interaction cascade across all four pillars simultaneously.

  9. Excellent bulk water chemistry does not guarantee clean surfaces. The 0.5 mg/L chlorine residual maintains the water column but may not penetrate established biofilms on pipe surfaces, fill media, or basin walls. The system’s pH determines how much of that chlorine is present as effective HOCl. 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.

  10. 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 biodispersant 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 – a pillar interaction that biocide alone cannot address.

⟵ 09 Microbiology (Problem Set) · Contents · 10 Suspended Solids ⟶

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