Dose Determines if Soluble Copper is a Nutrient or an Antimicrobial for Legionella pneumophila in Premise Plumbing

The effect of copper on Legionella pneumophila in potable water plumbing systems is dependent on its dose and water chemistry. For instance, prior research demonstrated that the presence of aluminum hydroxide from anode rods in water heaters can bind copper and render high doses non-toxic. On this basis it was also hypothesized that iron hydroxide would have similar effects and that lower levels of copper may act as a nutrient encouraging Legionella growth. Here we conducted complementary experiments at bench, microcosm and pilot-scale to evaluate the effect of copper speciation and dose on L. pneumophila. At bench-scale, the addition of 5 mg/L as Fe iron hydroxide to a solution with 1 mg/L copper decreased soluble copper from > 90% down to < 20% at pH 6.5-7. The reduction in soluble copper caused ~3-logs higher L. pneumophila culturability when iron was added with copper when compared to a condition with copper alone. In a 9-month microcosm test using simulated glass water heaters with PEX pipe, a complete range of copper doses (0, 4, 30, 250 and 2000 g/L) were tested in triplicate. Over the first phase of research covered herein, the L. pneumophila levels were low at the four lowest doses of copper, and non-detectable at the highest dose. Moreover, total cell counts were highest at 250 g/L copper, lowest at 2000 g/L copper, and in between these extremes at the lower copper doses. This ongoing experiment will continue for months after this thesis was complete. Pilot-scale experiments were conducted with anode rods removed from tank water heaters, to examine the effects of unprotected corrosion of the steel on iron release and Legionella pneumophila levels in systems with 1) added copper (WH-Cu), 2) copper and phosphate corrosion inhibitor (WH-Cu+PO4-3), 3) phosphate corrosion inhibitor alone (WH-PO4-3) and 4) a control with neither copper nor inhibitor (WH-Control). While there were slight differences in iron between the conditions, the iron concentration in the water of the tanks did not dramatically increase compared to when the powered anodes were still installed and reducing corrosion. Because the iron level was usually < 0.1 mg/L, the released iron dose was not high enough to reduce soluble copper. In fact, oddly, soluble copper increased by 37-183%, mostly likely because the installed anode rods were suppressing copper release. Consequently, with the anode rods removed, the dose of 2000 g/L copper still had a strong antimicrobial effect. The levels of L. pneumophila increased in the order WH-Cu (2.6-logs CFU/L) < WH-PO4-3 (5.1-logs CFU/L) ≈ WH-Control (5.1-logs CFU/L). The addition of phosphate precipitated some of the added copper, such that the condition with copper and phosphate [WH-Cu+PO4-3 (4.2-logs CFU/L)] had L. pneumophila in between the condition with copper alone and that with no added copper. When the copper dose in the pilot rig was reduced to 1000 g/L, Legionella increased somewhat in the system with added copper compared to the control, and L. pneumophila increased in the water heaters in the order WH-Cu (3.4-logs CFU/L) < WH-Cu+PO4-3 (4.3-logs CFU/L) < WH-PO4-3 (4.9-logs CFU/L) ≈ WH-Control (5.0-logs CFU/L). Overall, an antimicrobial effect of copper was maintained in the water heaters even after the anodes were removed and iron in the water increased slightly. If iron corrosion and release to water were much higher without the anode rods than observed in this study, it would be predicted that the higher iron would have reduced the copper antimicrobial effect.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Environmental Engineering

Department dc:contributor.department

Environmental Science and Engineering

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2022

Author dc:creator

• Finkelstein, Rachel Briana

Chairs dc:contributor.committeechair

• Edwards, Marc A.
• Pruden, Amy

Committee member dc:contributor.committeemember

• Falkinham, Joseph O. III

• Legionella pneumophila
• Copper
• Iron
• Water Chemistry

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:35390

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/115603