Controlling Legionella pneumophila growth in hot water systems by reducing dissolved oxygen levels

Figures & data

Figure 1. Diagram of hot water model system.

(1) Makeup water supply line; (2) check valve to prevent backflow of water; (3) flow meter with adjustment valve; (4) sampling port valve for Incoming Supply Water; (5) sampling port valve for Hot Water Return; (6) 5 µm filter; (7) membrane contactor; (8) vacuum pressure gauge; (9) vacuum pump; (10) high purity nitrogen supply tank; (11) water heater tank; (12) sampling port valve for Hot Water Tank; (13) circulating water pump; (14) dissolved oxygen probe; (15) branch CPVC line; (16) electronic solenoid valve; (17) sampling port valve for Distal Fixture. Total volume of system is approximately 200 L.

Figure 2. Hot water model system L. pneumophila inhibition below 0.3 mg/L DO.

(+) indicates amplification of L. pneumophila in the hot water system; (−) indicates inhibition of L. pneumophila in the hot water system; (*) indicates no data available from the prior week's sample as this was the first round of sampling; error bars reflect the highest and lowest values used to calculate the geometric mean of L. pneumophila concentrations; data without error bars represents a single sample value.

Table 1. Measurement data from hot water model system.

	Colonizing Phase			Scoping Phase			Threshold Testing Phase
Sample Location
Days Since Inoculation	23	29	36	43	50	57	64	71	78
Test Week ID	C1	C2	C3	S1	S2	S3	T1	T2	T3
Incoming/Supply Water
L. pneumophila (CFU/mL)	–	5	20	0.5	1.0	40	250	20	670
Dissolved Oxygen (mg/L)	4.74	5.11	5.84	4.65	4.20	4.47	5.60	4.60	4.75
Hot Water
L. pneumophila (CFU/mL)	88	22	45	24	134	615	14	1	32
Dissolved Oxygen (mg/L)	1.79	0.53	0.69	0.38	0.41	0.82	0.29	0.25	0.27
Distal Fixture
L. pneumophila (CFU/mL)	1,140	490	>3,000	2,310	2,320	>3,000	>1,500	2,930	>3,000
Dissolved Oxygen (mg/L)	4.60	4.17	6.38	6.73	4.03	4.02	6.80	4.66	4.30
*D-Value (at 42 ^OC) in Days	–	–	–	–	–	–	5.642C	5.442C	5.342C
Incoming/Supply Water
NTM (CFU/mL)+	5	–	–	–	–	–	–	–	324
Dissolved Oxygen (mg/L)	4.74	–	–	–	–	–	–	–	4.75
Hot Water
NTM (CFU/mL)++	66	–	–	–	–	–	–	–	<1
Dissolved Oxygen (mg/L)	1.79	–	–	–	–	–	–	–	0.27
Distal Fixture
NTM (CFU/mL)++	35	–	–	–	–	–	–	–	119
Dissolved Oxygen (mg/L)	4.60	–	–	–	–	–	–	–	4.30

Samples and measurements taken from the Incoming/Supply Water represent the water entering the model test system (Figure 1 Site 4) where DO levels were not measured but not controlled. Samples and measurements labeled as Hot Water represent a combination of water leaving the hot water heater tank (Figure 1 Site 12) combined with the Hot Water Return (Figure 1 Site 5) of the model test system; where DO levels were measured and controlled. Samples and measurements taken from the Distal Fixture represent the water discharged from the model test system (Figure 1 Site 17) through an automated kitchen faucet where DO levels were measured but not controlled. *D-Value is reported as the number of days to reduce L. pneumophila concentrations in the circulating hot water compared to the incoming water by 1-log at 42 °C. +Non-Tuberculosis Mycobacteria (NTM) values reflect total concentrations of M. llatzerense, M. chimaera/intracellulare, and M. gordonae. ++ values reflect total concentrations of M. chimaera/intracellulare and M. gordonae.

Data availability

The author confirms that the data supporting the findings of this study are available within the article [and/or] its supplementary materials.