A Novel Water Heating System for Pathogen Control.pdf

A Novel Water Heating System for Pathogen Control

David Yashar, PhD

Deputy Chief, Energy and Environment Division

The National Institute of Standards and Technology

This slide deck was presented at the Legionella Conference 2019 Building Water Systems: The Sustainability and Public Health Nexus Organized by the NSF International & National Environmental Health Association September 11-13, 2019 Westin Bonaventure Hotel & Suites, Los Angeles, CA

9/9/2020

Slide 2

• Background information

• Current treatment methods

• Basis for temperature control

• Analysis for basic system and system with mixing valve

• Proposed design and analysis

• Summary and next steps

Outline

Slide 3

• Legionella is present in all water sources

• Certain conditions promote growth of bacteria

• Temperature, biofilm/organic matter

• Infection through inhalation

• Susceptibility

• Legionnaires’ disease or Pontiac fever

• More than 6 000 cases per year reported in the US*

• Suspected underdiagnoses, likely 10x

• 10 % fatality rate

• The best control is to not allow it in building water systems in the first place

Background

*Source: Center for Disease Control - https://www.cdc.gov/legionella/fastfacts.html https://www.cdc.gov/legionella/fastfacts.html

Slide 4

• Chemical – typically chlorine

• Added at treatment facility

• Becomes less effective with time, little certainty of how long until POU

• Ionization, filtration, ultraviolet

• More complex approaches

• Thermal

Common Treatment Options

Temperature Range Notes

Dormant Range T < 20 °C (68 °F) Alive but inactive, does not multiply, much less likely to cause harm

Active Range 20 °C < T < 45 °C (68 °F < T < 113 °F)

Ideal range for growth

High Temp Range

T > 50 °C (122 °F) Bacteria has difficulty sustaining life

Slide 5

• Temperature control can be a very effective tool

• Focused on maintaining temperatures outside of active range

• Heat treatment effectiveness is related to temperature and the duration of exposure

Temperature Control

Decimal Reduction Rate – time to kill 90% of existing microorganisms

50 °C (122 °F) 2 h

60 °C (140 °F) 2 min

70 °C (158 °F) All microorganisms killed, almost instantly

Source: Legionella and the prevention of legionellosis, World Health Organization, 2007

Slide 6

• Requirements are set to protect against scalding

• Max temp typically in the range of 46 °C (115 °F) – 49 °C (120 °F)

• Exposure time that results in scalding is roughly 2 orders of magnitude faster than the decimal reduction rate

High Temperature Concern: Scalding

Exposure time to induce a burn injury

37 °C (100 °F) None

48 °C (120 °F) 5 min

52 °C (127 °F) 1 min

56 °C (133 °F) 15 s

60 °C (140 °F) 5 s

64 °C (148 °F) 2 s

68 °C (155 °F) 1 s

Source: Moritz and Henriques, Studies of Thermal Injuries, 1947

Slide 7

• Typical systems/setpoints invariably result in water within active range

• Heated to the lower end of the High Temp Range

• May be short duration

• Discharged into building piping network

• Heat rejection to building reduces temperature back into active range

• Can dwell or stagnate in piping network for a long time

Basic Water Heating System* Hot water to building 49 °C (120 °F)

Cold water supply ≈ 16 °C (60 °F)

Nutrient-rich sludge

Temperature variation within tank

49 °C (120 °F)

≈ 32 °C (90 °F)

* All temperatures are examples for illustrative purposes

Slide 8

Temperature Profile for Basic System

Time

Te m p er at u re

Active growth range

Bacteria killing range

Inside heating system

Enter building

Hot water distribution system

Slide 9

• Benefit

• Better temperature control

• Maintain higher temperature in tank

• Problem

• Dormant microorganisms may be reintroduced through mixing valve

• Hot water piping network - same situation as before, maybe with fewer organisms

• Option - Hot Water Return

• Easier to maintain high temperature in piping network

• Still introducing dormant microorganisms

Basic System with Mixing Valve*

> 49 °C (120 °F)

> 32 °C (90 °F)

49 °C (120 °F)

Slide 10

Temperature Profile System with Mixing Valve

Time

Te m p er at u re

Hot water return

Bacteria killing range

Inside heating system

Enter building

Hot water distribution system

Slide 11

• Raise tank temperature much higher

• > 71 °C (160 °F) should kill all live organisms

• Water-to-water heat exchanger

• Thermal energy taken from water leaving tank, reduce supply to safe temperature

• Energy recovered to preheat water entering tank, minimizes energy impact on system

• Unmixed streams prevent dormant microorganisms from entering piping network

• Fixed volume tank will not allow flow without heat removal

• Minimizes possibility of occupant exposure to ultra-hot water

Proposed System* Hot water to building 49 °C (120 °F)

Cold water supply ≈ 16 °C (60 °F)

> 71 °C (160 °F)

> 54 °C (130 °F)

≈ 38 °C (100 °F)

Slide 12

Temperature Profile for Proposed System

Time

Te m p er at u re

Bacteria killing range

Inside heating system

Enter building

Hot water distribution system

Heat exchanger heat absorption

Heat exchanger heat rejection

Slide 13

• Water-to-water heat exchanger between tank inlet and outlet

• Eliminate living microorganisms to protect against exposure to waterborne pathogens

• Recovers energy from ultra-hot water to protect against scalding

• Rejected energy from ultra-hot water is reused to preheat incoming water to conserve energy

• Need experimental data to investigate

• Effectiveness for legionella control

• Sensitivity to water flow rate and inlet temperature

• Other factors

• For practical application

• Additional equipment ($)

• Thicker insulation to reduce impact of higher storage temperature

• Training and education needs

Summary and next steps