How should the water circulation pipeline be constructed to avoid bacterial growth?

Water for hemodialysis treatment is not ordinary tap water; it must circulate continuously throughout the entire pipeline network.

Why is continuous circulation mandatory? Once the flow velocity is inadequate, biofilm will rapidly proliferate on the inner wall of pipelines.

After biofilm forms, conventional disinfection methods cannot completely eliminate microorganisms. Bacteria shelter within the extracellular polymeric substance (EPS) polysaccharide protective layer, exhibiting 100 to 1000 times greater resistance to chemical disinfection than planktonic bacteria.

This is the root cause of the common issue encountered in many hemodialysis centers: water quality meets standards immediately after disinfection yet exceeds microbial limits again within a few weeks.

Appendix B of YY/T 1269-2015 Routine Control Requirements for Water Treatment Equipment Used in Hemodialysis and Related Therapies explicitly stipulates that the minimum recirculation flow velocity at the far end of hemodialysis water circulation pipelines shall be controlled at 0.4572 m/s (1.5 ft/s).

This parameter originates from standards issued by the Association for the Advancement of Medical Instrumentation (AAMI), and is also formally cited in the dialysis facility survey guidelines released by the U.S. Centers for Medicare & Medicaid Services (CMS):

"A minimum velocity of 1.5 ft/s in the distal portion of a direct feed system is recommended when the system is operating under conditions of peak demand."—— U.S. CMS State Operations Manual, Appendix H: End-Stage Renal Disease (ESRD) Dialysis Facility Survey Guidance(State Operations Manual Appendix H)

However, if the piping network is sized and routed for 40 dialysis stations yet only 10 dialysis machines draw process water, the actual flow rate across the entire pipeline will be roughly one-quarter of the full-load flow, and the flow velocity at the distal end of the piping will fall substantially below 0.4572 m/s.

Far more dangerous than insufficient flow velocity are the 30 vacant water supply outlets where dialysis machines are not yet installed.

Without standardized disposal, these outlets will form individual dead legs with completely stagnant water flow. The U.S. CMS guidelines clearly state:

"Dead-end pipes and unused branches and taps that can trap fluid must be eliminated because they act as reservoirs of bacteria and are capable of continuously inoculating the entire volume of the system."

Dead legs trap fluid, serve as bacterial reservoirs, and continuously inoculate bacteria into the entire water system.

This represents sound engineering logic.

During the renovation phase, the water supply and return piping shall be fully installed to meet the full-load capacity of 40 dialysis stations in one go to avoid subsequent hidden troubles. Retrofitting piping with construction work for future capacity expansion will incur far higher costs than the incremental expense of one-time complete installation.

The main water treatment units can be deployed in phases. For the initial 10 dialysis stations, select main units with a water output capacity sized for 10 to 15 stations. Reserve mounting spaces and piping connections for main units; additional or upgraded main units can be added for future expansion without disrupting normal operation of the existing system.

This is a detail that is most frequently overlooked and prone to operational pitfalls.

The 30 water supply outlets without installed dialysis machines shall neither be left open nor casually blocked with cloth.

Correct practice: Seal the outlets securely with dedicated blank plugs compatible with the piping material, to ensure disinfectant can reach the outlets during disinfection and flushing water can flow through during rinsing. Reinstall the blank plugs once disinfection is completed.

If conditions permit, install short valve-connected pipe nipples at each unused outlet. Keep the valves closed under routine operation and open them during disinfection to allow disinfectant circulation, so as to minimize dead leg risks to the greatest extent.

The water consumption of only 10 dialysis stations cannot sustain the recirculation flow velocity required for a piping system sized for 40 stations.

The solution is to install a bypass valve at the distal end of the loop. Excess water bypasses the dialysis machines and flows back directly to the main unit through the bypass pipeline to form an independent high-speed recirculation loop.

No matter how many dialysis stations are actually in operation, the recirculation flow velocity of the whole piping system can be consistently maintained above 0.4572 m/s, eliminating low-flow stagnant zones.

This design requirement shall be proposed during the renovation phase and explicitly incorporated into the system design by the water treatment equipment manufacturer. Modifications after system installation should be avoided.

This step is prone to improper operation: maintenance personnel tend to believe that disinfection is unnecessary for the 30 unused dialysis station branches.

This practice is incorrect.

Disinfection shall cover the entire planned piping network, including all pipe segments that are not yet connected to dialysis machines.

In accordance with YY 0793.2-2023 and WS/T 854-2025, disinfection protocols shall be dynamically verified based on microbial monitoring data, rather than determining the disinfection scope subjectively.

During the phased opening period, the piping layout is relatively lengthy with low flow velocity, resulting in a higher risk of bacterial proliferation compared with full-load operation. The disinfection frequency shall not be reduced; it must be implemented strictly in accordance with the operation manual of the water treatment equipment, or even increased appropriately.

The phased opening period carries the highest water quality risks. The following recommendations are provided:

• For the first three months before clinical operation commences, raise the bacterial monitoring frequency from once monthly to once every two weeks.
• Collect samples simultaneously at the supply start and pipeline distal ends, and closely track any upward trend of microbial levels at the distal end.
• Refer to the intervention threshold specified in YY 0793.2-2023 (bacteria ≥ 50 CFU/mL). Initiate disinfection and root cause investigation immediately once the intervention threshold is reached, instead of waiting until microbial limits are exceeded.