Secondary water supply refers to a system in which water is delivered to high-rise buildings through intermediate storage and pressurization units—such as water tanks or reservoirs and booster pumps—when municipal water pressure is insufficient for direct distribution. This process functions analogously to a "relay" mechanism, ensuring continuous water delivery to upper floors.

Problems that may exist in secondary water supply

Risk of water pollution (the most critical issue)

Mainly stems from water storage facilities and pipelines.

Pollution of water tanks (pools):

1. Microbial growth: If water tanks (pools) are not cleaned and disinfected regularly, the inner walls will breed algae, bacteria (such as Escherichia coli), viruses, etc. The non-sealed lids will also allow mosquitoes and small animals to enter, causing pollution.

2. Impurities settling: Iron rust, sand and other impurities in the water settle at the bottom of the water tanks over time, affecting water quality.

3. Unqualified materials: If the materials of water tanks (such as cement, inferior fiberglass reinforced plastic) are not qualified, harmful substances may be released or the tanks may rust.

Pollution of water supply pipelines:

If the materials of the pressure pump, valves and pipelines are of poor quality (such as using outdated galvanized steel pipes), they are prone to rust and corrosion, resulting in excessive levels of heavy metals like iron and manganese, causing the water to turn yellow.

Pipeline rupture or leakage at the joints may lead to sewage backflow or the infiltration of underground pollutants.

The "dead water" phenomenon:

If the occupancy rate of buildings is low or the design of water tanks is unreasonable, water will remain in the tanks for an excessively long time, turning into "dead water". The residual chlorine will be completely evaporated, losing its disinfection effect, and creating conditions for the reproduction of microorganisms.

Unstable water pressure：

-Insufficient pressure: During peak water usage periods (such as in the morning and evening), high-rise users may experience reduced water pressure, slow water flow, or even water cut-offs. This is usually due to improper pump selection, equipment aging, or poor scheduling.

-Excessive pressure: Unstable pressure increase may cause significant fluctuations in the water supply network pressure, which could easily damage water-using appliances such as faucets and connection hoses in users' homes.

Hygiene Issues：

During the process of secondary water supply, hygiene problems may arise. This is mainly due to improper cleaning of water supply equipment or inadequate maintenance of the water supply network. Hygiene issues can lead to the proliferation of bacteria and the spread of viruses, which can have an impact on human health.

Indicators for Monitoring Water Quality in Secondary Water Supply Systems

1. Temperature: Temperature is a key parameter influencing water quality stability. The typical range for potable water is 10–30 °C. Elevated temperatures may promote microbial proliferation, while excessively low temperatures can affect chemical equilibrium and disinfection efficiency.

2. Turbidity: Turbidity measures the degree to which suspended particles—such as silt, organic and inorganic debris, plankton, microorganisms, and colloidal matter—obstruct the passage of light through water. For tap water, turbidity should generally remain below 0.5 NTU to ensure clarity and effective disinfection.

3. pH Value: pH indicates the acidity or alkalinity of water. The acceptable pH range for drinking water is 6.5–8.5. Values outside this range may increase pipe corrosion or scaling, potentially affecting both water quality and distribution system integrity.

4. Residual Chlorine: Residual chlorine refers to the concentration of free or combined chlorine remaining in water after disinfection. It serves as a critical indicator of ongoing disinfectant capacity. While adequate residual chlorine prevents microbial regrowth, excessive levels may cause unpleasant odor and potential health concerns.

5. Organic Matter: Organic matter includes naturally occurring compounds such as humic and fulvic acids, phenols, and other dissolved organics. These substances can impair taste and odor, consume disinfectants, and serve as substrates for microbial growth, thereby compromising water safety.

6. Microorganisms: The presence of bacteria, viruses, protozoa, and other pathogenic microorganisms is a primary concern in water quality management. Effective disinfection is essential to ensure microbial levels comply with national and international drinking water standards.