System Design, Field Implementation and Performance Validation

Abstract

With the rise of fourth-generation residential concepts that prioritize health, intelligence and ecological living, high-quality pipeline direct drinking water has become a core component of premium residential supporting facilities. This paper presents an innovative centralized pipeline direct drinking water system developed for fourth-generation housing projects, adopting a novel short-process purification train consisting of carbon molecular sieve filtration, hollow fiber nanofiltration and combined ozone-UV disinfection. The system is equipped with a reverse-return circulation pipe network, dual-channel metering meters, UV germicidal faucets and an intelligent monitoring platform to ensure continuous, safe and stable water supply. Validated by a full-scale 320-household residential project, the effluent fully complies with the Quality Standard for Purified Drinking Water (CJ 94-2005), with high removal rates of turbidity, total dissolved solids, total hardness and total bacteria, as well as excellent drinking taste. This solution overcomes the drawbacks of traditional decentralized point-of-use water purifiers, providing a reliable, cost-effective reference for high-end residential drinking water system design.

1. Project Background and Overview

1.1 Demand for High-Quality Drinking Water in Fourth-Generation Housing

Fourth-generation residential buildings integrate ecological greening, smart home systems and sustainable design concepts, aiming to build healthier, more human-centered living environments. As a basic element of residential health, drinking water quality has become a core indicator of high-end residential quality.

Traditional drinking water solutions in Chinese residential areas, including under-sink purifiers, bottled water and vending machines, have prominent pain points: decentralized treatment leads to uneven water quality, frequent filter replacement increases user burden, long-term idle use causes secondary pollution, and high wastewater ratio contradicts green low-carbon concepts. In contrast, centralized pipeline direct drinking water systems achieve unified purification, circulating supply and professional operation, which have gradually become the mainstream direction for premium community water supply, and perfectly align with the green, healthy and intelligent positioning of fourth-generation housing.

1.2 Project Profile and Raw Water Quality

The project is the first fourth-generation residential demonstration community in a province in eastern China, covering a total land area of 20,000 m² with 5 residential buildings and 320 households. Instead of the traditional decentralized under-sink purifier model, a centralized direct drinking water pump house and dedicated supply pipeline network are constructed to deliver purified water directly to each household.

Raw water is sourced from the municipal water supply network, which meets the Standards for Drinking Water Quality (GB 5749-2022) at the plant outlet. However, after approximately 20 km of long-distance pipeline transmission, secondary pollution leads to obvious quality degradation at the community end:

• Increased turbidity caused by pipeline aging and sediment accumulation
• Elevated total bacterial count indicating microbial reproduction in the pipe network
• High total hardness and total dissolved solids affecting drinking taste
• Potential residual chlorine by-products from pipe network transmission

The core objectives of the system are to eliminate secondary pollution risks, reduce water hardness to improve taste, retain beneficial mineral elements, and establish a full-chain safety guarantee from treatment to terminal water withdrawal.

2. Process Route Selection

Traditional direct drinking water systems usually adopt a multi-stage process of “pretreatment + advanced treatment + post-treatment” with more than ten filtration units, which has complex flow, high energy consumption and large equipment footprint. For this fourth-generation housing project, a novel short-process purification technology was selected after comprehensive comparison, replacing multi-stage filtration with integrated high-efficiency units.

The comparison of core process solutions is as follows:

The selected short-process route integrates three core functional units: carbon molecular sieve composite filtration, hollow fiber nanofiltration, and combined ozone-UV disinfection. While ensuring purification effect, it greatly shortens the process flow, reduces equipment footprint and operating energy consumption, and is more suitable for centralized direct drinking water systems in residential communities.

3. Core Purification Technology Principles

3.1 Carbon Molecular Sieve Composite Filtration

As the first-stage purification unit, the carbon molecular sieve composite filter element is processed by combining powdered activated carbon with polymer hot-melt pore-forming materials, integrating dual functions of physical interception and adsorption.

• Physical interception removes turbidity, rust, particulate matter and suspended impurities in raw water
• Adsorption function effectively removes residual chlorine, organic matter, disinfection by-products and peculiar smell
• The integrated structure avoids the problem of loose carbon powder leakage in traditional activated carbon filters

This unit replaces the combined functions of traditional multi-stage precision filtration and activated carbon adsorption, realizing efficient pretreatment in a single unit.

3.2 Hollow Fiber Nanofiltration Membrane Separation

Hollow fiber nanofiltration is the core deep purification unit of the system. Its membrane pore size is between ultrafiltration and reverse osmosis, with selective separation characteristics:

• Effectively retains macromolecular organic matter, colloids, algae, bacteria, viruses and heavy metal ions
• Moderately reduces total dissolved solids and total hardness to improve drinking taste
• Retains beneficial natural mineral elements such as calcium, magnesium, strontium, potassium and metasilicic acid in raw water, avoiding the health and taste problems of pure water produced by RO membranes
• Lower operating pressure than reverse osmosis membranes, with significantly reduced energy consumption and wastewater ratio

This technology balances purification safety and drinking water health attributes, and is particularly suitable for household direct drinking water scenarios.

3.3 Combined Ozone and UV Disinfection System

The system adopts a dual disinfection mechanism of ozone + ultraviolet to ensure microbial safety of the whole circulating pipe network:

• Ozone disinfection is applied in the purified water tank, which has a broad-spectrum bactericidal effect and can continuously inhibit bacterial reproduction in the circulating pipe network, with residual ozone decomposed by a dedicated destructor before water supply
• Ultraviolet sterilizer is installed at the water supply outlet to inactivate bacteria and viruses instantly before water enters the pipe network
• Point-of-use UV germicidal faucets are installed at user terminals as the last safety barrier to eliminate secondary pollution risks of the household branch pipe

The three-level disinfection system covers the whole process from water production, pipeline circulation to terminal water withdrawal, ensuring continuous microbial safety of the water supply system.

4. System Design and Engineering Implementation

4.1 Centralized Water Purification Pump House

The direct drinking water pump house is built independently adjacent to the secondary water supply pump house to reduce pipeline loss and facilitate operation and maintenance. The main treatment equipment has a rated water production capacity of 0.25 m³/h, with a water supply pressure of 70 m, adopting a non-zoned water supply mode with pressure reducing valves installed on lower floors to ensure stable terminal water pressure.

The pump house adopts a modular layout:

• Raw water tank and purified water tank are arranged at the rear of the treatment system, with antibacterial respirators and ozone disinfection devices configured
• The main purification system is centrally arranged in the middle of the pump house for easy maintenance and filter replacement
• Variable frequency water supply pump set is arranged beside the purified water tank, serving both water supply and circulation functions
• The intelligent control system is arranged independently to realize water-electricity separation and automatic operation of the whole system

The full-circulation water supply mode is adopted to avoid stagnant water in the pipe network, and all unused return water is sent back to the pump house for secondary purification before storage.

4.2 Reverse-Return Circulation Pipe Network

The water distribution pipeline adopts food-grade SUS304 thin-walled stainless steel pipes with crimping connections to prevent bacterial growth and secondary pollution. The pipe network adopts a reverse-return (same 程) design, which ensures equal total length of water supply and return pipes for each loop, realizing hydraulic balance of the whole network and consistent water quality at each terminal.

Key design measures for the pipe network:

• Automatic exhaust valves are installed at the highest point of each building’s water supply pipe network to avoid air accumulation
• Flow regulating valves are installed on branch return pipes to adjust hydraulic balance of each loop
• Supply and return risers are arranged in public pipe shafts, with embedded plastic-coated stainless steel pipes for horizontal branches to meet concealed installation requirements
• Drain ball valves are set at the lowest point of risers as water quality sampling ports

According to specification requirements, the length of non-circulating branch pipes is controlled within 1 m, far below the 6 m limit specified in the technical regulation, effectively avoiding water quality deterioration caused by stagnant water in household branches.

4.3 Terminal and Intelligent Supporting Facilities

• Dual-channel metering water meters: Ultrasonic dual-channel meters are installed at each household to measure supply and return flow simultaneously, with the difference calculated as actual water consumption. The meters have remote data transmission function and built-in check valves to prevent cross-flow pollution.
• UV germicidal faucets: Built-in high-efficiency UV sterilization elements realize instant disinfection when water flows through. The rated flow rate is increased to 0.1 L/s to shorten users’ water collection waiting time and improve use experience.
• Intelligent monitoring system: Online monitoring devices for turbidity, pH, ozone concentration, conductivity and temperature are installed on the main supply and return pipes, with real-time data transmitted to the property monitoring center. The system can trigger alarms in case of abnormal water quality and support remote operation and maintenance scheduling.

5. Operational Performance Validation

After the system was completed and put into operation, a qualified third-party testing agency was commissioned to conduct water quality testing at 5 random sampling points in the community. The long-term operation results are as follows.

5.1 Water Quality Performance

All effluent indicators fully meet the requirements of Quality Standard for Purified Drinking Water (CJ 94-2005), with core performance indicators as follows:

• Turbidity is reduced to below 0.08 NTU, with clear and colorless effluent
• Total dissolved solids are controlled at about 170 mg/L, and total hardness is stabilized at 75–95 mg/L, with an average value of 85 mg/L, which falls within the 10–100 mg/L range defined as “good-tasting water” by international drinking water research, with no bitter taste and excellent mouthfeel
• Total bacteria, total coliforms and thermotolerant coliforms are all not detected, with a 100% removal rate of microorganisms
• Heavy metal indicators such as lead, arsenic, cadmium and mercury are all far below the standard limits, and disinfection by-products such as chloroform are not detected

After 2 years of continuous operation, the system maintains stable effluent quality, and user satisfaction surveys show high recognition of water taste and safety.

5.2 Economic Benefit Analysis

• Household drinking cost: The metered charging price of the system is 350 RMB/ton, which is 40–76% lower than the 800–1500 RMB/ton cost of traditional bottled water, and also significantly lower than the comprehensive cost of household water purifiers including equipment purchase and filter replacement.
• System operation cost: The comprehensive operating cost is 8.95 RMB/ton, including water fee, electricity fee, filter replacement cost and labor cost. The short-process design greatly reduces the number of consumables and manual intervention requirements, and the intelligent automatic cleaning system further reduces operation and maintenance costs.
• Investment payback period: The estimated static investment payback period of the project is 6.5 years. With policy subsidies for energy-saving and emission-reduction equipment and carbon trading benefits, the comprehensive return rate can be increased by 8–12 percentage points.

6. Engineering Application Analysis

6.1 Applicable Scenarios

This short-process direct drinking water system is suitable for the following project types:

• Fourth-generation residential communities and high-end residential projects with requirements for drinking water quality
• High-end office buildings, hotels, hospitals and other public buildings with centralized direct drinking water demand
• Renovation projects of existing communities to upgrade drinking water quality
• Scenarios with limited equipment room space and requirements for compact system layout

6.2 Key Engineering Design Points

• Raw water quality investigation: Detailed raw water quality testing at the community end must be carried out in the early design stage, especially for projects in urban fringe areas with long water transmission distances, to avoid design deviation caused by secondary pollution of the pipe network.
• Pipe network hydraulic design: Reverse-return design is preferred to ensure hydraulic balance of each loop. The length of non-circulating branch pipes should be strictly controlled, and stainless steel or copper pipes that meet food-grade requirements should be selected to avoid secondary pollution.
• Zoning design: For high-rise residential projects, vertical zoning water supply should be adopted according to building height to ensure stable terminal water pressure and avoid excessive pressure bearing of low-floor pipelines.
• **Disinfection system matching: The combined disinfection scheme of circulating pipe network and terminal point-of-use disinfection should be adopted to cover the whole water supply chain and eliminate dead-angle pollution risks.

6.3 Operation and Maintenance Best Practices

• Establish a regular water quality testing mechanism, and conduct comprehensive water quality testing at least quarterly
• Replace core consumables such as carbon molecular sieve filter elements and nanofiltration membranes regularly based on comprehensive evaluation of differential pressure, cumulative water production and effluent quality
• Maintain normal operation of the circulation system to avoid long-term stagnant water in the pipe network
• Regularly check the performance of ozone generators, UV lamps and online monitoring instruments to ensure the stable operation of the safety guarantee system

6.4 Full Lifecycle Value

Compared with the decentralized household water purifier model, the centralized pipeline direct drinking water system has obvious advantages in full lifecycle: unified professional operation ensures stable water quality, avoids waste of resources caused by repeated purchase of household equipment, and reduces plastic waste from bottled water and wastewater from household purifiers, which conforms to the green and low-carbon concept of fourth-generation housing.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we believe that healthy, intelligent and integrated water supply solutions will become the core supporting facility for the next generation of high-end residential buildings. The short-process direct drinking water technology fully aligns with our technical philosophy of modular equipment manufacturing and smart water system integration.

Combined with SYNERAQUA’s skid-mounted equipment manufacturing capabilities, this purification system can be integrated into prefabricated modular pump house units, greatly shortening on-site construction periods and improving installation quality. Integrated with our SCADA automation platform and smart property management system, it can realize functions such as remote water quality monitoring, automatic filter replacement reminder and fault early warning, reducing manual operation and maintenance costs while improving management efficiency. We continue to optimize the membrane material performance and process integration scheme to provide more efficient, energy-saving and reliable centralized direct drinking water solutions for residential and commercial building projects.

8. Conclusion

The short-process pipeline direct drinking water system provides a high-quality drinking water solution matching the positioning of fourth-generation housing, with the following core conclusions:

1. The “carbon molecular sieve + hollow fiber nanofiltration + combined disinfection” short-process route achieves efficient water purification with a compact flow, retains beneficial mineral elements, and the effluent fully meets the quality standard for purified drinking water with excellent taste.
2. The system configuration of reverse-return circulation pipe network, dual-channel metering and terminal UV disinfection realizes full-chain water quality safety guarantee, effectively avoiding secondary pollution risks in the transmission and distribution process.
3. The centralized treatment mode significantly reduces household drinking costs, with stable operating costs and reasonable investment payback period, and has good economic benefits and promotion value.
4. The intelligent monitoring platform realizes unattended operation and remote management, which conforms to the intelligent and green development direction of fourth-generation residential buildings, and can be widely referenced for similar high-end residential direct drinking water system design.

FAQ

Q1: What is the difference between nanofiltration direct drinking water and reverse osmosis pure water?
Nanofiltration membranes have a larger pore size than reverse osmosis membranes. They can remove harmful substances such as heavy metals, bacteria and organic matter while moderately retaining beneficial mineral elements such as calcium and magnesium in water. Reverse osmosis membranes remove almost all dissolved substances to produce pure water, which has a flat taste and is not suitable for long-term daily drinking.

Q2: Why does the pipeline direct drinking water system adopt a circulating pipe network design?
The circulating pipe network ensures that water in the pipeline keeps flowing all the time, avoiding stagnant water that leads to bacterial reproduction and water quality deterioration. The reverse-return design further ensures balanced water pressure and consistent water quality at each terminal of the community.

Q3: Is pipeline direct drinking water more cost-effective than household water purifiers?
For users, the metered cost of pipeline direct drinking water is 40–76% lower than the comprehensive cost of bottled water and household water purifiers, and there is no need for users to replace filter elements or maintain equipment, which is more convenient and economical. For property managers, the centralized system has higher operation efficiency and lower per-unit management cost.

Q4: How does the system ensure the safety of terminal water quality?
The system establishes a three-level safety barrier: first, centralized purification and disinfection at the pump house; second, continuous circulating disinfection of the pipe network; third, point-of-use UV disinfection at the terminal faucet. Combined with real-time online water quality monitoring, it fully guarantees the drinking water safety of end users.