Process Optimization, Retrofitting Solutions and Techno-Economic Analysis

Abstract

As a critical water resource, groundwater plays a core role in urban water supply systems. Rising demand for high-quality drinking water has drawn increasing attention to high hardness levels in groundwater sources. To improve drinking water experience and supply quality, many existing water treatment plants are upgrading with advanced treatment processes to reduce effluent hardness. This paper explores the design key points of a dual-membrane hardness removal retrofitting project for a groundwater WTP, including process selection, flow configuration, core design parameters such as treatment scale and effluent targets, and targeted solutions for retrofitting challenges. A 200 μm self-cleaning filter before ultrafiltration (UF) and a 5 μm cartridge filter before nanofiltration (NF) are installed to effectively protect membrane modules, achieving a UF recovery rate of 96.5% and NF recovery rate of 80.2%. After the dual-membrane treatment, groundwater hardness is reduced from 440–470 mg/L to 20–30 mg/L. Blending membrane permeate with raw water in a mixing well for 1 minute achieves uniform water distribution and favorable economic efficiency. Water quality and quantity balance verification confirms that the total dissolved solids (TDS) of membrane concentrate is no higher than 2000 mg/L, meeting municipal sewer discharge requirements. Through targeted design and BIM 3D modeling, the project overcomes difficulties including limited land space and complex existing pipelines and facilities, successfully achieving the target of finished water hardness ≤200 mg/L. The project has a unit investment of 1550 yuan per ton of capacity, a total treatment cost of 1.22 yuan/m³, and an incremental water tariff of 1.34 yuan/m³. The design experience summarized in this project can provide reference and design ideas for similar groundwater upgrading projects.

1. Introduction

Groundwater accounts for approximately 18% of China’s total available water resources and 30% of total water consumption, with more than 290 cities relying on groundwater as their primary drinking water source. In northern China, groundwater supplies over 59.8% of urban water demand. Groundwater is valued for its stable quality, sufficient quantity and low conventional treatment cost, typically requiring only sedimentation and disinfection to meet basic drinking water standards.

However, with rising public demand for higher drinking water quality, high hardness in groundwater sources has become a prominent issue. Excessive calcium and magnesium ions, leached from mineral-rich rock formations, cause adverse impacts on daily life and industrial production: hard water reduces washing efficiency, damages fabric texture, and leads to scaling in boilers and pipe networks. Long-term consumption of excessively hard water is also associated with increased risk of urinary calculi, while extremely soft water is linked to higher prevalence of cardiovascular and metabolic diseases. Therefore, moderate hardness reduction through water plant upgrading is of great practical significance to improve public drinking water experience and health benefits.

Among existing hardness removal technologies, membrane processes have become the mainstream solution for drinking water advanced treatment, as they effectively reduce hardness while improving water taste and organoleptic quality. Compared with reverse osmosis (RO), nanofiltration offers lower operating pressure, lower energy consumption, and retains part of the beneficial minerals in water, making it more suitable for drinking water hardness reduction. This paper presents a full-scale groundwater WTP upgrading project adopting the UF+NF dual-membrane process, systematically summarizes its design highlights and operational performance, and provides technical reference for similar projects.

2. Project Overview and Retrofitting Challenges

2.1 Plant Profile

The target groundwater water treatment plant has a designed supply capacity of 60,000–70,000 m³/d, supplied by 4 well fields with complex source water conditions. Raw groundwater has a total hardness of 440–470 mg/L, with all other indicators meeting the requirements of Standards for Drinking Water Quality (GB 5749-2022) except hardness. The original process only included lift pumping and sodium hypochlorite disinfection, with no hardness removal capacity. Previously, the plant relied on blending with transferred surface water in the clear well to reduce finished water hardness to approximately 360 mg/L, which still falls short of high-quality drinking water standards and incurs high water transfer costs.

2.2 Key Retrofitting Difficulties

The primary technical challenge is selecting an appropriate hardness removal process and setting a reasonable effluent hardness target that balances water quality and investment cost. Additional design considerations include reasonable treatment scale, scientific process parameters, convenient operation and energy efficiency.

Furthermore, the plant is located in a built-up urban area with compact site layout. Existing facilities including clear wells, pump houses, chlorination rooms and power distribution buildings occupy most of the site, leaving only a 1000 m² landscape pond available for new construction, resulting in extremely tight land resources. The complex existing underground pipeline network also requires careful pipeline connection design, finished water distribution and protection of existing facilities during retrofitting.

3. Process Design and Parameter Determination

3.1 Hardness Removal Process Comparison and Selection

Mainstream hardness removal technologies include boiling, chemical softening, ion exchange and membrane filtration. A systematic comparison of three widely applied industrial processes is shown in Table 1.

Table 1 Comparison of Main Hardness Removal Processes

Ion exchange is unsuitable for large-scale municipal drinking water plants due to high operating cost and regeneration wastewater disposal issues. RO achieves thorough desalination but has high energy consumption and removes beneficial trace minerals, which is not ideal for drinking water supply. Nanofiltration, with a pore size between UF and RO, achieves 90–98% removal of divalent ions such as magnesium sulfate while retaining part of monovalent ions, with lower operating pressure and energy consumption than RO.

Considering treatment performance, construction cost and operating cost comprehensively, the ultrafiltration + nanofiltration dual-membrane process was selected for total hardness removal in this project.

3.2 Process Flow Configuration

Pre-treatment units are installed before both UF and NF membranes to protect membrane elements:

• A 200 μm self-cleaning filter is set before UF, which automatically cleans the filter screen based on pressure difference or timer, with features of low pressure loss, corrosion resistance and continuous water production.
• A 5 μm cartridge security filter is set before NF, to prevent membrane filament shedding from UF, algae growth in backwash water and concrete debris from damaging NF membranes.

A bypass pipeline is installed before the UF system for flexible operation: when raw water quality is favorable, raw water can directly enter the NF system to reduce energy consumption. The original groundwater supply system is fully retained for emergency water supply. The upgraded process flow is as follows:

3.3 Effluent Hardness Target Setting

Drinking water hardness standards vary significantly across countries. Both excessively high and excessively low hardness have adverse health impacts. Epidemiological studies indicate that 170 mg/L is an optimal total hardness value for drinking water from a health perspective.

Comprehensively considering the hardness level of peer water plants in the region and engineering investment, the project sets the finished water total hardness target at ≤ 200 mg/L, which balances drinking water safety, taste and economic cost.

3.4 Membrane System Design Parameters

Based on mainstream membrane product performance and reference project experience, core design parameters are determined as follows:

• Ultrafiltration system: External pressure filtration mode, designed system recovery rate of 90%, net flux of 49.84 L/(m²·h), transmembrane pressure (TMP) of 0.2 MPa, filtration cycle of 60 min (58 min production + 2 min air/water backwash), chemical cleaning cycle of once per month.
• Nanofiltration system: One-stage two-pass arrangement with a 2:1 membrane element ratio between first and second pass, designed system recovery rate of 80%, average flux of 21.6 L/(m²·h), operating pressure of 0.5–0.9 MPa. Designed hardness removal rate is 90%, nitrate removal rate 55%, TDS removal rate 50%. Antiscalant (5 mg/L) and reducing agent (2 mg/L) are continuously dosed to prevent calcium/magnesium scaling and membrane oxidation.

3.5 Project Scale Determination with Blending Scheme

To reduce engineering investment, the project adopts a “partial treatment + raw water blending” scheme: only part of the raw water is treated by the dual-membrane system, then blended with untreated raw water to meet the finished water hardness target.

Water quantity calculation under different membrane scales shows that a 40,000 m³/d membrane permeate output can be blended with 23,300 m³/d raw water, meeting the plant’s planned total supply capacity. Considering urban development, water demand reserve and land constraints, the membrane workshop is designed for a long-term scale of 50,000 m³/d, with equipment installed for a near-term capacity of 40,000 m³/d.

4. Core Design Highlights

4.1 Water Quality and Quantity Balance Verification

After determining the treatment scale and membrane performance parameters, water quality and quantity balance is verified for both near-term and long-term operation, to ensure that membrane concentrate meets the discharge standard for municipal sewers.

Verification results show that the TDS concentration of NF concentrate is controlled below 2000 mg/L, complying with the Wastewater Quality Standard for Discharge to Municipal Sewerage. Near-term total finished water supply reaches 63,300 m³/d with a hardness of 199.5 mg/L, fully meeting the design target. The balance calculation also provides a basis for concentrate discharge pipeline design and blending ratio adjustment.

4.2 Pipeline Connection and Uniform Blending Design

A DN1000 pipeline is diverted from the existing raw water main to the membrane system, with multiple control valves to flexibly switch between membrane treatment mode and direct supply mode under emergency conditions.

A new mixing and distribution well is designed to ensure uniform blending of NF permeate and untreated raw water. Membrane permeate is delivered by gravity through a new DN800 pipeline to the mixing well, where it is blended with raw water and disinfected for 1 minute, then distributed to two existing clear wells. Weir-type head control is set at the outlet of the NF product water tank and mixing well to prevent backflow of blended water during membrane backwash, eliminating the need for additional overflow pipelines.

4.3 Compact Membrane Workshop Layout Optimization

Due to extremely limited site area, a conventional single-story membrane workshop is not feasible. Two layout schemes (2-story vs. 3-story) were compared in the design stage, as shown in Table 2.

Table 2 Comparison of Membrane Workshop Layout Schemes

Comprehensively considering project benchmarking goals, aesthetics and functional requirements, the 3-story layout was adopted:

• Ground floor: process tanks and rotating equipment, including UF feed tank, permeate tank, NF product water tank, feed pumps, high-pressure pumps, backwash pumps and filters
• Middle floor: pipe gallery and chemical dosing room, with inter-stage booster pumps for NF
• Top floor: electrical control room and membrane stack area, with 10,000 m³/d capacity per membrane train for flexible operation

4.4 BIM 3D Design for Complex Pipeline Integration

The dual-membrane system has an extremely complex pipeline network, including feed, permeate, backwash, air scour and chemical cleaning pipelines for both UF and NF systems. To avoid pipeline collision in the limited 3-story space, Building Information Modeling (BIM) 3D design was adopted throughout the design and construction phases.

The 3D model provides intuitive visual guidance for installation, effectively eliminating conflicting pipeline layouts and reducing rework during construction. This technology significantly improves construction accuracy and efficiency for compact multi-story membrane facilities.

5. Techno-Economic Analysis and Operational Performance

5.1 Investment and Cost Analysis

The upgrading project has been completed and put into stable operation. Economic calculation results are as follows:

• Unit construction investment: 1550 yuan per ton of daily treatment capacity
• Total treatment cost: 1.22 yuan/m³
• Operating cost: 0.89 yuan/m³
• Incremental water tariff (considering internal rate of return and payback period): 1.34 yuan/m³

5.2 Full-Scale Operational Performance

Actual operational data confirms stable and reliable system performance:

• Ultrafiltration system recovery rate reaches 96.5%, with power consumption of 92 kWh per thousand tons of produced water
• Nanofiltration system recovery rate reaches 80.2%, with power consumption of 338 kWh per thousand tons of produced water
• NF permeate total hardness is 20–30 mg/L, corresponding to a hardness removal rate of 93.6–95.5%, exceeding the design target
• Finished water after blending has turbidity of 0.06–0.10 NTU, with hardness stably maintained below 200 mg/L, fully meeting and exceeding national drinking water standards

6. Engineering Application Insights

6.1 Applicable Scenarios

The UF+NF dual-membrane hardness removal process is particularly suitable for the following scenarios:

• Groundwater water treatment plants requiring hardness reduction upgrading, especially in northern China
• Retrofitting projects with limited site area and complex existing facilities
• Drinking water projects requiring moderate desalination while retaining beneficial minerals
• Plants pursuing high-quality water supply standards above national basic requirements

6.2 Key Design Considerations for Similar Projects

• Blending scheme verification: The partial treatment + blending solution can significantly reduce investment, but rigorous water quality calculation and balance verification must be conducted to ensure stable finished water quality under all operating conditions.
• Concentrate discharge assessment: TDS and other pollutant indicators of membrane concentrate must be checked against local sewer discharge standards in advance to avoid secondary environmental problems.
• Operational flexibility design: Bypass pipelines and multi-point valve control should be reserved to adapt to seasonal changes in raw water quality and reduce unnecessary operating energy consumption.
• Compact layout optimization: For land-constrained retrofitting projects, vertical multi-story layout combined with BIM 3D pipeline design is an effective solution to maximize space utilization.

6.3 Operation and Maintenance Best Practices

• Establish regular membrane performance monitoring mechanisms, tracking TMP, flux and rejection rate changes to arrange chemical cleaning in a timely manner.
• Optimize antiscalant dosage dynamically based on raw water hardness and recovery rate to prevent membrane scaling while reducing chemical consumption.
• Adjust the blending ratio of membrane permeate and raw water according to raw water quality fluctuations, to balance water quality standards and operating costs.
• Develop standardized membrane chemical cleaning procedures and long-term performance attenuation management plans to extend membrane service life to 5–7 years.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that UF+NF dual-membrane technology is the optimal solution for groundwater drinking water quality upgrading, balancing treatment efficiency, water quality safety and operating economy.

Our skid-mounted modular dual-membrane water purification units are prefabricated and factory-tested, which can greatly shorten on-site construction periods and reduce civil engineering investment for retrofitting projects. Equipped with our intelligent membrane operation management platform, the system realizes real-time monitoring of membrane fouling status, automatic adjustment of operating parameters and intelligent cleaning reminder, which can further improve system recovery rate by 3–5% and reduce comprehensive energy consumption by more than 10%. We also provide full-process drinking water advanced treatment solutions including pre-treatment, dual-membrane system and post-disinfection, helping water utilities achieve high-quality water supply upgrading efficiently and economically.

8. Conclusion

The UF+NF dual-membrane process has excellent applicability for hardness removal in groundwater water treatment plants, and will become one of the core advanced treatment technologies for high-quality drinking water supply in the future. The main conclusions of this project are as follows:

1. Pre-filtration units including 200 μm self-cleaning filter and 5 μm security filter can effectively protect membrane elements and ensure long-term stable operation of the dual-membrane system. Bypass pipelines significantly improve operational flexibility of the plant.
2. The “partial membrane treatment + raw water blending” water supply scheme can effectively reduce engineering investment. Accurate blending ratio calculation and water quality balance verification are critical design links, and uniform mixing facilities are necessary to ensure stable finished water quality.
3. For land-constrained retrofitting projects, a multi-story compact membrane workshop combined with BIM 3D pipeline design can effectively solve the contradiction between process requirements and limited land, with high application value for similar urban upgrading projects.
4. The project achieves stable finished water hardness below 200 mg/L, with a unit investment of 1550 yuan/ton and total cost of 1.22 yuan/m³, providing reliable reference for similar groundwater hardness removal projects.
5. Appropriate drinking water hardness is critical to public health, and further research is needed to establish unified optimal hardness standards for drinking water in China. For projects prioritizing cost over aesthetics, a 2-story membrane workshop layout is recommended to reduce civil investment and improve maintenance convenience.

FAQ

Q1: Why is nanofiltration selected instead of reverse osmosis for drinking water hardness removal?
Compared with reverse osmosis, nanofiltration has lower operating pressure and energy consumption, reducing operating costs by approximately 30–40%. More importantly, nanofiltration retains part of the beneficial monovalent minerals in water while removing more than 90% of calcium and magnesium hardness ions, producing healthier and better-tasting drinking water, which is more suitable for municipal water supply scenarios.

Q2: Is the blended water supply solution safe and stable?
Yes. The blending ratio is calculated based on the maximum raw water hardness and minimum NF removal rate, with sufficient safety margin. A dedicated mixing well with 1-minute hydraulic retention time ensures thorough and uniform mixing. The finished water hardness is continuously monitored online, and the blending ratio can be adjusted dynamically to ensure stable and compliant water quality.

Q3: How is the membrane concentrate disposed of in this project?
The TDS concentration of nanofiltration concentrate is verified to be below 2000 mg/L, which fully meets the discharge standard for municipal sewers. The concentrate is discharged to the municipal wastewater pipe network and treated at the urban wastewater treatment plant, with no additional environmental impact.

Q4: What is the typical service life of UF/NF membranes and the associated maintenance cost?
Under standard operation and regular maintenance, ultrafiltration membranes have a service life of 5–7 years, and nanofiltration membranes have a service life of 4–6 years. Annual maintenance costs including chemical cleaning, antiscalant and membrane replacement amortization account for approximately 30–40% of total operating costs. Standardized maintenance can effectively extend membrane life and reduce life-cycle costs.