Fine Design of Low Temperature and Low Turbidity Raw Water: Technical Analysis of Dalian River Water Treatment Project in Liaoning Province
Chen Shichu, Liang Jiawen, Cui Hechuan, Huang Jingchun, Wu Mengmeng, Chen Yongquan
（Foshan Hongjun Water Treatment Equipment Co., Ltd）
Summary
This article takes the low-temperature and low turbidity raw water in winter in Dalian, Liaoning Province as the research object, and proposes a refined and integrated water treatment process plan to address the problems caused by low water temperature (1-4 ℃) and low turbidity (<10 NTU), such as difficult flocculation, poor formation of alum flowers, and low sedimentation efficiency. The system has improved the treatment efficiency of low-temperature and low turbidity water by key technical measures such as extending flocculation time, controlling G value through graded stirring, strengthening GT value regulation through grid flocculation, reducing surface load of sedimentation tank, and selecting high-efficiency V-shaped filter. The project design scale is 10000 tons/day, and the effluent quality is stable and better than the requirements of the "Sanitary Standards for Drinking Water" (GB 5749-2022), providing successful experience and technical reference for water treatment processes under similar water quality conditions in cold northern regions of China.
keywords
Low temperature and low turbidity water; Strengthen flocculation; Grid flocculation tank; GT value control; V-shaped filter; Water treatment design
Project Background
The Dalian area in Liaoning Province belongs to a temperate monsoon climate, with long and cold winters (November to March of the following year). The raw water temperature can drop to 1-4 ℃, and the turbidity is significantly reduced (usually below 10 NTU during the low temperature period). Low water temperature leads to increased viscosity of water, weakened Brownian motion of particles, and increased zeta potential of colloidal particles, making it difficult for traditional flocculation processes to form effective alum flowers; Low turbidity means a lack of carrier particles required for flocculation, further exacerbating the difficulty of flocculation. To meet the daily demand of 10000 tons of high-quality drinking water in Dalian, we have designed an integrated water purification equipment for low-temperature and low turbidity raw water. This article focuses on introducing its core process design ideas and technical characteristics.
Raw water characteristics and design challenges
1.Low temperature effect: When the water temperature is below 5 ℃, the dynamic viscosity of water increases by about 50%, resulting in:
The collision efficiency of particles sharply decreases, and the flocculation effect is poor.
Alum flowers form slowly, have a loose structure, and are not easily settled.
2.Low turbidity characteristics: The concentration of suspended solids in the raw water is low, the distance between particles is large, and the collision probability is small, requiring better hydraulic conditions and flocculants to create collision opportunities.
3.Seasonal differences in water quality: The design should take into account the normal water quality in other seasons and require the process to have good regulatory flexibility.
Core process design and optimization
We have comprehensively strengthened and refined the conventional process to address the difficulty of low temperature and low turbidity water. The process flow is: reinforced flocculation reaction tank → grid flocculation tank → inclined tube sedimentation tank → V-shaped filter tank.
1. Strengthen the flocculation reaction tank (extended to 25 minutes)
Design concept: To address the core issue of slow reaction speed, we compensate for the negative impact of low temperature by extending the effective flocculation time.
Key parameters:
Reaction time: Designed to be 25 minutes (usually 15-20 minutes for conventional processes), providing sufficient time for the destabilization, collision, and growth of colloidal particles.
Graded mixing: Three stage vertical axis mechanical mixing is used, gradually reducing the mixing intensity (G value). The front section uses a higher G value (60-80 s ⁻¹) to promote mixing and collision, while the rear section uses a lower G value (20-30 s ⁻¹) to prevent breaking the already formed fragile alum.
2. Grid flocculation tank (GT value precise control)
Design concept: Utilizing grid holes to form micro vortices and increase the effective collision frequency of particles is a key technology to overcome the problem of low temperature and turbidity.
Strict control of GT value:
Front stage: High G-value (70-90 s ⁻¹), short time (T ≈ 2 min), GT value of about 1.0 × 10 ⁴ -1.6 × 10 ⁴, aimed at quickly completing drug diffusion and initial destabilization.
Mid section: Medium G value (40-50 s ⁻¹), medium time (T ≈ 10 min), GT value of about 2.4 × 10 ⁴ -3.0 × 10 ⁴, is the main growth section of alum flowers.
Later stage: Low G value (10-20 s ⁻¹), longer time (T ≈ 13min), GT value of about 0.8 × 10 ⁴ -1.6 × 10 ⁴, promotes the further growth and densification of alum flowers, preparing for settlement.
The total GT value of the entire flocculation process is strictly controlled within the classic range of 10 ⁴~10 ⁵, and tends towards the upper limit to enhance the flocculation effect.
3. Inclined tube settling tank
Design optimization:
Reduce surface load: Design the surface load to 6-7 m ³/(m ² · h) (usually 8-10), extend the hydraulic retention time, and adapt to the slow settling rate of alum at low temperatures.
Optimize water distribution area: Increase the height of the water distribution area, install rectification facilities, ensure uniform and stable water flow into the inclined pipe area, and prevent flocs from breaking in the pool.
Selection of inclined pipes: Adopt inclined pipes with a diameter of 50mm to ensure settlement efficiency while avoiding blockage problems that may occur at low temperatures due to small spacing.
4. V-shaped filter
Design concept: As the ultimate water quality guarantee, the V-shaped filter is very suitable as a terminal treatment unit for low-temperature and low turbidity water due to its high filtration accuracy and stable backwashing effect.
Design features:
Homogeneous filter material: Using 1.2m thick and 0.95-1.35mm particle size homogeneous quartz sand filter material, it has strong interception ability and can effectively ensure the turbidity of the effluent.
Air water backwashing: Adopting a flushing method of first air washing, then air water combined washing, and finally water rinsing, the flushing effect is good, the water consumption is low, and it can thoroughly clean the viscous flocs formed under low temperature conditions attached to the surface of the filter material.
Constant water level filtration: automatically adjusted by a clear water valve to ensure a constant filtration rate and stable effluent quality during the filtration cycle.
System integration and intelligent control
4.Optimization of drug administration:
Using ferrate or polyaluminum chloride (PAC) as the main coagulant, and compounding coagulants (such as activated silica) or polymer flocculants to enhance the flocculation effect at low temperatures.
Based on online monitoring of raw water temperature, turbidity, and flow rate, the dosage is automatically and accurately controlled to achieve energy conservation and consumption reduction.
5.Insulation measures: Take insulation and antifreeze measures for key tanks and pipelines to prevent heat loss and further decrease in water temperature.
6.Intelligent PLC control: The system integrates a PLC automatic control system to monitor the operating parameters of each process unit in real time (such as G value, GT value, head loss, turbidity, etc.), achieving refined operation management.
Conclusion
This project successfully solved the problem of low temperature and turbidity in winter river water in Dalian area by extending flocculation time, accurately controlling GT value, reducing sedimentation load, and selecting a combination technology route of high-efficiency V-shaped filter. The design embodies the core idea of "refinement and flexible regulation", and the stable effluent quality of the system is better than the requirements of the "Sanitary Standards for Drinking Water" (GB 5749-2022) (turbidity<0.3 NTU). This scheme provides valuable technical reference and successful cases for the treatment of similar water quality in cold regions of northern China.
Author: [Chen Shichu/Foshan Hongjun Water Treatment Equipment Co., Ltd.]
Position: Technical Director
Project location: Dalian, Liaoning
Project scale: 10000 tons/day
Keywords: low temperature and turbidity, flocculation design, GT value, V-shaped filter, water treatment technology
