In water supply engineering, a growth ring refers to the irregular ring-like buildup that gradually forms along the inner wall of a pipe during long-term service. Its development is not caused by a single factor. Physical, chemical, electrochemical, and microbiological processes all contribute to its formation over time.
Internal structure of a growth ring
From a physical standpoint, the deposit can generally be divided into three layers, arranged from the pipe wall outward:
- corrosion products
- post-precipitation layer
- biofilm layer
This layered structure reflects the combined effects of metal deterioration, mineral deposition, and microbial activity inside the distribution system.
Chemical characteristics
Tests on growth-ring material show that its moisture content is relatively high, typically above one-third, and in some cases it can reach one-half of the material.
X-ray diffraction (XRD) indicates that the main components are:
- goethite
- lepidocrocite
- ferrous sulfide
X-ray photoelectron spectroscopy (XPS) shows that the dominant elements are iron and sulfur. Among the measured elements, iron accounts for about 55.692% and oxygen about 40.846%.
These results show that growth rings are not just loose dirt deposits; they are chemically complex accumulations closely tied to corrosion and secondary precipitation.
Microbial composition
The microbial community commonly found in growth rings includes:
- iron bacteria
- sulfur bacteria
- Escherichia coli
The presence of these microorganisms is important because biological activity can accelerate or reinforce other corrosion mechanisms already occurring in the pipe.
Why growth rings form
Several conditions in water distribution systems promote the development of these deposits.
Post-precipitation inside the pipe
Some impurities enter the network in colloidal form or as true solutions. When flow velocity becomes low, these substances can settle and form deposits on the pipe wall. Common post-precipitation materials include:
- calcium (magnesium) carbonate deposits
- iron and manganese post-precipitates
- silicon and aluminum post-precipitates
Once these substances accumulate, they provide a foundation for further scaling, corrosion products, and microbial attachment.
Instability of water chemistry
As water moves through the distribution network, its chemical conditions may change. Variations in pH, water temperature, and ion concentration can destabilize the water and increase the likelihood of pipe corrosion.
Electrochemical corrosion
Water being conveyed through a supply system acts as an electrolyte. Under these conditions, corrosion of metal pipe materials is generally electrochemical in nature. The process is influenced mainly by:
- dissolved oxygen
- pH
- residual chlorine
- flow velocity
These factors affect the metal surface condition and the rate at which corrosion reactions proceed.
Microbiologically influenced corrosion
Microbial corrosion often occurs at the same time as electrochemical corrosion. In many cases, microorganisms contribute indirectly by affecting electrode potential and promoting concentration-cell action. In water supply pipes, the microorganisms most commonly associated with this kind of corrosion are iron bacteria and sulfur bacteria.
Effects on water quality and hydraulic performance
Growth rings affect both the quality of supplied water and the ability of the network to deliver it efficiently.
Impact on water quality
The main consequences include:
- increased concentrations of metallic elements
- changes in microbiological indicators
- changes in turbidity
As the deposits grow and interact with the passing water, they can release substances or harbor microorganisms that alter water quality.
Impact on flow capacity
Their hydraulic effects are also significant:
- higher flow resistance coefficients
- reduced effective flow cross-sectional area
- greater pump head requirements
In other words, the pipe becomes rougher and narrower inside, making water transport less efficient and increasing energy demand.
Approaches to control and treatment
Managing growth rings involves both prevention and removal. In practice, neither side can be ignored.
Control measures
pH control
pH has a strong influence on bacterial growth in water. Most bacteria begin to be inhibited when pH > 8.0 or pH < 5. At the same time, XPS results indicate the presence of calcium in growth rings. Calcium in water can readily form slightly soluble substances such as calcium carbonate, which then become part of the deposit.
For this reason, controlling pH can help assess, and to some degree regulate, the tendency of a pipe to undergo corrosion and scaling.
Improved water treatment processes
Better treatment performance can effectively control important finished-water indicators such as BOD and pH, reducing the conditions that favor deposition and corrosion after the water enters the network.
Pipe lining
Applying an internal lining is one of the principal ways to reduce corrosion and limit deposit growth.
- Cement mortar lining
Cement mortar lining relies on its own bonding strength and support from the pipe wall, giving it a stable structure. Its roughness coefficient is lower than that of a metal pipe surface. In addition to providing physical protection, it also offers chemical corrosion resistance because contact between cement and the metal wall creates a high-pH environment.
- Epoxy resin coating
Epoxy resin has good wear resistance, flexibility, and tightness. When mixed with a hardener, the reactive resin forms a coating film that is rapid-setting, strong, and durable. A single spray application with a thickness of 0.5-1 mm can meet corrosion-protection needs. After lining with a fast-curing epoxy resin, the pipe can be returned to service after about 2 hours of curing, followed by washing and drainage.
- Flexible hose lining
Flexible liner methods are used to address corrosion in old pipes. These include slip lining, inversion lining, the sock method, and methods using Poly-Pig to pull a polyurethane film through the pipe. All of these create a “pipe within a pipe” anticorrosion structure and can provide excellent protection. They are especially suitable for long pipe sections without branch lines, but they are not well suited to urban water supply pipelines.
Chemical treatment
The chemical agent method is used more often in circulating cooling-water pipes and some industrial production pipelines. For urban water supply systems, however, it is generally unsuitable because it is difficult to create a closed system, and water safety concerns make chemical dosing less appropriate.
Removal methods
Mechanical scraping
This method uses external force, often through devices such as steel cables, to drag scraping tools back and forth inside the water pipe. It is suitable for pipe diameters from 75 mm to 1000 mm, and the scraping length can reach 100-250 m.
Its limitations are also clear: pipe cutting and water shutdown may be required, and construction becomes difficult where valves, fittings, or other appurtenances are present. After scraping, the pipe should be lined immediately; otherwise, the freshly exposed surface may corrode even more easily.
Hydraulic flushing
Hydraulic cleaning uses high-velocity water flow under a certain pressure to scour the pipe interior. Its main advantages are convenience and operational simplicity. It is most suitable when the pipe wall contains only loose and soft deposits. Used regularly, it can help prevent these deposits from hardening into a more stable growth ring.
High-pressure jetting
In this method, a high-pressure water pump is connected to a hose, and a specially designed nozzle sprays the inside of the pipe being cleaned. It uses relatively little water and provides a good cleaning effect, making it suitable mainly for small- and medium-diameter pipes.
Air-pressure pulse cleaning
This technique uses a water-air mixture with continuously changing pressure to dislodge material attached to the pipe wall. It is particularly suitable for rust removal in urban water supply pipelines. Because rust removal is the foundation of pipeline rehabilitation, this method has practical importance in renovation work.
Experience has also shown that relying on only one cleaning method often does not produce ideal results. The composition of the scale and deposits inside the pipe should first be investigated so that a truly effective treatment approach can be selected.
As expectations for drinking water quality continue to rise, corrosion and deposit formation in distribution pipes remain a major challenge for water utilities. Addressing the problem effectively requires looking at finished-water quality improvement and the sanitary condition of the pipeline itself as a single, connected issue. Only then can the quality of supplied water be protected in a meaningful way.
