Forty Years Experience With Glass Lined Piping Systems In Wastewater Treatment Plants


Historically, there has been a continually recurring problem with accumulation of grease and solids on the interior of conventional piping systems that handle raw sludge and scum in sewage treatment plants. The adverse effects caused by this build-up are extremely costly; increased pressure drop becomes so great that the carrying capability of the line is severely diminished, sometimes with total pluggage, and pumping systems are often damaged due to high system backpressures and reduced flowrate.

In an attempt to keep the lines clear and free-flowing, costly cleaning methods are employed which are, at best, only temporary. These processes include manual rodding, pigging, steam lance cleaning and hot water flushing, all of which impact the use of the system and are also undesirable due to the nature of the material involved.

Other unsatisfactory alternatives have been evaluated in an attempt to limit the effect of interruptions for required cleaning and to allow the plant operations to continue. For instance, some plants have installed duplicate lines to allow for full plant throughput while one line is out of service for cleaning. Other systems have been designed with oversized piping in an attempt to extend the period between cleaning-related shutdowns.

Introduction of Glass Lined Pipe

The use of glass lined piping systems to combat this continuing problem came about somewhat by accident at the wastewater treatment plant in Eugene, OR. when it was noticed that the grease build-up did not occur in the Pyrex sight glass sections which had been placed in the raw sludge lines. During the design stage of a later addition to this plant, a search was made to find a piping system which exhibited these same smooth, non-stick characteristics. It was found that Porcelain Enameled "glass lined" pipe and fittings had been previously used in various industrial applications to protect the pipe from a highly corrosive environment. It was decided to install a test section of this glass lined piping in the problematic sludge line of the Eugene facility. This test section proved satisfactory over a period of several months with no accumulation on the glass lined surface even though there was continued build-up in the unlined pipe on each side. As a result of this test, glass lined pipe was used on raw sludge lines when the plant was expanded in 1961.

The operations personnel at the Eugene Plant indicated that the original Glass Lined Piping is still in use and has provided exceptional service. The only cleaning required for this system has been to remove build-up in the non-glass lined areas within the system such as the valves, expansion joints, etc.

It is interesting to note that significant research has been done in recent years with various valve manufacturers to develop a glass lined plug valve for use in sludge and scum systems. The results have been positive and now allow the installation of an essentially continuous Glass Lined System.

Description of Glass Lined Piping

The glass lining applied to these systems consists of a thin layer of fused glass material which binds itself integrally with the metallic substrate when fired at elevated temperatures. Typical thickness of the glass system is only 10-20 mils. Alternatively, this process is known as Vitreous or Porcelain Enamel and exhibits the following properties:

  • Superior Lubricity (smoothness and non-stick characteristics) - Hazen-Williams Coefficient of approximately 150. Does not diminish over time.
  • Corrosion Resistance in environments where the pH ranges from 3 to 10.
  • Abrasion Resistance - hardness on the Mohs scale of 6+, and a Rockwell exceeding 73.
  • Temperature Resistance - maintaining most properties up to 800° F.
  • Thermal Shock Resistance - instantaneous temperature changes exceeding 360° F.

To a large extent, these properties are due to the use of a "dual-fire" system using two frit formulations that, in ground coat, establish an excellent bond with the base metal and, in the cover coat, provide the durable hard glossy surface. In the ground coat application, following shotblast cleaning, the finely ground glass components liquify as the base metal is heated above 1400° F., flowing into the roughened microsurface of the iron. The glass becomes locked in place providing a fuseable interface for subsequent attachment of the fired cover coat glass. This durable bond cannot be achieved by other "low temperature" processes such as application of organic materials (epoxies, vinyls, polyethylene). Photomicrographic cross-sections of various types of lining systems clearly show this superior bond. The integrity of this bond has been proved many times in actual adverse service (vacuum conditions and corrosive or abrasive environments) where the glass will not begin to separate or delaminate from the base metal as will occur with some organic coatings.

As a result of the unusual durability displayed by the glass lined system under a variety of adverse service conditions, its uses are not limited to sludge and scum piping systems. It also has applications for transport of acidic or alkaline fluids, abrasive slurries and high temperature fluids. Furthermore, there are no toxic materials associated with the specially formulated glass and, therefore, no adverse impact on the environment. Most importantly, glass is the basic component of the lining and is both inexpensive and relatively stable in cost. While production of a high quality product leads to significant processing costs, as applied to wastewater treatment facilities, the total additional cost for glass lined components can typically be justified in less than one year.

Typical Installation Experience

A 1969 issue of The American City Magazine referred to the Westpoint Sewage Treatment Plant in Seattle, WA, in which two separate systems of glass lined pipe had been installed. The first commenced service in July 1968 with the second system following by one year. At the time of the 1969 article, neither system had required any type of cleaning and they continue to operate today with the original glass lined systems.

The large Metropolitan Waste Control Commission, Pigs Eye Sewage Treatment Facility in Minneapolis/St Paul provides significant indication of the benefits and cost effectiveness of the glass lined system. This plant underwent a major expansion in the mid-1970's including installation of glass lined piping in the sludge and scum lines. This expansion increased the plant capacity by 200 MGD to its current capacity of 300 MGD with an average throughput of 230 MGD. Prior to the installation of Glass Lined Piping, plant operators indicate that solids would adhere to all parts of the system such that a minimum of 250-300 man hours per month were required to keep the sludge and scum lines operational. However, since the introduction of glass lined piping systems at the Pigs Eye Plant, there is no buildup problem under normal operation, and cleaning requirements have been reduced to such an extent as to be insignificant.

It has been estimated that, had Glass Lined Piping not been installed, the manhour requirements to clean these systems following the expansion would have increased proportionately with the increased capacity of the plant. Consequently, it is estimated that the savings in cleaning labor experienced by the plant has easily justified the cost differential for Glass Lining in less than one year.

Heated Effluent at Pig's Eye

An additional problem was encountered at the Pig's Eye Facility due to the fact that the sludge and scum lines are not required to operate 100% of the time. Operators have found that heavier materials settle in the piping and accumulate at the bottom of the system when the system is offline. Even though this material does not adhere to the interior of the glass surface, the solids tend to harden in tees, elbows, etc., such that they are not carried with the normal flow when the system is restarted. Several years ago, in order to soften this sediment, a heat exchanger was installed which heats plant effluent water to approximately 140-160° F. One time per week, the heated effluent is circulated throughout the sludge and scum lines to soften any sediment, removing it with normal system flow. Of course, this simple system would not be successful without the non-stick properties of the glass lining which eliminates any possibility of adherence to the pipe surface.

Photograph #1 shows an untouched section of glass lined pipe which was recently removed from the scum piping after 8 years of service. As can be seen, only minimal traces of grease and solids are present in the sample. Photograph #2 shows the same piece after cleaning with Windex and a paper towel. There is no evidence of erosion or other deterioration of the porcelain enamel surface and the section still possesses the same surface gloss as originally installed. This is the key factor in the lubricity or non-stick characteristic of the product and demonstrates the durability of the hard glass surface. Photograph #3 shows a similar section taken from the sludge line after nearly 13 years of service. This section also displays only minimal film on the pipe, which, as shown in Photograph #4, was also easily removed. As with the scum lines, the glass lining was totally intact with no deterioration in gloss or loss of other properties occurring since its original installation.

Comparison with Organic Coatings

Various plant operators have made comparisons between glass lined pipe and piping that is coated with organic materials. Initially, while they both have comparable performance, abrasion of the softer plastic materials causes a loss of lubricity, thus limiting the suitability of the piping to withstand build-up over extended periods. Currently, the cost of glass piping systems is slightly higher than for organic lining systems, although this situation may change as costs increase for organic materials derived from oil and natural gas.

Other Applications

The key factors that assure a long service life for glass lined products are the relatively high hardness, the continuity of the glossy porcelain surface and the integral bond with the base metal. These characteristics make the glass lined pipe effective in a moderately abrasive environment such as slurries or systems where solids are suspended in a fluid, particularly if the constituents are relatively less hard than the glass (one of the hardest natural occurring materials). An example of this is the use of glass lined Venturi Tubes installed by a Canadian company for systems which transport a tailings slurry stream from stripper pumps in a heavy crude oil extraction process. Previous unlined Venturis had been subject to rapid deterioration due to the abrasive action of the slurry. In 1984, however, glass lined Venturi Tubes were introduced into the system and, to date, the tubes have not failed.

The tubes will be inspected during an upcoming shutdown for scheduled maintenance. Even if the lining has been eroded somewhat, assuming the base castings are intact, new spare Venturi tubes will be installed and a new glass lining will be applied to the existing tubes so that they can be returned to service at a later date. Specifics of this application are as follows:


Process: Heavy Oil Extraction
Application: Tailings slurry from stripper pumps
Constituents: 79% water, 17% solids (sand)
1.5% Naphtha, 2.5% Bitumen
56.1 Micron mean solids size
Flow Rate: 6,000 GPM Max.
4,300 GPM Avg.
Line Size: 14"
Velocity: Inlet = 12.5 ft./sec.
Throat = 25 ft./sec.
Time in Service: 60%


As a product, integral glass lining is not limited to application on pipe and fittings. Heat exchanger tubes, pump bodies, valves, sanitary floor receptacles, screw augers, etc., have all utilized this system. Practically any steel, cast iron or ductile iron system that is subjected to a hostile environment can economically be protected with one of a number of specially formulated glass systems.


Increasingly, engineering firms and end users have become aware of the many advantages of glass lined piping systems since the first applications in wastewater treatment plants during the early 1960's. This system is used extensively throughout the United States for the sludge and scum lines of nearly all new sewage treatment plant construction. Installations outside the U.S. are increasing as plant operators learn the beneficial features of glass lining. A partial listing of major Engineers which now recommend the use of glass lined piping includes Black & Veatch, Brown and CaIdwell, Burgess and Niple, CH2M Hill, Camp Dresser and McKee, John Carollo Associates, Gannett Fleming Cordry and Carpenter, Havens and Emerson, James M. Montgomery, Jones and Henry, Kennedy Jenks Chilton, Malcolm Pirnie, Metcalf & Eddy, Nolte and Associates, SEH, Montgomery Watso Harza, and Whitman Requardt and Associates.

40 years experience has proven the long term durability of the glass lined system. The major cost savings which result from elimination of costly cleaning operations easily justify the relatively low initial cost differential for installation of this system. In short, the unique characteristics of the economical Glass Lined System has made it standard for Wastewater Treatment facilities and is leading to development of new products and end uses to solve wide ranging problems relative to lubricity and resistance to corrosion, abrasion, temperature and thermal shock.

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