System – Wastewater

• Overview

The first modern sewers to carry away waste from cities and towns were constructed in Boston and Brooklyn in the late 1850s.  Earliest sewers go back to ancient Rome and the “Cloaca Maxima,” which was built entirely of stone in 510 BC.  That sewer still provides service 2,400 years later to this day – a spectacular feat and level of engineering and service that this nation’s collapsing and deteriorating sewer lines will never even remotely approach.

Those first sewers carried away the waste from cities and towns to be ultimately dumped into rivers and oceans.  Only much later, beginning in the 1900s, did that raw waste receive any form of treatment to reduce its impact to the environment.  Then more recently with the passage of the Clean Water Act in the early 1970s, effective and large scale wastewater treatment plants were constructed.  For that reason, most wastewater treatment plant piping only dates back to the 1970s and later.

Given the many unknowns related to potential aggressiveness of the waste as well as the estimated millions of gallons of water moved through the system every day, exceptionally heavy pipe was installed in the earlier plants.  Pipe and fittings were typically cast and flanged Class 250 and even Class 350 and Class C cast iron having beginning wall thicknesses at or exceeding 1.25 in.  Substantially lighter materials, however, are found at other components such as flow measuring devices, Dresser style couplings etc.

For older 1975 pipe, a Class 250-E cast iron 24 in. elbow would have a wall thickness of 1.250 in.  Yet newer Class 350 ductile iron pipe of the same size installed in the 1990s would have a beginning wall thickness of only 0.430 in. – or approximately 2/3 less.

• Common Problems

The movement of waste through a treatment plant transforms that waste in stages – beginning with high level aeration basins to clarifiers and tanks settling the activated sludge.  As the waste travels through the system it undergoes changes making it more and less corrosive, which in turn has a varying impact on the pipe.  Waste gas is also carried.

Operating pressures are generally low, although the large diameter pipe required to carry millions of gallons of waste per day (MGD) defines the need for heavier materials.  Unlike other piping systems typically fabricated from the same pipe material and schedule, wastewater systems may contain cast iron, ductile iron, carbon steel, and stainless steel of varying classes and specifications.  This mandates that any investigation address a wider selection of piping components in order to improve the chances of identifying areas of greatest weakness.

Also unlike the evaluation of steel pipe which requires corrosion to completely penetrate the pipe wall before leaking, cast iron and ductile iron pipe are more fragile, and are well known to split or fracture once internal corrosion weakens one area sufficiently to release the stress forces created during its manufacture.  As we have previously discussed for Sanitary Waste systems on a vastly smaller scale, cast iron pipe has a completely different failure mechanism which is not only impossible to predict, but also produces a far greater failure condition when it occurs.  Ductile iron pipe has a different grain structure and is less vulnerable to the form of failure common to older cast iron pipe.

• Major Threats

The deterioration of very heavy Class 250 pipe over a 45 year period to the point approaching failure having a beginning wall thickness of 1.03 in. is somewhat remote, and in fact our investigations have shown such older pipe to wear extremely well in such a corrosive environment.  Heavy duty case flanges having an 18 bolt pattern will never separate due to corrosion.  However, as we have documented for all other piping services, thinner materials with less rigid joining methods offer an immediate saving, and therefore, such massively thick cast iron pipe is no longer installed.

The major threat to wastewater systems, therefore is ongoing corrosion which can produce a much more significant impact against thinner and more corrosion susceptible piping.  Over decades of component replacement, expansion, and repairs, a wide assortment of piping materials may exist, with their vulnerabilities caused by widely varying corrosion conditions unknown to plant operators.

• Testing Focus
  

Due to the expanse of most facilities and the location of the pipe underground, any inspection is inherently difficult.  Then add to the logistical difficulties the varying piping classifications and materials in use, often undefined, along with the technical difficulty of ultrasonically testing very thick and corroded cast iron and ductile iron pipe.  With changing pipe specifications, we have had to track down historical piping specifications of some manufacturers from the 1920s in order to identify the correct pipe specification and its wall thickness.

Specific areas of focus for any wastewater piping investigation are:

• • • The back wall of elbows and tees
    • Reducing fittings
    • Venturis
    • Impact areas of a pump discharge
    • Dresser style couplings
    • Any newer installed pipe
    • Carbon steel pipe
    • Externally corroded pipe
    • Pipe of every known diameter
    • Pipe of every material
    • Grooved clamped pipe
    • Centrifugal pump housings
    • Threaded pipe

In addition, there are aspects of investigation which cannot be addressed through ultrasound such as the support or hanging of large diameter pipe.  Unlike a section of 6 in. condenser water pipe, tremendous stresses exist for a full length of 60 in. cast iron wastewater piping.  As the waste pipe naturally deteriorates, its strength decreases, which in turn impacts the suitability of any supports or hangers which were specified when the pipe was new.

© Copyright 2023 – William P. Duncan, CorrView International, LLC