Pumping Systems for wastewater, stormwater, industrial process water, and clean water have long used all available materials in “sumps” of various depths and diameter. In many cases, various water types with “off-gassing”, pH and temperature extremes, or other factors determine what sump material is best suited to the application. In most cases, however, the sump material is determined by “preference” or what an entity “has always used”. The tragedy of this approach is that it often makes no sense when factoring in the realistic life expectancy of the material.
A good example are concrete wastewater sumps operating in areas with high ambient temperatures. Hydrogen sulfide is famously corrosive, and unless the concrete is coated with special material it will be “eaten” by the hydrogen sulfide, dramatically reducing the lifespan of a concrete sump. Hydrogen sulfide corrosion ceases to be a primary concern in Fiberglass, Polymer Concrete, or HDPE sumps.
There are various sump materials readily available and have varying costs, difficulties, benefits, and realistic lifespans in various configurations across water types.
CONCRETE SUMPS
Precast concrete is the most common sump material used in all types of water conveyance in both the public and private sectors and for good reason. Precast concrete is common and readily available. It typically has various uplift and/or buoyancy calculations as well as a variety of other engineering “proof” that it will work in underground applications.
There are also several benefits of precast concrete in underground sumps including how it is readily available. Most risers, tops, and bases are stock components. The means of mounting hardware are also readily available and understood by contractors.
Precast concrete can be installed to very deep depths and has certain installation advantages in high ground water installations. It also has certain advantages in traffic rated scenarios. The material is familiar to all and perhaps lower in initial cost.
Unfortunately, the problems with concrete can be equally common and sometimes hard to justify when compared to other materials. The following are just some of the real and present problems with using concrete sumps.
Good (high quality) precast concrete is hard to find. Much of it is “pitted”, porous, and cracks easily or develops microfractures over time. Interior and exterior coating is often required and must be applied onsite to protect it from leaking, water saturation, off gassing, and other water content scenarios.
Precast concrete also comes in sections (barrels) which must be stacked in a specific order and configuration. Each section of the precast concrete sump must be sealed at the joint to prevent water ingress and egress.
Additionally, precast concrete is heavy and difficult to align in holes over 10’ deep. At large diameters a crane is often required to set each section of the overall structure.
FIBERGLASS
Fiberglass is commonly used for small diameter sumps in the 3-5’ range and has been in use in underground installations for decades. There is good reason for the past and continued use of fiberglass as a sump material, and like most sump materials, it comes in a variance of high and low-quality options.
Fiberglass, like concrete, is available in inside diameters (ID) ranging from 3’ to 12’ and in depths for underground installations from 4’ to well over 40’.
What’s different about fiberglass is that we can’t improve its quality with coatings and/or linings. Instead, determining the quality of fiberglass is based on factors such as meeting the American Water Work standards. These standards include maintaining a 5 to 1 safety factor against general buckling and meeting AWWA-D-120 and recognition by the Underwriters Laboratories.
Essentially, you pay for what you get in fiberglass. The top-quality fiberglass comes from companies who supply buried tanks for oil and gas. Buried oil and gas tanks must meet exceptional standards. Entities already making these tanks produce a much more durable and reliable product than may initially come to mind when thinking of “Fiberglass.” These companies specialize in underground sumps and tanks required for the storage of various caustic and/or dangerous liquids in either vertical or horizontal configurations.
The benefits of top-quality fiberglass include chemical stability. It’s compatible with a wide range of temperature and chemical conditions (IE hydrogen sulfide).
One piece construction reduces points of failure such as concrete barrel joints. Fiberglass lends itself to more prefabrication of internal mechanical and electrical components and it is available in both single wall and double wall configurations.
Difficulties with Fiberglass include the uplift. Groundwater or soil conditions may require securing vertical fiberglass sumps to a concrete base slab and/or “guy-wiring” down. This may also require specific types of backfill.
Fiberglass is also difficult to modify. Post-fabrication changes to the sump (invert and discharge locations, etc.) can be time and labor intensive.
During initial installation, the buoyancy of high groundwater can pose challenges and the project may require larger (diameter) top slabs in “traffic rated” applications.
Lastly, it may not be suitable for extremely high water-temperature or dramatic PH applications, but this can be overcome with special-grade resin at an increased cost.
HDPE
HDPE can be thick and very rigid. At first glance, it has a lot of the best qualities (and more) that we find in the best fiberglass without a dramatic difference in price. HDPE is formed and “welded”. In other words, the vertical section is a large diameter pipe while the top and bottom are made of flat sheets. These pieces are positioned and “welded” in place. HDPE sumps are typically thicker than fiberglass, quite rigid, and appear compatible with a variety of conventional mechanical fasteners. The ability to field modify and weld HDPE is a “promise” that makes it look like a long-term contender to high quality fiberglass. While HDPE is relatively new in the world of underground sumps, there are a few early adopters nationwide, both public and private.
The potential benefits of HDPE include field repair and modification which is generally much quicker to finish and resume operation than other materials.
Additionally, HDPE is one piece construction. No barrel joints as a potential point of failure. It lends itself to more prefabrication of internal mechanical and electrical components. Also, it is not susceptible to Hydrogen Sulfide corrosion and has an extended potential lifespan compared to concrete.
Some drawbacks of HDPE are that it’s more susceptible to accidental puncture than concrete. It is not recommended for high temperature applications and any repair work requires specialty tools and a trained operator to perform. A suitable heavy extrusion welder can cost quite a bit.
Uplift in certain groundwater or soil conditions may require securing sumps to a concrete base slab and/or “guy-wiring” down and it may require specific types of backfill. The buoyancy of high groundwater can pose challenges during initial installation.
Finally, it may require larger (diameter) top slabs in “traffic rated” applications.
STAINLESS STEEL
Another type of sump material is stainless steel, particularly 316L stainless steel. 316L has a higher degree of corrosion resistance than 304, and the “L” designation makes it a more weldable alloy. Stainless sumps consist of common sheet sizes rolled and welded to form tubes which form the “riser” section. Flat sheets are cut and welded to form the top and base sections. Any additional bracing or features (such as an access hatch) can easily be welded or bolted in place.
There are various benefits to using stainless steel sumps like the high corrosion resistance and compatibility with a wide range of chemicals including hydrogen sulfide. It can be cut, formed, welded, etc. into almost any size and shape needed and it can be supplied as a single structure with internal components pre-installed.
The detractors to stainless steel sumps are that the price and availability of stainless steel can fluctuate dramatically especially if the material must be domestically sourced.
Additionally, the sump will need proper anchoring, anti-buoyancy measures, and additional reinforcement as the well depth increases.
POLYMER CONCRETE
Polymer Concrete bears many functional similarities to concrete with a few notable exceptions. Where the binding agent in concrete structures is cement, Polymer Concrete uses resin to bind the sand and aggregate. The rebar reinforcing Polymer Concrete is made of FRP. According to manufacturers of Polymer Concrete, all this makes for a sump material that is incredibly stable and durable over time.
Some of the advantages to using Polymer Concrete is there is no need for specialty coatings. Polymer Concrete can withstand a wide range of temperature and pH conditions. Additionally, permeability is positive as polymer concrete does not permeate with microfractures as with traditional concrete and an abrasion to the surface does not create a “weak” point in the structure susceptible to Hydrogen Sulfide corrosion or other chemical and temperature considerations (as opposed to coated concrete). It’s the same material all the way through. It is also lightweight and has a thinner wall compared to concrete but can be regarded in much the same way as traditional concrete when designing, installing, and modifying.
The downsides of polymer concrete include the high initial cost. Polymer Concrete is much more expensive than other sump materials, especially in large diameters.
The barrel joints are a potential point of failure like concrete. Thinner sump walls require consideration when mounting hardware. Also, there are limited manufacturing locations. Polymer Concrete is only produced in a handful of locations currently.
There is an enormous number of existing underground sump and related plumbing infrastructure in the US known to be failing. We also know that a significant portion of American water and sewer budgets are being spent to repair or prolong the life of this lacking and failing underground infrastructure. Water leaking (ingress and egress) compound the cost of maintaining this infrastructure. In many cases, old, active, and existing sumps have long since failed, yet are allowed to continue operation because the cost and difficulty to repair, and/or replace is simply too high.
If one was to truly evaluate the real lifetime of existing sumps based on their true structural and “leak proof” integrity we would find that many of the active, existing sumps have failed, and their true “lifetime” is (or should be) over.
CONSIDER THE OVERALL NEED
Sumps have never been available in more different and well-conceived materials than they are today. They are available in a wide range of depths and diameters, and each has their own true quality and lifespan.
One thing is certain, most of the “easy” (gravity based) land has been built on and consequently the number of new and future sumps for both active pumping systems and manholes will increase. The negative effects of all contaminants and off-gassing and the difficulty of treating all water types will continue to grow. With these considerations, and others, the need for sumps that last, perform well, and preserve their integrity longer will always be best value.
As sumps are built in increasingly unique and difficult locations, the viability of comfortable methods and familiar materials ought to be re-evaluated. New conditions will require novel thinking if we intend for our water conveyance systems to succeed in terms of lifespan, serviceability, and cost effectiveness for the entities who rely on them.
Romtec Utilities provides the design, manufacturing, and delivery of stormwater, wastewater, industrial water, and clean water pumping systems. Romtec engineers the entire system according to your specifications and your preferred components and brands. Each pumping system is unique, so Romtec provides the package experience with the customization of an engineered system. For more information, visit www.romtecutilities.com.