What are the differences between gravity, vacuum, and low-pressure sewer systems?

Wastewater collection systems are the infrastructure constructed to transport wastewater and solids from source such as residential, commercial and industrial areas to a treatment location prior to final discharge. There are three main types of sewer collection systems: gravity, vacuum, and low-pressure systems. This article provides a summary of each, including details why one may be selected over another.

Gravity sewer construction

Gravity sewer systems explained

The traditional gravity sewer system is the most common type designed to transport wastewater. This type of system has pipes laid with a downhill slope that use gravity to convey wastewater from homes and businesses towards a treatment plant. Gravity sewer systems have no power requirement from the source to the lift (pump) station and are cost-effective to install.

The design challenge occurs as the pipes must be constructed to match the local land contours and in flatter areas become progressively deeper throughout the network to maintain a continuing downhill slope. Engineers are familiar with designing this type of network and it results in discrete sewer catchments with gravity mains feeding to a wastewater lift station and forced main that transport the collected wastewater to a higher level and closer to the treatment plant.

Why design a different system?

The cost of construction and ongoing maintenance increases in areas that have a high-water table, hard/rocky ground, or aggressive acid sulphate soil. When engineers are looking to sewer new areas that have flat topography; gravity sewer networks are not always the optimal design after completing a triple-bottom line analysis.

How does a low-pressure sewer system work?

A low-pressure sewer (LPS) system uses a grinder pump to macerate and pump wastewater from homes and businesses. The wastewater is transported through small-diameter pipes under low pressure to a collection tank or treatment plant. Low-pressure sewer systems are ideal for areas with challenging topography, such as very flat areas, steep hills, or rocky terrain.

However, they require regular maintenance and are more expensive to install than gravity sewer systems. LPS systems require additional infrastructure on the customer’s property including the underground tank and pump, and need a control panel to alert if high levels or other faults occur. Electrical costs to power the pump are generally paid by the landowner.

Typical low-pressure grinder pump and storage tank

What are the advantages of a low-pressure sewer system?

Cost-effective installation: Low-pressure sewer systems can be installed at a lower cost compared to traditional gravity sewer systems. This is because they require smaller pipes and shallower excavation depths.

Flexible layout: Low-pressure sewer systems can be designed to follow the contour of the land, which means that they can be installed in areas with challenging terrain or where gravity sewer systems would be difficult to install.

Reduced inflow and infiltration: with a sealed system and no manholes, there are only very limited situations where wet weather would increase flow.

Reduced risk of pipe blockage: Low-pressure sewer systems are less likely to experience backups because they use individual grinder pumps to transport wastewater from each property to the main sewer line.

Reduced odours: The sealed nature of the low-pressure sewer system means that there is less opportunity for odours to escape from the system, leading to a more pleasant environment for residents.

Easy to expand: Low-pressure sewer systems can be easily expanded to accommodate new developments or additions, making them a flexible option for growing communities.

How does a vacuum sewer system work?

A vacuum sewer system uses a vacuum pump and differential pressure to remove wastewater from homes and businesses. Each property drains to an on-property collection pit, which is connected through the vacuum pipe system to a large vacuum pump station. When the collection pit valve opens, there is a pressure differential that sucks the wastewater from the pit into the connected pipe system, the pit outlet valve closes when the pit is emptied.

The pipe system is laid in a sawtooth profile that maintains a pressure differential and transports slugs of wastewater towards the vacuum station as valves in the system open with a blast of atmospheric pressure. The large vacuum pump at the end of the line will turn on and off intermittently to maintain the correct vacuum pressure range at the downstream end of the system.

Vacuum Sewer Schematic (Srstevens3, CC BY-SA 4.0)

What are the advantages of vacuum sewer system?

Cost-effective installation: Vacuum sewer systems only require shallow and narrow excavation compared to traditional gravity sewer systems resulting in a faster and cheaper construction cost.

Flexible layout: Vacuum sewer systems pipe routes are less affected by local topography and can be designed to follow the contour of the land, which means that they can avoid running through private property and be installed in areas with challenging terrain or where gravity sewer systems would be difficult to construct.

One pump station: Pumping is achieved by a single pump station at the end of the system.

No exfiltration: The vacuum nature of the system means that exfiltration should not occur.

Reduced odours: The sealed nature of the vacuum sewer system means that there is less opportunity for odours to escape from the system, leading to a more pleasant environment for residents.

About the Author

Mark Lee is the Business Development Manager (AUS/NZ) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.

What is the impact of drought on sewer pipe networks?

Drought conditions occur in many regions around the world, and has an impact on the performance on the sewer pipe network. Learn more.

Should I patch or line to repair my sewer pipe?

The decision around whether to patch or line a sewer pipe in poor condition can be challenging for city and utility engineers. Sewer pipe repairs are a critical task that requires careful consideration to ensure long-term results. Two common techniques frequently employed in the industry are patching and lining. Each method addresses specific issues and offers distinct advantages. In this blog post, we will delve into the world of sewer pipe repair, exploring the differences between patching and lining, and helping you make an informed decision for your specific needs.

Figure 1– Vitrified clay pipe with large cracking and breaking

When do sewer or stormwater pipes need repair?

Deteriorating sewer and stormwater pipes often have cracks, holes and other defects that affect the structural integrity of the pipe. Inflow and infiltration into older pipe networks is also a common problem. When a pipe is inspected and observed to have significant structural defects, engineers will need to decide on whether action is required to manage this risk. This will involve selecting a suitable repair or rehabilitation option, and deciding how soon this needs to occur.

Digging up and replacing with a new pipe is often cost prohibitive, a common way to extend the life of these underground pipes is to use trenchless renewal methods. This includes patching or lining a pipe to mitigate the existing structural defects.

What is a pipe patch?

Pipe patches are used for spot repairs along a pipe to fix a specific issue where full-length repair is not necessary. Pipe patches are often constructed from a cured-in-place-pipe (CIPP) material such as glass fibre patch that is hardened with fast curing resin during installation. There are also stainless-steel patches with rubber seal which mechanically lock into place on expansion in pipe.

Pipe patches can be installed across a large range of pipe diameters with the different patch lengths available depending on the situation (400mm-1200mm, 16in-4ft).

What is trenchless pipe relining, and how does it repair pipes?

The trenchless lining of pipes eliminates the need to dig down and repair long lengths of pipe which can be expensive and intrusive. Pipe lining options include:

Figure 2 – Cured-in-place pipe lining installed within an older host pipe (Courtesy M Tucker & Sons)

CIPP (Cured-In-Place Pipe) lining

CIPP (Cured-In-Place Pipe) lining is a method of rehabilitating existing pipes without the need for traditional excavation methods. It involves inserting a flexible liner that has been pre-coated with a resin into the damaged pipe. The new pipe uses an inversion technique to insert and inflate this new inner pipe and achieve bonding with the original host pipe wall. The result is a new, durable pipe within the existing one.

Fold-and-Form

Fold-and-Form lining is like CIPP, but rather than using an inversion method to insert the new plastic pipe (e.g., thermoformed PVC), it involves heating and then folding and forming the material into a flattened shape and pulling it into the existing pipe with a winch. It is then inflated, matching to the shape of the host pipe. The liner then cools to harden the thermoplastic material, creating a new pipe within the old pipe.

Figure 3 – Spiral Wound Lining inserted into an existing pipe

Spiral Wound Lining

Spiral Wound Lining is another common structural lining solution that can be used to rehabilitate defective pipes. It uses a continuous strip of profiled plastic (typically PVC or HDPE) that is mechanically wound into the host pipe and locked in place. There is no requirement for resin and can often be inserted under live flow conditions.

What to consider when choosing to patch or line an existing pipe that needs repair

Pipe patches are an excellent option to repair pipes with a specific defect at a few locations along an otherwise healthy pipe. Conversely, a pipe that has many defects from start to finish will most likely benefit more with full lining between maintenance holes. The grey area that engineers must consider is when the pipe falls somewhere between these two scenarios. It is good to have a discussion within an organisation around how many patches would be recommended before switching to a full trenchless lining solution. This will involve looking at the various patch and lining options available from the available contractors in your area. What will be the structural grade of your pipe after completing repairs, and has the observed risk been managed? Will the option chosen affect your asset valuations and recorded useful life?

Once you have answers to some of these questions it will be easier to settle on a more agreed organisational approach to whether patching or lining is the preferred option in each scenario.

Recording Repair Decisions and Completing Work

It is important that organisations have an efficient system in place that is able to record pipe repair decisions and package the required repair work into programs that ensures that budgets are allocated to the right areas of the network at the right time.

About the Author

Sci-Fi Manholes & Underground Sewer Tours

Just west of Frankfurt, Germany; the city of Wiesbaden holds some fascinating underground sewer history that has provided unique public access and insight into the underground word of wastewater over the last century. Situated on the northern bank of the Rhine River, engineers in Wiesbaden designed a network of underground canals between 1900 and 1907 to better service the city’s population. The large basket-handle shaped tunnels allowed for entry and traversing many parts of the underground flow path as wastewater made its way south towards a rake and sand trap treatment point before the Rhine River discharge 100m into the main Rhine channel.

Figure 1– Underground tours have been popular since construction. [Image by Patrick-Emil Zörnern](a)

A Combined Wastewater Network

To assist flow and flushing within the underground pipe system, the ‘Salzbachkanal’ was designed with controlled fresh surface water intakes from numerous local streams (or ‘bach’ in German) during low flow conditions. To reduce the frequency of blockages during low flow and on the shallow grades within the system, a higher-level storage tank supplied by stream flow was available to certain parts of the system. This meant that in only very rare circumstances would potable water be required for flushing. Part of the operation and maintenance of the system also meant that sewer workers would descend into the tunnels and clean the brick inverts with brushes and brooms.

A series of weirs were designed so in the event of high flow within the canal due to wet weather, the combined flow would spill to local rivers and streams once 5 times dry weather flow was reached, bypassing the treatment point.

Figure 2 – Confluence point of the Schwarzback/Rambach and Kesselbach/Wellritzbach canals. [Martin Kraft, MKr09174 Salzbachkanal (Wiesbaden), Cropped, CC BY-SA 4.0]

Winter Snow

The canal was even used to dispose of street snow that was collected and disposed through snow-chutes at strategic locations above the canal. The large canal cross-section and warmer wastewater flow meant the snow melted quickly and flowed onwards downstream.

Sewer Material

Located up to 7 metres below street level, the Salzbach Canal includes the famous basket-handle brick design which remains in mostly excellent condition to this day. This is in large part to its careful construction and material choice with a combination of clinker brick wall and hard-fired glazed stone. In addition to the more photographed parts of the network, there are concrete egg-shaped and circular glassed stoneware sections.

Figure 3 – Steep section of basket-handle canal cross section. [Image by Patrick-Emil Zörnern]⁽

A Unique Entry Point

This must be one of the most unique manhole covers we have seen. If there are other locations that use this six-piece triangular outward opening design with spiral staircase, we would love to see some photos. The local waste authority (ELW) has run 25 person tours of this historic underground network in the past, it isn’t clear if these tours are still available.

Figure 4 – One of the entry points to the Salzbachkanal [Image by Brül]^(c)

About the Author

Mark Lee is the Business Development Manager (Aus/Nz) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.

How to Get Rid of Tree Roots from Your Sewer and Stormwater Pipes: Rooting for Effective Solutions!

How to Get Rid of Tree Roots from Your Sewer and Stormwater pipes: Rooting for Effective Solutions!

Tree roots in sewer lines can be a real drain on the efficiency of your municipality’s wastewater management system. These pesky roots can infiltrate and obstruct sewer pipes, causing blockages, backups, and even structural damage. But fear not! In this article, we’ll explore some effective solutions for tackling tree roots in sewers, so you can keep your wastewater system flowing smoothly without any “rooty” interruptions!

The Root of the Problem: Understanding Tree Root Infiltration in Sewers

In this section, let’s discuss how tree roots find their way into sewer pipes and cause problems. As trees grow, their roots naturally seek out sources of moisture, including the small cracks and joints in sewer pipes. Over time, the roots can penetrate the pipe walls, creating a tangled mass that obstructs the flow of wastewater. This can result in backups, overflows, and costly repairs if not addressed promptly. Check out this recent video from a plumber extracting 20 feet of tree roots from a sewer pipe here.

Figure 1: Root ingress causing blockage

Best Practices for Tree Root Removal from sewer pipes

To tackle tree roots in sewers, it’s important to employ effective root minimising techniques. One commonly used method is using root-minimising chemicals that are formulated to destroy tree roots while being safe for the sewer system and the environment, when used properly. These chemicals can be applied through various means, such as foams or liquids, and are typically introduced into the sewer line through access points, such as cleanouts or manholes.

Going the Extra Mile: Preventative Measures to Keep Roots at Bay

Figure 2: Spiral Wound Lining of Pipe

While eliminating existing tree roots is essential, it’s equally important to take preventive measures to keep them from coming back. One effective approach is to install sewer pipe linings, such as cured-in-place pipe (CIPP) lining, which creates a durable barrier inside the pipe, preventing tree roots from infiltrating. This method can help extend the lifespan of your sewer pipes and reduce the need for frequent root-minimising treatments.

Staying Ahead of the Game: Regular CCTV Inspections for Early Detection

Figure 3: CCTV pipe inspection of post-lining work

Regular sewer CCTV inspections are a proactive approach to detect tree roots early on, before they cause significant damage. By using high-tech cameras to inspect the interior of sewer pipes, you can identify root intrusion, cracks, or other issues in real-time. This allows you to take prompt action and address the problem before it becomes a costly headache.

Conclusion

By employing root minimising techniques, implementing preventative measures, and conducting regular CCTV inspections, you can effectively manage and eliminate tree root intrusion in your municipality’s sewer system. So, don’t let tree roots “root” for clogs in your sewers, take action today and keep your wastewater management system flowing smoothly!

About the Author

Amanda Siqueira is an Australian civil and environmental engineer who has worked in design, construction and remediation of drainage and sewer pipes in Australia, New Zealand and the UK. She has passion for all things pipes and is also one of the Co-founders of VAPAR.

How Laser Scan Technology Enhances Small-Diameter Sewer Pipe Inspections

Photo courtesy: https://flic.kr/p/nSfp6K

Small diameter sewer pipes can be challenging to inspect and diagnose for issues due to their size and accessibility. The emergence of laser scan technology has provided an innovative solution for inspecting small diameter sewer pipes where CCTV inspections methods are not a viable solution.

What is it?

Laser scan technology uses a centrally placed rotating laser to create a digital 360-degree image of the pipe interior, providing accurate and detailed information about the structure and condition of the pipe. This method is suitable for pipes with diameters as small as 2 inches, making it an ideal solution for inspecting small diameter sewer pipes. Small diameter sewer pipes make up much of the network (https://www.vapar.co/sewer-pipe-material/).

How it works ?

The laser scan technology works by emitting a laser beam that rotates within the pipe. As the laser rotates, it captures millions of data points that are used to create a 3D image of the pipe’s interior. The resulting image is highly detailed and provides engineers with information on the pipe’s size, shape, and any defects or obstructions.

Benefits of laser scan technology

One of the significant benefits of using laser scan technology for small diameter sewer pipes is the non-invasive nature of the inspection. Traditional inspection methods often involve excavating the area around the pipe to access it. However, laser scanning can be performed with minimal disruption to the surrounding area, reducing the impact on communities and businesses.

Additionally, laser scan technology provides a more comprehensive view of the pipe’s interior compared to traditional methods. The highly detailed 3D images generated by laser scanning can identify defects and obstructions that may not be visible through CCTV inspection.

In general, in the construction industry, laser scanning has been shown to reduce construction time by up to 50%, according to a report by Trimble. This is due to the highly accurate and detailed information provided by laser scanning, which allows for better planning and coordination during the construction process.

Challenges of laser scan technology

Despite its benefits, laser scan technology for small diameter sewer pipes still faces challenges. The technology is expensive and requires specialized equipment and trained personnel to operate. The accuracy of the scan is also dependent on factors such as the condition of the pipe and the presence of debris, which can affect the quality of the resulting image. Additionally, interpretation of the data requires skilled engineers, and the high-resolution data generated by the laser scans can lead to large data files that require significant storage and processing power.

Advancement in the technology

Laser scanning technology is an exciting space and has made significant advancements in recent years, particularly in the inspection and assessment of sewer pipes. One of the key benefits of laser scanning is that it is a non-destructive testing method, meaning it does not require physical alterations to the pipe or surrounding environment. Additionally, portable devices have been developed, enabling inspections of previously inaccessible areas.

Automated analysis using machine learning and artificial intelligence is another benefit, which has enabled the development of software that can identify and classify defects. Finally, real-time data capture provides immediate analysis and decision-making, resulting in more accurate and efficient pipe inspections and maintenance practices.

Conclusion

In conclusion, laser scan technology offers an innovative solution for inspecting small diameter sewer pipes. With its non-invasive nature and highly detailed 3D images, it provides engineers with accurate and comprehensive information on the condition of the pipes. This technology has the potential to revolutionize the way we inspect and maintain small diameter sewer pipes.

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Sewer pipe useful life by pipe material

Vitrified Clay Pipes

Photo courtesy : https://www.flickr.com/photos/jitze1942/

What is useful life?

Asset useful life refers to the length of time an asset can stay in service. There are several factors that impact the useful life of an asset. For sewer pipes in particular, a pipes useful life can relate to the quality of installation, degree of maintenance, pipe material, type of use, and local environmental conditions.

What is the useful life of sewer pipes by pipe material?

Different types of sewer pipe materials have varying useful life estimates. For example, clay pipes typically have a useful life of 60 years, and PVC and concrete pipes have a useful life of 100 years.

Common Sewer Pipe Materials	Estimated Useful Life
Vitrified Clay	60 years ( ref: https://www.mdpi.com/2075-5309/13/4/952/pdf )
PVC	100 years (ref: https://www.uni-bell.org/Portals/0/E-NEWS/MediaFiles/pvc-pipe-longevity-report.pdf )
Concrete	100 years (ref: https://www.mdpi.com/2075-5309/13/4/952/pdf )

Concrete Pipe

Photo courtesy: https://www.flickr.com/photos/russellstreet/

What else impacts useful life of sewer pipes?

It should be noted that these are just estimates based on industry research and investigations. The actual useful life of sewer pipes can vary based on things like the quality of installation, the degree of maintenance, and the intensity of use, as well as local environmental and soil conditions.

How can you extend the useful life of a sewer pipe?

A recent example of when the useful life of sewer pipes came to an end was when a sewer pipe broke in North Carolina causing more than 4 million gallons of untreated wastewater to spill into a creek. Whilst the cause of the break is still under investigation, it’s likely a case of the pipe reaching the end of its useful life before a maintenance or renewal program could escalate it for further works.

Regular inspection and maintenance of sewer pipes can help extend their useful life and prevent premature failure. The frequency maintenance and renewal regularity should be dependent on each asset owners’ budgets and needs. Although the Water Environment Federation (WEF) recommends that sewer pipes be inspected at least once every five years, for many cities and water utilities, this is not practical. When an inspection is carried out, the areas of the catchment that are at coming to the end of its useful life should be prioritised.

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Why aren’t all sewer pipes round?

When people think of a sewer pipe, they will picture a cylindrical pipe with a round cross-section that has a constant diameter. Cylinders have many properties that lend themselves to being perfectly suited to transport fluid, in this case, wastewater.

Oval or Egg-shaped sewer

Photo courtesy : https://www.flickr.com/photos/gee01/

Why are round pipes the most common design?

The overwhelming majority of sewer pipes are round, and this is for good reason.

Strength – round pipes have no corners or weak joins.

Construction – manufacturing a round pipe is generally an easier process than other shapes. Die extrusion, spin-casting, and filament winding are all common processes used to manufacture pipes. A circular shape can be easily controlled when looking to achieve a consistent thickness and strength during production.

Economical – a cylindrical shape is able to provide the maximum volume for the material required and result in the most economical shape to fabricate.

When are other shapes used?

There are a range of non-circular shapes used in the construction of sewers, these include?

Ovoid (or egg-shaped) – one of the most common non-circular pipe is the ovoid sewer. These are popular in some combined sewer networks. With a narrower width at the bottom, this pipe shape is particularly useful to combine self-cleansing velocity during low flows (i.e., reduced blockages/settled deposits) and the capacity to transport larger volumes with a widening width in the upper half of the pipe.

Horseshoe – with a wider profile this design may suit a situation where a sewer line needs to be installed at a shallow depth or where the ground conditions do not allow for the installation of a more regular shape. The flatter invert also allows for easier conditions to manually enter and walk through a pipe.

Rectangular – rectangular, or box sewers, are sometimes used in combined systems where space is at a premium along buildings or other infrastructure. This shape is very common for stormwater culverts.

Parabolic – wider at the bottom than the top, parabolic sewers may be preferred where there is limited vertical clearance such as a bridge crossing.

U-Shaped – Looking like an inverted horseshoe, with a larger cross-sectional area than a circular sewer the U-shaped design allows for a greater total flow capacity and can help prevent backups and overflows during periods of heavy rainfall. During lower flows a higher velocity is achieved to aid self-cleansing.

Basket handle – like the ovoid design, this shape increases the velocity under low flow conditions to prevent settling of deposits. This shape was used in many parts of the original Parisian sewer; the horizontal flat sections provided a space for walkways or cleaning equipment.

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Why is stormwater management important?

Storm water (or rainwater) needs to be managed responsibly so that the impacts of high rainfall events are mitigated by the changing land use of urban cities. Before human urbanisation, most of the planet’s land surface was permeable.

A permeable surface is one that allows liquids (and gases) to pass through it. By way of example, grass is a permeable surface in so far that it allows water to infiltrate into the Earth below. This infiltration of rainfall allows the stormwater to enter the groundwater system, store in underground aquifers and slowly move through the underground soil matrix to rivers and oceans.

When we create impermeable surfaces (the opposite of permeable surfaces), we create a barrier to this important part of the water cycle, which can create future stormwater management issues.

How do you implement stormwater management in a catchment?

In cases where catchments have a high concentration or high percentage of impermeable surfaces there is an increased risk of flooding in that catchment. Good stormwater management reduces the effects of flooding risks on communities by creating structures and systems that mitigate flooding in high rainfall. These structures and systems include things like stormwater detention tanks, detention ponds (these can be an effective way to hold back the water which I that would ordinarily have permeated through the ground), or even constructed wetlands that also help to treat the stormwater as well as mitigate flooding.

3 examples of stormwater management options:

Flood mapping and public education – undertaking a flood study of catchments where there is an urban population and educating new and current property owners of any associated flood risk. This also includes including the flood risk of catchments into decision making about land development, particularly subdivision.
Offsetting flows from additional development – If additional impervious surfaces are added to the catchment, the associated increase in runoff should be managed to store the volume of water until the main storm event has passed. This is not always practical but should be encouraged where possible.
Restabilising stream and riverbanks and beds – restoring natural flow paths is just as important as ensuring those paths can handle the flow associated. Keeping the stream banks and stream beds vegetated and stabilised will increase the resilience of the stream system to cope in high rainfall events and reduce the risk of erosion and degradation of the stream ecosystem in the future.

Conclusion

Without good storm water management, there could be significant impacts on local communities and the greater environment. Flooding of private properties and damage to public infrastructure, as well as erosion and destabilization of stream and riverbanks are just a few of the impacts. Where there is a man-made impact on the stormwater flow in a catchment it is prudent for the public authority of that area to mitigate the risks to the community and the environment.

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What is asset condition grading and why is it needed?

If you’re working in an organisation that relies on assets to deliver reliable services , it’s likely you or someone in your team will need to carry out condition grading on those assets. This post will go through what condition grading is, what it’s significance is, what it is typically used for and how you can grade your assets.

Where did the 1 - 5 condition grading scale come from?

The 1 to 5 scale is a an internationally accepted rating system of categorising assets based on condition. The International Infrastructure Management Manual, 2006 is often referenced in relation to the 1-5 scale, where 1 is ‘Excellent’ and 5 is ‘Very Poor’. Some organisations choose to use a 1 to 10 scale, and some organisations use the scale an internationally accepted rating system , i.e. 5 is ‘Excellent’ and 1 is ‘Failed’. Ultimately if your organisation is clear about the definitions and criteria of each grading level and what you are using the data for, the grading should be fit for purpose. The scales can always be mapped to a smaller or larger scale as long as the data has been consistently applied and matches the new scale. So don’t worry if your organisation is not using the 1-5 rating scale. If your organisation is using this scale, then you’re in good company.

What is the point of condition grading?

Applying a condition grading to an asset is about establishing the performance of said asset in order to determine where it is on its life cycle i.e. the physical state . The resulting grade informs the user about how well or poorly that asset is performing and can indicate (in a relative sense) whether work may be required to improve the performance or useful life of the asset. In asset management, condition grading typically is used for the asset’s remaining life eg a pipe is in condition grade 4 and therefore it has 15 years left before it needs a major replacement.

The use of condition grading is now extended also to long term financial planning and asset valuations.

For example, if the organisation has 8km of pipes in condition 4 and 5, then it provides a simple and accurate basis of prioritising a capital plan over 5 years. Similarly, the condition grading can be used to perform a valuation based on life consumed by simply using the remaining life over a useful life as per the International Financial Reporting Standard 116.

A long-term financial plan and valuation is typically updated every 4 years, whilst the capital and maintenance plan would need a review every 12 months.

Figure 1 Indicative ‘degradation curve’ for a given asset.

The aim of the game is to understand where an asset is on its life curve based on the current condition/performance of the asset. Regular maintenance and timely renewals can avoid total failure as well as potentially extending the life of an asset.

What is the point of condition grading?

Aged based assessments

Due to the cost-prohibitive nature of undertaking condition grading on all assets, often the age of installation is used to indicate the asset’s performance. This is called the age-based method. This is when the degradation of an asset is assumed over a period of time, with the asset ‘age’ being the determining factor in performance. This method may be easier to adopt, when there may be installation dates readily available for most of the assets. Because undertaking inspection-based assessments on all assets can be cost-prohibitive, so often a representative subset of data is extrapolated out to give the condition of the overall asset class. However, this is unlikely to give you the actual condition of the assets, as assets rarely degrade uniformly based on age alone.

Inspection based assessments

Condition assessments can also be carried out based on inspection and/or testing of the asset to determine where the particular asset is on its expected life curve.

In the case of pipes, there are industry-specific documents and standards that provide guidance on how to carry out the condition assessment. The condition relates to the type of defects identified, the severity of these defects and even the number/frequency of them. This method accounts for the specific characteristics/defects of the asset to determine the condition. It provides data that is more accurate and, whilst expensive, is now more the norm given accountability, financial planning regulation, cost of litigation, project management and most importantly, evidence-based capital planning.

Other methods

It is possible to adopt an approach that is a hybrid of the above two, or even use quantitative methods such as degradation modelling. Regardless of which method you use to condition grade your assets, it’s helpful to understand what the data is going to be used for so that you can adopt the method that is going to maximise accuracy and minimise risk.

What are the differences between gravity, vacuum, and low-pressure sewer systems?

Gravity sewer systems explained

Why design a different system?

How does a low-pressure sewer system work?

What are the advantages of a low-pressure sewer system?

How does a vacuum sewer system work?

What are the advantages of vacuum sewer system?

About the Author

Should I patch or line to repair my sewer pipe?

When do sewer or stormwater pipes need repair?

What is a pipe patch?

What is trenchless pipe relining, and how does it repair pipes?

CIPP (Cured-In-Place Pipe) lining

Fold-and-Form

Spiral Wound Lining

What to consider when choosing to patch or line an existing pipe that needs repair

Recording Repair Decisions and Completing Work

About the Author

Sci-Fi Manholes & Underground Sewer Tours

A Combined Wastewater Network

Winter Snow

Sewer Material

A Unique Entry Point

About the Author

How to Get Rid of Tree Roots from Your Sewer and Stormwater pipes: Rooting for Effective Solutions!

The Root of the Problem: Understanding Tree Root Infiltration in Sewers

Best Practices for Tree Root Removal from sewer pipes

Going the Extra Mile: Preventative Measures to Keep Roots at Bay

Staying Ahead of the Game: Regular CCTV Inspections for Early Detection

Conclusion

About the Author

How Laser Scan Technology Enhances Small-Diameter Sewer Pipe Inspections

What is it?

How it works ?

Benefits of laser scan technology

Challenges of laser scan technology

Advancement in the technology

Conclusion

About the Author

Sewer pipe useful life by pipe material

What is useful life?

What is the useful life of sewer pipes by pipe material?

What else impacts useful life of sewer pipes?

How can you extend the useful life of a sewer pipe?

About the Author

Why aren’t all sewer pipes round?

Why are round pipes the most common design?

When are other shapes used?

About the Author

Why is stormwater management important?

How do you implement stormwater management in a catchment?

3 examples of stormwater management options:

Conclusion

About the Author

What is asset condition grading and why is it needed?

What is asset condition grading and why is it needed?

Where did the 1 - 5 condition grading scale come from?

What is the point of condition grading?

What is the point of condition grading?

Aged based assessments

Inspection based assessments

Other methods

About the Author

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