Keeping Things Flowing – Septicity in Foul Pumping Stations

Septicity is the product of stagnant waste sat in an anaerobic (oxygen-deprived) environment – its side effects are harmful to people and nature, and it is a risk in all foul pumping stations.

Effective pump station design can curb septicity, however what works changes over time due to wider societal factors impacting how waste flows through our foul water pumping stations, such as the introduction of low-flow home appliances.

Our article covers septicity, the potential effect of water saving appliances on the risk of septicity in pumping stations, and what can be done to offset this risk.

1. What is Septicity? – Process & Consequences
2. Is stagnation rising? –  Saving Water & Money with Low Flow Appliances
3. How to Prevent Pump Station Septicity? – Design Considerations
4. Conclusion

Septicity is the production of foul-smelling gasses from naturally occurring bacteria breaking down organic matter – there are three conditions essential for this process to occur:

• Organic matter – Present in sewage and foul water, organic matter provides nutrients that anaerobes (bacteria also present in sewage and foul water) use to power their fermentation. This process produces harmful gasses that are a health hazard to personnel and cause bad smells and corrosion.
• Anaerobic environment – An environment lacking oxygen, such as inside a pump station’s pipework or chamber, is ideal for anaerobes to thrive in because it provides the nitrates and carbon dioxide needed to fuel their anaerobic processes.
• Stagnation – A lack of flow through pipes encourages septicity. Allowing solids to accumulate within an anaerobic environment gives bacteria more time and nutrients to break down organic matter and produce septic gases.

Consequences of septicity:

• Unpleasant smells – Byproducts of pumping station septicity include methane and hydrogen sulfide gasses which cause the rotten egg smell that is synonymous with sewers.
• Health hazard – Hydrogen sulfide gas is a toxic, explosive and highly flammable gas, meaning septicity in pumping stations poses a high risk to life for maintenance personnel.
• Corrosion – If hydrogen sulfide reacts with water it can form sulfuric acid, a highly corrosive acid that damages metal, rocks and polymers within pumping stations which can lead to environmentally harmful leaks and costly breakdowns.

Organic matter and anaerobic conditions are inherent in every foul water pumping station as sewage passes through the system’s wet well and pipework, so the only thing left for septicity to occur is stagnation.

During the pumping station design phase, factors that could lead to stagnation – such as poor hydraulic design – are thoroughly analysed and addressed to minimise the risk of septicity. However, wider factors, such as a shift in our water usage habits, may be changing flow rates through our stations and increasing the potential for stagnation and septicity to occur.

As we learn more about the environment, our impact on it and the finite amount of resources available to us, more and more people are making the conscious effort to reduce their energy, gas and water consumption.

In turn, this has seen a change in the way our home appliances are designed – with manufacturers producing more eco-friendly goods that have built-in water saving features, such as low-flow toilets or smart washing machines that adjust water usage based on load size. Though this is great for the environment, it could also be leading to less reliable transport of waste through foul pumping stations.

This increased risk of sedimentation is amplified by the cost of utilities rising over the past three years and the current cost of living crisis, leading Brits to using water more conservatively in an effort to reduce their utility bills.

So, as the rate of flow through pumping stations changes and the chance for organic matter to stagnate increases, what can be done to manage the risk of septicity?

Pump cycles – For package and adoptable pump stations, estimating cycle times is crucial to determining the likelihood of septicity occurring. It highlights areas of risk in a station’s design and can inform changes that will prevent stagnation while still effectively managing incoming foul water.

Pump cycle frequency and duration should be based on the expected inflow of wastewater to the station, this will ensure waste is not held in the wet well or rising main for long enough to become septic.

Variable speed pumps and control units allow for fine-tuning of pumping rates based on demand, lowering the risk of septicity and optimising the station’s energy usage to decrease environmental impact and operating costs.

Wet well – A poorly configured wet well will cause foul water to stagnate. Optimising the position of the pump and the profile of the benching within the pumping chamber facilitates smooth and even flow of water to stop organic matter from accumulating.

Rising main – A station’s rising main is subject to the distance to be pumped. Because they are enclosed by nature, introducing stagnation into a rising main creates an anaerobic breeding ground that vastly increases the risk of septicity.

• Self-cleaning velocity – This is the minimum velocity at which liquid must travel through pipes to suspend solid matter and prevent sedimentation. The required self-cleaning velocity will differ for each pumping station depending on factors such as pipeline materials and diameter, the slope of the pipe and the size and weight of solid particles being transported. As a rule of thumb, Design Construction Guidance states that the minimum flow velocity to achieve a self-cleansing regime in foul water gravity sewers is 0.75 m per second at a one-third flow rate.
• Length – A shorter rising main will decrease retention time and stagnation, but it can also reduce a pump station’s capacity and increase pump cycling and wear. Hydraulic conditions must also be considered, as changing the length of the rising main will affect how foul water flows through and could lead to damaging pressure fluctuations.
• Diameter – The internal diameter of the pipework should be accurately sized to handle the site's flow demands. Too large and flow velocities are reduced, allowing solids to settle and promoting septicity. Too small and the station’s flow capacity is reduced, affecting its ability to effectively manage the site’s foul water output.
• Materials – Corrosion-resistant and smooth materials inside the rising main promote smooth flow through the pipework, helping to prevent sedimentation.

Good pumping station design can effectively alleviate problems of septicity; however, these options may not be feasible due to space and cost limitations. In these cases, chemical dosing may be suitable.

Chemical dosing – Chemical dosing is the automatic injection of chemicals into foul water networks to prevent anaerobic bacteria from growing, meaning septicity cannot occur even where stagnation exists.

Provisions for temporary, permanent and future chemical dosing can be added to foul water pumping stations at the design phase. However, for adoptable stations, each Water Authority has its own regulations for the inclusion of these facilities.

Some adopting authorities will not accept designs with chemical dosing included, as it suggests insufficient cycle times, poor design and incurs additional operating costs, whereas some water companies will accept designs including temporary and permanent chemical dosing provisions.

Evolving environmental awareness and utility costs pose a challenge for foul water pumping stations, with impacted flow rates increasing the risk of stagnation. Through accurate hydraulic analysis and careful consideration of the impact on cycle times, foul water can flow smoothly and without sedimentation for a septicity-free pumping station.

For a stress-free pumping station procurement process, for both package and adoptable models, speak to the experts in our T-T Pumping Stations® department today. They provide structural design, surge analysis and more to ensure your station is efficient, reliable and fit for purpose.

Call 01630 647200, email pumpingstations@ttpumps.com or complete our online enquiry form.