Today we are delighted to report the good news that water utility companies in the UK are achieving a significantly improved biogas yield from sewage sludge, with increases of up To 30% in their existing AD Plants, through improved reliability and simple upgrades to their main reactor tank mixing equipment.
Sewage sludge and its potential for biogas production
Sewage sludge is a residual, semi-solid by-product obtained from the treatment of wastewater. It consists of two main components: organic matter (like food waste, human waste, etc.) and water. Due to its high organic content, sewage sludge has been identified as a significant potential source for biogas production. Biogas, mainly composed of methane (CH4) and carbon dioxide (CO2), can be produced through the anaerobic digestion (AD) of organic matter in the sewage sludge.
Importance of biogas as a renewable energy source
Biogas is considered a renewable energy source because it is generated from organic materials that are continuously replenished, such as sewage sludge. Utilizing biogas can improve the sustainability of wastewater treatment plants by reducing the need for non-renewable energy sources and decreasing carbon dioxide emissions. Additionally, the utilization of biogas contributes to reducing the carbon footprint of water utility companies.
The intransigence of sewage sludge in biogas production
Sewage sludge is particularly intransigent for biogas production because the biomass in the sludge has already undergone the activated sludge process. This means that the remaining material in the sludge is quite resistant to further breakdown.
The activated sludge process involves aerating the sewage to encourage the growth of microorganisms that decompose the organic matter. Consequently, what remains in the sludge after this process has relatively robust cell walls that are difficult for the anaerobic digestion process to break down.
What is meant by “Advanced” Digestion (AD) Plants
Advanced digestion in this context refers to the pretreatment of incoming feed materials in an additional process stage, such as high-pressure and temperature pasteurization, ultrasonic cell destruction, and other pretreatment methods.
These pretreatment methods help in breaking down the cell walls of the organic matter in the sludge, making it easier for the anaerobic digestion process to occur.
Role of Reactor Tank Mixing Equipment in AD Plants
Reactor tank mixing equipment plays a crucial role in the AD process. Effective mixing ensures that the organic matter in the sludge is uniformly distributed throughout the reactor tank, optimizing contact between the microorganisms and the organic material. This, in turn, ensures efficient biogas production.
Additionally, proper mixing helps prevent the formation of dead zones (areas where there is no movement) and the accumulation of solids at the bottom of the reactor tank. Surface crust or scum creation is also avoided.
Contribution of improved reliability and upgrades in mixing equipment to increased biogas yield
Recent upgrades in mixing equipment, such as more efficient impellers and motor systems, have contributed to increased reliability and performance of the AD process. Enhanced mixing ensures optimal contact between the organic matter and microorganisms, leading to more efficient biogas production.
Consequently, improved reliability and upgrades in mixing equipment can contribute to increased biogas yield.
Impact and Benefits of a Higher Biogas Yield from Sewage Sludge
Environmental Benefits
– Reduction in Greenhouse Gas Emissions: Utilizing biogas as a renewable energy source helps in reducing the greenhouse gas emissions associated with the use of non-renewable energy sources.
– Waste Reduction: The AD process helps in reducing the volume of sewage sludge, which otherwise would need to be disposed of, often in landfills.
Economic Benefits
– Cost Savings: Utilizing biogas as a source of energy can lead to significant cost savings for water utility companies as it reduces the need for purchasing energy from external sources.
– Revenue Generation: Surplus biogas can be sold to the grid, creating an additional revenue stream for the water utility companies.
– Reduction in Disposal Costs: By reducing the volume of sewage sludge, the AD process helps in reducing the costs associated with sludge disposal.
The production of biogas from sewage sludge presents an environmentally friendly and economically viable option for water utility companies. Despite the challenges associated with the intransigence of sewage sludge, advancements in pretreatment methods and mixing equipment have contributed to overcoming these challenges and increasing biogas yields.
Now read our case study:
Press Release 24 October 2020:
Landia and Cambi help deliver a big upswing in biogas production for the water industry
Landia GasMix at Nigg pictured on the side of the reactor tank at the Scottish Water, Nigg WWTW.
Just south of Aberdeen, improvements continue to be made at Scottish Water’s Sludge Treatment Centre (STC) in Nigg – but at a site still approaching its full potential, 0.9~1.0MW per tonne of Dry Solids is already being consistently produced.
Part of its trailblazing route map to lead the water industry to net zero emissions and beyond by 2040 (five years ahead of the Scottish Government’s ambitious plan to become carbon neutral by 2045), Scottish Water’s go-ahead STC at Nigg is becoming a showpiece energy generation centre.
Scottish Water was a pioneer in 2001 when it commissioned the fifth-ever Cambi THP plant. Over the past two years, Cambi has modernised outdated control and measurement systems – and is currently upgrading the heat exchangers. The plant can treat up to 60 tonnes of dry solids per day, but further improvements could increase this considerably.
The externally mounted Landia GasMix at Nigg.
“A quarter to one third more biogas than we were previously”
On the subject of biogas Yield from sewage sludge, Simon Wrigglesworth of Scottish Water Services, Grampian, said:
“We’ve still got work to do, to deliver the full suite of improvements – and will keep fine-tuning – but since the refurbishment, taking everything into consideration, we are producing approximately a quarter to one third more biogas than we were previously”.
Increased Biogas Yield from Sewage Sludge
He added:
“We’ve come a long way since the old compressor mixing system at Nigg, which failed regularly. This resulted in the effective digester volume being reduced because the fouling up of vessels with an accumulation of solids lessened the amount of gas that we could produce”.
When the two 4,000m3 digesters at Nigg were cleaned out during the centre’s refurbishment, it was revealed that 45% of the volume had been lost due to grit and heavily compacted sludge. Central to the need to raise the biogas yield from sewage sludge and help to make this STC electrically self-sufficient and significantly reduce its carbon footprint was the important choice of a new mixing system.
“We looked at various options”,
continued Simon Wrigglesworth,
“but as has been proven, Landia, who were confident that their system could handle the thickness of the sludge, did exactly what they said they were going to do. There was nothing showy; they gave us a good feeling right from the start and provided an impressive whole life cost analysis. They were clearly very determined to bring about the benefits we required”.
Both digesters at Nigg are now fitted with two diametrically opposite Landia Chopper Pumps, which draw thick liquid from the bottom of the tank, where solids are chopped to accelerate the digestion process and prevent clogging of pipes and nozzles.
“A 25% to 30% increase in our gas production”.
A view of the Cambi Thermal Hydrolysis Plant (THP) at Nigg WWTW, Scottish Water.
In the first stage of the mixing process, the liquid is injected into the upper half of the tank, whilst biogas is aspirated from the top of the tank and mixed into the liquid. This not only has the benefit of reducing buoyancy at the surface of the liquid but also sees the rising gas bubbles continue to mix after the pumps are switched off.
With two 30kW systems on each digester, the installed power of the Landia mixing system equates to a very economical 15 watts per cubic meter. When factoring in reduced running times, as low as 20 minutes in the hour, energy savings are substantial. Reduced running times also mean that pump-wear components last significantly longer.
Cambi’s proven Thermal Hydrolysis Process (THP) exposes sewage sludge at Nigg to high temperatures (1600C to 1800C) and pressures (about 6 bars), typically for 20 to 30 minutes for each batch to ensure pathogen kill. Sterilised and hydrolysed sludge is passed to the flash tank, which operates at atmospheric pressure. The sudden pressure drop leads to substantial cell destruction of the organic matter in the sewage sludge, which is then cooled to its optimum temperature before it is fed to the digesters.
“Post-refurbishment”,
concluded Simon Wrigglesworth,
“we’ve now had a decent period of stabilisation, with no major hiccups in the performance or reliability of the Cambi and Landia equipment.”
“With a 25% to 30% increase in our gas production, we’re in such a better place”.
The following YouTube video about Biogas Yield from Sewage Sludge provides a quick explanation of the biogas yield performance improvements achievable through Cambi sludge pre-treatment by hydrolysis when the system is introduced for the first time:
There is so much of sewage in the world today with increasing population that must be recycled to biogas for energy and the solid matter as compost for growing food crops. Could someone suggest a simple domestic system to do it in a rural setting please?
The most complete system includes a toilet that may use the fluid from the digester for flushing is that manufactured by HomeBiogas, an Israeli-based company.
They also provide a stove that can be bought for the use of the biogas produced in cooking.
To put it another way, sewage waste is the best energy source; it outperforms wind and solar (both of which have erratic power supply).
There is so much of sewage in the world today with increasing population that must be recycled to biogas for energy and the solid matter as compost for growing food crops. Could someone suggest a simple domestic system to do it in a rural setting please?
The most complete system includes a toilet that may use the fluid from the digester for flushing is that manufactured by HomeBiogas, an Israeli-based company.
They also provide a stove that can be bought for the use of the biogas produced in cooking.