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A better whey to handle dairy residues

13 February 2017

Suzanne Gill reports from Europe’s largest dairy industry bioenergy plant, which combines effluent treatment and energy production. 

First Milk is the largest dairy farmer co-operative in the UK. Its members' farms stretch from Central Scotland to the South of England and from West Wales to East Anglia. The company handles around two billion litres of milk every year from its 2,600 dairy farmer members.

In common with many dairy businesses the company has faced a variety of challenges, due mainly to continuing milk price volatility. However, business for First Milk has improved significantly in the past few years as a result of the turnaround plan executed by the management team, along with an increased focus on delivering operational efficiencies and quality improvements. One such project has taken place at the company’s Lake District Creamery site, where a variety of cheddar cheeses are produced in addition to a range of whey protein products.

The Lake District Creamery needed to find a solution to allow it to better handle its production residues to enable clean water discharge to the local river. Replacing an existing aerobic treatment plant was a priority and it was felt that the installation of an onsite anaerobic digestion (AD) plant would offer the most cost-effective and environmentally acceptable solution. 

In an agreement between with funding and management partner, Renewables Unlimited and plant builder and operator Clearfleau, Lake District Biogas was set up as a business with the purpose of building, owning and operating the proposed plant on behalf of First Milk. The plant is designed to offer a five-year return on investment.

Clearfleau, as a specialist in onsite industrial AD processes designed to treat liquid residues with a high FOG (Fats, Oils and Grease) content, designed the bioenergy plant for the Lake District Creamery – the largest to be built on a European dairy processing site – to combine effluent treatment with bioenergy generation. 

With funding for the plant coming from outside the business and by outsourcing the plant’s operation and maintenance, First Milk has been able to refocus attention onto its core cheese production activities.  

The first part of the project required Clearfleau to upgrade an existing aerobic treatment plant before starting work on a new AD plant designed to treat all the process residues from the cheese creamery.  

The creamery residues are pumped from the dairy to balance tanks at the treatment plant which is situated adjacent to the dairy.  An important feature of the Clearfleau AD process is the solids management control system which ensures that feedstock is blended to the correct residue strength, before it is fed into one of the two digester tanks on the site. This ensures optimal biogas output, which, according to Richard Gueterbock, marketing director at Clearfleau, is around 15% more than competitive high-rate systems.

Producing bio-methane
Fed with around 350,000 litres of whey permeate from the creamery every day the Lake District Biogas plant can produce 16,000m3 of bio-methane every working day, with a methane concentration of at least 55%. Biogas is stored in the gas dome before upgrading to bio-methane. While 80% of the biogas is fed to a membrane based bio-methane upgrade unit where it is converted into bio-methane with a comparable thermal value to North Sea gas, the remaining 20% is fed into the AD plant’s combined heat and power (CHP) unit which provides power to run the plant.   

The liquid anaerobic digestion (AD) system can achieve a reduction in Chemical Oxygen Demand (COD) load of greater than 95% for residual sewer discharge.  However, discharge of the cleansed digestate to watercourse then requires further post-digestion aerobic treatment which is also supplied by Clearfleau. The downstream treatment of the permeate takes place in the original, upgraded and enhanced aerobic plant where it also undergoes chemical treatment for nutrient removal.

By feeding the up-graded bio-methane into the gas grid, the facility produces over £3m per annum in cost-savings and revenue, while also supplying up to 25% of the creamery’s energy requirements.  On-site digestion provides up to £2m per annum in net revenue (after operating costs) from savings, incentives and gas sales.

As it is pumped into the digester, feed from the balance tanks is mixed with heated sludge returned from the solids separation process, plus nutrients and other returned liquors. After commissioning the digester temperature is held at a steady 38°C. Accurate monitoring of the reactor ensures that the correct temperature and pH is always maintained and pH correction can be made as the effluent enters the reactor. An external macerator mixing pump ensures that solids always remain in suspension in the digester tanks.

While the effluent is pumped out of each reactor tank for heating and mixing purposes, degraded solids are removed from the sludge in a separation system, where polymer is added to assist the separation process. Cleansed effluent from the separation system is then further ‘polished’ before it is discharged. Polished water can be re-used on site as grey water or discharged to a water course. Solids are sent to local farms for use as a nutrient-rich soil improver.

Biogas produced by the anaerobic digestion process is stored in the space above the two reactors in a flexible bio-dome which comprises a two-layer cover. The outer cover is kept inflated by blowers, with the inner one moving independently – depending upon the volume of biogas. 

Optimal energy recovery
Commenting on the Clearfleau liquid AD solution, Gueterbock said. “Our technology employs adaptations of existing AD technology. The resulting solution ensures optimal energy recovery from the feedstock and minimal COD in the digestate. Also, the modular design of the plant allows it to fit into a more confined space than other solutions, which makes it possible to install AD plants closer to the production process, even on smaller sites with restricted space.”

The technique provides biogas for energy, cleansed water which can be returned to watercourses  and bio-solids which can be used as fertiliser. “It has allowed First Milk to more cost-effectively clean its process residues,” continued Gueterbock. It is a legislative requirement for food companies to manage waste. An AD solution can help this cleaning process to pay for itself through the production of energy. “First Milk buys back the gas from the grid as a renewable energy so it is purchasing it at less cost too,” said Gueterbock. 

Asked why the gas is sent to the grid instead of being turning into electricity he said. “Sending the gas to the grid ensures that 100% of the energy is utilised.  If biogas is used to produce electricity only 35% of the energy value of the gas is converted to power, although some of the surplus CHP heat can be used and receives Renewable Heat Incentive (RHI). However, this option is only available for larger biogas plants and gas will be fed to boilers or CHP engines on smaller sites.” 

On average 350,000l of whey permeate – containing milk whey and proteins – is now processed at the Lake District Biogas facility every day and the two digester tanks are designed to handle up to 420,000 litres. “As a next step we are also looking at how to make use of the wash water used in the dairy too which also contains a percentage of proteins, fats and sugars that could be used to create more bio-methane,” concludes Gueterbock. 

The facility offers a good example of how First Milk is continuing to progress its turnaround plans, by saving costs and improving the operational efficiency of every corner of the business. By utilising innovative AD technology the Lake District Creamery has succeeded in reducing the cost of handling its production residues, while generating renewable energy for use on site. The new AD plant is efficient, produces low greenhouse gas emissions, and has eliminated the need for hundreds of tanker journeys to and from the plant to remove the whey permeate for off-site recycling.

The final word goes to Gordon Archer, director of Lake District Biogas. He said: "This on-site bio-energy facility, which is generating energy from cheese residues is enabling our partner, First Milk, to save costs and improve the operational efficiency of its milk processing site.”


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