Sustainability is woven into the fabric of Muntons’ manufacturing process. The United Kingdom-based maker of malt, malt extracts and other malted products reduces its environmental impact while enhancing profitability with an on-site anaerobic digestion plant at its processing facility in Stowmarket, England, creating a novel closed-loop system that converts its liquid malt waste into biofertilizer that its barley farmers can use. But to truly maximize the capabilities of its anaerobic digestion operations, Muntons installed a pasteurization system that enhances the quality of that biofertilizer, generates substantial energy savings and further reduces the company’s carbon footprint.
Muntons expanded its manufacturing operations by building a $7.5 million on-site anaerobic digestion plant in 2014 after its analysis showed that 60 percent of the carbon footprint of its supply chain came from the artificial fertilizer used by its barley growers. To lessen its environmental impact, the company decided to turn its liquid malt waste into organic digestate fertilizer by treating the waste with anaerobic digestion, which is a series of biological processes whereby microorganisms break down biodegradable material in the absence of oxygen. Muntons could then sell the biofertilizer to its network of barley farmers, essentially closing the loop between farm and fork and diminishing the company’s carbon footprint.
But things didn’t go quite as planned. When Muntons began converting its liquid malt waste into biofertilizer, the company initially had to truck it off and apply it to fallow farmland because of a recent change to U.K. environmental regulations. Muntons’ barley farmers could not use the anaerobic digestate on fertile land at first because the company did not initially pasteurize it as required by PAS110 regulations. This U.K. law requires pasteurization of anaerobic digestate based on hazard analysis critical control points principles to ensure that the resulting biofertilizer does not contain toxic compounds and biological pathogens.
To create biofertilizer that its barley farmers can use on their crops and complies with U.K. environmental standards, Muntons commissioned the 3 Tank Batch Sludge Pasteurizer System with Energy Recovery from HRS Heat Exchangers as part of its anaerobic digestion plant. While the resulting digestate from the new pasteurization system is not certified to PAS110 standards yet, Muntons is currently allowed to distribute its biofertilizer to its barley farmers for free because it is pasteurized and made from barley that is traceable. Once it is granted an End of Waste Certification from the U.K. Environment Agency, Muntons will be able to sell its biofertilizer to its barley growers, bringing in extra revenue for the company. Muntons expects to receive the certification in 2019.
“We want our biofertilizer to compete in the agricultural market with the likes of PAS110 digestate and other biosolids,” says Nigel Davies, technical and sustainability director for Muntons. “Even though we are considered low risk and all our feedstock is traceable, food-grade grains, we learned from the U.K. regulators that extra protection has to be built in to stop other less scrupulous operators generating poor-quality fertilizers. Gaining the End of Waste Certification is a much more positive message when selling the new fertilizer as a product, not a waste. It also reassures local farmers that we will not contaminate their land.”
Tailored for efficiency
Unlike conventional anaerobic digestion plants, Muntons’ facility uses a combination of anaerobic and aerobic processes to treat about 53,000 gal of liquid malt waste a day. With both treatments, Muntons significantly reduces the chemical oxygen demand in the waste prior to the effluent being discharged to local waterways in accordance with environmental standards. The remaining sludge is then pasteurized in the 3 Tank Batch Sludge Pasteurizer System with Energy Recovery.
The HRS pasteurization system efficiently processes about 6 tons of sludge per hour. The sludge is heated by pumping it through two heat exchangers. The first heat exchanger is used for energy recovery, and the second heat exchanger is heated for pasteurization of the unpasteurized sludge. The heated sludge is then deposited into one of three tanks and is held at pasteurization temperature for one hour. While one tank is being filled, the second tank holds the sludge at 158⁰F. At the same time, the third tank is emptied.
According to Davies, the energy efficiency of the 3 Tank Batch Sludge Pasteurizer System with Energy Recovery reduces the plant’s energy consumption by about 40 percent. The three-tank process ensures the tanks remain heated at all times so the unit operates continuously, preventing the tanks from cooling down and requiring another heat load to restart. It also allows the system to run at half flow rate should the volume of digestate stock reduce. And as part of an energy recovery section incorporated into the process to make the pasteurization unit even more efficient, an intermediate water circuit transfers the heat from the pasteurized sludge in the tanks to the colder unpasteurized sludge, using heat that would have otherwise been wasted.
“After two hours, you have three tanks that basically fill, hold and empty. That’s a continuous process after two hours. So we’re not wasting heat,” Davies says. “We’re using regenerated heat, and we’re not letting anything cool down.”
Energy efficiency extends beyond the tanks. Waste cooling water from the combined heat and power engine of the anaerobic digestor is used to heat the sludge in the corrugated tube-in-tube heat exchangers. Heating the sludge in the heat exchangers is much more energy efficient than heating an entire tank of digestate. So the sludge arrives in the tanks already heated. The tanks merely maintain the pasteurization temperature of the heated sludge for one hour.
“We heat in the heat exchanger rather than in the tank because heating the tank is very inefficient,” says Matt Hale, international sales and marketing director for HRS Heat Exchangers.
In addition to using heat more efficiently, the 3 Tank Batch Sludge Pasteurizer System with Energy Recovery further reduced Muntons’ carbon emissions with less truck movements for waste disposal. When the company produced unpasteurized biofertilizer, Muntons’ trucks drove at least 140,000 miles a year to dispose of it on fallow farmland. “That cost us about $150,000 a year just to truck it off-site,” Davies says.
Closing the loop
With the pasteurization system installed in the anaerobic digestion plant, Muntons has been able to create a circular economy that not only benefits the company but also its farmers and local community. The pasteurization system helps Muntons convert 88,000 tons of liquid malt waste a year into biofertilizer that it can distribute to its network of barley growers, who produce the 275,000 tons of barley it needs to make 198,000 tons of Muntons’ malt each year. In the process, Muntons’ anaerobic digestion plant has helped the company reduce 800 tons of carbon emissions, generate 14 percent of the facility’s electricity, and save more than $3.35 million in electricity and waste disposal costs to date, according to Davies.
“We like the circular argument that can be made with the pasteurization unit,” Davies says. “We now have a waste material that is high in nitrogen, phosphorous and potassium, which has also been pasteurized and has a lot of water taken out of it. It’s an ideal fertilizer now for the growers of our barley that we make malt out of.”