Up to 90% of slurry is water and reducing the water content lessens the overall volume and storage requirements. HRS Heat Exchangers Ltd helped develop an evaporation process that reduced slurry volumes by 60% and created potential revenue streams for farmers.
The management and treatment of manures and slurries from livestock production is becoming an increasing issue around the world. The potential of manure to cause environmental harm and contribute to climate change when not correctly stored, handled or applied is significant.
However, manures and slurries also represent a valuable resource and can return valuable crop nutrients to farmland in a way which does not rely on energy-intensive industrial processing. Because of this, depending on how it is managed, manure can either exacerbate or mitigate climate change and environmental pollution.
Huge challenge In the European Union some 1.4 billion tonnes of manure is produced each year, the majority of which needs some form of management. This can range from simple, relatively low-cost methods, such as appropriate storage enabling it to be applied at the correct time, to more complex techniques such as treatment using separation and concentration technologies or anaerobic digestion (AD). However, estimates suggest that less than 10% of the EU’s manure is currently actively managed.
Manure management is particularly required in the most concentrated areas of livestock production, as in parts of France, Germany, The Netherlands and Spain. On a national basis, France is the largest producer of manure accounting for more than 263 million tonnes of pig, poultry and cattle manure each year. However, on a per-area basis The Netherlands has the largest production as its annual figure of 72.5 million tonnes is produced by a country with an area of just 40,000 km2, creating real challenges in terms of utilisation and disposal.
Furthermore, changing farming patterns across Europe are creating new challenges, in particular in countries such as Spain and Poland where intensive pig production is increasing. While the number of pig farms in Spain declined by 61% between 1999 and 2009, the actual number of pigs increased by 12% over the same period, and the majority of indoor production comes from slatted floor systems. Pig slurry now accounts for more than 20% of Spain’s total recorded manure production.
Benefits There are various reasons for managing manure, including reducing volumes to save costs, reduce pollution and prevent odour issues. A sow (with one litter of piglets) will produce 4.3 tonnes of slurry per year, meaning that a herd of 100 sows will produce one tanker truck’s worth of waste each and every month. For a country like the Netherlands, which has 12 million pigs, that’s 1.4 million truck journeys a year. Anything which can be done to reduce this number (cutting both costs and emissions) will be beneficial, not just to an individual business, but to the global environment.
In many countries there are environmental restrictions on the application of manures to land, such as Nitrate Vulnerable Zones (NVZs) in the UK, which are designed to prevent the leeching of nitrates and other nutrients into water. These often specify ‘closed periods’ when manure and other fertiliser cannot be applied, meaning that farms must have sufficient storage capacity for any manure generated during these times, or that manure must be sent elsewhere.
As up to 90% of pig and cattle slurry is water, reducing the water content is an effective way to reduce the overall volume and the required storage capacity. Furthermore, being able to store manure so that it can be applied when the nutrients it contains will be of maximum benefit is good agricultural practice and can help to maintain or improve crop yields, as well as reducing potential odours and gaseous emissions during application.
These challenges led to the creation of the EU-funded ‘EfficientHeat’ project in 2011, which looked at an ‘Integrated and cost-effective solutions to reduce the volume of pig slurry; minimise pollutant emissions and process energy consumption’. The project, which was co-ordinated by HRS Sales and product development director Arnold Kleijn, looked at how to improve the situation for pig producers in Spain, many of whom were reliant on off-site treatment plants to reduce the volume of slurry and manure produced. With transport costs accounting for 60% of the total processing cost, it was felt that there must be a cheaper and less cumbersome alternative.
Speeded process By the time the project was concluded in 2013, not only had it cut treatment costs by 40%, but it had also speeded up the treatment process, reduced energy consumption by 25%, cut slurry volumes by 60% and created potential revenue streams for farmers in terms of nutrients and potential biogas.
Arnold Kleijn explains why evaporation was the obvious choice to reduce the slurry volumes: “We used evaporation because the thermal energy needed for evaporation can often be obtained from nearby combined heat and power plants at little or no cost. One of the keys to success was improving the scraping action in the evaporator and preventing the concentrated manure from sticking to the surfaces. This increases heat transfer rates two- to threefold, increasing efficiency and speeding up the evaporation process. By combining the various technologies, we reduced slurry volume by up to 60%, resulting in less storage requirements and fewer tanker journeys.”
Another aim of the project was to devise a solution that was of a small enough scale to be built on farms, something that was accomplished by HRS Heat Exchangers’ engineering team. They used their extensive experience of evaporation and concentration systems to devise a treatment system which can be used directly on manures and slurries, or, in the case of on-farm biogas plants, on the digestate which remains after the AD process.
The treated manure also contains struvite, a valuable source of phosphorous and nitrogen. It can be used by farmers as a valuable organic fertiliser in its own right, or the struvite can be extracted via chemical treatment, providing a valuable product for use in the fertiliser industry.
Research into practice Following the success of the EfficientHeat project, HRS Heat Exchangers continued to develop the technology to improve its efficiency and make it suitable for practical use on farms. The company utilised its Unicus Series scraped-surface heat exchanger technology to provide the necessary evaporation and used acid dosing to reduce volatile ammonia in the process. This reduces odours and also creates ammonium sulphate [(NH4)2 SO4], which is useful as a crop fertiliser. Not only does ammonium sulphate supply useful sulphur to the crop, but it is much less likely to volatilise than urea when applied to crops.
HRS has installed a number of practical manure and digestate management systems. The first of these was commissioned in Spain and treats up to 12 tonnes and hour of pig manure from a number of nearby farms. The system has reduced volumes and therefore storage and treatment requirements, while the concentrated manure is returned to the farms for use as fertiliser. Since this first plant, HRS has since been involved in four other plants which manage and concentrate digestate from anaerobic digestion plants using a very similar process.