Integrated pig manure digestate processing for direct injection of organic liquid fertiliser into irrigation systems

PROJECT DESCRIPTION

BACKGROUND

Spain is Europes second largest producer of pork with 99 561 pig farms and more than 26 million pigs that generate 70 million kg of manure per day (2.7kg/animal per day). Pig manure can have significant benefits for agriculture, if treated correctly and in an environmentally respectful way. It is a valuable fertiliser rich in organic nutrients that become available to crops immediately after application. However, the excess of manure available in intensive pig breeding areas, along with a lack of land to spread it on, needs addressing. Many anaerobic digestion plants have nevertheless been established to convert the enormous amount of pig slurries into biogas and digestate. Biogas can be transformed into renewable energy, while the digestate has untapped potential.

OBJECTIVES

LIFE Smart Fertirrigation aimed to demonstrate the environmental and economic feasibility of innovative pig manure digestate treatment at biogas plants in order to produce liquid and solid biofertiliser. It planned to optimise the treatment of both manure liquid and solid fraction so that after internal recycling of nutrients, the liquid fraction can be directly injected into irrigation systems as organic fertiliser. By replacing mineral fertilisation in a cost-efficient way, opportunities for biogas producers and farmers would be created. Reducing the use of mineral fertilisers would also cut greenhouse gas emission and prevent soil acidification and eutrophication.

The digestate treatment process would be made up of three main phases: mechanical separation of the digestates solid and liquid fractions; extra filtration of liquid fraction to remove suspended solids and prevent clogging, making it suitable for direct injection into the irrigation system; and drying out of the solid fraction with the excess heat from the biogas production process and later ammonia treatment in an innovative pilot biological treatment plant.

In addition, the project aimed to reduce phosphorous levels in pig manure at source by adding phytase enzymes to the pig feed. Innovative phytase enzymes can significantly reduce excreted phosphate in manure thus preventing over enrichment.

RESULTS

The LIFE Smart Fertirrigation project demonstrated the environmental and economic feasibility of a filtration prototype for the liquid fraction of digestate from pig manure produced at a biogas plant, along with its application as organic fertiliser when it is injected into different irrigation systems. Filtrated digestate from biogas plants and filtrated pig manure from pig farms were shown to be effective in irrigated experimental farms. It also demonstrated the feasibility of the prototype for drying solid fraction.

Additionally, the project showed that by substituting inorganic fertilisers in the project area it was possible to avoid CO2 emissions. Nutrient absorption capacity of crops was found to be higher thanks to organic fertilisation. The projects methodology also reduces nitrogen and phosphorous in the ecosystem, including phosphorus in pig manure. By preserving the presence of nitrogen and phosphorous in the liquid fraction, the organic load in wastewater from pig manure was also improved. The project moreover demonstrated several key socioeconomic benefits, including energy and chemical savings through the biological treatment of ammonia in the gas stream from the process of drying the solid fraction.

Specific results included:

Establishment of the use of 640 t/year of mineral fertiliser for the bottom and top mineral fertiliser in all areas. Liquid biofertilisers are only valid for the top dressing fertilisation. All mineral fertiliser was replaced in the top dressing, higher than the expected 70%;

Cultivation of 75 ha with the substitution of around 7 100 kg of Nitrogen Fertiliser Unit (UFN) from mineral fertiliser (around 95 kg of UFN/ha). Considering a traditional liquid mineral fertiliser (NAC 27%; with 27% nitrogen content and a price of 0.3/kg), around 26 200 kg of mineral fertiliser have been replaced at a total savings of 7 860;

A 20% to 75% increase in nutrient absorption and crop yield with the fertirrigation application of the new liquid biofertilisers compared to a control that included mineral fertilisers. The project methodology offers a greater bioavailability of nitrogen in the biofertiliser along with additional micronutrients that are not provided by the mineral fertiliser;

A 17% reduction in phosphorus excreted in the slurry, along with a 12% reduction in the average excretion of nitrogen. The added enzymes breaking down phytic acid could help the animals digest other nutrients than phosphorus such as amino acids; and

A great reduction in emissions associated with digestate and treated slurry (56 times less than those associated with mineral fertilisers).

A list of the required amounts of liquid fertiliser for each crop type tested in the project was also created to foster uptake from interested stakeholders, as well as a step-by-step guide on how to implement the integrated approach. The project also actively engaged other farmers to demonstrate the high replicability of its results, reaching at least 500 relevant stakeholders (pig breeders, biogas producers and irrigating farmers).

Further information on the project can be found in the project's layman report (see "Read more" section).