UK research supports AD food-based digestate use as fertilizer

By Katie Fletcher | February 11, 2016

U.K.-based WRAP recently released results from The Digestate and Compost in Agriculture (DC-Agri) field experiments, providing an evidence base to support the use of food-based digestate—a product of anaerobic digestion (AD)—and composts by farmers and growers as renewable fertilizers.

The research demonstrates that digestates and composts can increase yields and reduce costs with no negative impacts on crop quality or safety, and that compost can increase soil organic matter more quickly than other organic materials. The DC-Agri data also enables farmers to predict how much nitrogen food-based digestate will supply to crops, but underlines the high economic and environmental cost of applying it when crops do not require nitrogen.

 “Digestate and compost are valuable renewable products of our food and garden waste recycling processes,” said Richard Swannell, director at WRAP. “These new findings show the benefits that using the products correctly can bring, and for the first time farmers and growers have the evidence to make informed decisions about their fertilizer use. This is a significant step forward for both the anaerobic digestion and composting industries and for farmers and growers.”

The DC-Agri project was commissioned in 2010 and the core of the experiments ran across three growing seasons, with supplementary research completed last year. Data was collected from 22 experimental sites throughout Wales, Scotland and England.

According to the research summary, in 2010 when the report was commissioned, food-based digestate was a “little understood, little used, ‘novel’ material.” Built on the data yielded from the field experiments, these resources aim to enable farmers to maximize the fertilizer value of more than 1.5 million metric tons of food waste that is processed by AD in the U.K. every year.

 “Digestate is an excellent biofertilizer which can help farmers to reduce their costs,” said Martin Rogers, environment policy advisor of NFU. “With the recent rapid growth in AD infrastructure, it is becoming increasingly available to farmers across the U.K. It is therefore important that anaerobic digestate is managed appropriately to maintain quality standards and achieve the best results. The DC-Agri experiments provide evidence to help farmers make informed decisions about its use.”

The field experiments were undertaken using whole food-based digestate, livestock slurry, green compost, green/food compost and farmyard manure. Research included measuring fertilizer value, crop quality, soil quality and emissions to the environment.

One of the focus areas was to determine the benefits of digestate as a nitrogen fertilizer. The nitrogen use efficiency (NUE) was calculated to determine the efficiency with which the nitrogen applied is taken up by the crop after losses are taken into account. The average NUE of food-based digestate, applied in spring using a bandspreader, was c. 55 percent of total nitrogen applied, as measured in replicated field experiments. When food-based digestate was bandspread in the autumn, this was reduced to c. 15 percent of total nitrogen, highlighting the effect of nitrogen losses due to overwinter nitrate leaching. Livestock slurry-based digestate applied in spring had a mean NUE of c. 50 percent, which also decreased to c. 15 percent of total nitrogen applied for autumn applications.

The ‘nutrient boost’ the research described organic materials providing soil was valued at £55 ($79.60) to £160 per hectare, taking into account the value of bagged fertilizer saved and the cost of spreading (but not sourcing) the organic materials.

As for crop quality, measurements of crops grown with compost and digestate were taken against specific grain weight, grain protein content and the oil content of rape seed. It was found the quality was just as good as crops grown using bagged fertilizers and that crops grown using compost and digestate were found to be safe following assessments of crop metal concentrations, mycotoxins in cereal grains and organic material contamination of cut grass. The research found that combining digestate or compost with bagged fertilizers in nutrient management plans can lead to higher yields. The experimental sites demonstrated grain yields from winter cereal crops increasing by 7 to 10 percent.

Soil organic matter, soil biology, physical properties of soil and heavy metals and organic contaminants in soil were examined in the soil quality portion of the research. Greater ammonia emissions were found from application of food-based digestates (c. 40 percent of total nitrogen applied), according to the report. This was partly due to the greater ammonium content of the food-based digestate and partly to its elevated pH (mean pH 8.3). Emissions from ammonia were reduced on grassland, however, where digestate (and cattle slurry) was applied with a trailing hose and particularly when it was applied via shallow injection. Methane emissions from digestates were lower than from livestock slurry. Leaching losses and atmospheric emissions related to compost can also be found in the report.

In response to the WRAP report, the U.K. Renewable Energy Association released comments supporting the five-year study.

“Anaerobic digestion and composting play an important and growing role in our efforts towards decarbonization and in creating a more resource efficient economy,” said Jeremy Jacobs, technical director of the REA. “The DC-Agri report will add precision to the push to reduce the U.K.’s reliance on fossil fuel-based fertilizers.”

Jacobs added, “There is serious room for growth particularly in AD, but additional government support is needed. Food waste is collected separately in Scotland and Wales in both the commercial and household sectors, which supports the industry. Whereas the U.K. now has over 240 sites in operation, Germany already has over 8,000. The government should introduce U.K.-wide mandatory food waste collections to help us catch up.”

A summary of the key research findings and the full DC-Agri project reports can be downloaded here.

WRAP also recently published guides to good practice, which can be read here.