ESR10: A calculator to determine application rates of REFLOW fertilisers

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A calculator to determine application rates of REFLOW fertilisers


ESR 10, Wenxuan Shi, is developing a calculator to determine the optimised application rates of REFLOW fertilisers based on crop nutrients requirements, soil status, and fertiliser properties. It will give farmers and agricultural advisors a guidance to use the REFLOW fertilisers.

REFLOW fertilisers are produced from recycled dairy processing waste. Dairy industry is one of the largest agricultural sectors in the EU and generates huge amounts of wastewater. Since the wastewater generally comprises dilutions of milk residues, it contains high nutrient content, especially phosphorus (P). Therefore, dairy wastewater is an important source for P recovery. To meet discharge limits, dairy wastewater must be treated before discharge and thus a large volume of dairy processing sludge (DPS) is generated. Approximately 3.8 million tonnes are generated annually in the EU, which is equivalent to about 155 million tonnes of milk production. Raw DPS can be further processed into struvite, biochar, and incineration ashes, collectively referred to as STRUBIAS. All the DPS and these STRUBIAS products are categorised as REFLOW products.


Since I started working in the REFLOW project, I have collected different types of REFLOW products from dairy wastewater treatment plants in Ireland and other REFLOW ESRs. DPS can be categorised into three types including Al-DPS, Ca-DPS, and Fe-DPS based on the lime and metal salt (Al and Fe salts) addition during wastewater treatment. Struvite is a P mineral precipitated from P-rich streams. Biochar and hydrochar are obtained from the thermochemical conversion of biomass in an oxygen-depleted atmosphere. Ashes are formed through the incineration of bio-based materials by oxidation

Collecting dairy processing sludge in the dairy wastewater treatment plant

Different RELOW fertilizers

All the samples were tested for their physical and chemical properties in the Teagasc laboratory: The physical properties included dry matter and organic matter., and chemical properties included pH, nutrient contents, and heavy metal contents.

Sample preparation in the lab


All the three types of DPS examined were rich in the major plant nutrients, especially rich in P, but they were not significant sources of K due to the elutriating effect of wastewater treatment on soluble components. The Al-DPS and Fe-DPS contain more plant-available N (ammonia nitrogen) than Ca-DPS since the lime addition in the wastewater treatment causes losses of NH3. The concentrations of heavy metals in the tested DPS samples were well below the EU regulatory guideline values for biosolids.

Chemical properties of DPS samples
Sample Type TN NH4-N TP TK Ca Fe Al Cu Ni Pb Cd Zn Hg          
g kg-1 mg kg-1
Al-DPS 57.6±8.8 3.8±1.8 34.7±11.1 7.4±3.9 41.8±20.0 1.4±1.0 34.5±16.6 2.4-34 <0.6-10.6 <2-4 <0.15-0.3 19-199.6 <0.1
Fe-DPS 45.6±12.9 3.8±1.2 40.7±4.1 9.2±4.1 78.1±50.4 111.9±37.3 0.8±0.9 3.0-15.8 4.6-19.4 2.8-9.3 <0.15 92.6-180.6 <0.1
Ca-DPS 30.8±20.5 1.6±1.2 52.8±35.3 4.1±2.2 155.9±60.5 1.3±1.0 0.8±0.5 1.0-45.9 1.3-22.3 <2-9.6 <0.15-0.4 15.9-267.1 <0.1
EU limit 1000-1750 300-400 750-1200 20-40 2500-4000 16-25

All the STRUBIAS samples contain high amounts of P contents. After slow pyrolysis reduced the contents of total C, N, and S in the resultant chars, as during thermal treatment, significant amounts of feedstock-bound elements are volatilised in the form of CO, CO2, NH3, and hydrocarbon compounds HCN and H2S. However, the losses of C and N were much lower during hydrochar production than pyrolysis, because pyrolysis resulted in the emission of gases at higher concentrations.

Furthermore, the higher the pyrolysis temperature, the lower the C and N in the biochars. The concentration of total K increased relative to temperature because of the inorganic association of K with DPS. Total concentration of the other elements, including Na, Ca, Fe, and Mg, increased after pyrolysis treatment. After the incineration by oxidation, the contents of total C and N in ash significantly decreased, while all the other elements including nutrients and heavy metal concentration increased, which means they are concentrated in the ash. Across all the REFLOW fertiliser samples, the highest concentrations of heavy metals were in ash and the lowest were in struvite, but they were under upper limits of the EU regulation for fertilisers.

The high TP concentration in the DPS and STRUBIAS products examined in this study meant that maximum legal application rates for each soil P index were determined by the TP concentration of the material. The calculator also considers the soil nutrient status, which is classified into index levels ranging from 1 to 4 depending on the quantity of the nutrient in the soil that is available to the crop in Ireland. The soil N index system is determined by the soil N supply status, while the plant available P is measured using Morgan’s reagent.

The application rate (in tonnes ha-1 y-1) for REFLOW fertiliser products used was determined based on the index level of the soil and the recommended N and P application rates for different crops from Teagasc Greenbook. In addition, the EU legal limits of metals are also considered in the calculator to avoid excess heavy metals applied into soil with REFLOW fertilisers.

The calculator to determine application rates of REFLOW fertilisers


The calculation of application rates only considered TP and TN rather than the plant-available N and P in the REFLOW fertilisers, which are more indicative of their performance as fertiliser replacements. Future research should focus on establishing the fertiliser equivalence values (FEV) of REFLOW products using pot and field trials. Completion and incorporation of such research into the calculator for all DPS and STRUBIUS product and crop types would lead to more accurate incorporation of bio-based fertilizers into nutrient management planning. The pot trial to determine both N and P FEV is carried out with REFLOW fertilisers in a greenhouse at Teagasc, Johnstown Castle. The trial went with two different crops, ryegrass and spring wheat. All the types of REFLOW fertilisers are included in. The results from the pot trial will be used to optimise the application rates calculation.

Pot trial to determine fertiliser equivalence value of REFLOW fertilisers




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