Can bio-fertilisers from dairy processing waste become the next generation phosphorus bio-fertiliser?

One man’s waste is another man’s treasure - while dairy processing waste doesn’t look or smell very nice, it’s a rich source of macro nutrients. These nutrients could have use as soil fertiliser. Researcher Karen Daly gives more information on a research project in Johnstown Castle

Abbreviations: Ca Calcium; DPS Dairy processing sludge; Mg Magnesium; P Phosphorus;

What are bio-fertilisers?

Dairy processing waste is a rich source of macro nutrients such as P, N, K, S and Ca, Mg. These could have a use as soil fertiliser either in its raw form or as a new product from thermal or chemical treatment.  Dairy processing waste or sludge is an ‘end-of-waste’ material following waste treatment from processing plants producing yogurt, cheese and other products. This waste, and any products derived from it, are known as bio-fertiliser.

At Teagasc Johnstown Castle we examined 5 recycled P bio-fertilisers, some from bio-chemically treated waste - Dairy processing sludge, Struvite, Biochar - and some derived from thermal treatment - Ash, Hydrochar.  Struvite was produced in a stirred batch reactor at 22 ◦C with the reaction time of 1 h and stirring rate of 60 rpm. Biochar was produced from a mixture of DPS and spruce wood chips. Hydrochar was produced from Iron-treated DPS by placing the waste material into s reactor at 200 ◦C for 2 hours. Ash was produced by combustion of the Biochar using a furnace at high temperature of 650 ◦C.  In an incubation experiment, we applied these products to a phosphorus deficient soil, and added a phosphorus isotope or tracer (33P) to track the mechanism of soil P turnover and measure P availability afterward.

What is soil phosphorus turnover?

Phosphorus turnover in soil is the rate at which P moves from the soil solution onto the soil matrix into exchangeable P pools (solid phase) and back out again. Fertiliser P applied to soil is dissolved in the soil-water, and can move onto the soil matrix or solid part of the soil to build up P availability and store P. This is a reversible process. Available P and stored P on the soil particles can be moved back into the soil-water when there is a demand for it by growing crops, and then back again onto the soil matrix when there is no further demand for it. Chemical fertiliser P dissolves quickly and moves rapidly onto the soil matrix, unless there is an immediate demand for it by the plants. Then when crops demand P, it becomes immediately available to them. When we use recycled forms of P, or bio-fertiliser, the mechanism of P moving in and out, from solution to solid phase, looks similar but happens at a slower rate of P turnover. 

Do all bio-fertiliser have the same effect on P availability?

Raw dairy waste and products produced without thermal processing have a high potential to replace mineral P fertilisers and build up available P in P deficient soils. However the slower P turnover rates compared to mineral fertilisers also means slower build up rates. This indicates that we need to allow more time to allow for a build-up of the available and exchangeable P pools.

Thermally treated dairy waste is subjected to extremely high temperatures and often have high concentration of multivalent metals. This interacts with P availability which makes them ineffective at building up the available P to an agronomic target level soils. We also found that applying thermally treated waste (Ash and Hydrochar) actually lowered the P availability, which lead to immobilisation of the available P further restricting P availability for crops.  

Conclusion

The study concluded that while dairy waste can go through different treatment processes to generate a bio-fertiliser product, the best results for improving P availability came from the raw waste itself, followed by chemically treated products. Therefore, we don’t need the high energy costs of thermal treatment of dairy waste when the raw or chemically treated product is effective. 

Read the published article on this research here. 

See below images following treatment

Acknowledgement: This research project (REFLOW) has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 814258.