Fertiliser equivalent value of dairy processing sludge-derived STRUBIAS products using ryegrass (Lolium perenne L.) and spring wheat (Triticum aestivum)

Abstract Struvite, biochar and ash products (collectively known as STRUBIAS) products derived from different waste streams are used as fertilisers in agriculture. Raw dairy processing sludges (DPS) show promise as bio-based fertilisers, but their STRUBIAS-derived equivalents have not yet been tested as fertilisers. The objective of this study was to calculate the equivalence of phosphorus mineral fertiliser equivalency (P-MFE) using the apparent P recovery (APR) method for Fe-DPS and DPS-derived struvites (Struvite1-4), hydrochars (HC1-3) and ash and, for the first time, to quantify their suitability as fertilisers for ryegrass (Lolium perenne L.) and wheat (Triticum aestivum). The results of the P-MFE pot trials showed that the products can be divided into two groups: (1) a range of products that can (i.e., Struvite1-3) and (2) cannot (i.e., Struvite 4, HC1-3, ash and Fe-DPS) be considered as fertilisers. In the first group, the P-MFE ranged from 66.8 to 76.7% for ryegrass and from 77.9 to 93.5% for spring wheat grain. In the second group, the P-MFE ranged from 7.8 to 58.3% for ryegrass and from -34.5 to -151.3% for spring wheat grain. Processing solutions are available to overcome agronomic performance deficits for some products. These include, for example, the avoidance of Fe dosing salts (in the case of struvite) by using biological methods of P removal or utilisation of oxalic acid during struvite precipitation, which removes Fe from the process chain and produce higher yields. Future policy and research must be aware that not all STRUBIAS products are suitable as fertilisers and therefore need to be tested individually.