Solid fraction of separated digestate as soil improver : implications for soil fertility and carbon sequestration

Purpose This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO(2)and mineral N. At specified intervals, CO(2)evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results The various SFs showed similar patterns of C mineralisation (15-26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg-1. Conclusions The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation

Fosfaatvormen in compost en andere organische meststoffen

Compost and other organic fertilizers contain phosphorus and their application rates should therefore comply with the P application rate standards for agricultural soil. Phosphorus in compost is exempted from this regulation for 50% (with a maximum of 3.5 kg P2O5/kg dry matter) due to the fact that compost consist partly of soil originating from garden waste. This project assessed phosphorus speciation in composts and several other organic fertilisers and relates the results to the potential availability of phosphorus for crop uptake

Effectiveness of iron sludge and fulvic acid for prevention of iron chlorosis in soybean

Drinking water company Vitens produces iron sludge and fulvic acid as by-products of their drinking water production process. This study investigated the effectiveness of iron sludge as a source of iron for soybean. A pot experiment was performed with a soybean variety that is susceptible for Fe chlorosis, grown on a clay soil from Tricht (Netherlands) and a calcareous soil from Canaveralejo (Spain). In the Tricht soil, no symptoms of iron chlorosis were observed in any treatment. In the Canaveralejo soil, soybean suffered from iron chlorosis, and iron sludge, with or without fulvic acid, reduced symptoms of iron chlorosis and yield loss as compared to the control treatment. However, iron sludge was not as effective as Fe-HBED since this treatment gave an even higher yield and showed no symptoms of iron chlorosis during the course of the experiment

Gebruik van ijzerwater en fulvozuur als ijzermeststof - laboratoriumtesten

Bij de productie van drinkwater uit grondwater worden grote hoeveelheden ijzerslib geproduceerd. Dit rapport bevat resultaten van lab-experimenten die een eerste verkenning vormen naar de mogelijkheden om ijzerslib, al dan niet in combinatie met fulvozuur, te gebruiken als ijzermeststof in de land- en tuinbouw

Perspectieven voor de afzet van (fosfaat-verarmd) zuiveringsslib naar de landbouw

Het project heeft als doel om nieuwe afzetroutes te formuleren waarbij zuiveringsslib op een duurzame wijze wordt verwerkt tot meststoffen en bodemverbeteraars zodat waardevolle nutriënten en organische stof worden hergebruikt

De enkelvoudige versus de gecombineerde indicator voor bepaling van de fosfaattoestand van de bodem : toetsing op data van veeljarige veldproeven op grasland en bouwland

In the Netherlands, the application of phosphorus (P) from fertilising products is regulated through application rate limits targeting equilibrium fertilisation and a neutral P status of the soil meaning that a higher P application rate limit is assigned to soils with a low P status and vice versa. Until 2021, the soil P status was derived based on one single soil P indicator namely P-AL-value for grassland and Pw-value for arable land. From 2021 onwards, the soil P status will be derived from the combination of P-AL-value(indicator for the P capacity) and P-CaCl2 –value (indicator for P intensity) enabling to differentiate based on the P buffering capacity of the soil. Data from long term field trials supply valuable data on trends in soil P indicators in relation to crop uptake and P application rate. This report assesses trends in the single P indicator (Pw-value or P-AL-value) and combined indicator (P-AL-value and P-CaCl2-value) in relation to P fertilisation rate and P uptake rate using data from long term field experiments on grassland and arable land

Speciation of P in solid organic fertilisers from digestate and biowaste

Phosphorus (P) is a finite resource and its reuse in organic fertilisers made from biowaste and manure should therefore be encouraged. The composition of solid organic fertilisers (SOFs) depends on the type of feedstock and processing conditions, and this may affect P speciation and hence P availability. Phosphorus speciation was assessed in eighteen different SOFs produced from biowaste and digestate. Available P was determined in 10 mM CaCl2 extracts at a fixed pH of 5.5 and at a fixed total P concentration in the suspension. P was dominantly present as inorganic P (>80% of total P). There was a strong variation in the Fe content of the SOFs and hence in the fraction of P bound to reactive Fe/Al-oxides (PFe). The fraction of total P soluble at pH 5.5 correlated negatively with PFe pointing to fixation of P by metal salts added during processing, or by soil mineral particles in case garden waste was processed. Therefore, the use of iron salts in processing plants should be avoided. In addition, the presence of P in poorly soluble precipitates lowered the fraction of easily available P. Overall, this study shows that Pt alone is not a good indicator for the agronomic efficiency of SOFs due to large differences in P speciation among SOFs

Phosphorus recovery from co-digested pig slurry : development of the RePeat process

In the Netherlands, the agricultural sector produces more manure than can be applied on agricultural land within the P application standards. The excess amount of manure should be exportedto other regions with a demand for P fertilisers. An alternative for current manure export is manure processing in which manure is processed into valuable fertiliser products. This reports describes a process in which the solid fraction of co-digested manure is processed into a concentrated P fertiliser and a nutrient-poor organic soil improver. The recovered P fertiliser can be used as a secondary raw material for fertiliser production or for export whereas the soil improver can be used on arable soils in the nearby region of the manure treatment plant. The separation process, called RePeat, consists of a acidification and dilution step to extract P from the solid fraction followed by a base addition step to recovery P. The proof of principle was given in a previous report (Schoumans et al., 2017). This reports describes additional laboratory and pilot tests (Chapter 2-4). Attention was given to the reuse of water within the process, the dewatering of calcium phosphate versus struvite and the selection of separation equipment for a demonstration plant. Chapter 5 gives the process mass balances calculated using a mass balance model. Chapter 6 assesses the quality of the organic soil improver in terms of its carbon- and nitrogen mineralisation rate. An example of a business case for the process is included in Chapter 7. The results were used to design a demonstration plant for the RePeat process to be built at Groot Zevert Vergisting B.V. in Beltrum