Browsing by Subject "Recycled fertilizer"

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Alternative phosphorus resources from urban waste as fertilization
(2023) You, Yawen; Müller, Torsten
Phosphorus (P) is an essential macronutrient for plants. Plant roots assimilate P in soil mainly in the form of orthophosphates as H2PO4- and HPO42-. Due to the high reactivity, orthophosphates generally exist at low concentrations in soils that have high P sorption capacity. Besides the indigenous P in soil, fertilizers manufactured from phosphate rock are the main source of P to ensure a satisfactory yield in agricultural production. However, phosphate rock is a limited reserve with uneven quality and is geographically restricted. Technologies for recovering and reusing the P from waste streams were therefore developed to alleviate the dependency on this critical raw material and to promote sustainable solutions. Sewage sludge, which contains most of the P from wastewater, has great potential to produce P-rich products. However, the evaluation of their P availabilities to plants by simple chemical extraction of the product is difficult because they often contain different P species that do not easily dissolve in water. In the first chapter, three types of recycled P fertilizers derived from sewage sludge were tested first in the greenhouse using maize in two different substrates and were incubated in soil for 0, 22, and 56 days. Untreated sewage sludge ash (SSA), Na-treated SSA, and struvite were tested here. Untreated SSA failed to promote the growth of young maize, while Na-treated SSA and struvite achieved similar biomass as mineral P fertilizer. The pre-incubation time had a negative impact on the P use efficiency of recycled fertilizers. Although the P availability of untreated SSA was very low, it might be a potential substitute for phosphate rock to produce fertilizers. In Chapter II, the P availability and heavy metal contamination risk of superphosphate produced with untreated SSA in the lab were investigated. It was found that the superphosphate produced with the mixture of 25% SSA and 75% rock phosphate had a similar P use efficiency as the superphosphate produced with 100% rock phosphate, indicating untreated SSA could be a suitable substitution of rock phosphate in the P fertilizer production. Despite the heavy metal accumulations in soil and plant being minimal, the Pb and Cu concentration in untreated SSA exceeded the maximum limit according to the EU regulation on fertilizers and therefore its use is restricted in fertilizer production. The separation of industrial and municipal sludge before incineration is recommended to obtain SSAs with high P concentrations but less heavy metal. In Chapter III, the P availability of granulated struvite as affected by fertilizer application methods in comparison to di-ammonium phosphate (DAP) was investigated under field conditions. The experiment was conducted in one field in 2020 and repeated in an adjacent field in 2021. Two-year maize results showed an increase of 30% in maize yield and P content when struvite was placed, indicating that fertilizer placement enhanced the efficiency of granulated struvite. Struvite-placed had similar P use efficiency as DAP-placed, and both treatments led to significantly higher yield and P content of maize than no-P control. The residual effect of fertilizer treatment was evaluated with faba bean (Vicia faba) and triticale (Triticosecale Wittm. ex A. Camus.) as subsequent crops after maize. No significant difference in yield and P content was found between struvite-placed and DAP-placed. Nevertheless, this chapter demonstrated that placed struvite can replace DAP as P fertilizer in maize cultivation. In Chapter IV, the sensitivity of three P extraction methods to different P species was investigated to provide insights into the characterization of current soil P tests to plant P availability. Three soil P tests were compared: calcium acetate-lactate (CAL), Olsen, and diffusive gradients in thin films (DGT). Results showed that a portion of added orthophosphates was immediately fixed in the soil and cannot be extracted by any of the methods. The acidic CAL method may overestimate immediately plant-available P of insoluble calcium phosphate like Ca3(PO4)2. The most suitable method to determine immediately available P might be the Olsen and DGT method. To conclude, this dissertation demonstrated the P availability of recycled P fertilizers derived from sewage sludge and possible strategies to enhance their P use efficiencies. It provided agronomic evidence on the feasibility of replacing phosphate rock-derived P fertilizers with recycled fertilizers and insight into its land application. With the recently revised EU regulation on fertilizing products, it can be expected that recycled fertilizers will soon share the market with mineral fertilizers and help develop sustainable agriculture.
Towards a sustainable nutrient management in organic farming : closing the nutrient gap with recycled fertilizers from urban waste
(2022) Reimer, Marie; Möller, Kurt
Nutrient scarcity is one of the main challenges in arable organic farming. Yet, little is known about the current supply and need of nutrients on organic farms and even less about the nutrient sources utilized by organic farmers. However, most stakeholders within the organic sector agree that additional nutrients, preferably from recycled sources, such as urban waste materials, are needed. In this thesis, the current need and use of nutrients (N, P, K, Mg, S) in the organic farming systems was investigated by performing a meta-analysis of previous studies and two farm gate nutrient budget studies across Europe. Further, the effect of recycled fertilizers from urban waste, such as compost from household and green waste, human urine and sewage sludge on crop yield, nutrient balances, soil fertility, and risk of contamination with potentially toxic elements (PTEs) were examined. To this end, three long-term field trials using different recycled fertilizers were investigated and combined with results of a simulation using the soil-plant-atmosphere model DAISY. The results of this thesis show that the organic cropping system within Europe operates under nutrient limited conditions, which limits the yield potential and can cause soil nutrient depletion, especially of P. Farms that relied to a high extend (>60%) on biological nitrogen fixation for their N supply were particularly prone to the risk of soil P and K depletion. Further, 17% of external N inputs derived from the conventional livestock system, which is often considered contentious. Omitting these would further increase the nutrient gap. Therefore, changes to the contemporary practice are needed to ensure sustainability in the organic nutrient management. First, a better distribution of nutrients within the organic sector is needed. In particular, to avoid nutrient surpluses in one farm type (e.g., livestock or vegetable farms), while other farm types (e.g., arable farms) experience nutrient deficits. Further, due to N losses during processing the nutrient composition of organic fertilizers does not match the crops’ nutrient offtake. Digestates from biogas plant show the closest resemblance. However, to avoid nutrient imbalances an adequate use of external inputs that is tailored to the specific farm’s nutrient demands and reliance on biological N fixation is necessary. Increased awareness of tools like nutrient budgeting among farmers and advisors could facilitate achieving a more balanced nutrient management. Still, additional nutrients are needed to close the current nutrient gap and to substitute animal manures from conventional origin. Recycled fertilizers from urban waste represent a suitable nutrient source to this end. Sewage sludge and human urine performed similarly to cattle slurry with N recovery rates of about 0.5 – 0.6 and household waste compost had similar values to straw-rich animal manures with recovery rate of about 0.3. Nitrogen losses after field application ranged between 34-55% of the applied N amount, with nitrate leaching being the main loss pathway. Total N losses were slightly smaller for compost and cattle manure and were accompanied by a higher soil N accumulation of about 25% of applied N. Similar to the accumulation of soil N, compost also resulted in the highest soil C sequestration. Using cattle manures and sewage sludge showed a smaller effect, while cattle slurry did not cause a soil C increase. Most concerns related to the use of recycled fertilizers derive from the risk of contaminants, such as potentially toxic elements (PTEs). Compost and sewage sludge fertilization can lead to a higher amount of PTEs in the soil. However, significant changes in crop PTE uptake were rare due to low PTE bioavailability. The risk to human health and soil environment associated with PTEs through recycled fertilizer application is therefore neglectable. Urban waste can also be refined by incineration or precipitation processes to ensure less contamination, yet this results in nutrient (e.g., N) and organic matter loss. An argument can therefore be made for the use of raw materials if they lie within the contamination threshold values. In conclusion, the organic nutrient management in Europe requires more external nutrient inputs. Recycled fertilizers from urban wastes are an adequate source in terms of yield effect and soil fertility to close the nutrient gap and to substitute animal manures from conventional origin. However, the infrastructure and availability of recycled fertilizers need to be improved and suitable policy making is needed to fulfil the whole potential of these nutrient sources by for example permitting fertilizers derived from human excreta or the strategical placement of biogas plants which recycle urban wastes.