Sustainable manure management via anaerobic digestion integrated with struvite precipitation
Bibliographic record
Abstract
The amount of livestock waste produced is monumental due to the increase in intensive agriculture. The manure management stage is one of the largest contributors to the carbon footprint of the Canadian livestock sector. To mitigate these harmful effects, various manure management strategies have been undertaken. Anaerobic digestion (AD) is one of the methods used to treat manure sustainably, producing biogas (which has multiple applications) and digestate, a valuable by-product. Historically, digestate was applied to farmlands as a source of fertilizer. However, excessive application of digestates on agricultural lands leads to environmental problems: eutrophication, greenhouse gas (GHG) emissions, heavy metal, and pathogen contamination. To diminish the mentioned problems, the focus has shifted to finding alternate methods to treat these digestates that can reduce their impact but also give value-added products. Struvite precipitation is one such method wherein the produced crystals can be applied as a fertilizer. The struvite crystals contain good levels of magnesium, nitrogen, and phosphorus (elements crucial for a crop’s growth), with other benefits such as relatively low solubility, compact storage, easy transportation, lower GHG emissions, lower soil and water contamination due to reduced leaching while giving the same effect as conventional fertilizers in crop yield. Furthermore, struvite crystal production has been underutilized using agricultural waste streams which contain a considerable amount of nutrients that can be recovered and recycled, especially anaerobic digestates of nitrogen-rich livestock waste streams. This thesis aims to ascertain struvite production in an agricultural scenario from anaerobic digestates of varying compositions to encourage valorizing wastes for the recovery of nutrients and to implement sustainable circular economic principles. This is done by first devising a protocol to validate the conditions: pH 9, 1.5 Mg/P, and 240 rpm. Digestates obtained from the co-digestion of different wastes and mono-digestion of poultry manure were initially tested, where 3 sets of trials were done to see the efficacy of different phosphorus salts on nitrogen recovery and mass production. Nitrogen (N) recovery of ≥95% was achieved for all the digestates, also causing a substantial volatile fatty acids reduction. While the recovery was on par, the mass of crystals produced varied with the digestates, with a range of 0.33g/10 ml to 1g/10 ml being produced. To advance the idea of bio-economic sustainability, integration of AD with struvite precipitation is proposed. To determine its feasibility, the efficiency of high solids anaerobic digester using co-digested wastes was assessed for one representative cycle of operation of 77 days with more emphasis on the struvite precipitation, which was conducted at the end of the cycle to maximize recovery of nutrients and energy. The AD presented good results, with a maximum specific methane yield of 1.26 L CH4/g CODs fed for D1 (poultry and dairy manures and corn silage) and 1.49 L CH4/ CODs fed for D2 (poultry, dairy, swine manures and corn silage), which was achieved at the end of the respective operation cycle. The N recovery was very high (around 98-99%), producing 0.67 g/10 ml for all the phosphorus salts. The overall results of this study show that high N recovery is possible in co-digested wastes when struvite precipitation is used, with further opportunities to scale up and integrate this with AD to create a closed nutrient loop cycle. The goal is to show that struvite has immense potential in the agricultural sector, promoting a circular economy as it maximizes energy and nutrient production. The advantage of the struvite compound outweighs the direct usage of digestate as a fertilizer. Detailed research can be pursued to scale up the integration process and assess its capabilities as a fertilizer
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".