Value-added products from urine: Enriched compost and stabilized liquid fertilizer
The Rich Earth Institute is pioneering the use of sanitized human urine as an innovative and sustainable fertilizer. Wide-scale reuse of urine as fertilizer can provide a stably-priced, locally produced, and sustainable source of fertilizer for farms, while directly alleviating the growing problem of nutrient pollution of surface waters by septic systems and wastewater treatment facilities.
One challenge relating to the storage of urine and its eventual application as a fertilizer is the expense of transporting and storing large volumes of liquid urine until is is needed as a fertilizer. Co-composting urine along with a solid substrate presents the opportunity to incorporate the nutrients from urine into compost while driving off excess moisture as vapor during the composting process. The resulting product, finished compost, can be stored inexpensively and applied using familiar equipment and methods. Our objective for this experiment was to concentrate nutrients from urine by evaporating excess water through composting, and to produce a compost that has a higher nitrogen:phosphorus ratio than typical animal manure compost.
This report covers work done in the fall of 2016 under a one-year extension of this project. It contains preliminary findings from a benchtop composting trial, conducted as a follow-up to trials done in 2015 under our SARE project ONE14-218 (Urine as fertilizer: Maximizing hay yield and enriching low-N composts). These included a successful trial conducted at a temperature of 30°C, and an unsuccessful trial at 50°C which was hampered by reduced aeration conditions caused by a mechanical failure, as reported in our final report for that project.
Due largely to the relatively lower moisture content of the leaf substrate compared with the manure/shavings substrate, the leaves were able to absorb and retain roughly 2.4 the volume of liquid per volume of substrate as the manure/shavings mixture. Viewed on a mass basis, the difference is even greater: over the first 45 days of composting, net liquid absorption of the manure/shavings mixture averaged 0.33 L urine/kg of matrix material (wet weight), compared to 1.4 L/kg for leaves.
Because of the high performance of the leaf substrate, it will be used exclusively in the remaining composting trial using larger compost volumes, still to be conducted for this SARE research project.
Our previous trial was conducted at 30°C, using four feedstock combinations of horse manure, wood shavings, and shredded leaves, each formula then being mixed with pasteurized urine. In that trial, the leaf substrate was the most effective at retaining nitrogen in the finished product (retaining 83% of feedstock N), while horse manure/wood shavings recipes performed poorly (retaining 45% to 53%). Of the manure/wood shavings recipes, respiration rate was highest for the recipe containing the highest manure fraction and lowest for the recipe with the lowest manure fraction. This difference in respiration rate was reflected in the degree of degradation apparent in each end product.
These results informed the choice of compost recipes used to fill the four vessels in the new 50°C trial, which is described in this report. Based on good performance in the 30°C trial, two substrates were chosen for continued study in the 50°C trial: the leaf subtrate, and the manure/wood shaving substrate with the highest manure fraction (85% manure/15% shavings w/w).
Along with the two substrates under investigation, we also tested two urine products: pasteurized urine, as was used previously, and a new urine concentrate made using reverse osmosis. This addition represented another strategy: partial dewatering of urine through reverse osmosis before addition to the compost, which would allow more nutrients to be added to the compost without adding excessive moisture.
In the 50°C trial, two replicate vessels were filled with 85%/15% manure/shavings mix, and the other two vessels were filled with leaves, resulting in four vessels total. One of the two manure/shavings replicates and one of the two leaf replicates were wetted with pasteurized urine, while the other replicate of each substrate type was wetted with a pasteurized urine concentrate, produced using a reverse osmosis process that concentrated the urine by a factor of roughly 4.5. The four combinations are shown in Table 1.
Table 1: Compost recipes for the 50°C composting trial
Each substrate mixture was placed in a 20 liter aeration vessel, wetted to capacity with treated urine, and incubated it at 50°C. Bedded horse manure and dry hardwood leaves were chosen because of their high C:N ratios, which would be improved with urine. An adjustable air supply ensured adequate aeration. Automated monitoring of the oxygen levels in exhaust air provided feedback to the aeration system, which was programmed to adjust the aeration rate of each vessel individually to maintain at least 10% oxygen in the exhaust.
The trial was initiated on 11/15/16 and is currently still underway. Determination of the fraction of N retained, the C:N ratio, and the levels of plant macro- and micro-nutrients will occur after the trial concludes, but preliminary data related to moisture retention are available for discussion.
There were major differences between the manure/shavings compost and the leaf compost in terms of the source and fate of moisture. Because of the high moisture content of the manure (79%), the manure/shavings substrate contained more moisture (69%) than the leaf substrate (12%) before the addition of urine. After urine addition, the manure recipes contained about 80% moisture, while the leaf recipes contained only about 60% moisture. Despite its lower total moisture level, the leaf recipe contained about 20% more urine by volume than the manure/shavings recipe. As a result of the high moisture content of the manure, only approximately 45% of the total moisture in the manure/shavings recipes came from urine, while urine was the source of about 90% of the moisture in the leaf recipe.
Over the first 45 days of the trial, all leachate was collected from the four vessels, and recycled back into the compost as needed to maintain adequate composting moisture. Vessels 1 and 2 (containing the manure/shavings substrate) produced significant leachate, (1.2 and 0.9 L, respectively,) and remained wet to the touch. Vessels 3 and 4 (containing the leaf substrate) dried considerably during that period, allowing all leachate to be recycled back into the compost, and an additional 300 and 500 mL of water to be added to vessels 3 and 4 respectively.
Table 2 shows the starting mass and moisture content of each recipe, as well as figures relating to net urine absorption. Negative leachate production masses reflect the addition of water to maintain adequate moisture for composting.
Table 2: Moisture properties of the four composts
Impacts and Contributions/Outcomes
The intent of these urine composting trials is to asses the ability of different substrates to absorb urine and incorporate its constituent plant nutrients into a solid compost matrix. Therefore it is desirable that the composts produce no excess leachate, beyond what can be recycled back into the compost and reabsorbed. Production of net leachate would run counter to the objective of incorporating the urine nutrients into a solid matrix.
Due largely to the relatively lower moisture content of the leaf substrate compared with the manure/shavings substrate, the leaves were able to absorb and retain over twice the volume of urine per volume of subtrate as the manure/shavings mixture. Viewed on a mass basis the difference is even greater: net liquid absorption of the manure/shavings mixture averaged 0.33 L urine/kg of matrix (wet weight), compared to 1.4 L/kg for leaves. Increasing the ratio of shavings to manure would increase the ability of the mixture to absorb urine—but as was established in our 30°C composting trials, such an adjustment simultaneously reduces the rate of composting, resulting in a not-fully decomposed product even after several months of incubation.
For co-composting urine, leaves appear to perform better than a manure/shavings mixture as measured by two metrics: absorption and incorporation of the largest volume of urine with the smallest volume of solid substrate (as established in this trial) and retention in the finished compost of nitrogen originating in the urine (as established in the 30°C trial discussed in our final report for ONE14-218). At the conclusion of the current 50°C trial we will determine total nitrogen retention of each mixture, which will shed further light on the nitrogen retention capabilities of the different mixtures.
Based on these findings, the final composting trial conducted under this SARE grant (trials in vessels containing roughly one cubic yard of compost) will be conducted using leaves exclusively as the substrate, replicating the recipes used in vessels 3 and 4. One large vessel will be amended with pasteurized urine, and another using pasteurized urine concentrate.
Farm-scale implications will be explored in the final report, based on final analysis of the compost from this trial, and findings from the larger-scale composting trial to be conducted before the conclusion of this project.
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