Evaluation of sewage-sludge based products for indoor basil production

Evaluation of sewage-sludge based products for indoor basil production A. Shenk*, R. Gorji*, S. Schwede* and M. Odlare* * Future Energy Center, Mälardalens Universitet, Universitetsplan 1, Västerås, 72223 SE (E-mail: aubrey.she[email protected]; reyhan[email protected]; Sebastian.schwed[email protected]; [email protected];) Abstract As human population continues to rise, more sustainable methods of food production are required. Current practices of producing nitrogen fertilizer from the Haber-Bosch process have been affected recently by geopolitical strife, rising natural gas prices and the global pandemic of 2020. This work has investigated the use of several municipal wastewater streams as mineral fertilizer replacements on a nitrogen basis. Waste activated sludge and digested sludge were transformed using pyrolysis at low (300°C) and high (600°C) temperatures to produce biochar products that were applied to pot experiments growing basil. Reject water from dewatering digested sewage sludge was also used after filtration and two different ozone treatment for sterilization as liquid fertilizer options. Though the products did not perform as well as the mineral fertilizer control, the sludge based fertilizers were able to provide enough nutrients for the herbs to grow. Further experimentation on the proper dosing of these products has the potential to produce a material competitive with mineral fertilizer. Keywords (maximum 6 in alphabetical order) Biochar; circular economy; nitrogen recovery; pot experiment; sustainability; wastewater INTRODUCTION Nitrogen is an element essential to all life on Earth as a key component of nucleic acids, proteins and chlorophyll. Though abundant in the atmosphere, it is not in a form readily available to plants, thus fertilizers must be produced that transform nitrogen into bioavailable forms to increase crop production. As of 2015 it was estimated that approximately 3.84 billion people could be sustained without nitrogen fertilizers (“World Population Supported by Synthetic Nitrogen Fertilizers,” n.d.). As our populations continue to increase, the percentage of those reliant on fertilizers for their daily food will also increase. However, increased use of nitrogen fertilizers also comes at a cost both monetarily and environmentally. The production of ammonium from the Haber Bosch process generated 438.5 Mt CO2e in 2018, which accounted for 38.8% of the total emissions of the supply chain of nitrogen fertilizers. The overall N fertilizer supply chain including production, transportation and field emissions accounted for 10.6% of the global agricultural emissions for the same year (Menegat, Ledo, and Tirado 2022). Though nitrogen is important and required for increased agricultural production, too much nitrogen in the natural environment is responsible for eutrophication, which can lead to loss of biodiversity and decreased water quality. Wastewater treatment plants use nitrification-denitrification to remove ammonium from water to prevent eutrophication of receiving water bodies, which requires an estimated 4.0 kWh/kg N reduced (Malovanyy et al. 2022). To close the nitrogen loop around agriculture, we propose utilizing wastewater products as a source of nitrogen (and other nutrients) to produce edible foods. Sweden currently recovers nutrients from approximately 30% of the sewage sludge produced in the country through direct application to arable land, but to meet the expected legislation requiring 15% nitrogen recovery from wastewater this would need to increase to 80% (Malovanyy et al. 2022). Alternative streams high in nitrogen like reject water could and should also be considered to help contribute to this goal. Experimental work around the safety of using wastewater products in food production is of the utmost importance, and this work contributes to the growing knowledge around the topic. MATERIALS AND METHODS This work was performed in an indoor farming set up using 1 L pots and 1 mm particle size agriperlite media. Ten basil seeds were sewn at the start of the experiment and wetted with deionized water until sprouting. After sprouting, the fertilization began with 100 mL of solution three times a week for 8 weeks. The biochar pots were irrigated with 100 mL of deionized (DI) water at each fertilization. The control consisted of only agriperlite media and was fertilized with 100 mL of an optimal mineral fertilizer solution containing N, P, K (220, 65, 200 mg/L respectively) as well as micronutrients Mg, S, Ca, Cu, Zn, Fe, Bo, Mn and Na. Four solid products derived from sewage sludge at two different stages of processing were tested in this experiment. Waste activated sewage sludge and digested sludge were provided by MälarEnergi’s wastewater treatment plant in Västerås, Sweden. Sewage sludge was first dried at 80°C until constant mass, then pyrolyzed in two groups. The first set was pyrolyzed at 300°C for 7 hours. The second set was pyrolyzed at 300°C for 2 hours, then cooled and followed by another pyrolysis at 600°C to increase the adsorption capacity of the material for nitrogen loading. This process was inspired by (Schlederer, Martín-Hernández, and Vaneeckhaute 2024) which showed that pyrolyzing in two stages allows for the recovery of more bio oil and aqueous pyrolysis liquid while still producing a more carbonized and stable char. After pyrolysis all materials were washed with deionized water in three stages to remove potential inhibitory tars and oils on the surface of the biochar. The biochar was mixed into the top 3 cm of agriperlite media and applied such that the concentration of available nitrogen would match the total nitrogen applied during the experiment of the liquid fertilizer control (770 mg). The reject water was also collected from MälarEnergi’s wastewater treatment plant, as the liquid stream coming from dewatering of the digested sludge. It was vacuum filtered using 125µm filters, then split into three groups: filtered reject water, high ozone treatment and low ozone treatment. The two ozone treatments were perfomed using an ozone generator by PBS generation, where ozone was created and diffused through the reject water using a bubbler at the bottom of a tank. The first treatment lasted 30 minutes, and the second treatment was only 10 minutes. The primary motivation for ozonation was for sterilization of the wastewater, since there are harmful microorganisms present at this stage in the wastewater treatment plant that must be removed before its use for human food production. RESULTS AND DISCUSSION Processing of solid materials Pyrolysis of waste activated sludge and digestate was determined to be incomplete by visual inspection of the materials. An obvious color change from the raw material (brown) to carbonized material (black) was noted for a portion of each pyrolysis run, but due to the heating mechanism and design of the oven the effect was uneven, leaving some material less or completely uncarbonized. The material in its various states of carbonization was mixed before applying to the plants. The main motivation for pyrolysis was to immobilize heavy metals inherent in the sewage sludge, so an under-carbonized material might pose a risk of heavy metal leakage into the environment, potentially entering the biomass. ICP analysis of the biomass and biochars will quantify this risk. (Kominko, Gorazda, and Wzorek 2022) tested the mobility of heavy metals into biomass of maize and rape after fertilization with sewage sludge-poultry litter ash or sludge-mineral fertilizer mixtures and found that the levels of heavy metals (especially Ni and Cd) increased but were characterized by a low pollution level. There was no information about what part of the plant was affected most by the heavy metal accumulation. If the non-edible parts of the biomass uptake the metals there should not be a health risk for consuming the leaves of the herb. Ozonation of Reject Water The water quality parameters measured were in accordance with EU legislation (EU)2020/741 outlining requirements for water reuse in irrigation and agriculture, and included turbidity, biological oxygen demand (BOD5), and suspended solids. Chemical oxygen demand (COD) is not part of legislation but is said to be affected by ozone treatments so it was included in our analysis (Yasar et al. 2007). Table 1. Results of ozone treatment on reject water Sample E.coli (CFU/100mL) Turbidity COD (Cr) BOD5 (ATU) Suspended solids N (est) mg/L Reject 2200 86 800 110 89 1500 Low ozone 800 86 740 160 100 30 High ozone <10 34 780 140 45 30 Regulation <10 <5 - <10 <10 - The high ozone treatment successfully irradicated the microorganisms in the reject water, but the low ozone treatment did not. Neither ozone treatment was able to decrease the other water quality parameters to be below the limits set by legislation (less than 10 for all parameters except turbidity which should be less than 5 mg/L). Additionally, the ozone treatments decreased the available nitrogen content in the reject water to a point where the materials were not viable for plant production. Basil growth Out of the four biochars tested, the digestate at 300°C/600°C had the highest plant height, number of leaves and total fresh weight of all the biochars tested, though it did not compete with the control. Comparing between the two pyrolysis treatment regimes, the two-stage pyrolysis of the material performed better than the material that was only pyrolyzed at 300°C. Following harvesting of the material, the biochar and substrates will be analyzed to help shed light on why this material performed the best. Additional analysis of the plant material will track the heavy metal content to determine where in the basil the metals concentrate. Figure 2. Growth parameters of basil with different fertilizing materials CONCLUSION (PRELIMINARY) The results from this experiment indicate that there are nutrients present in sewage sludge-based products that are bioavailable. Though the experiment was designed around providing ample nitrogen to the plants, there must be other nutrients available as well for the plants to grow. Analysis of the materials will give a better picture of what nutrients were available in what quantites, and in which parts of the plant the heavy metals accumulate. If the heavy metals do not accumulate in the plant at all, or accumulate in the roots it can be considered safe for human consumption, at least on the basis of heavy metals. If however the contaminants concentrate in the edible portion of the plants, additional treatment to remove heavy metals from the wastewater materials will need to be considered before implementation could proceed. Additional processing for reject water must be considered to address the microbial contamination while leaving as much of the nutrients as possible in the material. UV light treatment may prove to be a better technique for this application than ozonation. 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