The use of raw sanitary sewage (RS) in fertigation offers a sustainable approach to nutrient provision for crops, potentially reducing reliance on mineral fertilizers and addressing water needs. Its year-round availability, particularly in areas with established sewage systems, makes it an attractive resource. While techniques like spraying and dripping are common, furrow irrigation presents advantages when using RS due to its reduced risk of emitter blockage from chemical, physical, and biological agents, and improved sanitation safety compared to spraying. However, conventional furrow fertigation, where nutrients are applied at a single point, can lead to nutrient accumulation and pollution near the application site. This study focuses on the impact of RS application position within closed-end furrows on the productivity and nutrient uptake of elephant grass, a significant forage crop in Brazil. Prior research has examined the impact of various factors on elephant grass productivity, including fertilization, cutting age, and season, but the response to RS application remains less explored. Therefore, this work aims to compare the effects of alternating and non-alternating RS application positions in closed-end furrows on elephant grass productivity and macronutrient extraction over a three-year period.

Existing literature supports the potential of wastewater reuse in agriculture, particularly in urban settings where water scarcity is a concern. Studies have highlighted the benefits of wastewater treatment for agricultural applications and the need for improved application techniques. Furrow irrigation, despite its seemingly rudimentary nature, offers advantages over other methods when using raw wastewater, as it mitigates the risk of emitter clogging and contamination. However, research consistently demonstrates that conventional furrow fertigation results in uneven nutrient distribution, leading to accumulation at the application point and potential environmental concerns. While the impact of various factors (fertilizer type and dose, cutting age, season, etc.) on elephant grass productivity is documented, the plant's response to sanitary sewage application is relatively understudied. This gap in knowledge necessitates the present investigation.

This three-year field experiment was conducted at the Sewage Treatment Station of COPASA ETE-Onça near Belo Horizonte, Brazil. Elephant grass (*Pennisetum purpureum*) was planted in June 2016 and subjected to four treatments in a completely randomized design with seven replicates: (i) Conventional mineral fertilization (CMF) with treated water (TWN) applied without alternation; (ii) CMF with TW (TWA) applied with alternation; (iii) Fertigation with RS and TW (TFN) without alternation; and (iv) Fertigation with RS and TW (TFA) with alternation. Each plot (72 m²) comprised three furrows and four planted rows. CMF doses were established following recommendations for N, P, and K. RS fertigation was performed weekly, aiming for a sodium dose of 300 kg ha⁻¹ year⁻¹. Evapotranspiration was used to determine plant water requirements. Alternating application involved switching application points between the furrow ends. Weekly RS samples were analyzed for Na, TKN, TP, and TK using established methods (APHA). Plant samples were collected after each cutting (approximately five per year) and analyzed for dry matter, TN, P, and K (EMBRAPA methodology). Data were analyzed using ANOVA followed by the Tukey test (p<0.05) and linear regression.

The RS application provided significantly higher nitrogen than conventional fertilization (647 kg ha⁻¹ year⁻¹ vs 300 kg ha⁻¹ year⁻¹), while phosphorus was close to recommendations and potassium was lower. The TFA treatment (alternating RS application) yielded the highest average dry matter production (29.9 Mg ha⁻¹ year⁻¹), exceeding the other treatments (17.5-22.2 Mg ha⁻¹ year⁻¹) by approximately 30%. TFA also resulted in significantly higher average macronutrient extraction: 688 kg ha⁻¹ year⁻¹ N, 102 kg ha⁻¹ year⁻¹ P, and 508 kg ha⁻¹ year⁻¹ K. These values were considerably higher than those in TWN, TWA, and TFN treatments. Nutrient content per unit mass of dry matter also demonstrated higher N and K in TFA compared to other treatments. Linear regression analysis revealed a negative correlation between distance from the application point and dry matter yield and nutrient extraction in TFN, unlike TFA, which showed no significant correlation, indicating even distribution. The TFA treatment achieved approximately 100% recovery of applied macronutrients, minimizing the risk of leaching. Comparison with existing literature showed similar or higher nutrient extraction in TFA compared to various studies using different fertilization methods.

The results strongly support the hypothesis that alternating the application point of raw sanitary sewage in closed-end furrows significantly enhances elephant grass productivity and nutrient uptake. The improved nutrient distribution in TFA, compared to the accumulation observed in TFN, accounts for this difference. The higher dry matter yield and nutrient extraction in TFA aligns with previous research showing a positive linear relationship between nitrogen application and elephant grass yield. The considerable nutrient extraction in TFA, exceeding the applied amounts, further suggests efficient nutrient uptake and a reduced risk of environmental contamination. While potassium levels in RS were lower than the recommended CMF application, the overall nutrient balance in TFA is highly beneficial, demonstrating the potential of RS as a sustainable fertilizer source. The findings highlight the importance of optimized application methods for maximizing resource use and minimizing environmental risks when using wastewater in agriculture.

This study demonstrates the substantial benefits of alternating raw sanitary sewage application in closed-end furrows for improving elephant grass productivity and nutrient extraction. The TFA treatment significantly outperformed other methods, achieving a 30% increase in dry matter yield and ensuring efficient nutrient uptake, minimizing leaching risks. This research underscores the importance of system management in maximizing the benefits of wastewater reuse in agriculture. Future research could investigate other variables like flow rate, soil texture, and different plant species to further refine the understanding and application of this sustainable farming practice.

The study was conducted at a single location with specific soil and climate conditions, potentially limiting the generalizability of findings to other contexts. The use of raw sewage could raise concerns regarding potential pathogen transfer although the method employed presents better sanitation safety, without contamination of plants and system operators compared to application by spraying. Further research is warranted to assess the long-term effects of RS application on soil health and potential risks.