Introduction:
Managing municipal sewage sludge has evolved into a critical environmental and economic challenge globally due to its constantly increasing volume and the complex issues surrounding its safe and cost-effective disposal. Conventional methods like landfilling or incineration are often expensive and carry significant environmental risks, including the release of greenhouse gases and potential contamination of soil and groundwater. This situation underscores the urgent need for finding sustainable and circular economy solutions for resource recovery. The utilization of processed sewage sludge (biosolids) as a soil amendment or agricultural fertilizer offers a promising sustainable pathway. This material is rich in essential macronutrients like nitrogen, phosphorus, and potassium, which are vital for plant growth. Furthermore, its high organic matter content effectively enhances the soil's physical structure, improves water retention capacity, and stimulates microbial activity. This practice not only provides a valuable outlet for waste management but also significantly reduces agriculture’s reliance on costly synthetic fertilizers, contributing to greater overall system sustainability. However, potential risks, particularly the presence of heavy metals and pathogens, necessitate careful evaluation of its safety and efficacy before widespread agricultural application.

Material and Methods:
This study evaluates the effectiveness and risks of using sewage sludge from the wastewater treatment plant in the city of Tiran, Isfahan province, for growing summer vegetables. The research was conducted in 2024 using three different mixtures of soil and dried sludge at weight ratios of 0%, 20%, and 40% for the cultivation of tomatoes, eggplants, and peppers in a greenhouse. The Tiran wastewater treatment plant, located at 32°38'21.88" N and 51°11'12.20" E, uses an anoxic-aerobic biological process with an average flow of 864 cubic meters per day to treat the wastewater of its covered population. Plants were grown in pots with a height of 40 cm and a diameter of 30 cm, holding approximately 3.5 kg of soil. Seedlings of each vegetable were surface-sterilized with a 2% (w/v) sodium hypochlorite solution for 10 seconds before planting. For each scenario, four pots were initially cultivated. After one month, one pot with the least similarity to the others was removed, leaving nine pots per product for further research. During the growth period, weeds were manually removed, and no other fertilizers or chemicals were used. The pots were irrigated with municipal drinking water every three days for the first 30 days, and then every four to five days, until the soil reached its field capacity. Key soil characteristics, including pH, electrical conductivity (ECe), organic carbon, total nitrogen, available phosphorus, and heavy metals (lead and cadmium), were analyzed. Standard methods such as the Kjeldahl method for total nitrogen and atomic absorption spectrophotometry (model B1100 Perkin Elmer) for heavy metals were used. Plant growth parameters, including the total number of fruits, total fresh fruit weight, number of branches, main stem diameter, and plant height, were measured during the harvest period, which began 60 days after planting and continued for 30 days. The data were statistically analyzed using SPSS software. 

Results and Discussion:
The addition of sludge reduced the soil pH from 6.78 to 6.7 and 6.5 in the 20% and 40% sludge treatments, respectively, although these changes were not statistically significant (p>0.05). The application of 20% and 40% sludge increased organic carbon levels by 128% and 258%, respectively. Total nitrogen content also significantly increased from 0.08 g/kg in the control sample (0% sludge) to 1.82 g/kg in the 40% sludge treatment. The concentration of available phosphorus rose from 13 mg/kg to 3400 mg/kg, highlighting the sludge's potential as a rich source of nutrients. Statistical analysis revealed significant differences (p<0.005) in growth parameters, including plant height and stem diameter, with increasing sludge percentage. Maximum plant height and stem diameter were recorded at 73 cm and 19 mm, respectively, in the 40% sludge treatment. Furthermore, plants grown in the 40% sludge mixture showed an average 70% increase in fruiting compared to the control group. The heavy metal content in the sludge, specifically lead and cadmium, was measured at 2.8 and 1.4 mg/kg, respectively, which were below permissible limits. The processing method involving aeration and sunlight exposure successfully reduced pathogen levels in the sewage sludge

Conclusion:
The findings of this research indicate that applying sludge at concentrations of 20% and 40% not only enriches the soil with essential nutrients like nitrogen, phosphorus, and potassium but also improves its fertility. Higher sludge concentrations positively correlate with improved plant growth metrics, including total fruit production, plant height, and stem diameter, emphasizing its importance as a nutrient-rich organic amendment. This study alleviates concerns about heavy metal accumulation, as the observed levels were within safe limits, ensuring the safety of this agricultural practice. Overall, the processed sewage sludge effectively enhances the growth of the three selected plants and improves soil fertility without increasing heavy metals or pathogens. The study highlights the importance of preventing industrial wastewater and agricultural runoff from entering urban sewage networks and using physical dewatering processes without chemical polymers to produce safe sludge.