Assessment of the chemical and biological safety of vermicompost and water produced at Rulindo Faecal Sludge Treatment Plant in Rwanda

Abstract

Growing global concerns about environmental sustainability and public health have emphasized the need for safe and effective waste management solutions. Key challenges, such as insufficient faecal sludge treatment, inadequate policies and regulations, and weak institutional frameworks, hinder sustainable sanitation in developing countries. In low- and middle-income countries like Rwanda, where sewer networks are often lacking, on-site treatment and reuse of faecal sludge have emerged as critical strategies to address sanitation challenges while promoting resource recovery. Vermicomposting, a biological process that utilizes tiger worms to decompose organic waste, has gained attention as an eco-friendly method for converting faecal sludge into valuable products like compost and treated water. However, the chemical and biological safety of these by-products remains a significant concern, particularly regarding potential contamination of organic matter, nutrients, pathogens, heavy metals, or other harmful substances. The Rulindo Faecal Sludge Treatment Plant in Rwanda uses vermicomposting technology to treat faecal sludge. Its outputs, vermicompost and treated water, show potential for agricultural and non-potable applications, but their safety must be thoroughly assessed to ensure they pose no risks to the environment or human health. There have been fewer research works in sub-Saharan Africa focused on assessing FSTP wastewater, black soldier fly larvae use in FSTPs, faecal sludge valorization, and faecal sludge management technology and ultimate applications. Even as yet, there has been no discussion of chemical and biological safety assessment of vermicompost and water released from FSTPs in Rwanda. Therefore, the aim of this study is to assess the chemical and biological safety of vermicompost and water produced in Rulindo FSTP, offering useful information on their suitability for sustainable reuse. Physical, chemical, and biological parameters were extensively checked. Average results of the physical parameters in FSTP wastewater are as follows: pH = 7.83, turbidity at 13 NTU, total suspended solids (TSS) = 7.5 mg/L, and dissolved oxygen (DO) = 3.74 mg/L. Chemical parameters included chemical oxygen demand (COD) of 198.9 mg/L and removal efficiency of 94.34% and biochemical oxygen demand (BOD) of 38.5 mg/L and removal efficiency of 94.16%. In addition, the total nitrogen (TN) was 305.25 mg/L, nitrates (NO₃⁻) were 225.53 mg/L, total phosphorus (TP) was 8.38 mg/L, potassium (K) was 397.5 mg/L, calcium (Ca) was 12.83 mg/L, magnesium (Mg) was 103.03 mg/L, and sulfur (S) was 58.95 mg/L. Biological analysis of the wastewater indicated high levels of total coliforms, faecal coliforms, Salmonella and Shigella with a removal efficiency of 93.54%, 55%, 91.75% and 98.09% respectively, and the v absence of E. coli (about 100% removal efficiency). Although some physical and chemical parameters of the wastewater comply with Rwanda Utilities Regulatory Agency (RURA) and FAO guidelines, high nutrient levels and microbial contaminants, including total coliforms, faecal coliforms, salmonella, and shigella (exceeding 1,000 CFU/100 mL), raise concerns about its suitability for irrigation. To identify water quality, significant indices such as Sodium Adsorption Ratio (SAR), Kelly's Ratio (KR), Soluble Sodium Percentage (SSP), and Magnesium Adsorption Ratio (MAR) were ascertained. SAR, KR, and SSP values indicate the water as suitable for irrigation. However, the MAR value of 88.92% exceeding the tolerance limit of 50% due to the high content of magnesium suggests otherwise. In addition, the Wilcox diagram that classifies irrigation water in terms of salinity and sodium hazards positions produced effluent in class C4S1, which is not suitable for irrigation due to high salinity. These findings conclude that the wastewater from the Rulindo Faecal Sludge Treatment Plant is not suitable for irrigation unless further technical adjustments are made. On the other hand, the solid vermicompost produced at the Rulindo STP was analyzed using the Gravimetric Ignition method for proximate analysis. The results revealed a moisture content of 58.21%, total solids of 41.79%, volatile organic solids (VOS) of 39.18%, total organic carbon (TOC) of 22.73%, total nitrogen of 1.96%, and a carbon-tonitrogen (C/N) ratio of 12:1. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used for the chemical analysis, identified macronutrients, micronutrients, and heavy metals. Detected macronutrients included phosphorus (P) at 1.747%, K at 0.248%, Ca at 2.203%, Mg at 0.392%, and S at 0.531%. Micronutrients such as iron (Fe) at 2.565% and manganese (Mn) at 0.058% were also identified. Heavy metals detected included lead (Pb) at 0.004%, titanium (Ti) at 0.043%, vanadium (V) at 0.004%, and zinc (Zn) at 0.052%. According to the RURA guidelines for faecal sludge management, FAO guidelines for organic fertilizers, and other national standards, the vermicompost produced at Rulindo FSTP is of good quality for agricultural use and is chemically safe due to the absence of toxic heavy metals. Taken together, the study found that the Rulindo FSTP effectively reduces most contaminants to acceptable levels. However, nutrient concentrations in the treated wastewater remain high, and biological pathogens require further disinfection to meet acceptable standards. To support sustainable waste management practices and contribute to Rwanda’s environmental sustainability goals, additional research studies are recommended to complement this work.