Assessing the nutritive value, safety and consumers’ perceptions of co-compost made from dewatered sewage sludge and organic green waste.

Abstract

The increasing global population which is estimated to reach 9.6 billion in 2050 implies the need to increase food, feed, and fuel production on the limited agricultural lands while carefully managing waste. Chemical fertilisers have primarily been used to enhance plant productivity because they have a high nutrient composition, are water-soluble, readily available to plants, easy to apply, and transport. Although these chemical fertilisers have these benefits, they often leach into the water surface contaminating underground water bodies and causing eutrophication in water courses. Moreover, the Harber-Bosch process which produces these chemical fertilisers is energy-intensive and contributes to greenhouse gas emissions. Furthermore, small-scale farmers in Sub-Saharan Africa often apply less than 10 kg/ha of nitrogen. On the other hand, commercial farmers in developed countries apply excessive amounts of chemical fertilisers, which subsequently degrade the soil and lead to organic matter loss. The nutrients are transported from farms to urban settlements in the form of food and feed leaving the soil nutrient-depleted. In cities, food, market, and human waste are often landfilled in hazardous landfills; however, they pose health risks and environmental pollution if improperly managed. Co-composting human excreta including dewatered sewage sludge (DSS), and organic green waste (OGW) can prevent the stockpiling of sludge in the wastewater treatment plants (WWTP) and divert OGW from landfills. Moreover, co-composting can recover essential nutrients from residual plant materials and human excreta while subsequently reducing improper waste disposal and management. The first objective of the study investigated the effect of mixing ratio and turning frequency on the production of high nutritive value and pathogen-free co-compost when using dewatered sewage sludge and organic green waste as feedstocks. The study employed different turning frequencies and mixing ratios to determine the optimum co-composting conditions. The study found that there were significant differences between treatments in pH, electrical conductivity, organic matter, and organic carbon. The highest turning frequency (7 days turning interval) resulted in low pathogen load contamination in two different mixing ratios however it was characterized by low organic matter compared to less frequently turned (28 days turning interval) co-compost piles. The study found all treatments except high turning frequency and low sludge proportion (1DSS:3OGW_28TF) complied with the World Health Organization and some treatments complied with the Fertiliser Act standards of viable helminth eggs in the matured co-compost. The second objective of the study established a novel, cost-effective method for assessing the phytotoxicity of co-compost through a rhizotron experiment. In context, co-compost treatments outlined in Chapter 3 were compared to sandy loam soil and Gromor commercial compost. Cucumber (Cucumis sativus) was used as a test crop because of its sensitivity to phytochemicals that inhibit growth and development of root traits and shoot. The root system architecture measured were convex hull area (CHA), root branch count (RBC), total root length (TRL), root system depth (RSD), root system width (RSW), root mass ratio (RMR), shoot mass ratio (SMR), root-shoot mass ratio (RSMR), shoot-root mass ratio (SRMR), leaf area index (LAI), and leaf number (LN). The study found that co-compost treatments with a higher proportion of sludge resulted into higher above ground biomass and it was positively associated with availability of nutrients in the growing media. The convex hull area was high in the sandy loam soil and roots length was high in the commercial Gromor compost. The third objective of this study investigated the effect of the combined application of cocompost and synthetic fertiliser on the growth, development, and nutritional composition of maize (Zea mays). The maize was grown on two contrasting soils but both acidic with different physical properties a sandy loam and a clay-loam soil. The sandy loam soil was collected from KwaDinabakubo (KD) in Durban while the clay loam soil was collected in Bishopstowe (BS) Pietermaritzburg. The study found that combining co-compost with chemical fertilisers at half the recommended rate improved visual growth parameters of maize including plant height, chlorophyll content, maize cobs, and harvest index. Moreover, the application of co-compost in the sandy loam soil improved organic carbon and organic matter and led to harvestable grain in the poor soil. Essential minerals including nitrogen (N), calcium (Ca), and zinc (Zn) were high in maize grains with the treatment combination of co-compost and synthetic fertiliser in both sandy loam and clay loam soil. These results have provided information and solutions to farmers faced with low productivity because of poor soil organic matter and organic carbon. The fourth objective of this study assessed the attitudes, perceptions, and willingness of university students to purchase and consume food produced using co-compost. This study aimed at validating assumptions and negative stigmas associated with acceptance of food produced using human excreta-derived fertilisers. The study found that willingness to purchase and consume food produced using co-compost was mainly influenced by students’ awareness of the use of human excreta for agricultural purposes. Moreover, the study found that there were differences across gender and male students were more willing to purchase and consume food produced using co-compost compared to female students. The study further unveiled that willingness to consume food produced using co-compost is affected by the type of food or crops grown using this type of soil amendment. In context, students/consumers were less willing to consume food produced using co-compost if it was eaten raw and grew below ground. Interestingly, students were more willing to eat food produced using co-compost of it grows above ground, if it was eaten cooked and if it undergoes processing. These results will assist farmers and policymakers in carefully choosing the types of crops that can be grown or produced using co-compost. In context, the study revealed that through co-composting challenges of waste management in landfills and wastewater treatment plants can be addressed with an added benefit of ensuing circular nutrient economy. The study further developed a novel approach to determine and assess phytotoxic compost in the matured co-compost using a rhizotron as it shows the growth of both roots and shoots. This approach is useful in identifying phytochemical compounds in the co-compost that can inhibit the growth of crops. The study found that sandy loam and clay loam soil can be improved by integrating synthetic fertilisers and co-compost. The study further recommends simulating awareness campaigns and developing dissemination strategies to enhance public understanding of fertilizers derived from human excreta.