The suitability of organic coagulants for wastewater treatment and their impact on sewage sludge biological stabilization and utilization as soil amendment

The treatment of municipal wastewater often employs inorganic coagulants (IC) to remove contaminants such as dissolved and particulate organic matter and nutrients through the coagulation/flocculation process. However, the use of IC is linked to concerns related to sludge treatment and disposal. This thesis investigated the suitability of applying organic coagulants (OC) as alternatives to IC in the treatment of municipal wastewater. Additionally, the impact of replacing IC with OC on the biological stabilization (i.e., anaerobic digestion (AD) and composting) of the resulting sludge and the effect of anaerobically digested sludge utilization in soil conditioning was evaluated. Laboratory investigations (i.e., jar-tests), pilot-plant systems (AD and composting) and soil microplot experiments (fertilization and leaching) were utilized. Monitoring involved the analysis of various parameters in different matrices (water, soil, sludge, and plant), including organic matter measured as biochemical oxygen demand (BOD), nutrients (i.e., nitrogen (N) and phosphorous (P), metals, and micropollutants (e.g., pharmaceuticals). Other factors monitored were water turbidity, pH, gas production, feedstock temperature, leaf growth, etc. In general, the results showed that OC with a high molecular weight and very high charge density can potentially act as sole coagulants, removing up to 88% of BOD and 87% of phosphate from wastewater. Regarding the stabilization of produced sludge, the type of coagulant affected both the AD and the composting processes. For example, during AD, natural OC exhibited the highest methane production. While evaluating the effect of coagulant type on the recirculation of sludge to land, the OC-precipitated AD sludge (biosolids) showed the highest total phosphorus and total nitrogen discharge load in leaching experiments. In addition, OC-biosolid-fertilized grasses showed a higher nutrient content in leaves compared to that of the IC biosolid-fertilized grasses.