Wastewater generated at domestic sites must be treated to reduce the level of contaminants that are released with the wastewater into the natural environment. These contaminants include compounds that remove oxygen from water and nutrients that collectively prevent the natural functioning of water bodies. Homesites that are unable to be connected to a sewer system have traditionally used on-site septic tank/drainfield combination systems to treat and dispose of the wastewater into the native soil system. The wider community now expects a higher level of performance to address the environmental problems associated with wastewater disposal. New on-site wastewater treatment technology capable of improved performance is therefore required.
Vermicompost filtration is a new technology that treats wastewater and solid organic waste at homesites. A population of earthworms living in the system converts solid organic waste inputs into earthworm casts. A vermicompost filter bed develops over time as the casts accumulate in the system beneath the layer of degrading solid waste. Wastewater generated intermittently in the home is applied onto the filter bed surface where it passes down through the bed before exiting the system. The treatment system is housed within a tank that is usually buried below ground. As vermicompost filtration system was new to scientific investigation, little was known of the performance capability of the system or the mechanisms that underpin the treatment process.
This dissertation describes an evaluation of the performance and the sustainability of performance of vermicompost filtration systems. This evaluation was conducted primarily by systematically comparing system inputs with outputs. Vermicompost filtration systems must be capable of operating with variable input load rates and climatic conditions as these variables can not be controlled at individual homesites. A research program to quantify the performance and sustainability of the system at various input load rates and temperatures was therefore developed. Some of the mechanisms behind the treatment process were also investigated. This investigation was conducted experimentally and through mathematical modeling where the model structure was used to help explain mechanisms of the process.
Short-term experiments were conducted on small-scale, 25 cm diameter filter beds to establish the processes that occur in the system and determine the effect of filter bed depth on treatment performance. The reactors were segmented to enable wastewater samples to be taken at variable bed depths down to 50 cm. The physical and chemical properties of vermicompost filter beds were also established using similar reactors. The long-term performance and sustainability of the system was determined by conducting a carbon balance around pilot-scale, 90 cm diameter vermicompost filter bed reactors. A purpose-built experimental facility was constructed to measure soluble, particulate and gaseous carbon that entered and exited three vermicompost filtration reactors operating at 13, 20 and 27°C.
The solid waste at the surface of the filter bed was found to leach oxygen demand and PO4 into the wastewater flowing through it. The oxygen demand was subsequently removed from the wastewater during passage through the filter. There were no mechanisms for PO4 removal therefore the phosphorus that was leached from the solid waste left the reactor in the effluent. Nitrification occurred throughout the filter bed depth profile. Denitrification also occurred when the level of biological oxygen demand leached from the solids was high. The simultaneous nitrification-denitrification (SND) process was previously not known to occur in vermicompost filtration systems and could lead to new applications for the technology. The best treatment performance occurred when the filter bed depth was 50 cm. The SND process was successfully modeled using Monod Kinetics.
Increasing the solid waste loading rates whilst maintaining the influent carbon concentration had no significant effect on effluent quality. Effluent quality was maintained providing undegraded carbon did not accumulate in the reactor over long time periods. Accumulation of undegraded carbon within the system led to a decline in effluent quality and ultimately to hydraulic failure of the filter bed. At all other times, the reactors operated under aerobic conditions which was demonstrated by the absence of CH4 in the reactor exhaust gas. It was found the accumulation of undegraded carbon was a function of ambient air temperature within which the reactors were operating. Increasing the soluble carbon concentration whilst maintaining the solid waste loading rate resulted in reduced effluent quality. The population of the earthworm species Eisenia fetida in the pilot-scale reactors was sustainable in the reactors at 13 and 20°C. However, they could not be sustained at 27°C. It was concluded that the performance of the reactor would therefore be unsustainable at this temperature as the earthworms form an essential role within the vermicompost filtration system.
The performance of the vermicompost filtration system was explained in part by the physical and chemical characteristics of the vermicompost filter bed. In contrast to the degrading solid organic waste, the vermicompost was a stable medium that did not leach significant quantities of oxygen demand or nutrients into wastewater flowing through it. The filter bed comprised of internal voidage that accounted for between 60 and 78% of the bed volume at varying bed depth. The wastewater that filled this voidage remained in the reactor for long time periods relative to wastewater that flowed through the external voidage. The wastewater leaving the reactors was therefore a combination of water that had bypassed the matrix through the earthworm burrows and water that had passed through the earthworm cast matrix. Only wastewater held in the filter matrix was representative of the full treatment capacity of the vermicompost filtration system.
The vermicompost filtration system is an effective waste treatment system that reduces most of the contaminants applied to the system. The application of solid waste to the system is required to form the filter bed which subsequently supports the wastewater treatment process. A wastewater treatment system that treats solid organic waste at the home has practical advantages, as a separate waste treatment system is not required for organic solids. The solid waste however, also leaches contaminants that must be removed from the wastewater before they leave the system in the effluent. The solid waste loading rate should therefore be controlled to prevent this occurring.
This thesis describes the application of systematic scientific thinking to a novel design of on-site waste treatment process. The thesis is in a discipline that does not usually receive such scrutiny. The findings are the first to be presented in the scientific literature on vermicompost filtration systems. The results establish the performance and sustainablity of the system and are discussed in relation to the range of input loading rates and temperatures expected at homesites. Recommendations for changes to the design and operational procedures and the future direction of experimental work are also discussed.