Browsing by Author "Gaszynski, Chris"

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    Open Access
    A System-Wide Model for Solid Waste Separation and Food Waste Discharge to Sewer Systems
    (2022) Angula, Shalongo; Ikumi, David S; Gaszynski, Chris
    The emergence of the circular economy, together with the changing paradigms in resource and environmental management, has resulted in a call to (1) convert wastewater treatment plants (WWTPs) to water resource recovery facilities (WRRF) and (2) diversion of organic waste (i.e., food waste) from landfills. Due to excess anaerobic digester (AD) capacities at most WWTPs, it has been proposed to co-digest sewage sludge and food waste in the AD at WWTPs to enhance resources (mainly energy and nutrient) recovery. However, suitable options for sustainable food waste handling (i.e., separation and transport) and the characteristics of food waste have not been established, particularly in South Africa. Without characterising food waste, it is impossible to include it in WWTP simulation models. In this study, food waste and sewage sludges were limited to the household food waste and primary sludge categories, respectively. A detailed review on alternative sustainable solid waste separation and food waste transport systems was carried out and a review paper was submitted to Environmental Challenges journal for publication. The experimental investigation of the study focused on identifying the biodegradable organic composition of selected substrates, i.e., primary sludge (PS), food waste, and PS and food waste blend. PS was obtained from Belville Wastewater Treatment Works (BWWTW) in Cape Town, while food waste was manually simulated. The augmented biomethane potential (ABMP) experiment was used to obtain the required research data. The ABMP data was used to determine the substrates' biodegradable composition. The biodegradable composition was carried out using two approaches. The first approach is the mass balanced bioprocess stoichiometric calculations, which used the measured start and end concentrations of the ABMP experiment. The second approach is the parameter estimation procedure, which used the mass balanced steady-state AD model of Ikumi et al. (2015) together with its built-in parameter estimation function. With the exception of methane COD produced, there were slight to moderate differences between the measured and modelled experiment results. However, the modelled experiments produced significantly higher methane COD than the measured experiment, suggesting a high-level error associated with the gas measurement. As a result, the modelled experiment and the substrates' composition obtained using parameter estimation were chosen to be more accurate and reliable. The results revealed that anaerobic digestion of food waste and co-digestion of primary sludge and food waste produces 41% and 21% more methane than the anaerobic digestion of primary sludge, respectively. The methane produced is equivalent to the potential energy recoverable. These findings suggest that diverting food waste from landfills to WWTP's AD systems can potentially enhance energy recovery. This highlights the potential need to revise urban wastewater systems to include solid waste separation and food waste diversion to AD systems to enhance resource recovery.
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    Development of an Integrated thermal hydrolysis process - Anaerobic Digestion (THPAD) Model
    (2022) Olando, Alexander; Ikumi, David; Gaszynski, Chris
    Historically, anaerobic digestion is one of the most common processes used to treat sludge generated from wastewater treatment plant (WWTP) processes. However, with the exponential increase in populations, which implies an increase in WWTP loads, the amount of waste generated poses an imminent problem to the handling capacity of current anaerobic digesters. Subsequently, there has been a lot of research into various physical and chemical processes that would allow for a more efficient sludge handling mechanism. Studies have reported various advantages associated with digesting sludge at higher temperatures known as thermophilic temperatures. These advantages include increased sludge handling capacity, a higher degree of sludge biodegradability and subsequently increased methane production and better sludge dewatering characteristics implying cheaper sludge transportation costs just to mention a few. However, despite the advantages associated with thermal treatment, this technology has not yet been proven in a South African context. This project involved the development of an integrated thermal hydrolysis process (THP) and anaerobic digestion (AD) model capable of simulating these processes at elevated temperatures. A comparative desktop case study of the existing AD facility at the Cape Flats wastewater treatment works (CFWWTW) in Western Cape, South Africa was investigated following the City of Cape Town's (CCT) initiative to retrofit a THP unit to the anaerobic digesters to help deal with the increase in sludge handling capacity. A comparison was therefore carried out, investigating the base case scenario of maintaining the existing conventional mesophilic anaerobic digesters (MAD) and retrofitting a THP unit to the conventional anaerobic digesters (THPAD). A steady-state THP and AD model was developed and used in conjunction with an integrated dynamic THP and modified AD (termed as the Extended-UCTSDM3P) model for simulating both the conventional MAD and THPAD processes. This allowed for a comparison of results not only between the two processes, but also the two types of models. These models were then used to simulate the treatment of a mixture of primary sludge (PS) and waste activated sludge (WAS) at a ratio of 60:40 with the WAS obtained from a Nitrification Denitrification Biological Excess Phosphorus Removal (NDBEPR) activated sludge treatment. The AD models, therefore, accounted for the increased phosphorus concentration as a result of iv polyphosphates (PP) breakdown and consequently the possible precipitation of struvite (MgNH4PO4) from the AD liquor. The results showed that the THPAD configuration allowed the digesters to process 2.3 times more sludge than with the conventional mesophilic anaerobic digesters. Furthermore, the methane production in the THPAD was conservatively calculated to be 2.5 times higher than the MAD. This implied an increased potential for use of the methane gas as an alternative source of energy in the wastewater treatment plants. Given that no laboratory experiments were carried out, the results were based on theoretical scenarios and knowledge collected from an extensive literature review. However, given the capacity, flexibility and detail the model has been developed to, different scenarios in the anaerobic digestion process can be investigated and valuable practical insight extracted. Furthermore, through calibration with accurate meaningful data from a pilot or full-scale plant, the developed model is a tool that could be used in predicting digester performance.
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    The Economic and Technical Feasibility of the Application of Partial Nitritation Anammox Technology Over Conventional Nitrification Denitrification for the Treatment of Sidestream Liquor at Cape Flats WWTP
    (2021) Jelliman, Shanen; Ikumi, David; Gaszynski, Chris
    The objective of this study was to provide a comparative analysis for treating sidestream liquor from a future regional anaerobic digestion (AD) facility (Thermal Hydrolysis Process (THP)+AD) at Cape Flats WWTP in South Africa. The study focused on comparing a conventional nitrification denitrification (Modified Ludzack-Ettinger (MLE) configuration) with a novel partial nitritation/ anammox (PN/A) process. The sidestream liquor was characterised by mass balance over the AD process. Steady-state models were used to predict equipment sizes and determine the process efficiency for each treatment technology. The models were evaluated using an effluent quality index (EQI) and operational cost index (OCI). The capital cost of each treatment solution was calculated and used to perform an economic lifecycle cost analysis (LCCA). The study concluded that the sidestream liquor (untreated) would recycle 3 415 kg TKN/d back to the mainstream process which represented 29% of the design capacity of the plant. The MLE process required a 53% smaller reactor volume but consumed 93% more energy and produced 64% more sludge which resulted in the MLE OCI being 10.2 times higher than the PN/A process. The total capital cost of the PN/A and MLE processes were calculated to be R139 537 000 and R117 420 000 respectively. Although the MLE process costs 16% less to implement initially, the LCCA over a 20-year operational period indicated that the net present value of the MLE process is 2.8 times higher than the PN/A process.