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Title: Biological nutrient removal from municipal sewage using acti-zyme: recovering biogas and bio-solids from sewage sludge
Authors: Manyuchi, Musaida Mercy 
Keywords: Acti-zyme;bio-augmentation;biogas;bio-nutrients;bio-solids;sewage treatment;statistical modelling;techno-economic analysis
Issue Date: 2016
Publisher: Cape Peninsula University of Technology
Abstract: Water scarcity is a global problem hence the need for sustainable wastewater management. Sewage, a form of wastewater is being disposed-of to river bodies untreated. Additionally, disposal of sewage sludge, a by-product from the sewage treatment process, is resulting in landfilling problems. This study focused on the sustainable anaerobic treatment of sewage, coharnessing biogas and bio-solids as value added products utilizing Acti-zyme, an enzyme biocatalyst through bio-augmentation. Emphasis was given to the optimum sewage treatment conditions for removal of bio-nutrients, biogas and bio-solids generation, kinetic and statistical modelling of the bio-nutrient removal in sewage as well as the biogas and bio-solids production from sewage sludge. A techno-economic analysis was then done to check the viability of applying this technology on a large scale. The biochemical properties for Acti-zyme were characterized for potential use in anaerobic sewage treatment with the aim of producing biogas. Acti-zyme was then used for sewage treatment at a temperature of 37 Β°C, agitation rate of 60 rpm, Acti-zyme loadings of 0-0.070 g/L and retention times of 0-60 days. The total Kjeldahl nitrogen (𝑇𝐾𝑁), biochemical oxygen demand (𝐡𝑂𝐷5), total suspended solids (𝑇𝑆𝑆), total dissolved solids (𝑇𝐷𝑆), electrical conductivity (𝐸𝐢), 𝑝𝐻, chloride ions concentration (𝐢𝐼 βˆ’), total phosphorous (𝑇𝑃), sulphate ions concentration (𝑆𝑂4 2βˆ’), dissolved oxygen (𝐷𝑂) and the chemical oxygen demand (𝐢𝑂𝐷) of sewage were measured using standard methods. The bio-nutrient removal ratios from the sewage were determined and statistical modelling was carried out for the bio-nutrient removal ratios: The 𝐢𝑂𝐷⁄𝐡𝑂𝐷5, 𝐡𝑂𝐷5⁄𝑇𝐾𝑁, 𝐢𝑂𝐷/𝑇𝐾𝑁 and the 𝐢𝑂𝐷/𝑇𝑃. The sewage sludge was anaerobically digested using Acti-zyme in order to obtain biogas and bio-solids. Sewage sludge loading of 5-10 g/L.d and mesophilic and thermophilic temperatures of 37 Β°C and 55 Β°C were applied. The biogas quantity produced was measured using the water displacement method. Samples of the biogas were analysed for bio-methane (𝐢𝐻4), carbon dioxide (𝐢𝑂2) and traces gases composition using gas chromatography. The bio-solids obtained were tested for nitrogen, phosphorous and potassium (𝑁𝑃𝐾) content using 𝑒𝑣 βˆ’ 𝑣𝑖𝑠 spectrophotometry. Kinetic modelling was carried out in MATLAB R2013A to simulate bio-methane production from sewage sludge. Statistical models for anaerobic sewage sludge digestion for generation of biogas and bio-solids utilizing Acti-zyme, were then simulated from the experimental data. SPSS Statistics 19.0 was used as the statistical modelling package at a p-value of 0.05. Capital budgeting techniques were then used for techno-economic assessment of sewage treatment recovering biogas and bio-solids. Acti-zyme was found to be immotile and contained catalase, proteolytic enzymes and amylase. Acti-zyme did not promote 𝐻2𝑆 production, making it useful in sewage treatment producing biogas. Sewage treatment using Acti-zyme resulted in >60% decrease of the sewage contaminants through bio-augmentation. Optimum sewage treatment conditions were obtained at 0.050 g/L Acti-zyme loading and retention time of 40 days. The 𝐢𝑂𝐷⁄𝐡𝑂𝐷5, 𝐡𝑂𝐷5⁄𝑇𝐾𝑁, 𝐢𝑂𝐷/𝑇𝐾𝑁 and the 𝐢𝑂𝐷/𝑇𝑃 ratios obtained were > 1.2, 4.0, 8.0 and 15.0 respectively. The 𝐢𝑂𝐷⁄𝐡𝑂𝐷5, 𝐡𝑂𝐷5⁄𝑇𝐾𝑁, 𝐢𝑂𝐷/𝑇𝐾𝑁 and the 𝐢𝑂𝐷/𝑇𝑃 bio-nutrient removal models were developed. Optimum biogas production was obtained at a sewage sludge loading of 7.5 g/L.d and Acti-zyme loading of 0.050 g/L with a 78% bio-methane composition was achieved at mesophilic temperatures of 37 Β°C. Bio-solids with 8.17, 5.84 and 1.34 % of 𝑁𝑃𝐾 respectively were produced. The bio-methane production was simulated to the linear, exponential, logistics kinetic, exponential rise to a maximum, first order exponential model and the modified Gompertz kinetic models. The logistics kinetic model accurately simulated the bio-methane production with a k-value of 0.073 day-1 . Furthermore, linear, quadratic, compound and exponential statistical models were tested and validated for the biogas and the bio-solids generation. The quadratic statistical models were significant for simulating biogas and bio-solids production respectively. A sewage plant with a capacity of 19.6 600 ML/day was considered for techno-economic assessment, with an operating efficiency of 60% and a life span of 20 years. $5.125/day of Actizyme were required for production of 12 769.69 kg/day for biogas costing $1.50/kg and 672.08 kg/day of bio-solids costing $16.00/50kg. A net benefit of $5 656 363.92 per annum for using the Acti-zyme technology in sewage digestion was forecasted, whilst a net energy of 1 387.33 KWh was set to be produced. An investment of $22 199 501.40 was required for kick-starting the project. A positive net present value of $1 186 239.23 was realized with an internal rate of return of 17.6% and a payback period of 5.9 years. For breakeven to be realised, only 183 059.16 KWh must be produced. The techno-economic assessment indicated it is viable to treat sewage using Acti-zyme as bio-augmentation catalyst; co-harnessing biogas and bio-solids as valued added products to an extent of contributing 0.04% to the Zimbabwe gross domestic product.
Description: Thesis (DTech (Chemical Engineering))--Cape Peninsula University of Technology, 2016
Appears in Collections:Chemical Engineering - Doctoral Degrees

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