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Biological nutrient removal from municipal sewage using acti-zyme: recovering biogas and bio-solids from sewage sludge
Author(s)
Manyuchi, Musaida Mercy
Date Issued
2016
Type
Thesis
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 °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 (𝐵𝑂𝐷5), total suspended solids (𝑇𝑆𝑆), total dissolved solids (𝑇𝐷𝑆), electrical
conductivity (𝐸𝐶), 𝑝𝐻, chloride ions concentration (𝐶𝐼
−), total phosphorous (𝑇𝑃), sulphate ions
concentration (𝑆𝑂4
2−), dissolved oxygen (𝐷𝑂) and the chemical oxygen demand (𝐶𝑂𝐷) 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 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁, 𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/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 °C and 55 °C were
applied. The biogas quantity produced was measured using the water displacement method.
Samples of the biogas were analysed for bio-methane (𝐶𝐻4), carbon dioxide (𝐶𝑂2) and traces
gases composition using gas chromatography. The bio-solids obtained were tested for nitrogen,
phosphorous and potassium (𝑁𝑃𝐾) content using 𝑢𝑣 − 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 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁,
𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/𝑇𝑃 ratios obtained were > 1.2, 4.0, 8.0 and 15.0 respectively.
The 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁, 𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/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 °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.
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 °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 (𝐵𝑂𝐷5), total suspended solids (𝑇𝑆𝑆), total dissolved solids (𝑇𝐷𝑆), electrical
conductivity (𝐸𝐶), 𝑝𝐻, chloride ions concentration (𝐶𝐼
−), total phosphorous (𝑇𝑃), sulphate ions
concentration (𝑆𝑂4
2−), dissolved oxygen (𝐷𝑂) and the chemical oxygen demand (𝐶𝑂𝐷) 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 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁, 𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/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 °C and 55 °C were
applied. The biogas quantity produced was measured using the water displacement method.
Samples of the biogas were analysed for bio-methane (𝐶𝐻4), carbon dioxide (𝐶𝑂2) and traces
gases composition using gas chromatography. The bio-solids obtained were tested for nitrogen,
phosphorous and potassium (𝑁𝑃𝐾) content using 𝑢𝑣 − 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 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁,
𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/𝑇𝑃 ratios obtained were > 1.2, 4.0, 8.0 and 15.0 respectively.
The 𝐶𝑂𝐷⁄𝐵𝑂𝐷5, 𝐵𝑂𝐷5⁄𝑇𝐾𝑁, 𝐶𝑂𝐷/𝑇𝐾𝑁 and the 𝐶𝑂𝐷/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 °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.
Additional information
Thesis (DTech (Chemical Engineering))--Cape Peninsula University of Technology, 2016
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