Biological nutrient removal from municipal sewage using acti-zyme: recovering biogas and bio-solids from sewage sludge

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.