Sustainable Engineering Practice & Design

Project M: Mixing Analysis of Primary Wastewater Digester

Note: This is a shared Project between OENG1116 and OENG1118. Many of the assessments will be shared between OENG1116 and OENG1118 in that for a given assessment you will need to prepare one piece of work (e.g. Group Proposal Report, Group Project Report) that covers the requirements of both OENG1116 and OENG1118. Marking of these assessments shall be performed separately for each course, but your group's overall recommendations take into account findings from both the Modelling and Simulation and Sustainability Assessment aspects of the project. BLUE text below indicates tasks that shall be assessed by OENG1116 only, RED text below indicates tasks that shall be assessed by OENG1118 only, and BLACK text indicates tasks that shall be assessed by both courses.
1.0 Project Task/Outline

With increasing population density and demands on water resources in general, water and wastewater treatment play an increasingly pivotal role in modern society. At the Western Treatment Plant, the primary digester acts to digest suspended solids and biomass into inert materials and methane for on-­-site power generation that may be used by the plant or sold back to the grid. It has been recognized that the efficiency of the complex chain of biochemical digestion reactions within the digester are heavily impacted by the mixing conditions within the digester. It is suspected that the current mixing conditions in the primary digester at the Western Treatment Plant are suboptimal, and may be attributed to the highly viscous nature of the activated sludge that arises from the high biomass content. As such, whilst it has been recognized that the primary digester could benefit from improved mixing conditions, conventional approaches to accelerate fluid mixing such as gas spargers or mixing impellers are not appropriate for this application.

Your Engineering Team has been charged with the task of assessing the current mixing conditions within the primary digester and develop and assess alternate mixing approaches under laminar flow conditions such as the use of internal baffles and/or steady or transient fluid extraction and reinjection, the outcomes of which will be communicated to the client via a professional engineering report which outlines the project recommendations and feasibility.
Your Engineering Team based within a professional engineering firm is interested in undertaking this assessment project that is comprised of the following tasks:

- Develop a computational fluid dynamics (CFD) model of the flow conditions within the primary digester under the different mixing approaches and develop a measure of the mixing efficiency across these cases

- Estimate the energy consumption of the alternate approaches and develop recommendations based upon the offset between mixing and energy efficiency.

- Conduct a preliminary sustainability assessment to quantify and qualify impacts of the solution on the economy, environment, and society

- Conduct a multi-­-criteria analysis to decide on an optimal variant of the solution

- Make and justify recommendations as to whether the project should proceed based on the outcomes of the tasks above

This information will allow the benefits of the project to be estimated and the overall feasibility of the project to be gauged. As detailed in the assessment descriptions for OENG1116 and OENG1118, your Engineering Team will first develop a proposal that will act as a competitive bid for this assessment project. Assuming that this bid is successful, your Engineering Team will then undertake the assessment project itself. The assessment project tasks are outlined in detail as follows:

Typical design questions -Mixing Analysis

1. Find or estimate details of the primary digester at the Western Treatment Plant, including
a. Digester dimensions and geometry
b. Input flow rate
c. Digester fluid rheology

2. Using a suitable software package develop a two-­-dimensional (2D) preliminary CFD model of the existing primary digester at the Western Treatment Plant
a. Verify model through convergence testing of mesh and time-­-stepping
b. Solve steady-­-state flow in digester
c. Develop measure of mixing efficiency of digester and solve steady-­-state mixing efficiency

3. From 2D model above, identify regions of poor mixing in the primary digester and suggest potential interventions to improve mixing efficiency, such as
a. Internal baffles and mixing elements
b. Judicious extraction and re-­-injection of fluid

4. Model digester flow and mixing over several different mixing protocols and measure
a. Mixing efficiency and robustness

b. Energy consumption

5. Develop recommendations for improvement of the mixing efficiency in the primary digester and feasibility. Comment on shortcomings of model and scope for improvement.

Typical design questions - Sustainability Assessment

Environment

For all products in the scenarios:

1. Formulate a single functional unit.
2. Determine a single system boundary.

For the products in each scenario:

3. Identify the major material-­- and energy-­-conversion processes across the product life cycle.

4. Determine the major material and energy flows.

5. Draw a process flow diagram that includes:
processes
elementary flows of materials and energy
intermediate flows of materials and energy

6. Calculate the largest environmental impacts.

7. Calculate the sensitivity of parameters with high contributions and high uncertainty, and of assumptions.

Economy

For the solution in each scenario:

1. Identify sources of cost and revenue across the product life cycle.

2. Determine the economic costs and revenues.

3. Determine the useful life of the product.

4. Calculate the payback period and net present value of the product.

Society

For the solution in each scenario:

1. Identify the societal impacts from the project (both positive and negative impacts).

Multi-­-criteria decision analysis

1. Identify a variety of criteria or objectives with justifications for multi-­-criteria analysis.
2. Determine the appropriate weightings for each criterion/objective with justifications.
3. Determine the measure of the effectiveness of the project/alternative.

Software tools

1. Fluid dynamics software:
- ANSYS-­-CFX
2. Modelling equations and theoretical underpinnings

Discuss the governing equations that will be solved and their theoretical underpinnings. Identify and discuss assumptions involved and their validity.

3. Numerical techniques and software packages

Discuss the numerical techniques you will employ to solve the governing equations and the software package used. Discuss issues regarding accuracy and stability of numerical solutions. Identify possible ways to cross-­-validate numerical predictions

Attachment:- merged.pdf