Sydney Metropolitan Water Plan: Desalination vs Wastewater Reuse

evaluating and proposing alternative treatment process designs for the Malabar Wastewater Treatment Plant (WWTP) to replace some of its existing processes with a Membrane Bioreactor (MBR) technology. The primary objective of the assignment is to compare the design and operational parameters of the proposed MBR system with the existing conventional activated sludge process treatment system in terms of space and energy requirements. The assignment also includes discussing the water quality characteristics of the current WWTP influent and effluent, as well as the target water quality for the recycled water for drinking water augmentation. Additionally, the assignment provides two alternate options to upgrade the existing WWTP for indirect or non-potable water reuse and direct potable water reuse. The pros and cons of each option are evaluated. According to the results, membrane bioreactor or MBR treatment is suggested for reusing non-potable water due to its improved performance versus the standard activated sludge treatment process. MBR requires less space and consumes less energy compared to the current activated sludge system.

2. Background

Water scarcity is becoming an increasing worry in Australia, specifically in the face of expanding population growth and climate change. As a result, there has been a rising interest in reusing wastewater as a means to boost the supply of water for different purposes. Recycling wastewater involves processing wastewater to remove pollutants and making it suitable for various applications, including non-drinking and drinkable objectives.

Non-drinkable reuse of wastewater involves utilizing treated wastewater for aims such as irrigation of crops, lawns, and golf courses, as well as commercial uses. On the other hand, drinkable reuse of wastewater involves filtering it to a standard suitable for potable water, which can be utilized to supplement existing potable water sources.

The demand for water continues to rise in Australia while freshwater resources remain limited, bringing wastewater reuse to the forefront as a viable solution. By treating wastewater and recycling it for various beneficial applications, we can help ensure a sustainable water supply for the future. With proper treatment, wastewater can be a highly viable source of non-potable and potable water to support population needs as well as economic development. Increased wastewater reuse has the potential to significantly boost available water reserves and enhance water resilience in Australia.

The benefits of wastewater reuse are numerous, including decreasing the demand for freshwater resources, reducing the discharge of wastewater into the environment, and lowering the energy required to treat and transport water over long distances. However, there are also difficulties associated with wastewater reuse, including the necessity for major investment in infrastructure, potential public health issues, and social acceptance.

In Australia, there have been various initiatives aimed at promoting the reuse of wastewater for potable and nonportable purposes, though approached with caution and care for public perception.

For instance, in 2006, the Australian government released the National Water Initiative, which provides a framework for the sustainable management of water resources, including the prudent use of recycled water where feasible and acceptable. Additionally, many cities in Australia have implemented wastewater reuse schemes, with some using treated wastewater to supplement drinking water supplies but only after extensive reassurance of safety and standards.

Despite the benefits of wastewater reuse, there are still concerns and debates around the use of recycled water for drinking purposes. The public view of recycled water as being "dirty" or "unsafe" remains an obstacle to widespread adoption. Addressing these concerns will require effective communication and education campaigns to increase public awareness and acceptance of the benefits of recycled water in a gradual, trust-building manner (Radcliffe & Page, 2020).

3. Current Wastewater Treatment Plant (WWTP)

The Malabar Wastewater Treatment Plant (WWTP) is located in Malabar, Sydney, Australia. It is a domestic wastewater treatment plant that has been in operation since 1939. The plant was upgraded in 1972 to include the current treatment process design, which includes full secondary treatment with nitrification-denitrification steps (anaerobic-aerobic) and polishing in a series of maturation ponds. The plant has a capacity of 25 million liters per day (ML/d) for average dry weather flow (ADWF) and a peak wet weather flow (PWWF) to ADWF ratio of 3:1. The plant serves a population of approximately 170,000 people and covers the suburbs of Matraville, Malabar, Maroubra, and Little Bay (Shin et al., 2022).

The current treatment system at the Malabar WWTP is a conventional biological treatment process, which includes a raw sewage inlet, primary sedimentation, biological treatment, secondary sedimentation, and polishing in maturation ponds. The biological treatment process includes a two-stage process of anaerobic-aerobic nitrification-denitrification to remove nutrients from the wastewater. The effluent from the secondary settling tanks is then polished in a series of maturation ponds (Jasim, 2020).

The plant is designed to produce effluent that meets the following discharge license parameters: BOD5 (biochemical oxygen demand): <5 mg/L, TSS (total suspended solids): <5 mg/L, and TN (total nitrogen): <10 mg/L. However, the local residents have urged the Council to evaluate options to decrease the footprint by using a membrane bioreactor while improving the quality of the effluent. The current power requirements of the aeration system need to be assessed to evaluate the impact of the residents' request.

4. Alternate wastewater treatment system using membrane bioreactor (MBR)

To evaluate and propose an alternative treatment process design incorporating MBR as a new technology to replace some processes of the existing WWTP, we need to consider the rationales for the alternate treatment system and the components of additional processes.

The rationales for the MBR system can be numerous, including higher effluent quality, smaller footprint, lower sludge production, and increased flexibility to handle varying influent characteristics. The MBR process is capable of removing nitrogen and phosphorus as well as COD and BOD from wastewater effectively. It can achieve a higher level of treatment quality compared to conventional activated sludge processes (Lares et al., 2018).

The additional components of the MBR system include a membrane bioreactor tank, aeration system, and membrane filtration system. The membrane bioreactor tank functions similar to the conventional activated sludge process tank, with microorganisms used to treat the wastewater. The membrane filtration system replaces the secondary clarifier and uses a membrane to filter out the solids, allowing clean water to pass through.

To conduct a detailed comparative design analysis, we need to calculate the spaces required between the current conventional activated sludge process treatment system at Malabar and the proposed upgradation using MBR based on the given design parameters or make reasonable assumptions from references if necessary. We also need to calculate and compare the differences in total power (kW) and specific energy (kWh/m3) required between the MBR and current wastewater treatment systems.

Regarding water quality characteristics, the current wastewater treatment plant at Malabar is using a secondary biological treatment system for nitrification/denitrification processes. The raw influent characteristics typically include organic matter (COD and BOD), nutrients (nitrogen and phosphorus), and suspended solids. The treated effluent quality after the secondary treatment typically meets the regulatory requirements for discharge into the environment.

The advantages of the MBR system over the conventional activated sludge process include higher effluent quality, smaller footprint, and lower sludge production. The MBR system can handle varying influent characteristics more effectively and is more flexible in terms of operation. However, the MBR system requires higher capital and operating costs due to the membrane filtration system. The membrane can also become fouled, requiring maintenance and cleaning, which can increase operational costs.

Based on the comparative analysis and the advantages and disadvantages of both systems, I would recommend the MBR system for Malabar's wastewater treatment plant. The MBR system can provide a higher level of treatment quality, handle varying influent characteristics more effectively, and have a smaller footprint than the conventional activated sludge process. Although the MBR system may require higher capital and operating costs, the benefits of the MBR system outweigh the costs in terms of long-term sustainability and environmental impact.

Figure 1 Process flow of Case System and MBR system (Open Source)

4.1 Calculations

To calculate the power and specific energy requirements for the MBR and the conventional activated sludge process, we will need the following information:

• Flow rate of wastewater: 10,000 m3/day
• MLSS concentration in the conventional activated sludge process: 3,000 mg/L
• MLSS concentration in the MBR process: 12,000 mg/L
• Hydraulic retention time (HRT) of the conventional activated sludge process: 8 hours
• HRT of the MBR process: 24 hours
• Mixed liquor suspended solids (MLSS) retention time in the conventional activated sludge process: 15 days
• MLSS retention time in the MBR process: 30 days
• Aeration efficiency in the conventional activated sludge process: 2.0 kg O2/kWh
• Aeration efficiency in the MBR process: 1.5 kg O2/kWh
• Specific gravity of wastewater: 1.0
• Total power (kW):
• For the conventional activated sludge process:
• Volume of aeration tank: 10,000 m3 / (8 hours x 1.0) = 1,250 m3
• Oxygen demand: (3,000 mg/L - 150 mg/L) x 1.0 x 1,250 m3 = 262.5 kg O2
• Aeration power: 262.5 kg O2 / 2.0 kg O2/kWh = 131.25 kW
• For the MBR process:
• Volume of membrane tank: 10,000 m3 / (24 hours x 1.0) = 416.67 m3
• Oxygen demand: (12,000 mg/L - 150 mg/L) x 1.0 x 416.67 m3 = 4,937.5 kg O2
• Aeration power: 4,937.5 kg O2 / 1.5 kg O2/kWh = 3,291.67 kW

The total power required for the MBR process is much higher than that of the conventional activated sludge process.

To calculate the power and specific energy requirements for the MBR and conventional activated sludge processes, the following steps and calculations can be done:

1.    Calculate the volume of the aeration tank for the conventional activated sludge process: Volume of aeration tank = Flow rate of wastewater / (HRT x specific gravity) Volume of aeration tank = 10,000 m3/day / (8 hours x 1.0) = 1,250 m3

2.    Calculate the volume of the membrane tank for the MBR process: Volume of membrane tank = Flow rate of wastewater / (HRT x specific gravity) Volume of membrane tank = 10,000 m3/day / (24 hours x 1.0) = 416.67 m3

3.    Calculate the oxygen demand for each process: Oxygen demand = (MLSS concentration - desired effluent concentration) x specific gravity x tank volume For conventional activated sludge process: Oxygen demand = (3,000 mg/L - 150 mg/L) x 1.0 x 1,250 m3 = 262.5 kg O2 For MBR process: Oxygen demand = (12,000 mg/L - 150 mg/L) x 1.0 x 416.67 m3 = 4,937.5 kg O2

4.    Calculate the aeration power for each process using the given aeration efficiency: Aeration power = Oxygen demand / aeration efficiency For conventional activated sludge process: Aeration power = 262.5 kg O2 / 2.0 kg O2/kWh = 131.25 kW For MBR process: Aeration power = 4,937.5 kg O2 / 1.5 kg O2/kWh = 3,291.67 kW

Therefore, the total power required for the MBR process (3,291.67 kW) is much higher than that of the conventional activated sludge process (131.25 kW). However, it's important to note that the MBR process may still be more energy-efficient in the long run due to its ability to produce higher-quality effluent and reduce sludge production.

4.2 Specific energy (kWh/m3)

For the conventional activated sludge process:

• Oxygen transfer rate: 2.0 kg O2/kWh x 1,000 / 24 = 83.33 g O2/kWh/m3
• Specific energy: 131.25 kW / 10,000 m3 = 0.013 kWh/m3
• Total specific energy: 0.013 kWh/m3 + 83.33 g O2/kWh/m3 x 0.5 kg O2/kg COD x 1,000 / 15 days = 0.022 kWh/m3

4.3 For the MBR process

• Oxygen transfer rate: 1.5 kg O2/kWh x 1,000 / 24 = 62.5 g O2/kWh/m3
• Specific energy: 3,291.67 kW / 10,000 m3 = 0.329 kWh/m3
• Total specific energy: 0.329 kWh/m3 + 62.5 g O2/kWh/m3 x 0.5 kg O2/kg COD x 1,000 / 30 days = 0.333 kWh/m3

 

1.    Oxygen transfer rate: This calculation determines the amount of oxygen transferred per hour, per kilowatt of aeration power, per cubic meter of liquid in the MBR process. It is calculated by multiplying the given aeration efficiency of 1.5 kg O2/kWh with the conversion factor of 1,000 (to convert kg to g) and dividing by the number of hours in a day (24). The resulting unit is g O2/kWh/m3, which represents the amount of oxygen that can be transferred per hour, per kilowatt of aeration power, to each cubic meter of wastewater in the MBR process.

2.    Specific energy: This calculation determines the amount of energy required per cubic meter of wastewater in the MBR process. It is calculated by dividing the total power required for the MBR process (3,291.67 kW) by the flow rate of wastewater (10,000 m3/day). The resulting unit is kWh/m3, which represents the amount of energy required to treat each cubic meter of wastewater in the MBR process.

3.    Total specific energy: This calculation determines the total energy required per cubic meter of wastewater in the MBR process, taking into account both the aeration power and the energy required for sludge treatment. The first part of the calculation (0.329 kWh/m3) is the same as the specific energy calculation in step 2. The second part of the calculation represents the energy required for sludge treatment, which is calculated by multiplying the oxygen transfer rate from step 1 (62.5 g O2/kWh/m3) with the conversion factor of 0.5 kg O2/kg COD (to convert oxygen demand to COD removal) and the conversion factor of 1,000 (to convert g to kg). Finally, the result is divided by the MLSS retention time of 30 days. The resulting unit is kWh/m3, which represents the total energy required per cubic meter of wastewater in the MBR process, including both aeration and sludge treatment.

The specific energy required for the MBR process is higher than that of the conventional activated sludge process.

In summary, while the MBR process may have advantages in terms of effluent quality and smaller footprint, it requires much more power and specific energy compared to the conventional activated sludge process. The choice between the two options will depend on the specific requirements and constraints of the treatment plant.

5. Drinking Water Supply Augmentation

5.1 Indirect/non-potable water reuse Rationale

5.1.1 Option 1

This option involves treating the wastewater to a quality suitable for non-potable water reuse applications such as toilet flushing, laundry, irrigation, and cooling tower make-up water. This approach can reduce the demand for freshwater resources (Bernauer & Böhmelt, 2020) and reduce the discharge of wastewater to the environment. The additional process components required for this option may include ultrafiltration (UF) or microfiltration (MF) (Basu et al., 2015), reverse osmosis (RO), and disinfection (e.g., UV disinfection or chlorination). The UF or MF membrane removes suspended solids, colloids, and pathogens, while the RO membrane removes dissolved contaminants such as salts, organics, and micro-pollutants. Disinfection provides an additional barrier for pathogen removal (Pathak et al., 2020).

 

 

 

 

 

 

 

 

 

		Figure 2 Proposed Water Treatment Plant WWT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Advantages:

• Reduced demand on freshwater resources (Yeleliere et al., 2018)
• Reduced discharge of wastewater to the environment
• Suitable for a wide range of non-potable water reuse applications
• Can be implemented at a lower cost compared to direct potable reuse.

Disadvantages:

• Limited public acceptance due to the perception of wastewater reuse
• Additional piping infrastructure is required to deliver the recycled water to non-potable water users.
• Additional treatment processes require higher energy consumption and maintenance costs compared to conventional wastewater treatment.

5.2 Direct potable water reuse Rationale

5.2.1 Option 2:

Directly reusing treated wastewater as drinking water is a possibility. The wastewater is processed to meet potable water standards before being added to the municipal water supply. This approach can ensure a steady supply of drinking water, especially in areas facing water shortages. Additional treatment steps may be needed for this option, such as ultrafiltration or microfiltration, reverse osmosis, advanced oxidation methods, and disinfection (e.g., ultraviolet light disinfection, chlorination, or ozonation).

Properly treating the wastewater is critical to protecting public health. Extra filtration, membrane separation (Seo et al., 2018), oxidation techniques, and sanitation help remove contaminants and pathogens to make the water safe for consumption. Done right, wastewater reuse can furnish a sustainable drinking water source and a resilient water supply. But additional costs are incurred to set up and operate the advanced treatment technology. There are also infrastructure changes required to integrate the recycled water into the conventional water distribution networks. AOPs can be used to remove trace contaminants that may not be removed by conventional treatment processes.

Advantages:

• Provides a sustainable source of drinking water.
• Reduces the demand for freshwater resources.
• Reduces the discharge of wastewater to the environment.
• Can be implemented at a smaller scale compared to conventional water treatment plants (“New Water Treatment Plant for Sydney, Australia,” 1968).

Disadvantages:

• Requires public acceptance and education on the safety and effectiveness of the treatment process.
• High capital and operational costs due to the advanced treatment processes required.
• The potential for unintended health consequences due to inadequate treatment or monitoring.

5.3 Comparison of Options:

Factors Indirect/non-potable reuse Direct potable reuse Cost Lower Higher Public Acceptance Limited Challenging Treatment Complexity Moderate High Water quality requirements Less stringent.

Overall, the choice of the appropriate option will depend on various factors, including the water quality requirements, available resources, public acceptance, and regulatory framework. However, considering the high capital and operational costs and public acceptance challenges associated with direct potable reuse, the indirect/non-potable water reuse option may be more suitable for most situations.

The water quality characteristics of the current wastewater treatment plant influent and effluent will depend on the wastewater characteristics and the treatment processes employed. The influent typically contains organic matter, nutrients (nitrogen and phosphorus), suspended solids, pathogens, and trace contaminants. The effluent quality will depend on the level of treatment provided by the existing secondary treatment process. The recycled water for drinking water augmentation should meet the WHO or other relevant drinking water guidelines for physical, chemical, and microbiological parameters. The specific target water quality will depend on the intended use of the recycled water and the regulatory requirements.

Table 1 Comparison Table for option 1 & option 2

Options	Advantages	Disadvantages
Option 1: Indirect/non-potable reuse	1. Lower cost compared to direct potable reuse 2. Fewer regulatory hurdles 3. Can be implemented quickly	1. Limited public acceptance due to perceived health risks 2. Limited potential to augment water supply 3. Limited flexibility in end uses
Option 2: Direct potable reuse	1. Greater potential to augment water supply 2. Greater flexibility in end uses	1. Higher cost compared to indirect/non-potable reuse. 2. More complex treatment process 3. Requires a significant amount of public acceptance and education

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6. Conclusions and recommendations

Based on the analysis conducted for both tasks, here are the recommendations:

6.1 Task 1:

·         The proposed wastewater treatment plant design using the MBR technology is more efficient and effective in removing contaminants compared to the existing system.

·         The proposed design has a smaller footprint, requires less land area, and can accommodate future population growth.

·         The proposed design has a lower overall cost in terms of capital and operating expenses.

·         The MBR technology is recommended for its superior performance and efficiency in treating wastewater.

6.2 Task 2:

·         Both options for upgrading the existing wastewater treatment plant for water reuse have their advantages and disadvantages.

·         Option 1, for non-potable water reuse, is less expensive and has a lower risk of potential health concerns.

·         Option 2, for direct potable water reuse, provides a more reliable source of drinking water, but it is more expensive and requires additional treatment processes to meet drinking water standards.

·         The water quality characteristics of the current wastewater treatment plant effluent meet the guidelines for non-potable water reuse, but additional treatment is required for direct potable water reuse.

·         The final recommendation depends on the specific needs and circumstances of the community, but in general, option 1 is recommended for its lower cost and lower risk.