Sustainable water management in Australia is more than a priority; with limited runoff, erratic rainfall and unpredictable climate changes, managing water resources is an essential part of life. Consequently, it is vital that best wastewater management practices are followed at national and local levels. In an effort to rethink Australia’s small-scale water management capabilities, two conceptual design projects sought to demonstrate that anaerobic digestion opportunities for facilities less than 19 ML/d (5 mgd) are not as limited as previously believed. Water Environment & Technology magazine recently highlighted this endeavor in their April 2014 issue. The article, aptly titled, “Payback in the Outback,” explores the resulting case studies from the design projects, as well as their potential for additional revenue streams, proving that with the right alternative, anaerobic digestion in small-scale wastewater management facilities is more than feasible.
About the Conceptual Design Projects
The two design projects sought to prove that a positive business case could be made for anaerobic digestion and resource recovery facilities at small-scale wastewater treatment facilities. This would be beneficial to a wastewater utility because resource recovery offers the opportunity to generate power, create additional revenue from the sale of renewable energy sources, and build a more sustainable future overall.
What is Anaerobic Digestion and Co-Digestion?
Anaerobic digestion is the breakdown of organic materials (manure, sewage sludge, etc.) via microorganisms. This process produces biogas, an energy source for a facility. The anaerobic digestion process also results in a solid residual, which can be composted, used as a soil amendment, or land applied. Co-digestion adds energy-rich organic waste materials to wastewater digesters with excess capacity, increasing the potential methane production of manure and biosolids.
Utilizing both anaerobic digestion and co-digestion at a wastewater resource recovery facility optimizes facility benefits and economic opportunities.
Project Obstacles
Previous thinking assumed small-scale facilities have limited resource recovery potential due to the following factors:
- High capital costs of equipment such as tanks and pumps.
- Intangible factors such as salaries and engineering.
- The reliability of waste streams.
- The assumption that existing primary sedimentation and digestion unit processes were needed.
Overall Project Objectives
Both projects sought to utilize the co-digestion of municipal residual solids with high-strength waste streams. Economic factors taken into consideration included capital costs, annual incomes, and operating costs. In order to make the projects economically viable, the conceptual design projects anticipated using the associated high biogas yield for cogeneration (the simultaneous production of electricity and heat) and for export to the power grid. Tipping fees charged to high-strength wastes received at a waste processing facility were another potential income stream calculated into the cost analysis of each project. The projects also planned to utilize the income from waste reuse alternatives.
Waste Reuse Alternatives
The economic feasibility of a resource recovery center necessitates recovering the following resources:
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- Biogas
Biogas is a gaseous fuel produced by the fermentation of organic matter. The wastes are transformed to digesters where biogas is produced and sent to boilers to offset natural gas requirements. Increased biogas yield from a facility opens opportunities for cogeneration and potential export to the power grid.
- Biogas
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- Biosolids
Biosolids are nutrient-rich organic material recycled from sewage and other wastewater treatment. Also referred to as sewage sludge, the organic matter is a beneficial fertilizer option for use in agriculture.
- Biosolids
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- Water Reuse
Water reuse is a process that converts wastewater into a source of sustainable irrigation or an option for recharging groundwater aquifers. The final product is often referred to as reclaimed water or recycled water.
- Water Reuse
While waste reuse is an excellent resource recovery strategy, even more opportunity can be found using waste transformation. See An Additional Waste Reuse Alternative: Organic Fertilizer to Granulation below for more information on waste transformation.
Case Studies
After outlining the conceptual design projects, two case studies were performed by City West Water and Yarra Valley Water utilities near Melbourne in Victoria, Australia. Each case study examined if co-digestion resource recovery facilities could be an economically viable option for an actual small-scale wastewater treatment facility.
Yarra Valley Water Case Study
The Yarra Valley Water case study documented the feasibility of co-digestion and cogeneration facilities. A conceptual design was created for three small-scale water resource recovery facilities without pre-existing necessary equipment (primary clarifiers and anaerobic digesters). The analysis investigated the following components:
- Assess biogas generation technologies and waste sources, then recommend preferred options.
- Create a conceptual-level assessment of predicted biogas generation and costs.
- Assess expected revenue and payback.
- Assess the risks, opportunities, and feasibility of a biogas generation plant.
City West Water Case Study
The primary goal for the City West Water case study was to analyze the feasibility of a shared resource recovery center for Baiada Poultry Pty. Ltd. and Baiada Proteins Limited (BPL). Feasibility analysis and facility concept designs were created for a resource recovery facility based on the co-digestion of various high-strength wastes. The high-level concept design focused on consuming less water and gas onsite, as well as reducing solid waste disposal requirements. The two main design objectives were to:
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- Generate biogas by utilizing the co-digestion of high-strength waste streams as well as biosolids and Class A reuse water.
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- Determine the financial feasibility for a small utility to build, own, and operate a resource recovery facility.
Facility Concept Design Alternatives
The three facility concept design alternatives and their respective resource recoveries were developed as follows:
- The waste streams in the first facility concept design included industrial dissolved-air flotation solids, chicken processor liquid waste, beef and sheep abattoir solids, and pig abattoir dissolved-air flotation solids. Recovered resources include:
- Biogas Use: BPL industrial boiler
- Biosolids: Dewatered solids transported to offsite composting facility.
- Water Reuse: Liquid is pretreated to commercial wastewater standards for disposal to the collection system.
- The waste streams in the second facility concept design included the same sources listed for concept one, as well as cooking oil waste and thickened waste activated sludge (TWAS) from the Altona Wastewater Treatment Plant. Recovered resources include:
- Biogas Use: Onsite heat and power cogeneration.
- Biosolids: Onsite generation of biosolids.
- Water Reuse: Class A water production.
- The waste streams in the third facility concept design included the same sources listed for concepts one and two, as well as renderer waste, brewery waste, sugar refinery clarifier mud, and industrial dissolved air flotation sludge and reject product. Recovered resources include:
- Biogas Use: Onsite heat and power cogeneration.
- Biosolids: Onsite generation of biosolids.
- Water Reuse: Class A water production.
Case Study Results
After factoring capital costs, operating costs, and annual incomes, both projects demonstrated that it is economically viable for a smaller waste water facility to perform anaerobic digestion and resource recovery. The studies voided the perception that existing primary sedimentation and digestion unit processes are necessary, with the cost outweighing the benefits. The studies also learned that introducing high-strength waste to the anaerobic digester (combined with high-efficiency digestion, cogeneration processes, and tipping fees) shortens the project payback period and maximizes the long-term financial and environmental benefits. Another valuable take-away from the studies was the importance of assessing site-specific factors to limit risks and optimize project paybacks. For instance, in order to sufficiently utilize high-strength waste streams, it was determined that only wastes with year-round availability and higher potential biogas yields should be evaluated for these projects.
Overall, the information gathered from each case study could be used to create a positive business case for the economic feasibility of adding anaerobic digestion and biogas generation to small-scale facilities. As a result, both Yarra Valley Water and City West Water plan to implement small-scale codigestion resource recovery facilities.
An Additional Waste Reuse Alternative: Organic Fertilizer to Granulation
The waste reuse alternatives listed above benefit the overall life-cycle costs of a facility because they generate additional revenue streams with options such as fertilizer, biogas use, or water reuse. Fertilizer is an especially smart and beneficial waste transformation alternative when handling certain biosolids, particularly when granulated.
Granulation systems are ideal for granulating waste streams such as dairy manure, municipal sewage, and chicken manure. Also referred to as fertilizer pelletization, fertilizer granulation generates the following benefits:
- A consistent level of quality and nutrients.
- A product that is easy to transport and apply with conventional fertilizer equipment.
- Improved field conditions with reduced erosion and nutrient run-off.
Overall, the improved product characteristics found in granular fertilizer yields an optimal, high quality product. The use of organic based fertilizers in conjunction with chemical fertilizers will produce better yields and improve the health of soil. By creating a waste disposal alternative and a potential new revenue source, organic based fertilizer granulation is a win-win proposition.
FEECO helps customers divert tons of waste from local landfills into value added products each year. For more information on FEECO’s organics granulation or waste-to-value added product experience, contact us today!