Reliable wastewater treatment is necessary to protect water sources and ensure the long-term stability of the environment. Conventional wastewater treatment throughout the industrialized world requires significant energy input to produce an effluent that can be discharged to the environment. In fact, according to the U.S. Environmental Protection Agency, more than 3 percent of all electricity used in the U.S. is to treat water and wastewater. When considering that the U.S. consumed 3,886 billion kilowatt hours in 2014, the amount of electricity expended during wastewater treatment is astonishing.
Not all aspects of wastewater treatment are energy intensive. Plants equipped with anaerobic digestion (AD) have the ability to produce energy. The anaerobic bugs, which thrive in an AD system, break down organic material and convert it to biogas. Biogas is rich in methane and can be converted to renewable energy.
In wastewater treatment plants, this organic material is the indigenous sludge in the raw wastewater and is also generated through the overall treatment process. In practice, anaerobically digesting the sludge and converting the biogas to electricity with combined heat and power (CHP) units only cover a portion of the plants’ parasitic load.
With sustained, global energy conservation concerns, greenhouse gas emission reductions and the need for long-term energy certainty, wastewater authorities are implementing energy-neutral operation practices. Typically, this would include optimizing equipment and process operations to minimize energy requirements. A growing number of plants, however, are implementing CHP’s to produce electricity and also looking for ways to increase biogas production. A prime strategy for this is codigestion.
Implementing codigestion, not just biology
The premise of codigestion is simple: Digest multiple organic materials within the same digester. Sounds easy enough since AD is a natural process. The “bugs” do all the work and are not selective. If the material is organic, they will eat it. However, challenges are involved in codigestion concerning materials handling and integration. Anaerobic digesters at municipal wastewater treatment plants were designed to digest sludge, while being able to add other material, for example, food scraps from a kitchen table were not thought of originally.
The first challenge when implementing a codigestion system is determining what organic materials are available in a reasonable catchment area. Understanding the amount of material and the type is important. These impact the following:
- Selecting the equipment added to receive the materials
- The applicability of the existing digester infrastructure to handle the material blend
- Handling the resultant biosolids
- Digester loading rate design, feeding regime and operational stability
Once the available organic material is understood, assessing how much of the material can be added is next. If the plant is operating at a much lower than designed capacity, chances are surplus digester volume is present. Plants that are capacity limited, however, may need to implement advanced digestion processes for sludge — such as biological or thermal hydrolysis — to free up capacity or add digester volume.
The available digester volume is a factor in determining the additional material, but so is the organic loading rate. Digester loading rates are designed conservatively around sludge because of its dilute nature. When implementing codigestion, loading rates can be practically increased, because external organic materials can be more concentrated with solids, especially volatile solids. This would allow for the digester volume to be more efficiently used provided the digester equipment, such as heat exchangers and mixers, can handle the new conditions.
With increased solids, existing digester mixing systems may be inadequate to properly mix the tank contents, which could impact the biogas yield and the potential for settling. Digester heating systems must also be able to tolerate the increased solids content.
At the onset of a codigestion program, the types of waste streams that will be available to the site, the condition of the material and the need to condition and separate contaminants must be considered. This is key to ensure the long-term success of any codigestion program. Separation equipment should be selected to treat the material that it receives. For instance if a plant received fats, oil and grease exclusively, the separation equipment considerations are different than if it received food waste from a grocery store. In addition, some contaminants (such as plastics and metals) will need to be separated. The organic material will have inherent grit and grit created during the separation process. To ensure that this grit does not affect the digester mechanical equipment or deposit in the digester, it must be effectively removed. A clean, consistent slurry, which can be provided to a digestion system at reliable rates along with the sludge is necessary to ensure long-term stability.
While the purpose of codigestion is that external organic material is digested within the same tank as sludge, the impact on makeup of the resultant biosolids must be considered. Depending on local regulations or the requirements of potential off-takers, a preference for separate sludge biosolids and the external organic material digestate, may need to be considered. In this case, operational practice must allow for dedicated and isolated digester volume for the external material. Plant owners can still benefit from the additional biogas, and to some this principle may simplify implementation.
A natural fit worth implementing
While challenges certainly exist when implementing a codigestion program, the natural fit is obvious in terms of the optimal use of existing assets and the positive impact on long-term environmental concerns. Some AD technology provides biological hydrolysis for sludge and for the digestion of food waste. The economic and environmental drivers are here to stay and technology is able. Transforming wastewater treatment plants into energy centers is not the future, it is now.
Michael Theodoulou, PE, is a senior product manager with GE Water & Process Technologies, with more than 16 years of experience in developing, commercializing and integrating innovative technologies, including advanced anaerobic digestion solutions. He can be reached at Michael.firstname.lastname@example.org.