Utilities are actively working to decrease energy usage at wastewater treatment plants (WWTPs). They are increasingly interested in incorporating carbon diversion processes as a means to accomplish this goal. The market for carbon diversion is expanding, and in this regard, GWI (Global Water Intelligence) investigates the advantages and obstacles associated with its implementation.

Understand the carbon diversion in the context on WWTPs

Carbon diversion, also known as enhanced primary treatment, is gaining traction as a crucial tool in the water sector’s pursuit of energy efficiency. It offers utilities a means to maximize the value of their operations by simultaneously reducing aeration energy consumption and boosting biogas generation. Various carbon diversion methods are emerging in the market to achieve these dual benefits.

In wastewater treatment plants, aeration stands out as the most energy-intensive stage. It utilizes blowers to introduce air, creating optimal conditions for aerobic biological treatment processes. While crucial for effective wastewater treatment, this step offers significant potential for optimization to achieve strong results while minimizing energy usage.

Carbon diversion intervenes precisely at this point. It involves reducing the organic load on secondary treatment systems by diverting a higher portion of biochemical oxygen demand (BOD) towards the sludge line during primary treatment. Consequently, this reduces the aeration requirements for subsequent biological processes downstream.

Wastewater treatment plants, which employ anaerobic digestion, can capitalize on the additional benefit of carbon diversion by increasing biogas production by redirecting more organic matter to the sludge at an early stage of the treatment process. Depending on the operating conditions and the specific method of enhanced primary treatment employed, these plants could potentially achieve a 20-65% increase in biogas generation while simultaneously reducing aeration energy consumption by 10-30%.

How chemicals can important role in carbon diversion at WWTPs?

There are three types of carbon diversion methods: chemically enhanced, biologically enhanced, and filtration-based. Chemically-enhanced carbon diversion involves adding chemicals to clarifiers to promote faster settling of suspended solids. This process forms larger or heavier flocs that settle to the bottom of the tank, without requiring extensive infrastructure modifications.

However, the use of additional chemicals in chemically-enhanced carbon diversion incurs extra operational costs for utilities. These costs need to be carefully considered in relation to the benefits of reducing energy expenses for aeration. Another challenge with chemically-enhanced treatment is the difficulty of recovering chemically-bound phosphorus, which has become a significant concern for utilities, especially in Europe.

Ballasted clarification is a variation of chemically-enhanced primary treatment. It utilizes microsand or magnetic materials to enhance the settling of particles that would typically be too small to settle in traditional gravity-driven tanks. While chemical coagulants and flocculants are readily available, leading providers of ballasted clarification systems, such as Veolia Water Technologies, Evoqua, and WesTech Engineering, offer innovative solutions. These systems continuously reuse the materials that enhance the settling process, thereby reducing operational costs compared to purely chemical-based processes.

Infiltration technology to reduce chemical reliance

Utilities are increasingly seeking chemical-free solutions for carbon diversion. One emerging approach involves the use of filters to improve primary treatment by removing more suspended solids before biological processes. Researchers and wastewater treatment practitioners are employing various types and configurations of filters for this purpose, demonstrating promise in achieving desirable outcomes. Notably, cloth disc filters and compressible media filters have demonstrated effectiveness.

For instance, Aqua-Aerobic Systems has introduced the AquaPrime product, which is an adapted version of its tertiary disc filter. This innovative filter incorporates specially engineered cloth media and increased basin depth, making it well-suited for enhanced primary treatment applications.

Another technology addressing this need is the Proteus Primary filter developed by Tomorrow Water. This filter utilizes a cross-shaped media design that enhances the filtration surface area by 50%. Large-scale wastewater treatment plants in Korea have successfully implemented carbon diversion, as well as pilot projects in locations like Milwaukee in the United States. Additionally, Tomorrow Water offers the option of incorporating chemical enhancements into the filter to address the challenges posed by unpredictable peak flows resulting from climate change-induced wet weather events.

Advancements in biological processes – A potential alternative option.

Biological treatment processes for carbon diversion present a promising opportunity in the primary treatment market. Unlike physical and chemical processes that only eliminate suspended material, biological processes can also remove soluble matter, which constitutes a significant portion of municipal wastewater.

One prevalent configuration of biologically-enhanced primary treatment is the A-stage process. This approach involves establishing a small high-rate activated sludge system in the primary reactor. Bacteria within this system absorb soluble carbon from the wastewater without metabolizing it, ultimately settling into the waste sludge along with the excess carbon.

Another option is the A/B process, which represents a modification to the traditional activated sludge process. This method utilizes a combination of adsorption and bio-oxidation to divert carbon. The short retention time in the A/B process leads to reduced energy consumption for aeration, while soluble material is rapidly absorbed without being broken down.

Innovations within this market are focused on integrating treatment processes in novel ways. An exemplary instance is the alternating activated adsorption (AAA) clarifier, developed by NEWhub Corp. This technology combines the A-stage process with contact stabilization, directing the flow to the bacteria layer. Here, bacteria bind carbon to their surface and undergo biosorption. The AAA clarifier is particularly suitable for smaller treatment plants and has gained recognition in Europe, with plans to explore markets in the US and Australia.

Importance of the harmonization between different WWTPs processes for better results

Implementing enhanced primary treatment to reduce energy consumption is an attractive option, but utilities face a complex decision due to the crucial role of carbon in nutrient removal. If too much carbon is removed at the primary stage, additional carbon sources will be needed for biological processes to meet discharge standards.

The tightening of nutrient removal regulations worldwide highlights the importance of considering these standards. In Europe, for instance, the proposed updates to the Urban Waste Water Directive aim to significantly reduce total nitrogen and phosphorus limits. While utilities may prioritize efficiency and cost savings, their main focus will be on meeting these stringent regulatory limits.

Emerging methods of nutrient removal offer the potential to reduce carbon requirements in secondary treatment. One promising option is anaerobic ammonium oxidation, although widespread commercial adoption is still a distant goal. Nonetheless, it could eventually allow utilities to allocate more carbon to sludge. Currently, utilities must carefully balance the competing priorities of energy reduction and meeting strict nutrient removal requirements.

The immediate markets for carbon diversion are larger utilities in Europe, the US, and East Asia, where efforts to improve the sustainability and efficiency of wastewater treatment processes are gaining momentum. As more utilities adopt carbon diversion practices, they demonstrate how to harness the synergies that accelerate the development of energy-positive wastewater treatment plants with a smaller environmental footprint.

Reference: Original article (HOW COULD CARBON DIVERSION UNLOCK ENERGY SAVINGS FOR WWTPS? by AuqaTech) with Knowledge partner GWI published on Thursday, 11 May 2023.