Possibilities For Reducing Aeration Through Carbon Diversion Technologies
Process intensification is a standard term used in engineering that can be applied broadly. For instance, wastewater treatment intensification could be defined as any system that significantly outperforms conventional designs. New approaches are needed to intensify treatment of wastewater within existing infrastructure, because sustainable increases to wastewater treatment capacity and capability are necessary.
In 2015, a group of scientists, engineers, entrepreneurs, and other wastewater professionals gathered to accelerate, develop, demonstrate, and further implement innovative technologies to enhance recovery of water, nutrients, energy, heat, and other valuable products at water resource recovery facilities (WRRFs) at reduced costs. Over 30 new technology developers shared their innovations with nearly 150 leading experts and practitioners from utilities, consulting firms, universities, regulators, and other areas of the industry. Among the technologies examined was carbon diversion — using enhanced primary treatment, filtration, or high-rate systems to increase carbon resource recovery.
Recently, several carbon diversion technologies discussed during that gathering have been accepted to the Leaders Innovation Forum for Technology (LIFT) Technology Scan process. LIFT Technology Scans identify and evaluate innovative technologies to inform water facility owners, funders, advisors, and end users in order to promote early adoption of the technologies. They offer technology providers an optimal platform to introduce their emerging, precommercial, and newly commercialized technologies.
Diverting organics from the biological oxidation process, which demands large amounts of energy for aeration, and redirecting them to anaerobic digestion, where these organics can actually help produce energy, could significantly alter treatment facilities. In fact, studies in A Guide to Net-Zero Energy Solutions for WRRFs (WE&RF ENER1C12) indicate that carbon diversion using chemically enhanced primary treatment or A-stage processes could help plants achieve energy neutrality.
However, the specific impact of carbon-diversion technologies will depend on a number of factors, including a plant’s configuration and its current level of energy efficiency — and because carbon is required for biological nutrient removal (BNR), there is also the lingering issue of meeting nutrient discharge limits. If mainstream shortcut nitrogen removal processes can be developed, the demand for carbon would decrease drastically, paving the way for carbon diversion to reach its full potential.
A recently completed Water Environment & Reuse Foundation research report, State of Knowledge and Workshop Report: Intensification of Resource Recovery Forum (TIRR1R15), summarizes a suite of carbon diversion technologies and their apparent technology readiness levels. In that report, the technologies are divided into three types: primary effluent filtration, biologically enhanced primary treatment, and anaerobic treatment alternatives.
Primary effluent filtration technologies under consideration include ones by Schreiber, BKT, Trojan Technologies, and ClearCove Systems. Schreiber has taken its Fuzzy Filter, a compressed media filtration technology that is well-established for use in tertiary treatment and wet weather management, and adapted it for use in primary effluent filtration. Full-scale piloting and demonstrations are under way at the Dry Creek and Linda County treatment plants in California. Schreiber reports that primary filtration yields a 37 percent increase in digester gas production, a 25 percent decrease in blower power requirement, and a 34 percent increase in secondary process capacity. BKT’s biofiltration system (BBF™), an upflow process system that has previously been used for wet weather treatment, is now being applied to primary filtration. The BBF unit uses expanded polypropylene as a floating media layer for filtration. A BBF pilot plant has been operating for primary and wet weather treatment for 12 months using a small coagulant dose to enhance filtration. Two full-scale BBF systems (320 MGD total) have been designed for wet weather treatment in Seoul, Korea. Trojan Technologies Salsnes Filter is a rotating belt filter that removes suspended solids and provides thickening and dewatering up to 30 percent dry weight. Trojan estimates that the filter requires 10 percent of the land of a primary clarifier and that existing clarifiers could be transformed into extra secondary tank capacity with the addition of the filter. The company offers a demonstration unit for site evaluation. ClearCove Systems’ enhanced primary treatment (EPT) technology performs screening, grit removal, primary clarification, and equalization in one single step. This proprietary technology would completely replace headworks in order to divert nearly all organics to the digester. ClearCove’s findings indicate that the EPT system yields three times more biogas than a thickened sludge sample and produces an energy savings of 52 percent for aeration. In order to meet carbon demands for BNR, ClearCove suggests that online control could divert effluent from EPT to secondary treatment. The New York State Energy Research and Development Authority (NYSERDA) conducted a technology demonstration project in Ithaca, NY.
The biologically enhanced primary treatment technologies reviewed thus far include A-stage processes, alternating activated adsorption and an Evoqua system. A-stage processes have been used since the 1970s. A high-rate activated sludge process using adsorption to maximize carbon capture (the A-stage) is followed by secondary treatment that can perform BNR (the B-stage). The A-stage uses an aeration tank with a high surface-loading rate, aeration for a short hydraulic retention time (~ 0.5 hr), and a sludge retention time of 0.1 to 0.5 day. The A/B process is compatible with nitrogen removal, especially if shortcut pathways are used, which have a lower carbon demand. Although the process was designed to reduce overall volume of treatment, today it is mainly used to improve the energy balance or increase plant capacity. There are more than 20 full-scale installations in Europe and a few in the United States. Alternating activated adsorption (AAA) is a new configuration of the A-stage process consisting of two alternating sequencing batch reactors that can be retrofitted into rectangular basins. The Captivator System® blends waste-activated sludge with incoming wastewater in an aerated contact tank prior to dissolved air flotation. This system uses biosorption to capture soluble biochemical oxygen demand not captured in primary treatment, along with particulate biochemical oxygen demand. Evoqua claims the system reduces aeration energy requirements by 40 percent while increasing biogas production by 40 percent and has a footprint that is one-fifth that of conventional primary treatment. A 32-MGD installation has been operating since 2013 in Pima County, AZ. An example from Philadelphia was also presented in which two out of five existing primary clarifiers would be transformed to the Captivator, and the remaining three would be converted to secondary aeration to meet a pending nitrification requirement. A 300,000-GPD pilot plant is also available for on-site evaluations.
There is a growing interest in anaerobic treatment of domestic wastewater due to the potential to reduce energy demand and increase energy recovery. Anaerobic systems also have low overall sludge production, which can save capital and operational costs. Anaerobic treatment has been studied for more than four decades, and the upflow anaerobic sludge blanket (UASB) reactor is widely applied in South America. More recent interest is on the development of anaerobic membrane bioreactors. The major disadvantages of anaerobic treatment are the reduction of sulfur and production of H2S (a corrosive and odorous gas), the production of supersaturated dissolved methane (a potent greenhouse gas), and the presence of residual organics. Anaerobic treatment is also less efficient at low temperatures, which may limit application. If anaerobic treatment were realized, nutrient removal would be integrated as a downstream treatment process. Traditional BNR is challenging with anaerobic effluents, and new denitrification processes are being developed that use H2S and methane as reducing equivalents (in lieu of carbon). Nitrite shunt and deammonification are also promising processes when used downstream of anaerobic mainstream treatment.
While there are technologies that show promise for energy savings, there still is a need for additional research into the full potential of carbon diversion technologies that would have the most benefit to WRRFs. WE&RF’s research will advance our knowledge and promote early adoption of technologies. For more information on carbon diversion technologies accepted by LIFT, visit the WE&RF website for available research reports at www.werf.org.