Guest Column | June 1, 2016

Opportunity To Evaluate Three Technologies For Achieving Class A Biosolids With Reduced Capital And Operating Costs

Compost

By Water Environment & Reuse Foundation (WE&RF)

The Water Environment & Reuse Foundation (WE&RF) has a significant opportunity for municipal water resource recovery facilities (WRRFs) seeking to upgrade their solids treatment systems to produce Class A biosolids in a cost-effective and technically sound manner. WRRFs can take part in piloting three cost-effective technologies suitable for all plant sizes.

For utilities seeking to minimize capital, operating, and maintenance expenditures, and/or to minimize their energy footprint, the most suitable approach to achieving Class A biosolids may not be to add another energy-intensive, high-tech, and expensive treatment process to their biosolids management train.

A Low-Cost Low-Tech (LCLT) Methods approach will be appealing to WRRF plants of all sizes, by applying techniques being evaluated in a WE&RF funded project at Michigan Technological University. High-tech analysis is being conducted to refine a number of promising low-tech techniques under a range of process conditions. The LCLT methods include lagoon storage, air drying, and cake storage. Resulting mathematical models can be used by plant personnel and regulators to predict reduction in different Pathogen Indicator Organisms (PIOs) and appropriate treatment criteria for specific biosolids characteristics at ambient environmental conditions. Initial work will establish lagoon storage studies for anaerobic digestion for vector attraction reduction (VAR) and dewatering by belt-filter press to simulate a high-solids long-term storage process.

For the subsequent phases, WE&RF is seeking cost-share partners who will benefit from these LCLT methods. These phases will include the necessary lab analysis to estimate inactivation constants, and apply these inactivation constants under different conditions in pilot projects. Pilot-scale studies of four LCLT Class A biosolids treatment processes will be performed according to the established treatment criteria. An abundance of PIOs and environmental factors will be monitored to validate the predictions based on the laboratory studies and mathematical modeling.

A mathematical model of PIO inactivation rate will be developed for each PIO. In addition to being useful for predicting PIO inactivation under a wide range of conditions — for example, for storage temperature, high and low levels of 25°C and 4°C­ — the models developed in this research will identify the combinations of conditions that are optimal for PIO inactivation. This tool will be especially useful for utility managers seeking to design and optimize LCLT Class A biosolids treatment processes.

Once validated, the models can be used by plant personnel, regulators, and others to predict how long it will take to reduce different PIOs to Class A levels, and then select the most conservative treatment criteria for their biosolids and environmental conditions.

Small to mid-size WRRFs may benefit from the BioElectro Process being evaluated by the City of Houston and Concordia University. The BioElectro Process retrofits existing aerobic digesters to enhance function by reducing existing digester volumes, improving the dewatering process, and reducing construction and operational costs.

Preliminary studies have shown that temperature could rise to 50°C and 70°C within less than 30 minutes to an hour, respectively, using BioxyS™ and a voltage gradient less than 5V/cm. The heat generated from the exothermic reactions facilitates subsequent thermophilic digestion, decreases the time needed for biosolids stabilization, and assures biosolids disinfection. The BioElectro process thus provides the potential to retrofit existing aerobic digesters to achieve a Class A biosolids through disinfection and thermophilic digestion without increasing the existing digester volume.

The BioElectro process can be cost effective for smaller utilities due to its low construction costs, potential to reduce existing digester volumes, and changes to sludge properties that improve dewatering operations and reduce operational costs. For example, capital costs are projected to be in excess of $9M for using existing technology to retrofit the aerobic ambient temperature process at the 13 MGD Munster WWTP, St. Bernard Parish (LA) to a Class A product process. But, using the BioElectro process prior to the existing aerobic digestion process would be less than $2M, and the Parish would see further reductions in O&M costs.

Support is sought for two critical phases of large bench-scale testing. The first phase will 1) refine electrode requirements, 2) establish chemical additives and dosing, 3) refine electrical contact time, and 4) assess impacts on biosolids dewatering. The second phase will establish requirements for full-scale piloting, projected capital costs, and operational costs. The technology will be of special interest to managers of smaller facilities, or larger utilities that manage small facilities.

For larger (>100 MGD) facilities, a Dallas Water and Texas A&M team is evaluating Electron Beam Enhanced Anaerobic Digestion (E-Beam) technology, which offers a small footprint and modular design. Empirical data on high-energy E-Beam technology in hydrolyzing sewage sludge for enhanced biogas production allows optimization of E-Beam dose and specific total solids content to minimize methane gas production and identify the by-products (nitrogen and phosphorus) of highest commercial value.

For larger (>100 MGD) facilities, E-Beam technology holds great promise, offering a small footprint, a modular design, and reduced capital cost requirements. The technology uses high-energy electrons to cost effectively hydrolyze municipal wastes enhancing methane production from sewage, improving sludge dewaterability, reducing sludge viscosity, and reducing sludge residence times. 

The E-Beam research team developed a proposal based on positive preliminary data from high-energy (five million electron volts (MeV) to 10 MeV) linear accelerators. Significant improvements in E-Beam technology capable of generating deep penetrating electrons are adequate for effective municipal waste treatment, meaning E-Beam could be a viable technology for enhanced anaerobic digestion.

This project is building on past WE&RF work, Disinfecting and Stabilizing Biosolids using E-Beam and Chemical Oxidants (Project U4R06), which demonstrated enhanced stabilization and disinfection of municipal biosolids when high-energy, 10 MeV electron beam technology is coupled with oxidants such as chlorine dioxide and ferrate. Research results indicated that 10 MeV E-beam is capable of cost effectively inactivating bacterial and viral pathogens in aerobically and anaerobically digested biosolids. Significant reduction was observed in microbial pathogens and estrogenic compounds when E-beam was combined with ferrate.

The project will obtain empirical data to evaluate the applicability of high-energy E-Beam technology to hydrolyze sewage sludge for enhanced biogas production, both to optimize E-Beam dose and specific total solids content to minimize methane gas. The project will also identify the by-products (significant pools of nitrogen and phosphorus) of the sludges processed with the E-Beam technology to identify those of highest commercial value. Control systems will be automated, with precise process monitoring.  Ideal partner organizations are those interested in increasing methane gas production for their facilities larger than 100 MGD with anaerobic digestion.

The widespread adoption of sustainable biosolids treatment methods stands to benefit from the improved understanding of how pathogen organisms are inactivated (e.g., including bacteria, viruses, and helminths). With increasing community awareness, plant personnel and regulators look to demonstrate the effectiveness of these methods from the environmental and public health perspective. The projects outlined here make significant progress with regards to both the technical and practical understanding of production of Class A biosolids.

Potential partners interested in exploring these technologies should contact Allison Deines at adeines@werf.org.

Image credit: "Compost," U.S. Department of Agriculture © 2014, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/