Comparison of Ballasted Activated Sludge Technologies

Overview
Ballasted Activated Sludge (BAS) technologies combine the benefits of enhanced settling and increased capacity with the familiarity of a Conventional Activated Sludge (CAS) system which many operations staff are already comfortable with. BAS technologies can make maximum use of existing infrastructure at many existing conventional treatment facilities. Recent innovations in BAS technologies have expanded the range of alternatives for process design engineers to consider, including some which merge characteristics of both BAS and aerobic granular sludge (AGS) technology into single system.
Background
The primary benefit of BAS is that it can dramatically increase the settling rate of the biomass (Young et al, 2013). Due to this high settling rate, ballasted processes can significantly increase secondary clarifier solids loading rates, which can, in turn, allow higher Mixed Liquor Suspended Solids (MLSS) concentrations to be carried in the biological reactors. The higher MLSS concentrations in the biological reactors allow for either higher Solids Retention Time (SRT) in the process, for greater treatment capacity, or some combination of both compared to a CAS process with the same tank volume. Therefore, BAS can typically achieve treatment objectives with less process reactor volume and less secondary clarifier area than conventional processes. A variety of inert media has been tested as potential ballast to bind with biological floc to enhance the settling characteristics of activated sludge. A commercially viable system using magnetite (Fe3O4), a readily available inert form of iron ore with a specific gravity of 5.2 for ballasting activated sludge was patented in 2010 by Cambridge Water Technologies (now owned by Evoqua) and as of 2021 the Magnetite Ballasted Activated Sludge (MBAS) process had more than 20 full-scale installations around the world. A more recently proposed innovation to this technology has experimented with the use of stacked tray grit separator units to separate the heavier magnetite ballasted floc faster, and in a smaller footprint, than is achievable with conventional secondary clarifiers (Gilmore, 2021). This would allow the technology to further shrink its footprint. Another recent innovation in BAS is the use of kenaf as the inert ballast media. Kenaf is a natural organic cellulosic plant fiber that is high in both surface area and absorptivity. Due to its high surface area, the kenaf core is capable of growing thick biofilms, some of which develops in the form of granules with high specific gravity which increases the sludge settling characteristics similar to what is achieved with magnetite ore ballast. However, the kenaf media also serves as a place for a biofilm to grow, such that a significant amount of the total biomass in the system consists of the biofilm growing on the suspended kenaf as opposed to strictly relying on the suspended biomass in a CAS or MBAS (Boltz et al, 2021). The use of kenaf media for activated sludge ballast has been patented by Nuvoda as the Mobile Organic Biofilm (MOB) process. Like CAS, reactor influent is combined with return activated sludge and treated in process reactors. Like the MBAS process, the system is initially charged with the inert media (in this case kenaf instead of magnetite) and then the bulk of the media is recycled back from the secondary clarifiers to the front of the process reactors in the return sludge while a fraction is recovered and recycled from the waste sludge. Unlike the MBAS, the biological activity is divided between suspended biomass and the biofilm. Therefore, the MLSS in the MOB process can be kept much lower than the MBAS or other alternatives. With both types of BAS technology, efficient recovery and reuse of the media is key to sustainable economic performance. In the case of MBAS, magnetite is recovered from waste sludge using shear mills followed by rotating magnetic drums to separate out the ballast from the biological solids. In the case of MOB, kenaf is recovered by pumping the waste sludge through rotary drum screens. The first full-scale municipal treatment installation of MOB was at Moorefield, West Virginia in 2016. MOB technology has been more recently piloted at larger utilities, including Clean Water Services in Oregon (Schauer, 2021).
Cost Comparison of Technologies for Target Installation
Concept design and lifecycle cost estimates were developed for the application of MBAS and MOB technologies at a 20 mgd capacity wastewater treatment facility in the Chesapeake Bay Region. These technologies were compared to each other and to the originally planned CAS upgrade of the facility.
These applications were applied to an existing CAS facility consisting of preliminary and primary treatment, 4-stage Bardenpho process reactors for biological nitrogen removal, and circular secondary clarifiers. The existing facility had severe limitations in secondary clarifier capacity and the Owner was planning to construct three additional secondary clarifiers and an additional process reactor in order to reduce solids loading rates and allow SRT targets to be reliably maintained in the process reactors, based on historical SVI values. Applying MBAS and MOB technology to this facility would reduce the number of new secondary clarifiers required and eliminate the need to construct an additional process reactor. On the other hand, MBAS and MOB technology required supplemental mixing in the process reactors and the addition of a ballast media recovery facility as well as periodic addition of new ballast media to keep the system fully charged. Design criteria for the concept designs are summarized in Table 1. The 20-year Operation & Maintenance Net Present Value and Project Lifecycle Costs of the concept designs are summarized in Figures 1 and 2.
Conclusion Based on the analysis, all three technologies offered similar lifecycle project cost for this target application. However, MBAS and MOB technologies provide reduced site footprint and lower projected operating costs. On the other hand, both MBAS and MOB require the operation of a media recovery unit to recover media from the waste sludge for reuse. The MOB system has significantly lower oxygen demands thanks to the lower SRT and improved oxygen transfer rates, and the higher yield of waste sludge can potentially yield more biogas, so that this system would be closer to energy neutrality than the other two systems. In this case, both MBAS and MOB was able to avoid the construction of a new process reactor and secondary clarifier, thus saving significant construction cost that was offset by the cost of the installed equipment. With optimization, it may be possible to further reduce the initial capital cost of applying these technologies and allow them to be more cost competitive compared with conventional approaches. At facilities where there is no room to add reactors and/or clarifiers, these technologies would offer an option for expansion while maintaining the treated effluent quality.