The significance of the harvest step is easy to underestimate. In a typical manufacturing train, human embryonic kidney 293 (HEK293) cells are cultivated in bioreactors and transfected to produce viral vectors, after which a chemical lysis step releases the particles from the cells. What remains is a murky soup of viral vectors, host cell proteins, host cell DNA, and fragmented cellular material. Before chromatography-based purification can begin, this crude lysate must be clarified, and the efficiency of that clarification determines how much product survives and how smoothly downstream operations run. At laboratory scales of up to roughly 10 liters, centrifugation handles the job well, but single-use centrifuge technologies cannot deliver the required centrifugal forces at production scale, forcing manufacturers to rely on filtration instead.

Depth filtration has long been the industry standard for this task. These filters contain a porous multilayer matrix of fibrous material and filter aids such as diatomaceous earth, and they are valued for their high retention capacity and their ability to remove not only cell debris but also meaningful quantities of host cell proteins and DNA. However, the Roche team identified a critical weakness when they systematically screened ten depth filters or filter combinations using lysate from rAAV8-producing cultures in 1 to 3 liter test volumes. Capacity values ranged widely, from 20 to 300 liters per square meter of filter area, with turbidity reductions generally exceeding 90 percent for the better performers. The synthetic Millistak+ HC Pro D0SP filter emerged as the clear favorite, achieving approximately 300 liters per square meter, followed by Clarisolve 20MS at roughly 240 and Millistak+ HC Pro C0SP at 120 liters per square meter.

The problem lay not in performance but in geometry. The largest filter area available per fully closed single-use capsule of the best-performing depth filter was limited to 0.027 square meters. Scaling calculations for a 500 liter harvest revealed that closed processing would require roughly 50 such lab-scale capsules, an impractical proposition, while the cassette-based alternative that provides adequate area requires a multi-use filter holder whose disassembly cannot be performed under closed conditions. This matters because rAAV vector variants classified as risk group 2 fall under German biosafety regulations that prescribe a fully closed system whenever no separate space exists for the disassembly and cleaning of equipment outside the production area. A fully closed system, as defined in the study, fully isolates cells and viral vectors from the environment throughout processing.

Alluvial filtration, also known as body feed filtration, offered a way around this constraint. Rather than relying on a fixed filter matrix alone, the technique adds diatomaceous earth directly to the feed, where it forms a porous, incompressible cake on the filter sheet that acts as the working filtration medium. This cake resists blockage, supports high flow rates of around 300 liters per square meter per hour or more, and enables efficient impurity removal. The Roche team, building on earlier small-scale work conducted at merely 50 milliliters by other groups, optimized the method across combinations of filter sheets, diatomaceous earth grades, and dosing ratios. The winning configuration paired the PURAFIX CH 31 filter sheet with Celpure C 300 diatomaceous earth at a ratio of 2 grams of filter aid per 3 grams of biological wet mass, the latter determined gravimetrically from centrifuged samples of each feed.

In head-to-head small-scale comparisons using 1 to 3 liter feeds, the optimized alluvial method matched the best depth filter in capacity at approximately 300 liters per square meter while surpassing it on nearly every other measure. Alluvial filtration achieved complete removal of host cell DNA, a 75 percent reduction in host cell protein, 97 percent turbidity reduction, and product recoveries of 95 percent for viral genomes and 97 percent for capsids. The absolute turbidity after filtration was slightly higher than with the leading depth filter, 63 versus 40 nephelometric turbidity units, but remained comfortably within the range suitable for subsequent sterile filtration. The method also operated at a recommended flux of 480 liters per square meter per hour, more than three times the depth filter recommendation, translating into substantially shorter processing times.

The real test came at scale. The researchers processed lysate from bioreactors at 10, 100, and 500 liters, spanning process development, initial supplies, and pilot-scale study material. The runs covered three rAAV variants, rAAV2, rAAV8, and rAAV9, providing a stress test of robustness across different vector serotypes and feedstock compositions. Average filter capacities came in at 223, 256, and 229 liters per square meter for the three scales respectively, a remarkably consistent band. Product recovery and turbidity reduction stayed high at every scale with no decline at 500 liters, and standard deviations remained acceptable. Impurity reduction for host cell DNA and protein showed larger variability, a deliberate trade-off in favor of capacity, which the authors judged acceptable since robust lysate processing, not maximal impurity clearance, is the critical objective at this initial stage.

Pressure profiles from a representative 500 liter run told a story of a process operating well within its limits. Pressure before the alluvial filter rose to 1.15 bar at the start of filtration and then held nearly constant, while downstream pressure stabilized at 0.75 bar and flow declined only marginally from 12 to 10 liters per minute, a drop attributed to the centrifugal pump rather than filter clogging. The entire 405 liter filtration finished in approximately 30 minutes, matching the calculated projection, and pressure never approached the 1.5 bar safety limit. The filter cake volume also stayed below the module maximum, indicating that the system’s full capacity was never reached and that the process could tolerate even higher-turbidity feedstocks. A sterile filtration stage downstream of the alluvial filter performed without blocking or product loss, confirming the filtrate was fit for purification.

Safety engineering rounded out the design. Because filtration trains are generally classified as high-risk unit operations prone to equipment failure and pressure buildup, the pilot-scale setup incorporated single-use pumps, sterile couplings, and pressure-rated tubing, along with continuous monitoring by pressure and flow sensors. If system pressure reached the 1.5 bar maximum, flow was automatically reduced to prevent exceedance. Before any viral vector-containing fluid entered the train, all connections were checked, filters were vented during a water pre-rinse, and the housing and connections were validated by a water pressure test. Disassembly was performed by tube welding and sealing, allowing the used filter to be removed and autoclaved for viral inactivation without ever opening the system.

The study’s authors conclude that alluvial filtration meets both the performance and biosafety requirements that the best available depth filter could not satisfy at pilot scale, offering up to 2.71 square meters of filter area in a single closed module where the leading depth filter capsule provides only 0.027 square meters. The method does demand roughly an hour of additional preparation to measure biological wet mass and suspend the filter aid, but this is more than compensated by filtration times cut more than threefold. As demand for rAAV-based gene therapies continues to climb, the ability to harvest viral vectors safely, quickly, and at the 500 liter scale within a fully closed single-use train may prove a meaningful step toward industrial-scale production of these transformative medicines.

Subject of Research: Alluvial filtration for scalable, closed-system harvest clarification of recombinant adeno-associated viral vectors in gene therapy manufacturing

Article Title: Alluvial filtration as a scalable and safe single-use technology for the harvest of viral vectors

Article References: Wiesner, J., Bauer, A., & Eisenkraetzer, D. (2026). Alluvial filtration as a scalable and safe single-use technology for the harvest of viral vectors. Applied Microbiology and Biotechnology, 110(1), Article 293. https://doi.org/10.1007/s00253-026-14035-1

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14035-1

Keywords: alluvial filtration, rAAV vectors, gene therapy, viral vector manufacturing, depth filtration, diatomaceous earth, harvest clarification, single-use technology, bioprocessing, HEK293 cells, pilot-scale production, biosafety

The research, conducted by a team from DrM Dr. Müller AG and the Zurich University of Applied Sciences, set out to compare cyclical cake filtration against two established approaches: conventional depth filtration and alluvial filtration. Depth filtration, the current industry standard, removes particles by forcing the harvest through a porous bed of filter media in which contaminants are trapped in tortuous channels. It works, but it comes with well-known liabilities. The filter bed gradually clogs as it loads with solids, which means engineers must size filters generously to guarantee that a full batch can be processed before the flow stalls. Oversized filter areas translate directly into higher consumable costs and more waste. Depth filters can also reduce yield through non-specific adsorption, in which product molecules bind to the filter media and are lost before they ever reach the chromatography columns downstream.

Alluvial filtration was developed to ease some of these pressures. In this configuration, a protective layer of diatomaceous earth, a porous sediment derived from fossilized algae, is built up on the filter surface before the harvest is introduced. The diatomaceous earth cake acts as a sacrificial shield, capturing the bulk of the particulate load and protecting the underlying filter media from rapid fouling. This extends the filtration capacity considerably compared with bare depth filtration. Yet even alluvial filtration has limits, and the authors of the new study identified an opportunity to go further by exploiting the same diatomaceous earth chemistry in a fundamentally more dynamic way.

Cyclical cake filtration takes the protective cake concept and turns it into a repeating cycle. Rather than building a single cake and running the batch until it is exhausted, the system forms a diatomaceous earth layer on a filter cloth, processes harvest through it, and then regenerates the cloth for the next cycle. This multi-cycle cloth regeneration is the core innovation. Because the filter medium is repeatedly refreshed during operation, the process can sustain high flow rates for extended periods without the progressive clogging that plagues depth filters. In the experiments reported in the study, cyclical cake filtration maintained higher flow rates than depth filtration and showed no clogging under the conditions tested, a striking result given that clogging is precisely the failure mode that forces manufacturers to oversize their equipment.

Clarification alone is not enough, however. A harvest step must also strip out impurities that would otherwise burden downstream purification. The researchers augmented the cyclical cake filtration process with diatomaceous earth and a range of ion-exchange resins, allowing the filter cake to act not just as a physical sieve but as a chemical polishing stage. With this combination, the filtrate turbidity dropped as low as 2.8 nephelometric turbidity units, an exceptionally clear result for a primary clarification step, while product yields remained between 91 and 94 percent. The process reduced DNA concentrations by up to 95 percent and host cell protein levels by up to 62 percent, figures that approach the performance of conventional two-stage clarification trains. For process developers, that means a single, well-designed filtration operation could potentially simplify or shorten the sequence of steps that normally follows the bioreactor.

Scalability is where many promising lab-scale technologies stumble, and the team addressed this concern directly. They ran filtration studies across three scales, spanning an 18-fold increase in filter area, and found that flow rate behavior and filtrate quality were comparable across the range. Product yield stayed consistent from the smallest to the largest configuration, while impurity levels and turbidity showed only minor variation. This kind of scale-up predictability is essential for biopharmaceutical manufacturers, who must transfer processes from development-scale equipment to commercial facilities with confidence that performance will carry over. A filtration technology whose behavior changes unpredictably with area would introduce exactly the kind of process risk that regulators and manufacturers work hard to eliminate.

The commercial implications become vivid when the numbers are projected to manufacturing scale. For a hypothetical batch of 2,000 liters of CHO culture, the study estimated that cyclical cake filtration would require a primary clarification filter area of just 0.88 square meters. Alluvial filtration, by comparison, would need 16.8 square meters for the same task, and conventional depth filtration would demand 28.5 square meters. The difference is not incremental; it is roughly a 19-fold reduction against alluvial filtration and more than 30-fold against depth filtration. Smaller filter areas mean fewer disposable filter cartridges, less storage space, lower shipping and handling burdens, and a smaller volume of solid waste at the end of each batch. In an industry under growing pressure to reduce both cost of goods and environmental footprint, those are consequential advantages.

To capture the trade-off between speed and footprint in a single figure of merit, the authors combined processing time and filter area into a proposed efficiency parameter. When the three technologies were evaluated through this lens, cyclical cake filtration came out on top. The result highlights a point that is easy to overlook in filtration comparisons: a technology that is fast but requires a huge filter area, or compact but slow, may look attractive on one metric while hiding costs on the other. By folding both dimensions into one parameter, the study offers process engineers a more honest basis for comparing clarification options and for modeling how a change in harvest strategy would ripple through facility design, consumable budgets, and batch scheduling.

The authors are careful to frame cyclical cake filtration as an alternative rather than a wholesale replacement. Depth filtration is deeply embedded in validated manufacturing processes, and any new harvest technology must clear regulatory and operational hurdles before adoption. It is also worth noting that two of the study’s authors are employees of DrM, which manufactures the filtration system evaluated, so the work comes from the technology’s own developers, although the findings are presented in a peer-reviewed, open-access venue with detailed methods that other groups can scrutinize and replicate. Independent confirmation at other sites and with different product molecules will be the natural next step for the field.

Even so, the study lands at a moment when the biomanufacturing industry is actively searching for intensification strategies. Continuous processing, perfusion cultures, and higher-titer fed-batch runs all push more solids through the harvest step, making the limitations of legacy clarification technologies more acute each year. If cyclical cake filtration performs as described at industrial scale, it could allow manufacturers to process harvests faster, with dramatically less filter media, lower waste volumes, and impurity removal that rivals multi-step trains. For an industry in which a single batch of a blockbuster biologic can be worth millions of dollars, a clarification step that protects yield, cuts consumable costs, and shrinks the equipment footprint is not a minor optimization. It is the kind of unglamorous engineering advance that quietly determines how affordable the next generation of antibody therapies can become.

Subject of Research: Cyclical cake filtration for clarification of Chinese hamster ovary cell culture harvests in biomanufacturing

Article Title: Increasing CHO cell harvest efficiency with cyclical cake filtration

Article References: Pynos, C. J.-J., Bucher, T., Mannone, A., Gopalakrishnan, A., & Ott, J. (2026). Increasing CHO cell harvest efficiency with cyclical cake filtration. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14021-7

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14021-7

Keywords: CHO cells, biomanufacturing, cell harvest, clarification, cyclical cake filtration, diatomaceous earth, depth filtration, alluvial filtration, host cell protein, impurity removal, scale-up, process efficiency