No single company dominates this field. The leading assignee in this dataset holds 13 of the 218 records in scope, and the top 5 assignees combined account for 22.9% of all records. That leaves the majority of filings spread across a long tail of companies, utilities and one university, each holding a handful of records. For freedom-to-operate purposes this means checking several mid-sized filers rather than clearing one dominant owner.

Filing peaked in 2017 at 15 records, dipped afterward, and has since risen again from 4 records in 2021 to 6 in 2024, a +50% increase over that span. The most recent one to two years in any patent dataset understate true activity because publication lags filing by roughly 18 months, so 2025 and 2026 figures should not be read as a slowdown. On the basis of the last complete years, momentum is building rather than fading.

US6254775B1, assigned to Renergy LLC and issued in 2001, claims a vertically upright digester vessel with a matrix supporting microorganism biomass, a slurry input above the matrix, a gas output at the top and an effluent output at the bottom. It specifies a preferred liquid height-to-diameter ratio of 2:1 and a fiberglass-reinforced plastic construction with a translucent blue gel-pigment coating for light filtering. It is among the most-cited records in this dataset, meaning later filings in this space have repeatedly had to design around its vessel geometry and biomass-support configuration.

Mechanical mixing hardware itself is comparatively thin: B01F, the IPC subclass most directly tied to mixing and stirring, appears in only 7.8% of the 218 records, against 77.1% for wastewater-treatment process claims and 43.6% for bioreactor apparatus claims. Specific configurations such as gas-lift dead-zone elimination, variable-speed impeller control tied to power input, and retrofit sludge-circulation loops for existing digesters show thinner filing density than the core digestion process claims. That gap is where new claims are more likely to clear prior art cleanly.

The United States receives the largest volume in this dataset at 71 filings, followed by Europe (EPO) at 29, WIPO/PCT at 25, Japan at 21, Canada at 16 and Australia at 12. The relatively thin PCT count compared to the US total suggests that many applicants file domestically first and do not always pursue the broader international route for mixing-specific claims. That pattern matters for competitors assessing exposure outside the US market.