In this dataset, a process-stability patent is one whose title or claims explicitly address maintaining stable digester operation under variable conditions — resistance to inhibition from ammonia or volatile fatty acid build-up, long-term stable operation across feedstock changes, or digester reliability more broadly. This is narrower than general anaerobic digestion filing, which also covers biogas yield, reactor design and feedstock pre-treatment without a stability claim. The search here combines those stability terms with IPC classes covering wastewater treatment, bioreactor apparatus and sludge digestion, so it captures the operational-robustness slice of the art specifically.

US4735724A, on a non-mixed vertical tower digester that passively concentrates solids, has accumulated citations over nearly four decades, which mechanically gives it more opportunities to be cited than a 2023 filing. Its foundational claims on solids retention and hydraulic retention time are also the kind of basic mechanism that later, narrower patents tend to reference when distinguishing their own claims. High citation count here should be read as a marker of influence within the searched corpus, not as evidence that the underlying technology is still the most commercially relevant approach.

The trend in this dataset suggests yes, with caveats. Filings rose from 6 in 2017 to a peak of 11 in 2020, dipped to a midpoint of 4 in 2022, and have climbed back to 7 in the most recent year tracked. Because publication typically lags filing by around eighteen months, that most recent figure is likely understated and will revise upward, so the underlying trajectory looks like renewed rather than declining interest.

The United States leads with 43 records in this dataset, but European (EPO) and international (WIPO/PCT) filings each account for 25, with India, Canada and China also represented at meaningful volumes. This spread means a filing strategy or freedom-to-operate search confined to a single office would miss a substantial share of the relevant art, particularly given the international WIPO/PCT filings that have not yet been routed to national phase in all jurisdictions.

The core wastewater-treatment classification (C02F) is heavily claimed, appearing in nearly all records, but adjacent branches are comparatively thin: grinding and pre-treatment for feedstock homogenisation, chemical detoxification and antidote dosing for inhibitory compounds, and membrane-based solids separation for digesters each show meaningfully lower filing density than the core class. These are plausible areas for a first-mover claim, particularly as co-digestion of mixed feedstocks increases the practical need for pre-treatment and inhibition-management technology.