– But not only AIP is important: When a submarine used to snort to recharge the battery it also refreshed the air on board. Now the snorting has been solved (partly) the next bottleneck is the air quality. CO2-scrubbers as used on nuclear subs are large and are consuming a lot of electricity, not suitable for an board AIP-submarine. The air quality is that much of an issue that in practice an AIP equipped submarine will still be snorting every 2 days or so to refresh the air. So not only AIP has to improve but also other systems iot make the AIP-submarine successful.

The Swedish Navy has operated submarines equipped with air independent propulsion for two
decades. This type of submarine can stay submerged for periods far longer than other nonnuclear
submarines are capable of. The air quality during longer periods of submersion has so far
not been thoroughly investigated. This study presents results for a number of air quality
parameters obtained during more than one week of continuous submerged operation. The
measured parameters are pressure, temperature, relative humidity, oxygen, carbon dioxide,
hydrogen, formaldehyde and other volatile organic compounds, ozone, nitrogen dioxide,
particulate matter and microbiological contaminants. The measurements of airborne particles
demonstrate that air pollutants typically occur at a low baseline level due to high air exchange
rates and efficient air-cleaning devices. However, short-lived peaks with comparatively high
concentrations occur, several of the sources for these have been identified. The concentrations of
the pollutants measured in this study do not indicate a build-up of hazardous compounds during
eight days of submersion. It is reasonable to assume that a substantial build-up of the
investigated contaminants is not likely if the submersion period is prolonged several times, which
is the case for modern submarines equipped with air independent propulsion.

Seems the Swedes can handle an 8-day submerged mission — and it seems to me that 8 days is not the maximum…

http://www.rsc.org/delivery/_ArticleLinking/DisplayHTMLArticleforfree.cfm?JournalCode=EM&Year=2006&ManuscriptID=b605331a&Iss=11