Recirculation polishing filtration is typically best done in an independent circuit with HIGH flow - TANK TURNOVER .... ie. recirculation volume at approx. 3-4 gallons per MINUTE (per 100 gallons of tank capacity) vs. a 10-15ÁM rated filter. This will result, due the many 'tank turnovers', in a residual or resident particle distribution 'in the tank' at essentially 'sub-micronic' levels. A Walbro type impulse diaphragm 'continuous duty rated' transfer pump is the most common and probably the best pump for this type application.
Recirculation polishing is best optimized at HIGH flow rate and LOW pressure drop. Such will ensure that if the tank releases a 'slug' of particulate (breaking loose from the tank walls), the high turnover rate will quickly restore ('recover') the tank back to acceptable particle levels.
Using the fuel line filters + 'return line' & engine's lift pump will accomplish virtually NO 'effective' filtration due to the low 'dilution' (statistical reduction) of particles in the tank as the lift pump on most small marine diesel engines are rated typically for 1-2 gallons per HOUR.
Summary for recirculation polishing : 3 - 5 gallons per MINUTE
(per 100 gallons) @ 10-15ÁM
in INDEPENDENT circuit - the filter being supplied with PRESSURE (NOT vacuum). Use standard INDUSTRIAL filter cartridges (either pleated or 'graded pore density depth-filters')@ 2.5" or 2.75" dia. X 10 inch length in common industrial filter housing with flat gasket filter seals; expect to pay ~$10-$15 per 10" filter. Performing polishing via the main fuel line is next to worthless due to the extremely low 'tank turnover'. To adequately 'dilute' and maintain the resident tank particles you would need to 'turnover' the tank about 10-20 times .... and at the common .75 gallons per HOUR of a common lift pump would take 20 X .75 X 100 = 1500 HOURS !!! At 3-4 gallons per MINUTE would take 10 to 20 X 4/60 X 100 = 65 to 130 HOURS. Note: if you would use a 2ÁM instead of a 10-15ÁM the above values would be ~10500 HOURS vs. 65 to 130 HOURS (@10-15ÁM) due to the much higher resistance (differential pressure drop) to flow in the 2ÁM filter !!!!
Low flow & very fine ÁM retention will NOT dilute a high particle load in a tank within a 'reasonable' time (Ref. - Newton's Law of Exponential Dilution/decay) ... dont waste your time and $$$$ on low flow and 'tight' filter ÁM to 'polish' a tank.
FYI - Ive been very deeply involved in 'filtration engineering' for over 30+ years.
Pumps: Walbro FR-Series Industrial and Marine Fuel Pumps from Fuel-Pumps.net