1. I thought that to have a filter with pores fine enough to remove viruses, there should be frequent clogging problems. Is that why some used a bacteria size pore and a secondary disinfectant step (e.g., iodine resin) to kill viruses.
The proprietary filter formulation used in Black Berkey purification elements makes up for the incredibly small pore size by utilizing an exponentially greater number of micro pores in each purification element than that found in other filter elements. This is the reason Black Berkey purification elements produce approximately 8 times more water per hour than do other elements. Therefore, while some of the surface pores will clog up from contaminates, it is difficult for such contamination to overwhelm all of the pores and so clog up the filter. Eventually the majority of surface pores may clog up slowing down the flow rate of the Black Berkey purification elements. When this happens the filter can be refreshed by simply brushing the outside of the element with a ScotchBrite pad or toothbrush. This will remove the surface pores that have become clogged. Therefore, in answer to your question, even though the filters have such small micro fine pores, the element will not clog up prematurely.
2. How does your filter in the Berkey system deal with viruses? Possibly with a series of filters with graded pore sizes? Or is the virus filtration due to the tendency of viruses to clump with bacteria and debris in the water?
The micro fine pores are so small that they prevent micro fine particulate such as bacteria, virus and even food coloring particulate from passing through the tortuous path that water is able to pass through. These contaminates are trapped in the microfine pores while the much smaller H20 molecules are not. Therefore, in answer to your question, this method separates the contaminates from the purified water.
3. Is the MS2 - Fr Coliphage still known to be a good indicator of virus filtration? At least one article suggests that it might not be. Do you have tests on any other viruses?
MS2 and Fr Coliphage are two separate virus with two different removal characteristics. That is why they are used as surrogates for other types of virus. These virus were selected, by both the EPA and the Military, because of their small size relative to other virus strains and the difficulty in removing both strains. They each are approximates 24-26 nanometers in size which makes them among the smallest of virus. These virus are comparable in size on that chart to the Rhonivirus. Therefore, in answer to your question, yes MS2 and Fr Coliphage are the ideal virus for use as indicators of a purification systems ability to remove virus.
The fact that Black Berkey purification elements have been tested to remove viruses to greater than the EPA purification standards suggests that contaminates larger in size, such as bacteria, should also be removed.
The MS-2 virus is 24-26 nm in size.
The Fr Coliphage virus is 25nm in size.
In other words, they are smaller than the hepatitis virus at 45mn and the rhinovirus at 30nm both referenced on the chart.