Suspended solids and pathogenic organisms. In theory viruses can pass through 0.2 micron pore but they are normally attached to other material and are prevented from passing. Yes No Simple and robust. Blind quickly if water contains suspended solids. Suspended solids are removed by scrubbing candle and scrubbing wears away ceramic material.
Candles are relatively expensive.
Yes Relatively easy to distribute and use, particularly in emergencies. Not locally available in many developing countries, have to be imported. Expensive for long term use. Dose depends on organic material, etc in water. Available chlorine in tablet can decline with age.