It provides the oxygen required by the microorganisms in the biofilm. Large suspended particles and pathogens are trapped in the top of the sand and partially plug the pore spaces between the sand grains. This causes the flow rate to decrease. Idle time typically comprises greater than 80% of the daily cycle; during this time, microbial attenuation processes are likely to be significant. Most removal occurs where water is in contact with the biofilm. The processes that occur in the biofilm have not been identified. When the standing water layer reaches the level of the outlet tube, the flow stops. Ideally, this should be high enough to keep the biofilm in the sand layer wet and allow oxygen to diffuse through the standing water to the biolayer. The pause period allows microorganisms in the biolayer to consume the pathogens and nutrients in the water. The rate of flow through the filter is restored as they are consumed. If the pause period is too long, the biolayer will consume all of the pathogens and nutrients and will die, reducing the efficiency of the filter when it is used again. The pause period should be between 1 and 48 hours. Pathogens in the non-biological zone die from a lack of nutrients and oxygen. Over time, particles accumulate between the filter's sand grains. As more water is poured, a biofilm forms along the top of the diffuser plate. Both of these occurrences cause a decrease in flow rate (clogging and bioclogging). Although slower flow rates generally improve water filtration due to idle time [APS1], it may become too slow for the users’ convenience. If flow rates fall below 0.1 litres/minute, it is recommended by CAWST to perform maintenance. The "swirl and dump", or wet harrowing cleaning technique, is used to restore flow rate. About is poured into the filter before cleaning (assuming the filter is empty). The upper layer of sand is then swirled in a circular motion. Dirty water from the swirling is dumped out and the sand is smoothed out at the top. This process is repeated until flow rate is restored. Cleaning the diffuser plate, outlet tube, lid, and outside surfaces of the filters regularly is also recommended. Long-term sustainability and efficacy of biosand filters depends on education and support from knowledgeable support personnel. Clean Water for Haiti, a non-profit in Haiti implements an education and follow-up program post-installation of the biosand filter. The program includes visits to the beneficiary homes after one, three and twelve months and another at 5 years from installation date.