Concrete filters are the most widespread type of biosand filter. Concrete is generally preferable to other materials because of the low cost, wide availability and the ability to be constructed on-site. The plans for the concrete filter are distributed openly by CAWST. Several versions have been developed. The CAWST Version 9 biosand filter is constructed with a higher maximum loading rate. Although the filtered water passes EPA water quality standards, it is not optimal. Recent research establishes that contact time between the water and the granular material is the leading determinant in purifying water. The CAWST Version 10 biosand filter takes this into account; the volume of the water reservoir is equal to the pore space volume of the sand layer. The maximum loading rate was decreased by 33% to ensure stagnant water is in constant contact with granular material.