Sand, anthracite, sandstone, and charcoal are used
as granular media for water filtration in POU
devices such as bucket, drum or barrel, roughing,
and cistern filters. These granular media are
usually negatively charged and may be mixed with
positively charged metal oxides and hydroxides of
iron, aluminum, calcium and magnesium for more
effective adsorption of negatively charged viruses
and bacteria. These filters may also contain
antibacterial elements such as silver.
Amount of Water Treated
Granular media and rapid rate filters are available
in a variety of scales for household and
community-level use. The most widely used
commercial bucket filtration system provides enough
water for 10 people per day. Drum or barrel filters
typically have a 200 liter capacity, though a
version designed by UNICEF provides 40 liters of
treated water a day. Roughing filters are operated
at relatively low flow rates, but typically large
enough to provide water for an entire community.
Granular media and rapid rate filters typically
remove up to 90 percent of turbidity and enteric
bacteria and more than 99 percent of larger
parasites. Roughing filters can remove 50 to 85
percent of bacteria in highly turbid water.
Combining the filter media with a positively
charged ingredient can result in up to 99 percent
viral and bacterial removal. Filters containing
granulated vegetable matter such as burnt rice hull
ash have been shown to reduce turbidity and general
bacteria by 90 percent, with E. Coli reductions
reported at 90 to 99 percent.
Ease of Use
Bucket filters consist of two or three buckets, one
of which is perforated at the bottom and filled
with granulated media. Water is passed through this
bucket into an empty bucket. Initially, enough
water should be passed through the filtering bucket
to clean the media. The media also needs to be
cleaned or replaced every few weeks to remove
accumulated contaminants and microbes. Drum or
barrel filters come in a variety of designs, most
of which have a pour-through design. These filters
require regular cleaning through backwashing, a
process of forcing water through the filter, which
may be technically difficult. Roughing filters also
require regular backwashing to work effectively.
Roughing filters are most often used on a community
level because of their need for operational and
maintenance skill and labor. If a regular
maintenance schedule is not followed, users may be
unaware that any of these filters needs cleaning
until signs of contamination are evident.
Bucket filters can be locally built at low cost
using sand or other local granular media.
Commercial bucket filters are also available. The
most widely used version consists of two 19 liter
buckets and filters and costs $50 US, with
replacement filters that cost $20 US. These bucket
filter systems are often subsidized by NGOs to make
them affordable. Drum or barrel filters cost $0.001
to $0.10 US per liter of water treated.