Jay Rajapakse

Simple Water Treatment: Sustainable Implications of Pebble Matrix Filtration (PMF)

Pebble Matrix Filtration (PMF) is a novel non-chemical, sustainable pretreatment method of protecting Slow Sand Filters (SSF) from high turbidity during heavy monsoons in tropical countries. After successful first laboratory trials in the UK, the technology was field tested with pilot trials in Papua New Guinea in the late 90’s and the results proved successful. During 2002-2004 another PMF field trial was successfully conducted in Serbia, and further trials in Mexico in 2006. 

Subsequently, the construction of the first full-scale PMF plant to protect an existing Slow Sand Filter system in Sri Lanka was completed in May 2008, although due to some construction difficulties, proper monitoring of the full-scale plant was commenced only in November 2008 by the National Water Supply and Drainage Board (NWSDB) of Sri Lanka.

The sustainability of this new technology might depend on availability and supply of pebbles and sand, both finite resources. In Sri Lanka there are two principal methods of obtaining pebbles and sand, namely dredging from rivers and beaches, and due to the scarcity of these resources the cost of pebbles is about 4-5 times higher than that of sand.  Under the Sri Lankan Mines and Minerals Act No 33 of 1992, the extraction and transport of pebbles and sand requires a licence, thus, particularly the purchase of pebbles is becoming more and more difficult and a search for an alternative to pebbles is a requirement of sustainability of Pebble Matrix Filtration. After some preliminary laboratory tests conducted in Colombo-Sri Lanka, Poznan-Poland and Cambridge-UK, a 100-year-old brick factory near Sudbury, Suffolk, has produced hand-made clay pebbles satisfying the PMF quality requirements.

With regard to sand as a filter media, for many years quarried sand has been the principal media-product traditionally used in water and wastewater filtration plants. However, recent research combined with pilot trials, has clearly established that crushed recycled glass, properly processed and graded for the purpose, is an equally or in some cases much more effective and environmentally friendly alternative to sand. This work also supports the previous research findings that tested recycled glass filter media perform as well as or better than sand. There are potential opportunities for recycled crushed glass and clay ball manufacturing processes in some countries where they can be used as filter media.

There is clear a clear possibility for Pebble Matrix Filtration technology to be transferred to other developing countries with similar high turbidity problems. These include India, South-East Asia, East and Central Africa and Latin America.