What Makes an Efficient Ceramic Water Filter?

An important but brief study was published last year by individuals working with the RDIC factory in Cambodia to find what factors make a more efficient ceramic water filter. In this study several empirical tests were preformed to investigate the relationships between filter mix ratios and firing schedules. Perhaps the most relevant results demonstrated by these tests demonstrated the lack of correlation between flow rate and bacterial removal efficiency. In the test, filter discs ranging from 2 to 18 liters per hour showed similar e-coli removal efficacy.

This has long been suspected in the industry where standard flow rate best practices specify a 1.5 to 3 liter range of acceptability. These flow rates were set in an attempt to ensure that new factories making filters were producing effective ceramic water filters without the ability to effectively test their real ability to remove bacteria.

Other Tendencies of Efficient Ceramic Water Filters

  • Increasing burnout to clay ratio seems to increase flow rate while reducing strength
  • Firing to a higher temperature may increase flow rates, (it is assumed that firing higher increases pore size)
  • Increasing firing temperature may result in a stronger filter.
  • Firing higher seemed to reduce bacterial removal effectively, but the correlation was weak.
  • There is strong variation in flow rates of filters formed in wet or dry season.
  • Similar flow rates can be achieved by 1: Increasing burnout to clay ratio and reducing burnout particle size 2: Reducing burnout to clay ratio and increasing burnout particle size.

This study clearly presents an argument for further testing of these same variables to produce a more efficient ceramic water filter.

The onus is now on each factory to further develop their manufacturing practices. If the established best practices are to be modified on a case by case basis it should only be by careful experimentation and with the partnership of an independent monitoring agency to ensure that filters being produced meet minimum bacterial removal standards.

Although these results direct us towards a path of study that develops efficient ceramic water filters, any operation in the manufacturing process which relies on additional mechanical or technological resources also removes a layer of appropriateness to the ceramic water filter technology as a whole.

Imagine, for instance, that to make a more effective filter we find that burnout must be screened to finer tolerance. The factory in Somalia must then find a new reliable source a new mesh for their screening machines. The mesh they are currently using comes from out of the country and is very expensive and difficult to obtain. I argue that although the finer mesh would result in a more effective filter, if the basic standards were to change to reflect the greater efficiency it would reduce the appropriateness of the technology as a whole.

Development of a more effective filter is a part of factory scalability, once production of a basic filter has been stabilized a factory is ready to begin the process of improvements. These improvements will be determined by the local environment. After all, there are plenty of more effective filters on the market when cost, manufacturing limitations, and community adaptability are removed from the qualifications.

Studies such as these are important for us to better understand the path ahead with regard to improvement/changes to ceramic water filter production practices. That said, many of the studies assessing these practices are conducted in research facilities using laboratory methods. While this is a necessary first step, it must be followed by a replication of the results in a working filter factory. If this was a part of all academic research there would be a lot less ambiguity.

Link to original study:
Isabelle Gensburger – October 2011

Investigation of the Critical Parameters in the Production of Ceramic Water Filters

Isabelle Gensburger

October 2011

The flow rate can be increased by:
1. increasing the porosity of the filter, by increasing the quantity of burn-out material in the clay mix; and
2. increasing the pore size, either by

changing the particle size distribution of the burnout material, or by

changing the maximum firing temperature.

The bacteria removal effectiveness is only compromised when increasing the pore size

Download (PDF, 1.08MB)

Batch Consistency – Ceramic Pot Water Filters

At Potters Without Borders our focus is on the transfer of skills required to manufacture strong, effective ceramic pot water filters. Manufacturing filters as an appropriate technology requires the utilization of locally sourced skills and raw materials. For technicians, the main challenge is to go through the process required to develop a working filter body within a short time, using greatly variable materials. Although these materials may differ between factories, it is very important to maintain batch consistency when selecting materials for what is essentially a health product.

Establishing a reliable mixture of clay, burnouts, and accessory materials requires a period of testing and troubleshooting. Although working mixtures have been established in other factory locations, the information gained from these experiments only give us a general idea of where to start. It is not only necessary to begin factory production with a lengthy testing period, but every time a new batch of raw materials comes into the factory, testing must be done to make sure they are of consistent source and quality.

Developing a system of record-keeping and filter numbering is a critical aspect of manufacture troubleshooting. Factories that have been in production for years discover, suddenly, peculiar changes in filter functioning or appearance. By going back through the logs we can discover if the change occured because of a change of material sourcing, or a firing schedule fluctuation.

When selecting a source of raw materials, one must consider that it is a finite resource. A feasability study attempts to identify problems of inconsistant availablilty, contaminated sources, or insufficient material for the lifetime of the factory. The variable nature of naturally sourced materials makes it impossible to produce an identical product every time, but following certain procedures allows us to produce a greater number of filters which fall within the range of acceptability.

Production protocols which have been extablished in the first months of setup should be recorded and maintained. Any improvements, or changes to manufacturing procedures must be recorded and re-produced through several operating cycles in order to be proven.

Finally, we must realize that producing a homogenous filter is beyond the scope of appropriate technology as it applies to these filters. What we want is to produce an effective filter as simply as possible with locally sourced materials and abilities. Consistency in materials, sourcing, and testing increases the percentage of effective filters in each production cycle.

Ceramic Water Filters – Batch Proportions

The first four filters from the PWB filter press. All produced using 50-50 mix of clay and sawdust. From left to right are, numbers: 1/18/07-4, 1/18/07-1, 1/18/07-2, 1/18/07-3. Details as follows:

PWB Filter Batch Tests:

Ceramic Water Filter Test Batch No. 1
Thursday January 18, 2007

Mix Proportions @ approximately 50-50 by volume
Plainsman Red Earth Clay 44lbs.
Screened Sawdust 7lbs.
Water 21 lbs 11.5 oz. (42.55%)
(62.31 lbs total mix used for 4 filters or 15.58 lbs per filter)

Clay and Sawdust were placed in mixer and mixed for 10 minutes. Water was added and mixed a further 10 minutes.

20lbs. 3 oz. of the resulting mix was used as a charge for each filter.

Four filters were made from this mix and were marked with Iron oxide with the date.

These tests are forthcomming:

Ceramic Water Filter Test Batch No. 2
Saturday January 20th, 2007
Plainsman Red Earth Clay 44lbs.
Screened Sawdust 6lbs.4.8oz
Water as needed (20 lbs )
Mix Proportion represents 10% less sawdust then mix No. 1

Ceramic Water Filter Test Batch No. 3
Date to be set
Plainsman Red Earth Clay 44lbs.
Screened Sawdust 5lbs. 9.6oz
Water as needed (20 lbs )
Mix proportion represents 20% less sawdust than mix No. 1.

Ceramic Water Filter Test Batch No. 4
Date to be set
Plainsman Red Earth Clay 44lbs.
Screened Sawdust 7lbs. 11.2oz
Water as needed (20 lbs )
Mix Proportion represents 10% more sawdust then mix No. 1

Ceramic Water Filter Test Batch No. 5
Date to be set
Plainsman Red Earth Clay 44lbs.
Screened Sawdust 8lbs. 6.4oz
Water as needed (20 lbs )
Mix Proportion represents 20% more sawdust then mix No. 1

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