Optimising Ceramic Water Filters for the Developing World - Part 1 by J. Auton

Summary Experimental design

Ceramic water filters provide households in the developing world with a cheap and simple method of purifying their water for drinking. However, many families complain that the water doesn’t flow fast enough to provide for all their needs.

• Both pot and disc-shaped filters are used in developing countries, but this study is based upon the pot-shaped design by the charity Potters the Peace

This project aims to increase their usefulness by exploring three methods of increasing the flow rate through the filters: reducing the thickness, increasing the porosity (via changing the clay mixture) and increasing the head.

• Financial and time constraints meant that I decided to run the majority of tests upon discs, but also make 2 pots to “scale up” the flow results to compare to Potters for Peace figures.

The preliminary results so far show that both thickness and porosity can increase the flow rate without impairing the water quality (so it is still safe to drink). However, increasing porosity is likely to be more effective than reducing thickness, as filters which are too thin are likely to break. Design of an insert to increase both the capacity and the head is ongoing.

What is a ceramic water filter?

Figure 1: Water is collected from streams, rivers or wells depending upon location and season

• Dirty water is a major cause of disease in the developing world

Ceramic filter element

• Ceramic water filters are made from a mixture of clay and sawdust (or some other combustible material). The sawdust burns away during firing to create pores for water to flow.

• Dirty water is cleaned by 3 mechanisms: - Direct filtration of particles on which bacteria can grow (but this means that the filters can clog over time and require regular scrubbing)

Receptacle for safe storage Tap dispenses clean water

- Adsorption onto the surface of pores, and chemical reactions with the clay

Table 1: Comparison of pot and disc designs Pot filter Disc filter Usage

Closest to actual usage by charity Potters for Peace Still need to make a few pots to “scale up” flow results

Clay volume Production method

Uses 6kg clay per pot Complicated: large plant pot used as mould, with a base section, walls made in two sections and a lip Discs require only 1kg clay each, hence reducing cost and mixing time. Very simple: clay/sawdust mixture is rolled out and pressed by hand into a disc-shaped mould.

6-7 days

Drying time before firing

9 days, as lip has to be added after removing from mould.

Firing (takes 2 days to heat and cool)

A small kiln owned by retired potter Derek Andrews can only fire one pot at a time

Filters made

Can fire 9 discs at a time on shelves in kiln - finely ground (colloidal) silver is added to pores after firing, an antibacterial agent believed to work by catalysing a reaction with cell walls.

• The cleaned water is stored in a bucket beneath the filter, and dispensed via a tap to prevent contamination from dirty cups or hands •

Advantages:

Cheap and simple to use, using local materials and skills which creates new business opportunities for local potters •

Disadvantages:

The flow rate is quite low (1-2 litres per hour), clay filters are vulnerable to breakage and users must be educated to prevent unwitting contamination during cleaning.

Figure 2: A ceramic filter in the home cleans the collected water ready to drink Figure 3: Filter elements can be pot, disc or candle shaped, but all use the same basic two-bucket filter system

14 disc filters made originally, plus 4 to replace broken filters

Changing mixture: (13mm thickness)

0%, 20%, 40%, 50%, 60% (x2 filters each)

Changing thickness (all using 40% sawdust by volume)

10mm, 13mm, 18mm (x 2 filters per thickness)

2 Pots:

13mm thickness with 40% sawdust

Notation: xx1 and xx2 denotes xx% at normal thickness eg 601 = 60% sawdust

Optimising Ceramic Water Filters for the Developing World - Part 2 by J. Auton

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Flow Results

As predicted, the higher porosity filters produced much higher flow rates with 60% being the best option.

Tests on filters with 0% sawdust illustrated that the sawdust is the major contributor to porosity, rather than natural pores due to firing at low temperature Several filters broke during preparation and testing, illustrating the problems of brittleness Pores get blocked over time, requiring scrubbing to rejuvenate flow rate Adding silver reduces the flow rate

Quality test results

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Water from the River Cam was passed through the filters over several weeks, before and after addition of silver

All filters show almost 100% reduction in total coliforms (bacteria which indicate faecal contamination and hence whether water is likely to cause disease) before and after adding silver There is a clear difference in pH between filtered and unfiltered water, indicating a chemical reaction within the pores Turbidity (quantity of particles suspended in the water) is reduced on average by 70-80% Zinc content was found to be too low to measure in river water Silver may possibly be leached from the filter pores: this will be measured in the next few weeks.

Capacity increased by 150-250%

Downward force (eg from a brick) onto cylinder cover to resist uplift

O-ring or other waterproof seal

Upwards water pressure on seal

Head increase using insert of 20-40cm Max head across filter = 20cm using bucket Filter flow rate increased

Design challenge:

Need to increase both HEAD and CAPACITY to make filters more useful Cylindrical insert design needs to be: • Made locally from cheap materials • Increase head across filter • Resist water overflow out of bucket