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Source: Homespun Environmental – Russ Michaud, Truth Sponsor & Contributor
This paper discusses the two main styles of ceramic filter cartridges that are currently on the market. These are the ‘candle’ filter and the ‘dome’ filter. I often get questions on whether one is better than the other and have also seen several DIY blogs on the internet showing both of these. I figured it was a good idea to write a short paper on this topic to add as a reference to the technical notes on my website and to help folks interested in researching this.
For those who may be unfamiliar with ceramic water filters, here is a brief overview.
The ceramic water filter cartridge consists of a ceramic shell, containing colloidal silver and enclosing some amount of activated carbon. The shell has very small pore openings that allow water to pass through but are too small for bacteria. The silver keeps bacteria from growing on the shell, and the activated charcoal adsorbs chemicals that are present in the water. This allows for filtering of particulates, organisms, and chemicals in a simple, low-cost, package. These filters have been used by NGOs and missionary groups throughout the world with good effect.
More details can be found here: http://www.homespunenvironmental.com/v/vspfiles/Low%20Cost%20Water%20Insurance.pdf
The picture to the left shows a standard filter set up using two buckets.
The chemical and bacterial filtering each last for different amounts of time, the bacterial filtering that is done by the pores in the ceramic will last as long as the shell is intact which can be a few years. The chemical filtering which is done by the activated carbon is a lot shorter and depends on the amount of carbon in the filter but normally is less than a year. When discussing the cartridge’s longevity the shorter chemical lifespan is the figure that is given.
Ceramic filter types are described by their dimensions: base diameter x filter height, (in the US these dimensions are given in inches). These filters are also commonly named for their shapes. Tall and skinny 2×10 filters are called ‘candle’ filters, while the shorter, rounded 4×4 filters are called ‘dome’ filters. These are shown in the pictures below.
The filters all share the same shell and carbon design and will provide the same basic functions. The big differences are the surface area of the shell, which correlates to how much water can flow through them in a given amount of time; and the space that is enclosed, which limits the amount of carbon that the cartridge contains (and directly sets the chemical lifespan of the cartridge).
Here is a table that compares the dimensions, surface area, and space enclosed for the candle and dome filters. (Notes for those who want to check these figures: the candle is a 9 inch cylinder and 1 inch dome, the dome filter is a 2 inch cylinder and a 2 inch dome, and that the wall thickness of these filters is approximately one quarter inch.)
Two things to be noted here are that the candle filter has approximately 25% more surface area and thus should have 25% more water flow than a dome filter. Also very importantly, the dome filter holds almost two times as much activated carbon as the candle filter and thus the chemical removal should last about twice as long.
An interesting point on the candle filter is that it often is not totally submersed in the water because it is so tall. When this is the case the surface area of the shell, and activated carbon above the water are not being utilized. (This also happens with the dome filter but to a much lesser extent.)
A simple way to improve the candle filter’s utilization is to install it into containers horizontally instead of vertically. This would allow for full use of the filter down to 2 inches of water. Unfortunately this is not possible in standard buckets as the candle filter is too large to be installed horizontally.
A simple solution to improve the flow rate of the dome filter is to install a siphon hose on the outlet spout. The increase in the flow rate is proportional to the vertical height of the hose. For a bucket system the hose can more than double the flow rate. Note that this siphon effect will also work on the candle filters. For those interested in researching this further you can look at the siphon effect here: http://en.wikipedia.org/wiki/Siphon
While both candle and dome filters have advantages, the dome filter seems to have the edge as the lower profile means that it is used at its maximum efficiency more of the time, it has almost twice as much carbon, and the flow rates can be increased using a simple siphon tube. If the candle filters are used, it is better if they are used in a horizontal configuration to ensure maximum effectiveness. Whichever filter shape is used a basic conclusion is that ceramic water filters are a good way to ensure potable water availability, even in the worst of times as has been proven in disaster conditions around the globe.
Homespun Environmental is a niche store specializing in practical water filter components and kits for the DIY’er. This allows our customers to realize substantial savings over factory made systems that many of our competitors offer and also allows for a high degree of customization of systems based of specific needs and local conditions. Along with water filter kits and components, technical details and application notes can be found on the company’s website: www.homespunenvironmental.com .