A reed bed should work regardless of the external rainfall conditions. However you may need to build it a little smaller than the Irish Codes suggest so that your evapotranspiration is limited in extended periods of very hot dry weather. To compensate for the smaller size you may wish to put in a percolation system after it that is able to handle either the most extreme droughts expected or the most extreme rainfall conditions. Ideally use a series of trenches following teh sizing specified in your local codes, but instead of 4" perforated pipes, place gutters face down on the trench base. You can make the gutters out of twin-walled 1' corrugated piping cut down the middle to form two runs of pipe 6" high each. Cover these over with soil, plant trees that will survive dry conditions, but which will appreciate the moisture nonetheless. If you want edibles, then be sure that everything you flush down to the system is food grade. Cleaning chemicals, detergents, cosmetics, soaps, shampoos etc.
Feidhlim Harty wrote:John, do you have a wetland or reed bed and do you notice any seasonal variation in effectiveness based on the variability of your rainfall patterns?
One end of our pond has a shallow shelf that I have planted in cattail. If the pond is full that area is about 7cm deep. However by August it can be dry bottom. Depending on seasonal variations the pond depth can drop from 3cm to 9cm.
William, by the sounds of it John's system is always moist enough to support Cat-tail, which is a pretty standard plant of soil based constructed wetland systems. I wouldn't bother increasing the grey water output - because it will coincide with a time of year when water is scarce and you should be conserving it all the more, or even putting it out on pots in the patio etc. I'd simply build a wetland that can cater for the variations in seasonal inputs.
For this you may be as well to use a 1' depth of soil over the clay or plastic liner base, rather than the standard 150-200mm soil depth. This is to retain a residual amount of water for when the season turns dry. You could always raise the outlet flow control unit in the last month prior to an anticipated dry spell, to maximise the retention of water in the system prior to the summer.
Let me know if you've any questions on how to go about it.
Is there a specific type of vegetation that is less likely to dry out the reed (or reed substitute) bed?
I want some sort of reed bed/drain field to deal with the output of a worm composting sewage system and in summer I fear reeds would die unless they receive extra irrigation. We get upto 9 mths with no rain, sometimes as little as 100mm rain a year, and temps upto 45C in the middle of the dry season. Would reeds be ok?
I'd actually bypass the reed bed element completely and use the effluent as an irrigation source within my garden. If you lay the piping out following standard codes for the surface area, then you should be ok legally. Then if you use a ribbit.ie splitter to get the effluent exactly where you want, you can use it to water your trees. Note that it should only be used as part of a subsurface system, so that you don't have contaminants at surface level.
If the runoff from the composting system is very concentrated you may have an issue with burning the plant roots with excess nitrogen. How much liquid volume would you expect to get in a given day? Is it all the water from the toilet? If so, that should be ample to dilute the nutrients present.
If this sounds appealing, let me know and I'll elaborate a little on the set up. Or visit oasis.net for more background. Art Ludwig's site on grey water reuse is a mine of information.