Tribewanted: Vorovoro

Hoy mates – I wanted to get this out ASAP before construction on the new reed bed is begun… while the current vertical flow reed bed is pretty cool, it is designed as just a small insitu mechanical treatment system. This is actually what most water treatment plants use on a larger scale – the one biggest problem with mechanical purification is clogging of the filtration media (the rocks). In large-scale applications, this is solved by providing a backflow device – water is forced up backwards to clean everything out every so often. It is also important to realize that the water will undergo very little – if any – chemical treatment. The main reason to use a reed bed – or more commonly called – constructed wetland treatment system, is because of the incredible mechanical and chemical treatment from the water. I am familiar with the first system ever built on an island, and the outflow at the end of the system is actually cleaner than tap water! In our current design, we might as well plant poupou or pineapple underneath the outflows because it isn’t actually getting any real benefit from the plants – it’s really just watering them. Enough with the problems, on to the basic design.

The biggest idea in the design of a proper wetland treatment system is the need for laminar flow with a really long residence time. This means that a guess must be made on the actual volume that will need to be treated… well, not really the volume, but instead the max flow of water. The idea is to create an inflow, a shallow basin in the middle, and then a point outflow location at the far end – forcing the water to travel horizontally through the system – with enough time for the plants/reeds to eat the critters and clean the water. The major design criteria here are a residence time of 24 hr minimum and a maximum velocity of 1 ft/second. To use these we need to make a assumption on the maximum volume of water going in to the system – any guesses??

To build the system, it can be set up in the same general matter… but instead of a deep pit, open to the barrel in the ground, the pit should be 2’ deep and at least 2x longer than it is wide. for the inflow, a pipe can be placed along the far edge of the box. There is really no need to use different sizes of stone as it isn’t necessary – clean sand (so that we aren’t making salt water in our reed beds and killing plants in the process) should be fine for the full depth. The box needs to be water proof for the whole box. At the far end of the box, a hole can be cut to allow for outflow. The hole should be covered with mesh to prevent sand from migrating out of the box, and the hole should only be below the height of the input by a short distance (maybe 1 inch). We don’t want water that flows in to be able to quickly flow through the box and right out the outlet, because we want our minimum 24 hours of residence time (the total volume of the box would flow out in a period of 24 hours – for example, we have an inflow of 200 gallons per day. To get a residence time of 24 hours, we need a volume of 200 gallons – about 27 cubic feet. Using our box 2x longer than wide, 2 feet deep, it would be about 2.75’ by 5.5’. Realistically, we’d want the volume to be about 2x that in order to double the residence time, and probably closer to 3x or 4x longer than it is wide.) At the other side of the outflow, we could still bury our drum in which to collect the water and then water the gardens. Plant selection is important as well – the ones we have in our bed are pretty good, but do you know what might actually be really good for the inflow side of the new box? Corn!

So in order to really determine the required sizes and design we need a maximum volume to treat… what is the proposed reed bed being used for in the village? Is it treating shower water as our is? If so, how often are they showering and how much water do they use??