Notes from Windward: #66


A Tale of Two Tanks

adapting two septic tanks to other uses

     When we have heavy equipment brought in to Windward, we have to pay "travel time," and so we work hard to set things up so that we can get a full day's work out of the machinery. In this case, we're talking about a machine special built to dig ditches and holes, and an operator who is highly skilled at making it do exactly what he wants it to. Watching Rick today, you'd have no problem believing that the could make that machine dance. It's always fun, and humbiling, to watch a real professional use heavy equipment to move a lot of earth.

Rick starts to dig the hole for the propagation green house tank

     We first had Rick dig up the gas line to the kitchen, a project which deserves it's own article. Once the old line was exposed, Rick moved down the hill to dig two holes large enough to each take a 1,250 gallon concrete septic tank. Since a tank of that capacity is nine feet long and six feet wide, that's a sizeable hole.

     The first tank will go under what will be our propagation greenhouse. This is where we'll take cuttings of plants such as lavender, root them and prepare them for planting in large beds. There are many plants, such as lavender, which can be grown from seed, but you're never sure of what sort of plant the offspring of a plant will prove to be -- with a cutting, the second plant is exactly the same as the first.

     Before the tank is set down into the hole, the bottom will be insulated with dense foam supported by treated lumber, the sides will be encased in dense foam, and then the entire package will be wrapped with that clear wrapping plastic that's used to hold pallets of goods together during shipping. Once it's properly located in the hole, the heavy, uninsulated concrete lid will be set in place. Later the green house will be assembled over the tank, and the tank filled with water.

Loading the excavated dirt into the dump truck

     Solar hot-water heating panels will be mounted along the peak of the roof of the greenhouse. When the sun shines, a solar panel will generate electricity to power a DC pump which will push water from the tank up and through the solar panels where it will be heated and then drain back down into the underground insulated holding tank.

     Because the holding tank is not insulated on top, and since the top of the tank is also the floor of the greenhouse, the heat stored during the day will rise up through the floor to keep the greenhouse warm at night.

     Here's the math on how that works. Solar incidence here is about a kilowatt per square meter. We'll have four square meters of collector space over the greenhouse gathering heat for about eight hours a day in the fall and spring, less of course during the heart of winter. In other words we can look for our insulated hot water tank to store a bit more than thirty kilowatt hours of heat. If we find we need more than that, we can always add more collector area or tap some waste heat from one of the power generation systems.

     Water stores a BTU per pound per degree, and concrete about half a pound per degree, and there are about 3,400 BTUs in a kilowatt, so since our 1,250 gallon concrete tank holds about 10,000 pounds of water, we'll be able to store about four kilowatt hours of heat per degree of temperature rise in the water stored in the underground, insulated tank.

the surplus dirt was hauled over to be used
for backfilling the east side of the cabin

     When the sun goes down, the power to the pump will shut off thereby allowing the water to drain back down from the roof panels and into the underground tank. That's necessary because a solar thermal panel radiates heat at night just as well as absorbs heat during the day, a factor which exposes the panel to the risk of being damaged by having water freeze and burst the copper tubing. With no water in the tubes, there's no danger of freezing, and the water won't return to the panel until there's enough solar radiation hitting the greenhouse to power the pump, which is to say, enough to warm up the panel. It's a design that's primarily passive, but which uses just enough renewable energy to insure that the system isn't damaged by the cold night sky.

     The surplus dirt was hauled over and dumped at the back door of the cabin to be use to backfill around the foundation.

Digging the hole for the mini-aquaponics' fish tank

     Once that hole was dug to size, the track hoe moved over to Vermadise and dug another similar hole, only not quite so deep. This septic tank will be used to hold the main body of water circulating in the mini-aquaponics system that we're building on the downhill side of Vermadise. Once we have the system parameters, such as pH and dissolved oxygen, stablized, we'll use this tank to grow koi, a fancy type of goldfish.

     Goldfish are carp and able to prosper in ground temperature water, a trait which is not true of the talapia which we'll be growing in our full-scale aquaponics system -- which is one of the reasons why it's important to first master the simpler systems before undertaking ever more complex systems.

     Carp will survive in ice covered water, whereas tilapia like water that's 75°, don't do well in water that's colder than 65°, and die when the water gets below 55°. That's also why tilapia aren't a threat to local fisheries the way they are in the many Southern states such as Florida and Texas.

     While the primary purpose of the mini-aquaponics system is to provide a real-time, hands-on learning experience, it's also good to create the system in such a way that it will be able to make a positive contribution to the food needs of the community.

using the surplus dirt to backfill
the east side of the cabin's foundation

     A core characteristic of sustainable systems involves being able to use the output of one activity as the input for another. In this case, surplus dirt generated by digging the second hole was loaded into the dump truck and hauled up to the dining hall and used to add more earth to the dining hall's earth-sheltered northern side.

     Once the fish tank hole was dug, the track hoe moved up to the cabin where it used its bucket to tuck the surplus dirt from the first hole up against the cabin's foundation.

Notes From Windward - Index - Vol. 66