Archive for the ‘Suction Harvesting’ Category

We Begin Suction Harvesting

Thursday, June 26th, 2008

In deference to the spawning habits of the resident fish (and the restrictions of our State permit) we wait each year until late June to begin suction harvesting.  The divers are still actually hand pulling the milfoil, scooping up roots and silt to remove the whole plants.  They then thrust the mass of root, silt, and plant into the vacuum hose, which sucks it up through the boat mounted pump.  It is spewed out of corresponding hoses into what we call the ‘trash rack,’ a sort of floating strainer which we can tow up the boat ramp and dump at the end of the day.

DEPLOYING THE HOSES FOR SUCTION HARVESTING

While milfoil plants are being fed into the hoses by two divers on the bottom, the other two stand in the trash rack and scrape the plants and fragments from the screen to keep it from clogging.

THE SUCTION HARVESTER AT WORK

The trash rack is a utility trailer equipped with an hydraulic dumper and fitted with styrofoam floats for buoyancy.  When full of wet milfoil it is very heavy.  We dump it daily when it is in use.

EMPTYING THE TRASH RACK ON A CHILLY MORNING

A New Silt Screen

Wednesday, August 15th, 2007

Ripstop nylon pocketA silt screen is used to contain loose fragments of milfoil and sediment churned up from the bottom during suction harvesting. This summer we fabricated a new silt screen, a great improvement over what we had been using.

50 foot sections of siltscreenThe whole apparatus consists of four fifty foot sections of fine mesh, with flotation at the top and weighted at the bottom. The mesh is a heavy weight nylon window screen, sold under various trade names and intended to keep Fido from scratching his way through a screen door. Or to leave the screens undamaged when Kitty climbs them.

Swimming noodlesFor flotation we chose foam swimming “noodles” of the kind widely available every summer. We used ripstop nylon fabric to attach them, looped above the screen to form a long pocket.

Tape for pullingWe sewed a strong nylon tape in a loop just below the float pocket which could be used to move the net segments around in the water. Below the tape you can see two of the grommets which can be used to link the siltscreen segments together.

Anchor tie pointThe bottom edge of the siltscreen is weighted with a length of stainless steel cable, sewn into a fold in the bottom of the netting. This picture shows one of the tie points to which we attach an anchor to keep the system from drifting.

Here is a shot of several sections being towed across the lake.

In the water

Suction Harvesting

Tuesday, July 3rd, 2007

Suction Harvester 6934 Where the milfoil growth is thick hand harvesting is not adequate. Since each plant has to be pulled at the roots and every part of the plant has to be completely removed from the lake, we employ a device like a huge underwater vacuum cleaner, called a suction harvester. A high volume pump mounted on the pontoon boat sucks water through two long four inch hoses.

Divers with Hoses 6929Divers with scuba gear take these hoses down to the root end of the plants. They can pull the plants with the roots, and the whole thing is sucked into the hose and whisked away. One of the continuing problems working on the lake bottom is diminished visibility when the silt on the bottom is disturbed. Pulling milfoil plants severely clouds the water, but the suction harvester pulls the silted water away too, making it much easier to see.

Trash Rack 6951The hoses empty out into a floating contraption attached to the pontoon boat, which the divers colorfully call the trash rack. I am not sure the name describes its function or its condition. In any case, the streams of water carrying milfoil and silt spray against its perforated sides. The milfoil stays inside, and the water escapes.

Climbing in 6939 Scraping 6944The two divers who are not at the moment underwater picking milfoil and stuffing it into the hoses climb into the trash rack and scrape the milfoil from the screen, keeping the openings from getting clogged with debris and the milfoil waste from overflowing into the lake.

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