Sampling a manure treatment lagoon
By: Ted Funk
There comes a time in every lagoon’s life that somebody is going to want to know what’s down there—beneath that murky purple surface, that bottom that hasn’t been exposed since the beginning of time. It may be that there is not even a record of how deep it was dug, or how it has been modified since.
There are good reasons to know: (1) how much working volume is remaining above the sludge blanket; (2) how far down the bottom is; (3) how many feet thick the sludge blanket is; (4) the nutrient content of the sludge blanket. An adequate working volume keeps the lagoon perking along with relatively little odor. Knowing the depth and nutrient content of the sludge in the lagoon allows you to plan for land-applying the lagoon contents at the appropriate nutrient rate. The lagoon sludge is a good, stable fertilizer, and does not have much odor compared to pit manure.
Now for the trick—how do you plumb the depths? Most producers have or can borrow a small fishing boat or canoe to paddle out onto the lagoon, but the equipment for getting depths and samples is another matter.
If it’s just depth information you need, there are two ways to find out—sonar, or a probe. A fish finder sonar can give you the approximate depth to the top of the “sludge blanket”, which has a fairly dense and abrupt change at the top but a soupy layer gradually increases in density the deeper you probe. Sludge depth estimates then assume that you know the total excavated depth of the lagoon. The manual method is to make a disk, 12” diameter or so, on the end of a probe, or on the end of a short rod (3-4 feet) tied to a rope; the flat disk lowered slowly in the liquid gives a better feel for sludge density differences as you lower the probe. The disk can be a 5-gallon bucket lid, for example; add some weight to get it to sink reliably.To get the depth to the lagoon bottom, you’ll need to use a small-diameter rigid probe to feel the bottom. (Be careful with the probe if the lagoon has a plastic liner!) My colleagues and I have used lengths of ¾” PVC conduit, hooked together with threaded fittings glued to the conduit. Thin-wall ¾” metal conduit would work also.
For the depth records, be sure you benchmark the top-of-liquid on the day you sample; otherwise you’ve introduced an unknown in your depth data. Get your bearings in the sampling boat by lining up your depth-measuring locations with points on the lagoon berm, and try to obtain at least three or four evenly-spaced sampling points across the lagoon.
The gadgeteers out there may prefer to make a remote-controlled lagoon sludge boat with a GPS-linked fish finder. By piloting the boat across the lagoon a few trips and putting the data collected in the right software, you can produce a map that shows the top of the sludge blanket across the entire storage. The same technique can be used to check the sludge level after de-sludging is done.Plus there is something to be said for the safety of having a “drone” boat do the work instead of paddling across the lagoon yourself. Contact the author if you want ideas on making your own boat.
While you are out there in the boat checking depth, it’s a good idea to sample the sludge. You’d be surprised how much fertilizer value is in the sludge layer—often ten or more times the phosphorus concentration that is in the liquid on top. But getting the sample is not a trivial task. There are a couple of ways to go at it, one being a tube with a “remote stopper” such that the bottom end of the tube can be opened at any depth desired and the tube will fill with the liquid from that depth, after which the stopper is replaced to capture the liquid in the tube. The tube is pulled up to the surface and the contents emptied into a sample container. Ideas and pictures of samplers can be found in the Midwest Plan Service publication “Manure Characteristics” at downloadable or paper copy for only $9.00.
For those of you who have a lot of liquid sampling to do, including pit sampling, you might prefer another way: put together a battery-operated vacuum pump, a liquid sampling trap, and a vacuum hose (such as a length of milking machine hose) with a weight on the end, to sample lagoon liquid and sludge at various depths. Working with vacuum “plumbing parts” is unfamiliar to some, so if you have questions on how to make a vacuum sampler, feel free to contact the author.
What to do with the information? Sludge depth and sludge nutrient contents of a lagoon vary with depth, and with distance from the inlet pipes. The sludge depth will not vary too much across the width of the lagoon once you get away from the inlet(s), because wave action and gravity tend to even out the solids as they slowly settle to the bottom. Sludge nutrients—nitrogen and phosphorus--get steadily more concentrated the deeper you sample in the sludge blanket. Potassium is soluble and fairly constant throughout the sludge. Get the surface liquid and sludge samples run at a lab, then use your sludge depth information to estimate the total quantity of sludge.
If the sludge depth is substantial compared to the overall depth of the lagoon, you may want or need to de-sludge the lagoon this fall. Large lagoons cannot be agitated completely from the lagoon bank, and a floating barge is the answer for getting the job done. Two types of lagoon barges are available. One has a cutter boom that “strip-mines” the sludge as the barge travels back and forth across the lagoon. The mined sludge is pumped to shore and land-applied directly via tanks or dragline, and the thin liquid above the sludge is left behind. The other type is a floating agitator that shoots a jet of lagoon liquid downward and stirs the sludge with the lagoon supernatant, making a more uniform liquid product that,before it settles again, is constantly pumped from the lagoonwith separate equipment and land applied. Both types of equipment are available via custom manure haulers who work in Illinois. You can find contact information through Uof I Extension at see the “commercial haulers” link.
De-sludging a lagoon is a substantial task that requires planning to line up the crop acreage needed. You may be surprised to find how much good fertilizer is resting at the bottom of the storage, and how much better your lagoon works once you get the sludge out and the working volume back. But the first thing is to find the depths and nutrient contents, and that means sampling.
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