July 22, 2019

In general, screw presses separate the “free” water from a flow, producing a fibrous press cake. The cake will hold what is referred to as “bound” water.  The cake fibers will hold water on the fiber surface, and also as water which is bound in the organic molecules of the fiber.

We frequently are asked to estimate the percent moisture which will be left in some press cake. Unless we have actual experience with the material involved, it is hard to predict with accuracy. That is why we run tests in Tampa on a weekly basis.

The case for dairy manure shows what we are up against. We know that manure press cake, as a rule, will have a moisture content of 65% to 70%. But it can easily fall out of this range. For example, calves are feed milled feeds, with smaller particles, so it is hard to get the moisture out of their manure. At the same time, if sawdust is the bedding used for the lactating animals, the low moisture content of the wood which gets blended in can result in press cake of only 60% moisture.

There are a lot of variables which affect press cake moisture content at a dairy farm. A given farm is either flush barn or scrape barn, and most do not have reliable control over the amount of milking parlor water or flush water being added at any time. When we go from one farm to another, other major considerations take over: diet of the cows, type of bedding, freshness and agitation of the manure, etc.

There are two variables which we know tend to offset each other:  (1) Flush barn manure will have a lower concentration of dissolved solids in it (compared to scrape barn) because solids will have been diffused into the flush water. That would tend to reduce the solids content of the press cake being produced. And (2) flush barn press cake will have a larger average particle size than scrape barn manure because the fines tend to get washed out with the press liquor during the pressing process. That would cause press cake from a flush barn to come out drier than scrape barn manure because the fine particles offer extra surface area to which water can attach. The relative weight of these two offsetting factors can only be guessed at.

The good news is that Vincent uses the interrupted flight screw design. That makes the press operate satisfactorily within a wider range of inbound consistencies. We do not change the recommended screw or screen selection of a manure press based on flush barn vs. scrape barn considerations. (Pressing News #105 describes this screw design.)

The proof of the pudding: Last year Vincent’s and a another firm’s presses were run side-by-side on effluent from a biogas digester. The other firm’s presses worked very well at times. But they were sensitive to changes in the consistency of the inbound flow. The project managers settled on the Vincent presses because they did better when the operating conditions varied.

Checking performance records, we see that we have run our KP-16 presses on dairy manure with everything from 35 to 800 gpm of press liquor being produced. The solids in the press cake all fall in a normal bracket despite such severe variations. But we are not able to predict in advance at what end of the bracket a given farm will fall.

Issue #315