There is a tremendous focus on animal waste. This is true around the world, especially in Europe and the United States. Dairy manure gets the most attention, closely followed by swine. University studies and EPA pronouncements come out on a monthly basis. Much of this work is labeled “nutrient management”. This has to do with the how, when, and where of disposition of the various nutrients in manure.

If I were going for a PhD in Ag Engineering, I would do my dissertation on “The Role of Dissolved Solids in Manure”. A basic characteristic of manure is that the moisture fraction in it contains dissolved solids. This characteristic deserves special attention because the dissolved solids in a mass cannot be separated by mechanical filtration. That is, if there are 3% dissolved solids in the water in manure, there will be 3% dissolved solids in the moisture in the both the fiber and liquid which are separated by separation equipment. This is basic to all conventional equipment such as sidehill screens, rotary drum screens, screw presses, belt presses, centrifuges, etc.

This means that a dissolved nutrient in manure cannot be concentrated (let alone separated) by running the manure through any of the aforementioned equipment.

Vincent engineers have gathered some interesting data in regards to dissolved solids:
Scrape barn manure measured 6º to 7º Brix.
Pond water measured 3º Brix.
Manure from a flush barn measured 4º Brix
Effluent from a dairy manure digester read about 3º Brix.
Human urine reads a real sharp 2º Bx.

This has an important effect on the moisture content of the press cake produced when manure is run through a screw press. We have seen manure press cake from a scrape barn that contains 68% moisture and 7 Bx. In round numbers, if before pressing this  same manure were diluted (by flushing) with pond water to where it had 4º Brix, then the press cake would measure 70% moisture.

In other words, the cake from flush barn manure will typically be two percentage points higher in moisture content than that from a scrape barn. This is because the moisture does not carry with it as many dissolved solids.

Separately, we queried Dr. Robert Burns of Iowa State University about the dissolved solids in manure. His reply has good insight on digester operation:

“You are on the right track in thinking about the bugs eating sugars. In reality, they are consuming carbon as a food source. Some carbon sources (like sugar) are very easy for bugs to utilize, while other, more complex carbon sources are harder to digest, and some may even be recalcitrant to the point that they can not be digested within the HRT that a given digester operates at. Unlike fruit juice (where most of the dissolved solids are sucrose), manure contains lots of complex compounds (like proteins, carbohydrates and lipids), so I believe that your hypothesis that the dissolved solids you are seeing with your refractometer are a mix of digestible and undigestible dissolved solids is the correct one.

“The question of should you press manure before or after digestion is one that can be answered pretty easily. My lab regularly runs Biochemical Methane Potentials (BMPs)which are an effective method to determine the anaerobic degradability of a given substrate. We run these on a fee basis for companies all of the time to determine what mix of substrates will yield the most biogas. You can take a look under the AD portion of our webpage https://www.abe.iastate.edu/daniel-andersen/dan-andersen/anaerobic-digestion-of-agricultural-wastes/ for more info. It would be pretty simple to run some BMPs on raw manure and then run some on the press liquor to see which yields more biogas.”

Robert T. Burns, Ph.D., P.E.
Professor
Agricultural & Biosystems Engineering
Iowa State University
3224 NSRIC, Ames, Iowa 50011
Email – rburns@iastate.edu https://www.abe.iastate.edu/daniel-andersen/dan-andersen/agricultural-waste-management-laboratory-services/

Note from Pressing News #190:
Brix is a unit of measurement named after Adolph Brix. It is used commonly by food technologists to measure the amount of sugar dissolved in water. It can be calculated by dividing the dissolved solids by the sum of the dissolved solids plus the water, all multiplied by 100. That is, Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water. ((Note that suspended (or insoluble) solids do not enter into the equation for calculating Brix.))

ISSUE # 221