Separation of Bedding Sand from Manure
By Shaun Jones
Environmental Science and Engineering
The rising costs of fuel and electricity affect everyone, and with farmers facing several economic
challenges, there must be a way for farmers to ease this burden. Anaerobic digesters convert a waste
product, manure, into methane rich biogas that may be burned to produce electricity and heat. In addition,
bedding choices affect the performance of digesters treating dairy waste. While organic bedding (e.g. saw
dust, straw or paper) represent a substrate that could be partially converted to biogas in the digester
inorganic bedding (e.g. sand) is non biodegradable and will settle within the digester. The benefits of
using sand, as cited by farmers, are increased cow comfort which results in increased productivity and
reduced diseases occurrences. In order for sand laden manure to be used with anaerobic digestion the sand
will have to be continuously removed from the treatment system in order to maintain the digester reaction
volume. Common sand separation processes such as screw presses rely on the dilution of the manure for
separation to occur. Since anaerobic digesters are designed by hydraulic residence time any dilution of
the incoming feed stream with extra water to the digester will result in an increased reactor size thus
increasing capital cost of the system.
The hypothesis of this research is that digester effluent, which has similar fluid properties than
water, can be used for dilution during manure/sand separation. Using digested effluent in lieu of clean
water would have no effect on the digester sizing and therefore would represent an economic method to
remove sand in a hybrid sand separator/digester system. Using the Tru-Grit Manure Sand Separator
(MSS) from Parkson Corporation, which is a commercially available sand separator, the main objective of
this study is to evaluate the effects of using digester effluent (instead of water) on sand separation.
The experimental matrix consists of four different sets of experiments: (1) Using the MSS unit in
its original operation as base case where manure enters the top of the unit and equal flow of water is
introduced from the bottom, (2) pre-mixing manure and water at a 1:1 dilution, (3) pre-mixing manure
and effluent at a 1:1 dilution and (4) introducing gas through the bottom of the unit that is charged with a
1:1 mixture of manure and digester effluent. The first set serves as a control of the unit with manure
entering at the top and water being sprayed through 1/8 inch water jets from the bottom resulting in a 1:1
dilution. A vertically mounted agitator rotating clockwise cause suspended solids to be pulled to the
surface to allow for additional separation time. The reactor volume of the MSS unit is 120 gallons; the
two runs of each test were run for 10 minutes each, at a manure flow rate of 6 gallons per minute (i.e.
total flow rate of 12 gpm at 1:1 dilution), resulting in one bed volume exchanges per run. Samples were
taken every minute; the samples were then analyzed for total solids, volatile solids and non-volatile
solids.
The results of the base line test show that 43% of the sand within the manure was removed. This
is considerably less than the expected removal percentage of 98%. One possible reason for the low
removal could be that approximately 200 pounds of sand per 225 gallons of manure settled out during
transport between the farm and Clarkson University. In the second test, 1:1 dilution with water, 40% of
sand was removed. Influent non-volatile solids content (i.e. sand) varied between 2.5 and 12 %. while
effluent concentrations decreased from initially 5% to below 3% over the course of one experiment
(Figure 1). Sand content of NHD manure has been measured to be between 10-15%. In order to have
minimal sand settling within the digester a target concentration of approximately 1% was established,
This target concentration could not be achieved during the short run-times. I
Without longer run times, it is difficult to determine any final trend. It is however possible to
compare the four different experimental conditions among each other. Baseline removal was considerably
less than anticipated, but the results of the second test do match. It is anticipated that this would continue
with a longer run time of the test as more sand would settle from both sets of samples. This just shows
2008, Environmental Engineering and Science, REU, Dr. Stefan Grimberg
that it is possible the target concentration could be met over time, if the pattern stayed the same over time.
With the current results and patterns, it does appear that the digester effluent can be used for dilution,
proving the hypothesis. Longer tests must be performed to confirm that observation.
Figure 1. Sand content of influent and effluent streams of the MSS. Influent manure stream was diluted
with fresh water at a dilution ratio of 1:1
2008, Environmental Engineering and Science, REU, Dr. Stefan Grimberg