Bedding dairy cows on sand improves animal health resulting in higher milk production and overall farm profitability. The resulting sand-laden dairy manure (SLDM), however, complicates manure management, causing premature equipment wear, clogging pipes and settling during storage. Sand separation systems (SSS) remove and reclaim sand from SLDM. The increased handling of manure and the addition of dilution water associated with the removal of sand alter the manure characteristics. Efficiency of sand separation and the resulting impact on anaerobic digestion (AD) has not been extensively evaluated.
The objective of this research was to determine the impact of sand manure separation on AD. To accomplish this objective, a technique to estimate separation efficiency first needed to be developed. The separation efficiency of sand, in combination with an understanding of the residual sand characteristics and the loss of volatile solids (VS), allows for solid's balances to be determined across the entire sand separation system (SSS) and AD. This balance can then be used to predict impact on AD performance. To verify the predictions, comparison to a full-scale, operating digester was conducted.
Mass balance was found not found to be possible due to the unstable flow rates of several SSS inputs and outputs. Consequently, a semi-empirical evaluation technique was developed that required a combination of industry standards and on-farm measurements. For the test farm, Green Meadow Farms, the overall fixed solids (FS) separation efficiency of 91 to 99% was estimated.
The average sand particle size remaining in the manure following the SSS, residual sand, was determined to be between 0.18 mm and 0.21 mm. Installed mixer power, theoretically could achieve the scour velocity for the residual sand average particle size, indicating that settling should be minimal. This was confirmed when one AD tank was emptied after fifteen months of operation revealing only 25 to 50 mm of sludge (sand and manure solids) accumulation.
During the sand separation process, a loss of VS from the manure stream was observed, however the change was not found to be a statistically significant treatment effect. The observed cumulative change in the mass of VS determined using the semi-empirical mass balance ranged from 33 to 53%. Changes in VS are important due to the direct correlation between VS and biogas potential. The theoretical electrical energy potential of the full-scale AD at the case study farm, which utilized SSS effluent as the feedstock, was 5,890 kWh/d. In 2008, the maximum electrical output of the full-scale system was achieved in July, when 5,505 kWh/d was produced or 93% of theoretical potential. The lost electrical generation revenue due to loss of VS throughout the SSS, assuming $0.08 kWh, was $123,200 per year.