# Physical Properties of Dairy Manure Pre- and Post-Anerobic Digestion
## 3 Results  
### Density versus Total Solids  
The linear models from the five farms that used manure solids as bedding and did not incorporate any off-farm wastes into their system were not significantly different and could be grouped together to develop one equation. Farm B (which used sand bedding) and farm F (which incorporated food waste as a co-digestion feedstock) were analyzed separately as they were significantly different than the rest. Farm B had a statistically significant higher average density at 1027 kg m\({}^{-3}\) while farm F had a statistically significant lower density at 997 kg m\({}^{-3}\) compared to the remaining farms.  
Figure 1 shows that manure density increased with increasing TS levels from 1.5% to 13.5%. The values varied within a range of 990 to 1065 kg m\({}^{-3}\) for pre-digested manure. The density of manure that contained sand and food waste did not follow the same trends as the other farms as shown in Figure 1. The density of manure with sand bedding was higher than all the other farms at each TS level, as sand has a higher relative density (1370 to 2323 kg m\({}^{-3}\)) than organic compounds in manure (400 m\({}^{-3}\)) [34].  
The trend of increasing density with increasing TS contents (from 2.5% to 14.2%) for undigested manure is in agreement with results published by [PERSON] and [PERSON] [20]. The authors generated a linearly increasing model for undigested beef manure with a TS range of 2.5% to 14.2%, as shown in Figure 2. The undigested manure density results are higher than those determined by [PERSON] et al. [22], who reported undigested dairy manure density for TS content ranging from 16.7% to  
Figure 1: Density of pre-digested manure as a function of total solids (TS) content.  
47.7%. The authors observed a decrease in density from 637 to 199 kg m\({}^{-3}\) until 44.4% solids, when the density values increased back to 238 kg m\({}^{-3}\) at 47.7% solids level.  
When examining the 1.5% to 13.5% TS results from this study with the 16.7% TS and higher results from [PERSON] et al. [22], it is apparent that the unprocessed dairy manure density reaches a critical maximum value after which the density value starts to decrease. Figures 2 and 3 fit a model to data from this study as a function of TS ranging from 1.5% to 50%. This model includes the additional samples for pre-digested dairy manure (farms H, I, and J) and the samples of the five farms that use manure solids as bedding, excluding farm B (using sand as bedding) and farm F (co-digesting manure with food waste). In this third order polynomial equation, the density increases until it reaches a critical point, 8.0% TS, where void space is created, and the density begins to drop.  
Figure 3: Density of pre-digested manure (farms A, C–G) as a function of TS content.  
Figure 2: Comparison of three models relating density to TS content for manure only (pre-digestion). The overall model considers samples only from farms A, C–G.  
Unlike pre-digested manure, the average density of post-digested manure at each farm is widely distributed with no significant differences. This change could be a result of the digestion process, which converts the digestible organic compound to biogas. The density of manure from farm B remains higher due to its sand content, but the differences are not statistically significant. Even though sand was removed before the digestion process, studies have reported that up to 3% of the sand may have entered and settled in the digester causing the large range of TS content [10]. The post-digested manure density also increased with increasing TS content ranging from 990 to 1056 kg m\({}^{-3}\), as shown in Figure 4. Data from farms A through F were used to develop one regression model while farm G was assessed separately according to Chow test results which indicated that farm G was significantly different. The lower density value of post-digested manure from farm G could be a result of the lower density of the chopped straw added in the digester [35].