# Physical Properties of Dairy Manure Pre- and Post-Anerobic Digestion
## 2 Materials and Methods  
Pre-digested and post-digested manure samples were collected from seven Wisconsin dairy farms with AD. Dairy cows were housed in free-stalls, with digested and separated manure solids used as bedding on six farms (farms A, C-G) and sand bedding used on one farm (farm B). At all sites, after excretion, dairy manure (both urine and feces) was collected from the barns with a scrape system and stored in a collection pit for less than 24 h before pumping into the digester. Farm F incorporated food waste with the manure, also known as co-digestion, and farm G added chopped straw to the digester influent stream to produce more bedding [31]. All farms had modified plug-flow digesters, which are defined flow-path digesters with mixing provided along the flow, except farm A and F which had completely mixed systems. All digesters were operated at mesophilic temperatures and were designed for retention times from 21 to 30 days.  
Samples were collected from both the collection pits before digestion and directly from the effluent line out of the digester. All samples were stored at 4 \({}^{\circ}\)C until analysis was completed. Prior to analysis, samples were agitated for 3 min using a drill mixer to ensure the incorporation of any settled solids. Bulk density was measured based on the sample mass in a 100 mL graduated cylinder. TS and VS were determined following gravimetric procedures found in standard methods [32]. All samples were initially analyzed for TS content (Table 2). Subsamples were then prepared with 1.5%, 3.0%, 4.5%, 6.0%, 7.5%, 9.0%, 10.5%, 12.0%, and 13.5% TS levels by either diluting with deionized (DI) water or oven drying at 105 \({}^{\circ}\)C. Additional pre-digested samples were collected from farm H, I, and J as excreted and oven dried. Subsamples with 16%, 18%, 20%, 25%, 33%, and 50% TS were made by adding DI water to the dried manure. Actual TS content was then verified for diluted or dried subsamples. Since the verified TS level for pre-digested and post-digested samples on each farm were not equal, the VS destroyed in this study was calculated by the difference between the linear model at a TS level of 8% which is a common TS content for the influent for mixed plug-flow digesters in Wisconsin [31].  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Farm ID** & **Substrate** & **Bedding Type** & **Digester** & **Pre-Digested TS (\%)** & **Post-Digested TS (\%)** \\ \hline A & Manure & Manure solids & Complete mix & 10.4 & 8.4 \\ B & Manure & Sand & Plug-flow & 6.6 & 6.0 \\ C & Manure & Manure solids & Plug-flow & 8.0 & 5.0 \\ D & Manure & Manure solids & Plug-flow & 6.4 & 5.1 \\ E & Manure & Manure solids & Plug-flow & 5.5 & 4.4 \\ F & Manure and food waste & Manure solids & Complete mix & 7.1 & 4.8 \\ G & Manure and chopped straw & Manure solids & Plug-flow & 7.6 & 6.2 \\ H & Excreted manure & NA & NA & 15.4 & NA \\ I & Excreted manure & NA & NA & 15.2 & NA \\ J & Excreted manure & NA & NA & 12.8 & NA \\ \hline \end{tabular}
\end{table}
Table 2: Farm management practices and total solids (TS) content of undiluted manure.  
The [PERSON]'s chi-squared test was used to evaluate if there are statistical differences (\(p<0.05\)) in density and TS content in manure between farms. Data for pre-digested and post-digested manure were analyzed separately. Each dataset was fit to a linear regression model with density as the dependent variable and the verified TS content as the independent variable. Chow tests were performed to select the data that could be used to develop a general model between density and TS content. These tests indicate whether the coefficients in two linear regressions on different data sets are equal. The final statistical model was selected using [PERSON]'s information criterion (AIC) forward stepwise. The impact of VS on density was also evaluated for pre- and post-digested samples separately. All the statistical analyses were conducted using R 3.0.1 [33].