Fresh swine manure contains high levels of suspended solids and low levels of dissolved solids (Evans et al., 1978). Once excreted from animals and stored anaerobically, the manure will naturally undergo decomposition. Dissolved solids are easily degradable organic materials and suspended solids are more difficult to degrade. Bacteria first consume the dissolved solids and, at the same time, hydrolyze and convert the suspended solids into dissolved solids to obtain a continuous food supply for their growth. This decomposition process leads to an increase of total dissolved solids (TDS) and a decrease of total suspended solids (TSS). Since manure separation only affects TSS with little effect on TDS (TDS are usually removed by biological treatment or coagulation), use of separation is not effective for removing solids from liquid manure after bacteria convert TSS to TDS. Therefore, the maximum separation efficiency is achieved if separation is performed before the bacteria in manure become fully established. An experiment was carried out to show the changes in TSS and TDS of fresh swine manure with different particle size ranges during 30 days of storage. The most effective separation time after manure is excreted may be inferred based on the data collected from this study.
Experimental procedure
Fresh liquid manure was divided into three different portions with particle size ranges of < 2.0 mm, < 1.4 mm, and < 1.0 mm. The particle size ranges were created by successive sieving of the fresh swine manure through a series of three ASTM standard wire screen sieves with openings of 2.0 mm, 1.4 mm, and 1.0 mm (ColeParmer Company, Chicago, Illinois). Two 35-gallon containers were used to transfer manure back and forth for each sieving. After each stage of separation, a sump pump was used to mix the sieved manure to keep solids suspended and, at the same time, one of three 91.62-cm tall Plexiglas columns (15.27 cm in diameter) was filled with manure of that specific particle size range, leaving approximately 10.18 cm of headspace. Once all the columns had been filled, each column was thoroughly stirred using a motorized paddle-stirrer and a sample was drawn from the homogenized slurry. This sampling technique was used every five days during the 30-day test period. The columns were placed in a dark room to simulate the conditions in storage pits and the room temperature was maintained between 18 and 22oC. For each sampling, total solids (TS), total suspended solids (TSS), and total dissolved solids (TDS) were measured.
Results and discussion
The concentrations of TSS and TDS for all the particle size ranges during the 30-day storage period are presented in Figure 1. The reduction of TSS and the increase in TDS were relatively small within the first 10 days of the test and there appeared to be increasing decomposition of TSS thereafter. This observation implies that the breakdown and liquefaction of TSS by microbes may not proceed at an appreciable rate for manure less than 10 days old in these particle size ranges. It is therefore recommended, that solid-liquid separation treatment should be performed within the first 10 days of manure storage to improve the separation efficiency.
Figure 1. The TSS and TDS concentration during the first 30-days of storage of fresh manure.
Also from Figure 1, the TSS concentrations started to decrease at a higher rate after 10 days of storage accompanied by increases in TDS for all particle size ranges. On day 25, the amount of TSS was almost equal to the amount of TDS for the manure with particle size < 2.0 mm. For the other two size ranges, the difference between the quantities of TSS and TDS also reached a minimum. A simple calculation based on the data presented in Figure 1 indicates that approximately 42%, 44%, and 31% of TSS in the manure for the three particle sizes of <2.0 mm, <1.4 mm, and <1.0 mm, respectively, were dissolved after 25 days of storage (Table 1).
Table 1. Percentage reduction in TSS during storage.
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According to Table 1, the concentrations of TSS did not vary much during the first 10 days, but increased tremendously on day 20 and 25. Thus, it is likely to be more efficient and economical to conduct separation on manure less than 10 days old to remove as much of TSS as possible.
Summary
According to results from this study, the solid-liquid separation technique, if applied to the treatment of liquid swine manure with solid particle sizes equal to or greater than 1.0 mm, should be performed within the first ten days after the manure is excreted in order to improve separation efficiency.
In the first 25 days of storage, about 31 to 42% of total suspended solids were biologically degraded and no longer available for mechanical separation. Therefore, use of separation techniques to treat manure that is older than 25 days may not be a cost effective practice in terms of removing organic solids that have potential for producing odors during storage.
Reference
Evans, M. R., R. Hissett, M. P. W. Smith, and D. F. Ellam. 1978. Characteristics of slurry from fattening pigs, and comparison with slurry from laying hens. Agriculture and Environment 4(1):77-83.
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