Evaluation of mechanical separation of pig and cattle slurries by a decanting centrifuge and a brushed screen separator
Image Caption: Centrifugal separator
Executive summary report
This report summary is a précis of a detailed report by Gilkinson and Frost (2007)1. Mechanical separation of animal slurry produces a liquid fraction with a lower dry matter concentration than the input slurry and a solid fraction with a higher dry matter concentration than the input slurry. Plant nutrients in the slurry are partitioned between the liquid and solid fractions. Differential partitioning occurs if one or more component of the input slurry is partitioned in excess of the weight/volume split between the liquid and solid fractions.
In the current work, AFBI-Hillsborough evaluated the performances of a brushed screen separator and a decanting centrifuge with pig and cattle slurries. The effects of adding coagulant and polyelectrolyte to the slurries (chemical treatment) on separator efficiencies were also evaluated.
For the brushed screen control treatment (no chemicals added), separation efficiency for all components measured was positively correlated with input pig slurry dry matter concentration. There was some differential partitioning of dry solids into the separated solid fraction; otherwise the brushed screen separator partitioned nutrients more or less in proportion to the fresh mass of the liquid and solid fractions. The weight of fresh solids produced from the brushed screen per tonne of slurry was dependent on the input slurry dry matter concentration e.g. with pig slurry dry matter concentrations of 25g/kg and 60g/kg, 10kg and 91kg fresh solids per tonne of slurry were produced respectively. Chemical treatment of slurry inputted to the brushed screen resulted in improved partitioning of total nitrogen (TN), total phosphorus (TP) and dry solids, though the effect was small.
The decanting centrifuge partitioned a much greater proportion of TN, TP and dry solids in pig slurry into the separated solid fraction than the brushed screen separator. The weight of fresh solids produced from the decanting centrifuge per tonne of pig slurry was dependent on input slurry dry matter concentration e.g. at pig slurry dry matter concentrations of 25g/kg and 60g/kg, 58kg and 111kg fresh solids per tonne of slurry were produced respectively. Chemical treatment of pig slurry further increased the quantity of fresh solids produced from the decanting centrifuge e.g. at 60g/kg slurry dry matter concentration, 185kg of fresh solids were produced per tonne of slurry. Without chemical additions, 79% of the TP in pig slurry and 64% of the TP in cattle slurry was partitioned to the separated solid fraction by the decanting centrifuge. Adding chemicals to slurry inputted to the decanting centrifuge increased the TP in pig and cattle slurry partitioned into the separated solids to 93% and 82% respectively but had very little effect on the partitioning of potassium (K) and ammonia nitrogen (NH3-N). The brushed screen separator without chemical addition transferred an average of 6% and 17% of the TN from pig and cattle slurry respectively into the solid fraction, increasing to 7% and 23% with chemical additions. The corresponding figures for the decanting centrifuge were 21% and 25% for pig and cattle slurry respectively, increasing to 34% and 41% with chemical additions.
For both separator types, adding chemicals to pig slurry to improve separation efficiency significantly increased the volume of supernatant by between 9% (medium rate of polyelectrolyte addition) and 28% (high rate of polyelectrolyte addition) as a result of dilution with water. The cost of the chemicals used in this experiment ranged from £1.50 (low rate coagulant/medium polyelectrolyte addition) to £3.74 (high rate coagulant/high polyelectrolyte addition) per tonne of slurry inputted to the separators. The increased volume of supernatant resulting from the high rate of polyelectrolyte addition would not be practical for many farms.
Pig slurry treated with chemicals prior to decanting centrifuge separation produced a supernatant that contained approximately 9g/kg dry matter content, 2g/kg total nitrogen and 0.04g/kg total phosphorus For an annual throughput of 4,000 tonnes of pig slurry it was estimated that the cost of separation, without chemicals, with the decanting centrifuge could be approximately £4.50 per tonne of input slurry and about £0.85 per tonne for the brushed screen. At this annual throughput of pig slurry and without chemical addition, the estimated costs for partitioning phosphorus and nitrogen into the separated solids could be in the order of £6,000/t of TP and £5,000/t TN for the decanting centrifuge and £13,000/t TP and £3,000/t TN for the brushed screen.
All these costs are dependant, inter alia, on the quantity of slurry separated per year, depreciation and interest charges. For example, at a throughput of 8,000 tonnes per year, costs could be approximately halved.
1Stephen Gilkinson and Peter Frost, 2007. Evaluation of mechanical separation of pig and cattle slurries by a decanting centrifuge and a brushed screen separator. AFBI-Hillsborough, September 2007
Full report of study
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