The effect of aeration and aerator design on the physical, chemical and fertiliser quality of cattle slurry

Abstract

Intensification of agricultural practices has led to the production of liquid slurries in sufficient volumes that prohibits year round application. To reduce the risk of environmental pollution from inappropriate land applications, slurry is stored along
with parlour and yard washings during the winter months, resulting in the accumulation of large volumea During storage, anaerobic conditions prevail, resulting in the breakdown of biological compounds into noxious and toxic chemicals which are
of primary concern in environmental pollution. The introduction of large scale anaerobic digesters on every firm is impractical, as extensive supervision of the digester is required. The problems associated with the application of anaerobically stored shiny back to land, such as noxious odours, ground and surface water pollution and crop scorching are no longer tolerated by the general public and government pollution agencies.
To alleviate some of the problems faced by farmers storing large quantities of slurry, aerobic treatment systems have been devised that can reduce the risk of pollution. Slurry aeration has become a widely used method in the treatment of cattle slurry, as it is effective in reducing odours, preventing crust formation and provides a semihomogenous mixture. The aeration system can vary dramatically in design and operation, from very simple bubble diffusers consisting of holes pierced into pipes which are laid across the floor of a tank, to complex Venturi systems which rely on the generation of negative pressures from tapered nozzles to draw air into a stream of slurry. The aeration systems used in this thesis are of a fine bubble difiliser and a mechanical surface aerator design. These systems were chosen because of their low capital and running costs.
Aeration efficiency was studied at three scales, laboratory (up to 1 litre), intermediate (up to 80 litres) and pilot (up to 10, 000 litres) using volumetric mass transfer coefficient, standard oxygen transfer rate and oxygenation efficiency as the testing
criteria. The fine bubble diffuser used throughout all the scale up experiments was of the same principle. Air was delivered through a compressor which was isolated outside of the reaction vessel. Sufficient air was delivered to the slurry at all scales to prevent the need for an additional mixing unit to maintain the solids in suspension. However, at pilot scale the design of the diffuser layout as critical in order to prevent biological fouling of the material. The mechanical surface aerator used at pilot scale was placed within the centre of the reaction vessel and could only be accessed by removing it from the tank. The mode of operation was to thaw liquid up through a central volumetric screw and release it radially outwards above the suthce of the liquid. The ability of the mechanical suthce aerator to transfer oxygen to both water and slurry was higher than
that of the fine bubble difibser, with concomitant higher kLa, SOTK and OE being recorded.
The introduction of oxygen into cattle slurry had two effects, the generation of a stable microbial culture and a major reduction in physical and chemical pollutants, such as chemical oxygen demand, total solids and total suspended solids. Each aeration device generated a microbial population which was distinctive in gross morphology and settling characteristic& Both microbial populations flocculated naturally and quickly settled to a clear liquor and brown sludge. Flocs generated through violent aeration were small and compact, unlike the large feathery flocs generated by the fine bubble diffliser.
The mechanical surface aerator achieved higher reductions in all the measured pollutants than the fine bubble difihiser, with reductions of 75% for COD of the mixed liquor for the mechanical surface aerator compared to 67.2 % for the fine bubble
diffuser.
Untreated cattle slurry has a high concentration of nitrogen species in various forms. If treated correctly the nitrogen in the system can be conserved and utilised for the benefit of a growing plant. A discontinuous aeration regime was chosen from the experimentation so that the pollution of ground and surface waters from high concentrations of oxidised nitrogen species such as nitrite and nitrate could be minimised. The suppression of detectable nitrite and nitrate generation was achieved
up to 6 days with both aeration devices. A greater fraction of the total nitrogen was removed from the system aerated with the mechanical surface aerator. The use of aerated shiny resulted in a higher fresh and dry weight yield for perennial rye grass when compared to unaerated slurry on a clay loam soil. The ability to predict the nitrogen value of aerated slurry would offset some of the cost for inorganic fertiliser.