Burying and spreading slurry of agricultural origin

The management of manure is a fundamental problem for livestock farms. Burying slurry is a good solution, able to limit emissions of ammoniacal nitrogen in the atmosphere and the spread of unpleasant odour. Systems with different characteristics and uses, produced by important industries in the sector, can be employed today, with significant advantages for both disposal and distribution on the ground of fertilizing substances

The consistency of livestock wastes is extremely varied, and basically depends on the animal species being considered, on the modalities of breeding and on the physical characteristics of the animals’ diet. In short, it ranges from manure with a high content of straw that is typically bovine (actually very rare today due to the difficulties in supplying straw and because of the high cost of cereal straw), to very diluted slurry, with a very low fertilizing content, in most cases produced by swines.

The diffusion of anaerobic digesters for the production of biogas also means it has become necessary to dispose of high quantities of waste material, digestate, that results from this process, and that  occupies, with regard to its consistency, an intermediate position among agricultural organic byproducts.

The need to correctly manage digestate has lead to the development of techniques aimed at optimizing its disposal, requiring the preventive separation of the solid fractions from the liquid component, and a subsequent mechanized production chain for the spread of both in the fields.

Especially with regard to the liquid component, the development of plants and then of the necessary machinery has recently undergone a substantial acceleration, aimed in particular at minimizing emissions of ammoniacal nitrogen into the atmosphere that cause the product to lose its nourishing properties, environmental pollution and the spread of smells that are unpleasant for the inhabitants of residential settlements close to the plots of land in which the manure is distributed.

Slurry burying techniques

Compared to a number of options, all of them more or less traditional, for superficial spreading (followed by more or less immediate soil tillage), burying slurry carried out already in the distribution phase no doubt represents an effective shift with regard to the solution of the problems. Leaving out the description of the solutions for spreading or depositing the waste liquid on the surface (in which case up to 80% of the ammoniacal nitrogen leaks into the atmosphere), the solutions that in the last years have attracted the most interest are the ones that allow to incorporate slurry in the ground immediately, at variable depths, by means of deposition or injection of the liquid into a furrow, that is variable in size.

With spreading in an open furrow, the slurry is deposited in small longitudinal cavities 6-8 cm deep, purposely created by blade or disc mouldboards. Obviously the distributed quantity must be commensurate with the dimensions of the furrow, that is, with the working depth, to prevent the slurry from overflowing and remaining on the surface.

With distribution in a closed furrow, on the other hand, the waste must be first injected into the soil, which is tilled with disks or teeth at a depth of about 12-15 cm, and then immediately covered with the same soil that has been loosened with other disks or rollers, so as to limit as much as possible the emissions of ammonium and the spread of odours. This way significant amounts of product can be incorporated, thanks to a more vigorous tilling of the soil.  

For deep injection anchors are used that create fissures that are more than 30 cm deep, on the bottom of which the slurry is deposited with dispensing tubes. Gas emissions are very limited, but the use of this equipment causes a significant modification of the structure of superficial layer of the soil, the traction required is costly, and the equipment is limited with regard to operational capacity.

The impact of emissions

Regardless of the technique used, ammonia volatilization during the phase of distribution of the livestock slurry is no doubt conditioned by various side factors, by the nature and technical peculiarities of the liquid waste, by climatic and soil conditions, by the amount of crop or vegetation residues in the soil at the moment of the intervention. The ammoniacal nitrogen leaks into the atmosphere are reduced in proportion to the time the slurry is exposed to the air. In other words, the deeper the deposition, the lesser the dispersion. However, the choice of the most suitable modality of distribution must be also based on other factors, such as the presence or absence of the crop to be fertilized.

Mechanization

In this context, a double evolution has taken place, which has no doubt interested the traditional liquid manure spreaders, which have become equipped with modules for burying liquid waste, but has also lead to the models designed especially for the task, which are almost all self-propelled and have a high working capacity.

Trailed slurry spreaders

In this case, the devices for traditional distribution (splash plates, sprayers, etc.) have been substituted with equipment for burying liquid waste.

In order to optimize the distribution routine, it is particularly important to have access to high capacity pumps, able to reduce as much as possible the time for loading the slurry. For instance, the single axis trailed model with a 5000 lt tank, equipped with two burying shares by Vendrame di Silea (TV) also mounts a pump with a capacity of 8500 lt/min, liquid cooled, with a Ø 150 mm sideboom suction pipe.

Vaia di Calvisano (BS) offers a diverse range of buriers, both with anchors and with discs, serrated and or smooth. Typically discs allow a finer tillage of the superficial layer of the soil and a more homogeneous distribution of the liquid waste. In detail, the disc harrow by Vaia is equipped with 14 discs with 13 dispensing tubes, for a total working width of 4 m. Alternatively, there is a bar with double smooth divergent discs mounted on a 4 point hydraulic lift that measures 2.5 m.  An interesting feature is the rear infrared video camera that allows control of distribution, both burying and superficial distribution.

Self-propelled slurry spreaders

The need to increase the operational capacity of slurry spreaders, also dictated by the limits set by the law with regard to the periods of time in which spreading in the fields is allowed, has lead to the production of self-propelled models that have been introduced in the market, equipped with very large tanks and high-performance pumps, designed especially to limit downtime as much as possible during the operational routine.

Alongside successful models such as the Xerion by Claas and the Challenger TerraGator by AGCO, among the systems designed by Italian companies the Talpa model has attracted much interest.

Commercialized by the group Maschio Gaspardo, based in Campodarsego (PD), with the Unigreen brand, it is a high capacity self-propelled slurry spreader, equipped with a tank for the liquid waste in stainless steel with a 12,000 l capacity, a 6000 l/min lobe pump, and offers a series of solutions for the distribution of the product both on bare soil and on land with crops. The high ground clearance (1.65 m), but also the hydraulically adjustable wheel base, allow the Talpa model to apply digestate and slurry during the booting stage of the crop, so the soil may absorb the nutrient. The burying device makes use of an articulated parallelogram frame with adjustable load spring with a large diameter floating toothed wheel which creates inter-row furrows to ensure inter-row penetration of the distribution of slurry.

A pair of rear convex closing discs to avoid the dispersion of the ammoniac into the atmosphere.

Moreover, Maschio Gaspardo also provides various distribution options with a special rear 3-point lift coupled with a cultivator share disc for autumn or spring pre-seeding.

The automotive side of the Talpa includes features which make the machine maneuverable and comfortable and a 250 hp engine and full hydrostatic power transmission for all six wheels with steering managed by a system that coordinates the central articulation between the front and rear body and the third axle. 

The cab, equipped with ROPS, is pressurized and fitted with an active carbon filter and completed by a touchscreen monitor with joystick and multi-control console for the best management of distribution. Moreover, a step facilitates access to the driver’s seat.

Umbilical spreading

The use of a network of dedicated hoses for the transport of slurry is a solution known for some time, a method with such very clear advantages such as reducing distribution time and avoiding the compacting of the soil caused by heavy tankers.

On the other hand, the critical features of the system are the limited distances for transporting the product underground which involves laying hose from the farm center to the fields unless a supplementary slurry pit is created as a lung. 

This means that the most advantageous method is to bring in hose reels of the type used for irrigation mounted on trolleys for distribution.

A pump drives the slurry through the hose buried in polyethylene ending in the field and coupled to the reel turbine mounted to a tractor mounted subsoiler at the other end. The slurry is injected continuously in troughs to the depth of 20-30 cm with injector shanks at distances of 30-40 cm the rate of up to 200 m³/h with no impact on the environment. Another advantage is that the system can be used for traditional irrigation and fertigation.

Irrigation slurry

The distribution of animal wastes through irrigation is not at all anything new but is not the best possible practice because of the release into the atmosphere of the nitrogen component and covering the crop with slurry.

There is now a solution for these drawbacks which consists of adding water to the slurry to the maximum concentration of 10%.

Another option under study would be to distribute slurry and water under rainfall using two separates jets, water on the surface and washing the the slurry on the plants below for solving the problem of covering the plants with slurry while effectively avoiding the release of  ammoniac into the atmosphere.