Sustainable distribution of slurry and digestate

Slurry spreaders have been quickly adapted for the need to distribute products of various origins on fields even with site specific methods while improving performance in maneuverability, safety and comfort, especially in transport

The use of livestock wastes in solid form as well as manure which becomes slurry when liquified has always been fundamental as organic fertilization for full field crops. Compared to traditional bedding for animals, the recent use of grain straw has led to a progressive lack of manure resulting in a great increase in slurry as the main production of livestock wastes. This has involved a number of updates on farms for handling temporary storage and distribution in the fields while taking into consideration that these procedures are often covered by highly detailed regulations such as those on logistics, management of the tanks of the contents, the environment as regards odors and especially the nitrogen content of the soil.

As a consequence, the agricultural machinery used for distribution in the field has undergone significant development for minimizing the problems referred to while increasing work capacities for improving the operation completed within the window of time allowed for this field procedure.

 

Nitrogen content and time window for distribution

The European Union Directive  91/676/ECC, better known as the Nitrogen Directive, is aimed at reducing and preventing direct or indirect pollution of water by nitrates of agricultural origin. Over time, the Directive has also led to national and regional measures to regulate the periods when it is possible to distribute organic types of nitrogen fertilizers in the field. Without getting into details, these measures provide a precise annual calendar for local distribution, called the nitrates calendar, associated with the nitrogen content accumulated in farmland for a rigorous discipline on allowing machinery in the fields. 

 

The problem of odors

Running parallel to the distribution of nitrogen fertilizers, especially those in slurry form, raises another problem of limiting the issue of odors disturbing people living in the immediate vicinity of a farm using this fertilization. It must be pointed out that the expansion of urban centers, not always harmoniously, bringing in land which has been traditionally dedicated to farming has created a series of  problems, including the one outlined. 

 

Biogas and the management of digestates

For a number of years, the rapid development in agriculture of anaerobic digestion for the production of biogas for fueling co-generation generators of electric and heating energy has led to the problem of how to handle the resulting byproducts, that is, the so called digestates. These digestates are routinely subjected to solid-liquid treatment for differentiating final use. Especially the liquid part raises the question of how to deal with above.

 

Sludge residues

Whether sludge residues are produced by urban or industrial treatment plants, their distribution in fields has been an interesting way for diversifying farming income for some time. Leaving out the important problem of the correct environmental management of this byproduct, it is fully evident that distribution must be carried out with the right machinery and with the suitable techniques.

 

The development of the slurry tanker

Briefly, the machine is built on a chassis with one or more axles, often set close to each other, on which a slurry tank is mounted with the necessary components for loading/unloading the liquid under pressure or low pressure and regulation of capacity and distribution. The fundamental difference among types of slurry spreaders is the way the liquid is loaded, either by using atmospheric pressure for loading/vacuuming the air in/from the tank.

 

Atmospheric press slurry spreader

A centrifugal pump siphons the liquid from the cistern and sends it under pressure to the distribution component located on the rear of the machine. In this case, the advanced filling of the tank is done with a turbine pump which loads the tank under pressure the same as outside pressure. This makes it possible to use a cistern with normal strength for a substantial reduction of empty weight as well as the cost of the equipment. However, this system is subjected to significant wear and the risk of clogging which is sometimes minimized by a chopping component which separates and homogenizes the solids.

 

Slurry spreaders with pressurized tanks

On the other hand, these spreaders are equipped with an external pump which lowers the pressure for siphoning the slurry from the cistern for loading while the pressure applied to the slurry created in the cistern enables unloading through the spreading. In this case, the ongoing stresses of pressure and low pressure raise the risk of explosion or implosion deformation. This means that the tank and related low pressure must have type approval and periodical testing for verification with the results reported in a specific booklet.

As for all trailed equipment allowed travel on public roads, these tankers must have type approval certification and subsequent registration requiring an eye hook for towing, a brake plant and light kit. Pressurized slurry spreaders are now undoubtedly those most widespread compared to those using atmospheric pressure because they feature a limited number of working components in direct contact with the slurry which reduces corrosion of the materials. 

 

Distribution

Traditional surface spreading is done by a spreader plate which breaks up the jet. Though these plates are simple components, they post various problems in operations due to a lack of uniformity in transversal spreading because of the significant fragmentation of the jet leading to drift and the release of strong smells. To avoid these problems, an oscillating jet system has been introduced with an oscillating brake-up plate and a bar which breaks up the jet.

Superficial or deep burying is an important alternative for spreading thanks to a more or less fine deposit of the slurry in the soil working with fixed or rotating components.

These solutions limit odors, reduce loss of nitrogen ammonia and contamination of the epigeal areas of the crop while eliminating risk of contamination of surface water by the slurry, improving the physical condition of the soil and making it possible to spread in proximity to housing. The drawbacks include more complicated machinery, a substantial increase in the power required for these operations and thus higher energy costs as well as the need to temporarily stop work on the land after the slurry operation.

Other than the distribution of a variable accural if the case is also site specific, the recent developments of slurry spreaders are aimed mainly at improving the performance capabilities thanks to the progressive increase in cistern capacities and higher pump operations for loading and spreading slurry. Also involved are the introduction of automated functions for managing routine work remotely. These changes have led to adding a series of devices to the tank for maximizing safety in transport, especially for traveling on public roads at high speeds.   

 

New features on the market

Vaia in Calvisano in the Province of Brescia, mounts hydraulic drawbars on various models of their slurry tankers capable of easily and continuously regulating the height of the eye hook to keep it as low as possible to benefit the stability of the tractor and the tanker.  Vendrame in Silea in the Province of Treviso, is marketing drawbars and steering wheels as well as controlled maneuvers for improving operations in the localized spreading of digestates on corn.

For raising the level of safety and comfort in transport on the top models in their range, Al.Pa.Car Pagliari in Squarzanella di Viadana in the Mantua Province is offering industrial type pneumatic suspension with levelling valves and damper spring completed by steering wheels, also in reverse, hydraulically controlled by a dedicated CPU.

In a similar way, Zaccaria in Modena provides hydraulic suspension mounts on independent springs which keep the load evenly distributed on all the tires in all conditions in the field and ensure more stable braking for stopping the spreader.

Overall, this can be completed with request for site specific distribution registration. The option of remote management of routine operations by radio control marks a significant step ahead as regards safety on the job. Many manufacturers offer this function along with remote control consoles located in the driver’s cab with dedicated radio control like what is marketed by Bonsegna in Nardò in the Lecce Province.

The spread of the ISOBUS protocol had played a fundamental role in automation, especially for the dialogue between the tractor and implement for improving efficiency in work on crops. Obviously slurry tankers have benefited from this innovation such as the models produced by Bossini in Carpenedolo in the Brescia Province equipped with IT interface between the spreader and the tractor with a single unified connection and a monitor for visualization.