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The re-use of agricultural wastewater

 

Effluent use in irrigation is a means of wastewater disposal and a resource for economic development. The practice of applying effluent through center pivots is quite common and effective.

In the past, the livestock waste was usually handled in a dry state or as thick slurry. At different times of the year, this waste was spread onto the land without really taking into account its impact on the soil or surface water and without knowing the quantities of nutrients applied in comparison with the crop requirements. The introduction of the Clean Water Act* as well as other legislative actions and significant changes in the numbers of head of livestock per farm, produced a situation which was quite different. Nowadays, it is becoming increasingly common for slurry producers not to have enough land or sufficient surface area and they have to rely on the neighbouring farms in order to get rid of it. Today’s dairy farms are faced with increasingly strict provincial and federal compliance policies associated with solid and liquid manure handling. The efficient and cost effective application of solid and liquid manure is a primary objective of every dairy operation.

 

The spreading of effluents with mobile irrigation equipment, such as pivots and lateral move machines, has been used successfully for a number of years.

 

Since the early 1980s the equipment and techniques used for irrigating with fresh water have changed dramatically and many of these changes have been incor­porated into mechanized equipment used for land applications (Gilley, 1983)

While these changes have brought significant improvements, in today’s world we must also take other issues into account, particularly the public perception of land application systems. Mobile irrigation equipment has been used for spreading muni­cipal, industrial and agricultural waste waters on the land. Mecha­nized irrigation, given its special features, is considered to have a number of advan­tages with regard to the application of reused waste water. Some of the more significant advantages are reduced labour require­ments, uniformity of appli­ca­tion, ease of handling for?large?volumes?of effluent and, above all, the capacity to promote the development of the crop with minimal negative im­pact. The pivots can ope­rate during those periods when there are adverse climatic conditions, which may?prevent?or?prove challenging for conven­tional waste handling me­thods requiring the move­ment of tractors and other equipment?across?the fields.

The possibility of having pivots which move around the field has been the most popular solution. Pivots getting bogged down or stuck in the mud was a common problem, gene­rally preventing the pivots from completing a full circle because of the wet spots. This disadvantage has generally been overcome with the introduction of special drive units that keep the pivots more mobile.

Using mobile mechanised irrigation equipment for spreading has been shown to have numerous advantages in many wastewater reuse projects. One of the key factors that contributes towards the success of a project is the adoption of an integrated approach during the research and design phase, taking into due consideration the equipment, agronomic principles, management procedures and the neighbours’ opinions, together with the inputs of the party producing the effluent.

 

 

Spreading of slurry on large fields with pivots has a number of advantages compared  with the use of slurry tankers.

 

There are no repeat runs across the soil and therefore there is no compaction, so it is thus possible to run the pivot across the field during the pre-sowing or pre-emergence stage. There is also the possibility of double use: slurry spreading - irrigation. The spreading process takes less time so there is a saving on time. The spreading capacity allows for large amounts to be dealt with. And the cost is competitive when there is more than 15,000 m3 to spread.

Spreading with the pivot during pre-emergence maximises the fertilising potential of the slurry. Warning: when the maize or corn is at the 4/5 leaf stage (some plants go to 6/7), it is better to apply the slurry when the sky is overcast to avoid scorching the plant. As the product to be applied is quite acidic and contains some particles, the pivot has to be adapted to this use. The slurry will pass through PVC or polyethylene tubes. The sprinklers will be made of thermoplastic (not bronze, in order to avoid the phenomenon of electrolysis) with a protected stainless steel spring. The sprinkler guns will be anodised or made of stainless steel. Slurry spreading is a fertilisation-type operation. It will, therefore, be necessary to ensure that the distribution along the length of the pivot is as uniform as possi­ble. The simplest solution is to have a polyethylene “liner” fitted inside the pivot pipes. One sprinkler gun is fitted per span, the size of the nozzle increasing with distance from the centre point.

 

The main parameters of a good spreading system.

 

The land spreading system should be adapted to the existing mana­gement and/or treatment process and there must be sufficient land available to receive the expected level of nutrients and cope with the hydraulic load, allowing for modifications to be made in the future. The design and planning process must take local constraints into account, such as odour, visual impact on the landscape, etc. The project must be reviewed periodically to ensure that it is opera­ting in accordance with the basic design concepts and the needs of the participating parties and a continuous training programme must be put in place for the employees of the engineering and com­pa­ny con­cerned, so that they will be familiar with the equipment, ba­sic concepts and agronomic aspects of a waste­water land application system.

For the pivots, the basic components are as follows:

The ability to distribute small amounts of waste water and effluent, thus facilitating the management of the lagoons (high pivot speed); remote control and monitoring (Sys­tems such as Field Sentry, Tracker and others); control panels with wind, rain and other sen­sors. We also can note the particular attention being paid to the sprinkler packages (spacing between the sprinklers must be as wide as possible so as to be able to operate large diameter nozzles, with the use of regulators or self-adjusting nozzles, so as to be able to determine the consequences of not using a regulator and look at the options put forward by the sprinkler manufacturers); the use of techniques that will prevent the pivots from getting bogged down (three-wheel drive, tracked drive units).

Using center pivots, slurry lagoon levels can be managed throughout the growing season compared to relying on custom applicators to pump water from the lagoon on their schedule. Center pivots allow dairy farmers to apply lagoon water when the crop can make the best use of the nutrients and water to maximize the production of feedstock for the cattle.

Some center pivots can be designed to fit a wide range of field sizes and shapes and incorporate a variety of options to best match the operational needs of the farm. The standard center pivot is approximately 400 meters long (approximately ¼ mile) and covers 50 hectares (124 acres). Fields that cannot accommodate a full circle pivot can be irrigated with a part circle machine.

 

Design of a center pivot must include an understanding of the solids content of the liquid manure, site specific soil properties, and the type of crops to be grown.

 

For trouble-free spray irrigation it is important that the manure is processed appropriately. The slurry is homogenized with a mixer, submersible mixer or CSP pump and separated afterwards.

Notwithstanding the correct timing of the application, it is also important to be aware of the composition of the nutrients in the slurry, i.e. it is important to know the N-P-K ratio (phosphates, potassium and nitrates) of the slurry and dilute it with water, if required. The slurry can also be applied undiluted. In practice, however, it is advisable to mix the slurry with water. The slurry should be applied as close to the ground as possible, in order to prevent unpleasant smells and reduce the loss of nutrients. Moreover, special nozzles are available for slurry irrigation. However, these are not necessary if the slurry has been separated beforehand. Nevertheless, after applying the slurry we advise that the pipes should be rinsed with clean water.”

In order to prevent the galvanised materials from being corroded by the slurry, the level of aggressiveness depending on the PH-value, most of the manufacturers offer a special polyethylene coating for all parts that come into contact with the water or slurry.

Solids content of the liquid manure will be dependent on the type of solids handling system employed by the dairy farm. In order to select the proper center pivot irrigation package, through which liquid manure will flow, the following questions should be evaluated: What method is used to remove manure from the barns; flushed or scraped? What type of bedding or mats are used?

Type of solids screening utilized; mechanical screen, gravity? Is a portion of the water recycled for flushing? Size, depth and number of lagoon cells? Will water be pumped from the surface or near the bottom of the lagoon?

An understanding of the solids handling system and the percent of solids to be pumped through the center pivot will allow for the selection of the appropriate irrigation package to handle the solids content of the water, with the lowest potential for nozzle plugging.

Knowing and understanding the types of soils across the field is important when selecting the center pivot flow rate. The flow rate of the center pivot will determine not only the range of application depths that can be applied, but also the instantaneous application rate at which water is applied to the soil. Selecting a flow rate that produces an instantaneous application rate (millimeters per hour) that best matches the water intake rate of the soil will reduce the potential for runoff during an irrigation event.

Application depths from a center pivot can be changed at the control panel to match crop water demand and soil moisture levels, thereby reducing the potential for ponding and surface runoff, a critical compliance requirement for the irrigation of liquid manure. Irrigation scheduling that best matches the water and nutrient needs of the crop during the growing season reduces the potential for the movement of nutrients through the soil profile, further reducing an possible impact on the groundwater and surface water.

 

 

Center pivots provide dairy farmers with the ability to manage liquid manure efficiently and cost effectively while at the same time maximizing the benefits and value of liquid manure nutrients through the production of feedstocks for the cattle.

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