Slurry treatment

Livestock development and current impact

Waste and the consequences of its disposal are becoming one of the main problems of the modern world due to the deterioration they cause to the environment.

The pig industry represents an important economic activity in Spain. According to data from the Ministry of Agriculture, Food and Environment, in 2013 there were more than 25 million pigs in Spain, 51% of which were located in the communities of Catalonia and Aragon.

This figure has been growing year after year over the last few decades, leading to the proliferation of industrial-type farms outside of land management criteria, which have contributed to the imbalance between livestock and agriculture. This fact has led to the emergence of numerous environmental pollution problems.

Livestock farms, primarily pig farms, are not immune to this issue, as they house a large number of animals, and often do not have enough land to absorb the large amount of waste, which poses serious management problems.

Legislation and controls are becoming increasingly strict in order to find a viable solution to a problem with serious environmental repercussions.

The Administration, intending to address the problems arising from the overdosing of manure as fertilizer in soils, has developed a very strict legislative framework that affects the operation of farms.

Thus, systems that treat effluents efficiently and are also economically viable are essential for the survival of current farms as well as for the promotion of new ones.

Problems arising from manure

The dual objective is to solve the problems posed by waste (odors, storage issues, pollution of waterways, groundwater, and soils), and on the other hand, to utilize the organic and fertilizing load that they possess for use in agriculture, providing a solution to a product that is difficult to eliminate.

The process that takes place on the farm is the intensive breeding of pigs, generally in closed cycles, with a huge generation of solid and liquid waste, which contain a high contaminating power towards soils, waters, and air.

The logical use for this waste is to apply it to the field as fertilizer, but due to the high concentration of farms, and therefore of manure, the amount of excrement becomes excessive, contaminating soils, waters, and emitting a large amount of generated gases into the atmosphere.

To date, most manure treatment plants have consisted of cogeneration plants, where natural gas is burned to thermally dry the manure and, with the residual combustion heat, electricity is generated that is sold and injected into the grid. However, the new reform of the electricity sector, which has substantially reduced the price of cogenerated kWh, as well as the increase in the price of natural gas, has made this technical-financial mechanism economically unsustainable.

In this context, the developing manure treatment systems must be, in addition to efficient and environmentally friendly, also economically sustainable without the help of artificial bonuses that could be withdrawn through legislative changes.

These treatment systems must be designed according to the operation of intensive farms, which use pressurized water as a cleaning system to wash away the waste. This technique, which facilitates cleaning and increases the sanitary conditions of the farm, conversely entails a high water consumption and generates a high flow of manure, which is nothing more than a liquid-pasty mixture of feces, wash water, and feed remnants.

The concentration of contaminants in manure depends on the size of the farm, as larger operations make more intensive use of water. In general, the contaminating load presents a high variability, as it depends on the production process (maternity, weaning, piglets, fattening, etc.), feeding, the age of the animals, etc. These factors make it essential to correctly characterize the effluents.

Treatments and applications of manure

There are different alternatives for managing manure, from the simplest (and often unfeasible) to the most efficient and competitive, using various technologies including ultrafiltration, biological treatments, physical-chemical treatment, or evaporation.

Below is a brief description of the different existing alternatives:

Direct agricultural application

This solution is only viable when the balance between agriculture and livestock is balanced.

When the livestock operation is industrial, the generation of waste is very high in proportion to the available land area, as regulations set the maximum amount of manure that can be dosed to the soil per unit area per year.

Direct thermal drying

Although it is a technically effective option, it entails high operating costs. Until now, it was the usual option in Spain, as the bonuses for cogeneration made it economically viable to burn natural gas to dry the waste and produce electricity.

With the reform of the electricity sector, it is no longer economically viable to burn natural gas to dry waste.

Composting

The solid fraction of manure, mixed with coconut fiber and peat, can be composted. The compost obtained can be used in the ecological and landscape restoration of soils degraded by grazing, agriculture, firewood collection, etc., where the organic matter content has been considerably reduced, and therefore, its fertility.

However, this is only a solution for the solid fraction, and although the quality of the compost is good, the additional cost of this alternative compared to others is only justifiable from an environmental benefit perspective.

Aerobic biological treatment + Physical-chemical treatment

Manure has a BOD5/COD ratio between 0.2 and 0.4 and a high nitrogen content relative to carbon. These two factors mean that, although they can be treated through an aerobic biological process with nutrient removal, the results improve dramatically if the aerobic process is combined with a physical-chemical system.

Although this option is technically effective, it is not the most economically competitive nor the simplest in terms of operation.

Biomethanization and evaporation of digestate

This alternative is the most interesting in terms of both economic and environmental sustainability.

The organic matter content of manure is not very high, but if mixed with organic waste of plant origin or any other waste containing carbonaceous material, it can undergo an anaerobic digestion or biomethanization process.

As a result, the following is obtained:

• A solid fraction (digested sludge), stabilized and sanitized, which can be used directly as fertilizer in agriculture.
• A liquid fraction, the digestate, which can be concentrated through a vacuum evaporation process, obtaining on one side water, and on the other side, a concentrated waste that can be valorized as fertilizer.
• A gaseous fraction, biogas, which can be used as fuel in a cogeneration process, where it is transformed into thermal energy and electrical energy. The thermal energy can be used both to keep the anaerobic digester operating at the optimal temperature (36-38 ºC) and to meet the heat requirements of the vacuum evaporator treating the digestate. The electrical energy can be self-consumed both in the livestock operation and in the manure treatment plant, reducing the operating costs of the activity.

Thus, industrial farms dedicated to pig farming have a problem of imbalance between the volume of manure generated and the available land area to assimilate it. The demanding regulations require that the generated manure be managed in an environmentally respectful manner, and for the financial balance of the activity, the treatment system must be economically competitive.

Of the different existing management alternatives, the most economically competitive, as well as environmentally friendly, is the treatment of manure through a biomethanization process. The final results yield a solid usable as fertilizer for agriculture, a liquid waste that can be valorized as fertilizer, water, and energy that is used to reduce the overall operating costs of the activity.