**2. Material and methods**

**1. Introduction**

Sludge in Spain [2] indicates that 1200 × 103

274 Organic Fertilizers - From Basic Concepts to Applied Outcomes

sation capacity of sludge and its environmental implications.

The adequate management of sludge from urban wastewater treatment plants is a critical global environmental concern due to population growth in urban centres, which increases the production of industrial and household waste and the amount of wastewater that must be processed by treatment plants. This issue is especially relevant, as more than 50% of the global population lived in urban centres in 2008 for the first time in the history of humankind [1]. The treatment of wastewater generates a semi-solid residue; the final disposal of this residue requires permanent technical and science-based solutions to prevent contamination of the natural environment. The most economical options that are available for the elimination of sludge entail its use as a fertiliser in agriculture or its disposal in landfills. The National Registry of Sewage

represents an increase of 62% since 1997. The use of sludge in agricultural soils has increased in recent years: it was applied to 65% of the agricultural surface area in 2006 and to 80% in 2013. In the latter year, the landfill sludge disposal decreased by 8%, whereas its incineration increased by 4%; however, these rates do not account for the use of sludge in non-agricultural soils.

The incentive to apply sludge in agricultural areas is significantly given the ability of this waste product to serve as a fertiliser and their soil positively influence. Conversely, the continual use of mineral fertilisers causes a loss of soil organic matter and other negative impacts (e.g. a decrease in soil fauna or contamination of water bodies) and reduces the capacity of soil to crop production. Considering its different components, the organic matter content in sludge improves the density, porosity and water retention capacity of soil and promotes the activation of microorganisms and soil enzymes. The biosolids application (sanitised, stabilised and dry) favours germination and plant growth, increases the production of various crops and improves the quantity of plant protein and dry matter. The application of this waste product can produce a seasonal shift that causes the early flowering/fruiting of crops (e.g. flax and cotton) and reduce the growing period. Different risks are associated with the chemical composition of sludge, which are primarily related to its heavy metal content. The use of sludge affects the final concentration, solubility (the relative presence of fulvic and humic acid may aid the removal of heavy metals) and final bioavailability of heavy metals in the soil. Similarly, an excessive accumulation of metals negatively influences the development of mycorrhizae. An uncontrolled biosolids application can provoke a soil nutrient excess (N, P) and potentially contaminate aquifers. Deficits in the levels of K and Mn in plants have also been detected, which signals the need for chemical fertilisers to maintain sustained medium-term and longterm production. The sludge use also favours the persistence of pesticides that are applied during crop cultivation [3]. Another consideration is the high variability in sludge composi‐ tion, which is dependent on the inputs of wastewater treatment plants, season and type of waste post-treatment. Therefore, the sludge chemical composition must be determined prior to its application in crop soils, and field assays should be performed to determine the fertili‐

Mkg m.s. of sludge was generated in 2013, which
