Sewage sludge as an organic amendment for quarry restoration: Effects on soil and vegetation

Abstract

Quarry restoration in Mediterranean environments usually needs organic amendments to improve the substrates used for technosol construction. Digested sewage sludges from municipal wastewater treatment plants are rich in organic matter, N, and P and constitute an available and economically interesting alternative for substrate amendment. However, their pollutant burden and labile organic matter content involve an environmental risk that must be controlled. Moreover, ecological succession in restored areas can be influenced by the use of sludge and should be assessed. To minimize these risks, a new sewage sludge dose criterion relating to its labile organic matter and heavy metal content has been established. Sewage sludge doses currently range between 10 and 50 Mg ha−1. In order to verify the suitability of this dose criterion, 16 areas rehabilitated using sewage sludge located in limestone quarries in a Mediterranean climate in Catalonia (NE Spain) have been assessed. These evaluations focused on physicochemical properties of rehabilitated soils, land degradation processes, and ecological succession. In the short term, 6 months after sludge application, an increment of organic matter content in the restored soils was observed, without significant increases in electrical conductivity or heavy metals content, and with a dense plant cover that contributes to effective soil erosion control. Two years after, ruderal plants were still present but later successional species colonized the restored zones in different degrees. These results suggest that sewage sludge, used as a soil amendment according to the proposed methodology, can safely improve technosol quality without constraints that compromise ecological succession.

Digestion Procedure and Determination of Heavy Metals in Sewage Sludge—An Analytical Problem

“…Therefore, stabilized sludge can be safely used in agriculture. Also, environmental risk involved was pointed out by Carabassa et al [7], who concluded that nitrate inflow to surface water reservoirs may lead to uncontrolled eutrophication. Fortunately, due to the intense nitrate uptake by plant roots during the growing season, this risk may only occur within the first four months after the sludge has been applied.…”

“…Sludge management includes the following methods e.g., landfilling, thermal processes, application to agricultural land, composting, wasteland reclamation and silviculture, use in the construction industry or recovery of rare earth metals [1,[4][5][6][7][8][9][10]. In 2014, total sewage sludge generated in Poland from industrial and municipal wastewater treatment plants was 967.4 thous.…”

Mid‐term effects on ecosystem services of quarry restoration with Technosols under Mediterranean conditions: 10‐year impacts on soil organic carbon and vegetation development

“…However, we only considered the first top 20 cm of soil in our carbon stock estimations. Consequently we underestimated the total SOC, especially on amended plots, due to the strong plant growth in the first years after Technosol construction (Carabassa et al ). This plant growth, which included trees, could have increased deep C through root growth, as shown by Simón et al () in mine Technosols in SE Spain 6 years after their establishment.…”

“…On the contrary, sludge‐amended Technosols still presented high N and P concentrations after 10 years, and in the specific case of N, more than in the short term after sludge amendment. Although nitrogen leaching is plausible with the sludge treatment, it would mainly take place during the first 4 months due to the high mineralization rates of organic N from sludge (Carabassa et al ). After this period, leaching should decrease quickly due to reduced sludge decomposition rates and the enhanced nitrate absorption by the growing vegetation (Tarrasón et al ; Jordán et al ).…”

“…When a Technosol approach is taken, sewage sludge is applied once, at a moderate dosage, to act both as fertilizer and organic amendment, and usually mixed with mining debris before their spreading as a topsoil layer in the restored areas. Sewage sludge is then an interesting option for valorizing mine spoils due to its fertilizing properties (Van‐Camp et al ) and its known positive effects on soil aggregate stability (Caravaca et al ; Ojeda et al ), soil water retention (Ojeda et al , ), and vegetation recovery (Moreno‐Peñaranda et al ; Ortiz & Alcañiz ; Carabassa et al ). However, sewage sludge application also requires strong supervision and monitoring due to certain environmental risks related to its potentially harmful heavy metals and persistent organic pollutant content (Düring & Gäth ; Carabassa et al ).…”