Recovery of humic acids from anaerobic sewage sludge: Extraction, characterization and encapsulation in alginate beads.

Wastewater production is rising all over the world and one of the most difficult problems is the disposal of sewage sludge (SS). It is known that SS contains certain quantities of added-value compounds, such as humic acids (HA) which in turn have beneficial effects on soil quality and plant growth. On the other hand, SS can retain many pollutants, such as heavy metals. The present work aimed to implement an HA alkaline extraction protocol from anaerobic sewage sludge (ASS). Subsequently, the HA were quantified in ASS, in HA extract and in commercial HA, used as a benchmark, which gave results of 12.53%, 26.87% and 77.87% (on dry matter basis), respectively. FESEM and EDX analyses on lyophilized HA extract confirmed that no heavy metals had passed into the extract. Afterwards, in order to allow controlled release of the HA in soils, alginate beads containing the HA extract were created. Finally, a pot experiment in a greenhouse was performed using Chilean lettuce plants (Lactuca sativa L.) treated with alginate-HA extract beads. At the end of the greenhouse experiments, the hypogean dry biomass of the treated plants was significantly higher than for non-treated plants. The relevance of this study relies not only on the exploitation of green chemistry principles, by converting a waste stream into a high-value product, but also on the application of an approach following a circular economy model.

Anaerobic digestates from sewage sludge used as fertilizer on a poor alkaline sandy soil and on a peat substrate: Effects on tomato plants growth and on soil properties.

Anaerobic digestates from sewage sludge (SSADs) are a by-product of the wastewater treatment process that still preserves a certain agronomic interest for its richness in plant nutrients and organic matter. Fertilizing properties of two liquid and two dewatered SSADs were tested on tomato plants (Solanum lycopersicum L.). Pot experiments were performed on sandy soil and peat substrate under greenhouse conditions with a SSADs application rate of 170 kg N/ha over a period of three months. Beneficial effects of SSADs were reported on different growth parameters, revealing an increase in biomass and height up to 37.5 and 6-folds over untreated control. No phytotoxic effect occurred on SSAD-exposed plants. Chemical analysis of soils treated with SSADs showed enrichment of macro- and micro-nutrients as well as organic matter. In some cases, the chemical characterization of leaves revealed an enhancement of uptaken macronutrients. This study contributed in general to deepen the knowledge on the short-term growing season fertilizing effects of SSAD. Despite the treatment dosage was calculated only on nitrogen requirements, the study highlighted the importance of the other nutrients and organic matter on plant growth.

Evaluation of anaerobic digestates from sewage sludge as a potential solution for improvement of soil fertility.

Sewage sludge production in European countries has widely raised in the last decade and its fate is currently landfilling, incinerators, composting or land application. To explore its agronomic potential, the main target of this work is to understand the effects of anaerobic digestates from sewage sludge (SSAD). To this aim, four different SSADs (two liquids and two dewatered) were characterized. On the liquid ones, Germination Index was evaluated through a plate bioassay with Lepidium sativum L. seeds; low concentrations of SSAD (2.5%) improved GI in one case, while at higher concentrations phytotoxic effects occurred in both. Then, pot experiments were set in climate chamber with Cucumis sativus L. grown for 30 days on two different substrates: a sandy, alkaline and poor soil, and peat substrate. All SSADs and a mineral fertilizer were used at three increasing dosages: 85, 170, 255 kg of nitrogen per hectare (kg N/ha). Results in terms of germination, dry biomass, chlorophyll content, net photosynthesis, stomatal conductance, CO2 concentration in substomatal cavity and root development were compared to a not treated control. All treatments gave results significantly higher or similar to control on all the parameters evaluated. Moreover, the intermediate nitrogen dosage (170 kg N/ha) generally showed the highest results compared to other dosages, especially for dewatered SSADs. All these results were much more evident for cucumber plants grown on an the alkaline, sandy and poor soil than on peat substrate, such demonstrating that SSADs have a fertilizing effect for plants growing on this kind of soil.