Is anaerobic digestate from sewage sludge a potential solution for improvement of poor soils?

Sewage sludge is the principal waste coming from wastewater treatment and its production in Europe has rather increased in last twenty years. Nowadays, anaerobic digestion (AD) is the most diffused technique to stabilize SS and achieve energy recovery. Anaerobic digestate from sewage sludge (SSAD) is the principal by-product of this process. In the present work, one of the SSAD destinations (i.e. agricultural use) is reprised and challenged, evaluating the fertilizing effects of SSAD on a soil depleted of organic matter and plant nutrients. To this aim, four different SSADs (two liquid and two dewatered) were characterized and used to set up pot experiments with cucumber (Cucumis sativus L.), grown on two different substrates: a poor and sandy soil, and an organic substrate. SSADs and a mineral fertilizer were utilised at three different dosages: 85, 170, 255 kg of nitrogen per hectare (kg N/ha). Different plant parameters were measured such as germination, dry biomass, chlorophyll content index (CCI), net photosynthesis, stomatal conductance, CO2 concentration in substomatal cavity and root development. Results showed that the evaluated parameters were in general significantly similar or higher than untreated control. Moreover, the 170 kg N/ha dosage generally showed the best results compared to other dosages, especially for dewatered SSADs. All these results were much more evident for cucumber plants grown on sandy soil than on peat substrate, assessing the desired fertilizing effect on a poor soil.