• Energy Research

Textile effluents are characterised by high content of recalcitrant compounds and are often discharged (together with municipal wastewater to increase their treatability) into centralized wastewater treatment plants with a complex treatment scheme. This paper reports the results achieved adopting a granular sludge system (sequencing batch biofilter granular reactor - SBBGR) to treat mixed municipal-textile wastewater. Thanks to high average removals in SBBGR (82.1% chemical oxygen demand, 94.7% total suspended solids, 87.5% total Kjeldahl nitrogen, 77.1% surfactants), the Italian limits for discharge into a water receiver can be complied with the biological stage alone. The comparison with the performance of the centralized plant treating the same wastewater has showed that SBBGR system is able to produce an effluent of comparable quality with a simpler treatment scheme, a much lower hydraulic residence time (11 h against 30 h) and a lower sludge production.

Civil buildings air conditioning systems comprise a massive source of energy consumption and greenhouse gas emissions. The experimental study we present is a full-scale demonstration of a "nearly zero energy facility" provided with a water-based air conditioning system and domestic hot water production unit, both supplied with thermal energy produced by a water sourced solar heat pump, whose hot and cold sources are partially fed by the leftover thermal bio-energy available in an innovative wastewater treatment system, whereas energy excess produced by solar heat pump is stored in phase change material units. Preliminary results showed that the innovative system, thanks to its particular features, allows wastewater heat extraction within the treatment process, and demonstrated its ability to exchange energy and, at the same time, to operate a thermal regulation of the treatment reactors, integrating the optimization of thermo-dependent biological processes with energy recovery systems.

This paper reports the results of an investigation aimed at evaluating the performance of an innovative technology (SBBGR system - Sequencing Batch Biofilter Granular Reactor), characterised by a low sludge production, for treating municipal wastewater at demonstrative scale. The results have shown that even at the maximum investigated organic load (i.e., 2.5 kg COD/m(3) d), the plant removed 80% of COD, total suspended solids and nitrogen content with relative residual concentrations lower than the Italian limits for discharge into soil. The process was characterised by a very low sludge production (i.e., 0.12-0.14 kg TSS/kg COD(removed)) ascribable to the high sludge age in the system (thetac >120 d). Molecular in situ detection methods and microscopy staining procedures were employed in combination with the traditional measurements (oxygen uptake rate and total protein content) to evaluate both the microbial activity and composition, and the structure of the biomass. A stable presence of active bacterial populations (mainly Proteobacteria) was found within compact and dense aggregates.

The integration of an off-grid solar-assisted heat pump (SHP) and a sequencing batch biofilter granular reactor (SBBGR) for thermal energy recovery from wastewater was assessed by means of a prospective life cycle assessment (LCA) and life cycle costing (LCC), by theoretically scaling up a pilot installation in Bari, Italy, to a full-scale unit designed for 5000 person-equivalents. The LCA and LCC included all activities in the life cycle of the SHP and wastewater treatment plant (WWTP), namely construction, operation and end-of-life. The thermal energy produced by the SHP was assessed as supplying heating and cooling for an air-conditioning system, displacing a conventional air-source heat pump powered by electricity from the grid. This integrated systemwas compared to a reference situation where wastewater is treated in a conventional WWTP applying activated sludge with no thermal energy recovery system, showing clear environmental benefits in all impact indicators, such as a 42% reduction in greenhouse-gas emissions and a cost reduction of 53%. Several sensitivity analyses confirmed these findings, with the exception of the price rebound effect, which showed that the lower cost of the integrated system could lead to overturning the environmental benefits. As a limitation of the study, the distribution of the supplied air-conditioning to meet a demand off-site theWWTP premises, such as in residential buildings or hotels, was not included. Therefore, our results constitute only a preliminary positive outcome that should be validated in a real-life application.

The organic fraction of municipal solid waste (OFMSW) is widely recognized as a possible substrate for anaerobic digestion processes. However, the heterogeneity of this matrix and the presence of slowly biodegradable compounds can slow down anaerobic digestion and reduce its performance. This study compares the effectiveness of different thermal pre-treatments in increasing OFMSW anaerobic digestibility. Thermal pre-treatments were compared with OFMSW shredding, considered as the minimum pre-treatment required in order to reduce particles size of the OFMSW. The pre-treatments were performed in autoclave (121 °C and 1.4 bar for 20 min) or in an ad hoc hydrolysis reactor designed for the experimental trial (140 °C and 7 bar for 30 min) with air or nitrogen as gas phase. The thermal pre-treatments affected methane yield (NmLCH4/gVS), depending on the pre-treatment strategy, with autoclaving allowing for an 80% increase with respect to the control run, and leading to a methane yield of 476 ± 194 NmLCH4/gVS. The pre-treatments in the hydrolysis reactor caused a loss of organic matter (due to its volatilization) reducing the organic loading rate of the digester. Nevertheless, the digester performance in terms of COD (chemical oxygen demand) and VSS (volatile suspended solid) removal showed limited differences among the pre-treatments applied and ranged on average 79–94%.

Urban wastewater is a valuable source of clean energy available for both building conditioning and hot sanitary water production, thus reducing primary energy demand and greenhouse gas emissions. In the present study, the integration of a highly efficient solar-assisted fully off-the-grid water-source heat pump (SHP) with a sequencing batch biofilter granular reactor (SBBGR) is tested on a pilot scale for recovering and reusing thermal energy generated during the depuration process. The prototype was designed to simulate wastewater production (240 L/d), domestic hot water (DHW) (152 L/d at 40 °C), and space heating (20-25 °C) energy demand for a one-person equivalent. Three set temperatures for heat extraction from the SBBGR were tested: 20, 14, and 10 °C. Heat extraction had limited effects on the average SBBGR performances. The SBBGR ensured a removal efficiency close to 90% for total suspended solids (TSS), chemical oxygen demand (COD), and ammonia, whereas a decrease in total nitrogen (TN) removal efficiency, namely from 75% to 71%, was observed with the operating temperature decrease. Energy recovery data suggested that the energy extracted from the SBBGR might cover the energy demand for DHW production or space heating from April to October. Thus, the collected energy data was modeled with the following purposes: highlighting the key parameters for optimizing energy recovery, quantifying the share of recoverable energy derived the microbial metabolism, and supporting or rejecting the scalability of the results. The model outcomes confirmed that the temperature difference between the sewage and heat extraction set point temperatures was the key parameter for energy recovery and succeeded in estimating the contribution of microbial metabolisms (i.e. about 3.2 kWh/m3 × d). However, the estimation of the full-scale recoverable energy was partially biased by the impact of the environmental conditions on the pilot.

Posidonia oceanica is the most abundant aquatic plant of the Mediterranean Sea where it plays great ecological importance. The accumulation of residues along the shore, however, creates a littering hardship for the territory due to their bad rotting smell and introduces an obstacle to the enjoyment of the beaches and tourist swimming. Posidonia oceanica residues may be valorized producing bioenergy by anaerobic digestion. Due to its high lignin content, however, a pretreatment step is required for enhance energy recovery. In the present study the effects of acid addition in the thermal hydrolysis step were evaluated in terms of energy balance, biogas production and solids reduction. The results obtained have shown that when thermal pretreatment was enhanced by adding hydrochloric acid (0.4% w/w), an improvement in methane production of 575% was obtained compared to thermal pretreatment only with specific biogas production as high as 0.241 ± 0.065 Nm per kgVS of wet Posidonia or 0.138 ± 0.056 Nm CH/kgVS. This result was ascribed to the defibration of lignocellulosic components operated by acidic thermal pretreatment which allowed the removal of 74%, 70% and 24% of cellulose, hemicellulose and lignin, respectively, during anaerobic digestion. The energy analysis carried out for treatment plants with capacity of 10 and 50 m/d has shown that acid addition in the thermal hydrolysis step allows the energy balance to turn from extremely negative (energy demand is 8 to 10 times greater than the one produced) to positive values, with process energy efficiencies ranging from 22 to 35% with regards to the size of the plant.

In the present study, the possibility of recovering both thermal energy and water for agricultural purposes from sewage is evaluated. A treatment plant, based on a sequencing batch biofilter granular reactor (SBBGR) followed by sand filtration and coupled with a solar wastewater source heat pump, was operated from September to November 2018 at a set-point temperature of 14 °C to verify the stability of heat recovery efficiency and the suitability of plant effluent to be reused for irrigation. Heat recovery did not influence the SBBGR treatment and disinfection efficiency, which removed about 90% of suspended solids, chemical and biochemical oxygen demand and ammonia, as well as 70% of total nitrogen, 3 log10 units of Escherichia coli and more than 1 log10 unit of Clostridium perfringens. Furthermore, after sand filtration, water quality complied with the standards for agricultural reuse currently in force in several countries. Energy extracted from SBBGR was mainly influenced by environmental conditions, affecting wastewater temperature, and also by wastewater composition, affecting the energy release due to bacterial metabolic activity for carbon and nitrogen removal. Notably, no evident deterioration of energy extraction efficiency from the SBBGR was observed, suggesting negligible fouling phenomena on the submerged thermal exchanger.