# Biomethane Potential of Sludges from a Brackish Water Fish Hatchery
###### Abstract  
The development of intensive aquaculture is facing the challenge of the sustainable management of effluents. The reproductive sectors (i.e., hatcheries) mainly use water recirculation systems (RAS), which discharge a portion of wastewater. Anaerobic digestion (AD) could reduce the environmental impact of this waste stream while producing biogas. The study is focused on the biochemical methane potential (BMP) of brackish fish hatchery sludges. Wastewater was concentrated by microfiltration and sedimentation and thickened sludges were treated in a BMP system with different inoculum/substrate (I/S) volatile solids ratios (from 50:1 to no inoculum). The highest I/S ratio showed the highest BMP (564.2 NmL CH\({}_{4}\)/g VS), while different I/S ratios showed a decreasing trend (319.4 and 127.7 NmL CH\({}_{4}\)/g VS, for I/S = 30 and I/S = 3). In absence of inoculum BMP resulted of 62.2 NmL CH\({}_{4}\)/g VS. The kinetic analysis (modified Gompertz model) showed a good correlation with the experimental data, but with a long lag-phase duration (from 14.0 to 5.5 days) in particular with the highest I/S. AD applied to brackish water sludges can be a promising treatment with interesting methane productions. For a continuous, full-scale application further investigation on biomass adaptation to salinity and on retention times is needed. Further experimental tests are ongoing. biomethane; sludges; aquaculture; brackish; fish; hatchery; anaerobic digestion 2020  
5 January 2021  
Bionethane Potential of Sludges from a Brackish Water Fish Hatchery  
[PERSON]  
1 Department of Agricultural, Food, Environmental and Animal Sciences (DHA), University of Udine Via delle Scienze 206, 33100 Udine, Italy; [PERSON]F.G.B. ); [EMAIL_ADDRESS] (G.F.)  
33100 Udine, Italy; [EMAIL_ADDRESS] (M.M. ); [EMAIL_ADDRESS] (D.G.) 40/10/2020055204 produced waste is very diluted and for this reason it is difficult to store or to submit to a proper treatment [4; 5]. Aerobic degradability of saline industrial wastewater was successfully demonstrated by comparing biological and chemical treatments in [PERSON] et al. (2014) [6] and by using biological sequencing batch reactors (SBRs) in Uygur (2006) [7], with particular focus on nutrient abatement. However, considering that aerobic processes are often characterized by high-energy demand and necessarily lack biogas production, in several cases anaerobic digestion (AD) treatment is preferred, particularly when concentrated effluents need to be treated. In the present research, anaerobic digestion (AD) was investigated for the treatment of sludges from RAS of intensive fish hatchery using brackish water (i.e., salinity 10-12 g/L), after sludge thickening by mechanical filtration and sedimentation. AD applied to marine or brackish aquaculture effluents could allow several advantages: the obtained biomethane can be used as an alternative source of renewable energy [8; 9]; the supernatant from AD may be recycled to RAS, saving water, salts, reducing energy costs and obtaining a lower final sludge volume [10]; a reduction of the atmospheric pollution and carbon footprint can be accomplished [11; 12]; and a reduction of sludge transport, disposal or reuse costs can be obtained [4; 13]. However, some critical aspects can be associated to the anaerobic treatment of these sludges. AD is known to be inhibited by high salinity mainly due to the presence of cations: in particular, a sodium concentration higher than 10 g/L strongly inhibits methanogenesis [14]. However, the anaerobic archaea are able to adapt to moderate saline conditions, where methane production can be even stimulated, rather than inhibited [15]. The high levels of sulfate, which is reduced to sulfide under anaerobic conditions, and of ammonia produced by the degradation of protein in feces and excess feed, can be other limiting factors in applying AD to aquaculture effluents [16].