Sequencing Batch Biofilter Granular Reactor (SBBGR) for wastewater treatment and irrigation reuse

In the last decades, both world governmental organizations and public opinion have become aware of water as a precious and limited resource. The increase in world population and climate change suggest the need for a more rational use of water, the reuse of treated wastewater turns out to be a promising strategy to preserve water resources and increase the availability of water in different sectors. The technologies used for wastewater treatment are based primarily on activated sludge systems. These technologies have several limitations such as the need for large treat-ment surfaces, low biomass settling properties and high excess sludge production, but above all, they do not guarantee sufficient pathogens removal in case of reuse of treated wastewater. Conventional wastewater treatments require a process consisting of several purification steps and need a disinfection phase to obtain microbiologically safe effluent aimed to irrigation. These disadvantages have prompted the scientific community to study new treatment systems able to guarantee the quality of treated water and the minimum environmental impact, in order to more easily meet water demand in agricultural sector. Among the new systems recently proposed that can comply with this request, the SBBGR system, developed by the Water Research Institute (IRSA) of the Italian National Research Council (CNR), seems to be a promising technology. The aim of this study is the evaluation of SBBGR as a system able to increases the simplification of the treatment scheme for treating and reusing municipal raw wastewater and improves the management of water demand and supply. In the present study, a larger group of physical, chemical and microbiological parameters was considered for evaluating the effectiveness of the SBBGR system for municipal wastewater treatment and agricultural reuse. SBBGR enhancement with sand filtration and chemical (by peracetic acid, PAA) or physical (by UV radiation) disinfection was also evaluated to have a high effluent quality needed for irrigation. The monitored parameters were chosen according to the current Italian regulation and their representativeness. In particular, the microbial indicators were chosen considering that most human pathogens that could be derived from the reuse of wastewater belong to the domains of bacteria, viruses and protozoa, and these microorganisms are characterized by different physiological characteristics and consequently different survival rates in wastewater treatment. Furthermore the spreading of antibiotic resistent genes (ARGs) into the environment has recently raised a great concern for the risk of wastewater reuse as vehicle for genes, for this the removal SBBGR capability of four genes (ermB, sul1, sul2, tetA) resistant to different classes of antibiotics was evaluated in effluent after biological treatment and after tertiary phase. The SBBGR resulted really efficient in removing suspended solids, COD, BOD and nitrogen with an average effluent concentration of 4, 40, 1 and 14 mg/L, respectively. Lower removal efficiency was observed for phosphorus with an average concentration in the effluent of 3.7 mg/L. Plant effluent was also characterized by an average electrical conductivity and sodium adsorption ratio of 0.88 mS/cm and 3.8, respectively. Therefore, according to these gross parameters, the SBBGR effluent was conformed to the national stand-ards required in Italy for agricultural reuse. Moreover, disinfection performances of the SBBGR was higher than that of conventional municipal wastewater treatment plants and met the quality criteria suggested by WHO (Escherichia coli <1000 CFU/100 mL) for agricultural reuse. In particular, the biological treatment by SBBGR removed, 2.5 ± 0.8 log units of total coliforms, 3.2 ± 1.1 log units of E. coli, 1.1 ± 0.4 log units of Clostridium perfringens, 1.4 ± 0.3 log units of Somatic coliphages, 1.5 ± 0.9 log units of Giardia and 1.8 ± 0.3 log units of Cryptosporidium. The investigated disinfection processes (UV and peracetic acid) resulted very effective for total coliforms, E. coli and somatic coliphages. In particular, a UV radiation and peracetic acid doses of 40 mJ/cm2 and 1 mg/L respectively reduced E. coli content in the effluent below the limit for agricultural reuse in Italy (10 CFU/100 mL). Conversely, they were both ineffective on C. perfringens spores. The SBBGR system is able to reduce 1-2 logarithmic units of ARGs. In particular, a removal of 1.6 ± 0.7 log units was recorded for ermB, 1.9 ± 0.8 LUR for sul1 and 2.2 ± 1.1 for tetA, while for sul2 there was a lower decrease compared to the other genes, 1.0 ± 0.4. No reduction of the ARGs amount normalized to the total bacteria content (16S rDNA) was obtained, indicating that these genes are removed together with total bacteria and not specifically eliminated. Enhanced ARGs removal was obtained by sand filtration, while no reduction was achieved by both UV and PAA disinfection treatments tested.