Co-treatment of synthetic faecal sludge and synthetic wastewater in an aerobic granular sludge laboratory reactor

Abstract

Many developing countries are facing the challenge of faecal sludge management. The limited sewer infrastructures, water scarcity and intermittent water services are the main reasons why on-site sanitation systems are their primary form of sanitation. Recently, a common practice is the co-treatment of septic/faecal sludge with municipal wastewater. Since aerobic granular sludge (AGS) systems present a compact and efficient treatment process for domestic and many types of industrial wastewater, this research aims to assess the effects of the co-treatment of a faecal sludge simulant with synthetic wastewater in a laboratory-scale AGS reactor. Two double wall column reactors (AGS-1 and AGS-2) were operated as SBRs, initially with a 3 hr cycle. Both reactors were inoculated with partly crushed granules from Vroomshoop WWTP, the Netherlands. The start-up of the reactors was repeated twice due to an algae contamination from the dilution water vessel that created a microalgae-bacteria symbiosis, slowing the granulation process and developing granules with a loose structure. During Phase II, AGS-1 was fed with synthetic wastewater and AGS-2 with synthetic wastewater and an E.coli media. The addition of the E.coli media had a noteworthy effect on the granule formation and stability in the reactor. A batch test was done under simplified conditions to assess the effects of its addition to the granulation process. SEM images showed differences in the biocoenosis and morphology of the granules between both systems. A synthetic faecal sludge simulant was developed based on reported recipes, and it was added to AGS-1 after 67 days of operation (Phase III). A chemical characterisation of the simulant was done, and its addition was at 4.2% of the influent volume of the system during 57 days. Operational conditions in AGS-1 were changed to a 4 hr cycle after its addition to enhance the biological processes. The system had average removal efficiencies of soluble COD, PO4 3--Pand NH4+-N of 89.2 ± 2.2, 97.8 ± 2.1 and 79.2 ± 15.9 %, respectively. Simultaneous nitrification and denitrification was not achieved throughout the experimental phase. A partial nitrification occurred in the system leading to the accumulation of NO2 in reactor. The removal of the particulate organic matter was mainly due to the protozoa grazing on the surface of the granules and on the partly degraded organics that were in the mixed liquor. The accumulation of solids from the simulant affected the settleability properties of the biomass and the effluent SS concentration. At the end of the experiments, the sludge bed was comprised of a mixture of granules and flocs, with an SVI5 of 36.8 ml/g. AGS-2 was run during 118 days as the control reactor. This system achieved stable conditions regarding COD and P removal after 68 days of operation. It had average removal efficiencies of COD, PO43--P and NH4+-N of 87.1 ± 6.6, 91.0 ± 10.2 and 52.5 ± 26.2 %, respectively. Towards the end of the study, it was achieved a simultaneous nitrification and denitrification in the system by lowering the DO concentrations from 50 to 20%. Overall, it was concluded that the co-treatment of low concentrations of faecal sludge could be done in an AGS process as long as the operational conditions of the system are adjusted for this type of substrate. Further studies need to be done to evaluate the stability of the granules under these conditions in long-term operations, and up to what extent its addition can be done without hindering the reactor performance