Effect of the co-treatment of synthetic faecal sludge and wastewater in an aerobic granular sludge system

Abstract

The co-treatment of two synthetic faecal sludges (FS-1 and FS-2) with municipal synthetic wastewater (WW) was evaluated in an aerobic granular sludge (AGS) reactor. After characterisation, FS-1 showed the following concentrations, representative for medium-strength FS: 12,180 mg TSS L⁻¹, 24,300 mg total COD L⁻¹, 93.8 mg PO₃-P L⁻¹, and 325 mg NH₄-N L⁻¹. The NO₃-N concentration was relatively high (300 mg L⁻¹). For FS-2, the main difference with FS-1 was a lower nitrate concentration (18 mg L⁻¹). The recipes were added consecutively, together with the WW, to an AGS reactor. In the case of FS-1, the system was fed with 7.2 kg total COD m⁻³d⁻¹ and 0.5 kg Nitrogen m⁻³d⁻¹. Undesired denitrification occurred during feeding and settling resulting in floating sludge and wash-out. In the case of FS-2, the system was fed with 8.0 kg total COD m⁻³d⁻¹ and 0.3 kg Nitrogen m⁻³d⁻¹. The lower NO₃-N concentration in FS-2 resulted in less floating sludge, a more stabilised granular bed and better effluent concentrations. To enhance the hydrolysis of the slowly biodegradable particulates from the synthetic FS, an anaerobic stand-by period was added and the aeration period was increased. Overall, when compared to a control AGS reactor, a lower COD consumption (from 87 to 35 mg g⁻¹ VSS h⁻¹), P-uptake rates (from 6.0 to 2.0 mg P g VSS⁻¹ h⁻¹) and NH₄-N removal (from 2.5 to 1.4 mg NH₄-N g VSS⁻¹ h⁻¹) were registered after introducing the synthetic FS. Approximately 40% of the granular bed became flocculent at the end of the study, and a reduction of the granular size accompanied by higher solids accumulation in the reactor was observed. A considerable protozoa Vorticella spp. bloom attached to the granules and the accumulated particles occurred; potentially contributing to the removal of the suspended solids which were part of the FS recipe.