Shipboard Slop Treatment by Means of Aerobic Granular Sludge: Strategy Proposal for Granulation and Hydrocarbons Removal

Abstract

This study investigates the possibility to achieve aerobic granulation for the treatment of saline shipboard slop wastewater to remove hydrocarbons. Two sequencing batch reactors (SBR) with a working volume of 3.5 L, were used. In the first reactor (R1), granules were previously cultivated with a stepwise increase of salinity and, subsequently, they were gradually adapted to hydrocarbons. In the second reactor (R2), granules were simultaneously adapted to a gradual increase of salinity and hydrocarbons. Both the reactors were operated with an organic loading rate (OLR) of 1.6 KgCOD∙m⁻³·d⁻¹, by setting a 6-hours cycle. The volumetric exchange ratio (VER) was fixed at 50% and the hydraulic retention time (HRT) was kept at 0.5 days. Five parallel phases of 30 days-duration were studied. When only slop was fed to reactor, in R1 the total suspended solids (TSS) and granules dimension decreased till about 4.7 g∙L⁻¹ and 1.25 mm respectively, probably due to a metabolic inhibition and a breakage of granules. In R2 the TSS concentration stabilized on a value next to 4.4 g∙L⁻¹ and no degranulation was observed (mean diameter ≈ 1 mm), probable due to a biological adaptation and a biodegradation of the adsorbed recalcitrant fraction. In R1, the reduction of total petroleum hydrocarbons (TPH) in liquid phase and the increase of TPH on granules, confirmed that a physical bioadsorption effect was dominant. In R2 a decreasing trend of TPH bioadsorbed on granules, together with a contextual depletion in liquid phase, suggested a probable occurrence of hydrolysis and biodegradation of the bioadsorbed hydrocarbons.