Shale wastewater, the wastewater produced in oil Shale processing, contains high levels of complex organic compounds. There are concerns for protecting the quality of the receiving environment when shale wastewater is disposed without appropriate treatment. Previous studies have demonstrated that a column packed with solid waste products (shale ash and overburden) is an effective process for shale wastewater treatment. However, column blockage is a major hindrance in sustaining long-term operation of this process.
This thesis aims to provide a better insight into the fundamentals of this packed-column for shale wastewater treatment. Three specific research objectives are pursued, namely:
· the roles of adsorption and biofilm in the successful treatment of the column
· the concentration profiles of shale wastewater organics and biomass down the column
· the cause of column blockage
The shale wastewater and shale ash investigated in this thesis originated from the Stage 1 Southern Pacific Petroleum Stuart oil shale project. The shale wastewater had an alkaline pH, high concentrations of TOC (4800 mg/L), TKN (630 mg/L), phenol (160 mg/L) and an insignificant amount of phosphorus. The investigation into the degradability of the shale wastewater organics was focused on a column study using a 0.81 m shale ash packed-column (SAPC), fed with the shale wastewater at a flow velocity of 0.040 m3/m2/d. The extent of biodegradation was measured by monitoring CO2 evolution from the reactor by using on-line respirometry technique. Biodegradation could then be clearly distinguished from physical hold up processes such as precipitation and adsorption. Biodegradation was proposed to occur in the bulk liquid, biofilm and adsorbed phase. To identify the relative importance of these phases, a series of experiments were conducted on systems that featured only one or two of these phases. Specifically, these experiments included a 0.81 m column packed with glass beads (GBPC), a 4 L sequencing batch biofilm reactor with shale ash (SBBR) and a 4 L sequencing batch suspended-growth reactor (SBR). A numerical model was developed to simulate the TOC profiles down the SAPC, based on independently determined kinetic parameters in the GBPC, SBBR and SBR experiments.
The major outcomes from this PhD project are:
· Biodegradation of the shale wastewater TOC occurred in a column with shale ash only. Overburden, as an inoculum in previous studies, was not required. This precluded one of the factors causing column blockage as overburden has dispersive clay nature.
· More than 96% of the shale wastewater TOC was removed by the SAPC and at least 86% was biodegraded. The biomass in the column at the end of the experiment was equivalent to 2% of the degraded TOC, but this low yield represents the net result of growth and endogenous decay processes. The TOC accumulated on the shale ash surface was approximately 10% of the influent TOC. The alkaline pH and ammonia (43 mg/L) in shale wastewater had no adverse influence on the performance of the SAPC.
· The modelling results showed that the mechanisms governing the treatment of shale wastewater in the SAPC could not be approximated by simply summing the degradation mechanisms occurring in the SBR, SBBR and GBPC. The rapid degradation of the readily biodegradable TOC in the top 0.1 m of the GBPC was not evident in the SAPC. The biofilm in the SAPC was of small significance in degradation of shale wastewater biorefractory organics; however, it was still critical in the SAPC because the shale ash surface beneath the biofilm played an important role in biodegradation of the shale wastewater organics, which would otherwise be biorefractory as in the controls where less than 65% TOC was degraded, regardless of operating conditions.
· All of the phenol in the shale wastewater was degraded within the top half of the SAPC. The biomass distribution in the SAPC corresponded well with the TOC degradation rate, with the largest biomass concentration present in the top 0.05 m of the column, where the highest TOC degradation rate was observed. This suggests that the required depth of the column is not more than 0.4 m at an organic loading rate of 0.2 kg TOC/m2/day.
· Based on the profiles of pressure drop down the columns, the blockage was found in the top 0.1 m of the columns. The pressure drop profiles were consistent with the profiles of biomass concentration down the column, with the largest pressure drop in the top zone of the column where the greatest biomass concentration was present. Microscopy observation showed that a relatively thin and patchy biofilm attached firmly to the shale ash in the upper region of the SAPC. In contrast, the biofilm on the glass beads in the upper region of the GBPC was extremely thick and gel-like. Biomass aggregates were accumulated between the voids of glass beads.