Microbial ecology of activated sludge and granular activated carbon communities treating oil and gas produced water

Abstract

In recent years, oil and gas (O&G) exploration has grown exponentially in the United States. Development in the industrial sector provokes competition with agricultural and public entities for natural resources, particularly the fresh water cycle. Typically sourced from local water supplies, energy production waste streams – specifically produced water – are disposed of through deep well injection. Implementation of advanced treatment technologies in the O&G industry presents a unique opportunity to maximize water reuse while minimizing environmental damage from increased seismic activity associated with deep well injection. These studies analyzed the microbial community development in and evaluated the performance of novel produced water treatment options. Two separate studies utilized a sequencing-batch membrane bioreactor (SB-MBR) and a biologically active filter (BAF) with granular activated carbon (GAC). Next-generation DNA sequencing and bioinformatic analyses were used to study the unique microbial shifts correlated with fluctuations in system performance. A pilot-scale SB-MBR treating residential wastewater in conjunction with up to 20% produced water by volume was investigated. Organic removal remained stable throughout the experiment despite increasing produced water fraction in the feed. The minimal variation in overall bioreactor microbial communities suggest the ability of seeded biological activity to adapt to 6–20% produced water incorporation. However, not all biological processes followed similar trends—ammonia removal declined from 85% to 55% removal. Phylogenetic results demonstrate a loss in the abundance of key nitrifying microorganisms in the bioreactors with greater produced water fractions. Biological processes recovered as produced water additions were reduced, indicating that an optimal ratio of produced water and residential wastewater to system performance is achievable. This work also addresses the performance of a BAF with GAC for the pretreatment of organic matter in produced water before membrane desalination. The biological community was seeded with native produced water microorganisms, making this system operationally unique. Two configurations were tested in two separate clear PVC columns, one operating under oxic conditions at ~0.25 L/min dissolved oxygen (DO), and the other under an anoxic condition. Phylogenetic data revealed the development of a diverse, aerobic and anaerobic microbial community. The presence of obligate anaerobic microorganisms in each column type may suggest oxygen microgradients inside GAC grains. Both columns were capable of degrading greater than 85% of the influent chemical oxygen demand (COD) suggesting that COD removal could be independent of aeration. System performance declined to 55% removal of COD in correlation with a loss in biological activity measured using ATP as a proxy. Results may indicate organic removal in each column as a factor of sorption and biological processes. Once microbial activity declined, sorption to the GAC alone was no longer sufficient in maintaining COD removal. Both studies concluded that the microbial community structure plays a pivotal role in determining overall system performance. Results suggest shifts in population abundance lead to the decreased removal of pollutants. By further understanding the microbial ecosystems involved in biological treatment, environmental controls can be optimized to maximize water reuse from unconventional O&G production. Both of these studies suggest that the combined chemical/physical/biological treatment of produced waters is indeed possible for beneficial surface use rather than deep well injection.