Biosolids recycling impact on biofilm extracellular enzyme activity and performance of hybrid rotating biological reactors

Francis Hassard,Jeremy Biddle,Elise Cartmell,Frédéric Coulon,Tom Stephenson

doi:10.1016/j.scitotenv.2019.135865

Francis Hassard, Jeremy Biddle + Show 3 more

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https://doi.org/10.1016/j.scitotenv.2019.135865

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Abstract

Biological processes for wastewater treatment is limited by extracellular enzyme activity (EEA) of the biofilm on polymeric substrates. The efficiency of biodegradation / biosorption mechanisms causing EEA and organic load removal in biofilms remains unknown. Our hypothesis was that the limiting step of biological process can be overcome by biostimulation and/or bioaugmentation of the return sludge in hybrid biofilm reactors, which leads to competition between suspended and attached bacteria and lower effective substrate to microrganism ratio. Therefore, we considered more active biosolids to perform best at enhancing reactor removal rate. To test this, the efficacy of recycling distinct bio-solids types considered to have different bacterial activity such as final effluent (FE), humus solids (HS) and recycle activated sludge (RAS) on performance improvements of rotating biofilm reactors (RBRs). These bio-solids were investigated under high organic loading rates (OLR) and solids loading rates (SLR) using pilot scale reactors receiving real municipal wastewaters. Controlled overloading of RBRs revealed that EEA improved with increasing OLR/SLR. High SLR (>3.3 kg Total Suspended Solids m−2 d−1) delayed and decreased the reduction of organic and inorganic removal rates in the biological processes which commonly occurs under high OLRs. This effect was more pronounced in the highest activity solids (RAS > HS > FE) suggesting the activity and function of bio-solids was critical to improve performance of RBRs. High OLR and SLR induced efficient denitrification and organics removal within the biofilm reactor at residence times of <5 min. Recycling active solids permitted EEA despite overloading which was critical to the performance of the RBRs.

Highlights

Extracellular enzyme activity (EEA) is needed to treat organic wastewater polymers as the majority are too large to be transported directly into bacteria, this is the case for biological processes containing floc forming activated sludge bacteria, granular sludge bacteria and biofilms (Burgess and Pletschke, 2008; Hassard et al, 2018)
The very high organic loading rate (OLR) treatment was the exception as the NO3-N concentration in the recycle activated sludge (RAS) recycle feed decreased by 67% to 8.2 ± 4.3 mg L−1, due to denitrification in the final clarifier of the activated sludge plant (ASP) at these very high recycle rates
The rotating biofilm reactors (RBRs) reactor DO decreased with higher OLRs and solids loading rates (SLR) (Tables 1 and 2) principally due to lower hydraulic retention time (HRT) for oxygenation and greater oxidation of bulk organics

Summary

Introduction

Extracellular enzyme activity (EEA) is needed to treat organic wastewater polymers as the majority are too large to be transported directly into bacteria, this is the case for biological processes containing floc forming activated sludge bacteria, granular sludge bacteria and biofilms (Burgess and Pletschke, 2008; Hassard et al, 2018). Polymers are transported through the water, second, the polymeric material is adsorbed to the biofilm, third the polymeric material undergoes a series of stepwise depolymerisation reactions and components are assimilated, stored or released by the bacteria These processes can be slow and have the potential to restrict the substrate removal rates which can be achieved in wastewater treatment. Biofilm processes have a genuine advantage over traditional flocculated systems through intensification of bacteria and their extracellular enzymes, but a biofilm will usually have a larger barrier to substrate diffusion than a smaller floc This is pertinent considering that wastewater bacteria have been shown to regulate EEA based on available substrates, electron acceptor conditions and their specific microbial growth rate (Li and Chróst, 2006; Hauduc et al, 2013; Shackle et al, 2000). This suggests, that EEA could be bioengineered (increased) through effective process control to improve removal rates in aerobic conditions (Confer and Logan, 1998; Hassard et al, 2016; Hassard et al, 2018)

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