Abstract
ABSTRACT In this study, the impact of applied solids retention time (SRT) on the biological performance of an anaerobic membrane bioreactor (AnMBR) treating synthetic dairy wastewater with high lipid content was assessed. Two side-stream AnMBR systems were operated at an SRT of 20 and 40 days (R20 and R40, respectively), equipped with an inside-out tubular membrane operated in cross-flow mode under full-scale operational conditions, i.e. crossflow velocity, transmembrane pressure, membrane flux. Successful operation was achieved and removal efficiencies of both reactors were up to 99% applying an organic loading rate (OLR) of 4.7 g COD L−1 d−1. No precipitation of lipids was observed throughout the operational period, keeping the lipids available for the anaerobic degradation. Long chain fatty acid (LCFA) accumulation was very modest and amounted 148 and 115 mg LCFA-COD per gram of volatile suspended solids (VSS) for R20 and R40, respectively. At an SRT of 40 days, a slightly better biological conversion was obtained. Periodically performed specific methanogenic activity (SMA) tests showed stabilization of the SMA for R40 sludge, whereas for R20 sludge the SMA continued to decrease. This study revealed a more stable reactor performance operating the AnMBR at an SRT of 40 days compared to 20 days.
Highlights
The dairy sector produces large quantities of wastewater, approximately 0.2 to 10 liters of wastewater per liters of processed milk [1,2,3]
Two side-stream anaerobic membrane bioreactor (AnMBR) systems were operated at an solids retention time (SRT) of 20 and 40 days (R20 and R40, respectively), equipped with an inside-out tubular membrane operated in cross-flow mode under full-scale operational conditions, i.e. crossflow velocity, transmembrane pressure, membrane flux
At the end of the operation of both reactors, the methanogenic activities on acetate, propionate, and butyrate decreased 26%, 77%, 50% for R20 and 46%, 13% and 14% for R40, showing a slightly higher decrease in the sludge activity on R20 compared to R40. Both reactors operating at different sludge retention times were characterized by a stable operation indicated by both an organic matter removal of more than 99%, and by a stable biogas production; these performances were much better when compared to other studies on AnMBR treating other types of wastewater [19], and to other high-rate anaerobic wastewater treatment (HRAWT) systems [10]
Summary
The dairy sector produces large quantities of wastewater, approximately 0.2 to 10 liters of wastewater per liters of processed milk [1,2,3]. The main constituents of dairy industrial wastewater include biodegradable carbohydrates (mainly lactose), as well as proteins and lipids [4,5,6]. The exact composition of dairy wastewater considerably differs per location (Table 1), depending both on the type of dairy product being produced, such as milk, butter, yoghurt, ice-cream, desserts, and/or cheese, and on the production methods, operations, and technologies available at each particular industry. Most dairy wastewaters are characterized by considerable amounts of fats, oil and grease (FOG) (Table 1) [7]. Karadag et al [8], reported FOG concentrations varying from 0.5 to 9.5 g L-1 and reported a detailed analysis of the long-chain fatty acids (LCFA) being present in dairy wastewater, mainly consisting of palmitic acid (23.5%), oleic acid (21%), and myristic acid (10.5%)
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