Volatile fatty acid production and application as external carbon source for denitrification

Abstract: By rethinking wastewater treatment plants (WWTPs) as resource recovery facilities, it is possible to de- velop the next generation of WWTPs. Moreover, it allows to accomplish environmental goals, such as reducing the CO2 footprint, and comply with increasing effluent standards regarding the concentration of nitrogen in a more sustainable way. This research study aims to analyse the possibility of recirculating carbon within WWTPs in form of volatile fatty acids (VFAs) produced by co-fermentation of primary sludge and food waste. The obtained fermentation liquid is utilised as carbon source to enhance the denitrification process in a post-anoxic denitrification plant setup. Two pilot scale fermentation reactors were semi-continuously operated, systematically varying only in pH. By controlling one reactor to pH 10, while the second reactor was operated without pH control, it was possible to assess the influence of the pH on the carbon recovery process. Despite the pH not being controlled in the second fermentation reactor, it adjusted itself to a stable pH around 5.4. The co-fer- mentation process was monitored by weekly analysis of the SCOD and total amount of VFAs (TVFA). While the alkaline conditions in the reactor operated at pH 10 allowed a higher hydrolysis of the sub- strate, the second reactor, operated without pH control, achieved a more distinct acidification, due to the lower pH. Consequently, the SCOD in the reactor operated without pH control contains a higher percentage of TVFA amounting to 64 % in comparison to the reactor operated at pH 10 with 40 % TVFA. Furthermore, the achieved degree of fermentation was assessed by calculating the net increase of TVFA per gram of VS, respectively VSS. A higher degree of fermentation was achieved without pH control, resulting in a higher VFA yield compared to the fermentation reactor operated at pH 10. Moreover, anal- ysis of the individual VFAs by gas chromatography showed distinct differences in the composition of the fermentation liquids. According to the findings, the reactor operated at pH 10 produced mainly acetic acid (61 %), followed by propionic acid (18 %) and n-butyric acid (14 %). In contrast, the fermentation reactor operated without pH control produced mainly n-caproic acid (47 %), followed by acetic acid (25 %) and n-butyric acid (16 %). Despite the similar fermentation substrate supplied to both reactors, the acidic conditions in the reactor operated without pH control allowed carboxylic acid chain elongation from acetic acid to n-caproic acid, resulting in the main difference of the fermentation liquids. The fermentation liquid of the two reactors was filtered, diluted to a concentration of 5 g COD/L and supplied as additional carbon source to enhance denitrification in two continuously operated pilot scale moving bed biofilm reactors (MBBR), applying a carbon-to-nitrogen ratio of 4.5. One of the denitrifica- tion MBBRs received the carbon recovered by fermentation at pH 10 as external carbon source, whereby the carbon source produced by fermentation without pH control was supplied to the other MBBR. The maximal achieved denitrification rate was quite similar for both MBBRs amounting to 3.25 g NO3- Neq/(m2·d) for the MBBR receiving the carbon source recovered by co-fermentation at pH 10 and 3.38 g NO3-Neq/(m2·d) for the MBBR receiving the VFA-mix obtained by co-fermentation without pH control. However, the MBBR provided with the carbon source recovered by co-fermentation under acidic conditions achieved a higher average denitrification rate of 2.5 g NO3-Neq/(m2·d), compared with the MBBR receiving carbon produced by co-fermentation at pH 10 (1.8 g NO3-Neq/(m2·d)). The lower efficiency of the MBBR supplied with additional carbon recovered by fermentation at pH 10 is caused by an accumulation of NO2-N during the denitrification process. This accumulation of NO2-N indicates suboptimal conditions, both due to the composition of the supplied carbon source and an overall higher pH during the denitrification process, which might supress facultative anaerobes, such as denitrifiers. Nevertheless, this study shows that both VFA-rich carbon sources obtained by co-fermentation of pri- mary sludge and food waste are suitable to enhance denitrification of municipal wastewater, with the carbon source recovered by fermentation without pH control achieving a higher denitrification effi- ciency.